Alain Michel Gardier

In the 1950s–60s, serotonergic psychedelic drugs were studied as potential adjuvants to psychotherapy to treat addiction and alcoholism. However, starting in the 70s, preclinical and clinical studies on psychedelics stopped for decades because legislation controlled its recreational use, citing their hallucinogenic and psychotomimetic effects, as well as their abuse potential. Amazingly, we are witnessing an impressive return of these drugs due to recent clinical trials suggesting a therapeutic potential of psychedelics, among them psilocybin, for treating patients with depression resistant to conventional antidepressant drugs. Yet, their underlying mechanisms of action remain incompletely elucidated. This review provides an update on seminal clinical trials using psilocybin, as well as preclinical work uncovering the pharmacological properties and experimental pharmacology of psilocybin and its active metabolite psilocin. These drugs are primarily serotonin 5‐HT _2A receptor (5‐HT2AR) agonists. Although there is a consensus that 5‐HT2AR activation mediates its psychedelic effects in human and rodent models of anxiety/depression, its role in psilocin's antidepressant effects remains controversial. This review also provides an overview of neurotransmitter systems, neuroplasticity, and neural circuits activated by psilocin. Further research in developing effective antidepressants for depression is prescient now more than ever, as according to the World Health Organization (WHO), depression will be the main cause of disability in 2030. Understanding the mechanisms through which psilocybin/psilocin would be an effective antidepressant is crucial to ultimately validate its therapeutic potential when combined with SSRIs/SNRIs in mood disorders.

( R,S )-ketamine (ketamine) has rapid and sustained antidepressant (AD) efficacy at sub-anesthetic doses in depressed patients. A metabolite of ketamine, including ( 2R,6R )-hydroxynorketamine (( 6 )-HNKs) has been reported to exert antidepressant actions in rodent model of anxiety/depression. To further understand the specific role of ketamine’s metabolism in the AD actions of the drug, we evaluated the effects of inhibiting hepatic cytochrome P450 enzymes on AD responses. We assessed whether pre-treatment with fluconazole (10 and 20 mg/kg, i.p.) 1 hour prior to ketamine or HNKs (10 mg/kg, i.p.) administration would alter behavioral and neurochemical actions of the drugs in male BALB/cJ mice with a highly anxious phenotype. Extracellular microdialysate levels of glutamate and GABA (Glu _ext , GABA _ext ) were also measured in the medial prefrontal cortex (mPFC). Pre-treatment with fluconazole altered the pharmacokinetic profile of ketamine, by increasing both plasma and brain levels of ketamine and ( R , S )-norketamine, while robustly reducing those of ( 6 )-HNKs. At 24 hours post-injection (t24h), fluconazole prevented the sustained AD-like response of ketamine responses in the forced swim test and splash test, as well as the enhanced cortical GABA levels produced by ketamine. A single ( 2R,6R )-HNK administration selectively rescued the antidepressant-like activity of ketamine in mice pretreated with fluconazole within 24 hours of treatment. Overall, these findings are consistent with an essential role of ( 6 )-HNK in mediating the sustained antidepressant-like effects of ketamine and suggest potential interactions between pharmacological CYPIs and ketamine during antidepressant treatment in patients.

La kétamine, un antagoniste non compétitif du récepteur N-méthyl-D-aspartate (R-NMDA) du glutamate, possède un effet antidépresseur rapide (dès 24 h post-dose) et prolongé (jusqu’à une semaine) dans la dépression résistante au traitement par des antidépresseurs « classiques » et dans les modèles rongeurs d’anxiété/dépression. Les arguments concernant ses mécanismes cellulaires et moléculaires sous-tendant son activité antidépressive viennent principalement d’études animales. Des débats persistent cependant sur le remodelage structurel des neurones frontocorticaux/hippocampiques et sur le rôle des neurotransmetteurs excitateurs/inhibiteurs impliqués dans cet effet comportemental observé chez l’animal. Les modifications neurochimiques et comportementales se maintiennent 24 h après l’administration de la kétamine, bien au-delà de sa demi-vie d’élimination plasmatique. L’avancée des connaissances sur les conséquences du blocage du R-NMDA permet de préciser les mécanismes sous-jacents impliquant (i) l’activation des récepteurs AMPA du glutamate, qui déclenche une cascade d’évènements intracellulaires dépendants de la cible mécanistique de la rapamycine, (ii) le facteur neurotrophique dérivé du cerveau et (iii) la synthèse de protéines synaptiques facilitant la plasticité synaptique (nombre d’épines dendritiques, synaptogenèse). Les cellules pyramidales glutamatergiques du cortex préfrontal médian sont principalement impliquées dans les effets thérapeutiques de la kétamine. La présente revue se concentre sur les anomalies des systèmes de neurotransmetteurs associées aux troubles dépressifs caractérisés, leur impact potentiel sur les circuits neuronaux et les effets bénéfiques de la kétamine. Les résultats d’études précliniques récentes devraient aider à orienter les futures études pour mieux préciser le mécanisme d’action des antidépresseurs d’action rapide et permettre ainsi le développement de nouvelles thérapies plus efficaces.

BACKGROUND: Serotonin (5-HT) receptors and N -methyl-D-aspartate receptors (NMDARs) have both been implicated in the pathophysiology of depression and anxiety disorders. Here, we evaluated whether targeting both receptors through combined dosing of ( R , S )-ketamine, an NMDAR antagonist, and prucalopride, a serotonin type IV receptor (5-HT ₄ R) agonist, would have additive effects, resulting in reductions in stress-induced fear, behavioral despair, and hyponeophagia. METHODS: A single injection of saline (Sal), ( R , S )-ketamine (K), prucalopride (P), or a combined dose of ( R , S )-ketamine and prucalopride (K+P) was administered before or after contextual fear conditioning (CFC) stress in both sexes. Drug efficacy was assayed using the forced swim test (FST), elevated plus maze (EPM), open field (OF), marble burying (MB), and novelty-suppressed feeding (NSF). Patch clamp electrophysiology was used to measure the effects of combined drug on neural activity in hippocampal CA3. c-fos and parvalbumin (PV) expression in the hippocampus (HPC) and medial prefrontal cortex (mPFC) was examined using immunohistochemistry and network analysis. RESULTS: We found that a combination of K+P, given before or after stress, exerted additive effects, compared to either drug alone, in reducing a variety of stress-induced behaviors in both sexes. Combined K+P administration significantly altered c-fos and PV expression and network activity in the HPC and mPFC. CONCLUSIONS: Our results indicate that combined K+P has additive benefits for combating stress-induced pathophysiology, both at the behavioral and neural level. Our findings provide preliminary evidence that future clinical studies using this combined treatment strategy may prove advantageous in protecting against a broader range of stress-induced psychiatric disorders.

We investigated the mechanisms underlying the effects of the antidepressant fluoxetine on behavior and adult hippocampal neurogenesis (AHN). After confirming our earlier report that the signaling molecule β‐arrestin‐2 (β‐Arr2) is required for the antidepressant‐like effects of fluoxetine, we found that the effects of fluoxetine on proliferation of neural progenitors and survival of adult‐born granule cells are absent in the β‐Arr2 knockout (KO) mice. To our surprise, fluoxetine induced a dramatic upregulation of the number of doublecortin (DCX)‐expressing cells in the β‐Arr2 KO mice, indicating that this marker can be increased even though AHN is not. We discovered two other conditions where a complex relationship occurs between the number of DCX‐expressing cells compared to levels of AHN: a chronic antidepressant model where DCX is upregulated and an inflammation model where DCX is downregulated. We concluded that assessing the number of DCX‐expressing cells alone to quantify levels of AHN can be complex and that caution should be applied when label retention techniques are unavailable.

Major depressive disorder (MDD) is a serious public health problem, as it is the most common psychiatric disorder worldwide. Antidepressant drugs increase adult hippocampal neurogenesis, which is required to induce some behavioral effects of antidepressants. Adult‐born granule cells in the dentate gyrus (DG) and the glutamate receptors subunits 2 (GluN2B) subunit of N‐methyl‐D‐aspartate (NMDA) ionotropic receptors play an important role in these effects. However, the precise neurochemical role of the GluN2B subunit of the NMDA receptor on adult‐born GCs for antidepressant‐like effects has yet to be elucidated. The present study aims to explore the contribution of the GluN2B‐containing NMDA receptors in the ventral dentate gyrus (vDG) to the antidepressant drug treatment using a pharmacological approach. Thus, (αR)‐(4‐hydroxyphenyl)‐(βS)‐methyl‐4‐(phenylmethyl)‐1‐piperidinepropanol (Ro25‐6981), a selective antagonist of the GluN2B subunit, was acutely administered locally into the ventral DG (vDG, 1 μg each side) following a chronic fluoxetine (18 mg/kg/day) treatment—known to increase adult hippocampal neurogenesis—in a mouse model of anxiety/depression. Responses in a neurogenesis‐dependent task, the novelty suppressed feeding (NSF), and neurochemical consequences on extracellular glutamate and gamma‐aminobutyric acid (GABA) levels in the vDG were measured. Here, we show a rapid‐acting antidepressant‐like effect of local Ro25‐6981 administration in the NSF independent of fluoxetine treatment. Furthermore, we revealed a fluoxetine‐independent increase in the glutamatergic transmission in the vDG. Our results suggest behavioral and neurochemical effects of GluN2B subunit independent of serotonin reuptake inhibition.