Identification of a Brainstem Circuit Controlling Feeding

The role of dorsal raphe nucleus neuropeptides in reward and aversion

The dorsal raphe nucleus is a critical node for affective and motivated circuits in the brain. Though typically known as a serotonergic hub, the dorsal raphe nucleus is also highly enriched in a variety of neuropeptides. Recent advances in biotechnology and behavioral modeling have led to a resurgence in neuropeptide research, allowing investigators to target unique peptide systems with unprecedented clarity. Here, we review and discuss multiple neuropeptide systems in dorsal raphe and consider how their activity may contribute to reward and aversion. While this is not an exhaustive review, this short overview will highlight the many opportunities available to refine our understanding of multiple dorsal raphe neuropeptides. By more thoroughly studying dorsal raphe neuropeptides, we will reveal novel pathways to design more effective therapeutics and tailor treatments for millions of patients.

Brainstem neuropeptidergic neurons link a neurohumoral axis to satiation

Hunger is evolutionarily hardwired to ensure that an animal has sufficient energy to survive and reproduce. Just as important as knowing when to start eating is knowing when to stop eating. Here, using spatially resolved single-cell phenotyping, we characterize a population of neuropeptidergic neurons in the brainstem's dorsal raphe nucleus (DRN) and describe how they regulate satiation. These neurons track food from sensory presentation through ingestion, integrate these signals with slower-acting humoral cues, and express cholecystokinin (CCK). These CCK neurons bidirectionally regulate meal size, driving a sustained meal termination signal with a built-in delay. They are also well positioned to sense and respond to ingestion: they express a host of metabolic signaling factors and are integrated into an extended network known to regulate feeding. Together, this work demonstrates how DRN CCK neurons regulate satiation and identifies a likely conserved cellular mechanism that transforms diverse neurohumoral signals into a key behavioral output.

Association Between Stress-Induced Weight Loss and Autophagy-Related Gene Expression in the Hippocampus and Midbrain of Depression Model Mice

AIM

Recent studies have implicated autophagy in both weight regulation and depression. This study aimed to investigate the relationship between stress-induced weight loss and autophagy-related gene expression in a mouse model of depression.

METHOD

Male C57BL/6 mice were subjected to a chronic immobilization stress (CIS) protocol for 14 days to induce depressive-like behavior. Body weight was measured before and after the CIS, and depressive-like behavior was assessed using the tail suspension test (TST). The expression levels of autophagy-related genes (Atg5, Atg7, Atg12, Becn1, Mmp9, Fkbp5, and Map1lc3b) in the hippocampus and midbrain were evaluated using reverse transcription-quantitative PCR (RT-qPCR). Serum cortisol levels were also measured.

RESULTS

The CIS resulted in significant weight loss and increased immobility time in the TST, indicating depressive-like behavior. Serum cortisol levels were not different between CIS-depression model and control mice. In the hippocampus, the expression levels of Fkbp5, Mmp9, and Map1lc3b were significantly higher in CIS-depression model mice than in control mice. In the midbrain, the expression levels of Fkbp5 and Mmp9 were significantly higher in CIS-depression model mice than in control mice. Increased autophagy-related gene expressions in CIS-depression model mice were consistent with the previous studies in the postmortem brains of patients with depression. A significant negative correlation was also found between Fkbp5 mRNA expression in the hippocampus and the weight change ratio before and after the CIS.

CONCLUSION

The findings suggest that enhanced autophagy may be related to the pathology of depression and that Fkbp5, an autophagy regulator, mediates stress-induced weight loss.

Dorsal raphe GABA-ergic neurons regulate the susceptibility to social transfer of pain in mice

Some individuals are more susceptible to developing or suffering from pain states than others. However, the brain mechanisms underlying the susceptibility to pain responses are unknown. In this study, we defined pain susceptibility by recapitulating inter-individual differences in pain responses in mice exposed to a paradigm of socially transferred allodynia (STA), and with a combination of chemogenetic, molecular, pharmacological and electrophysiological approaches, we identified GABA-ergic neurons in the dorsal raphe nucleus (DRN) as a cellular target for the development and maintenance of STA susceptibility. We showed that DRN GABA-ergic neurons were selectively activated in STA-susceptible mice when compared with the unsusceptible (resilient) or control mice. Chemogenetic activation of DRN GABA-ergic neurons promoted STA susceptibility; whereas inhibiting these neurons prevented the development of STA susceptibility and reversed established STA. In in vitro slice electrophysiological analysis, we demonstrated that melanocortin 4 receptor (MC4R) enriched in DRN GABA-ergic neurons was a molecular target for regulating pain susceptibility, possibly by affecting DRN GABA-ergic neuronal activity. These results establish the DRN GABA-ergic neurons as an essential target for controlling pain susceptibility, thus providing important information for developing conceptually innovative and more accurate analgesic strategies.

Novel neural pathways targeted by GLP-1R agonists and bariatric surgery

The glucagon-like peptide 1 receptor (GLP-1R) agonist semaglutide has revolutionized the treatment of obesity, with other gut hormone-based drugs lined up that show even greater weight-lowering ability in obese patients. Nevertheless, bariatric surgery remains the mainstay treatment for severe obesity and achieves unparalleled weight loss that generally stands the test of time. While their underlying mechanisms of action remain incompletely understood, it is clear that the common denominator between GLP-1R agonists and bariatric surgery is that they suppress food intake by targeting the brain. In this Review, we highlight recent preclinical studies using contemporary neuroscientific techniques that provide novel concepts in the neural control of food intake and body weight with reference to endogenous GLP-1, GLP-1R agonists, and bariatric surgery. We start in the periphery with vagal, intestinofugal, and spinal sensory nerves and then progress through the brainstem up to the hypothalamus and finish at non-canonical brain feeding centers such as the zona incerta and lateral septum. Further defining the commonalities and differences between GLP-1R agonists and bariatric surgery in terms of how they target the brain may not only help bridge the gap between pharmacological and surgical interventions for weight loss but also provide a neural basis for their combined use when each individually fails.

Serotonin neurons integrate GABA and dopamine inputs to regulate meal initiation

Obesity is a growing global health epidemic with limited orally administered therapeutics. Serotonin (5-HT) is one neurotransmitter which remains an excellent target for new weight-loss therapies, but a gap remains in understanding the mechanisms involved in 5-HT produced in the dorsal Raphe nucleus (DRN) and its involvement in meal initiation. Using an optogenetic feeding paradigm, we showed that the 5-HTDRN➔arcuate nucleus (ARH) circuit plays a role in meal initiation. Incorporating electrophysiology and ChannelRhodopsin-2-Assisted Circuit Mapping, we demonstrated that 5-HTDRN neurons receive inhibitory input partially from GABAergic neurons in the DRN, and the 5-HT response can be enhanced by hunger. Additionally, deletion of the GABAA receptor subunit in 5-HT neurons inhibits meal initiation with no effect on the satiation process. Finally, we identified the role of dopaminergic inputs via dopamine receptor D2 in enhancing the response to GABA-induced feeding. Thus, our results indicate that 5-HTDRN neurons are inhibited by synergistic inhibitory actions of GABA and dopamine, for the initiation of a meal.

Selective activation of SIGMAR1 in anterior cingulate cortex glutamatergic neurons facilitates comorbid pain in depression in male mice

Depression and comorbid pain are frequently encountered clinically, and the comorbidity complicates the overall medical management. However, the mechanism whereby depression triggers development of pain needs to be further elucidated. Here, by using the chronic restraint stress (CRS) mouse model of depression and comorbid pain, we showed that CRS hyperactivated the glutamatergic neurons in the anterior cingulate cortex (ACC), as well as increasing the dendrite complexity and number. Chemogenetic activation of these neurons can induce depression and pain, while chemogenetic blockade can reverse such depression-induced pain. Moreover, we utilized translating ribosome affinity purification (TRAP) in combination with c-Fos-tTA strategy and pharmacological approaches and identified SIGMAR1 as a potential therapeutic molecular target. These results revealed a previously unknown neural mechanism for depression and pain comorbidity and provided new mechanistic insights into the antidepressive and analgesic effects of the disease.

The neurobiology of parenting and infant-evoked aggression

Parenting behavior comprises a variety of adult-infant and adult-adult interactions across multiple timescales. The state transition from nonparent to parent requires an extensive reorganization of individual priorities and physiology and is facilitated by combinatorial hormone action on specific cell types that are integrated throughout interconnected and brainwide neuronal circuits. In this review, we take a comprehensive approach to integrate historical and current literature on each of these topics across multiple species, with a focus on rodents. New and emerging molecular, circuit-based, and computational technologies have recently been used to address outstanding gaps in our current framework of knowledge on infant-directed behavior. This work is raising fundamental questions about the interplay between instinctive and learned components of parenting and the mutual regulation of affiliative versus agonistic infant-directed behaviors in health and disease. Whenever possible, we point to how these technologies have helped gain novel insights and opened new avenues of research into the neurobiology of parenting. We hope this review will serve as an introduction for those new to the field, a comprehensive resource for those already studying parenting, and a guidepost for designing future studies.

Acquired hypoprolactinemia in men, possible phenotype

The physiological role of prolactin (PRL) in men is still not well defined. The pathological increase is characterized by sexual function impairment along with possible negative consequences in body composition and metabolic profile. Conversely, the clinical significance of reduced PRL levels was only partially investigated or mainly neglected. The present paper aims to summarize and critically discuss possible phenotypes characterizing male subjects with reduced PRL levels. When possible, meta-analytic results were provided. Available data derived from patients seeking medical care for sexual dysfunction as well as from cross-sectional and longitudinal studies showed that low PRL in males is associated with a worse metabolic phenotype (including diabetes mellitus), mood disturbances (including anxiety and depression), and sexual dysfunctions (including psychogenic erectile and ejaculatory dysfunctions). Whether or not these features are direct consequences of reduced PRL levels or whether the latter reflect other pathway impairments such as serotoninergic failure cannot be clarified. The present data, however, emphasize that a deficiency of PRL should be taken into account and need further investigations.

Inhibition of RtTg neurons reverses methamphetamine-induced attention deficits

Chronic methamphetamine (METH) use, a prevalent psychostimulant, is known to impair attention, yet the cellular mechanisms driving these deficits remain poorly understood. Here, we employed a rat model of repeated passive METH injections and evaluated attentional performance using the 5-choice serial reaction time task (5-CSRTT). Using single-nucleus RNA sequencing, immunofluorescence and in situ hybridization, we characterized the response of neurons in the reticulotegmental nucleus (RtTg) to METH exposure. Our results indicate that METH exposure disrupts RtTg neurons at the transcriptional level and results in an increased activation ratio of RtTg under 5-CSRTT conditions. Crucially, chemogenetic inactivation of these neurons or RtTg lesion attenuated METH-induced attention deficits, whereas their activation reproduced the deficits. These findings underscore the critical role of RtTg neurons in mediating METH-induced attention deficits, positioning RtTg as a promising therapeutic target for the treatment of attention deficits linked to chronic METH use.

Feedforward inhibition of stress by brainstem neuropeptide Y neurons

Resistance to stress is a key determinant for mammalian functioning. While many studies have revealed neural circuits and substrates responsible for initiating and mediating stress responses, little is known about how the brain resists to stress and prevents overreactions. Here, we identified a previously uncharacterized neuropeptide Y (NPY) neuronal population in the dorsal raphe nucleus and ventrolateral periaqueductal gray region (DRN/vlPAG) with anxiolytic effects in male mice. NPYDRN/vlPAG neurons are rapidly activated by various stressful stimuli. Inhibiting these neurons exacerbated hypophagic and anxiety responses during stress, while activation significantly ameliorates acute stress-induced hypophagia and anxiety levels and transmits positive valence. Furthermore, NPYDRN/vlPAG neurons exert differential but synergic anxiolytic effects via inhibitory projections to the paraventricular thalamic nucleus (PVT) and the lateral hypothalamic area (LH). Together, our findings reveal a feedforward inhibition neural mechanism underlying stress resistance and suggest NPYDRN/vlPAG neurons as a potential therapeutic target for stress-related disorders.

In vivo magnetogenetics for cell-type-specific targeting and modulation of brain circuits

Neuromodulation technologies are crucial for investigating neuronal connectivity and brain function. Magnetic neuromodulation offers wireless and remote deep brain stimulations that are lacking in optogenetic- and wired-electrode-based tools. However, due to the limited understanding of working principles and poorly designed magnetic operating systems, earlier magnetic approaches have yet to be utilized. Furthermore, despite its importance in neuroscience research, cell-type-specific magnetic neuromodulation has remained elusive. Here we present a nanomaterials-based magnetogenetic toolbox, in conjunction with Cre-loxP technology, to selectively activate genetically encoded Piezo1 ion channels in targeted neuronal populations via torque generated by the nanomagnetic actuators in vitro and in vivo. We demonstrate this cell-type-targeting magnetic approach for remote and spatiotemporal precise control of deep brain neural activity in multiple behavioural models, such as bidirectional feeding control, long-term neuromodulation for weight control in obese mice and wireless modulation of social behaviours in multiple mice in the same physical space. Our study demonstrates the potential of cell-type-specific magnetogenetics as an effective and reliable research tool for life sciences, especially in wireless, long-term and freely behaving animals.

GABA system in the prefrontal cortex involved in psychostimulant addiction

Drug addiction is a chronic and relapse brain disorder. Psychostimulants such as cocaine and amphetamine are highly addictive drugs. Abuse drugs target various brain areas in the nervous system. Recent studies have shown that the prefrontal cortex (PFC) plays a key role in regulating addictive behaviors. The PFC is made up of excitatory glutamatergic cells and gamma-aminobutyric acid (GABAergic) interneurons. Recently, studies showed that GABA level was related with psychostimulant addiction. In this review, we will introduce the role and mechanism of GABA and γ-aminobutyric acid receptors (GABARs) of the PFC in regulating drug addiction, especially in psychostimulant addiction.

SH2B1 Defends Against Energy Imbalance, Obesity, and Metabolic Disease via a Paraventricular Hypothalamus→Dorsal Raphe Nucleus Neurocircuit

SH2B1 mutations are associated with obesity, type 2 diabetes, and metabolic dysfunction-associated steatotic liver disease (MASLD) in humans. Global deletion of Sh2b1 results in severe obesity, type 2 diabetes, and MASLD in mice. Neuron-specific restoration of SH2B1 rescues the obesity phenotype of Sh2b1-null mice, indicating that the brain is a main SH2B1 target. However, SH2B1 neurocircuits remain elusive. SH2B1-expressing neurons in the paraventricular hypothalamus (PVHSH2B1) and a PVHSH2B1→dorsal raphe nucleus (DRN) neurocircuit are identified here. PVHSH2B1 axons monosynaptically innervate DRN neurons. Optogenetic stimulation of PVHSH2B1 axonal fibers in the DRN suppresses food intake. Chronic inhibition of PVHSH2B1 neurons causes obesity. In male and female mice, either embryonic-onset or adult-onset deletion of Sh2b1 in PVH neurons causes energy imbalance, obesity, insulin resistance, glucose intolerance, and MASLD. Ablation of Sh2b1 in the DRN-projecting PVHSH2B1 subpopulation also causes energy imbalance, obesity, and metabolic disorders. Conversely, SH2B1 overexpression in either total or DRN-projecting PVHSH2B1 neurons protects against diet-induced obesity. SH2B1 binds to TrkB and enhances brain-derived neurotrophic factor (BDNF) signaling. Ablation of Sh2b1 in PVHSH2B1 neurons induces BDNF resistance in the PVH, contributing to obesity. In conclusion, these results unveil a previously unrecognized PVHSH2B1→DRN neurocircuit through which SH2B1 defends against obesity by enhancing BDNF/TrkB signaling.

A Light-Responsive Neural Circuit Suppresses Feeding

Light plays an essential role in a variety of physiological processes, including vision, mood, and glucose homeostasis. However, the intricate relationship between light and an animal's feeding behavior has remained elusive. Here, we found that light exposure suppresses food intake, whereas darkness amplifies it in male mice. Interestingly, this phenomenon extends its reach to diurnal male Nile grass rats and healthy humans. We further show that lateral habenula (LHb) neurons in mice respond to light exposure, which in turn activates 5-HT neurons in the dorsal Raphe nucleus (DRN). Activation of the LHb→5-HTDRN circuit in mice blunts darkness-induced hyperphagia, while inhibition of the circuit prevents light-induced anorexia. Together, we discovered a light-responsive neural circuit that relays the environmental light signals to regulate feeding behavior in mice.

Regulation of stress-induced sleep perturbations by dorsal raphe VGLUT3 neurons in male mice

Exposure to stressors has profound effects on sleep that have been linked to serotonin (5-HT) neurons of the dorsal raphe nucleus (DR). However, the DR also comprises glutamatergic neurons expressing vesicular glutamate transporter type 3 (DRVGLUT3), leading us to examine their role. Cell-type-specific tracing revealed that DRVGLUT3 neurons project to brain areas regulating arousal and stress. We found that chemogenetic activation of DRVGLUT3 neurons mimics stress-induced sleep perturbations. Furthermore, deleting VGLUT3 in the DR attenuated stress-induced sleep perturbations, especially after social defeat stress. In the DR, VGLUT3 is found in subsets of 5-HT and non-5-HT neurons. We observed that both populations are activated by acute stress, including those projecting to the ventral tegmental area. However, deleting VGLUT3 in 5-HT neurons minimally affected sleep regulation. These findings suggest that VGLUT3 expression in the DR drives stress-induced sleep perturbations, possibly involving non-5-HT DRVGLUT3 neurons.

Hypothalamic neural circuits regulating energy expenditure

The hypothalamus plays a central role in regulating energy expenditure and maintaining energy homeostasis, crucial for an organism's survival. Located in the ventral diencephalon, it is a dynamic and adaptable brain region capable of rapid responses to environmental changes, exhibiting high anatomical and cellular plasticity and integrates a myriad of sensory information, internal physiological cues, and humoral factors to accurately interpret the nutritional state and adjust food intake, thermogenesis, and energy homeostasis. Key hypothalamic nuclei contain distinct neuron populations that respond to hormonal, nutrient, and neural inputs and communicate extensively with peripheral organs like the gastrointestinal tract, liver, pancreas, and adipose tissues to regulate energy production, storage, mobilization, and utilization. The hypothalamus has evolved to enhance energy storage for survival in famine and scarce environments but contribute to obesity in modern contexts of caloric abundance. It acts as a master regulator of whole-body energy homeostasis, rapidly adapting to ensure energy supplies for cellular functions. Understanding hypothalamic function, pertaining to energy expenditure, is crucial for developing targeted interventions to address metabolic disorders, offering new insights into the neural control of metabolic states and potential therapeutic strategies.

Deep 3D histology powered by tissue clearing, omics and AI

To comprehensively understand tissue and organism physiology and pathophysiology, it is essential to create complete three-dimensional (3D) cellular maps. These maps require structural data, such as the 3D configuration and positioning of tissues and cells, and molecular data on the constitution of each cell, spanning from the DNA sequence to protein expression. While single-cell transcriptomics is illuminating the cellular and molecular diversity across species and tissues, the 3D spatial context of these molecular data is often overlooked. Here, I discuss emerging 3D tissue histology techniques that add the missing third spatial dimension to biomedical research. Through innovations in tissue-clearing chemistry, labeling and volumetric imaging that enhance 3D reconstructions and their synergy with molecular techniques, these technologies will provide detailed blueprints of entire organs or organisms at the cellular level. Machine learning, especially deep learning, will be essential for extracting meaningful insights from the vast data. Further development of integrated structural, molecular and computational methods will unlock the full potential of next-generation 3D histology.

Virtual reality-empowered deep-learning analysis of brain cells

Automated detection of specific cells in three-dimensional datasets such as whole-brain light-sheet image stacks is challenging. Here, we present DELiVR, a virtual reality-trained deep-learning pipeline for detecting c-Fos+ cells as markers for neuronal activity in cleared mouse brains. Virtual reality annotation substantially accelerated training data generation, enabling DELiVR to outperform state-of-the-art cell-segmenting approaches. Our pipeline is available in a user-friendly Docker container that runs with a standalone Fiji plugin. DELiVR features a comprehensive toolkit for data visualization and can be customized to other cell types of interest, as we did here for microglia somata, using Fiji for dataset-specific training. We applied DELiVR to investigate cancer-related brain activity, unveiling an activation pattern that distinguishes weight-stable cancer from cancers associated with weight loss. Overall, DELiVR is a robust deep-learning tool that does not require advanced coding skills to analyze whole-brain imaging data in health and disease.

Molecular recording of calcium signals via calcium-dependent proximity labeling

Calcium ions serve as key intracellular signals. Local, transient increases in calcium concentrations can activate calcium sensor proteins that in turn trigger downstream effectors. In neurons, calcium transients play a central role in regulating neurotransmitter release and synaptic plasticity. However, it is challenging to capture the molecular events associated with these localized and ephemeral calcium signals. Here we present an engineered biotin ligase that generates permanent molecular traces in a calcium-dependent manner. The enzyme, calcium-dependent BioID (Cal-ID), biotinylates nearby proteins within minutes in response to elevated local calcium levels. The biotinylated proteins can be identified via mass spectrometry and visualized using microscopy. In neurons, Cal-ID labeling is triggered by neuronal activity, leading to prominent protein biotinylation that enables transcription-independent activity labeling in the brain. In summary, Cal-ID produces a biochemical record of calcium signals and neuronal activity with high spatial resolution and molecular specificity.

Neural circuits expressing the serotonin 2C receptor regulate memory in mice and humans

Declined memory is a hallmark of Alzheimer's disease (AD). Experiments in rodents and human postmortem studies suggest that serotonin (5-hydroxytryptamine, 5-HT) plays a role in memory, but the underlying mechanisms are unknown. Here, we investigate the role of 5-HT 2C receptor (5-HT2CR) in regulating memory. Transgenic mice expressing a humanized HTR2C mutation exhibit impaired plasticity of hippocampal ventral CA1 (vCA1) neurons and reduced memory. Further, 5-HT neurons project to and synapse onto vCA1 neurons. Disruption of 5-HT synthesis in vCA1-projecting neurons or deletion of 5-HT2CRs in the vCA1 impairs neural plasticity and memory. We show that a selective 5-HT2CR agonist, lorcaserin, improves synaptic plasticity and memory in an AD mouse model. Cumulatively, we demonstrate that hippocampal 5-HT2CR signaling regulates memory, which may inform the use of 5-HT2CR agonists in the treatment of dementia.

5-HT Neurons Integrate GABA and Dopamine Inputs to Regulate Meal Initiation

Obesity is a growing global health epidemic with limited effective therapeutics. Serotonin (5-HT) is one major neurotransmitter which remains an excellent target for new weight-loss therapies, but there remains a gap in knowledge on the mechanisms involved in 5-HT produced in the dorsal Raphe nucleus (DRN) and its involvement in meal initiation. Using a closed-loop optogenetic feeding paradigm, we showed that the 5-HTDRN→arcuate nucleus (ARH) circuit plays an important role in regulating meal initiation. Incorporating electrophysiology and ChannelRhodopsin-2-Assisted Circuit Mapping, we demonstrated that 5-HTDRN neurons receive inhibitory input partially from GABAergic neurons in the DRN, and the 5-HT response to GABAergic inputs can be enhanced by hunger. Additionally, deletion of the GABAA receptor subunit in 5-HT neurons inhibits meal initiation with no effect on the satiation process. Finally, we identified the instrumental role of dopaminergic inputs via dopamine receptor D2 in 5-HTDRN neurons in enhancing the response to GABA-induced feeding. Thus, our results indicate that 5-HTDRN neurons are inhibited by synergistic inhibitory actions of GABA and dopamine, which allows for the initiation of a meal.

Nausea-induced suppression of feeding is mediated by central amygdala Dlk1-expressing neurons

The motivation to eat is suppressed by satiety and aversive stimuli such as nausea. The neural circuit mechanisms of appetite suppression by nausea are not well understood. Pkcδ neurons in the lateral subdivision of the central amygdala (CeA) suppress feeding in response to satiety signals and nausea. Here, we characterized neurons enriched in the medial subdivision (CeM) of the CeA marked by expression of Dlk1. CeADlk1 neurons are activated by nausea, but not satiety, and specifically suppress feeding induced by nausea. Artificial activation of CeADlk1 neurons suppresses drinking and social interactions, suggesting a broader function in attenuating motivational behavior. CeADlk1 neurons form projections to many brain regions and exert their anorexigenic activity by inhibition of neurons of the parabrachial nucleus. CeADlk1 neurons are inhibited by appetitive CeA neurons, but also receive long-range monosynaptic inputs from multiple brain regions. Our results illustrate a CeA circuit that regulates nausea-induced feeding suppression.

Tonic noradrenergic input to neurons in the dorsal raphe nucleus mediates food intake in male mice

The dorsal raphe nucleus (DRN) is essential for the control of food intake. Efferent projections from the DRN extend to several forebrain regions that are involved in the control of food intake. However, the neurotransmitters released in the DRN related to the control of food intake are not known. We have previously demonstrated that a tonic α1 action on DRN neurons contributes to satiety in the fed rats. In this study we investigated the participation of norepinephrine (NE) signaling in the DRN in the satiety response. Intra-DRN administration of NE causes an increase in the 2-hour food intake of sated mice, an effect that was blocked by previous administration of yohimbine, an α2 antagonist. Similarly, Intra-DRN administration of clonidine, an α2 agonist, increases food intake in sated mice. This result indicates that in the satiated mice exogenous NE acts on α2 receptors to increase food intake. Furthermore, administration of phenylephrine, an α1 agonist, decreases food intake in fasted mice and prazosin, an α1 antagonist, increases food intake in the sated mice. Taken together these results indicate that, in a satiated condition, a tonic α1 adrenergic action on the DRN neurons inhibits food intake and that exogenous NE administered to the DRN acts on α2 adrenergic receptors to increase food intake. These data reinforce the intricate neuronal functioning of the DRN and its effects on feeding.

A midbrain GABAergic circuit constrains wakefulness in a mouse model of stress

Enhancement of wakefulness is a prerequisite for adaptive behaviors to cope with acute stress, but hyperarousal is associated with impaired behavioral performance. Although the neural circuitries promoting wakefulness in acute stress conditions have been extensively identified, less is known about the circuit mechanisms constraining wakefulness to prevent hyperarousal. Here, we found that chemogenetic or optogenetic activation of GAD2-positive GABAergic neurons in the midbrain dorsal raphe nucleus (DRNGAD2) decreased wakefulness, while inhibition or ablation of these neurons produced an increase in wakefulness along with hyperactivity. Surprisingly, DRNGAD2 neurons were paradoxically wakefulness-active and were further activated by acute stress. Bidirectional manipulations revealed that DRNGAD2 neurons constrained the increase of wakefulness and arousal level in a mouse model of stress. Circuit-specific investigations demonstrated that DRNGAD2 neurons constrained wakefulness via inhibition of the wakefulness-promoting paraventricular thalamus. Therefore, the present study identified a wakefulness-constraining role DRNGAD2 neurons in acute stress conditions.

Neuronal and Molecular Mechanisms Underlying Chronic Pain and Depression Comorbidity in the Paraventricular Thalamus

Patients with chronic pain often develop comorbid depressive symptoms, which makes the pain symptoms more complicated and refractory. However, the underlying mechanisms are poorly known. Here, in a repeated complete Freund's adjuvant (CFA) male mouse model, we reported a specific regulatory role of the paraventricular thalamic nucleus (PVT) glutamatergic neurons, particularly the anterior PVT (PVA) neurons, in mediating chronic pain and depression comorbidity (CDC). Our c-Fos protein staining observed increased PVA neuronal activity in CFA-CDC mice. In wild-type mice, chemogenetic activation of PVA glutamatergic neurons was sufficient to decrease the 50% paw withdrawal thresholds (50% PWTs), while depressive-like behaviors evaluated with immobile time in tail suspension test (TST) and forced swim test (FST) could only be achieved by repeated chemogenetic activation. Chemogenetic inhibition of PVA glutamatergic neurons reversed the decreased 50% PWTs in CFA mice without depressive-like symptoms and the increased TST and FST immobility in CFA-CDC mice. Surprisingly, in CFA-CDC mice, chemogenetically inhibiting PVA glutamatergic neurons failed to reverse the decrease of 50% PWTs, which could be restored by rapid-onset antidepressant S-ketamine. Further behavioral tests in chronic restraint stress mice and CFA pain mice indicated that PVA glutamatergic neuron inhibition and S-ketamine independently alleviate sensory and affective pain. Molecular profiling and pharmacological studies revealed the 5-hydroxytryptamine receptor 1D (Htr1d) in CFA pain-related PVT engram neurons as a potential target for treating CDC. These findings identified novel CDC neuronal and molecular mechanisms in the PVT and provided insight into the complicated pain neuropathology under a comorbid state with depression and related drug development.

Control of feeding by a bottom-up midbrain-subthalamic pathway

Investigative exploration and foraging leading to food consumption have vital importance, but are not well-understood. Since GABAergic inputs to the lateral and ventrolateral periaqueductal gray (l/vlPAG) control such behaviors, we dissected the role of vgat-expressing GABAergic l/vlPAG cells in exploration, foraging and hunting. Here, we show that in mice vgat l/vlPAG cells encode approach to food and consumption of both live prey and non-prey foods. The activity of these cells is necessary and sufficient for inducing food-seeking leading to subsequent consumption. Activation of vgat l/vlPAG cells produces exploratory foraging and compulsive eating without altering defensive behaviors. Moreover, l/vlPAG vgat cells are bidirectionally interconnected to several feeding, exploration and investigation nodes, including the zona incerta. Remarkably, the vgat l/vlPAG projection to the zona incerta bidirectionally controls approach towards food leading to consumption. These data indicate the PAG is not only a final downstream target of top-down exploration and foraging-related inputs, but that it also influences these behaviors through a bottom-up pathway.

Ketamine ameliorates activity-based anorexia of adolescent female mice through changes in GluN2B-containing NMDA receptors at postsynaptic cytoplasmic locations of pyramidal neurons and interneurons of medial prefrontal cortex

Anorexia nervosa (AN) is a mental illness with high rates of mortality and relapse, and no approved pharmacotherapy. Using the activity-based anorexia (ABA) model of AN, we previously showed that a single sub-anesthetic intraperitoneal injection of ketamine (30 mg/kg-KET, but not 3 mg/kg-KET), has an immediate and long-lasting effect of reducing anorexia-like behavior among adolescent female mice. We also showed previously that excitatory outflow from medial prefrontal cortex (mPFC) engages hunger-evoked hyperactivity, leading to the ABA condition of severe weight loss. Ketamine is known to target GluN2B-containing NMDARs (NR2B). Might synaptic plasticity involving NR2B in mPFC contribute to ketamine's ameliorative effects? We addressed this question through electron microscopic immunocytochemical quantification of GluN2B at excitatory synapses of pyramidal neurons (PN) and GABAergic interneurons (IN) in mPFC layer 1 of animals that underwent recovery from a second ABA induction (ABA2), 22 days after ketamine injection during the first ABA induction. The 30 mg/kg-KET evoked synaptic plasticity that differed for PN and IN, with changes revolving the cytoplasmic reserve pool of NR2B more than the postsynaptic membrane pool. Those individuals that suppressed hunger-evoked wheel running the most and increased food consumption during recovery from ABA2 the most showed the greatest increase of NR2B at PN and IN excitatory synapses. We hypothesize that 30 mg/kg-KET promotes long-lasting changes in the reserve cytoplasmic pool of NR2B that enables activity-dependent rapid strengthening of mPFC circuits underlying the more adaptive behavior of suppressed running and enhanced food consumption, in turn supporting better weight restoration.

TAAR1 agonists improve glycemic control, reduce body weight and modulate neurocircuits governing energy balance and feeding

OBJECTIVE

Metabolic Syndrome, which can be induced or exacerbated by current antipsychotic drugs (APDs), is highly prevalent in schizophrenia patients. Recent preclinical and clinical evidence suggest that agonists at trace amine-associated receptor 1 (TAAR1) have potential as a new treatment option for schizophrenia. Intriguingly, preclinical tudies have also identified TAAR1 as a novel regulator of metabolic control. Here we evaluated the effects of three TAAR1 agonists, including the clinical development candidate ulotaront, on body weight, metabolic parameters and modulation of neurocircuits implicated in homeostatic and hedonic feeding.

METHODS

Effects of TAAR1 agonists (ulotaront, RO5166017 and/or RO5263397) on body weight, food intake and/or metabolic parameters were investigated in rats fed a high-fat diet (HFD) and in a mouse model of diet-induced obesity (DIO). Body weight effects were also determined in a rat and mouse model of olanzapine-, and corticosterone-induced body weight gain, respectively. Glucose tolerance was assessed in lean and diabetic db/db mice and fasting plasma glucose and insulin examined in DIO mice. Effects on gastric emptying were evaluated in lean mice and rats. Drug-induced neurocircuit modulation was evaluated in mice using whole-brain imaging of c-fos protein expression.

RESULTS

TAAR1 agonists improved oral glucose tolerance by inhibiting gastric emptying. Sub-chronic administration of ulotaront in rats fed a HFD produced a dose-dependent reduction in body weight, food intake and liver triglycerides compared to vehicle controls. In addition, a more rapid reversal of olanzapine-induced weight gain and food intake was observed in HFD rats switched to ulotaront or RO5263397 treatment compared to those switched to vehicle. Chronic ulotaront administration also reduced body weight and improved glycemic control in DIO mice, and normalized corticosterone-induced body weight gain in mice. TAAR1 activation increased neuronal activity in discrete homeostatic and hedonic feeding centers located in the dorsal vagal complex and hypothalamus with concurrent activation of several limbic structures.

CONCLUSION

The current data demonstrate that TAAR1 agonists, as a class, not only lack APD-induced metabolic liabilities but can reduce body weight and improve glycemic control in rodent models. The underlying mechanisms likely include TAAR1-mediated peripheral effects on glucose homeostasis and gastric emptying as well as central regulation of energy balance and food intake.

Seeking satiety: From signals to solutions

Remedies for the treatment of obesity date to Hippocrates, when patients with obesity were directed to "reduce food and avoid drinking to fullness" and begin "running during the night." Similar recommendations have been repeated ever since, despite the fact that they are largely ineffective. Recently, highly effective therapeutics were developed that may soon enable physicians to manage body weight in patients with obesity in a manner similar to the way that blood pressure is controlled in patients with hypertension. These medicines have grown out of a revolution in our understanding of the molecular and neural control of appetite and body weight, reviewed here.

Neural adaption in midbrain GABAergic cells contributes to high-fat diet-induced obesity

Overeating disorders largely contribute to worldwide incidences of obesity. Available treatments are limited. Here, we discovered that long-term chemogenetic activation of ventrolateral periaqueductal gray (vlPAG) GABAergic cells rescue obesity of high-fat diet-induced obesity (DIO) mice. This was associated with the recovery of enhanced mIPSCs, decreased food intake, increased energy expenditure, and inguinal white adipose tissue (iWAT) browning. In vivo calcium imaging confirmed vlPAG GABAergic suppression for DIO mice, with corresponding reduction in intrinsic excitability. Single-nucleus RNA sequencing identified transcriptional expression changes in GABAergic cell subtypes in DIO mice, highlighting Cacna2d1 as of potential importance. Overexpressing CACNA2D1 in vlPAG GABAergic cells of DIO mice rescued enhanced mIPSCs and calcium response, reversed obesity, and therefore presented here as a potential target for obesity treatment.

Optogenetic control of neural activity: The biophysics of microbial rhodopsins in neuroscience

Optogenetics, the use of microbial rhodopsins to make the electrical activity of targeted neurons controllable by light, has swept through neuroscience, enabling thousands of scientists to study how specific neuron types contribute to behaviors and pathologies, and how they might serve as novel therapeutic targets. By activating a set of neurons, one can probe what functions they can initiate or sustain, and by silencing a set of neurons, one can probe the functions they are necessary for. We here review the biophysics of these molecules, asking why they became so useful in neuroscience for the study of brain circuitry. We review the history of the field, including early thinking, early experiments, applications of optogenetics, pre-optogenetics targeted neural control tools, and the history of discovering and characterizing microbial rhodopsins. We then review the biophysical attributes of rhodopsins that make them so useful to neuroscience - their classes and structure, their photocycles, their photocurrent magnitudes and kinetics, their action spectra, and their ion selectivity. Our hope is to convey to the reader how specific biophysical properties of these molecules made them especially useful to neuroscientists for a difficult problem - the control of high-speed electrical activity, with great precision and ease, in the brain.

Serotonergic regulation of appetite and sodium appetite

Serotonin is a neurotransmitter that is synthesized and released from the brainstem raphe nuclei to affect many brain functions. It is well known that the activity of raphe serotonergic neurons is changed in response to the changes in feeding status to regulate appetite via the serotonin receptors. Likewise, changes in volume status are known to alter the activity of raphe serotonergic neurons and drugs targeting serotonin receptors were shown to affect sodium appetite. Therefore, the central serotonin system appears to regulate ingestion of both food and salt, although neural mechanisms that induce appetite in response to hunger and sodium appetite in response to volume depletion are largely distinct from each other. In this review, we discuss our current knowledge regarding the regulation of ingestion - appetite and sodium appetite - by the central serotonin system.

Unraveling the serotonin saga: from discovery to weight regulation and beyond - a comprehensive scientific review

The prevalence of obesity is rapidly increasing worldwide, while the development of effective obesity therapies lags behind. Although new therapeutic targets to alleviate obesity are identified every day, and drug efficacy is improving, adverse side effects and increased health risks remain serious issues facing the weight-loss industry. Serotonin, also known as 5-HT, has been extensively studied in relation to appetite reduction and weight loss. As a result, dozens of upstream and downstream neural targets of 5-HT have been identified, revealing a multitude of neural circuits involved in mediating the anorexigenic effect of 5-HT. Despite the rise and fall of several 5-HT therapeutics in recent decades, the future of 5-HT as a therapeutic target for weight-loss therapy looks promising. This review focuses on the history of serotonin, the state of current central serotonin research, previous serotonergic therapies, and the future of serotonin for treating individuals with obesity.

Parallel streams of raphe VGLUT3-positive inputs target the dorsal and ventral hippocampus in each hemisphere

The hippocampus (HP) receives neurochemically diverse inputs from the raphe nuclei, including glutamatergic axons characterized by the expression of the vesicular glutamate transporter type 3 (VGLUT3). These raphe-HP VGLUT3 projections have been suggested to play a critical role in HP functions, yet a complete anatomical overview of raphe VGLUT3 projections to the forebrain, and in particular to the HP, is lacking. Using anterograde viral tracing, we describe largely nonoverlapping VGLUT3-positive projections from the dorsal raphe (DR) and median raphe (MnR) to the forebrain, with the HP receiving inputs from the MnR. A limited subset of forebrain regions such as the amygdaloid complex, claustrum, and hypothalamus receives projections from both the DR and MnR that remain largely segregated. This highly complementary anatomical pattern suggests contrasting roles for DR and MnR VGLUT3 neurons. To further analyze the topography of VGLUT3 raphe projections to the HP, we used retrograde tracing and found that HP-projecting VGLUT3-positive neurons (VGLUT3^HP ) distribute over several raphe subregions (including the MnR, paramedian raphe, and B9 cell group) and lack co-expression of serotonergic markers. Strikingly, double retrograde tracing experiments unraveled two parallel streams of VGLUT3-positive projections targeting the dorsal and ventral poles of the HP. These results demonstrate highly organized and segregated VGLUT3-positive projections to the HP, suggesting independent modulation of HP functions such as spatial memory and emotion-related behavior.

Neural and Molecular Investigation into the Paraventricular Thalamic-Nucleus Accumbens Circuit for Pain Sensation and Non-opioid Analgesia

The paucity of medications with novel mechanisms for pain treatment combined with the severe adverse effects of opioid analgesics has led to an imperative pursuit of non-opioid analgesia and a better understanding of pain mechanisms. Here, we identify the putative glutamatergic inputs from the paraventricular thalamic nucleus to the nucleus accumbens (PVT^Glut→NAc) as a novel neural circuit for pain sensation and non-opioid analgesia. Our in vivo fiber photometry and in vitro electrophysiology experiments found that PVT^Glut→NAc neuronal activity increased in response to acute thermal/mechanical stimuli and persistent inflammatory pain. Direct optogenetic activation of these neurons in the PVT or their terminals in the NAc induced pain-like behaviors. Conversely, inhibition of PVT^Glut→NAc neurons or their NAc terminals exhibited a potent analgesic effect in both naïve and pathological pain mice, which could not be prevented by pretreatment of naloxone, an opioid receptor antagonist. Anterograde trans-synaptic optogenetic experiments consistently demonstrated that the PVT^Glut→NAc circuit bi-directionally modulates pain behaviors. Furthermore, circuit-specific molecular profiling and pharmacological studies revealed dopamine receptor 3 as a candidate target for pain modulation and non-opioid analgesic development. Taken together, these findings provide a previously unknown neural circuit for pain sensation and non-opioid analgesia and a valuable molecular target for developing future safer medication.

An iterative neural processing sequence orchestrates feeding

Feeding requires sophisticated orchestration of neural processes to satiate appetite in natural, capricious settings. However, the complementary roles of discrete neural populations in orchestrating distinct behaviors and motivations throughout the feeding process are largely unknown. Here, we delineate the behavioral repertoire of mice by developing a machine-learning-assisted behavior tracking system and show that feeding is fragmented and divergent motivations for food consumption or environment exploration compete throughout the feeding process. An iterative activation sequence of agouti-related peptide (AgRP)-expressing neurons in arcuate (ARC) nucleus, GABAergic neurons in the lateral hypothalamus (LH), and in dorsal raphe (DR) orchestrate the preparation, initiation, and maintenance of feeding segments, respectively, via the resolution of motivational conflicts. The iterative neural processing sequence underlying the competition of divergent motivations further suggests a general rule for optimizing goal-directed behaviors.

Autonomic instability, arrhythmia and visual impairment in a new presentation of MTFMT-related mitochondrial disease

Mitochondrial methionyl-tRNA formyltransferase (MTFMT) is required for the initiation of translation in mitochondria. Pathogenic variants in MTFMT have been described in association with clinical presentations with Leigh syndrome, as well with as multisystem involvement (particularly cardiac and ocular involvement). There is a spectrum of severity, but many reported presentations have been milder with a better prognosis than other pathogenic variants associated with Leigh syndrome. We describe the case of a 9-year-old boy homozygous for a pathogenic MTFMT variant (c.626C > T/p.Ser209Leu) who presented with hypertensive crisis on a background of hyperphagia and visual impairment. His clinical course was complicated by supraventricular tachycardia and severe autonomic instability, requiring intensive care unit admission. He also developed seizures, neurogenic bladder and bowel and had a markedly abnormal eye examination with bilateral optic atrophy. Magnetic resonance image brain showed abnormal high T2/fluid-attenuated inversion recovery signal within the dorsal brainstem and in the right globus pallidus with some reduced diffusivity. Despite recovery from the acute neurological and cardiac manifestations, he has ongoing deficits in his gross motor skills and continues to have hyperphagia with rapid weight gain (approx. 20 kg in 2 years). Ophthalmic findings are persistent. This case expands the phenotype associated with MTFMT disease.

Ketamine ameliorates activity-based anorexia of adolescent female mice through changes in the prevalence of NR2B-containing NMDA receptors at excitatory synapses that are in opposite directions for of pyramidal neurons versus GABA interneurons In medial prefrontal cortex

A previous study showed that a single sub-anesthetic dose of ketamine (30 mg/kg-KET, IP) has an immediate and long-lasting (>20 days) effect of reducing maladaptive behaviors associated with activity-based anorexia (ABA) among adolescent female mice. This study sought to determine whether synaptic plasticity involving NR2B-containing NMDA receptors (NR2B) at excitatory synapses in the prelimbic region of medial prefrontal cortex (mPFC) contributes to this ameliorative effect. To this end, quantitative electron microscopic analyses of NR2B-subunit immunoreactivity at excitatory synapses of pyramidal neurons (PN) and GABAergic interneurons (GABA-IN) were conducted upon layer 1 of mPFC of the above-described mice that received a single efficacious 30 mg/kg-KET (N=8) versus an inefficacious 3 mg/kg-KET (N=8) dose during the food-restricted day of the first ABA induction (ABA1). Brain tissue was collected after these animals underwent recovery from ABA1, then of recovery from a second ABA induction (ABA2), 22 days after the ketamine injection. For all three parameters used to quantify ABA resilience (increased food consumption, reduced wheel running, body weight gain), 30 mg/kg-KET evoked synaptic plasticity in opposite directions for PN and GABA-IN, with changes at excitatory synapses on GABA-IN dominating the adaptive behaviors more than on PN. The synaptic changes were in directions consistent with changes in the excitatory outflow from mPFC that weaken food consumption-suppression, strengthen wheel running suppression and enhance food consumption. We hypothesize that 30 mg/kg-KET promotes these long-lasting changes in the excitatory outflow from mPFC after acutely blocking the hunger and wheel-access activated synaptic circuits underlying maladaptive behaviors during ABA.

Dorsal raphe serotonergic neurons suppress feeding through redundant forebrain circuits

OBJECTIVE

Serotonin (5HT) is a well-known anorexigenic molecule, and 5HT neurons of dorsal raphe nucleus (DRN) have been implicated in suppression of feeding; however, the downstream circuitry is poorly understood. Here we explored major projections of DRN^5HT neurons for their capacity to modulate feeding.

METHODS

We used optogenetics to selectively activate DRN^5HT axonal projections in hypothalamic and extrahypothalamic areas and monitored food intake. We next used fiber photometry to image the activity dynamics of DRN^5HT axons and 5HT levels in projection areas in response feeding and metabolic hormones. Finally, we used electrophysiology to determine how DRN^5HT axons affect downstream neuron activity.

RESULTS

We found that selective activation of DRN^5HT axons in (DRN^5HT → LH) and (DRN^5HT → BNST) suppresses feeding whereas activating medial hypothalamic projections has no effect. Using in vivo imaging, we found that food access and satiety hormones activate DRN^5HT projections to LH where they also rapidly increase extracellular 5HT levels. Optogenetic mapping revealed that DRN^5HT → LH^vGAT and DRN^5HT → LH^vGlut2 connections are primarily inhibitory and excitatory respectively. Further, in addition to its direct action on LH neurons, we found that 5HT suppresses GABA release from presynaptic terminals arriving from AgRP neurons.

CONCLUSIONS

These findings define functionally redundant forebrain circuits through which DRN^5HT neurons suppress feeding and reveal that these projections can be modulated by metabolic hormones.

Regulation of wakefulness by GABAergic dorsal raphe nucleus-ventral tegmental area pathway

The dorsal raphe nucleus (DRN) has previously been proved to be involved in the regulation of the sleep-wake behavior. DRN contains several neuron types, such as 5-HTergic and GABAergic neurons. GABAergic neurons, which are the second largest cell subtype in the DRN, participate in a variety of neurophysiological functions. However, their role in sleep-wake regulation and the underlying neural circuitry remains unclear. Herein, we used fiber photometry and synchronous electroencephalogram (EEG)/electromyography (EMG) recording to demonstrate that DRN GABAergic neurons exhibit high activities during wakefulness and low activities during NREM sleep. Short-term optogenetic activation of DRN GABAergic neurons reduced the latency of NREM-to-wake transition and increased the probability of wakefulness, while long-term optogenetic activation of these neurons significantly increased the amount of wakefulness. Chemogenetic activation of DRN GABAergic neurons increased wakefulness for almost 2 h and maintained long-lasting arousal. In addition, inhibition of DRN GABAergic neurons with chemogenetics caused a reduction in the amount of wakefulness. Finally, similar to the effects of activating the soma of DRN GABAergic neurons, optogenetic stimulation of their terminals in the ventral tegmental area (VTA) induced instant arousal and promoted wakefulness. Taken together, our results illustrated that DRN GABAergic neurons are vital to the induction and maintenance of wakefulness, which promote wakefulness through the GABAergic DRN-VTA pathway.

Variability in Sleep Timing and Dietary Intake: A Scoping Review of the Literature

The objective of this scoping review was to summarize previous studies which examined the effect of day-to-day variability in sleep timing and social jetlag (SJL) on dietary intake. A systematic literature search was conducted in PubMed, Embase, and Clarivate Analytics Web of Science and we identified 22 records. No difference in caloric and macronutrient intake between SJL groups was observed in studies that enrolled healthy young adults. However, studies that enrolled participants with obesity and obesity-related chronic conditions reported a higher caloric intake and a higher intake of carbohydrates, total fat, saturated fats, and cholesterol in participants with SJL than in those without. Most studies reported a lower quality of diet, a delayed mealtime, and eating jetlag in participants with SJL vs. those without SJL. No correlation of day-to-day variability in sleep timing with average caloric intake was observed, but bed-time variability was negatively associated with diet quality. Methodological issues have been identified in sources assessed including study design, power calculation, population enrolled, and tools/metrics used for sleep timing variability assessment. Future well powered longitudinal studies, with clear protocols, standardized metrics, including all age groups from general population are needed to clarify the dietary intake consequences of variability in sleep timing.

Pharmacological targeting of glutamatergic neurons within the brainstem for weight reduction

Food intake and body weight are tightly regulated by neurons within specific brain regions, including the brainstem, where acute activation of dorsal raphe nucleus (DRN) glutamatergic neurons expressing the glutamate transporter Vglut3 (DRN^Vglut3) drive a robust suppression of food intake and enhance locomotion. Activating Vglut3 neurons in DRN suppresses food intake and increases locomotion, suggesting that modulating the activity of these neurons might alter body weight. Here, we show that DRN^Vglut3 neurons project to the lateral hypothalamus (LHA), a canonical feeding center that also reduces food intake. Moreover, chronic DRN^Vglut3 activation reduces weight in both leptin-deficient (ob/ob) and leptin-resistant diet-induced obese (DIO) male mice. Molecular profiling revealed that the orexin 1 receptor (Hcrtr1) is highly enriched in DRN Vglut3 neurons, with limited expression elsewhere in the brain. Finally, an orally bioavailable, highly selective Hcrtr1 antagonist (CVN45502) significantly reduces feeding and body weight in DIO. Hcrtr1 is also co-expressed with Vglut3 in the human DRN, suggesting that there might be a similar effect in human. These results identify a potential therapy for obesity by targeting DRN^Vglut3 neurons while also establishing a general strategy for developing drugs for central nervous system disorders.

Pathway-specific GABAergic inhibition contributes to the gain of resilience against anorexia-like behavior of adolescent female mice

Anorexia nervosa is one of the most debilitating mental illnesses that emerges during adolescence, especially among females. Anorexia nervosa is characterized by severe voluntary food restriction and compulsive exercising, which combine to cause extreme body weight loss. We use activity-based anorexia (ABA), an animal model, to investigate the neurobiological bases of vulnerability to anorexia nervosa. This is a Mini-Review, focused on new ideas that have emerged based on recent findings from the Aoki Lab. Our findings point to the cellular and molecular underpinnings of three ABA phenomena: (1) age-dependence of ABA vulnerability; (2) individual differences in the persistence of ABA vulnerability during adolescence; (3) GABAergic synaptic plasticity in the hippocampus and the prefrontal cortex that contributes to the suppression of the maladaptive anorexia-like behaviors. We also include new data on the contribution to ABA vulnerability by cell type-specific knockdown of a GABA receptor subunit, α4, in dorsal hippocampus. Although the GABA system recurs as a key player in the gain of ABA resilience, the data predict why targeting the GABA system, singularly, may have only limited efficacy in treating anorexia nervosa. This is because boosting the GABAergic system may suppress the maladaptive behavior of over-exercising but could also suppress food consumption. We hypothesize that a sub-anesthetic dose of ketamine may be the magic bullet, since a single injection of this drug to mid-adolescent female mice undergoing ABA induction enhances food consumption and reduces wheel running, thereby reducing body weight loss through plasticity at excitatory synaptic inputs to both excitatory and inhibitory neurons. The same treatment is not as efficacious during late adolescence but multiple dosing of ketamine can suppress ABA vulnerability partially. This caveat underscores the importance of conducting behavioral, synaptic and molecular analyses across multiple time points spanning the developmental stage of adolescence and into adulthood. Since this is a Mini-Review, we recommend additional literature for readers seeking more comprehensive reviews on these subjects.

VGLUT3 neurons in median raphe control the efficacy of spatial memory retrieval via ETV4 regulation of VGLUT3 transcription

The raphe nucleus is critical for feeding, rewarding and memory. However, how the heterogenous raphe neurons are molecularly and structurally organized to engage their divergent functions remains unknown. Here, we genetically target a subset of neurons expressing VGLUT3. VGLUT3 neurons control the efficacy of spatial memory retrieval by synapsing directly with parvalbumin-expressing GABA interneurons (PGIs) in the dentate gyrus. In a mouse model of Alzheimer's disease (AD mice), VGLUT3→PGIs synaptic transmission is impaired by ETV4 inhibition of VGLUT3 transcription. ETV4 binds to a promoter region of VGLUT3 and activates VGLUT3 transcription in VGLUT3 neurons. Strengthening VGLUT3→PGIs synaptic transmission by ETV4 activation of VGLUT3 transcription upscales the efficacy of spatial memory retrieval in AD mice. This study reports a novel circuit and molecular mechanism underlying the efficacy of spatial memory retrieval via ETV4 inhibition of VGLUT3 transcription and hence provides a promising target for therapeutic intervention of the disease progression.

The Multifaceted Melanocortin Receptors

The 5 known melanocortin receptors (MCs) have established physiological roles. With the exception of MC2, these receptors can behave unpredictably, and since they are more widely expressed than their established roles would suggest, it is likely that they have other poorly characterized functions. The aim of this review is to discuss some of the less well-explored aspects of the 4 enigmatic members of this receptor family (MC1,3-5) and describe how these are multifaceted G protein-coupled receptors (GPCRs). These receptors appear to be promiscuous in that they bind several endogenous agonists (products of the proopiomelanocortin [POMC] gene) and antagonists but with inconsistent relative affinities and effects. We propose that this is a result of posttranslational modifications that determine receptor localization within nanodomains. Within each nanodomain there will be a variety of proteins, including ion channels, modifying proteins, and other GPCRs, that can interact with the MCs to alter the availability of receptor at the cell surface as well as the intracellular signaling resulting from receptor activation. Different combinations of interacting proteins and MCs may therefore give rise to the complex and inconsistent functional profiles reported for the MCs. For further progress in understanding this family, improved characterization of tissue-specific functions is required. Current evidence for interactions of these receptors with a range of partners, resulting in modulation of cell signaling, suggests that each should be studied within the full context of their interacting partners. The role of physiological status in determining this context also remains to be characterized.

The crosstalk between bone remodeling and energy metabolism: A translational perspective

Genetics in model organisms has progressively broken down walls that previously separated different disciplines of biology. One example of this holistic evolution is the recognition of the complex relationship that exists between the control of bone mass (bone remodeling) and energy metabolism in mammals. Numerous hormones orchestrate this crosstalk. In particular, the study of the leptin-mediated regulation of bone mass has not only revealed the existence of a central control of bone mass but has also led to the realization that sympathetic innervation is a major regulator of bone remodeling. This happened at a time when the use of drugs aiming at treating osteoporosis, the most frequent bone disease, has dwindled. This review will highlight the main aspects of the leptin-mediated regulation of bone mass and how this led to the realization that β-blockers, which block the effects of the sympathetic nervous system, may be a viable option to prevent osteoporosis.