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Background and Purpose: Psychedelics elicit prosocial, antidepressant and anxiolytic effects via neuroplasticity, neurotransmission and neuro‐immunomodulatory mechanisms. Whether psychedelics affect the brain endocannabinoid system and its extended version, the endocannabinoidome (eCBome) or the gut microbiome, remains unknown. Experimental Approach: Adult C57BL/6N male mice were administered lysergic acid diethylamide (LSD) or saline for 7 days. Sociability was assessed in the direct social interaction and three chambers tests. Prefrontal cortex and hippocampal endocannabinoids, endocannabinoid‐like mediators and metabolites were quantified via high‐pressure liquid chromatography with tandem mass spectrometry (HPLC‐MS/MS). Neurotransmitter levels were assessed via HPLC‐UV/fluorescence. Gut microbiome changes were investigated by 16S ribosomal DNA sequencing. Key Results: LSD increased social preference and novelty and decreased hippocampal levels of the N‐acylethanolamines N‐linoleoylethanolamine (LEA), anandamide (N‐arachidonoylethanolamine) and N‐docosahexaenoylethanolamine (DHEA); the monoacylglycerol 1/2‐docosahexaenoylglycerol (1/2‐DHG); the prostaglandins D2 (PGD2) and F2α (PGF2α); thromboxane 2 and kynurenine. Prefrontal eCBome mediator and metabolite levels were less affected by the treatment. LSD decreased Shannon alpha diversity of the gut microbiota, prevented the decrease in the Firmicutes:Bacteroidetes ratio observed in saline‐treated mice and altered the relative abundance of the bacterial taxa Bifidobacterium, Ileibacterium, Dubosiella and Rikenellaceae RC9. Conclusions and Implications: The prosocial effects elicited by repeated LSD administration are accompanied by alterations of hippocampal eCBome and kynurenine levels, and the composition of the gut microbiota. Modulation of the hippocampal eCBome and kynurenine pathway might represent a mechanism by which psychedelic compounds elicit prosocial effects and affect the gut microbiome. [ABSTRACT FROM AUTHOR]

The melatonin system and circadian disruption have well‐established links with aggressive behaviors; however, the biological underpinnings have not been thoroughly investigated. Here, we aimed at examining the current knowledge regarding the neurobiological and psychopharmacological involvement of the melatonin system in aggressive/violent behaviors. To this end, we performed a systematic review on Embase and Pubmed/MEDLINE of preclinical and clinical evidence linking the melatonin system, melatonin, and melatoninergic drugs with aggressive/violent behaviors. Two blinded raters performed an independent screening of the relevant literature. Overall, this review included 38 papers distributed between clinical and preclinical models. Eleven papers specifically addressed the existing evidence in rodent models, five in fish models, and 21 in humans. The data indicate that depending on the species, model, and timing of administration, melatonin may exert a complex influence on aggressive/violent behaviors. Particularly, the apparent contrasting findings on the link between the melatonin system and aggression/violence (with either increased, no, or decreased effect) shown in preclinical models underscore the need for further research to develop more accurate and fruitful translational models. Likewise, the significant heterogeneity found in the results of clinical studies does not allow yet to draw any firm conclusion on the efficacy of melatonin or melatonergic drugs on aggressive/violent behaviors. However, findings in children and in traits associated with aggressive/violent behavior, including irritability and anger, are emerging and deserve empirical attention given the low toxicity of melatonin and melatonergic drugs. [ABSTRACT FROM AUTHOR]

Melatonin (MLT) is widely used to treat sleep disorders although the underlying mechanism is still elusive. In mice, using wheel‐running detection, we found that exogenous MLT could completely recover the period length prolonged by N‐methyl‐D‐aspartate receptor (NMDAR) impairment due to the injection of the NMDAR antagonist MK‐801, a preclinical model of psychosis. The analysis of the possible underlying mechanisms indicated that MLT could regulate the homeostatic state in the ventrolateral preoptic nucleus (VLPO) instead of the circadian process in the suprachiasmatic nucleus (SCN). In addition, our data showed that MK‐801 decreased Ca2+‐related CaMKII expression and CREB phosphorylation levels in the VLPO, and MLT could rescue these intracellular impairments but not NMDAR expression levels. Accordingly, Gcamp6 AAV virus was injected in‐vivo to further monitor intracellular Ca2+ levels in the VLPO, and MLT demonstrated a unique ability to increase Ca2+ fluorescence compared with MK‐801‐injected mice. Additionally, using the selective melatonin MT2 receptor antagonist 4‐phenyl‐2‐propionamidotetralin (4P‐PDOT), we discovered that the pharmacological effects of MLT upon NMDAR impairments were mediated by melatonin MT2 receptors. Using electroencephalography/electromyography (EEG/EMG) recordings, we observed that the latency to the first nonrapid eye movement (NREM) sleep episode was delayed by MK‐801, and MLT was able to recover this delay. In conclusion, exogenous MLT by acting upon melatonin MT2 receptors rescues sleep phase delayed by NMDAR impairment via increasing intracellular Ca2+ signaling in the VLPO, suggesting a regulatory role of the neurohormone on the homeostatic system. [ABSTRACT FROM AUTHOR]

Melatonin (MLT) levels fluctuate according to the external light/dark cycle in both diurnal and nocturnal mammals. We previously demonstrated that melatonin MT2 receptor knockout (MT2−/−) mice show a decreased nonrapid eye movement sleep over 24 hours and increased wakefulness during the inactive (light) phase. Here, we investigated the role of MT2 receptors in physiological light/dark cycle fluctuations in the activity of dorsal raphe nucleus (DRN) serotonin (5‐HT) neurons and anxiety‐ and depression‐like behavior. We found that the 5‐HT burst‐firing activity was tonically reduced across the whole 24 hours in MT2−/− mice compared with MT2+/+ mice. Importantly, the physiological changes in the spontaneous firing activity of DRN 5‐HT neurons during the light/dark cycle were nullified in MT2−/− mice, with a higher DRN 5‐HT neural firing activity during the light phase in MT2−/− than in MT2+/+ mice. The role of MT2 receptors over DRN 5‐HT neurons was confirmed by acute pharmacological studies in which the selective MT2 receptors agonist UCM1014 dose dependently inhibited DRN 5‐HT activity, mostly during the dark phase. Compared with MT2+/+, MT2−/− mice displayed an anxiety‐like phenotype in the novelty‐suppressed feeding and in the light/dark box tests; while anxiety levels in the light/dark box test were lower during the dark than during the light phase in MT2+/+ mice, the opposite was seen in MT2−/− mice. No differences between MT2+/+ and MT2−/− mice were observed for depression‐like behavior in the forced swim and in the sucrose preference tests. These results suggest that MT2 receptor genetic inactivation impacts 5‐HT neurotransmission and interferes with anxiety levels by perturbing the physiologic light/dark pattern. [ABSTRACT FROM AUTHOR]

The role of melatonin in humans still remains unclear. Uncertainties persist about its effects on neurophysiology regarding its levels in human cerebrospinal fluid (CSF), as the bulk of knowledge on this subject mainly derives from studies conducted on animals. In this study, CSF was micro-sampled with a simple, new method from each cerebral ventricle of patients undergoing neuroendoscopy for hydrocephalus. Our purpose was to measure CSF melatonin levels and determine possible differences in its concentration among various significant areas in the cerebral ventricles (e.g. pineal recess, pituitary recess, lateral ventricle, fourth ventricle) and lumbar cistern. From 2002 to 2004, 10 hydrocephalic patients were operated on using a neuroendoscopic technique. The CSF specimens were investigated for melatonin concentrations (free plus protein-bound) after deproteinization; the measurement technique was high-performance liquid chromatography. The preliminary data obtained with this endoscopic micro-sampling technique (applied to humans for the first time) suggest that melatonin is more concentrated within the ventricles and its highest concentration is found in the third ventricle (IIIv), although the difference detected between the CSF of the IIIv and that of the pineal recess was not significant. [ABSTRACT FROM AUTHOR]

The pharmacological potential of targeting selectively melatonin MT1 or MT2 receptors has not yet been exploited in medicine. Research using selective MT1/MT2 receptor ligands and MT1/MT2 receptor knockout mice has indicated that the activation of MT2 receptors selectively increases non‐rapid eye movement (NREM) sleep whereas MT1 receptors seem mostly implicated in the regulation of REM sleep. Moreover, MT1 knockout mice show an increase in NREM sleep, while MT2 knockout a decrease, suggesting an opposite role of these two receptors. A recent paper in mice by Sharma et al (J Pineal Res, 2018, e12498) found that MT1 but not MT2 receptors are expressed on orexin neurons in the perifornical lateral hypothalamus (PFH). Moreover, after injecting melatonin or luzindole into the mouse PFH, the authors suggest that melatonin promotes NREM sleep because activates PFH MT1 receptors, which in turn inhibit orexin neurons that are important in promoting arousal and maintaining wakefulness. In this commentary, we have critically commented on some of these findings on the bases of previous literature. In addition, we highlighted the fact that no conclusions could be drawn on the melatonin receptor subtype mediating the effects of melatonin on sleep because the authors used the non‐selective MT1/MT2 receptors antagonist luzindole. More solid research should further characterize the pharmacological function of these two melatonin receptors in sleep. [ABSTRACT FROM AUTHOR]