Classically, psychedelics have been defined to include drugs such as lysergic acid diethylamide (LSD), mescaline, phenylcyclohexyl piperidine (PCP), ibogaine, 3,4-methylenedioxymethamphetamine (MDMA), psylocibin and ketamine, because each of these compounds produces alterations to sensory, self, time and space perception that are “so alien to everyday experience that they shed new light on the workings of these everyday mental functions”. Although more recent attempts have been made to subcategorize psychedelics on the basis of the subjective character of the altered state that they induce (for example, hallucinogenic, empathogenic, oneirogenic or dissociative), their chemical structure (for example, tryptamines, phenethylamines or arylcyclohexamines), or their principal binding target (for example, serotonin receptor 2A (5-HT_2AR), monoamine transporter, κ-opioid receptor (KOR) or N-methyl-D-aspartate receptor (NMDAR)), the importance of these categories for therapeutic applications remains unclear, since psychedelics that span the diversity of classification systems have shown remarkable promise for the treatment of addiction, post-traumatic stress disorder (PTSD) and depression. Thus, identification of a common neurobiological mechanism that can account for the shared therapeutic effects of psychedelics is an obvious priority for translational neuroscience.

During specific periods of brain development, the nervous system exhibits heightened sensitivity to ethologically relevant stimuli, as well as increased malleability for synaptic, circuit and behavioural modifications. These mechanistically constrained windows of time are called critical periods and neuroscientists have long sought methods to reopen them for therapeutic benefit. Recently, we have discovered a novel critical period for social reward learning and shown that the empathogenic psychedelic MDMA is able to reopen this critical period. This mechanism shares a number of features with the therapeutic effects of MDMA-assisted psychotherapy for the treatment of PTSD, including rapid onset, durability and context dependence. At the same time, cocaine does not reopen the social reward learning critical period, and since cocaine does not share the psychedelics’ therapeutic profile, these results lend further support for the view that the reinstatement of social reward learning in adulthood underlies the therapeutic efficacy of MDMA.

Whether the ability of MDMA to reopen the critical period for social reward learning generalizes across psychedelics remains an open question. MDMA is classified as an ‘empathogen’ because its acute subjective effects are distinctly prosocial in quality. The fact that this quality is not shared by hallucinogenic psychedelics such as psilocybin and LSD, dissociative psychedelics such as ketamine, or oneirogenic psychedelics such as ibogaine challenges the idea that these drugs could reopen the social reward learning critical period. However, the psychotropic effects of MDMA include an altered state of consciousness shared by all psychedelics, and if it is this characteristic rather than its prosocial properties that embodies the subjective experience of reopening critical periods, then the ability to reinstate social reward learning in adulthood might generalize across psychedelics.

Critical period reopening is a shared property

To test whether the ability of MDMA to reopen the social reward learning critical period generalizes across psychedelics, we began by examining the effect of psilocybin pretreatment on the magnitude of social reward learning in adulthood using the social reward conditioned place preference (sCPP) assay (Extended Data Fig. 1). We administered a single intraperitoneal (i.p.) dose of psilocybin(0.3 mg kg⁻¹) to adult male mice (at postnatal day 96 (P96)) and 48 h later (at P98), we assessed the magnitude of sCPP (Fig. 1a). Mice pretreated with psilocybin, but not saline, exhibited a significant sCPP at P98 (Fig. 1b–d). To formally designate ‘open’ and ‘closed’ states of this critical period, we next generated a natural spline regression model to previously published data with knots at P35 and P98 (P = 1.003 × 10⁻⁶; root mean square error (r.m.s.e.) = 0.19; R² = 0.11), as shown in Extended Data Fig. 2. When compared with this derived curve, the magnitude of sCPP in saline-treated mice did not deviate significantly from the closed state (P = 0.72), whereas the fit derived from psilocybin-treated mice demonstrated a significant mean shift (P = 1.12 × 10⁻⁶) in range of the open state (Fig. 1e). Similarly, pretreatment with LSD (i.p. 1 µg kg⁻¹) but not saline, also reopened the critical period for social reward learning (saline P = 0.90, LSD P = 1.76 × 10⁻⁹) (Fig. 1f–i). Next, we examined the effects of ketamine (i.p. 3 mg kg⁻¹) and ibogaine (i.p. 40 mg kg⁻¹). Mice pretreated with either drug also exhibited sCPP in adulthood (P = 8.78 × 10⁻⁴ and P = 3.17 × 10⁻⁵, respectively) (Fig. 1k–m). As with MDMA, these effects were dose-dependent (Extended Data Fig. 3). In juveniles, MDMA (i.p. 10 mg kg⁻¹) pretreatment did not lead to a further increase the magnitude of social reward learning (Extended Data Fig. 3). In contrast to its effects on social reward learning behaviour, pretreatment with psychedelics had no effect on the magnitude of two addiction-like behaviours: cocaine reward learning and amphetamine-induced locomotor sensitization (Extended Data Fig. 4). Together, these studies demonstrate that as with empathogenic psychedelic, hallucinogenic, oneirogenic and dissociative psychedelics are able to reopen the critical period for social reward learning.'

Duration of the psychedelic open state

The duration of acute subjective effects and the durability of the therapeutic response vary considerably across psychedelics. For example, in humans, the acute subjective effects of ketamine last 30–120 min, whereas its antidepressant effectslast for 1 week. By contrast, the subjective effects of psilocybin and MDMA last for 3–6 h, whereas the acute effects of LSD and ibogaine persist for 8–10 h and 36–72 h, respectively; these long-lasting subjective effects correspond to highly durable therapeutic effects that last months to years. Previously, we showed that MDMA-induced critical period reopening lasts for two weeks, but returns to the closed state by four weeks. Here, to further probe the time course of the critical period open state induced by psychedelics, we examined the duration of critical period reopening following treatment with ketamine, psilocybin, LSD and ibogaine (Fig. 2a). One week following psychedelic treatment, psilocybin-treated mice, but not those treated with ketamine, exhibited significant social reward learning (Fig. 2b–e). Two weeks following psychedelic treatment, the social reward learning critical period remained open for both psilocybin- and LSD-treated mice (Fig. 2f–i). At three weeks, LSD-treated mice, but not those treated with psilocybin, exhibited significant social reward learning (Fig. 2j–m), whereas at four weeks, the social reward learning critical period remained open for mice treated with ibogaine but not those treated with LSD (Fig. 2n–q). For each psychedelic, we examined at least three time points; increasing the LSD dose to 50 µg kg⁻¹ did not extend the duration of the open state (Extended Data Fig. 5). As shown in Fig. 3, the progressively longer-lasting open states induced by ketamine (Figs. 1f–i and 2b–e and Extended Data Fig. 5), followed by psilocybin (Fig. 2b–i), MDMA¹¹ (Extended Data Fig. 5), LSD (Fig. 2j–q) and ibogaine (Fig. 2n–q and Extended Data Fig. 5) are proportional to the duration of the acute subjective effects of these drugs in humans. These results provide a mechanistic explanation for the importance of the post-treatment integration period for clinical implementation of psychedelics, and inform the design of novel compounds for clinical applications.

Metaplasticity, not hyperplasticity

Dynamic regulation of the extent to which synaptic plasticity can be induced is called ‘metaplasticity’ and is thought to be one of the mechanisms underlying the establishment of critical periods. Previously, we showed that oxytocin induces a novel form of presynaptically expressed long-term depression, and implicated this plasticity in encoding social reward learning. Here, to determine whether the ability to induce metaplastic upregulation of oxytocin plasticity generalizes across psychedelics, we pretreated adult mice with either saline, cocaine or psychedelics. Forty-eight hours or two weeks later we prepared ex vivo acute slices containing the nucleus accumbens (NAc) and conducted whole-cell voltage-clamp recordings from medium spiny neurons (MSNs) (Fig. 4a–c). A 10-min bath application of oxytocin induced a significant decrease in the frequency (Fig. 4d–k) but not the amplitude (Fig. 4l–s) of miniature excitatory post-synaptic currents (mEPSCs) following pretreatment with MDMA, LSD, psilocybin, ketamine and ibogaine, but not with saline or cocaine, at 48 h; this metaplasticity persisted for 2 weeks in the LSD pretreatment group, but not in the ketamine pretreatment groups. We did not observe significant changes in baseline mEPSC amplitude or frequency following pretreatment with psychedelics in the NAc or in layer 5 of the medial prefrontal cortex (mPFC) (Extended Data Fig. 6). Together, these results provide evidence that psychedelics induce metaplasticity rather than hyperplasticity, a distinction that is especially important for designing biomarkers to test therapeutic profiles and abuse liability of novel compounds.

5-HT_2AR is not the universal mechanism

The serotonin receptor 5-HT_2AR, first identified by its binding to LSD, mediates alterations of perception and cognition induced by ‘serotonergic psychedelics’ such as LSD and psilocybin. Furthermore, MDMA is thought to trigger synaptic efflux of serotonin through its binding at the serotonin transporter SERT, and some of the effects of ketamine are reportedly mediated by 5-HT_2AR. Thus, we sought to determine the role of 5-HT_2AR in reopening the social reward learning critical period with LSD, psilocybin, MDMA and ketamine. We administered psychedelics intraperitoneally in P96 adult mice either alone or in combination with ketanserin (HTR-A, 0.1 mg kg⁻¹)—the 5-HT_2AR antagonist used in human studies—which we injected 30 min before the psychedelic (Extended Data Fig. 7). Pre-treatment with either LSD or psilocybin induced reinstatement of sCPP measured 48 h later, and this effect was blocked by co-administration of ketanserin (Extended Data Fig. 7). However, MDMA-induced reinstatement of sCPP persisted in the presence of ketanserin (Extended Data Fig. 7). Similarly, co-administration of ketanserin did not block ketamine-induced reinstatement of social reward learning in adulthood (Extended Data Fig. 7). These results demonstrate that whereas 5-HT_2ARs are required for LSD- and psilocybin-induced reopening of the social reward learning critical period (with potential contributions from serotonin 2B and 2C receptors, since ketanserin also has affinity at these serotonin receptor 2 subtypes), MDMA and ketamine reinstate social reward learning in a 5-HT_2AR-independent manner. Although some have argued that psychedelics that bind 5-HT_2AR (such as LSD and psilocybin) should be classified separately from those that do not (such as MDMA and ketamine), these results identify a novel property (critical period reopening) that coheres the category of psychedelics but violates the 5-HT_2AR-binding boundary. Thus, combined with the data presented in Figs. 1 and 2, these results support the continued use of the established naming convention for psychedelics, rather than subclassification or renaming based on receptor binding or subjective properties.

β-arrestin-2 is not the universal mechanism

Recent studies indicate that prolonged binding at the 5-HT_2AR by LSD triggers β-arrestin-2 (β-arr2)-biased signalling over canonical G-protein signalling. Moreover, the effects of MDMA and ibogaine are also thought to be mediated by metabotropic G-protein-coupled receptors (GPCRs). Although the therapeutic effects of ketamine are thought to be mediated by ionotropic NMDA receptors, the metabotropic glutamate receptor 5 has also been implicated. To test the hypothesis that β-arr2-biased signalling mediates the ability of psychedelics to reopen the social reward learning critical period, we examined their effects in commercially available β-arr2-knockout (KO) mice. We began by determining baseline sCPP in juvenile and adult β-arr2-KO mice and found that these mice exhibited the normal maturational profile of social reward learning (Extended Data Fig. 8). Next, we compared the magnitude of sCPP in adult (P98) β-arr2 wild-type and β-arr2-KO mice 48 h following administration of psychedelic drugs (Extended Data Fig. 9). LSD and MDMA reopened the social reward learning critical period in wild-type mice but did not do so in β-arr2-KO mice (Extended Data Fig. 9). Conversely, ketamine and ibogaine were able to reinstate social reward learning in both wild-type and β-arr2-KO mice (Extended Data Fig. 9). Together, these results demonstrate that whereas β-arr2 signalling is required for LSD- or MDMA-induced reopening of the social reward learning critical period, ketamine or ibogaine reinstate social reward learning in a β-arr2-independent manner.

Psychedelics induce remodelling of the ECM

Since psychedelics as a class all reopen the social reward learning critical period (Fig. 1) even though these drugs act on a diverse array of principal binding targets (Extended Data Fig. 7) and biochemical signalling pathways (Extended Data Fig. 9), we reasoned that the common mechanism that enables critical period reopening might be downstream of these cellular processes. Furthermore, given the durability of the response (Fig. 2), we hypothesized that psychedelics may modulate the expression of specific genes or pathways. To test this hypothesis, we carried out RNA sequencing of the microdissected NAc 48 h and 2 weeks after pretreatment with either saline, cocaine, ketamine, LSD or MDMA. We collected total mRNA from each sample and made strand-specific libraries for each of three replicates from each condition. Transcript-level abundances were collapsed to gene-level expression estimates for model fitting.

To directly compare treatment-related transcriptional changes specific to the shared ability of psychedelics to reopen the social reward learning critical period, we analysed the gene expression dataset between conditions in which the critical period is in the open state (48 h and 2 weeks after LSD treatment, 48 h after ketamine treatment, and 48 h after MDMA treatment) versus conditions where the critical period remains in or returns to the closed state (48 h and two weeks after saline treatment, 48 h and two weeks after cocaine treatment, and two weeks after ketamine treatment). Using this approach, we identified 65 genes that were significantly differentially expressed (likelihood ratio test; Benjamini–Hochberg-corrected q ≤ 0.1) (Fig. 5). Gene set enrichment analysis of this list identified significant enrichment of ontologies associated with endothelial development, regulation of angiogenesis, vascular development and tissue morphogenesis. Of note, many of the top scoring genes are components of the extracellular matrix (ECM) or have been implicated in its remodelling, including: Fn1), Mmp16(, Trpv4, Tinagl1, Nostrin, Cxcr4, Adgre5, Robo4 and Sema3g. Additionally, the differentially expressed gene set includes the immediate early genes (IEGs) Fos, Junb, Arc and Dusp. When we did not control for the psychedelic-specific psychoactive response (saline versus all drug conditions, including cocaine), we identified 39 differentially expressed genes (Benjamini–Hochberg-corrected q ≤ 0.15) (Extended Data Fig. 10); however, enrichment analysis identified no significant ontologies associated with this gene set, and only 6 genes (Hspa12b, Sema3g, Eng, Flt4, Cavin1 and Ube4b) overlapped with the differentially expressed genes in the open state versus closed state dataset shown in Fig. 5. These results provide evidence that the shared ability of psychedelics to reopen the social reward learning critical period converges at transcriptional regulation of the ECM. On the basis of these findings, our working model (Fig. 6) posits that psychedelics act at a diverse array of binding targets (such as SERT, 5-HT_2AR, NMDA and KOR), to trigger a downstream signalling response that leads to activity-dependent (perhaps via IEG-mediated coincidence detection) degradation of the ECM, which in turn is the permissive event that enables metaplasticity. In this model, transcriptional upregulation of ECM components (for example, FN1) and downregulation of ECM proteolytic enzymes (for example, MMP-16), reflects the homeostatic response to these long-lasting cellular changes. Together, these results demonstrate novel biological effects (behavioural, temporal, electrophysiological and molecular) that—similar to therapeutic effects—are shared across psychedelics.

Conclusions

These studies provide a novel conceptual framework for understanding the therapeutic effects of psychedelics, which have shown significant promise for treating a wide range of neuropsychiatric diseases, including depression, PTSD and addiction. Although other studies have shown that psychedelics can attenuate depression-like behaviours and may also have anxiolytic, anti-inflammatory and antinociceptive properties, it is unclear how these properties directly relate to the durable and context dependent therapeutic effects of psychedelics. Furthermore, although previous in vitro studies have suggested that psychedelic effects might be mediated by their ability to induce hyperplasticity, this account does not distinguish psychedelics from addictive drugs (such as cocaine, amphetamine, opioids, nicotine and alcohol) whose capacity to induce robust, bidirectional, morphological and physiological hyperplasticity is thought to underlie their addictive properties. Moreover, our ex vivo results (Fig. 4 and Extended Data Fig. 6) are consistent with in vivo studies, which demonstrate that dendritic spine formation following administration of psychedelics is both sparse and context dependent, suggesting a metaplastic rather than a hyperplastic mechanism. Indeed, previous studies have also directly implicated metaplasticity in the mechanism of action of ketamine. At the same time, since our results show that psychedelics do not directly modify addiction-like behaviours (Extended Data Fig. 4), they provide a mechanistic clue that critical period reopening may be the neural substrate underlying the ability of psychedelics to induce psychological flexibility and cognitive reappraisal, properties that have been linked to their therapeutic efficacy in the treatment of addiction, anxiety and depression.

Although the current studies have focused on the critical period for social reward learning, critical periods have also been described for a wide variety of other behaviours, including imprinting in snow geese, song learning in finches, language learning in humans, as well as brain circuit rearrangements following sensory or motor perturbations, such as ocular dominance plasticity and post-stroke motor learning. Since the ability of psychedelics to reopen the social reward learning critical period is independent of the prosocial character of their acute subjective effects (Fig. 1), it is tempting to speculate that the altered state of consciousness shared by all psychedelics reflects the subjective experience of reopening critical periods. Consistent with this view, the time course of acute subjective effects of psychedelics parallels the duration of the open state induced across compounds (Figs. 2 and 3). Furthermore, since our results point to a shared molecular mechanism (metaplasticity and regulation of the ECM) (Figs. 4–6) that has also been implicated in the regulation of other critical periods, these results suggest that psychedelics could serve as a ‘master key’ for unlocking a broad range of critical periods. Indeed, recent evidence suggests that repeated application of ketamine is able to reopen the critical period for ocular dominance plasticity by targeting the ECM. This framework expands the scope of disorders (including autism, stroke, deafness and blindness) that might benefit from treatment with psychedelics; examining this possibility is an obvious priority for future studies.

Methods

Mice

Male wild-type mice were bred in house and weaned at 3 weeks or obtained from Jackson Laboratories (stock no. 000664). β-arr2-KO mice (stock no. 011130) were obtained from Jackson Laboratories, bred in house and weaned at 3 weeks of age. All mice were inbred to the C57BL/6J congenic ‘wild-type’ strain (as opposed to outbred ‘true wilds’, which were not used in this study). Congenic strains are generated by backcrossing for a minimum of 10 generations, a standard that is derived from the congenic interval, and the theoretical estimate that by the 10th generation, 99.99% of the congenic strain background will be from the recipient inbred. Although the β-arr2-KO mouse (Jackson Laboratories stock no. 011130), was originally derived on the 129X1/SvJ background, it was backcrossed to the C57BL/6J congenic strain at Jackson Laboratories (https://www.jax.org/strain/011130). All mice were maintained on a 12 h:12 h natural light:dark cycle, starting at 07:30 with food and water provided ad libitum. All behavioural experiments were conducted during the same circadian period (07:30–19:30) in a dedicated, sound- and odour-controlled behavioural testing room, which is separated from the vivarium, and no other experiments were conducted simultaneously in the same room. Sample size was estimated based on previous work and published literature. Experimenters were blind to the condition when subjective criteria were used as a component of data analysis, and control and test conditions were interleaved. Mice were randomly assigned to experimental and control groups. All procedures complied with the animal care standards set forth by the National Institutes of Health and were in accordance with protocols approved by the Johns Hopkins University Animal Care and Use Committee.

sCPP assay

The protocol for sCPP was adapted from previously published work. Mice were socially housed (3–5 males) in a cage containing corncob bedding (Anderson Cob, 0.25 inch cob, Animal Specialties and Provisions) until the pre-determined age for sCPP testing. Each mouse was used for only one behavioural time point. At the pre-determined age, mice were placed in an open field activity chamber (ENV-510, Med Associates) equipped with infrared beams and a software interface (Activity Monitor, Med Associates) to monitor the position of the mouse. The apparatus was partitioned into two equally sized zones using a clear Plexiglas wall, with a 5 cm diameter circular hole at the base; each zone contained one type of novel bedding (Alpha-Dri, Animal Specialties and Provisions or Kaytee Soft Granule, Petco). The amount of time spent freely exploring each zone was recorded during 30-min test sessions. For example, a score of 900 means that the mouse spent exactly 50% of its time on each of the two beddings, whereas a score of 1,800 means that it spent the full 30 min in the bedding that would be subsequently assigned as the social conditioning cue, and no time in the bedding that would be assigned as the isolation conditioning cue. After an initial pre-conditioning trial to establish baseline preference for the two sets of bedding cues, mice were assigned to receive social conditioning (with cage mates) for 24 h on one type of bedding, followed by 24 h of isolation conditioning (without cage mates) on the other bedding cue. To assure unbiased design, chamber assignments were counterbalanced for side and bedding cues. Immediately after the isolation conditioning, a 30-min post-conditioning trial was conducted to establish preference for the two conditioned cues. CPP is a learned association between a condition (for example, social) and a cue (bedding). It does not require scent from the other mice, as the bedding itself serves as the cue. Exclusion criteria for this behaviour are strictly defined as a pre-conditioning preference score of >1.5 or <0.5. Mice are never excluded based on the quality of their social interactions. Pre-conditioning versus post-conditioning social preference scores were considered significant if paired Student’s t-test P values were less than 0.05. Comparisons between experimental conditions were made using both normalized social preference scores (time spent in social zone post-treatment divided by pre-treatment) and subtracted social preference scores (time spent in social zone post minus pre); these were considered significant if unpaired Student’s t-test P values were <0.05. All experiments were performed during the mouse rest period (light cycle), since pilot experiments revealed that sCPP is most robust if assayed during this period. Prior to i.p. drug treatment experiments (MDMA, LSD, psilocybin, ketamine or ibogaine hydrochloride), mice were habituated to the injection procedure with daily saline i.p. injections in the home cage. Pharmacological delivery schedules were counterbalanced for type of drug. Unless otherwise stated (Fig. 2 and Extended Data Fig. 5), for pretreatment, experiments mice were tested 48 h after the injection to allow for complete clearance of the drug. For the experiment testing involvement of the 5-HT_2AR, the 5-HT_2AR antagonist ketanserin was administered i.p. 30 min prior to the injection of the drug tested.

Electrophysiology

Subjects received an i.p. injection of either LSD (1 µg kg⁻¹), ketamine (3 mg kg⁻¹), psilocybin (0.3 mg kg⁻¹), MDMA (10 mg kg⁻¹), ibogaine (40 mg kg⁻¹) or saline. Forty-eight hours after drug treatment, either parasagittal slices containing the NAc core (250 µm thick) or coronal slices containing the PL/IL region of the mPFC (250 µm thick) were prepared from C57BL/6 mice using standard procedures. In brief, after mice were anaesthetized with isoflurane and decapitated, brains were quickly removed and placed in ice-cold low-sodium, high-sucrose dissecting solution (228 mM sucrose, 26 mM NaHCO₃, 11 mM glucose, 2.5 mM KCl, 1 mM NaH₂PO₄, 1 mM MgSO₄, 0.5 mM CaCl₂). Slices were collected with a Leica VT 1200s vibrating microtome. Slices were allowed to recover for a minimum of 60 min in a submerged holding chamber (∼25 °C) containing artificial cerebrospinal fluid (ACSF) consisting of 119 mM NaCl, 2.5 mM KCl, 2.5 mM CaCl₂, 1.3 mM MgCl₂, 1 mM NaH₂PO₄, 11 mM glucose and 26.2 mM NaHCO₃. For hyperplasticity recordings (Extended Data Fig. 6), slices were removed from the holding chamber and placed into the recording chamber, where they were continuously perfused with oxygenated (95% O₂, 5% CO₂) ACSF at 2 ml min⁻¹ at 25 °C. For metaplasticity recordings (Fig. 4), slices were removed from the holding chamber and incubated first for 10 min in oxygenated ACSF containing picrotoxin (50 µM, Sigma), followed by 10-min incubation in oxygenated ACSF containing both picrotoxin and oxytocin (1 µM, Tocris) before being placed into the recording chamber. Whole-cell voltage-clamp recordings from MSNs or layer V pyramidal cells were obtained under visual control using a 40× objective. The NAc core was identified by the presence of the anterior commissure, and the PL/IL region of the mPFC was identified by the presence of the forceps minor of the corpus callosum. Recordings were made with electrodes (2.5–4.0 MΩ) filled with 115 mM CsMeSO₄, 20 mM CsCl, 10 mM HEPES, 0.6 mM EGTA, 2.5 mM MgCl, 10 mM sodium phosphocreatine, 4 mM sodium ATP, 0.3 mM sodium GTP and 1 mM QX-314. Miniature EPSCs were collected at a holding potential of −70 mV in the presence of tetrodotoxin (0.5 μM, Tocris Biosciences) and picrotoxin (50 μM, Sigma). Two minutes after break-in, 30-s blocks of events (total of 200 events per cell) were acquired and analysed using the Recording Artist plugin in Igor Pro software with threshold parameters set at 5 pA amplitude and <3 ms rise time. All events included in the final data analysis were verified visually. Data were analysed by multivariate analysis of variance (MANOVA) with three independent variables (drug, brain area and age) and two dependent variables (frequency and amplitude). Likelihood ratio test performed comparing the full model using treatment, age, and structure to a reduced model using age and structure. All calculations were performed in either GraphPad Prism 9 or the R programming language and are available as Supplementary Code 1 and in the repository at https://github.com/genesofeve/DolenPsychedelicOpenState.

RNA extraction and sequencing

Male wild-type C57BL6/J mice were injected i.p. with LSD, ketamine, cocaine (20 mg kg⁻¹) or saline solution either 2 weeks or 48 h before the mice were euthanized. At P98 to P112, mice were euthanized, brains were rapidly removed and ~1mm thick coronal slice (n = 3 mice per condition) containing the nucleus accumbens were sectioned using a mouse brain matrix. To microdissect the NAc, slices were placed in a petri dish containing ice-cold ACSF (125 mM NaCl, 2.5 mM KCl, 2 mM CaCl₂, 1 mM MgCl₂, 1.25 mM NaH₂PO₄, 10 mM glucose and 26 mM NaHCO₃) supplemented with RNase inhibitor and oxygenated with carbogen gas (95% O₂ and 5% CO₂) to pH 7.3–7.4. The NAc was identified using the anterior commissure and other structural markers. Between each dissection, blades were replaced and all the instruments and the matrix were cleaned with a solution containing RNase inhibitor. Following dissection, tissue was immediately placed into 0.5 ml Trizol and subjected to a 15 s burst with a tissue homogenizer to lyse the cells. Samples were kept on ice prior to storage at −20 °C. Total RNA were extracted using the RNeasy Kit from Qiagen. The quality of purified RNA was assessed via both a nanodrop and 2100 Bioanalyzer from Agilent. Library preparation was performed using a TruSeq Stranded mRNA kit (Illumina) using the recommended protocol. Individual dual-indexed libraries were quality controlled, pooled, and sequenced on the NovaSeq 6000 platform on a single S1 flowcell to an average depth of 76,841,745 (±8,066,939.82) paired-end 100 bp reads per sample. Reads were pseudoaligned to the mouse GENCODE vM25 (ref. ⁷¹) reference transcriptome using kallisto (v0.46.2) with 100 bootstrapped samples and 6 threads. Defaults were used for all other parameters. Estimated transcript-level abundances were collapsed to gene-level expression estimates and analysed using the sleuth (v0.30.0) R/Bioconductor package. To identify genes with differential expression as a function of samples where the critical period is reopened we performed a likelihood ratio test comparing a full model which included batch, and critical period to a reduced model that only included batch. Time was not used as an explanatory variable in this model fitting. Using this test, we identified 65 genes as significantly differentially expressed at a 10% false discovery rate (Benjamini–Hochberg-corrected q ≤ 0.1). To identify genes with differential expression as a function of any drug treatment (including cocaine) versus saline we performed a likelihood ratio test comparing a full model that included batch, and ‘treated vs untreated’ to a reduced model that only included batch. Using this test, we identified 39 genes as significantly differentially expressed at a 15% false discovery rate (Benjamini–Hochberg-corrected q ≤ 0.15). Time was not used as an explanatory variable in this model fitting. Raw data will be made publicly available (Gene Expression Omnibus accession numbers: GSE230679 and GSM7231202–GSM7231228). Code to reproduce the RNA-seq analysis and associated figures is provided as Supplementary Code 2 and in the repository at https://github.com/genesofeve/DolenPsychedelicOpenState.

Statistics

All statistical details can be found in the figure legends, including the type of statistical analysis used, P values, n, degrees of freedom, t values and f values. Sample sizes were not predetermined by statistical methods; instead they were estimated based on the previously published literature. Data distributions were assumed to be normal. Homogeneity of variance was tested using Levene’s test for equality of variances. Comparisons between experimental manipulations were made using a two-tailed Students t-test (paired or unpaired, and with or without Welch’s correction as appropriate) and MANOVA for comparisons between multiple outcome measures, with P < 0.05 considered significant.

Linear, β-spline, loess smoothing and natural spline models evaluated on the previously published time course of normalized social preference scores. Loess smoothing yielded a pseudoinverse at age 41.695 and a knot point of 35 was chosen for both β-spline and natural spline models. The natural spline outperformed the β-spline (adjusted R² of 0.1053 versus 0.5554, respectively) with fewer parameters. Residuals were plotted against fitted values and age to check model assumptions. Leave one out cross validation was also used to assess model fit. Control data from all new experiments was used as test data via the predict R function. RSME and R²values were comparable between the original model and the new data. Two-way t-tests to compare means of controls groups against matched or binned time periods was done to confirm fit to new data. The full model including coefficients for splines, experiment and condition was constructed and tested against reduced models with the final reduced model being reported. MANOVA analysis was carried out using multivariate linear models and the ANOVA function. All statistical comparisons were carried out in the R programming language and can be found in Supplementary Codes 3 and 4 as well as in the repository at https://github.com/genesofeve/DolenPsychedelicOpenState.

Psychedelics Reopen the Social Reward Learning Critical Period

Historically, psychedelics have encompassed compounds such as lysergic acid diethylamide (LSD), mescaline, and psilocybin, recognized for their capacity to profoundly alter perception of sensory input, self, time, and space. While contemporary efforts have attempted to categorize these substances based on their subjective effects (e.g., hallucinogenic, empathogenic) or molecular targets, the clinical relevance of such distinctions remains unclear. Notably, psychedelics across diverse classifications have demonstrated significant therapeutic potential for conditions including addiction, post-traumatic stress disorder (PTSD), and depression. Identifying a common neurobiological mechanism shared among these compounds is therefore a crucial objective for translational neuroscience.

During specific developmental stages, the brain exhibits heightened responsiveness to relevant stimuli and increased capacity for neural and behavioral adaptation. These limited timeframes are termed critical periods. Researchers have actively sought methods to re-establish these periods for therapeutic purposes. Recent investigations have identified a novel critical period crucial for social reward learning. It has been demonstrated that the empathogenic psychedelic 3,4-methylenedioxymethamphetamine (MDMA) can reopen this specific critical period. This mechanism exhibits characteristics akin to the therapeutic effects observed in MDMA-assisted psychotherapy for PTSD, including rapid onset, sustained benefits, and context-dependent outcomes. In contrast, cocaine does not induce the reopening of this social reward learning critical period, supporting the notion that the re-establishment of social reward learning in adulthood underpins MDMA's therapeutic efficacy.

A key question remains whether MDMA's ability to reopen the critical period for social reward learning extends to other psychedelic substances. MDMA is classified as an 'empathogen' due to its distinct prosocial acute effects. This characteristic is not universally shared by hallucinogenic psychedelics like psilocybin and LSD, dissociative psychedelics such as ketamine, or oneirogenic psychedelics like ibogaine. This raises doubt about their capacity to reopen the social reward learning critical period. However, all psychedelics induce an altered state of consciousness. If this shared characteristic, rather than specific prosocial properties, mediates the reopening of critical periods, then the reinstatement of social reward learning in adulthood may be a generalized property across the psychedelic class.

Critical Period Reopening is a Shared Property

To ascertain if the capacity for social reward learning critical period reopening extends beyond MDMA to other psychedelics, initial studies examined the effect of psilocybin on social reward learning in adult mice using a conditioned place preference assay. Administration of a single dose of psilocybin to adult mice resulted in significant social reward learning, a response not observed in saline-treated controls. Further analysis confirmed that psilocybin-treated mice exhibited a magnitude of social reward learning indicative of an "open" critical period, significantly deviating from the "closed" state observed in saline-treated animals.

Subsequent investigations revealed similar results with other psychedelics. Pretreatment with LSD, ketamine, and ibogaine also reopened the critical period for social reward learning in adult mice, demonstrating dose-dependent effects. Notably, these psychedelics did not enhance social reward learning in juvenile mice, where the critical period is naturally open. Furthermore, in contrast to their effects on social reward learning, psychedelic pretreatment did not influence two addiction-related behaviors: cocaine reward learning or amphetamine-induced locomotor sensitization. These findings collectively demonstrate that empathogenic, hallucinogenic, oneirogenic, and dissociative psychedelics all share the ability to reopen the critical period for social reward learning.

Duration of the Psychedelic Open State

The duration of acute subjective effects and therapeutic benefits varies considerably among psychedelics. For instance, ketamine's acute effects last 30-120 minutes, while its antidepressant action can persist for a week. Psilocybin and MDMA's subjective effects last 3-6 hours, whereas LSD and ibogaine effects can extend to 8-10 hours and 36-72 hours, respectively, often correlating with highly durable therapeutic outcomes spanning months to years. Prior research indicated that MDMA-induced critical period reopening lasts for approximately two weeks.

To further explore the temporal profile of the critical period "open" state induced by various psychedelics, the duration of reopening was assessed following treatment with ketamine, psilocybin, LSD, and ibogaine. One week post-treatment, psilocybin-treated mice, but not ketamine-treated mice, exhibited significant social reward learning. By two weeks, the critical period remained open for both psilocybin- and LSD-treated mice. At three weeks, LSD-treated mice still showed significant social reward learning, unlike psilocybin-treated mice. At four weeks, the social reward learning critical period remained open exclusively for mice treated with ibogaine. These progressively longer-lasting open states, from ketamine to ibogaine, directly correlate with the reported duration of their acute subjective effects in humans. These results provide a mechanistic rationale for the importance of the post-treatment integration period in clinical psychedelic applications and can inform the development of new therapeutic compounds.

Metaplasticity, Not Hyperplasticity

The dynamic regulation of synaptic plasticity, known as 'metaplasticity,' is believed to contribute to the establishment of critical periods. Prior research has shown that oxytocin induces a specific form of long-term depression, implicated in social reward learning. To determine if the ability to induce metaplastic upregulation of oxytocin plasticity generalizes across psychedelics, adult mice were pretreated with saline, cocaine, or various psychedelics. Brain slices containing the nucleus accumbens were then prepared to record miniature excitatory post-synaptic currents (mEPSCs).

Exposure to oxytocin induced a significant decrease in the frequency of mEPSCs following pretreatment with MDMA, LSD, psilocybin, ketamine, and ibogaine, but not with saline or cocaine. This metaplasticity was observed 48 hours post-treatment and persisted for two weeks in the LSD group. No significant changes in baseline mEPSC amplitude or frequency were observed. These findings suggest that psychedelics induce metaplasticity rather than a general increase in synaptic excitability (hyperplasticity). This distinction is particularly important for designing biomarkers to assess the therapeutic profiles and potential for abuse of novel compounds.

5-HT2AR Is Not the Universal Mechanism

The serotonin receptor 5-HT2AR, known for its interaction with LSD, mediates the perceptual and cognitive alterations induced by 'serotonergic psychedelics' like LSD and psilocybin. While MDMA is thought to influence serotonin through the serotonin transporter (SERT) and some ketamine effects are reportedly linked to 5-HT2AR, the precise role of this receptor in critical period reopening required investigation. Experiments were conducted to determine the involvement of 5-HT2AR in the reopening of the social reward learning critical period by LSD, psilocybin, MDMA, and ketamine.

Co-administration of ketanserin, a 5-HT2AR antagonist, blocked the critical period reopening induced by LSD and psilocybin. However, ketanserin did not prevent MDMA- or ketamine-induced reinstatement of social reward learning. These results indicate that while 5-HT2ARs are necessary for the effects of LSD and psilocybin in reopening the social reward learning critical period, MDMA and ketamine achieve this effect independently of 5-HT2AR. Despite arguments for classifying psychedelics based on 5-HT2AR binding, these findings identify critical period reopening as a shared property that transcends this receptor-binding boundary. Therefore, these results support the continued use of established psychedelic naming conventions rather than subclassification based solely on receptor binding or subjective properties.

β-arrestin-2 Is Not the Universal Mechanism

Recent research suggests that prolonged 5-HT2AR binding by LSD can trigger β-arrestin-2 (β-arr2)-biased signaling. The effects of MDMA and ibogaine are also thought to involve metabotropic G-protein-coupled receptors (GPCRs), and while ketamine's therapeutic actions primarily involve ionotropic NMDA receptors, metabotropic glutamate receptor 5 has also been implicated. To test if β-arr2-biased signaling mediates the ability of psychedelics to reopen the social reward learning critical period, their effects were examined in β-arr2-knockout (KO) mice.

Baseline social reward learning in juvenile and adult β-arr2-KO mice exhibited a normal developmental profile. However, when adult β-arr2-KO mice were treated with psychedelics, a differential pattern emerged. LSD and MDMA effectively reopened the social reward learning critical period in wild-type mice, but this effect was abolished in β-arr2-KO mice. Conversely, ketamine and ibogaine successfully reinstated social reward learning in both wild-type and β-arr2-KO mice. These results demonstrate that β-arr2 signaling is essential for LSD- or MDMA-induced critical period reopening, but ketamine and ibogaine facilitate social reward learning reinstatement through a β-arr2-independent mechanism.

Psychedelics Induce Remodeling of the ECM

Given that all psychedelics share the ability to reopen the social reward learning critical period despite acting on diverse molecular targets and biochemical pathways, it was hypothesized that their common mechanism might lie downstream of these initial cellular processes. Considering the durability of the observed responses, it was further hypothesized that psychedelics might modulate gene expression or specific signaling pathways. To investigate this, RNA sequencing was performed on the nucleus accumbens of mice treated with saline, cocaine, ketamine, LSD, or MDMA.

To isolate transcriptional changes directly related to critical period reopening, gene expression data from "open" states (e.g., after ketamine, LSD, or MDMA treatment) were compared to "closed" states (after saline or cocaine treatment, or when the critical period returned to closed). This analysis identified 65 genes that were significantly differentially expressed. Gene set enrichment analysis revealed significant enrichment in biological processes associated with endothelial development, angiogenesis, vascular development, and tissue morphogenesis. Notably, many of the top-scoring genes are components of the extracellular matrix (ECM) or are involved in its remodeling. This suggests that the shared ability of psychedelics to reopen the social reward learning critical period converges on the transcriptional regulation of the ECM. The proposed model posits that psychedelics, acting via various binding targets, trigger a downstream signaling cascade leading to activity-dependent degradation of the ECM. This ECM degradation is hypothesized to be the permissive event enabling metaplasticity, with subsequent transcriptional changes in ECM components reflecting a homeostatic response. These findings collectively reveal novel biological effects (behavioral, temporal, electrophysiological, and molecular) that are consistently observed across different psychedelics, mirroring their shared therapeutic properties.

Conclusions

These studies introduce a novel conceptual framework for comprehending the therapeutic effects of psychedelics, which show substantial promise for treating a wide array of neuropsychiatric disorders, including depression, PTSD, and addiction. While other research has highlighted psychedelics' capacity to reduce depression-like behaviors and potentially confer anxiolytic, anti-inflammatory, and antinociceptive properties, the direct link between these properties and the durable, context-dependent therapeutic effects of psychedelics remains unclear.

Furthermore, previous in vitro studies have suggested that psychedelic effects might involve inducing hyperplasticity. However, this explanation fails to differentiate psychedelics from addictive substances like cocaine or opioids, whose addictive properties are thought to be driven by robust, bidirectional morphological and physiological hyperplasticity. In contrast, the current ex vivo findings align with in vivo studies demonstrating that psychedelic-induced dendritic spine formation is sparse and context-dependent, supporting a metaplastic rather than a hyperplastic mechanism. Indeed, metaplasticity has also been directly implicated in the mechanism of action for ketamine.

The present results, demonstrating that psychedelics do not directly alter addiction-like behaviors, offer a mechanistic insight: critical period reopening may represent the neural substrate underlying psychedelics' ability to promote psychological flexibility and cognitive reappraisal. These properties have been consistently linked to their therapeutic efficacy in addressing addiction, anxiety, and depression. While these studies focused on the critical period for social reward learning, such periods are well-documented for diverse behaviors, including language acquisition in humans and neural circuit reorganization following sensory perturbations.

The observation that psychedelics' capacity to reopen the social reward learning critical period is independent of their acute prosocial effects suggests that the shared altered state of consciousness induced by all psychedelics might reflect the subjective experience of critical period reopening. This view is supported by the parallel between the duration of psychedelics' acute subjective effects and the duration of the induced "open" state across compounds. Moreover, the identification of a shared molecular mechanism—metaplasticity and ECM regulation—which has also been implicated in the regulation of other critical periods, suggests that psychedelics could serve as a "master key" for unlocking a broad spectrum of critical periods. Emerging evidence, for example, indicates that repeated ketamine administration can reopen the critical period for ocular dominance plasticity by targeting the ECM. This framework broadens the potential scope of disorders, including autism, stroke, deafness, and blindness, that might benefit from psychedelic-assisted treatments, highlighting a crucial direction for future research.

Methods

The investigations utilized male wild-type and β-arrestin-2 (β-arr2)-knockout mice, housed under controlled conditions with ad libitum access to food and water. All animal procedures adhered to established animal care standards and approved protocols. Experiments were conducted during the mice's light cycle, and experimenters were blinded to treatment conditions where subjective criteria were involved.

Key behavioral assessments included the social reward conditioned place preference (sCPP) assay, which measures a learned association between social interaction and a specific environmental cue (bedding). Mice underwent pre-conditioning to establish baseline preference, followed by social and isolation conditioning, and then a post-conditioning test. Drug treatments (MDMA, LSD, psilocybin, ketamine, ibogaine) were administered intraperitoneally, with behavioral testing typically occurring 48 hours later to ensure drug clearance. For studies involving the 5-HT2AR antagonist ketanserin, it was administered 30 minutes prior to the psychedelic.

Electrophysiological recordings were performed on brain slices containing the nucleus accumbens or medial prefrontal cortex. Whole-cell voltage-clamp recordings from specific neurons (medium spiny neurons or layer V pyramidal cells) were used to measure miniature excitatory post-synaptic currents (mEPSCs) in the presence of tetrodotoxin and picrotoxin. For metaplasticity studies, slices were incubated with picrotoxin and oxytocin. Additionally, RNA sequencing was conducted on microdissected nucleus accumbens tissue to analyze gene expression changes. Total mRNA was extracted, and libraries were prepared for sequencing to assess transcript-level abundances and identify differentially expressed genes, particularly those associated with the extracellular matrix. Statistical analyses, including Student's t-tests and multivariate analysis of variance (MANOVA), were performed to compare experimental conditions and evaluate statistical significance.

Psychedelics and Social Learning

Psychedelic compounds, including substances like LSD, psilocybin, MDMA, and ketamine, are known for their profound effects on perception, self-awareness, time, and space. While various systems attempt to categorize these drugs based on their subjective effects (e.g., hallucinogenic, empathogenic) or chemical structures, the importance of these distinctions for therapeutic applications remains unclear. This is because many psychedelics, despite their differences, have shown significant promise in treating conditions such as addiction, post-traumatic stress disorder (PTSD), and depression. Therefore, identifying a common brain mechanism that explains these shared therapeutic benefits is a major goal in neuroscience.

During specific periods of brain development, the nervous system becomes highly receptive to new experiences and more adaptable to changes in brain connections and behaviors. These limited timeframes are known as "critical periods," and scientists have long sought ways to reactivate them for therapeutic purposes. Recent research identified a novel critical period for learning social rewards. It was also found that MDMA, an empathogenic psychedelic, can reopen this specific critical period. This mechanism shares characteristics with the therapeutic effects of MDMA-assisted psychotherapy for PTSD, including rapid onset, long-lasting effects, and dependence on the context.

In contrast, cocaine does not reopen this social reward learning critical period, and it also lacks the broad therapeutic profile of psychedelics. This supports the idea that restoring social reward learning in adulthood is a key factor in MDMA's effectiveness. However, it was an open question whether the ability to reopen this critical period extends to other types of psychedelics, especially since MDMA is known for its prosocial effects, a quality not typically shared by all other psychedelics. If the general altered state of consciousness, rather than just prosocial properties, is what facilitates critical period reopening, then this ability might be a shared trait among all psychedelics.

Critical period reopening is a shared property

To investigate whether the ability of MDMA to reopen the social reward learning critical period applies to other psychedelics, studies began by examining the effect of psilocybin on adult mice. Mice pretreated with psilocybin showed significant social reward learning, indicating an open critical period, while saline-treated mice did not. Similar results were observed with LSD, ketamine, and ibogaine. All these psychedelics, when administered to adult mice, successfully reopened the critical period for social reward learning. These effects were also found to be dose-dependent. Notably, these psychedelics did not enhance addiction-related behaviors like cocaine reward learning or amphetamine-induced locomotion. These findings suggest that empathogenic, hallucinogenic, oneirogenic, and dissociative psychedelics all share the capacity to reopen the critical period for social reward learning.

Duration of the psychedelic open state

The duration of both the acute subjective effects and the long-term therapeutic response varies considerably among psychedelics. For instance, ketamine's subjective effects last for a short period, but its antidepressant effects can last about a week. Psilocybin and MDMA have subjective effects lasting a few hours, while LSD and ibogaine's effects persist much longer, with these longer subjective durations often correlating with more durable therapeutic benefits lasting months to years. Previous research indicated that MDMA-induced critical period reopening lasts for two weeks. Further studies examined the duration of the open state after treatment with ketamine, psilocybin, LSD, and ibogaine. One week after treatment, psilocybin-treated mice still showed social reward learning, unlike ketamine-treated mice. At two weeks, both psilocybin- and LSD-treated mice maintained an open critical period. By three weeks, only LSD-treated mice continued to show this effect, and at four weeks, the critical period remained open only for ibogaine-treated mice. These results show a progressive increase in the open state duration, mirroring the acute subjective effects of these drugs in humans. This provides a biological explanation for the importance of post-treatment integration periods in psychedelic therapy and can guide the development of new compounds.

Metaplasticity, not hyperplasticity

The dynamic control over how much synaptic plasticity can occur is called "metaplasticity," and it is thought to be a key mechanism behind critical periods. Prior research showed that oxytocin induces a specific type of long-term depression in brain connections, important for social reward learning. To see if psychedelics could generally enhance this oxytocin-related plasticity, adult mice were pretreated with saline, cocaine, or various psychedelics. Brain tissue analyses showed that pretreatment with MDMA, LSD, psilocybin, ketamine, and ibogaine, but not saline or cocaine, led to a significant metaplastic upregulation of oxytocin plasticity in a specific brain region (the nucleus accumbens). This metaplasticity was observed 48 hours later and persisted for two weeks in the LSD group. Importantly, no significant changes in baseline brain activity were observed. These findings suggest that psychedelics induce metaplasticity (a controlled change in the brain's ability to be plastic) rather than hyperplasticity (excessive, uncontrolled plasticity), a crucial distinction for developing safe and effective new therapies and assessing their potential for abuse.

5-HT2AR is not the universal mechanism

The serotonin receptor 5-HT2AR, known for mediating the perceptual and cognitive changes caused by "serotonergic psychedelics" like LSD and psilocybin, was investigated for its role in reopening the social reward learning critical period. While LSD and psilocybin-induced reopening was blocked by a 5-HT2AR antagonist (ketanserin), MDMA and ketamine continued to reopen the critical period even in the presence of ketanserin. This demonstrates that although 5-HT2ARs are necessary for the effects of LSD and psilocybin in this context, MDMA and ketamine act independently of this receptor. These results highlight that critical period reopening is a shared property across psychedelics that is not limited by their initial receptor binding profiles, supporting the broad categorization of psychedelics rather than sub-classifying them based solely on receptor interactions or subjective effects.

β-arrestin-2 is not the universal mechanism

Recent studies suggest that prolonged binding of LSD to the 5-HT2AR receptor can trigger a specific signaling pathway involving β-arrestin-2 (β-arr2). The effects of MDMA and ibogaine are also thought to involve certain G-protein-coupled receptors. To determine if β-arr2-biased signaling is a universal mechanism for psychedelics reopening the social reward learning critical period, researchers examined their effects in β-arr2-knockout mice. It was found that LSD and MDMA reopened the critical period in normal mice but not in β-arr2-knockout mice, indicating a requirement for β-arr2 signaling. Conversely, ketamine and ibogaine successfully reinstated social reward learning in both normal and β-arr2-knockout mice, showing their effects are independent of β-arr2. These findings further demonstrate that while critical period reopening is a common outcome, the specific internal cellular pathways leading to this effect can vary among different psychedelics.

Psychedelics induce remodelling of the ECM

Given that psychedelics, as a group, all reopen the social reward learning critical period despite acting on various initial targets and biochemical pathways, researchers hypothesized that the common underlying mechanism might occur further downstream in cellular processes. Considering the long-lasting nature of the response, they also suggested that psychedelics might influence the expression of specific genes or pathways. To test this, RNA sequencing was performed on brain tissue from mice treated with saline, cocaine, ketamine, LSD, or MDMA. Analysis focused on gene expression changes unique to the "open" state of the critical period. This analysis identified 65 genes that were significantly altered. Gene enrichment analysis revealed that these genes were strongly associated with processes like endothelial development, blood vessel formation, tissue shaping, and notably, many were components of the extracellular matrix (ECM) or involved in its remodeling. This includes genes like Fn1 and Mmp16. Additionally, immediate early genes such as Fos and Arc were identified. These results strongly suggest that the shared ability of psychedelics to reopen the social reward learning critical period converges on the regulation of the ECM at a genetic level. A working model proposes that psychedelics act on diverse targets, leading to a signaling response that causes the breakdown of the ECM. This ECM degradation then permits metaplasticity, which enables the critical period to reopen. The later increase in ECM components and decrease in ECM-degrading enzymes would represent the brain's homeostatic response to these long-lasting cellular changes.

Conclusions

These studies offer a new way to understand the therapeutic effects of psychedelics, which show great promise for treating various brain disorders like depression, PTSD, and addiction. While psychedelics are known to have other effects, it has been unclear how these relate to their long-lasting, context-dependent therapeutic benefits. Previous in vitro studies suggested that psychedelics might induce hyperplasticity (excessive brain flexibility), but this does not differentiate them from addictive drugs, which also cause robust hyperplasticity associated with addiction. The current findings, however, point to metaplasticity, a more controlled and regulated form of brain plasticity, which is consistent with in vivo studies showing that changes in brain structure after psychedelic use are subtle and context-dependent. This aligns with the idea that critical period reopening, rather than uncontrolled hyperplasticity, is the neural basis for psychedelics' ability to promote psychological flexibility and cognitive reframing—qualities linked to their effectiveness in treating addiction, anxiety, and depression.

While the current research focused on social reward learning, critical periods exist for many other behaviors, such as language acquisition in humans or brain reorganization after injury. Since the ability of psychedelics to reopen the social reward learning critical period is independent of their specific prosocial effects, it is plausible that the altered state of consciousness shared by all psychedelics is actually the subjective experience of reopening these critical periods. This idea is supported by the parallel between the duration of acute subjective effects and the length of the induced "open state" across different compounds. Furthermore, since the research points to a shared molecular mechanism (metaplasticity and ECM regulation) that has also been implicated in other critical periods, it suggests that psychedelics could act as a "master key" to unlock a broad range of critical periods. Indeed, recent evidence indicates that repeated ketamine application can reopen the critical period for ocular dominance plasticity by targeting the ECM. This framework expands the potential scope of disorders, including autism, stroke, deafness, and blindness, that might benefit from psychedelic treatment, making this an important area for future research.

Methods

The research involved using male wild-type and β-arrestin-2-knockout mice. All animal procedures followed ethical guidelines. The social conditioned place preference (sCPP) assay was used to assess social reward learning, measuring mice's preference for bedding associated with social interaction versus isolation. Electrophysiology was performed on brain slices to measure changes in synaptic plasticity, specifically miniature excitatory post-synaptic currents, after psychedelic treatment. RNA sequencing was used to analyze gene expression changes in specific brain regions, identifying genes related to the extracellular matrix. Statistical analyses were conducted to determine the significance of the findings, including comparisons between treatment groups and various models to assess critical period states.

Psychedelic drugs, such as lysergic acid diethylamide (LSD), psilocybin, MDMA, and ketamine, are known for causing significant changes in how a person perceives senses, self, time, and space. These experiences are very different from everyday life and can offer new insights into how the mind works. While there have been efforts to group psychedelics based on their effects (like causing hallucinations or empathy), their chemical makeup, or how they interact with the brain (such as targeting the serotonin receptor 2A), it is still unclear if these different categories matter for medical use. This is because many types of psychedelics have shown great promise in treating conditions like addiction, post-traumatic stress disorder (PTSD), and depression. Therefore, finding a shared brain mechanism that explains why these diverse psychedelics have similar healing effects is an important goal for neuroscience research.

During certain stages of brain development, the nervous system becomes highly responsive to important environmental cues. At these times, the brain is more flexible in making changes to its connections and shaping behavior. These specific developmental phases are called "critical periods." Scientists have long been looking for ways to reactivate these periods for medical purposes. Recent research has identified a new critical period related to learning social rewards and found that MDMA, a psychedelic known for its empathy-producing effects, can reopen this period. This ability of MDMA to reopen a critical period has several similarities to its healing effects in treating PTSD through therapy, including how quickly it works, how long its effects last, and its dependence on the situation. In contrast, cocaine does not reopen this social reward learning critical period, and since cocaine does not have the same therapeutic benefits as psychedelics, these findings support the idea that reactivating social reward learning in adulthood is a key part of MDMA's effectiveness.

It has been unknown if MDMA's ability to reopen the critical period for social reward learning applies to other psychedelics. MDMA is known as an ‘empathogen’ because its immediate effects make people feel more social. Other psychedelics like psilocybin and LSD (which cause hallucinations), ketamine (which causes dissociation), or ibogaine (which can cause dream-like states) do not necessarily share this prosocial quality. This raises the question of whether these other drugs could also reopen the social reward learning critical period. However, all psychedelics cause an altered state of consciousness. If this shared characteristic, rather than MDMA's prosocial effects, is what allows critical periods to reopen, then it is possible that many psychedelics could help adults relearn social rewards.

Critical period reopening is a shared property

To determine if other psychedelics, beyond MDMA, could also reopen the social reward learning critical period, researchers studied the effects of psilocybin on adult mice. Using a test that measures social reward learning, it was found that mice given psilocybin showed significant social reward learning, unlike those given a saline solution. This suggested that psilocybin reopened the critical period. Further tests confirmed that the social reward learning observed in psilocybin-treated mice was similar to the 'open' state of this critical period, while saline-treated mice remained in the 'closed' state. Similarly, LSD also reopened the critical period for social reward learning in adult mice. The effects of ketamine and ibogaine were also studied, and both drugs similarly reopened the social reward learning critical period in adulthood. Like MDMA, these effects depended on the dose given. However, giving MDMA to young mice did not further boost their social reward learning. Importantly, while psychedelics affected social reward learning, they did not impact behaviors linked to addiction, such as cocaine reward learning or movement increases caused by amphetamine. These findings collectively show that many types of psychedelics—empathy-producing, hallucinogenic, dream-like, and dissociative—can reopen the critical period for social reward learning.

Duration of the psychedelic open state

The immediate effects of psychedelics and how long their therapeutic benefits last can vary greatly. For instance, ketamine's noticeable effects last for a short time, but its antidepressant effects can last for a week. Psilocybin and MDMA's effects last a few hours, while LSD and ibogaine's effects can last much longer, from several hours to days. These longer-lasting subjective experiences often align with therapeutic benefits that endure for months or even years. Previous research showed that MDMA's effect on reopening the critical period lasted for two weeks. This study further explored how long the critical period remained open after treatment with ketamine, psilocybin, LSD, and ibogaine. It was found that psilocybin kept the critical period open for at least one week, and psilocybin and LSD kept it open for two weeks. LSD-treated mice still showed an open critical period at three weeks, and ibogaine kept it open for four weeks. The duration of the open critical period for each psychedelic was found to correspond to how long their immediate effects are felt in humans. These findings help explain why a period of integration after psychedelic treatment is important in clinical settings and can guide the development of new treatments.

Metaplasticity, not hyperplasticity

The brain's ability to adjust how easily its connections can change is called 'metaplasticity,' and this process is believed to be key to critical periods. Prior research had linked a specific type of brain plasticity, involving the hormone oxytocin, to social reward learning. To see if psychedelics could trigger this kind of metaplasticity, adult mice were given either saline, cocaine, or a psychedelic drug. Brain tissue was later examined, and it was found that oxytocin caused significant changes in brain cell activity in mice treated with MDMA, LSD, psilocybin, ketamine, and ibogaine, but not in those treated with saline or cocaine. This metaplasticity was observed 48 hours after treatment and, for LSD, continued for two weeks. These findings suggest that psychedelics cause metaplasticity rather than 'hyperplasticity,' which refers to an uncontrolled increase in brain plasticity. This difference is important for developing ways to measure the therapeutic effects of new drugs and their potential for excessive use.

5-HT2AR is not the universal mechanism

The serotonin receptor 5-HT2AR is known to be involved in how 'serotonergic psychedelics' like LSD and psilocybin change perception and thinking. MDMA is believed to affect serotonin levels, and some of ketamine's effects might also involve 5-HT2AR. Researchers investigated whether this receptor was necessary for LSD, psilocybin, MDMA, and ketamine to reopen the social reward learning critical period. When mice were given a drug called ketanserin, which blocks 5-HT2AR, it prevented LSD and psilocybin from reopening the critical period. This indicates that 5-HT2AR is required for their effects. However, ketanserin did not stop MDMA or ketamine from reopening social reward learning. These findings show that while 5-HT2AR is crucial for LSD and psilocybin's effects, MDMA and ketamine can reopen the critical period through a different pathway that does not involve this receptor. Even though some scientists suggest classifying psychedelics based on whether they bind to 5-HT2AR, these results identify critical period reopening as a common feature that unites all psychedelics, regardless of their interaction with this specific receptor. This supports keeping the current way of naming psychedelics, rather than creating new classifications based on which receptors they bind to or their subjective properties.

β-arrestin-2 is not the universal mechanism

Recent research suggests that when LSD binds to the 5-HT2AR, it activates a specific signaling pathway involving a protein called β-arrestin-2 (β-arr2). MDMA and ibogaine's effects are also thought to involve similar signaling pathways. While ketamine's therapeutic effects are typically linked to other types of brain receptors, another receptor, metabotropic glutamate receptor 5, has also been implicated. To investigate if β-arr2 signaling is the common mechanism behind psychedelics' ability to reopen the social reward learning critical period, the drugs' effects were tested in mice genetically engineered to lack β-arr2. Initial tests showed that these β-arr2-deficient mice developed social reward learning normally. However, when adult mice were given psychedelics, LSD and MDMA reopened the critical period in normal mice but not in the β-arr2-deficient mice. This indicates that β-arr2 signaling is necessary for LSD and MDMA to have their effect. In contrast, ketamine and ibogaine were still able to restore social reward learning in both normal and β-arr2-deficient mice, showing that their effects do not depend on β-arr2. These findings suggest that while β-arr2 signaling is required for LSD and MDMA's ability to reopen the critical period, ketamine and ibogaine act through different, β-arr2-independent pathways.

Psychedelics induce remodelling of the ECM

Even though different psychedelics act on various targets in the brain and use different signaling pathways, they all seem to reopen the social reward learning critical period. This led researchers to believe that a common mechanism, located further down the biological chain, might be responsible. Also, given how long these effects last, it was proposed that psychedelics might change how specific genes or pathways are expressed. To investigate this, RNA sequencing was performed on brain tissue from mice treated with saline, cocaine, ketamine, LSD, or MDMA.

By comparing gene activity in brain states where the critical period was open (after psychedelic treatment) versus when it was closed (after saline or cocaine treatment, or when the critical period had returned to its closed state after ketamine treatment), 65 genes were identified that showed significantly different activity. Further analysis revealed that these genes are linked to processes like blood vessel development and tissue formation. Notably, many of these genes are part of the extracellular matrix (ECM), a network of molecules that provides structural support to cells, or are involved in changing its structure. This suggests that the shared ability of psychedelics to reopen the critical period involves changes in the ECM at the genetic level. Based on these findings, a model proposes that psychedelics, acting on various targets, trigger a response that leads to the breakdown of the ECM. This breakdown then allows the brain to become more flexible, a state known as metaplasticity. The body's response to these lasting cellular changes includes increasing the production of ECM components and decreasing the enzymes that break it down. Overall, these studies reveal new shared biological effects of psychedelics—at the behavioral, temporal, electrical, and molecular levels—that are similar to their therapeutic benefits.

Conclusions

These studies offer a new way to understand the therapeutic benefits of psychedelics, which show significant potential for treating various mental health conditions like depression, PTSD, and addiction. While other research indicates that psychedelics can lessen depression-like behaviors and may have anti-anxiety, anti-inflammatory, and pain-relieving qualities, how these specific properties relate to the lasting, context-dependent therapeutic effects is not fully clear. Earlier lab studies proposed that psychedelics might work by causing 'hyperplasticity,' a kind of uncontrolled brain flexibility. However, this explanation does not differentiate psychedelics from addictive drugs like cocaine or alcohol, which also cause strong brain plasticity thought to lead to addiction. The current findings, which suggest metaplasticity rather than hyperplasticity, align with observations that brain changes after psychedelic use are specific and dependent on the situation. Since the results also show that psychedelics do not directly change addiction-like behaviors, they hint that reopening critical periods might be the brain process that allows psychedelics to promote psychological flexibility and improved thinking, abilities linked to their success in treating addiction, anxiety, and depression.

Although this research focused on the critical period for social reward learning, critical periods exist for many other behaviors, such as language learning in humans or brain reorganization after injury. Because psychedelics can reopen the social reward learning critical period regardless of their immediate prosocial effects, it is possible that the altered state of consciousness experienced with all psychedelics is actually the feeling of these critical periods reopening. This idea is supported by the fact that the duration of a psychedelic's immediate effects matches how long the critical period remains open. Furthermore, since the findings point to a shared molecular mechanism (metaplasticity and ECM regulation) that has also been linked to the regulation of other critical periods, it suggests that psychedelics could act as a ‘master key’ to unlock a wide variety of critical periods. For example, recent evidence shows that ketamine can reopen the critical period for vision problems by targeting the ECM. This new understanding broadens the range of disorders, including autism, stroke, deafness, and blindness, that might benefit from psychedelic treatments. Exploring these possibilities is an important next step for future research.

Methods

Mice

For these studies, male mice of a standard lab strain were used, including a special strain that lacked the β-arrestin-2 gene. All mice were kept under controlled conditions with a regular light-dark cycle, and they had constant access to food and water. Behavioral experiments were performed at consistent times in a quiet, controlled room. The number of mice used for each experiment was based on previous research. To ensure fair and unbiased results, researchers were unaware of which treatment each mouse received during parts of the study, and mice were randomly assigned to different experimental groups. All animal care and research procedures followed strict ethical guidelines and were approved by the appropriate committees.

sCPP assay

The social reward conditioned place preference (sCPP) assay was used to measure social learning. Mice were placed in a chamber divided into two sections, each with a different type of bedding. Their preference for each bedding was recorded. Then, mice spent 24 hours with cage mates on one type of bedding (social conditioning) and 24 hours alone on the other bedding (isolation conditioning). After this, their preference for the beddings was tested again. A stronger preference for the bedding associated with social interaction indicated social reward learning. To ensure valid results, chamber setups were varied, and mice were prepared for drug injections with daily saline injections. In most cases, mice were tested 48 hours after receiving a drug to ensure the substance had cleared their system. For specific tests involving the 5-HT2AR receptor, a blocking drug called ketanserin was given 30 minutes before the psychedelic.

Electrophysiology

To study the electrical activity of brain cells, mice were given either a psychedelic drug or saline. Forty-eight hours later, thin slices of specific brain regions, such as the nucleus accumbens or medial prefrontal cortex, were prepared. These slices were kept in a special fluid to maintain their health. For some experiments, oxytocin was added to the fluid to see its effect on cell activity. Using a technique called whole-cell voltage-clamp recording, researchers measured the tiny electrical signals between brain cells. The frequency and strength of these signals were analyzed to understand how the drugs affected brain plasticity. Specialized software was used to collect and process the electrical data.

RNA extraction and sequencing

To analyze gene activity, male mice were injected with LSD, ketamine, cocaine, or saline. After a set period, their brain tissue, specifically the nucleus accumbens, was collected. RNA, which carries genetic instructions, was extracted from these samples. The quality of the RNA was checked, and it was then prepared for sequencing. This process involved converting the RNA into libraries that could be read by a high-throughput sequencing platform. The data from the sequencing was analyzed using specialized software to estimate gene expression levels. Researchers then compared gene activity between samples where the critical period was considered 'open' (after psychedelic treatment) and 'closed' (after saline or cocaine, or when the critical period had returned to its closed state). This comparison helped identify genes whose activity levels changed significantly depending on whether the critical period was open. The raw data from this gene sequencing study will be made publicly available for other scientists to review.

Statistics

All study results were analyzed using standard statistical methods to ensure their validity. The choice of statistical tests, such as t-tests for comparing groups or MANOVA for multiple measurements, was based on the specific type of data and experimental design. A p-value of less than 0.05 was typically considered statistically significant, indicating that the observed results were unlikely to have occurred by chance. Various statistical models were used to analyze the data, including those that account for changes over time. Model assumptions were checked, and cross-validation techniques were employed to confirm the accuracy of the models. These detailed statistical analyses were performed using specialized software.

Psychedelics Reopen Learning Times

Psychedelic drugs like LSD, MDMA, and psilocybin change how people see, feel, and think. For a long time, doctors have tried to use them to help with serious mental health problems such as addiction, depression, and post-traumatic stress disorder (PTSD). Our brains have special times, called "critical periods," when they can learn things very easily and change their connections. Scientists have been looking for ways to open these special learning times again to help people. Recent studies showed that MDMA can reopen a critical period for social learning. This means the brain becomes better at learning how to connect with others, which helps explain why MDMA can be useful in therapy.

Shared Effects and Brain Changes

New research shows that many other psychedelics, like psilocybin, LSD, ketamine, and ibogaine, can also reopen this special social learning time in the brain. They do this in mice, making them better at learning social rewards. This effect is specific to social learning and does not make addiction-like behaviors worse. How long these drugs keep the learning period "open" in the brain matches how long their helpful effects last in people. For example, drugs that have longer-lasting effects also keep the learning period open for a longer time. The brain changes caused by psychedelics are specific. They help the brain get ready to change in a helpful, controlled way, which is called "metaplasticity." This is different from how addictive drugs affect the brain, which simply make too many changes. Even though different psychedelics work through different "switches" in the brain, they all lead to this same ability to reopen the learning period.

A Common Way They Work

Because psychedelics work through many different pathways in the brain, scientists looked for a common final step. They found that these drugs seem to change the "scaffolding" around brain cells, known as the extracellular matrix (ECM). This scaffolding usually keeps brain cells in place, but psychedelics make it more flexible. This change in the ECM allows the brain to have metaplasticity, meaning it can learn and change more easily during this reopened period.

Important Findings

These studies give a new way to understand how psychedelics help people with mental health problems. They show that these substances open special learning times in the brain, which is different from how addictive drugs work. This ability to "unlock" learning periods means psychedelics could be like a "master key" for helping with many other brain problems. This might include issues with learning, or even problems with senses like hearing and sight. More research is needed to explore these exciting possibilities.