Orexin and MCH neurons: regulators of sleep and metabolism

Melanin-concentrating hormone: A promising target for antidepressant treatment

Depression represents a complex neuropsychiatric disorder with an escalating global health burden, characterized by heterogeneous pathophysiology and profound impairments in cognitive-emotional functioning. Current treatment methods have limited efficacy in some individuals and may induce undesirable side effects, necessitating the exploration of novel therapeutic targets and techniques. Emerging research has identified neuropeptide systems as pivotal regulators of mood-related circuits, with melanin-concentrating hormone (MCH) signaling emerging as a particularly promising candidate for antidepressant development. The potential involvement of MCH in the pathophysiology of depression was first proposed over two decades ago. Since then, accumulating evidence from recent studies has progressively illuminated its multifaceted roles in modulating depressive behaviors and underlying neurobiological mechanisms. This review systematically analyzes the mechanistic interplay between MCH signaling and depression pathophenotypes, including its relationship with the hypothalamic-pituitary-adrenal (HPA) axis, neurotransmitter systems, synaptic plasticity, and the regulation of sleep-wakefulness. Particular emphasis is placed on advancing the therapeutic rationale for MCH receptor 1 (MCHR1) antagonists, which demonstrate rapid-onset antidepressant efficacy in preclinical studies compared to traditional agents. Nonetheless, the antidepressant mechanism of the MCH system still requires further elucidation to confirm its therapeutic potential.

Prolactin in sleep and EEG regulation: New mechanisms and sleep-related brain targets complement classical data

The role of prolactin in sleep regulation has been the subject of extensive research over the past 50 years, resulting in the identification of multiple, disparate functions for the hormone. Prolactin demonstrated a characteristic circadian release pattern with elevation during dark and diminution during light. High prolactin levels were linked to non-rapid eye movement sleep and electroencephalogram delta activity in humans. Conversely, hyperprolactinemia showed strong correlation with REM sleep in rodent studies. Prolactin may be implicated in the alterations in female sleep patterns observed during the reproductive cycle, it may play a role in the REM sleep enhancement following stress and in sleep-related immunological processes. In conclusion, prolactin appears to have a sleep-promoting role, particularly during the dark phase. However, it does not appear to play a central and coherent role in sleep regulation, as observed in some neuropeptides such as orexin. Conversely, its principal function may be to facilitate situational, yet adaptive, changes in sleep patterns in response to challenging physiological phases, such as those associated with stress, immunological challenges, or the reproductive cycle. Neuronal substrates for prolactin-mediated sleep effects remain unknown; however, recent rodent sleep studies may provide insights into the potential sites of these effects.

Assessing the causal association of sleep abnormalities with preeclampsia and eclampsia: a Mendelian randomization analysis

BACKGROUND

Preeclampsia and eclampsia are severe pregnancy disorders marked by hypertension and potential organ damage. The etiological basis of preeclampsia and eclampsia is not fully understood. Previous studies have revealed a link between sleep abnormality and preeclampsia/eclampsia, but the causal relationship remains unclear. In this study, we explored the genetic links between sleep and preeclampsia/eclampsia using genome-wide association study (GWAS) summary data and Mendelian randomization (MR) analysis.

METHODS

RNA sequence dataset GSE114691 was downloaded from the Gene Expression Omnibus database, comprising placental tissues from patients with preeclampsia and controls. Differential expression analysis was conducted with R (v4.2.3) and DESeq2 (v1.38.3). Gene set enrichment analysis (GSEA) was carried out using HTSanalyzeR2. GWAS summary data on preeclampsia/eclampsia and genetic markers for sleep abnormality were sourced from the FinnGen Consortium and IEU genetic databases. The Mendelian randomization analysis was conducted with TwoSampleMR (v0.6.2), and the inverse variance weighted (IVW) approach was employed as the principal method.

RESULTS

GSEA analysis revealed that the orexin receptor pathway showed heightened expression in the preeclampsia group versus controls. The random-effects IVW results showed that sleeplessness/insomnia has a genetic causal relationship with preeclampsia (OR = 2.08, 95% CI: 1.07-4.06, p = 0.0318), while sleep duration has evidence of regulating eclampsia (OR = 0.09, 95% CI: 0.01-0.67, p = 0.0187).

CONCLUSION

This study provides significant evidence for a genetic causal association between sleep abnormalities and preeclampsia/eclampsia. [Figure: see text].

Effects of Paradoxical Sleep Deprivation on MCH and Hypocretin Systems

Melanin-concentrating hormone (MCH) and hypocretins (Hcrt) 1 and 2 are neuropeptides synthesized in the lateral hypothalamic area by neurons that are critical in the regulation of sleep and wakefulness. Their receptors are located in the same cerebral regions, including the frontal cortex and hippocampus. The present study aimed to assess whether 96 hours of paradoxical sleep deprivation alters the functioning of the MCH and hypocretin systems. To do this, in control rats with normal sleep (CTL) and in rats that were deprived of paradoxical sleep (SD), we quantified the following parameters: 1) levels of MCH and hypocretin-1 in the cerebrospinal fluid (CSF); 2) expression of the prepro-MCH ( Pmch ) and prepro-hypocretin ( Hcrt ) genes in the hypothalamus; 3) expression of the Mchr1 and Hcrtr1 genes in the frontal cortex and hippocampus; and 4) expression of the Hcrtr2 gene in the hippocampus. These measures were performed at 6 Zeitgeber time (ZT) points of the day (ZTs: 0, 4, 8, 12, 16, and 20). In the SD group, we found higher levels of MCH in the CSF at the beginning of the dark phase. In the frontal cortex, sleep deprivation decreased the expression of Hcrtr1 at ZT0 . Moreover, we identified significant differences between the light and dark phases in the expression of Mchr1 and Hcrtr1 , but only in the CTL animals . We conclude that there is a day/night modulation in the expression of components of the MCH and hypocretin systems, and this profile is affected by paradoxical sleep deprivation.

Supraspinal glycinergic neurotransmission in pain: A scoping review of current literature

The neurotransmitter glycine is an agonist at the strychnine-sensitive glycine receptors. In addition, it has recently been discovered to act at two new receptors, the excitatory glycine receptor and metabotropic glycine receptor. Glycine's neurotransmitter roles have been most extensively investigated in the spinal cord, where it is known to play essential roles in pain, itch, and motor function. In contrast, less is known about supraspinal glycinergic functions, and their contributions to pain circuits are largely unrecognized. As glycinergic neurons are absent from cortical regions, a clearer understanding of how supraspinal glycine modulates pain could reveal new pharmacological targets. This review aims to synthesize the published research on glycine's role in the adult brain, highlighting regions where glycine signaling may modulate pain responses. This was achieved through a scoping review methodology identifying several key regions of supraspinal pain circuitry where glycine signaling is involved. Therefore, this review unveils critical research gaps for supraspinal glycine's potential roles in pain and pain-associated responses, encouraging researchers to consider glycinergic neurotransmission more widely when investigating neural mechanisms of pain.

GABALAGEN Facilitates Pentobarbital-Induced Sleep by Modulating the Serotonergic System in Rats

Gamma-aminobutyric acid (GABA) is one of the inhibitory neurotransmitters with beneficial effects including sedative properties. However, despite various clinical trials, scientific evidence regarding the impact on sleep of orally ingested GABA, whether natural or synthesized through biological pathways, is not clear. GABALAGEN (GBL) is the product of fermented collagen by Lactobacillus brevis BJ20 (L. brevis BJ20) and Lactobacillus plantarum BJ21 (L. plantarum BJ21), enriched with GABA and characterized by low molecular weight. The aim of this study was to investigate the effect of GBL on sleep improvement via a receptor binding assay in a pentobarbital-induced sleep-related rat model. We utilized a pentobarbital-induced sleep-related rat model to conduct this research. The present study investigated the sedative effects of GBL through electroencephalography (EEG) analysis in the pentobarbital-induced sleep animal model. Exploration of the neural basis of these positive effects involved evaluating orexin in the brain via immunohistochemical methods and 5-HT in the serum using an enzyme-linked immunosorbent assay (ELISA). Furthermore, we conducted a binding assay for 5-HT2C receptors, as these are considered pivotal targets in the mechanism of action for sleep aids. Diazepam (DZP) was used as a positive control to compare the efficacy of GBL. Results: In the binding assay, GBL displayed binding affinity to the 5-HT2C receptor (IC50 value, 5.911 µg/mL). Administration of a low dose of GBL (GBL_L; 100 mg/kg) increased non-rapid eye movement sleep time and decreased wake time based on EEG data in pentobarbital-induced rats. Administration of a high dose of GBL (GBL_H; 250 mg/kg) increased non-rapid eye movement sleep time. Additionally, GBL groups significantly increased concentration of the 5-HT level in the serum. GBL_H decreased orexin expression in the lateral hypothalamus. Conclusion: Overall, the sedative effect of GBL may be linked to the activation of serotonergic systems, as indicated by the heightened affinity of the 5-HT2C receptor binding and elevated levels of 5-HT observed in the serum. This suggests that GBL holds promise as a novel compound for inducing sleep in natural products.

Effect of Earthing Mats on Sleep Quality in Rats

Grounding, a therapeutic technique involving direct contact with the earth, has been proposed by various studies to potentially have beneficial effects on pressure, sleep quality, stress, inflammation, and mood. However, the scientific evidence supporting its sedative effects remains incomplete. This study examined the sedative effectiveness of an earthing mat on sleep quality and investigated the underlying neural mechanisms using electroencephalography (EEG) analysis in rodents, focusing on orexin and superoxide dismutase (SOD) levels in the brain. Rats were randomly assigned to four groups: the naïve normal group (Nor), the group exposed to an earthing mat for 7 days (A-7D), the group exposed to an earthing mat for 21 days (A-21D), and the group exposed to an electronic blanket for 21 days (EM). EEG results revealed that the A-21D group exhibited significantly reduced wake time and increased rapid eye movement (REM), non-rapid eye movement (NREM), and total sleep time compared to the Nor group (p < 0.05). Moreover, the A-21D group demonstrated a significant increase in NREM sleep (p < 0.001), REM sleep (p < 0.01), and total sleep time (p < 0.001), along with a decrease in wake time compared to the EM group (p < 0.001). The orexin level in the A-21D group was significantly lower compared to the Nor group (p < 0.01), while SOD1 expression was markedly elevated in the A-21D group compared to the Nor group (p < 0.001). These results suggest that the earthing mat may represent a promising new method for promoting sleep quality and could serve as an effective therapeutic technique.

Dorsolateral septum GLP-1R neurons regulate feeding via lateral hypothalamic projections

OBJECTIVE

Although glucagon-like peptide 1 (GLP-1) is known to regulate feeding, the central mechanisms contributing to this function remain enigmatic. Here, we aim to test the role of neurons expressing GLP-1 receptors (GLP-1R) in the dorsolateral septum (dLS; dLSGLP-1R) that project to the lateral hypothalamic area (LHA) on food intake and determine the relationship with feeding regulation.

METHODS

Using chemogenetic manipulations, we assessed how activation or inhibition of dLSGLP-1R neurons affected food intake in Glp1r-ires-Cre mice. Then, we used channelrhodopsin-assisted circuit mapping, chemogenetics, and electrophysiological recordings to identify and assess the role of the pathway from dLSGLP-1R →LHA projections in regulating food intake.

RESULTS

Chemogenetic inhibition of dLSGLP-1R neurons increases food intake. LHA is a major downstream target of dLSGLP-1R neurons. The dLSGLP-1R→LHA projections are GABAergic, and chemogenetic inhibition of this pathway also promotes food intake. While chemogenetic activation of dLSGLP-1R→LHA projections modestly decreases food intake, optogenetic stimulation of the dLSGLP-1R→LHA projection terminals in the LHA rapidly suppresses feeding behavior. Finally, we demonstrate that the GLP-1R agonist, Exendin 4 enhances dLSGLP-1R →LHA GABA release.

CONCLUSIONS

Together, these results demonstrate that dLS-GLP-1R neurons and the inhibitory pathway to LHA can regulate feeding behavior, which might serve as a potential therapeutic target for the treatment of eating disorders or obesity.

Melanin-concentrating hormone receptor 1 is discarded by exosomes after internalization

Melanin-concentrating hormone (MCH) receptor 1 (MCHR1), a G protein-coupled receptor, is poised for interaction with its ligands on the plasma membrane. Analyses of MCHR1 knockout mice suggest that this receptor could be a therapeutic target for the treatment of appetite disorders, glucose metabolism, psychiatric disorders, and inflammation. Binding of MCH to MCHR1 initiates calcium signaling, which is subsequently attenuated through receptor internalization. However, the ultimate destiny of the receptor post-internalization remains unexplored. In this study, we report the extracellular secretion of MCHR1 via exosomes. The recruitment of MCHR1 to exosomes occurs subsequent to its internalization, which is induced by stimulation with the ligand MCH. Although a highly glycosylated form of MCHR1, potentially representing a mature form, is selectively recruited to exosomes, the MCHR1 transferred into other cells does not exhibit functionality. The truncation of MCHR1 at the C-terminus not only impairs its response to MCH but also hinders its recruitment to exosomes. These findings imply that functional MCHR1 could be secreted extracellularly via exosomes, a process that may represent a mechanism for the termination of intracellular MCHR1 signaling.