Simulating combined monoaminergic depletions in a PD animal model through a bio-constrained differential equations system.

Introduction: Historically, Parkinson's Disease (PD) research has focused on the dysfunction of dopamine-producing cells in the substantia nigra pars compacta, which is linked to motor regulation in the basal ganglia. Therapies have mainly aimed at restoring dopamine (DA) levels, showing effectiveness but variable outcomes and side effects. Recent evidence indicates that PD complexity implicates disruptions in DA, noradrenaline (NA), and serotonin (5-HT) systems, which may underlie the variations in therapy effects. Methods: We present a system-level bio-constrained computational model that comprehensively investigates the dynamic interactions between these neurotransmitter systems. The model was designed to replicate experimental data demonstrating the impact of NA and 5-HT depletion in a PD animal model, providing insights into the causal relationships between basal ganglia regions and neuromodulator release areas. Results: The model successfully replicates experimental data and generates predictions regarding changes in unexplored brain regions, suggesting avenues for further investigation. It highlights the potential efficacy of alternative treatments targeting the locus coeruleus and dorsal raphe nucleus, though these preliminary findings require further validation. Sensitivity analysis identifies critical model parameters, offering insights into key factors influencing brain area activity. A stability analysis underscores the robustness of our mathematical formulation, bolstering the model validity. Discussion: Our holistic approach emphasizes that PD is a multifactorial disorder and opens promising avenues for early diagnostic tools that harness the intricate interactions among monoaminergic systems. Investigating NA and 5-HT systems alongside the DA system may yield more effective, subtype-specific therapies. The exploration of multisystem dysregulation in PD is poised to revolutionize our understanding and management of this complex neurodegenerative disorder.

The activation of D2-like dopamine receptors increases NMDA currents in the dorsal raphe serotonergic neurons.

The dorsal raphe nucleus (DRN) receives dopaminergic inputs from the ventral tegmental area (VTA). Also, the DRN contains a small population of cells that express dopamine (DRNDA neurons). However, the physiological role of dopamine (DA) in the DRN and its interaction with serotonergic (5-HT) neurons is poorly understood. Several works have reported moderate levels of D1, D2, and D3 DA receptors in the DRN. Furthermore, it was found that the activation of D2 receptors increased the firing of putative 5-HT neurons. Other studies have reported that D1 and D2 dopamine receptors can interact with glutamate NMDA receptors, modulating the excitability of different cell types. In the present work, we used immunocytochemical techniques to determine the kind of DA receptors in the DRN. Additionally, we performed electrophysiological experiments in brainstem slices to study the effect of DA agonists on NMDA-elicited currents recorded from identified 5-HT DRN neurons. We found that D2 and D3 but not D1 receptors are present in this nucleus. Also, we demonstrated that the activation of D2-like receptors increases NMDA-elicited currents in 5-HT neurons through a mechanism involving phospholipase C (PLC) and protein kinase C (PKC) enzymes. Possible physiological implications related to the sleep-wake cycle are discussed.

Differential Impact of Serotonin Signaling Methylphenidate on Young versus Adult: Insights from Behavioral and Dorsal Raphe Nucleus Neuronal Recordings from Freely Behaving Rats.

Methylphenidate (MPD) remains a cornerstone pharmacological intervention for managing ADHD, yet its increasing usage among ordinary youth and adults outside clinical contexts necessitates a thorough investigation into its developmental effects. This study seeks to simultaneously investigate the behavioral and neuronal changes within the dorsal raphe (DR) nucleus, a center of serotonergic neurons in the mammalian brain, before and after the administration of varying doses of acute and chronic MPD in freely behaving young and adult rats implanted with DR recording electrodes. Wireless neuronal and behavioral recording systems were used over 10 consecutive experimental days. Eight groups were examined: saline, 0.6, 2.5, and 10.0 mg/kg MPD for both young and adult rats. Six daily MPD injections were administered on experimental days 1 to 6, followed by a three-day washout period and MPD re-administration on experimental day 10 (ED10). The analysis of neuronal activity recorded from 504 DR neurons (DRNs) in young rats and 356 DRNs in adult rats reveals significant age-dependent differences in acute and chronic MPD responses. This study emphasizes the importance of aligning electrophysiological evaluations with behavioral outcomes following extended MPD exposure, elucidating the critical role of DRNs and serotonin signaling in modulating MPD responses and delineating age-specific variations in young versus adult rat models.

Chronic Social Defeat Stress Induces Pathway-Specific Adaptations at Lateral Habenula Neuronal Outputs.

The lateral habenula (LHb) has emerged as a pivotal brain region implicated in depression, displaying hyperactivity in human and animal models of depression. While the role of LHb efferents in depressive disorders has been acknowledged, the specific synaptic alterations remain elusive. Here, employing optogenetics, retrograde tracing, and ex vivo whole-cell patch-clamp techniques, we investigated synaptic transmission in male mice subjected to chronic social defeat stress (CSDS) at three major LHb neuronal outputs: the dorsal raphe nucleus (DRN), the ventral tegmental area (VTA), and the rostromedial tegmental nucleus (RMTg). Our findings uncovered distinct synaptic adaptations in LHb efferent circuits in response to CSDS. Specifically, CSDS induced in susceptible mice postsynaptic potentiation and postsynaptic depression at the DRN and VTA neurons, respectively, receiving excitatory inputs from the LHb, while CSDS altered presynaptic transmission at the LHb terminals in RMTg in both susceptible and resilient mice. Moreover, whole-cell recordings at projection-defined LHb neurons indicate decreased spontaneous activity in VTA-projecting LHb neurons, accompanied by an imbalance in excitatory-inhibitory inputs at the RMTg-projecting LHb neurons. Collectively, these novel findings underscore the circuit-specific alterations in LHb efferents following chronic social stress, shedding light on potential synaptic adaptations underlying stress-induced depressive-like states.

[Central inflammatory mechanism of celastrol in intervention of obesity-depression comorbidity in mice from amygdala-dorsal raphe nucleus].

This study aims to further elucidate the efficacy targets of celastrol(CEL) intervention in central inflammation in mice with obesity-depression comorbiditiy, based on the differential mRNA expression in the amygdala(AMY) and dorsal raphe nucleus(DRN) after CEL intervention. C57BL/6J mice were randomly divided into a normal diet group(Chow), a obesity-depression comorbidity(COM) group, and low-, medium-, and high-dose CEL groups(CEL-L, CEL-M, CEL-H, 0.5, 1.0, 2.0 mg·kg~(-1)). The Chow group received a normal diet, while the COM group and CEL-L, CEL-M, CEL-H groups received a high-fat diet combined with chronic stress from wet bedding. After 10 weeks of feeding, the mice were orally administered CEL for three weeks. Subsequently, the AMY and DRN of mice in the Chow, COM, and CEL-H groups were subjected to transcriptome analysis, and the intersection of target differentially expressed genes in both nuclei was visualized using a Venn diagram. The intersected genes were then imported into STRING for protein-protein interaction(PPI) analysis, and Gene Ontology(GO) analysis was performed using DAVID to identify the core targets regulated by CEL in the AMY and DRN. Independent samples were subjected to quantitative real-time PCR(qPCR) to validate the intersection genes. The results revealed that the common genes regulated by CEL in the AMY and DRN included chemokine family genes Ccl2, Ccl5, Ccl7, Cxcl10, Cxcr6, and Hsp70 family genes Hspa1a, Hspa1b, as well as Myd88, Il2ra, Irf7, Slc17a8, Drd2, Parp9, and Nampt. GO analysis showed that the top 5 nodes Ccl2, Cxcl10, Myd88, Ccl5, and Irf7 were all involved in immune-inflammation regulation(P<0.01). The qPCR results from independent samples showed that in the AMY, compared with the results in the Chow group, chemokine family genes, Hsp70, Myd88, Il2ra, Irf7, Slc17a8, Parp9, and Nampt were significantly up-regulated in the COM group, with Drd2 showing a decreasing trend; these pathological changes were significantly improved in the CEL-H group compared to the COM group. In the DRN, compared with the results in the Chow group, chemokine family genes, Hsp70, Myd88, Il2ra, Irf7, Parp9, and Nampt were significantly down-regulated, while Slc17a8 was significantly up-regulated in the COM group; compared with those in the COM group, Cxcr6, Irf7, and Drd2 were significantly up-regulated, while Slc17a8 was significantly down-regulated in the CEL-H group. In both the AMY and DRN, the expression of Irf7 by CEL showed both inhibition and activation in a dose-dependent manner(R~2 were 0.709 8 and 0.917 2, respectively). These findings suggest that CEL can effectively improve neuroinflammation by regulating bidirectional expression of the same target proteins, thereby intervening in the immune activation of the AMY and immune suppression of the DRN in COM mice.

Maternal fluoxetine impairs synaptic transmission and plasticity in the medial prefrontal cortex and alters the structure and function of dorsal raphe nucleus neurons in offspring mice.

Selective serotonin (5-HT) reuptake inhibitors (SSRIs) are commonly prescribed to women during pregnancy and breastfeeding despite posing a risk of adverse cognitive outcomes and affective disorders for the child. The consequences of SSRI-induced excess of 5-HT during development for the brain neuromodulatory 5-HT system remain largely unexplored. In this study, an SSRI - fluoxetine (FLX) - was administered to C57BL/6 J mouse dams during pregnancy and lactation to assess its effects on the offspring. We found that maternal FLX decreased field potentials, impaired long-term potentiation, facilitated long-term depression and tended to increase the density of 5-HTergic fibers in the medial prefrontal cortex (mPFC) of female but not male adolescent offspring. These effects were accompanied by deteriorated performance in the temporal order memory task and reduced sucrose preference with no change in marble burying behavior in FLX-exposed female offspring. We also found that maternal FLX reduced the axodendritic tree complexity of 5-HT dorsal raphe nucleus (DRN) neurons in female but not male offspring, with no changes in the excitability of DRN neurons of either sex. While no effects of maternal FLX on inhibitory postsynaptic currents (sIPSCs) in DRN neurons were found, we observed a significant influence of FLX exposure on kinetics of spontaneous excitatory postsynaptic currents (sEPSCs) in DRN neurons. Finally, we report that no changes in field potentials and synaptic plasticity were evident in the mPFC of the offspring after maternal exposure during pregnancy and lactation to a new antidepressant, vortioxetine. These findings show that in contrast to the mPFC, long-term consequences of maternal FLX exposure on the structure and function of DRN 5-HT neurons are mild and suggest a sex-dependent, distinct sensitivity of cortical and brainstem neurons to FLX exposure in early life. Vortioxetine appears to exert fewer side effects with regards to the mPFC when compared with FLX.

Arginine vasopressin activates serotonergic neurons in the dorsal raphe nucleus during neonatal development in vitro and in vivo.

Birth stress is a risk factor for psychiatric disorders and associated with exaggerated release of the stress hormone arginine vasopressin (AVP) into circulation and in the brain. In perinatal hippocampus, AVP activates GABAergic interneurons which leads to suppression of spontaneous network events and suggests a protective function of AVP on cortical networks during birth. However, the role of AVP in developing subcortical networks is not known. Here we tested the effect of AVP on the dorsal raphe nucleus (DRN) 5-hydroxytryptamine (5-HT, serotonin) system in male and female neonatal rats, since early 5-HT homeostasis is critical for the development of cortical brain regions and emotional behaviors. We show that AVP is strongly excitatory in neonatal DRN: it increases excitatory synaptic inputs of 5-HT neurons via V1A receptors in vitro and promotes their action potential firing through a combination of its effect on glutamatergic synaptic transmission and a direct effect on the excitability of these neurons. Furthermore, we identified two major firing patterns of neonatal 5-HT neurons in vivo, tonic regular firing and low frequency oscillations of regular spike trains and confirmed that these neurons are also activated by AVP in vivo. Finally, we show that the sparse vasopressinergic innervation in neonatal DRN originates exclusively from cell groups in medial amygdala and bed nucleus of stria terminalis. Hyperactivation of the neonatal 5-HT system by AVP during birth stress may impact its own functional development and affect the maturation of cortical target regions, which may increase the risk for psychiatric conditions later on.

Electroacupuncture Alleviates Memory Deficits in APP/PS1 Mice by Targeting Serotonergic Neurons in Dorsal Raphe Nucleus.

OBJECTIVE: Alzheimer's disease (AD) has become a significant global concern, but effective drugs able to slow down AD progression is still lacked. Electroacupuncture (EA) has been demonstrated to ameliorate cognitive impairment in individuals with AD. However, the underlying mechanisms remains poorly understood. This study aimed at examining the neuroprotective properties of EA and its potential mechanism of action against AD. METHODS: APP/PS1 transgenic mice were employed to evaluate the protective effects of EA on Shenshu (BL 23) and Baihui (GV 20). Chemogenetic manipulation was used to activate or inhibit serotonergic neurons within the dorsal raphe nucleus (DRN). Learning and memory abilities were assessed by the novel object recognition and Morris water maze tests. Golgi staining, western blot, and immunostaining were utilized to determine EA-induced neuroprotection. RESULTS: EA at Shenshu (BL 23) and Baihui (GV 20) effectively ameliorated learning and memory impairments in APP/PS1 mice. EA attenuated dendritic spine loss, increased the expression levels of PSD95, synaptophysin, and brain-derived neurotrophic factor in hippocampus. Activation of serotonergic neurons within the DRN can ameliorate cognitive deficits in AD by activating glutamatergic neurons mediated by 5-HT1B. Chemogenetic inhibition of serotonergic neurons in the DRN reversed the effects of EA on synaptic plasticity and memory. CONCLUSION: EA can alleviate cognitive dysfunction in APP/PS1 mice by activating serotonergic neurons in the DRN. Further study is necessary to better understand how the serotonergic neurons-related neural circuits involves in EA-induced memory improvement in AD.

Dorsal raphe nucleus-hippocampus serotonergic circuit underlies the depressive and cognitive impairments in 5×FAD male mice.

BACKGROUND: Depressive symptoms often occur in patients with Alzheimer's disease (AD) and exacerbate the pathogenesis of AD. However, the neural circuit mechanisms underlying the AD-associated depression remain unclear. The serotonergic system plays crucial roles in both AD and depression. METHODS: We used a combination of in vivo trans-synaptic circuit-dissecting anatomical approaches, chemogenetic manipulations, optogenetic manipulations, pharmacological methods, behavioral testing, and electrophysiological recording to investigate dorsal raphe nucleus serotonergic circuit in AD-associated depression in AD mouse model. RESULTS: We found that the activity of dorsal raphe nucleus serotonin neurons (DRN5-HT) and their projections to the dorsal hippocampal CA1 (dCA1) terminals (DRN5-HT-dCA1CaMKII) both decreased in brains of early 5×FAD mice. Chemogenetic or optogenetic activation of the DRN5-HT-dCA1CaMKII neural circuit attenuated the depressive symptoms and cognitive impairments in 5×FAD mice through serotonin receptor 1B (5-HT1BR) and 4 (5-HT4R). Pharmacological activation of 5-HT1BR or 5-HT4R attenuated the depressive symptoms and cognitive impairments in 5×FAD mice by regulating the DRN5-HT-dCA1CaMKII neural circuit to improve synaptic plasticity. CONCLUSIONS: These findings provide a new mechanistic connection between depression and AD and provide potential pharmaceutical prevention targets for AD.

Neural Circuit Mechanisms of Sinisan formula for the Treatment of adolescent Depression: prefrontal cortex to dorsal raphe nucleus.

ETHNOPHARMACOLOGICAL RELEVANCE: Sinisan formula (SNSF), documented in the classic books Shanghan Lun, is known for its ability to regulate liver-qi and treat depression. However, its underlying mechanism, particularly its effects on dynamic real-time neuron activity and circuits remains to be fully elucidated. AIM OF THE STUDY: This study aimed to investigate the antidepressant effect of SNSF and its central nervous system mechanism on depression-like behaviors, focusing on the prefrontal cortex (PFC) to dorsal raphe nucleus (DRN) neural circuit in a stress-induced adolescent animal model. MATERIALS AND METHODS: SNSF comprised four herbs, the root of Bupleurum chinense DC., the root of Paeonia lactiflora Pall., the fruit of Citrus aurantium L., the rhizome of Glycyrrhiza uralensis Fisch., in equal propotions. The adolescent depression animal model was induced by maternal separation (MS) and chronic restraint stress (CRS). In-vivo multichannel physiological electrodes were implanted into the PFC on PND 28 and animals were recorded 5 times during PND 35-46. From PND 47, the behavioral tests were performed to evaluate the antidepressant efficacy of SNSF. Subsequently, brain tissue was collected for Western blot and immunofluorescence staining analysis. Retro virus was injected into the DRN to explore sources of projections received by serotonergic (5-HTergic) neurons. And the PFC-to-DRN circuit was activated or inhibited through chemogenetic techniques to investigate the effects of SNSF on depression-like behaviors. RESULTS: Administration of SNSF for 18 days effectively alleviated depression-like behaviors in MS&CRS adolescent mice. The PFC emerged as the primary glutamatergic projection source of the DRN5-HT neurons. Following SNSF administration for 13/15/18 days, there was an increase in the firing rate of excitatory neurons and excitatory/inhibitory (E/I) ratio in the PFC. MS&CRS stress let to a reduction in the density of 5-HT+ and CaMKII + neurons in the DRN, accompanied by an increase in the density of GAD + neurons in the DRN, while SNSF administration reversed the alterations. Chemogenetic activation of the PFC-to-DRN circuit rescued the depression-like behaviors induced by MS&CRS, whereas suppression of this circuit attenuated the antidepressant effect of SNSF. CONCLUSIONS: SNSF significantly mitigated depression-like behaviors in MS&CRS mice. SNSF exerts its antidepressant effects by increasing the E/I ratio in the PFC and enhancing glutamatergic projections from the PFC to the DRN.

Exploring the role of environmental enrichment and early life adversity on emotional development.

Early life adversity has been linked with a higher probability of developing behavioral impairments and environmental manipulation is a strategy that may reduce the negative effects of exposure to adversity in early life. Here, we focused on exploring the influence of environmental enrichment (EE) as a protective factor in the context of early life adversity. We hypothesized that 24 hours of maternal deprivation (MD), in the second week of life, could induce anxiety-like behavior alterations and that exposure to EE could induce resilience to these behaviors due to alterations in the serotonergic system. Male Wistar rats were exposed to MD, on postnatal days 11 and 13, and to EE, after weaning. In adulthood, we performed a series of behavioral tests for fear, anxiety, and locomotor activity. We also measured the levels of serotonin in the amygdala and dorsal raphe nucleus. Our results revealed that MD does not impact fear behavior or the levels of serotonin, while EE decreases locomotor activity in a novel environment and enhances exploration in the predator odor test. EE also decreases serotonin in the amygdala and increases its turnover rate levels. Our findings provide insights into the critical timeframe during which stress exposure impacts the development and confirm that exposure to EE has an independent and protective effect for anxiety-like behaviors later in life.

Dorsal raphe nucleus to basolateral amygdala 5-HTergic neural circuit modulates restoration of consciousness during sevoflurane anesthesia.

The advent of general anesthesia (GA) has significant implications for clinical practice. However, the exact mechanisms underlying GA-induced transitions in consciousness remain elusive. Given some similarities between GA and sleep, the sleep-arousal neural nuclei and circuits involved in sleep-arousal, including the 5-HTergic system, could be implicated in GA. Herein, we utilized pharmacology, optogenetics, chemogenetics, fiber photometry, and retrograde tracing to demonstrate that both endogenous and exogenous activation of the 5-HTergic neural circuit between the dorsal raphe nucleus (DR) and basolateral amygdala (BLA) promotes arousal and facilitates recovery of consciousness from sevoflurane anesthesia. Notably, the 5-HT1A receptor within this pathway holds a pivotal role. Our findings will be conducive to substantially expanding our comprehension of the neural circuit mechanisms underlying sevoflurane anesthesia and provide a potential target for modulating consciousness, ultimately leading to a reduction in anesthetic dose requirements and side effects.

Serotonergic and dopaminergic neurons in the dorsal raphe are differentially altered in a mouse model for parkinsonism.

Parkinson's disease (PD) is characterized by motor impairments caused by degeneration of dopamine neurons in the substantia nigra pars compacta. In addition to these symptoms, PD patients often suffer from non-motor comorbidities including sleep and psychiatric disturbances, which are thought to depend on concomitant alterations of serotonergic and noradrenergic transmission. A primary locus of serotonergic neurons is the dorsal raphe nucleus (DRN), providing brain-wide serotonergic input. Here, we identified electrophysiological and morphological parameters to classify serotonergic and dopaminergic neurons in the murine DRN under control conditions and in a PD model, following striatal injection of the catecholamine toxin, 6-hydroxydopamine (6-OHDA). Electrical and morphological properties of both neuronal populations were altered by 6-OHDA. In serotonergic neurons, most changes were reversed when 6-OHDA was injected in combination with desipramine, a noradrenaline (NA) reuptake inhibitor, protecting the noradrenergic terminals. Our results show that the depletion of both NA and dopamine in the 6-OHDA mouse model causes changes in the DRN neural circuitry.

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.

Dorsal raphe neurons integrate the values of reward amount, delay, and uncertainty in multi-attribute decision-making.

The dorsal raphe nucleus (DRN) is implicated in psychiatric disorders that feature impaired sensitivity to reward amount, impulsivity when facing reward delays, and risk-seeking when confronting reward uncertainty. However, it has been unclear whether and how DRN neurons signal reward amount, reward delay, and reward uncertainty during multi-attribute value-based decision-making, where subjects consider these attributes to make a choice. We recorded DRN neurons as monkeys chose between offers whose attributes, namely expected reward amount, reward delay, and reward uncertainty, varied independently. Many DRN neurons signaled offer attributes, and this population tended to integrate the attributes in a manner that reflected monkeys' preferences for amount, delay, and uncertainty. After decision-making, in response to post-decision feedback, these same neurons signaled signed reward prediction errors, suggesting a broader role in tracking value across task epochs and behavioral contexts. Our data illustrate how the DRN participates in value computations, guiding theories about the role of the DRN in decision-making and psychiatric disease.

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.

Interactions between Lateral Hypothalamic Orexin and Dorsal Raphe Circuitry in Energy Balance.

Orexin/hypocretin terminals innervate the dorsal raphe nucleus (DRN), which projects to motor control areas important for spontaneous physical activity (SPA) and energy expenditure (EE). Orexin receptors are expressed in the DRN, and obesity-resistant (OR) rats show higher expression of these receptors in the DRN and elevated SPA/EE. We hypothesized that orexin-A in the DRN enhances SPA/EE and that DRN-GABA modulates the effect of orexin-A on SPA/EE. We manipulated orexin tone in the DRN either through direct injection of orexin-A or through the chemogenetic activation of lateral-hypothalamic (LH) orexin neurons. In the orexin neuron activation experiment, fifteen minutes prior to the chemogenetic activation of orexin neurons, the mice received either the GABA-agonist muscimol or antagonist bicuculline injected into the DRN, and SPA/EE was monitored for 24 h. In a separate experiment, orexin-A was injected into the DRN to study the direct effect of DRN orexin on SPA/EE. We found that the activation of orexin neurons elevates SPA/EE, and manipulation of GABA in the DRN does not alter the SPA response to orexin neuron activation. Similarly, intra-DRN orexin-A enhanced SPA and EE in the mice. These results suggest that orexin-A in the DRN facilitates negative energy balance by increasing physical activity-induced EE, and that modulation of DRN orexin-A is a potential strategy to promote SPA and EE.

Deep learning models for atypical serotonergic cells recognition.

BACKGROUND: The serotonergic system modulates brain processes via functionally distinct subpopulations of neurons with heterogeneous properties, including their electrophysiological activity. In extracellular recordings, serotonergic neurons to be investigated for their functional properties are commonly identified on the basis of "typical" features of their activity, i.e. slow regular firing and relatively long duration of action potentials. Thus, due to the lack of equally robust criteria for discriminating serotonergic neurons with "atypical" features from non-serotonergic cells, the physiological relevance of the diversity of serotonergic neuron activities results largely understudied. NEW METHODS: We propose deep learning models capable of discriminating typical and atypical serotonergic neurons from non-serotonergic cells with high accuracy. The research utilized electrophysiological in vitro recordings from serotonergic neurons identified by the expression of fluorescent proteins specific to the serotonergic system and non-serotonergic cells. These recordings formed the basis of the training, validation, and testing data for the deep learning models. The study employed convolutional neural networks (CNNs), known for their efficiency in pattern recognition, to classify neurons based on the specific characteristics of their action potentials. RESULTS: The models were trained on a dataset comprising 27,108 original action potential samples, alongside an extensive set of 12 million synthetic action potential samples, designed to mitigate the risk of overfitting the background noise in the recordings, a potential source of bias. Results show that the models achieved high accuracy and were further validated on "non-homogeneous" data, i.e., data unknown to the model and collected on different days from those used for the training of the model, to confirm their robustness and reliability in real-world experimental conditions. COMPARISON WITH EXISTING METHODS: Conventional methods for identifying serotonergic neurons allow recognition of serotonergic neurons defined as typical. Our model based on the analysis of the sole action potential reliably recognizes over 94% of serotonergic neurons including those with atypical features of spike and activity. CONCLUSION: The model is ready for use in experiments conducted with the here described recording parameters. We release the codes and procedures allowing to adapt the model to different acquisition parameters or for identification of other classes of spontaneously active neurons.

High-fat diet, microbiome-gut-brain axis signaling, and anxiety-like behavior in male rats.

Obesity, associated with the intake of a high-fat diet (HFD), and anxiety are common among those living in modern urban societies. Recent studies suggest a role of microbiome-gut-brain axis signaling, including a role for brain serotonergic systems in the relationship between HFD and anxiety. Evidence suggests the gut microbiome and the serotonergic brain system together may play an important role in this response. Here we conducted a nine-week HFD protocol in male rats, followed by an analysis of the gut microbiome diversity and community composition, brainstem serotonergic gene expression (tph2, htr1a, and slc6a4), and anxiety-related defensive behavioral responses. We show that HFD intake decreased alpha diversity and altered the community composition of the gut microbiome in association with obesity, increased brainstem tph2, htr1a and slc6a4 mRNA expression, including in the caudal part of the dorsomedial dorsal raphe nucleus (cDRD), a subregion previously associated with stress- and anxiety-related behavioral responses, and, finally, increased anxiety-related defensive behavioral responses. The HFD increased the Firmicutes/Bacteroidetes ratio relative to control diet, as well as higher relative abundances of Blautia, and decreases in Prevotella. We found that tph2, htr1a and slc6a4 mRNA expression were increased in subregions of the dorsal raphe nucleus in the HFD, relative to control diet. Specific bacterial taxa were associated with increased serotonergic gene expression in the cDRD. Thus, we propose that HFD-induced obesity is associated with altered microbiome-gut-serotonergic brain axis signaling, leading to increased anxiety-related defensive behavioral responses in rats.

Serotonergic dysfunction may mediate the relationship between alcohol consumption and Alzheimer's disease.

The impact of Alzheimer's disease (AD) and its related dementias is rapidly expanding, and its mitigation remains an urgent social and technical challenge. To date there are no effective treatments or interventions for AD, but recent studies suggest that alcohol consumption is correlated with the risk of developing dementia. In this review, we synthesize data from preclinical, clinical, and epidemiological models to evaluate the combined role of alcohol consumption and serotonergic dysfunction in AD, underscoring the need for further research on this topic. We first discuss the limitations inherent to current data-collection methods, and how neuropsychiatric symptoms common among AD, alcohol use disorder, and serotonergic dysfunction may mask their co-occurrence. We additionally describe how excess alcohol consumption may accelerate the development of AD via direct effects on serotonergic function, and we explore the roles of neuroinflammation and proteostasis in mediating the relationship between serotonin, alcohol consumption, and AD. Lastly, we argue for a shift in current research to disentangle the pathogenic effects of alcohol on early-affected brainstem structures in AD.