Serotonin excites hippocampal CA1 GABAergic interneurons at the stratum radiatum-stratum lacunosum moleculare border

Psilocybin and 2C-B at Encoding Distort Episodic Familiarity

BACKGROUND

As research on psychedelics (hallucinogenic serotonin receptor 2A agonists) progresses, it is important to delineate the reliability of supposedly unique effects across this drug class. One such effect is how psychedelics impair the formation (i.e., encoding) of hippocampal-dependent recollections (retrieval of specific details) while potentially enhancing the encoding of cortical-dependent familiarity (a feeling of knowing that a stimulus has been previously experienced).

METHODS

In a double-blind, placebo-controlled, within-participants study (N = 20), we tested the acute effects of 2 distinct psychedelics, psilocybin and 2C-B, on the encoding of emotional episodic memories. During acute drug effects, participants viewed negative, neutral, and positive pictures. The following day (while sober), participants completed 2 separate memory tests for these pictures.

RESULTS

Using computational models of memory confidence, we found trends for psilocybin and 2C-B at encoding to impair estimates of recollection that were supported by other measures/analyses. Surprisingly, psilocybin and 2C-B at encoding impaired estimates of familiarity, but these impairments were likely due to a misattribution of heightened familiarity, because both drugs at encoding selectively increased familiarity-based false alarms, especially for negative and positive stimuli. Psilocybin and 2C-B at encoding also tended to impair estimates of metamemory (understanding one's own memory) for negative and neutral memories but enhanced estimates of metamemory for positive memories, although these effects were less reliable in additional analyses.

CONCLUSIONS

Despite differences in their chemistry, pharmacology, and subjective effects, both psilocybin and 2C-B distorted episodic familiarity, suggesting a common neurocognitive mechanism across psychedelics that may drive other phenomena.

Synthetic surprise as the foundation of the psychedelic experience

Psychedelic agents, such as LSD and psilocybin, induce marked alterations in consciousness via activation of the 5-HT2A receptor (5-HT2ARs). We hypothesize that psychedelics enforce a state of synthetic surprise through the biased activation of the 5-HTRs system. This idea is informed by recent insights into the role of 5-HT in signaling surprise. The effects on consciousness, explained by the cognitive penetrability of perception, can be described within the predictive coding framework where surprise corresponds to prediction error, the mismatch between predictions and actual sensory input. Crucially, the precision afforded to the prediction error determines its effect on priors, enabling a dynamic interaction between top-down expectations and incoming sensory data. By integrating recent findings on predictive coding circuitry and 5-HT2ARs transcriptomic data, we propose a biological implementation with emphasis on the role of inhibitory interneurons. Implications arise for the clinical use of psychedelics, which may rely primarily on their inherent capacity to induce surprise in order to disrupt maladaptive patterns.

Effects of Chronic Combined Treatment with Ketanserin and Fluoxetine in B6.CBA-D13Mit76C Recombinant Mice with Abnormal 5-HT1A Receptor Functional Activity

The recombinant B6.CBA-D13Mit76C mouse strain is characterized by an altered sensitivity of 5-HT1A receptors and upregulated 5-HT1A gene transcription. Recently, we found that in B6.CBA-D13Mit76C mice, chronic fluoxetine treatment produced the pro-depressive effect in a forced swim test. Since 5-HT2A receptor blockade may be beneficial in treatment-resistant depression, we investigated the influence of chronic treatment (14 days, intraperitoneally) with selective 5-HT2A antagonist ketanserin (0.5 mg/kg), fluoxetine (20 mg/kg), or fluoxetine + ketanserin on the behavior, functional activity of 5-HT1A and 5-HT2A receptors, serotonin turnover, and transcription of principal genes of the serotonin system in the brain of B6.CBA-D13Mit76C mice. Ketanserin did not reverse the pro-depressive effect of fluoxetine, while fluoxetine, ketanserin, and fluoxetine + ketanserin decreased the functional activity of 5-HT1A receptors and Htr1a gene transcription in the midbrain and hippocampus. All tested drug regimens decreased the mRNA levels of Slc6a4 and Maoa in the midbrain. These changes were not accompanied by a significant shift in the levels of serotonin and its metabolite 5-HIAA. Notably, ketanserin upregulated enzymatic activity of tryptophan hydroxylase 2 (TPH2). Thus, despite some benefits (reduced Htr1a, Slc6a4, and Maoa transcription and increased TPH2 activity), prolonged blockade of 5-HT2A receptors failed to ameliorate the adverse effect of fluoxetine in the case of abnormal functioning of 5-HT1A receptors.

Harnessing psilocybin: antidepressant-like behavioral and synaptic actions of psilocybin are independent of 5-HT2R activation in mice

Depression is a widespread and devastating mental illness and the search for rapid-acting antidepressants remains critical. There is now exciting evidence that the psychedelic compound psilocybin produces not only powerful alterations of consciousness, but also rapid and persistent antidepressant effects. How psilocybin exerts its therapeutic actions is not known, but it is widely presumed that these actions require altered consciousness, which is known to be dependent on serotonin 2A receptor (5-HT2AR) activation. This hypothesis has never been tested, however. We therefore asked whether psilocybin would exert antidepressant-like responses in mice and, if so, whether these responses required 5-HT2AR activation. Using chronically stressed male mice, we observed that a single injection of psilocybin reversed anhedonic responses assessed with the sucrose preference and female urine preference tests. The antianhedonic response to psilocybin was accompanied by a strengthening of excitatory synapses in the hippocampus-a characteristic of traditional and fast-acting antidepressants. Neither behavioral nor electrophysiological responses to psilocybin were prevented by pretreatment with the 5-HT2A/2C antagonist ketanserin, despite positive evidence of ketanserin's efficacy. We conclude that psilocybin's mechanism of antidepressant action can be studied in animal models and suggest that altered perception may not be required for its antidepressant effects. We further suggest that a 5-HT2AR-independent restoration of synaptic strength in cortico-mesolimbic reward circuits may contribute to its antidepressant action. The possibility of combining psychedelic compounds and a 5-HT2AR antagonist offers a potential means to increase their acceptance and clinical utility and should be studied in human depression.

Somatostatin interneurons activated by 5-HT2A receptor suppress slow oscillations in medial entorhinal cortex

Serotonin (5-HT) is one of the major neuromodulators present in the mammalian brain and has been shown to play a role in multiple physiological processes. The mechanisms by which 5-HT modulates cortical network activity, however, are not yet fully understood. We investigated the effects of 5-HT on slow oscillations (SOs), a synchronized cortical network activity universally present across species. SOs are observed during anesthesia and are considered to be the default cortical activity pattern. We discovered that (±)3,4-methylenedioxymethamphetamine (MDMA) and fenfluramine, two potent 5-HT releasers, inhibit SOs within the entorhinal cortex (EC) in anesthetized mice. Combining opto- and pharmacogenetic manipulations with in vitro electrophysiological recordings, we uncovered that somatostatin-expressing (Sst) interneurons activated by the 5-HT_2A receptor (5-HT_2AR) play an important role in the suppression of SOs. Since 5-HT_2AR signaling is involved in the etiology of different psychiatric disorders and mediates the psychological effects of many psychoactive serotonergic drugs, we propose that the newly discovered link between Sst interneurons and 5-HT will contribute to our understanding of these complex topics.

Cajal-Retzius cells and GABAergic interneurons of the developing hippocampus: Close electrophysiological encounters of the third kind

In contrast to the large number of studies investigating the electrophysiological properties and synaptic connectivity of hippocampal pyramidal neurons, granule cells, and GABAergic interneurons, much less is known about Cajal-Retzius cells. In this review article, we discuss the possible reasons underlying this difference, and review experimental work performed on this cell type in the hippocampus, comparing it with results obtained in the neocortex. Our main emphasis is on data obtained with in vitro electrophysiology. In particular, we address the bidirectional connectivity between Cajal-Retzius cells and GABAergic interneurons, examine their synaptic properties and propose specific functions of Cajal-Retzius cell/GABAergic interneuron microcircuits. Lastly, we discuss the potential involvement of these microcircuits in critical physiological hippocampal functions such as postnatal neurogenesis or pathological scenarios such as temporal lobe epilepsy.

Hippocampal GABAergic Inhibitory Interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10-15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.

5-HT2A receptor deficiency alters the metabolic and transcriptional, but not the behavioral, consequences of chronic unpredictable stress

Chronic stress enhances risk for psychiatric disorders, and in animal models is known to evoke depression-like behavior accompanied by perturbed neurohormonal, metabolic, neuroarchitectural and transcriptional changes. Serotonergic neurotransmission, including serotonin_2A (5-HT_2A) receptors, have been implicated in mediating specific aspects of stress-induced responses. Here we investigated the influence of chronic unpredictable stress (CUS) on depression-like behavior, serum metabolic measures, and gene expression in stress-associated neurocircuitry of the prefrontal cortex (PFC) and hippocampus in 5-HT_2A receptor knockout (5-HT2A−/−) and wild-type mice of both sexes. While 5-HT2A−/− male and female mice exhibited a baseline reduced anxiety-like state, this did not alter the onset or severity of behavioral despair during and at the cessation of CUS, indicating that these mice can develop stress-evoked depressive behavior. Analysis of metabolic parameters in serum revealed a CUS-evoked dyslipidemia, which was abrogated in 5-HT2A−/− female mice with a hyperlipidemic baseline phenotype. 5-HT2A−/− male mice in contrast did not exhibit such a baseline shift in their serum lipid profile. Specific stress-responsive genes (Crh, Crhr1, Nr3c1, and Nr3c2), trophic factors (Bdnf, Igf1) and immediate early genes (IEGs) (Arc, Fos, Fosb, Egr1-4) in the PFC and hippocampus were altered in 5-HT2A−/− mice both under baseline and CUS conditions. Our results support a role for the 5-HT_2A receptor in specific metabolic and transcriptional, but not behavioral, consequences of CUS, and highlight that the contribution of the 5-HT_2A receptor to stress-evoked changes is sexually dimorphic.