Negative affective bias (a tendency to over-weight negative information) is a core mechanism that underlies depressive disorders. Psilocybin has shown substantial potential as an antidepressant agent, yet since its mechanism is poorly understood, researchers investigated the mechanism by which it modulates negative affective bias in rats.

Rats were given and affective bias task (ABT). Here, they were trained to associate different digging cues with a food reward while either in a neutral or negative (drug induced) state. Rats were then given psilocybin or vehicle infusion to the pre-limbic subregion of the medial prefrontal cortex (PrL). When then given a choice between digging substrates learned in a neutral or negative condition, rats given the vehicle infusion showed a negative affective bias (preference against the substrate learned in a negative condition), however, this effect was significantly reduced in rats given psilocybin infusion.

When identifying the mechanism behind psilocybin’s effect, researchers noted a bi-directional effect. Neurons communicating with the amygdala (CA) had their excitatory input suppressed; however, nearby neurons communicating with other regions of the cortex had their inputs strengthened, thus creating somewhat of a redistributing effect. This effect was shown to be dependent on both 5HT1a and 5HT2a receptor activity, since blocking these receptors with receptor specific antagonist also blocked the previously seen reduction of excitatory CA projections previously shown.

To demonstrate the causal nature of the CA circuit in modifying negative affective bias, researchers genetically silenced amygdala projecting neurons, allowing for the replication of psilocybin’s effect without having to administer it., This showed the same reduction in effect of negative affective bias confirming the modulation of pathways projecting to the amygdala as central in psilocybin’s mechanism.

Mice were presented with varying tones, (4, 8 and 13 hz), with 4 and 13hz tones being associated with either water (positive stimuli), or air puffs (negative stimuli). Neurons in the auditory cortex of these mice were then imaged both before and after psilocybin / saline treatment in order to examine how psilocybin effected the activity patterns of auditory neurons.

Across both psilocybin and saline groups, there was a uniform reduction auditory neuron responsiveness to tones, with researchers attributing this as a result of the injection procedure itself. When imaging auditory neurons response to different stimuli instead of tones, the psilocybin treated mice showed a reduction in responsiveness specifically to air-puffs but not the water stimulus.

To investigate psilocybin’s effect on tone responsiveness without the injection confound, the design was modified so that post-injection imaging would be done the same day, 1 day and 8 days after psilocybin administration. Instead of using both water and air puff stimuli, tones were only associated with the negative air-puff, with 1 tone being paired before the psilocybin treatment, and a different tone being used for pairing immediately after treatment during the window in which psilocybin was active

Mice given psilocybin showed a larger reduction in neuron signalling specifically towards the tone paired before the psilocybin treatment, with the reduction in activity persisting at 8 days. Additionally, this reduction in spiking was specific to the tone which had been consolidated for a week prior, with this reduction effect not being extended to the latter tone.

Based on these findings researchers suggest that psilocybin is not simply dampening auditory signalling, but instead preferentially dampening well consolidated aversive sensory representations.

Fragile X Syndrome (FXS) is the most common inherited intellectual disability and is the leading cause of monogenic autism spectrum disorder (ASD). Both FXS and ASD have serotonergic dysregulation as a consistent feature, due to serotonins role in neurodevelopment, and synaptic maturation. Although psilocybin largely acts on the 5HT2a receptor (serotonergic), its wider effects include the promotion of neural plasticity, via increased BDNF signalling.

These researchers had previously shown that microdosing psilocybin in the same rat model as used in this study improved novel object recognition, with this study aiming to identify the mechanisms behind this. To do this, during the microdose treatment period, rats were given a 5HT1a, and 5HT2a receptor antagonist before each dose, as well as a TrkB antagonist (0.5mg/kg & 1mg/kg). All rats were then tested with a novel object recognition task, to assess the recovering of visual learning abilities.

As expected, psilocybin was able to improve object recognition in FXS rats, with this effect persisting in both rats given a 5HT1a and 5HT2a antagonist before each dose. On the other hand, rats given a TrkB antagonist (1mg/kg) in combination with psilocybin microdose, did not show recovered ability in the novel object recognition task; however, this was only in rats that experienced the higher dose of TrkB antagonists, not those with a dose of 0.5mg/kg.

Since object recognition abilities remained were still recovered in rats given 5HT1a & 5HT2a receptor antagonists, researchers suggest that the mechanism by which psilocybin rescues these cognitive abilities is independent of serotonin receptor activation. Instead, since object recognition was not recovered in rats given a stronger TrkB antagonist, which would disrupt BDNF signalling, researchers suggest that this BDNF signalling pathway is how psilocybin rescues these cognitive deficits.

By separating the therapeutic effects of psilocybin from hallucinatory pathways, the findings of this research show promising evidence for microdosing psilocybin as a potential future treatment.

While both 2C-B and Psilocybin are psychedelics that primarily act on 5-HT2a receptors, there are several differences in how these drugs each act in the brain. One of these differences is that 2C-B may cause lower levels of dysphoria, making it a better therapeutic alternative. Because of this, researchers aimed to map the differences in how drugs affect brain organisation.

To test this, participants were given a dose of 2C-B, Psilocybin and a placebo, each in a different session. They then underwent a resting-state MRI and completed a variety of questionnaires, with various connectivity measures being taken including: static functional connectivity (sFC), global connectivity (gFC), dynamic connectivity variability (dFC).

Both drugs reduced within network sFC within the visual and default mode networks while increasing between - network and subcortical to cortical connectivity. Psilocybin produced more extensive increases in between-network sFC, particularly within the default mode and somatomotor networks. Meanwhile, 2C-B showed connectivity increases in transmodal frontoparietal regions. Dynamic connectivity variance was robustly reduced across the whole brain by both drugs, though psilocybin's reductions were spatially more widespread. Measures of entropy increased comparably under both compounds, concentrated in bilateral extrastriate visual cortex and anterior thalamic nuclei, with no significant between-drug differences.

Analysis involving PET scan data of receptor density showed that reductions in dFC in both drug conditions strongly predicted by 5-HT2A receptor density, while increases in entropy aligned inversely with 5-HT1A expression. Drug differences in affinity for monoamine transporters were credited as the primary predictor of spatial differences in sFC, with DAT being responsible for differences in sFC between psilocybin and 2C-B, and SERT being responsible for differences in entropy between the two.

Through partial least squares analysis, a latent variable emerged, accounting for 33% of the variance in subjective experience. This latent variable was characterized by decreases in transmodal cortices and increased in unimodal sensorimotor regions. Based on these findings, authors suggest 2C-B engages the same core network-level mechanisms psilocybin, while its secondary monoaminergic interactions being responsible for spatial differences in sFC.

The orbitofrontal cortex (OFC) is implied in the pathology of disorders such as obsessive-compulsive disorder and addiction, 2 conditions psilocybin has been an effective treatment of. Despite this, little is known about how psilocybin interacts with the OFC, to produce its therapeutic effects. Because of this, researchers investigated the impact that single dose of psilocybin has on OFC cells.

Mice received a single dose of psilocybin at 1 mg/kg or saline. Twenty-four hours later, OFC tissue was collected for gene expression profiling . Separate groups of mice underwent patch-clamp recordings of layer 5 pyramidal neurons, parvalbumin-positive (PV+) interneurons, and somatostatin-positive (SST+) interneurons in OFC slices.

The Layer 5 pyramidal cells showed the greatest gene expression, with these largely impacting synapse formation proteins, such as glutamate receptor subunits. Despite this, PV+ and SST+ cells did not show the same change in gene expression, despite these 3 neurons forming a microcircuit, where PV & SST+ interneurons mediate the output of the pyramidal neuron.

Electrophysiological results confirmed a similar picture, with reduced firing rates and decreased spontaneous excitatory signals in pyramidal neurons. Which is consistent with the finding of reduced glutamate receptor subunit expression. While SST+ interneurons showed no change synaptic behaviour, PV+ interneurons showed increased excitability. Since PV+ neurons in this microcircuit act to inhibit the activity of pyramidal cells, authors interpreted this as contributing to a collectively reduction in OFC output.

In mice pre-treated with 5-HT2A antagonists, researchers noted the complete abolition all these neuronal changes, confirming the changes were 5-HT2A receptor dependent. Additionally, silencing 5-HT2a receptor in pyramidal neurons prevented psilocybin’s downregulation of the glutamate receptor subunits, thus removing the antidepressant effect in these mice. When the same thing was done in PV+ neurons, there was only a partial reduction in antidepressant effect, suggesting layer 5 pyramidal neurons are the primary target mediating the effects of psilocybin in the OFC.

There is still much unknown about Psilocybin’s mechanism on the brain. The subjective experience when taking psychedelics can range massively, without there being any concrete predictors or biomarkers that can indicate what the subjective experience may look like.

In this study researchers took EEG measures on participants both during resting state and again 60 minutes after administration. They were assessed for 5 minutes with eyes open and for 5 minutes with eyes closed. Lighting and ambient music was used to create pleasant scenes. Participants also filled in the ASC-5 questionnaire, which quantifies the subjective effects across dimensions of: Boundlessness, Dreadful Ego Dissolution, Visual Restructuralization, Auditory Alterations and Vigilance Reduction.

When the five ASC 5 dimensions were broken down, each showed a distinct neural fingerprint. Every ASC-5 item correlated positively with widespread power changes, with items such as Oceanic Boundlessness being tightly linked to increased gamma power particularly in limbic areas, and Anxious Ego Dissolution was more associated with occipital activity.

Psilocybin also reshaped the Default Mode Network (DMN) Compared with placebo, slow frequency (theta, alpha) power dropped across the DMN while fast frequency (beta, gamma1, gamma2) power rose, especially in frontal, temporal and limbic zones. Connectivity analysis revealed a marked increase in long range links within the DMN at these high frequency bands, whereas low frequency connections weakened. The net effect was a DMN that became more globally synchronized at beta gamma ranges, reflecting the network’s heightened integration during the psychedelic state.

Additionally. resting state EEG proved to be a surprisingly strong predictor of how intensely participants rated their psychedelic trip. The researchers first examined baseline power across theta, alpha, beta and gamma bands and linked those values to later ASC 5 scores, showing that higher pre drug activity in fast (beta/gamma) frequencies foretold stronger subjective effects1. Notably, frontal and limbic regions that displayed elevated baseline beta and most gamma power were tied to a more intense psychedelic experience.

Overall, this study provides exciting initial biomarkers of psychedelic experiences, which has the potential for improved and tailored clinical intervention.

Psilocybin traditionally has a bioavailability of around 52%, as well as a metabolic elimination rate which can vary. This is a problem when medicating with psilocybin, as it can lead to inconsistencies in what the appropriate dose would be.

An alternative formulation of psilocybin, (L-130), has been found with a bioavailability of 99%. Here, researchers use a rat model to demonstrate that it is not only safe but also showed expected improvements in anxiety

Rats were split into 5 groups, one acting as an unstressed control, with the remaining four receiving either saline or L-130 at daily and weekly intervals. Non-control rats were exposed to varying mild stressors over 28 days to induce anxiety-like symptoms, before undergoing blood work and behavioural assessments.

Safety results were encouraging, with no notable differences in blood-related markers, weight, or feed consumption across groups. Daily L-130 treatment significantly reduced cortisol levels relative to stressed controls, approaching those of unstressed animals.

Behaviourally, this group outperformed controls across all three anxiety tasks. Weekly dosing showed limited and inconsistent effects across measures, and head twitch responses indicating hallucinogenic activity were low and confined to the daily group. No tolerance developed over the 28-day period.

These findings suggest L-130 as a strong candidate for further pharmacological development. Its tighter dosage control, enabled by near-complete bioavailability, could allow for at home treatments while minimising the risk of unwanted hallucinogenic effects — making structured, precise clinical application considerably more achievable than with traditional psilocybin

The claustrum is a small subcortical structure sitting between the insular cortex and striatum, densely packed with serotonin 5-HT2A receptors. Despite long being theorised as a hub for sensory integration and consciousness, how psychedelics actually alter its neurochemistry had never been directly examined.

This study used in vivo microdialysis in freely moving rats to measure how psilocin and the synthetic compound 25I-NBOMe altered extracellular neurotransmitter levels directly within the claustrum. Samples were collected over two hours, with all changes expressed relative to baseline. A wet dog shake test was also used to measure hallucinogenic activity.

Both compounds showed elevated 5-HT (serotonin) levels, but 25I-NBOMe produced the strongest cumulative response. Notably, the higher psilocin dose resulted in lower 5-HT release than the lower dose, an effect the authors attribute to dose-dependent recruitment of inhibitory 5-HT1A receptors, which appear to dampen serotonergic output as a form of negative feedback. This self-limiting mechanism is absent with 25I-NBOMe, whose selective and high-affinity 5-HT2A agonism drives prolonged, unmodulated serotonergic activation.

The glutamate findings further distinguished the two compounds. 25I-NBOMe induced a more pronounced and sustained elevation in extracellular glutamate, whereas psilocin's response appeared to reach a ceiling effect at higher doses, again pointing toward inhibitory counter-regulation via 5-HT1A engagement. The authors highlight the resulting GLU/GABA ratio as a critical marker with psilocin at lower doses preserving excitatory-inhibitory balance, and 25I-NBOMe shifting the claustrum decisively toward excitatory dominance.

Taken together, these findings position the claustrum as a neurochemical convergence point as an active integrator of multiple transmitter systems simultaneously, as opposed to a simple sensory relay station.

It is largely established that psychedelics can have beneficial therapeutic effects across various psychiatric conditions, including major depressive disorder. Despite this, the exact mechanism that underpin this effect is not fully understood.

Rats were first trained in a reward learning essay. Here they were given various combinations of ceramic bowls which contained different amounts of a food reward (0, 1, or 2 pellets). This was done in a sequence which trained the rats to recognise how much reward was associated with each bowl creating a positive choice bias. This meant rats were more likely to choose the bowl that gave the most reward.

Rats were then split into 3 groups, with 2 groups being treated daily (for 14 days) with interferon alpha to induce a depression like phenotype. The other group remained as a control.

After 14 days, rats were re-tested in their reward learning environment. In rats where a depression phenotype was induced, a blunting of their positive choice bias was shown. This meant that while control rats maintained a preference for higher reward options, these rats were indifferent to higher or lower reward options.

Within the 2 groups of depressed rats, one group received psilocybin treatment, whereas the other group was given a salient solution to act as a control. In the psilocybin treated group, a significant reversal of the learning impairment was shown by 24 hours and persisted for up to 7 days.

Here, researchers demonstrate that psilocybin is able to rapidly restore depression induced deficits in reward learning. They then go on to suggest that this reversal is a potential mechanism by which psilocybin exerts its antidepressant effect. Specifically they reference the nucleus accumbens as a potential site where these changes may take place, as this is an area already implied in reward learning.

Researchers demonstrate Benzodiazepines negate long term antidepressant effect in rat models

Researchers gave rats either Saline (S), Saline and Psilocybin (S/P), or Psilocybin with prior administration of Lorazepam (L/P), a benzodiazepine that is often psychiatrically prescribed to manage anxiousness.

The antidepressant effects of each condition was measured via the forced swim test , a common measure of antidepressant effect in rodent models. This was done at 3 and 14 weeks, after administration.

At 3 weeks, both S/P and L/P groups were significantly more active in the forced swim test than the Saline control, suggesting an antidepressant effect. At week 14 the S/P group maintained significantly greater movement, demonstrating a long-term antidepressant effect which was expected. The L/P group showed reduced activity at week 14, in line with the Saline control group.

While this finding suggests the long term antidepressant effect of Psilocybin is mitigated by Lorazepam, researchers note that this is merely an exploratory study. The lack of positive control, to measure Lorazepam’s effect on the forced swim test, as well as a lack of locomotor assessments means there should be caution surrounding these findings.

Despite this, these findings still have interesting implications for treatment protocols involving benzodiazepines, as well as give further insight to the neural mechanisms that underlie psilocybin’s antidepressant effect