Role of the dorsal medial habenula in the regulation of voluntary activity, motor function, hedonic state, and primary reinforcement

Mu-opioid receptor activation in the habenula modulates synaptic transmission and depression-like behaviors

Opioid system dysregulation in response to stress is known to lead to psychiatric disorders including major depression. Among three different types of opioid receptors, the mu-type receptors (mORs) are highly expressed in the habenula complex, however, the action of mORs in this area and its interaction with stress exposure is largely unknown. Therefore, we investigated the roles of mORs in the habenula using male rats of an acute learned helplessness (aLH) model. First, we found that mOR activation decreased both excitatory and inhibitory synaptic transmission onto the lateral habenula (LHb). Intriguingly, this mOR-induced synaptic depression was reduced in an animal model of depression compared to that of controls. In naïve animals, we found an unexpected interaction between mORs and the endocannabinoid (eCB) signaling occurring in the LHb, which mediates presynaptic alteration occurring with mOR activation. However, we did not observe presynaptic alteration by mOR activation after stress exposure. Moreover, selective mOR activation in the habenula before, but not after, stress exposure effectively reduced helpless behaviors compared to aLH animals. Our observations are consistent with clinical reports suggesting the involvement of mOR signaling in depression, and additionally reveal a critical time window of mOR action in the habenula for ameliorating helplessness symptoms.

Topographic Organization of Glutamatergic and GABAergic Parvalbumin-Positive Neurons in the Lateral Habenula

Parvalbumin-expressing (PV) neurons, classified by their expression of the calcium-binding protein parvalbumin, play crucial roles in the function and plasticity of the lateral habenular nucleus (LHb). This study aimed to deepen our understanding of the LHb by collecting information about the heterogeneity of LHb PV neurons in mice. To achieve this, we investigated the proportions of the transmitter machinery in LHb PV neurons, including GABAergic, glutamatergic, serotonergic, cholinergic, and dopaminergic neurotransmitter markers, using transcriptome analysis, mRNA in situ hybridization chain reaction, and immunohistochemistry. LHb PV neurons comprise three subsets: glutamatergic, GABAergic, and double-positive for glutamatergic and GABAergic machinery. By comparing the percentages of the subsets, we found that the LHb was topographically organized anteroposteriorly; the GABAergic and glutamatergic PV neurons were preferentially distributed in the anterior and posterior LHb, respectively, uncovering the anteroposterior topography of the LHb. In addition, we confirmed the mediolateral topography of lateral GABAergic PV neurons. These findings suggest that PV neurons play distinct roles in different parts of the LHb along the anteroposterior and mediolateral axes, facilitating the topographic function of the LHb. It would be interesting to determine whether their topography is differentially involved in various cognitive and motivational processes associated with the LHb, particularly the involvement of posterior glutamatergic PV neurons.

High-throughput ultrasound neuromodulation in awake and freely behaving rats

Transcranial ultrasound neuromodulation is a promising potential therapeutic tool for the noninvasive treatment of neuropsychiatric disorders. However, the expansive parameter space and difficulties in controlling for peripheral auditory effects make it challenging to identify ultrasound sequences and brain targets that may provide therapeutic efficacy. Careful preclinical investigations in clinically relevant behavioral models are critically needed to identify suitable brain targets and acoustic parameters. However, there is a lack of ultrasound devices allowing for multi-target experimental investigations in awake and unrestrained rodents. We developed a miniaturized 64-element ultrasound array that enables neurointerventional investigations with within-trial active control targets in freely behaving rats. We first characterized the acoustic field with measurements in free water and with transcranial propagation. We then confirmed in vivo that the array can target multiple brain regions via electronic steering, and verified that wearing the device does not cause significant impairments to animal motility. Finally, we demonstrated the performance of our system in a high-throughput neuromodulation experiment, where we found that ultrasound stimulation of the rat central medial thalamus, but not an active control target, promotes arousal and increases locomotor activity.

Locomotor pattern generation and descending control: a historical perspective

The ability to generate and control locomotor movements depends on complex interactions between many areas of the nervous system, the musculoskeletal system, and the environment. How the nervous system manages to accomplish this task has been the subject of investigation for more than a century. In vertebrates, locomotion is generated by neural networks located in the spinal cord referred to as central pattern generators. Descending inputs from the brain stem initiate, maintain, and stop locomotion as well as control speed and direction. Sensory inputs adapt locomotor programs to the environmental conditions. This review presents a comparative and historical overview of some of the neural mechanisms underlying the control of locomotion in vertebrates. We have put an emphasis on spinal mechanisms and descending control.

Effects of the T-type calcium channel CaV3.2 R1584P mutation on absence seizure susceptibility in GAERS and NEC congenic rats models

RATIONALE

Low-voltage-activated or T-type Ca²⁺ channels play a key role in the generation of seizures in absence epilepsy. We have described a homozygous, gain of function substitution mutation (R1584P) in the Ca_V3.2 T-type Ca²⁺ channel gene (Cacna1h) in the Genetic Absence Epilepsy Rats from Strasbourg (GAERS). The non-epileptic control (NEC) rats, derived from the same original Wistar strains as GAERS but selectively in-breed not to express seizures, are null for the R1584P mutation. To study the effects of this mutation in rats who otherwise have a GAERS or NEC genetic background, we bred congenic GAERS-Cacna1hNEC (GAERS null for R1584P mutation) and congenic NEC-Cacna1hGAERS (NEC homozygous for R1584P mutation) and evaluated the seizure and behavioral phenotype of these strains in comparison to the original GAERS and NEC strains.

METHODS

To evaluate seizure expression in the congenic strains, EEG electrodes were implanted in NEC, GAERS, GAERS^-Cacna1hNEC without the R1584P mutation, and NEC^{-Cacna1hGAERS} with the R1584P mutation rats. In the first study, continuous EEG recordings were acquired from week 4 (when seizures begin to develop in GAERS) to week 14 of age (when GAERS display hundreds of seizures per day). In the second study, the seizure and behavioral phenotype of GAERS and NEC^{-Cacna1hGAERS} strains were evaluated during young age (6 weeks of age) and adulthood (16 weeks of age) of GAERS, NEC, GAERS^-Cacna1hNEC and NEC^{-Cacna1hGAERS}. The Open field test (OFT) and sucrose preference test (SPT) were performed to evaluate anxiety-like and depressive-like behavior, respectively. This was followed by EEG recordings at 18 weeks of age to quantify the seizures, and spike-wave discharge (SWD) cycle frequency. At the end of the study, the whole thalamus was collected for T-type calcium channel mRNA expression analysis.

RESULTS

GAERS had a significantly shorter latency to first seizures and an increased number of seizures per day compared to GAERS^-Cacna1hNEC. On the other hand, the presence of the R1584P mutation in the NEC^{-Cacna1hGAERS} was not enough to generate spontaneous seizures in their seizure-resistant background. 6 and 16-week-old GAERS and GAERS^-Cacna1hNEC rats showed anxiety-like behavior in the OFT, in contrast to NEC and NEC^{-Cacna1hGAERS}. Results from the SPT showed that the GAERS developed depressive-like in the SPT compared to GAERS^-Cacna1hNEC, NEC, and NEC^{-Cacna1hGAERS}. Analysis of the EEG at 18 weeks of age showed that the GAERS had an increased number of seizures per day, increased total seizure duration and a higher cycle frequency of SWD relative to GAERS^-Cacna1hNEC. However, the average seizure duration was not significantly different between strains. Quantitative real-time PCR showed that the T-type Ca²⁺ channel isoform Ca_V3.2 channel expression was significantly increased in GAERS compared to NEC, GAERS^-Cacna1hNEC and NEC^{-Cacna1hGAERS}. The presence of the R1584P mutation increased the total ratio of Ca_V3.2 + 25/-25 splice variants in GAERS and NEC^{-Cacna1hGAERS} compared to NEC and GAERS^-Cacna1hNEC.

DISCUSSION

The data from this study demonstrate that the R1584P mutation in isolation on a seizure-resistant NEC genetic background was insufficient to generate absence seizures, and that a GAERS genetic background can cause seizures even without the mutation. However, the study provides evidence that the R1584P mutation acts as a modulator of seizures development and expression, and depressive-like behavior in the SPT, but not the anxiety phenotype of the GAERS model of absence epilepsy.

Genetic and epigenetic studies of opioid abuse disorder - the potential for future diagnostics

INTRODUCTION

Opioid use disorder (OUD) is a global problem that often begins with prescribed medications. The available treatment and maintenance plans offer solutions for the consumption rate by individuals leaving the outstanding problem of relapse, which is a major factor hindering the long-term efficacy of treatments.

AREAS COVERED

Understanding the neurobiology of addiction and relapse would help identifying the core causes of relapse and distinguish vulnerable from resilient individuals, which would lead to more targeted and effective treatment and provide diagnostics to screen individuals who have a propensity to OUD. In this review, we cover the neurobiology of the reward system highlighting the role of multiple brain regions and opioid receptors in the development of the disorder. We also review the current knowledge of the epigenetics of addiction and the available screening tools for aberrant use of opioids.

EXPERT OPINION

Relapse remains an anticipated limitation in the way of recovery even after long period of abstinence. This highlights the need for diagnostic tools that identify vulnerable patients and prevent the cycle of addiction. Finally, we discuss the limitations of the available screening tools and propose possible solutions for the discovery of addiction diagnostics.

Electroacupuncture alleviates traumatic brain injury by inhibiting autophagy via increasing IL-10 production and blocking the AMPK/mTOR signaling pathway in rats

Autophagy, switched by the AMPK/mTOR signaling, has been revealed to contribute greatly to traumatic brain injury (TBI). Electroacupuncture (EA) is a promising therapeutic method for TBI, however, the underlying mechanism is still unclear. Herein, we hypothesize that the therapeutic effect of EA on TBI is associated with its inhibition on AMPK/mTOR-mediated autophagy. Sprague-Dawley rats were randomly divided into three groups: sham, TBI, and TBI + EA. TBI model was established by using an electronic controlled cortical impactor. Rats were treated with EA at 12 h after modeling, 15 min daily for 14 consecutive days. EA was applied at the acupuncture points Quchi (LI 11), Hegu (LI4), Baihui (GV20), Guanyuan (CV4), Zusanli (ST36) and Yongquan (KI1), using dense-sparse wave, at frequencies of 1 Hz, and an amplitude of 1 mA. After 3, 7 and 14 days of modeling, the modified neurological severity scale (mNSS), rota rod system, and Morris Water Maze (MWM) test showed that EA treatment promoted neurological function recovery in TBI rats. Moreover, EA treatment alleviated brain edema, pathological damage, neuronal apoptosis in TBI rats. EA improved abnormal ultrastructure, including abnormal mitochondrial morphology and increased autophagosomes, in the brain neurons of TBI rats, as measured by transmission electron microscopy, and the concentration of adenosine triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP). Western blot and immunohistochemistry (IHC) assays were performed to measure the protein levels of interleukin 10 (IL-10), autophagy-related proteins and key proteins in the AMPK/mTOR signaling pathway. EA treatment increased IL-10 production, inhibited the AMPK/mTOR signaling, and inhibited excessive autophagy in TBI rats. Additionally, AMPK inhibitor Compound C treatment had similar effects to EA. Both AMPK agonist AICAR and IL-10 neutralizing antibody treatments reversed the effects of EA on the related protein levels of autophagy and the AMPK/mTOR signaling pathway, and abolished the protective effects of EA on TBI rats. In conclusion, EA treatment promoted neurological function recovery and alleviated pathological damage and neuronal apoptosis in TBI rats through inhibiting excessive autophagy via increasing IL-10 production and blocking the AMPK/mTOR signaling pathway.

Cell-type-specific population dynamics of diverse reward computations

Computational analysis of cellular activity has developed largely independently of modern transcriptomic cell typology, but integrating these approaches may be essential for full insight into cellular-level mechanisms underlying brain function and dysfunction. Applying this approach to the habenula (a structure with diverse, intermingled molecular, anatomical, and computational features), we identified encoding of reward-predictive cues and reward outcomes in distinct genetically defined neural populations, including TH+ cells and Tac1+ cells. Data from genetically targeted recordings were used to train an optimized nonlinear dynamical systems model and revealed activity dynamics consistent with a line attractor. High-density, cell-type-specific electrophysiological recordings and optogenetic perturbation provided supporting evidence for this model. Reverse-engineering predicted how Tac1+ cells might integrate reward history, which was complemented by in vivo experimentation. This integrated approach describes a process by which data-driven computational models of population activity can generate and frame actionable hypotheses for cell-type-specific investigation in biological systems.

Stria medullaris innervation follows the transcriptomic division of the habenula

The habenula is a complex neuronal population integrated in a pivotal functional position into the vertebrate limbic system. Its main afference is the stria medullaris and its main efference the fasciculus retroflexus. This neuronal complex is composed by two main components, the medial and lateral habenula. Transcriptomic and single cell RNAseq studies have unveiled the morphological complexity of both components. The aim of our work was to analyze the relation between the origin of the axonal fibers and their final distribution in the habenula. We analyzed 754 tracing experiments from Mouse Brain Connectivity Atlas, Allen Brain Map databases, and selected 12 neuronal populations projecting into the habenular territory. Our analysis demonstrated that the projections into the medial habenula discriminate between the different subnuclei and are generally originated in the septal territory. The innervation of the lateral habenula displayed instead a less restricted distribution from preoptic, terminal hypothalamic and peduncular nuclei. Only the lateral oval subnucleus of the lateral habenula presented a specific innervation from the dorsal entopeduncular nucleus. Our results unveiled the necessity of novel sorts of behavioral experiments to dissect the different functions associated with the habenular complex and their correlation with the distinct neuronal populations that generate them.

Gene Expression Profiling of the Habenula in Rats Exposed to Chronic Restraint Stress

Chronic stress contributes to the risk of developing depression; the habenula, a nucleus in epithalamus, is associated with many neuropsychiatric disorders. Using genome-wide gene expression analysis, we analyzed the transcriptome of the habenula in rats exposed to chronic restraint stress for 14 days. We identified 379 differentially expressed genes (DEGs) that were affected by chronic stress. These genes were enriched in neuroactive ligand-receptor interaction, the cAMP (cyclic adenosine monophosphate) signaling pathway, circadian entrainment, and synaptic signaling from the Kyoto Encyclopedia of Genes and Genomes pathway analysis and responded to corticosteroids, positive regulation of lipid transport, anterograde trans-synaptic signaling, and chemical synapse transmission from the Gene Ontology analysis. Based on protein-protein interaction network analysis of the DEGs, we identified neuroactive ligand-receptor interactions, circadian entrainment, and cholinergic synapse-related subclusters. Additionally, cell type and habenular regional expression of DEGs, evaluated using a recently published single-cell RNA sequencing study (GSE137478), strongly suggest that DEGs related to neuroactive ligand-receptor interaction and trans-synaptic signaling are highly enriched in medial habenular neurons. Taken together, our findings provide a valuable set of molecular targets that may play important roles in mediating the habenular response to stress and the onset of chronic stress-induced depressive behaviors.

Functional α7 nicotinic acetylcholine receptors in GABAergic neurons of the interpeduncular nucleus

The interpeduncular nucleus (IPN) plays a key role in nicotine dependence and is involved in regulation of fear responses, affective states, and novelty processing. IPN neurons express nicotinic acetylcholine receptors (nAChR) and receive strong cholinergic innervation from the ventral medial habenula. Dorsal medial habenula neurons are primarily peptidergic, releasing substance P (SP) mainly onto IPN neurons in the lateral subnucleus (IPL). IPL neurons are sensitive to SP, but it is not known if they are involved in cholinergic transmission like other IPN neurons. We examined nAChR subunit gene expression in IPL neurons, revealing that Chrna7 (α7 nAChR subunit) is expressed in a subset of GABAergic IPL neurons. In patch-clamp recordings from IPL neurons, ACh-evoked inward currents were attenuated by methyllycaconitine (α7 nAChR antagonist) and potentiated by NS1738 (α7 Type I positive allosteric modulator). We confirmed α7 functional expression in IPL neurons by also showing that ACh-evoked currents were potentiated by PNU-120596 (Type II positive allosteric modulator). Additional pharmacological experiments show that IPN neurons expressing α7 nAChRs also express α3β4 nAChRs. Finally, we used 2-photon laser scanning microscopy and nicotine uncaging to directly examine the morphology of IPL neurons that express α7 nAChRs. These results highlight a novel aspect of α7 nAChR neurobiology, adding to the complexity of cholinergic modulation by nAChRs in the IPN.

The regulation of PKA signaling in obesity and in the maintenance of metabolic health

The cAMP-dependent protein kinase (PKA) system represents a primary cell-signaling pathway throughout systems and across species. PKA facilitates the actions of hormones, neurotransmitters and other signaling molecules that bind G-protein coupled receptors (GPCR) to modulate cAMP levels. Through its control of synaptic events, exocytosis, transcriptional regulation, and more, PKA signaling regulates cellular metabolism and emotional and stress responses making it integral in the maintenance and dysregulation of energy homeostasis. Neural PKA signaling is regulated by afferent and peripheral efferent signals that link specific neural cell populations to the regulation of metabolic processes in adipose tissue, liver, pancreas, adrenal, skeletal muscle, and gut. Mouse models have provided invaluable information on the roles for PKA subunits in brain and key metabolic organs. While limited, human studies infer differential regulation of the PKA system in obese compared to lean individuals. Variants identified in PKA subunit genes cause Cushing syndrome that is characterized by metabolic dysregulation associated with endogenous glucocorticoid excess. Under healthy physiologic conditions, the PKA system is exquisitely regulated by stimuli that activate GPCRs to alter intracellular cAMP concentrations, and by PKA cellular localization and holoenzyme stability. Adenylate cyclase activity generates cAMP while phosphodiesterase-mediated cAMP degradation to AMP decreases cAMP levels downstream of GPCRs. Chronic perturbations in PKA signaling appear to be capable of resetting PKA regulation at several levels; in addition, sex differences in PKA signaling regulation, while not well understood, impact the physiologic consequences of metabolic dysregulation and obesity. This review explores the roles for PKA signaling in the pathogenesis of metabolic diseases including obesity, type 2 diabetes mellitus and associated co-morbidities through neural-peripheral crosstalk and cAMP/PKA signaling pathway targets that hold therapeutic potential.

Chronic infusions of mecamylamine into the medial habenula: Effects on nicotine self-administration in rats

The habenula is an epithalamic structure through which descending connections go from the telencephalon to the brainstem, putting it in a key location to provide feedback control over the ascending projections from the brainstem to the telencephalon. The medial habenula has a high concentration of nicotinic receptors. We assessed the role of medial habenular nicotinic receptors for nicotine self-administration (SA) in female young adult Sprague-Dawley rats. The rats had bilateral chronic infusion cannulae placed into the medial habenula nucleus. Each cannula was connected to a slow delivery osmotic minipump to chronically infuse mecamylamine (100 µg/side/day) or vehicle for four consecutive weeks. The rats were tested for nicotine SA for the first two weeks of mecamylamine infusion. Then, they had one week of enforced abstinence, during which they had no access to the nicotine SA. Finally, they had one week of resumed nicotine SA access. There was a significantly differential mecamylamine effects in animals with lower and higher pretreatment baseline nicotine SA. Rats with lower baseline nicotine SA levels showed a nearly significant mecamylamine-induced reduction in SA while those with higher baseline levels of SA showed a significant mecamylamine-induced increase in nicotine SA. This study determined that medial habenular nicotinic receptors are important for nicotine reinforcement. Baseline level of performance makes a crucial difference for the involvement of habenular mechanisms in nicotine reinforcement with nicotinic activation being important for maintaining nicotine self-administration for those with lower levels of baseline self-administration and the opposite effect with subjects with higher levels of baseline self-administration.

Habenula orphan G-protein coupled receptors in the pathophysiology of fear and anxiety

The phasic emotion, fear, and the tonic emotion, anxiety, have been conventionally inspected in clinical frameworks to epitomize memory acquisition, storage, and retrieval. However, inappropriate expression of learned fear in a safe environment and its resistance to suppression is a cardinal feature of various fear-related disorders. A significant body of literature suggests the involvement of extra-amygdala circuitry in fear disorders. Consistent with this view, the present review underlies incentives for the association between the habenula and fear memory. G protein-coupled receptors (GPCRs) are important to understand the molecular mechanisms central to fear learning due to their neuromodulatory role. The efficacy of a pharmacological strategy aimed at exploiting habenular-GPCR desensitization machinery can serve as a therapeutic target combating the pathophysiology of fear disorders. Originating from this milieu, the conserved nature of orphan GPCRs in the brain, with some having the highest expression in the habenula can lead to recent endeavors in understanding its functionality in fear circuitry.

Discovery of TAK-041: a Potent and Selective GPR139 Agonist Explored for the Treatment of Negative Symptoms Associated with Schizophrenia

The orphan G-protein-coupled receptor GPR139 is highly expressed in the habenula, a small brain nucleus that has been linked to depression, schizophrenia (SCZ), and substance-use disorder. High-throughput screening and a medicinal chemistry structure-activity relationship strategy identified a novel series of potent and selective benzotriazinone-based GPR139 agonists. Herein, we describe the chemistry optimization that led to the discovery and validation of multiple potent and selective in vivo GPR139 agonist tool compounds, including our clinical candidate TAK-041, also known as NBI-1065846 (compound 56). The pharmacological characterization of these GPR139 agonists in vivo demonstrated GPR139-agonist-dependent modulation of habenula cell activity and revealed consistent in vivo efficacy to rescue social interaction deficits in the BALB/c mouse strain. The clinical GPR139 agonist TAK-041 is being explored as a novel drug to treat negative symptoms in SCZ.

Role of endocannabinoid signaling in a septohabenular pathway in the regulation of anxiety- and depressive-like behavior

Enhancing endocannabinoid signaling produces anxiolytic- and antidepressant-like effects, but the neural circuits involved remain poorly understood. The medial habenula (MHb) is a phylogenetically-conserved epithalamic structure that is a powerful modulator of anxiety- and depressive-like behavior. Here, we show that a robust endocannabinoid signaling system modulates synaptic transmission between the MHb and its sole identified GABA input, the medial septum and nucleus of the diagonal band (MSDB). With RNAscope in situ hybridization, we demonstrate that key enzymes that synthesize or degrade the endocannabinoids 2-arachidonylglycerol (2-AG) or anandamide are expressed in the MHb and MSDB, and that cannabinoid receptor 1 (CB1) is expressed in the MSDB. Electrophysiological recordings in MHb neurons revealed that endogenously-released 2-AG retrogradely depresses GABA input from the MSDB. This endocannabinoid-mediated depolarization-induced suppression of inhibition (DSI) was limited by monoacylglycerol lipase (MAGL) but not by fatty acid amide hydrolase. Anatomic and optogenetic circuit mapping indicated that MSDB GABA neurons monosynaptically project to cholinergic neurons of the ventral MHb. To test the behavioral significance of this MSDB-MHb endocannabinoid signaling, we induced MSDB-specific knockout of CB1 or MAGL via injection of virally-delivered Cre recombinase into the MSDB of Cnr1loxP/loxP or MgllloxP/loxP mice. Relative to control mice, MSDB-specific knockout of CB1 or MAGL bidirectionally modulated 2-AG signaling in the ventral MHb and led to opposing effects on anxiety- and depressive-like behavior. Thus, depression of synaptic GABA release in the MSDB-ventral MHb pathway may represent a potential mechanism whereby endocannabinoids exert anxiolytic and antidepressant-like effects.

Habenula GPR139 is associated with fear learning in the zebrafish

G-protein coupled receptor 139 (GPR139) is an evolutionarily conserved orphan receptor, predominantly expressing in the habenula of vertebrate species. The habenula has recently been implicated in aversive response and its associated learning. Here, we tested the hypothesis that GPR139 signalling in the habenula may play a role in fear learning in the zebrafish. We examined the effect of intraperitoneal injections of a human GPR139-selective agonist (JNJ-63533054) on alarm substance-induced fear learning using conditioned place avoidance paradigm, where an aversive stimulus is paired with one compartment, while its absence is associated with the other compartment of the apparatus. The results indicate that fish treated with 1 µg/g body weight of GPR139 agonist displayed no difference in locomotor activity and alarm substance-induced fear response. However, avoidance to fear-conditioned compartment was diminished, which suggests that the agonist blocks the consolidation of contextual fear memory. On the other hand, fish treated with 0.1 µg/g body weight of GPR139 agonist spent a significantly longer time in the unconditioned neutral compartment as compared to the conditioned (punished and unpunished) compartments. These results suggest that activation of GPR139 signalling in the habenula may be involved in fear learning and the decision-making process in the zebrafish.

Evidence of Altered Habenular Intrinsic Functional Connectivity in Pediatric ADHD

Objective: The habenula is a small region in the epithalamus that contributes to the regulation of midbrain dopaminergic circuits implicated in attention-deficit hyperactivity disorder (ADHD). This investigation aims to evaluate the intrinsic functional connectivity (iFC) of the habenula in children with ADHD. Method: A total of 112 children (5-9 years; 75 ADHD, 37 healthy comparisons) completed anatomical and resting-state functional magnetic resonance imaging (MRI) scans. Habenula regions of interest (ROIs) were identified individually on normalized T1-weighted anatomical images. Seed-based iFC analyses and group comparisons were conducted for habenula ROIs, as well as thalamic ROIs to test the specificity of habenula findings. Results: Children with ADHD exhibited reduced habenula-putamen iFC compared with healthy comparisons. Group differences in thalamic iFC showed no overlap with habenular findings. Conclusion: These preliminary findings suggest that habenula-putamen iFC may be disrupted in children with ADHD. Further work is needed to confirm and elucidate the role of this circuit in ADHD pathophysiology.

Down-regulation of habenular calcium-dependent secretion activator 2 induces despair-like behavior

Calcium-dependent secretion activator 2 (CAPS2) regulates the trafficking and exocytosis of neuropeptide-containing dense-core vesicles (DCVs). CAPS2 is prominently expressed in the medial habenula (MHb), which is related to depressive behavior; however, how MHb neurons cause depressive symptoms and the role of CAPS2 remains unclear. We hypothesized that dysfunction of MHb CAPS neurons might cause defects in neuropeptide secretion and the activity of monoaminergic centers, resulting in depressive-like behaviors. In this study, we examined (1) CAPS2 expression in the habenula of depression animal models and major depressive disorder patients and (2) the effects of down-regulation of MHb CAPS2 on the animal behaviors, synaptic transmission in the interpeduncular nucleus (IPN), and neuronal activity of monoamine centers. Habenular CAPS2 expression was decreased in the rat chronic restraint stress model, mouse learned helplessness model, and showed tendency to decrease in depression patients who died by suicide. Knockdown of CAPS2 in the mouse habenula evoked despair-like behavior and a reduction of the release of DCVs in the IPN. Neuronal activity of IPN and monoaminergic centers was also reduced. These results implicate MHb CAPS2 as playing a pivotal role in depressive behavior through the regulation of neuropeptide secretion of the MHb-IPN pathway and the activity of monoaminergic centers.

Distinct and Overlapping Patterns of Acute Ethanol-Induced C-Fos Activation in Two Inbred Replicate Lines of Mice Selected for Drinking to High Blood Ethanol Concentrations

The inbred high drinking in the dark (iHDID1 and iHDID2) strains are two replicate lines bred from the parent HS/Npt (HS) line for achieving binge levels of blood ethanol concentration (≥80 mg/dL BEC) in a four-hour period. In this work, we sought to evaluate differences in baseline and ethanol-induced c-Fos activation between the HS, iHDID1, and iHDID2 genetic lines in brain regions known to process the aversive properties of ethanol. Methods: Male and female HS, iHDID1, and iHDID2 mice underwent an IP saline 2 3 g/kg ethanol injection. Brain sections were then stained for c-Fos expression in the basolateral/central amygdala (BLA/CeA), bed nucleus of the stria terminals (BNST), A2, locus coeruleus (LC), parabrachial nucleus (PBN), lateral/medial habenula (LHb/MHb), paraventricular nucleus of the thalamus (PVT), periaqueductal gray (PAG), Edinger–Westphal nuclei (EW), and rostromedial tegmental nucleus (RMTg). Results: The iHDID1 and iHDID2 lines showed similar and distinct patterns of regional c-Fos; however, in no region did the two both significantly differ from the HS line together. Conclusions: Our findings lend further support to the hypothesis the iHDID1 and the iHDID2 lines arrive at a similar behavior phenotype through divergent genetic mechanisms.

Loss of habenular Prkar2a reduces hedonic eating and increases exercise motivation

The habenula (Hb) is a bilateral, evolutionarily conserved epithalamic structure connecting forebrain and midbrain structures that has gained attention for its roles in depression, addiction, rewards processing, and motivation. Of its 2 major subdivisions, the medial Hb (MHb) and lateral Hb (LHb), MHb circuitry and function are poorly understood relative to those of the LHb. Prkar2a codes for cAMP-dependent protein kinase (PKA) regulatory subunit IIα (RIIα), a component of the PKA holoenzyme at the center of one of the major cell-signaling pathways conserved across systems and species. Type 2 regulatory subunits (RIIα, RIIβ) determine the subcellular localization of PKA, and unlike other PKA subunits, Prkar2a has minimal brain expression except in the MHb. We previously showed that RIIα-knockout (RIIα-KO) mice resist diet-induced obesity. In the present study, we report that RIIα-KO mice have decreased consumption of palatable, “rewarding” foods and increased motivation for voluntary exercise. Prkar2a deficiency led to decreased habenular PKA enzymatic activity and impaired dendritic localization of PKA catalytic subunits in MHb neurons. Reexpression of Prkar2a in the Hb rescued this phenotype, confirming differential roles for Prkar2a in regulating the drives for palatable foods and voluntary exercise. Our findings show that in the MHb decreased PKA signaling and dendritic PKA activity decrease motivation for palatable foods, while enhancing the motivation for exercise, a desirable combination of behaviors.

Decreased habenular PKA signaling and altered localization of PKA catalytic subunits in medial habenula dendrites caused by Prkar2a deletion led to increased voluntary running and decreased sucrose solution intake in mice.

Can transsynaptic viral strategies be used to reveal functional aspects of neural circuitry?

Viruses have proved instrumental to elucidating neuronal connectivity relationships in a variety of organisms. Recent advances in genetic technologies have facilitated analysis of neurons directly connected to a defined starter population. These advances have also made viral transneuronal mapping available to the broader neuroscience community, where one-step rabies virus mapping has become routine. This method is commonly used to identify inputs onto defined cell populations, to demonstrate the quantitative proportion of inputs coming from specific brain regions, or to compare input patterns between two or more cell populations. Furthermore, the number of inputs labeled is often assumed to reflect the number of synaptic connections, and these viruses are commonly believed to label strong synapses more efficiently than weak synapses. While these maps are often interpreted to provide a quantitative estimate of the synaptic landscape onto starter cell populations, in fact very little is known about how transneuronal transmission takes place. We do not know how these viruses transmit between neurons, if they display biases in the cell types labeled, or even if transmission is synapse-specific. In this review, we discuss the experimental evidence against or in support of key concepts in viral tracing, focusing mostly on the use of one-step rabies input mapping and related methods. Does spread of these viruses occur specifically through synaptic connections, preferentially through synapses, or non-specifically? How efficient is viral transneuronal transmission, and is this efficiency equal in all cell types? And lastly, to what extent does viral labeling reflect functional connectivity?

The increased density of the habenular neurons, high impulsivity, aggression and resistant fear memory in Disc1-Q31L genetic mouse model of depression

Mood disorders affect nearly 300 million humans worldwide, and it is a leading cause of death from suicide. In the last decade, the habenula has gained increased attention due to its major role to modulate emotional behavior and related psychopathologies, including depression and bipolar disorder, through the modulation of monoamines' neurotransmission. However, it is still unclear which genetic factors may directly affect the function of the habenula and hence, could contribute to the psychopathological mechanisms of mood disorders. Disrupted-In-Schizophrenia-1 (DISC1) gene is among robust gene-candidates predisposing to major depression, bipolar disorder and schizophrenia in humans. DISC1-Q31L, a well-established genetic mouse model of depression, offers a unique opportunity for translational studies. The current study aimed to probe morphological features of the habenula in the DISC1-Q31L mouse line and detect novel behavioral endophenotypes, including the increased emotionality in mutant females, high aggression in mutant males and deficient extinction of fear memory in DISC1 mutant mice of both sexes. The histological analysis found the increased neural density in the lateral and medial habenula in DISC1-Q31L mice regardless of sex, hence, excluding direct association between the habenular neurons and emotionality in mutant females. Altogether, our findings demonstrated, for the first time, the direct impact of the DISC1 gene on the habenular neurons and affective behavior in the DISC1-Q31L genetic mouse line. These new findings suggest that the combination of the DISC1 genetic analysis together with habenular neuroimaging may improve diagnostics of mood disorder in clinical studies.

T-Type Calcium Channels Contribute to Burst Firing in a Subpopulation of Medial Habenula Neurons

Action potential (AP) burst firing caused by the activation of low-voltage-activated T-type Ca²⁺ channels is a unique mode of neuronal firing. T-type channels have been implicated in diverse physiological and pathophysiological processes, including epilepsy, autism, and mood regulation, but the brain structures involved remain incompletely understood. The medial habenula (MHb) is an epithalamic structure implicated in anxiety-like and withdrawal behavior. Previous studies have shown that MHb neurons fire tonic APs at a frequency of ∼2–10 Hz or display depolarized low-amplitude membrane oscillations. Here, we report in C57BL/6J mice that a subpopulation of MHb neurons are capable of firing transient, high-frequency AP bursts mediated by T-type channels. Burst firing was observed following rebounding from hyperpolarizing current injections or during depolarization from hyperpolarized membrane potentials in ∼20% of MHb neurons. It was rarely observed at baseline but could be evoked in MHb neurons displaying different initial activity states. Further, we show that T-type channel mRNA, in particular Ca_v3.1, is expressed in the MHb in both cholinergic and substance P-ergic neurons. Pharmacological Ca_v3 antagonism blocked both burst firing and evoked Ca²⁺ currents in MHb neurons. Additionally, we observed high-frequency AP doublet firing at sustained depolarized membrane potentials that was independent of T-type channels. Thus, there is a greater diversity of AP firing patterns in MHb neurons than previously identified, including T-type channel-mediated burst firing, which may uniquely contribute to behaviors with relevance to neuropsychiatric disease.

The interactions of diet-induced obesity and organophosphate flame retardant exposure on energy homeostasis in adult male and female mice

Previously, sex-dependent alterations in energy homeostasis were reported in adult mice fed a standard chow attributed to exposure to a mixture of organophosphate flame retardants (OPFRs) via estrogen receptors (ERα). In this study, adult male and female mice (C57BL/6J; Taconic) were treated with the same mixture of OPFRs (1 mg/kg each of tricresyl phosphate (TCP), triphenyl phosphate (TPP), and tris(1-3-dichloro-2propyl)phosphate (TDCPP)) for 7 weeks on a low-fat diet (LFD, 10% kcal fat) or a high fat (HFD, 45% kcal fat) in a diet-induced obesity model. Consistent with our previous observations, OPFRs altered weight gain in males, differentially with diet, while females remained unaffected. OPFR treatment also revealed sex-dependent perturbations in metabolic activity. During the night (approximately 0100-0400 hr), males exhibited elevated activity and oxygen consumption, while in females these parameters were decreased, irrespective of diet. OPFR disrupted feeding behavior and abolished diurnal water intake patterns in females while increasing nighttime fluid consumption in males. Despite no marked effect of OPFRs on glucose or insulin tolerance, OPFR treatment altered circulating insulin and leptin in females and ghrelin in males. Data indicate that adult OPFR exposure might influence, and perhaps exacerbate, the effects of diet-induced obesity in adult mice by altering activity, ingestive behavior, and metabolism.

Untangling the dorsal diencephalic conduction system: a review of structure and function of the stria medullaris, habenula and fasciculus retroflexus

The often-overlooked dorsal diencephalic conduction system (DDCS) is a highly conserved pathway linking the basal forebrain and the monoaminergic brainstem. It consists of three key structures; the stria medullaris, the habenula and the fasciculus retroflexus. The first component of the DDCS, the stria medullaris, is a discrete bilateral tract composed of fibers from the basal forebrain that terminate in the triangular eminence of the stalk of the pineal gland, known as the habenula. The habenula acts as a relay hub where incoming signals from the stria medullaris are processed and subsequently relayed to the midbrain and hindbrain monoaminergic nuclei through the fasciculus retroflexus. As a result of its wide-ranging connections, the DDCS has recently been implicated in a wide range of behaviors related to reward processing, aversion and motivation. As such, an understanding of the structure and connections of the DDCS may help illuminate the pathophysiology of neuropsychiatric disorders such as depression, addiction and pain. This is the first review of all three components of the DDCS, the stria medullaris, the habenula and the fasciculus retroflexus, with particular focus on their anatomy, function and development.

Transcriptional and Spatial Resolution of Cell Types in the Mammalian Habenula

The habenula complex is appreciated as a critical regulator of motivated and pathological behavioral states via its output to midbrain nuclei. Despite this, transcriptional definition of cell populations that comprise both the medial habenular (MHb) and lateral habenular (LHb) subregions in mammals remain undefined. To resolve this, we performed single-cell transcriptional profiling and highly multiplexed in situ hybridization experiments of the mouse habenula complex in naive mice and those exposed to an acute aversive stimulus. Transcriptionally distinct neuronal cell types identified within the MHb and LHb, were spatially defined, differentially engaged by aversive stimuli, and had distinct electrophysiological properties. Cell types identified in mice also displayed a high degree of transcriptional similarity to those previously described in zebrafish, highlighting the well-conserved nature of habenular cell types across the phylum. These data identify key molecular targets within habenular cell types and provide a critical resource for future studies.

β4-Nicotinic Receptors Are Critically Involved in Reward-Related Behaviors and Self-Regulation of Nicotine Reinforcement

Nicotine addiction, through smoking, is the principal cause of preventable mortality worldwide. Human genome-wide association studies have linked polymorphisms in the CHRNA5-CHRNA3-CHRNB4 gene cluster, coding for the α5, α3, and β4 nicotinic acetylcholine receptor (nAChR) subunits, to nicotine addiction. β4*nAChRs have been implicated in nicotine withdrawal, aversion, and reinforcement. Here we show that β4*nAChRs also are involved in non-nicotine-mediated responses that may predispose to addiction-related behaviors. β4 knock-out (KO) male mice show increased novelty-induced locomotor activity, lower baseline anxiety, and motivational deficits in operant conditioning for palatable food rewards and in reward-based Go/No-go tasks. To further explore reward deficits we used intracranial self-administration (ICSA) by directly injecting nicotine into the ventral tegmental area (VTA) in mice. We found that, at low nicotine doses, β4KO self-administer less than wild-type (WT) mice. Conversely, at high nicotine doses, this was reversed and β4KO self-administered more than WT mice, whereas β4-overexpressing mice avoided nicotine injections. Viral expression of β4 subunits in medial habenula (MHb), interpeduncular nucleus (IPN), and VTA of β4KO mice revealed dose- and region-dependent differences: β4*nAChRs in the VTA potentiated nicotine-mediated rewarding effects at all doses, whereas β4*nAChRs in the MHb-IPN pathway, limited VTA-ICSA at high nicotine doses. Together, our findings indicate that the lack of functional β4*nAChRs result in deficits in reward sensitivity including increased ICSA at high doses of nicotine that is restored by re-expression of β4*nAChRs in the MHb-IPN. These data indicate that β4 is a critical modulator of reward-related behaviors.SIGNIFICANCE STATEMENT Human genetic studies have provided strong evidence for a relationship between variants in the CHRNA5-CHRNA3-CHRNB4 gene cluster and nicotine addiction. Yet, little is known about the role of β4 nicotinic acetylcholine receptor (nAChR) subunit encoded by this cluster. We investigated the implication of β4*nAChRs in anxiety-, food reward- and nicotine reward-related behaviors. Deletion of the β4 subunit gene resulted in an addiction-related phenotype characterized by low anxiety, high novelty-induced response, lack of sensitivity to palatable food rewards and increased intracranial nicotine self-administration at high doses. Lentiviral vector-induced re-expression of the β4 subunit into either the MHb or IPN restored a "stop" signal on nicotine self-administration. These results suggest that β4*nAChRs provide a promising novel drug target for smoking cessation.

Differential expression of five prosomatostatin genes in the central nervous system of the catshark Scyliorhinus canicula

Five prosomatostatin genes (PSST1, PSST2, PSST3, PSST5, and PSST6) have been recently identified in elasmobranchs (Tostivint et al., General and Comparative Endocrinology, 2019, 279, 139-147). In order to gain insight into the contribution of each somatostatin to specific nervous systems circuits and behaviors in this important jawed vertebrate group, we studied the distribution of neurons expressing PSST mRNAs in the central nervous system (CNS) of Scyliorhinus canicula using in situ hybridization. Additionally, we combined in situ hybridization with tyrosine hydroxylase (TH) immunochemistry for better characterization of PSST1 and PSST6 expressing populations. We observed differential expression of PSST1 and PSST6, which are the most widely expressed PSST transcripts, in cell populations of many CNS regions, including the pallium, subpallium, hypothalamus, diencephalon, optic tectum, midbrain tegmentum, and rhombencephalon. Interestingly, numerous small pallial neurons express PSST1 and PSST6, although in different populations judging from the colocalization of TH immunoreactivity and PSST6 expression but not with PSST1. We observed expression of PSST1 in cerebrospinal fluid-contacting (CSF-c) neurons of the hypothalamic paraventricular organ and the central canal of the spinal cord. Unlike PSST1 and PSST6, PSST2, and PSST3 are only expressed in cells of the hypothalamus and in some hindbrain lateral reticular neurons, and PSST5 in cells of the region of the entopeduncular nucleus. Comparative data of brain expression of PSST genes indicate that the somatostatinergic system of sharks is the most complex reported in any fish.

Anatomical and single-cell transcriptional profiling of the murine habenular complex

The lateral habenula (LHb) is an epithalamic brain structure critical for processing and adapting to negative action outcomes. However, despite the importance of LHb to behavior and the clear anatomical and molecular diversity of LHb neurons, the neuron types of the habenula remain unknown. Here, we use high-throughput single-cell transcriptional profiling, monosynaptic retrograde tracing, and multiplexed FISH to characterize the cells of the mouse habenula. We find five subtypes of neurons in the medial habenula (MHb) that are organized into anatomical subregions. In the LHb, we describe four neuronal subtypes and show that they differentially target dopaminergic and GABAergic cells in the ventral tegmental area (VTA). These data provide a valuable resource for future study of habenular function and dysfunction and demonstrate neuronal subtype specificity in the LHb-VTA circuit.

Brn3/POU-IV-type POU homeobox genes-Paradigmatic regulators of neuronal identity across phylogeny

One approach to understand the construction of complex systems is to investigate whether there are simple design principles that are commonly used in building such a system. In the context of nervous system development, one may ask whether the generation of its highly diverse sets of constituents, that is, distinct neuronal cell types, relies on genetic mechanisms that share specific common features. Specifically, are there common patterns in the function of regulatory genes across different neuron types and are those regulatory mechanisms not only used in different parts of one nervous system, but are they conserved across animal phylogeny? We address these questions here by focusing on one specific, highly conserved and well-studied regulatory factor, the POU homeodomain transcription factor UNC-86. Work over the last 30 years has revealed a common and paradigmatic theme of unc-86 function throughout most of the neuron types in which Caenorhabditis elegans unc-86 is expressed. Apart from its role in preventing lineage reiterations during development, UNC-86 operates in combination with distinct partner proteins to initiate and maintain terminal differentiation programs, by coregulating a vast array of functionally distinct identity determinants of specific neuron types. Mouse orthologs of unc-86, the Brn3 genes, have been shown to fulfill a similar function in initiating and maintaining neuronal identity in specific parts of the mouse brain and similar functions appear to be carried out by the sole Drosophila ortholog, Acj6. The terminal selector function of UNC-86 in many different neuron types provides a paradigm for neuronal identity regulation across phylogeny. This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Invertebrate Organogenesis > Worms Nervous System Development > Vertebrates: Regional Development.

From controlled to compulsive drug-taking: The role of the habenula in addiction

Addiction is now recognized as a neurobiological and cognitive brain disorder and is generally viewed as a switch from recreational or voluntary to compulsive substance use despite aversive consequences. The habenula, composed of medial (MHb) and lateral (LHb) domains, has been implicated in regulating behavioral flexibility and anxiety-related behaviors and is considered a core component of the brain "anti-reward" system. These functions position the habenula to influence voluntary behaviors. Consistent with this view, emerging evidence points to alterations in habenula activity as important factors to contributing the loss of control over the use of drugs of abuse and the emergence of compulsive drug seeking behaviors. In this review, we will discuss the general functions of the MHb and LHb and describe how these functional properties allow this brain region to promote or suppress volitional behaviors. Then, we highlight mechanisms by which drugs of abuse may alter habenular activity, precipitating the emergence of addiction-relevant behavioral abnormalities.

Toward an understanding of the habenula's various roles in human depression

The habenula is an evolutionarily conserved structure in the vertebrate brain. Lesion and electrophysiological studies in animals have suggested that it is involved in the regulation of monoaminergic activity through projection to the brain stem nuclei. Since studies in animal models of depression and human functional imaging have indicated that increased activity of the habenula is associated with depressive phenotypes, this structure has attracted a surge of interest in neuroscience research. According to pathway- and cell-type-specific dissection of habenular function in animals, we have begun to understand how the heterogeneity of the habenula accounts for alteration of diverse physiological functions in depression. Indeed, recent studies have revealed that the subnuclei embedded in the habenula show a wide variety of molecular profiles not only in neurons but also in glial cells implementing the multifaceted regulatory mechanism for output from the habenula. In this review, we overview the known facts on mediolateral subdivision in the habenular structure, then discuss heterogeneity of the habenular structure from the anatomical and functional viewpoint to understand its emerging role in diverse neural functions relevant to depressive phenotypes. Despite the prevalent use of antidepressants acting on monoamine metabolisms, ~30% of patients with major depression are reported to be treatment-resistant. Thus, cellular mechanisms deciphering such diversity in depressive symptoms would be a promising candidate for the development of new antidepressants.

Medial habenula cholinergic signaling regulates cocaine-associated relapse-like behavior

Propensity to relapse, even following long periods of abstinence, is a key feature in substance use disorders. Relapse and relapse‐like behaviors are known to be induced, in part, by re‐exposure to drug‐associated cues. Yet, while many critical nodes in the neural circuitry contributing to relapse have been identified and studied, a full description of the networks driving reinstatement of drug‐seeking behaviors is lacking. One area that may provide further insight to the mechanisms of relapse is the habenula complex, an epithalamic region composed of lateral and medial (MHb) substructures, each with unique cell and target populations. Although well conserved across vertebrate species, the functions of the MHb are not well understood. Recent research has demonstrated that the MHb regulates nicotine aversion and withdrawal. However, it remains undetermined whether MHb function is limited to nicotine and aversive stimuli or if MHb circuit regulates responses to other drugs of abuse. Advances in circuit‐level manipulations now allow for cell‐type and temporally specific manipulations during behavior, specifically in spatially restrictive brain regions, such as the MHb. In this study, we focus on the response of the MHb to reinstatement of cocaine‐associated behavior, demonstrating that cocaine‐primed reinstatement of conditioned place preference engages habenula circuitry. Using chemogenetics, we demonstrate that MHb activity is sufficient to induce reinstatement behavior. Together, these data identify the MHb as a key hub in the circuitry underlying reinstatement and may serve as a target for regulating relapse‐like behaviors.

In vivo Characterization of a Selective, Orally Available, and Brain Penetrant Small Molecule GPR139 Agonist

Recently, our group along with another demonstrated that GPR139 can be activated by L-phenylalanine (L-Phe) and L-tryptophan (L-Trp) at physiologically relevant concentrations. GPR139 is discretely expressed in brain, with highest expression in medial habenula. Not only are the endogenous ligands catecholamine/serotonin precursors, but GPR139 expressing areas can directly/indirectly regulate the activity of catecholamine/serotonin neurons. Thus, GPR139 appears expressed in an interconnected circuit involved in mood, motivation, and anxiety. The aim of this study was to characterize a selective and brain penetrant GPR139 agonist (JNJ-63533054) in relevant in vivo models. JNJ-63533054 was tested for its effect on c-fos activation in the habenula and dorsal striatum. In vivo microdialysis experiments were performed in freely moving rats to measure basal levels of serotonin or dopamine (DA) in prefrontal cortex (mPFC) and nucleus accumbens (NAc). Finally, the compound was profiled in behavioral models of anxiety, despair, and anhedonia. The agonist (10–30 mg/kg, p.o.) did not alter c-fos expression in medial habenula or dorsal striatum nor neurotransmitter levels in mPFC or NAc. JNJ-63533054 (10 mg/kg p.o.) produced an anhedonic-like effect on urine sniffing, but had no significant effect in tail suspension, with no interaction with imipramine, no effect on naloxone place aversion, and no effect on learned helplessness. In the marble burying test, the agonist (10 mg/kg p.o.) produced a small anxiolytic-like effect, with no interaction with fluoxetine, and no effect in elevated plus maze (EPM). Despite GPR139 high expression in medial habenula, an area with connections to limbic and catecholaminergic/serotoninergic areas, the GPR139 agonist had no effect on c-fos in medial habenula. It did not alter catecholamine/serotonin levels and had a mostly silent signal in in vivo models commonly associated with these pathways. The physiological function of GPR139 remains elusive.

The Thalamus Regulates Retinoic Acid Signaling and Development of Parvalbumin Interneurons in Postnatal Mouse Prefrontal Cortex

GABAergic inhibitory neurons in the prefrontal cortex (PFC) play crucial roles in higher cognitive functions. Despite the link between aberrant development of PFC interneurons and a number of psychiatric disorders, mechanisms underlying the development of these neurons are poorly understood. Here we show that the retinoic acid (RA)-degrading enzyme CYP26B1 (cytochrome P450 family 26, subfamily B, member 1) is transiently expressed in the mouse frontal cortex during postnatal development, and that medial ganglionic eminence (MGE)-derived interneurons, particularly in parvalbumin (PV)-expressing neurons, are the main cell type that has active RA signaling during this period. We found that frontal cortex-specific Cyp26b1 knock-out mice had an increased density of PV-expressing, but not somatostatin-expressing, interneurons in medial PFC, indicating a novel role of RA signaling in controlling PV neuron development. The initiation of Cyp26b1 expression in neonatal PFC coincides with the establishment of connections between the thalamus and the PFC. We found that these connections are required for the postnatal expression of Cyp26b1 in medial PFC. In addition to this region-specific role in postnatal PFC that regulates RA signaling and PV neuron development, the thalamocortical connectivity had an earlier role in controlling radial dispersion of MGE-derived interneurons throughout embryonic neocortex. In summary, our results suggest that the thalamus plays multiple, temporally separate roles in interneuron development in the PFC.

Kisspeptin Neurons in the Arcuate Nucleus of the Hypothalamus Orchestrate Circadian Rhythms and Metabolism

Successful reproduction in female mammals is precisely timed and must be able to withstand the metabolic demand of pregnancy and lactation. We show that kisspeptin-expressing neurons in the arcuate hypothalamus (Kiss1^ARH) of female mice control the daily timing of food intake, along with the circadian regulation of locomotor activity, sleep, and core body temperature. Toxin-induced silencing of Kiss1^ARH neurons shifts wakefulness and food consumption to the light phase and induces weight gain. Toxin-silenced mice are less physically active and have attenuated temperature rhythms. Because the rhythm of the master clock in the suprachiasmatic nucleus (SCN) appears to be intact, we hypothesize that Kiss1^ARH neurons signal to neurons downstream of the master clock to modulate the output of the SCN. We conclude that, in addition to their well-established role in regulating fertility, Kiss1^ARH neurons are a critical component of the hypothalamic circadian oscillator network that times overt rhythms of physiology and behavior.

The Role of the Medial Habenula Cholinergic System in Addiction and Emotion-Associated Behaviors

The habenula is a complex nucleus composed of lateral and medial subnuclei, which connect between the limbic forebrain and midbrain. Over the past few years, the lateral habenula has received considerable attention because of its potential roles in cognition and in the pathogenesis of various psychiatric disorders. Unlike extensively studied lateral habenula, anatomically and histologically distinct medial habenula remains largely understudied. The medial habenula can be further subdivided into a dorsal region containing excitatory neurons that express the tachykinin neuropeptide substance P and a ventral region containing dense cholinergic neurons. Although the medial habenula is the source of one of the major cholinergic pathways in the brain, relatively few studies have been conducted to understand its roles. Recently, however, the medial habenula cholinergic system has attracted more attention because of its potential to provide therapeutic targets for the treatment of nicotine withdrawal symptoms, drug addiction, and various mood disorders. Here, we discuss the role of the medial habenula cholinergic system in brain function.

Medial habenula maturational deficits associate with low motivation for voluntary physical activity

The habenula is a small, diencephalic structure comprised of distinct subnuclei which receives inputs from the limbic forebrain and sends projections to various regions in the midbrain, making this region well positioned to influence reward and motivation. Genetic ablation of the dorsal medial habenula is known to decrease voluntary wheel-running in mice. However, the extent to which the medial habenula (MHb) mediates wheel-running motivation in the context of high or low motivation for voluntary physical activity remains to be determined. In so, we utilized 5-week-old female rats selectively bred to voluntarily run high (HVR) or low (LVR) distances in order to determine if inherent differences in medial habenula maturation accompany inherent differences in wheel-running motivation. We report a significantly higher expression of genes associated with MHb development (Brn3a, Nurr1, Tac1, and Kcnip) in HVR versus LVR rats. Furthermore, there was a positive correlation between Brn3a and Nurr1 expression and run distance in HVR, but not LVR rats. Similarly, NeuN and Synapsin 1, markers of neuronal maturation, were higher in HVR compared to LVR rats. Lastly, dendritic density was determined to be higher in the MHb of HVR versus LVR rats, while LVR rats showed a higher percentage of thin spines, suggesting a higher prevalence of immature dendrites in LVR rats. Taken together, the above findings highlight the involvement of MHb in driving the motivation to be physically active. Given pandemic levels of global physical inactivity, the role of the MHb offers a novel potential to improve our global health.

Transcriptomic Characterization of the Human Habenula Highlights Drug Metabolism and the Neuroimmune System

Due to size and accessibility, most information about the habenula is derived from rodent studies. To better understand the molecular signature of the habenula we characterized the genes that have high expression in the habenula. We compared anatomical expression profiles of three normal adult human brains and four fetal brains. We used gene set enrichment analyses to determine if genes annotated to specific molecular functions, cellular components, and biological processes are enriched in the habenula. We also tested gene sets related to depression and addiction to determine if they uniquely involve the habenula. As expected, we observed high habenular expression of GPR151, nicotinic cholinergic receptors, and cilia-associated genes (medial division). Genes identified in genetic studies of smoking and associated with nicotine response were enriched in the habenula. Genes associated with major depressive disorder did not have enriched expression in the habenula but genes negatively correlated with hedonic well-being were, providing a link to anhedonia. We observed enrichment of genes associated with diseases that are comorbid with addictions (hematopoiesis, thrombosis, liver cirrhosis, pneumonia, and pulmonary fibrosis) and depression (rheumatoid arthritis, multiple sclerosis, and kidney disease). These inflammatory diseases mark a neuroimmune signature that is supported by genes associated with mast cells, acute inflammatory response, and leukocyte migration. We also found enrichment of cytochrome p450 genes suggesting the habenula is uniquely sensitive to endogenous and xenobiotic compounds. Our results suggest the habenula receives negative reward signals from immune and drug processing molecules. This is consistent with the habenular role in the "anti-reward" system and suggests it may be a key bridge between autoimmune disorders, drug use, and psychiatric diseases.

Medial Habenula-Interpeduncular Nucleus Circuit Contributes to Anhedonia-Like Behavior in a Rat Model of Depression

The habenula is a nuclear complex composed of the lateral habenula (LHb) and medial habenula (MHb), two distinct structures. Much progress has been made to emphasize the role of the LHb in the pathogenesis of depression. In contrast, relatively less research has focused on the MHb. However, in recent years, the role of the MHb has begun to gain increasing attention. The MHb connects to the interpeduncular nucleus (IPN) both morphologically and functionally. The MHb-IPN pathway plays an important role in regulating higher brain functions, including cognition, reward, and decision making. It indicates a role of the MHb in the pathogenesis of depression. Thus, we investigated the role of the MHb-IPN pathway in depression. MHb metabolic activity was increased in the chronic unpredictable mild stress (CUMS)-exposed rat model of depression. MHb lesions in the CUMS-exposed rats reversed anhedonia-like behavior, as observed in the sucrose preference test, and significantly downregulated the elevated metabolic activity of the IPN. Substance P (SP)-containing neurons of the MHb were found to innervate the IPN and to be the main source of SP in the IPN. SP content of IPN tissue of the CUMS-exposed rats was increased and MHb lesions reversed this change. In the in vitro experiment, firing rate recordings showed that SP perfusion increased the activity of IPN neurons. Our results suggest that hyperactivity of the MHb-IPN circuit is involved in the anhedonia-like behavior of depression, and that SP mediates the effect of the MHb on IPN neurons.

Nonfunctional mutant Wrn protein leads to neurological deficits, neuronal stress, microglial alteration, and immune imbalance in a mouse model of Werner syndrome

Werner syndrome (WS) is a premature aging disorder caused by mutations in a RecQ-family DNA helicase, WRN. Mice lacking part of the helicase domain of the WRN orthologue exhibit many phenotypic features of WS, including metabolic abnormalities and a shorter lifespan. Yet, little is known about the impact of WRN mutations on the central nervous system in both humans and mouse models of WS. In the current study, we have performed a longitudinal behavioral assessment on mice bearing a Wrn helicase deletion. Behavioral tests demonstrated a loss of motor activity and coordination, reduction in perception, increase in repetitive behavior, and deficits in both spatial and social novelty memories in Wrn mutant mice compared to age-matched wild type mice. These neurological deficits were associated with biochemical and histological changes in the brain of aged Wrn mutant mice. Microglia, resident immune cells that regulate neuronal plasticity and function in the brain, were hyper-ramified in multiple regions involved with the behavioral deficits of Wrn mutant mice. Furthermore, western analyses indicated that Wrn mutant mice exhibited an increase of oxidative stress markers in the prefrontal cortex. Supporting these findings, electron microscopy studies revealed increased cellular aging and oxidative stress features, among microglia and neurons respectively, in the prefrontal cortex of aged Wrn mutant mice. In addition, multiplex immunoassay of serum identified significant changes in the expression levels of several pro- and anti-inflammatory cytokines. Taken together, these findings indicate that microglial dysfunction and neuronal oxidative stress, associated with peripheral immune system alterations, might be important driving forces leading to abnormal neurological symptoms in WS thus suggesting potential therapeutic targets for interventions.

Agmatine modulates spontaneous activity in neurons of the rat medial habenular complex-a relevant mechanism in the pathophysiology and treatment of depression?

The dorsal diencephalic conduction system connects limbic forebrain structures to monaminergic mesencephalic nuclei via a distinct relay station, the habenular complexes. Both habenular nuclei, the lateral as well as the medial nucleus, are considered to play a prominent role in mental disorders like major depression. Herein, we investigate the effect of the polyamine agmatine on the electrical activity of neurons within the medial habenula in rat. We present evidence that agmatine strongly decreases spontaneous action potential firing of medial habenular neurons by activating I1-type imidazoline receptors. Additionally, we compare the expression patterns of agmatinase, an enzyme capable of inactivating agmatine, in rat and human habenula. In the medial habenula of both species, agmatinase is similarly distributed and observed in neurons and, in particular, in distinct neuropil areas. The putative relevance of these findings in the context of depression is discussed. It is concluded that increased activity of the agmatinergic system in the medial habenula may strengthen midbrain dopaminergic activity. Consequently, the habenular-interpeduncular axis may be dysregulated in patients with major depression.

BRN3-type POU Homeobox Genes Maintain the Identity of Mature Postmitotic Neurons in Nematodes and Mice

Many distinct regulatory factors have been shown to be required for the proper initiation of neuron-type-specific differentiation programs, but much less is known about the regulatory programs that maintain the differentiated state in the adult [1-3]. One possibility is that regulatory factors that initiate a terminal differentiation program during development are continuously required to maintain the differentiated state. Here, we test this hypothesis by investigating the function of two orthologous POU homeobox genes in nematodes and mice. The C. elegans POU homeobox gene unc-86 is a terminal selector that is required during development to initiate the terminal differentiation program of several distinct neuron classes [4-13]. Through post-developmental removal of unc-86 activity, we show here that unc-86 is also continuously required throughout the life of many neuron classes to maintain neuron-class-specific identity features. Similarly, the mouse unc-86 ortholog Brn3a/POU4F1 has been shown to control the initiation of the terminal differentiation program of distinct neuron types across the mouse brain, such as the medial habenular neurons [14-20]. By conditionally removing Brn3a in the adult mouse central nervous system, we show that, like its invertebrate ortholog unc-86, Brn3a is also required for the maintenance of terminal identity features of medial habenular neurons. In addition, Brn3a is required for the survival of these neurons, indicating that identity maintenance and survival are genetically linked. We conclude that the continuous expression of transcription factors is essential for the active maintenance of the differentiated state of a neuron across phylogeny.

Chrna5-Expressing Neurons in the Interpeduncular Nucleus Mediate Aversion Primed by Prior Stimulation or Nicotine Exposure

Genetic studies have shown an association between smoking and variation at the CHRNA5/A3/B4 gene locus encoding the α5, α3, and β4 nicotinic receptor subunits. The α5 receptor has been specifically implicated because smoking-associated haplotypes contain a coding variant in the CHRNA5 gene. The Chrna5/a3/b4 locus is conserved in rodents and the restricted expression of these subunits suggests neural pathways through which the reinforcing and aversive properties of nicotine may be mediated. Here, we show that, in the interpeduncular nucleus (IP), the site of the highest Chrna5 mRNA expression in rodents, electrophysiological responses to nicotinic acetylcholine receptor stimulation are markedly reduced in α5-null mice. IP neurons differ markedly from their upstream ventral medial habenula cholinergic partners, which appear unaltered by loss of α5. To probe the functional role of α5-containing IP neurons, we used BAC recombineering to generate transgenic mice expressing Cre-recombinase from the Chrna5 locus. Reporter expression driven by Chrna5Cre demonstrates that transcription of Chrna5 is regulated independently from the Chrna3/b4 genes transcribed on the opposite strand. Chrna5-expressing IP neurons are GABAergic and project to distant targets in the mesopontine raphe and tegmentum rather than forming local circuits. Optogenetic stimulation of Chrna5-expressing IP neurons failed to elicit physical manifestations of withdrawal. However, after recent prior stimulation or exposure to nicotine, IP stimulation becomes aversive. These results using mice of both sexes support the idea that the risk allele of CHRNA5 may increase the drive to smoke via loss of IP-mediated nicotine aversion.SIGNIFICANCE STATEMENT Understanding the receptors and neural pathways underlying the reinforcing and aversive effects of nicotine may suggest new treatments for tobacco addiction. Part of the individual variability in smoking is associated with specific forms of the α5 nicotinic receptor subunit gene. Here, we show that deletion of the α5 subunit in mice markedly reduces the cellular response to nicotine and acetylcholine in the interpeduncular nucleus (IP). Stimulation of α5-expressing IP neurons using optogenetics is aversive, but this effect requires priming by recent prior stimulation or exposure to nicotine. These results support the idea that the smoking-associated variant of the α5 gene may increase the drive to smoke via loss of IP-mediated nicotine aversion.

Systemic and Intra-Habenular Activation of the Orphan G Protein-Coupled Receptor GPR139 Decreases Compulsive-Like Alcohol Drinking and Hyperalgesia in Alcohol-Dependent Rats

GPR139 is an orphan G protein-coupled receptor (GPCR) that is expressed mainly in the brain, with the highest expression in the medial habenula. The modulation of GPR139 receptor function has been hypothesized to be beneficial in the treatment of some mental disorders, but behavioral studies have not yet provided causal evidence of the role of GPR139 in brain dysfunction. Because of the high expression of GPR139 in the habenula, a critical brain region in addiction, we hypothesized that GPR139 may play role in alcohol dependence. Thus, we tested the effect of GPR139 receptor activation using the selective, brain-penetrant receptor agonist JNJ-63533054 on addiction-like behaviors in alcohol-dependent male rats. Systemic administration of JNJ-63533054 (30 mg/kg but not 10 mg/kg, p.o.) reversed the escalation of alcohol self-administration in alcohol-dependent rats, without affecting water or saccharin intake in dependent rats or alcohol intake in nondependent rats. Moreover, systemic JNJ-63533054 administration decreased withdrawal-induced hyperalgesia, without affecting somatic signs of alcohol withdrawal. Further analysis demonstrated that JNJ-63533054 was effective only in a subgroup of dependent rats that exhibited compulsive-like alcohol drinking. Finally, site-specific microinjection of JNJ-63533054 in the habenula but not interpeduncular nucleus (IPN) reduced both alcohol self-administration and withdrawal-induced hyperalgesia in dependent rats. These results provide robust preclinical evidence that GPR139 receptor activation reverses key addiction-like behaviors in dependent animals, suggest that GPR139 may be a novel target for the treatment of alcohol use disorder, and demonstrate that GPR139 is functionally relevant in regulating mammalian behavior.

Relevance of Rodent Models of Depression in Clinical Practice: Can We Overcome the Obstacles in Translational Neuropsychiatry?

The diagnosis of a mental disorder generally depends on clinical observations and phenomenological symptoms reported by the patient. The definition of a given diagnosis is criteria based and relies on the ability to accurately interpret subjective symptoms and complex behavior. This type of diagnosis comprises a challenge to translate to reliable animal models, and these translational uncertainties hamper the development of new treatments. In this review, we will discuss how depressive-like behavior can be induced in rodents, and the relationship between these models and depression in humans. Specifically, we suggest similarities between triggers of depressive-like behavior in animal models and human conditions known to increase the risk of depression, for example exhaustion and bullying. Although we acknowledge the potential problems in comparing animal findings to human conditions, such comparisons are useful for understanding the complexity of depression, and we highlight the need to develop clinical diagnoses and animal models in parallel to overcome translational uncertainties.

Blocking p38 Signaling Reduces the Activation of Pro-inflammatory Cytokines and the Phosphorylation of p38 in the Habenula and Reverses Depressive-Like Behaviors Induced by Neuroinflammation

Increasing evidence has demonstrated that neuroinflammation contributes to the development of depressive-like behaviors, in both animal models and human patients; however, the brain areas and signaling pathways involved are still elusive. Recent studies have suggested novel roles of the habenula in the onset of depression and other psychiatric disorders; however, there is no evidence for whether the habenula has a function in neuroinflammation-induced depression. Using an animal model of depression, which is induced by the repeated central administration of lipopolysaccharide (LPS), we examined whether cytokine expression and p38 signal activation in the habenula were involved in the depressive-like behaviors. Body weight, saccharin preference test, and tail suspension test were used to measure depressive-like behaviors. Immunohistochemistry, quantitative-polymerase chain reaction (q-PCR), and western blot were used to measure the expression of tumor necrosis factor-α (TNF-α), interleukin-10 (IL-10), and the phosphorylation of p38 in the habenula. The results showed that central LPS administration induced depressive-like behaviors, characterized by anhedonia in the saccharin preference test and increased immobility in the tail suspension test. Central LPS administration also significantly increased the p-p38 level in microglial cells and increased TNF-α expression in the habenula. Treatment with fluoxetine, a widely prescribed antidepressant, or SB203580, a p38-specific inhibitor, reversed the depressive-like behaviors, normalized the alterations in p-p38 and TNF-α levels and increased the levels of the anti-inflammatory cytokine IL-10 in the habenula. The present findings suggest that the habenula is involved in the pathophysiology of behavioral depression induced by neuroinflammation, and the p38 pathway may serve as a novel mechanism-based target for the treatment of inflammation-related depression.

The dorsal diencephalic conduction system in reward processing: Spotlight on the anatomy and functions of the habenular complex

The dorsal diencephalic conduction system (DDC) is a highly conserved pathway in vertebrates that provides a route for the neural information to flow from forebrain to midbrain structures. It contains the bilaterally paired habenular nuclei along with two fiber tracts, the stria medullaris and the fasciculus retroflexus. The habenula is the principal player in mediating the dialogue between forebrain and midbrain regions, and functional abnormalities in this structure have often been attributed to pathologies like mood disorders and substance use disorder. Following Matsumoto and Hikosaka seminal work on the lateral habenula as a source of negative reward signals, the last decade has witnessed a great surge of interest in the role of the DDC in reward-related processes. However, despite significant progress in research, much work remains to unfold the behavioral functions of this intriguing, yet complex, pathway. This review describes the current state of knowledge on the DDC with respect to its anatomy, connectivity, and functions in reward and aversion processes.

Voluntary exercise and depression-like behavior in rodents: are we running in the right direction?

Acute or chronic exposure to stress can increase the risk to develop major depressive disorder, a severe, recurrent and common psychiatric condition. Depression places an enormous social and financial burden on modern society. Although many depressed patients are treated with antidepressants, their efficacy is only modest, underscoring the necessity to develop clinically effective pharmaceutical or behavioral treatments. Exercise training produces beneficial effects on stress-related mental disorders, indicative of clinical potential. The pro-resilient and antidepressant effects of exercise training have been documented for several decades. Nonetheless, the underlying molecular mechanisms and the brain circuitries involved remain poorly understood. Preclinical investigations using voluntary wheel running, a frequently used rodent model that mimics aspects of human exercise training, have started to shed light on the molecular adaptations, signaling pathways and brain nuclei underlying the beneficial effects of exercise training on stress-related behavior. In this review, I highlight several neurotransmitter systems that are putative mediators of the beneficial effects of exercise training on mental health, and review recent rodent studies that utilized voluntary wheel running to promote our understanding of exercise training-induced central adaptations. Advancements in our mechanistic understanding of how exercise training induces beneficial neuronal adaptations will provide a framework for the development of new strategies to treat stress-associated mental illnesses.