Conserved expression of the GPR151 receptor in habenular axonal projections of vertebrates

Proton-sensing ion channels, GPCRs and calcium signaling regulated by them: implications for cancer

Extracellular acidification of tumors is common. Through proton-sensing ion channels or proton-sensing G protein-coupled receptors (GPCRs), tumor cells sense extracellular acidification to stimulate a variety of intracellular signaling pathways including the calcium signaling, which consequently exerts global impacts on tumor cells. Proton-sensing ion channels, and proton-sensing GPCRs have natural advantages as drug targets of anticancer therapy. However, they and the calcium signaling regulated by them attracted limited attention as potential targets of anticancer drugs. In the present review, we discuss the progress in studies on proton-sensing ion channels, and proton-sensing GPCRs, especially emphasizing the effects of calcium signaling activated by them on the characteristics of tumors, including proliferation, migration, invasion, metastasis, drug resistance, angiogenesis. In addition, we review the drugs targeting proton-sensing channels or GPCRs that are currently in clinical trials, as well as the relevant potential drugs for cancer treatments, and discuss their future prospects. The present review aims to elucidate the important role of proton-sensing ion channels, GPCRs and calcium signaling regulated by them in cancer initiation and development. This review will promote the development of drugs targeting proton-sensing channels or GPCRs for cancer treatments, effectively taking their unique advantage as anti-cancer drug targets.

The Roles of Histamine Receptor 1 (hrh1) in Neurotransmitter System Regulation, Behavior, and Neurogenesis in Zebrafish

Histamine receptors mediate important physiological processes and take part in the pathophysiology of different brain disorders. Histamine receptor 1 (HRH1) is involved in the development of neurotransmitter systems, and its role in neurogenesis has been proposed. Altered HRH1 binding and expression have been detected in the brains of patients with schizophrenia, depression, and autism. Our goal was to assess the role of hrh1 in zebrafish development and neurotransmitter system regulation through the characterization of hrh1-/- fish generated by the CRISPR/Cas9 system. Quantitative PCR, in situ hybridization, and immunocytochemistry were used to study neurotransmitter systems and genes essential for brain development. Additionally, we wanted to reveal the role of this histamine receptor in larval and adult fish behavior using several quantitative behavioral methods including locomotion, thigmotaxis, dark flash and startle response, novel tank diving, and shoaling behavior. Hrh1-/- larvae displayed normal behavior in comparison with hrh1+/+ siblings. Interestingly, a transient abnormal expression of important neurodevelopmental markers was evident in these larvae, as well as a reduction in the number of tyrosine hydroxylase 1 (Th1)-positive cells, th1 mRNA, and hypocretin (hcrt)-positive cells. These abnormalities were not detected in adulthood. In summary, we verified that zebrafish lacking hrh1 present deficits in the dopaminergic and hypocretin systems during early development, but those are compensated by the time fish reach adulthood. However, impaired sociability and anxious-like behavior, along with downregulation of choline O-acetyltransferase a and LIM homeodomain transcription factor Islet1, were displayed by adult fish.

New medications development for smoking cessation

Diseases associated with nicotine dependence in the form of habitual tobacco use are a major cause of premature death in the United States. The majority of tobacco smokers will relapse within the first month of attempted abstinence. Smoking cessation agents increase the likelihood that smokers can achieve long-term abstinence. Nevertheless, currently available smoking cessation agents have limited utility and fail to prevent relapse in the majority of smokers. Pharmacotherapy is therefore an effective strategy to aid smoking cessation efforts but considerable risk of relapse persists even when the most efficacious medications currently available are used. The past decade has seen major breakthroughs in our understanding of the molecular, cellular, and systems-level actions of nicotine in the brain that contribute to the development and maintenance of habitual tobacco use. In parallel, large-scale human genetics studies have revealed allelic variants that influence vulnerability to tobacco use disorder. These advances have revealed targets for the development of novel smoking cessation agents. Here, we summarize current efforts to develop smoking cessation therapeutics and highlight opportunities for future efforts.

Understanding the habenula: A major node in circuits regulating emotion and motivation

Over the last decade, the understanding of the habenula has rapidly advanced from being an understudied brain area with the Latin name 'habena" meaning "little rein", to being considered a "major rein" in the control of key monoaminergic brain centers. This ancient brain structure is a strategic node in the information flow from fronto-limbic brain areas to brainstem nuclei. As such, it plays a crucial role in regulating emotional, motivational, and cognitive behaviors and has been implicated in several neuropsychiatric disorders, including depression and addiction. This review will summarize recent findings on the medial (MHb) and lateral (LHb) habenula, their topographical projections, cell types, and functions. Additionally, we will discuss contemporary efforts that have uncovered novel molecular pathways and synaptic mechanisms with a focus on MHb-Interpeduncular nucleus (IPN) synapses. Finally, we will explore the potential interplay between the habenula's cholinergic and non-cholinergic components in coordinating related emotional and motivational behaviors, raising the possibility that these two pathways work together to provide balanced roles in reward prediction and aversion, rather than functioning independently.

G protein-coupled receptor 151 regulates glucose metabolism and hepatic gluconeogenesis

Human genetics has been instrumental in identification of genetic variants linked to type 2 diabetes. Recently a rare, putative loss-of-function mutation in the orphan G-protein coupled receptor 151 (GPR151) was found to be associated with lower odds ratio for type 2 diabetes, but the mechanism behind this association has remained elusive. Here we show that Gpr151 is a fasting- and glucagon-responsive hepatic gene which regulates hepatic gluconeogenesis. Gpr151 ablation in mice leads to suppression of hepatic gluconeogenesis genes and reduced hepatic glucose production in response to pyruvate. Importantly, the restoration of hepatic Gpr151 levels in the Gpr151 knockout mice reverses the reduced hepatic glucose production. In this work, we establish a previously unknown role of Gpr151 in the liver that provides an explanation to the lowered type 2 diabetes risk in individuals with nonsynonymous mutations in GPR151.

The mu opioid receptor and the orphan receptor GPR151 contribute to social reward in the habenula

The mu opioid receptor (MOR) and the orphan GPR151 receptor are inhibitory G protein coupled receptors that are enriched in the habenula, a small brain region involved in aversion processing, addiction and mood disorders. While MOR expression in the brain is widespread, GPR151 expression is restricted to the habenula. In a previous report, we created conditional ChrnB4-Cre × Oprm1^fl/fl (so-called B4MOR) mice, where MORs are deleted specifically in Chrnb4-positive neurons restricted to the habenula, and shown a role for these receptors in naloxone aversion. Here we characterized the implication of habenular MORs in social behaviors. B4MOR^-/- mice and B4MOR^+/+ mice were compared in several social behavior measures, including the chronic social stress defeat (CSDS) paradigm, the social preference (SP) test and social conditioned place preference (sCPP). In the CSDS, B4MOR^-/- mice showed lower preference for the social target (unfamiliar mouse of a different strain) at baseline, providing a first indication of deficient social interactions in mice lacking habenular MORs. In the SP test, B4MOR^-/- mice further showed reduced sociability for an unfamiliar conspecific mouse. In the sCPP, B4MOR^-/- mice also showed impaired place preference for their previous familiar littermates after social isolation. We next created and tested Gpr151^-/- mice in the SP test, and also found reduced social preference compared to Gpr151^+/+ mice. Altogether our results support the underexplored notion that the habenula regulates social behaviors. Also, our data suggest that the inhibitory habenular MOR and GPR151 receptors normally promote social reward, possibly by dampening the aversive habenula activity.

Circuits regulating pleasure and happiness - focus on potential biomarkers for circuitry including the habenuloid complex

INTRODUCTION

The multiplicity and complexity of the neuronal connections in the central nervous system make it difficult to disentangle circuits that play an essential role in the development or treatment of (neuro)psychiatric disorders. By choosing the evolutionary development of the forebrain as a starting point, a certain order in the connections can be created. The dorsal diencephalic connection (DDC) system can be applied for the development of biomarkers that can predict treatment response.

MATERIALS AND METHODS

After providing a brief introduction to the theory, we examined neuroanatomical publications on the connectivity of the DDC system. We then searched for neurochemical components that are specific for the habenula.

RESULTS AND DISCUSSION

The best strategy to find biomarkers that reflect the function of the habenular connection is to use genetic variants of receptors, transporters or enzymes specific to this complex. By activating these with probes and measuring the response in people with different functional genotypes, the usefulness of biomarkers can be assessed.

CONCLUSIONS

The most promising biomarkers in this respect are those linked to activation or inhibition of the nicotine receptor, dopamine D4 receptor, μ-opioid receptor and also those of the functioning of habenular glia cells (astrocytes and microglia).

Analyzing human knockouts to validate GPR151 as a therapeutic target for reduction of body mass index

Novel drug targets for sustained reduction in body mass index (BMI) are needed to curb the epidemic of obesity, which affects 650 million individuals worldwide and is a causal driver of cardiovascular and metabolic disease and mortality. Previous studies reported that the Arg95Ter nonsense variant of GPR151, an orphan G protein-coupled receptor, is associated with reduced BMI and reduced risk of Type 2 Diabetes (T2D). Here, we further investigate GPR151 with the Pakistan Genome Resource (PGR), which is one of the largest exome biobanks of human homozygous loss-of-function carriers (knockouts) in the world. Among PGR participants, we identify eleven GPR151 putative loss-of-function (plof) variants, three of which are present at homozygosity (Arg95Ter, Tyr99Ter, and Phe175LeufsTer7), with a cumulative allele frequency of 2.2%. We confirm these alleles in vitro as loss-of-function. We test if GPR151 plof is associated with BMI, T2D, or other metabolic traits and find that GPR151 deficiency in complete human knockouts is not associated with clinically significant differences in these traits. Relative to Gpr151+/+ mice, Gpr151-/- animals exhibit no difference in body weight on normal chow and higher body weight on a high-fat diet. Together, our findings indicate that GPR151 antagonism is not a compelling therapeutic approach to treatment of obesity.

Role of Habenula in Social and Reproductive Behaviors in Fish: Comparison With Mammals

Social behaviors such as mating, parenting, fighting, and avoiding are essential functions as a communication tool in social animals, and are critical for the survival of individuals and species. Social behaviors are controlled by a complex circuitry that comprises several key social brain regions, which is called the social behavior network (SBN). The SBN further integrates social information with external and internal factors to select appropriate behavioral responses to social circumstances, called social decision-making. The social decision-making network (SDMN) and SBN are structurally, neurochemically and functionally conserved in vertebrates. The social decision-making process is also closely influenced by emotional assessment. The habenula has recently been recognized as a crucial center for emotion-associated adaptation behaviors. Here we review the potential role of the habenula in social function with a special emphasis on fish studies. Further, based on evolutional, molecular, morphological, and behavioral perspectives, we discuss the crucial role of the habenula in the vertebrate SDMN.

Neurobiological Mechanisms of Nicotine Reward and Aversion

Neuronal nicotinic acetylcholine receptors (nAChRs) regulate the rewarding actions of nicotine contained in tobacco that establish and maintain the smoking habit. nAChRs also regulate the aversive properties of nicotine, sensitivity to which decreases tobacco use and protects against tobacco use disorder. These opposing behavioral actions of nicotine reflect nAChR expression in brain reward and aversion circuits. nAChRs containing α4 and β2 subunits are responsible for the high-affinity nicotine binding sites in the brain and are densely expressed by reward-relevant neurons, most notably dopaminergic, GABAergic, and glutamatergic neurons in the ventral tegmental area. High-affinity nAChRs can incorporate additional subunits, including β3, α6, or α5 subunits, with the resulting nAChR subtypes playing discrete and dissociable roles in the stimulatory actions of nicotine on brain dopamine transmission. nAChRs in brain dopamine circuits also participate in aversive reactions to nicotine and the negative affective state experienced during nicotine withdrawal. nAChRs containing α3 and β4 subunits are responsible for the low-affinity nicotine binding sites in the brain and are enriched in brain sites involved in aversion, including the medial habenula, interpeduncular nucleus, and nucleus of the solitary tract, brain sites in which α5 nAChR subunits are also expressed. These aversion-related brain sites regulate nicotine avoidance behaviors, and genetic variation that modifies the function of nAChRs in these sites increases vulnerability to tobacco dependence and smoking-related diseases. Here, we review the molecular, cellular, and circuit-level mechanisms through which nicotine elicits reward and aversion and the adaptations in these processes that drive the development of nicotine dependence. SIGNIFICANCE STATEMENT: Tobacco use disorder in the form of habitual cigarette smoking or regular use of other tobacco-related products is a major cause of death and disease worldwide. This article reviews the actions of nicotine in the brain that contribute to tobacco use disorder.

Functions of habenula in reproduction and socio-reproductive behaviours

Habenula is an evolutionarily conserved structure in the brain of vertebrates. Recent reports have drawn attention to the habenula as a processing centre for emotional decision-making and its role in psychiatric disorders. Emotional decision-making process is also known to be closely associated with reproductive conditions. The habenula receives innervations from reproductive centres within the brain and signals from key reproductive neuroendocrine regulators such as gonadal sex steroids, gonadotropin-releasing hormone (GnRH), and kisspeptin. In this review, based on morphological, biochemical, physiological, and pharmacological evidence we discuss an emerging role of the habenula in reproduction. Further, we discuss the modulatory role of reproductive endocrine factors in the habenula and their association with socio-reproductive behaviours such as mating, anxiety and aggression.

Update on GPCR-based targets for the development of novel antidepressants

Traditional antidepressants largely interfere with monoaminergic transport or degradation systems, taking several weeks to have their therapeutic actions. Moreover, a large proportion of depressed patients are resistant to these therapies. Several atypical antidepressants have been developed which interact with G protein coupled receptors (GPCRs) instead, as direct targeting of receptors may achieve more efficacious and faster antidepressant actions. The focus of this review is to provide an update on how distinct GPCRs mediate antidepressant actions and discuss recent insights into how GPCRs regulate the pathophysiology of Major Depressive Disorder (MDD). We also discuss the therapeutic potential of novel GPCR targets, which are appealing due to their ligand selectivity, expression pattern, or pharmacological profiles. Finally, we highlight recent advances in understanding GPCR pharmacology and structure, and how they may provide new avenues for drug development.

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.

GPR151 in nociceptors modulates neuropathic pain via regulating P2X3 function and microglial activation

Neuropathic pain is a major health problem that affects up to 7-10% of the population worldwide. Currently, neuropathic pain is difficult to treat due to its elusive mechanisms. Here we report that orphan G protein-coupled receptor 151 (GPR151) in nociceptive sensory neurons controls neuropathic pain induced by nerve injury. GPR151 was mainly expressed in nonpeptidergic C-fiber dorsal root ganglion (DRG) neurons and highly upregulated after nerve injury. Importantly, conditional knockout of Gpr151 in adult nociceptive sensory neurons significantly alleviated chronic constriction injury (CCI)-induced neuropathic pain-like behavior but did not affect basal nociception. Moreover, GPR151 in DRG neurons was required for CCI-induced neuronal hyperexcitability and upregulation of colony-stimulating factor 1 (CSF1), which is necessary for microglial activation in the spinal cord after nerve injury. Mechanistically, GPR151 coupled with P2X3 ion channels and promoted their functional activities in neuropathic pain-like hypersensitivity. Knockout of Gpr151 suppressed P2X3-mediated calcium elevation and spontaneous pain behavior in CCI mice. Conversely, overexpression of Gpr151 significantly enhanced P2X3-mediated calcium elevation and DRG neuronal excitability. Furthermore, knockdown of P2X3 in DRGs reversed CCI-induced CSF1 upregulation, spinal microglial activation, and neuropathic pain-like behavior. Finally, the co-expression of GPR151 and P2X3 was confirmed in small-diameter human DRG neurons, indicating the clinical relevance of our findings. Together, our results suggest that GPR151 in nociceptive DRG neurons plays a key role in the pathogenesis of neuropathic pain and could be a potential target for treating neuropathic pain.

G protein-coupled receptor GPR151 is involved in trigeminal neuropathic pain through the induction of Gβγ/extracellular signal-regulated kinase-mediated neuroinflammation in the trigeminal ganglion

ABSTRACT

Trigeminal nerve injury-induced neuropathic pain is a debilitating chronic orofacial pain syndrome but lacks effective treatment. G protein-coupled receptors (GPCRs), especially orphan GPCRs (oGPCRs) are important therapeutic targets in pain medicine. Here, we screened upregulated oGPCRs in the trigeminal ganglion (TG) after partial infraorbital nerve transection (pIONT) and found that Gpr151 was the most significantly upregulated oGPCRs. Gpr151 mRNA was increased from pIONT day 3 and maintained for more than 21 days. Furthermore, GPR151 was expressed in the neurons of the TG after pIONT. Global mutation or knockdown of Gpr151 in the TG attenuated pIONT-induced mechanical allodynia. In addition, the excitability of TG neurons was increased after pIONT in wild-type (WT) mice, but not in Gpr151-/- mice. Notably, GPR151 bound to Gαi protein, but not Gαq, Gα12, or Gα13, and activated the extracellular signal-regulated kinase (ERK) through Gβγ. Extracellular signal-regulated kinase was also activated by pIONT in the TG of WT mice, but not in Gpr151-/- mice. Gene microarray showed that Gpr151 mutation reduced the expression of a large number of neuroinflammation-related genes that were upregulated in WT mice after pIONT, including chemokines CCL5, CCL7, CXCL9, and CXCL10. The mitogen-activated protein kinase inhibitor (PD98059) attenuated mechanical allodynia and reduced the upregulation of these chemokines after pIONT. Collectively, this study not only revealed the involvement of GPR151 in the maintenance of trigeminal neuropathic pain but also identified GPR151 as a Gαi-coupled receptor to induce ERK-dependent neuroinflammation. Thus, GPR151 may be a potential drug target for the treatment of trigeminal neuropathic pain.

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.

GABAARα2 is Decreased in the Axon Initial Segment of Pyramidal Cells in Specific Areas of the Prefrontal Cortex in Autism

Some forms of Autism Spectrum Disorder, a neurodevelopmental syndrome characterized by impaired communication and social skills as well as repetitive behaviors, are purportedly associated with dysregulation of the excitation/inhibition balance in the cerebral cortex. Through human postmortem tissue analysis, we previously found a significant decrease in the number of a gamma-aminobutyric acid (GABA)ergic interneuron subtype, the chandelier (Ch) cell, in the prefrontal cortex of subjects with autism. Ch cells exclusively target the axon initial segment (AIS) of excitatory pyramidal (Pyr) neurons, and a single Ch cell forms synapses on hundreds of Pyr cells, indicating a possible role in maintaining electrical balance. Thus, we herein investigated this crucial link between Ch and Pyr cells in the anatomy of autism neuropathology by examining GABA receptor protein expression in the Pyr cell AIS in subjects with autism. We collected tissue from the prefrontal cortex (Brodmann Areas (BA) 9, 46, and 47) of 20 subjects with autism and 20 age- and sex-matched control subjects. Immunohistochemical staining with antibodies against the GABA_A receptor subunit α2 (GABA_ARα2) - the subunit most prevalent in the Pyr cell AIS - revealed a significantly decreased GABA_ARα2 protein in the Pyr cell AIS in supragranular layers of prefrontal cortical areas BA9 and BA47 in autism. Downregulated GABA_ARα2 protein in the Pyr cell AIS may result from decreased GABA synthesis in the prefrontal cortex of subjects with autism, and thereby contribute to an excitation/inhibition imbalance. Our findings support the potential for GABA receptor agonists asa therapeutic tool for autism.

Circuits and functions of the lateral habenula in health and in disease

The past decade has witnessed exponentially growing interest in the lateral habenula (LHb) owing to new discoveries relating to its critical role in regulating negatively motivated behaviour and its implication in major depression. The LHb, sometimes referred to as the brain's 'antireward centre', receives inputs from diverse limbic forebrain and basal ganglia structures, and targets essentially all midbrain neuromodulatory systems, including the noradrenergic, serotonergic and dopaminergic systems. Its unique anatomical position enables the LHb to act as a hub that integrates value-based, sensory and experience-dependent information to regulate various motivational, cognitive and motor processes. Dysfunction of the LHb may contribute to the pathophysiology of several psychiatric disorders, especially major depression. Recently, exciting progress has been made in identifying the molecular and cellular mechanisms in the LHb that underlie negative emotional state in animal models of drug withdrawal and major depression. A future challenge is to translate these advances into effective clinical treatments.

The habenular G-protein-coupled receptor 151 regulates synaptic plasticity and nicotine intake

The habenula, an ancient small brain area in the epithalamus, densely expresses nicotinic acetylcholine receptors and is critical for nicotine intake and aversion. As such, identification of strategies to manipulate habenular activity may yield approaches to treat nicotine addiction. Here we show that GPR151, an orphan G-protein-coupled receptor (GPCR) highly enriched in the habenula of humans and rodents, is expressed at presynaptic membranes and synaptic vesicles and associates with synaptic components controlling vesicle release and ion transport. Deletion of Gpr151 inhibits evoked neurotransmission but enhances spontaneous miniature synaptic currents and eliminates short-term plasticity induced by nicotine. We find that GPR151 couples to the G-alpha inhibitory protein Gα_o1 to reduce cyclic adenosine monophosphate (cAMP) levels in mice and in GPR151-expressing cell lines that are amenable to ligand screens. Gpr151- knockout (KO) mice show diminished behavioral responses to nicotine and self-administer greater quantities of the drug, phenotypes rescued by viral reexpression of Gpr151 in the habenula. These data identify GPR151 as a critical modulator of habenular function that controls nicotine addiction vulnerability.

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.

GPR31 and GPR151 are activated under acidic conditions

Recent studies have revealed that not only proton-sensing channels, but also one family of G protein-coupled receptors (GPCRs) comprising OGR1, GPR4, G2A and TDAG8 are responsible for the sensing of extracellular protons, or pH. Here, we report that two other GPCRs, GPR31 and GPR151, were also activated in acidic condition. Elevated pH of assay mixtures resulted in a remarkable increase in [35S]GTPγS binding by GPR31-Giα and GPR151-Giα fusion proteins in a narrow range between pH 6 and 5. Our reporter gene assays with CHO cells expressing recombinant GPR31 or GPR151 also showed that activation was maximal at pH ∼5.8. Although these results from in vitro and cellular assays revealed slightly different pH sensitivities, all of our results indicated that GPR31 and GPR151 sensed extracellular protons equally well as other proton-sensing GPCRs.

The Opioid Crisis and the Future of Addiction and Pain Therapeutics

Opioid misuse and addiction are a public health crisis resulting in debilitation, deaths, and significant social and economic impact. Curbing this crisis requires collaboration among academic, government, and industrial partners toward the development of effective nonaddictive pain medications, interventions for opioid overdose, and addiction treatments. A 2-day meeting, The Opioid Crisis and the Future of Addiction and Pain Therapeutics: Opportunities, Tools, and Technologies Symposium, was held at the National Institutes of Health (NIH) to address these concerns and to chart a collaborative path forward. The meeting was supported by the NIH Helping to End Addiction Long-Term^SM (HEAL) Initiative, an aggressive, trans-agency effort to speed scientific solutions to stem the national opioid crisis. The event was unique in bringing together two research disciplines, addiction and pain, in order to create a forum for crosscommunication and collaboration. The output from the symposium will be considered by the HEAL Initiative; this article summarizes the scientific presentations and key takeaways. Improved understanding of the etiology of acute and chronic pain will enable the discovery of novel targets and regulatable pain circuits for safe and effective therapeutics, as well as relevant biomarkers to ensure adequate testing in clinical trials. Applications of improved technologies including reagents, assays, model systems, and validated probe compounds will likely increase the delivery of testable hypotheses and therapeutics to enable better health outcomes for patients. The symposium goals were achieved by increasing interdisciplinary collaboration to accelerate solutions for this pressing public health challenge and provide a framework for focused efforts within the research community. SIGNIFICANCE STATEMENT: This article summarizes key messages and discussions resulting from a 2-day symposium focused on challenges and opportunities in developing addiction- and pain-related medications. Speakers and attendees came from 40 states in the United States and 15 countries, bringing perspectives from academia, industry, government, and healthcare by researchers, clinicians, regulatory experts, and patient advocates.

Overexpression of Spexin 1 in the Dorsal Habenula Reduces Anxiety in Zebrafish

Spexin (SPX) is an evolutionarily conserved neuropeptide that is expressed in the mammalian brain and peripheral tissue. Two orthologs are present in the teleost, SPX1 and SPX2. SPX1 is involved in reproduction and food intake. Recently, SPX1 neurons have been found to be located in the specific nuclei of dorsal habenula (dHb) and to project into the interpeduncular nucleus (IPN), in which galanin receptor 2a/2b (GALR2a/2b) expression was also observed. This indicates that habenula SPX1 neurons may interact with GALR2a/2b in the IPN; however, the function of SPX1 in the dHb-IPN neuronal circuit remains unknown. To determine the role of SPX1 in the dHb-IPN neural circuit, we generated transgenic zebrafish overexpressing SPX1 specifically in the dHb. We found that transgenic zebrafish overexpressing SPX1 in the dHb had anxiolytic behaviors compared with their wildtype siblings. Furthermore, quantitative PCR revealed that mRNA expression of galr2a and galr2b in the IPN and serotonin-related genes in the raphe was upregulated in the brains of transgenic zebrafish. Taken together, our data suggest that SPX1 function in the dHb-IPN neural circuits is implicated in the regulation of anxiety behaviors via modulation of the serotoninergic system in zebrafish.

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.

Demethylation of G-Protein-Coupled Receptor 151 Promoter Facilitates the Binding of Krüppel-Like Factor 5 and Enhances Neuropathic Pain after Nerve Injury in Mice

G-protein-coupled receptors are considered to be cell-surface sensors of extracellular signals, thereby having a crucial role in signal transduction and being the most fruitful targets for drug discovery. G-protein-coupled receptor 151 (GPR151) was reported to be expressed specifically in the habenular area. Here we report the expression and the epigenetic regulation of GRP151 in the spinal cord after spinal nerve ligation (SNL) and the contribution of GPR151 to neuropathic pain in male mice. SNL dramatically increased GPR151 expression in spinal neurons. GPR151 mutation or spinal inhibition by shRNA alleviated SNL-induced mechanical allodynia and heat hyperalgesia. Interestingly, the CpG island in the GPR151 gene promoter region was demethylated, the expression of DNA methyltransferase 3b (DNMT3b) was decreased, and the binding of DNMT3b with GPR151 promoter was reduced after SNL. Overexpression of DNMT3b in the spinal cord decreased GPR151 expression and attenuated SNL-induced neuropathic pain. Furthermore, Krüppel-like factor 5 (KLF5), a transcriptional factor of the KLF family, was upregulated in spinal neurons, and the binding affinity of KLF5 with GPR151 promoter was increased after SNL. Inhibition of KLF5 reduced GPR151 expression and attenuated SNL-induced pain hypersensitivity. Further mRNA microarray analysis revealed that mutation of GPR151 reduced the expression of a variety of pain-related genes in response to SNL, especially mitogen-activated protein kinase (MAPK) signaling pathway-associated genes. This study reveals that GPR151, increased by DNA demethylation and the enhanced interaction with KLF5, contributes to the maintenance of neuropathic pain via increasing MAPK pathway-related gene expression.SIGNIFICANCE STATEMENT G-protein-coupled receptors (GPCRs) are targets of various clinically approved drugs. Here we report that SNL increased GPR151 expression in the spinal cord, and mutation or inhibition of GPR151 alleviated SNL-induced neuropathic pain. In addition, SNL downregulated the expression of DNMT3b, which caused demethylation of GPR151 gene promoter, facilitated the binding of transcriptional factor KLF5 with the GPR151 promoter, and further increased GPR151 expression in spinal neurons. The increased GPR151 may contribute to the pathogenesis of neuropathic pain via activating MAPK signaling and increasing pain-related gene expression. Our study reveals an epigenetic mechanism underlying GPR151 expression and suggests that targeting GPR151 may offer a new strategy for the treatment of neuropathic pain.

Expression map of 78 brain-expressed mouse orphan GPCRs provides a translational resource for neuropsychiatric research

Orphan G-protein-coupled receptors (oGPCRs) possess untapped potential for drug discovery. In the brain, oGPCRs are generally expressed at low abundance and their function is understudied. Expression profiling is an essential step to position oGPCRs in brain function and disease, however public databases provide only partial information. Here, we fine-map expression of 78 brain-oGPCRs in the mouse, using customized probes in both standard and supersensitive in situ hybridization. Images are available at http://ogpcr-neuromap.douglas.qc.ca. This searchable database contains over 8000 coronal brain sections across 1350 slides, providing the first public mapping resource dedicated to oGPCRs. Analysis with public mouse (60 oGPCRs) and human (56 oGPCRs) genome-wide datasets identifies 25 oGPCRs with potential to address emotional and/or cognitive dimensions of psychiatric conditions. We probe their expression in postmortem human brains using nanoString, and included data in the resource. Correlating human with mouse datasets reveals excellent suitability of mouse models for oGPCRs in neuropsychiatric research.

Aliza Ehrlich et al. report the fine-mapping of orphan GPCR (oGPCR) transcripts in the mouse brain using in situ hybridization and provide a public resource for data mining. The authors also mapped 25 selected oGPCRs in human brains, identifying oGPCRs with high correlation between species and potential roles in neuropsychiatric disorders.

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.

Comprehensive Identification and Spatial Mapping of Habenular Neuronal Types Using Single-Cell RNA-Seq

The identification of cell types and marker genes is critical for dissecting neural development and function, but the size and complexity of the brain has hindered the comprehensive discovery of cell types. We combined single-cell RNA-seq (scRNA-seq) with anatomical brain registration to create a comprehensive map of the zebrafish habenula, a conserved forebrain hub involved in pain processing and learning. Single-cell transcriptomes of ∼13,000 habenular cells with 4× cellular coverage identified 18 neuronal types and dozens of marker genes. Registration of marker genes onto a reference atlas created a resource for anatomical and functional studies and enabled the mapping of active neurons onto neuronal types following aversive stimuli. Strikingly, despite brain growth and functional maturation, cell types were retained between the larval and adult habenula. This study provides a gene expression atlas to dissect habenular development and function and offers a general framework for the comprehensive characterization of other brain regions.

Neuropeptidomics of the Rat Habenular Nuclei

Conserved across vertebrates, the habenular nuclei are a pair of small symmetrical structures in the epithalamus. The nuclei functionally link the forebrain and midbrain by receiving input from and projecting to several brain regions. Each habenular nucleus comprises two major asymmetrical subnuclei, the medial and lateral habenula. These subnuclei are associated with different physiological processes and disorders, such as depression, nicotine addiction, and encoding aversive stimuli or omitting expected rewarding stimuli. Elucidating the functions of the habenular nuclei at the molecular level requires knowledge of their neuropeptide complement. In this work, three mass spectrometry (MS) techniques-liquid chromatography (LC) coupled to Orbitrap tandem MS (MS/MS), LC coupled to Fourier transform (FT)-ion cyclotron resonance (ICR) MS/MS, and matrix-assisted laser desorption/ionization (MALDI) FT-ICR MS-were used to uncover the neuropeptide profiles of the rodent medial and lateral habenula. With the assistance of tissue stabilization and bioinformatics, a total of 262 and 177 neuropeptides produced from 27 and 20 prohormones were detected and identified from the medial and lateral habenula regions, respectively. Among these neuropeptides, 136 were exclusively found in the medial habenula, and 51 were exclusively expressed in the lateral habenula. Additionally, novel sites of sulfation, a rare post-translational modification, on the secretogranin I prohormone are identified. The results demonstrate that these two small brain nuclei have a rich and differentiated peptide repertoire, with this information enabling a range of follow-up studies.

Monosynaptic retrograde tracing of neurons expressing the G-protein coupled receptor Gpr151 in the mouse brain

GPR151 is a G‐protein coupled receptor for which the endogenous ligand remains unknown. In the nervous system of vertebrates, its expression is enriched in specific diencephalic structures, where the highest levels are observed in the habenular area. The habenula has been implicated in a range of different functions including behavioral flexibility, decision making, inhibitory control, and pain processing, which makes it a promising target for treating psychiatric and neurological disease. This study aimed to further characterize neurons expressing the Gpr151 gene, by tracing the afferent connectivity of this diencephalic cell population. Using pseudotyped rabies virus in a transgenic Gpr151‐Cre mouse line, monosynaptic afferents of habenular and thalamic Gpr151‐expressing neuronal populations could be visualized. The habenular and thalamic Gpr151 systems displayed both shared and distinct connectivity patterns. The habenular neurons primarily received input from basal forebrain structures, the bed nucleus of stria terminalis, the lateral preoptic area, the entopeduncular nucleus, and the lateral hypothalamic area. The Gpr151‐expressing neurons in the paraventricular nucleus of the thalamus was primarily contacted by medial hypothalamic areas as well as the zona incerta and projected to specific forebrain areas such as the prelimbic cortex and the accumbens nucleus. Gpr151 mRNA was also detected at low levels in the lateral posterior thalamic nucleus which received input from areas associated with visual processing, including the superior colliculus, zona incerta, and the visual and retrosplenial cortices. Knowledge about the connectivity of Gpr151‐expressing neurons will facilitate the interpretation of future functional studies of this receptor.

The role of the habenula in the transition from reward to misery in substance use and mood disorders

The habenula (Hb) is an evolutionary well-conserved structure located in the epithalamus. The Hb receives inputs from the septum, basal ganglia, hypothalamus, anterior cingulate and medial prefrontal cortex, and projects to several midbrain centers, most importantly the inhibitory rostromedial tegmental nucleus (RMTg) and the excitatory interpeduncular nucleus (IPN), which regulate the activity of midbrain monoaminergic nuclei. The Hb is postulated to play a key role in reward and aversion processing across species, including humans, and to be implicated in the different stages of transition from recreational drug intake to addiction and co-morbid mood disorders. The Hb is divided into two anatomically and functionally distinct nuclei, the lateral (LHb) and the medial (MHb), which are primarily involved in reward-seeking (LHb) and misery-fleeing (MHb) behavior by controlling the RMTg and IPN, respectively. This review provides a neuroanatomical description of the Hb, discusses preclinical and human findings regarding its role in the development of addiction and co-morbid mood disorders, and addresses future directions in this area.

Specific connections of the interpeduncular subnuclei reveal distinct components of the habenulopeduncular pathway

The habenulopeduncular pathway consists of the medial habenula (MHb), its output tract, the fasciculus retroflexus, and its principal target, the interpeduncular nucleus (IP). Several IP subnuclei have been described, but their specific projections and relationship to habenula inputs are not well understood. Here we have used viral, transgenic, and conventional anterograde and retrograde tract-tracing methods to better define the relationship between the dorsal and ventral MHb, the IP, and the secondary efferent targets of this system. Although prior studies have reported that the IP has ascending projections to ventral forebrain structures, we find that these projections originate almost entirely in the apical subnucleus, which may be more appropriately described as part of the median raphe system. The laterodorsal tegmental nucleus receives inhibitory inputs from the contralateral dorsolateral IP, and mainly excitatory inputs from the ipsilateral rostrolateral IP subnucleus. The midline central gray of the pons and nucleus incertus receive input from the rostral IP, which contains a mix of inhibitory and excitatory neurons, and the dorsomedial IP, which is exclusively inhibitory. The lateral central gray of the pons receives bilateral input from the lateral IP, which in turn receives bilateral input from the dorsal MHb. Taken together with prior studies of IP projections to the raphe, these results form an emerging map of the habenulopeduncular system that has significant implications for the proposed function of the IP in a variety of behaviors, including models of mood disorders and behavioral responses to nicotine.

Grainyhead-like 3 (Grhl3) deficiency in brain leads to altered locomotor activity and decreased anxiety-like behaviors in aged mice

The highly conserved Grainyhead-like (Grhl) family of transcription factors, comprising three members in vertebrates (Grhl1-3), play critical regulatory roles during embryonic development, cellular proliferation, and apoptosis. Although loss of Grhl function leads to multiple neural abnormalities in numerous animal models, a comprehensive analysis of Grhl expression and function in the mammalian brain has not been reported. Here they show that only Grhl3 expression is detectable in the embryonic mouse brain; particularly within the habenula, an organ known to modulate repressive behaviors. Using both Grhl3-knockout mice (Grhl3^-/- ), and brain-specific conditional deletion of Grhl3 in adult mice (Nestin-Cre/Grhl3^flox/flox ), they performed histological expression analyses and behavioral tests to assess long-term effects of Grhl3 loss on motor co-ordination, spatial memory, anxiety, and stress. They found that complete deletion of Grhl3 did not lead to noticeable structural or cell-intrinsic defects in the embryonic brain; however, aged Grhl3 conditional knockout (cKO) mice showed enlarged lateral ventricles and displayed marked changes in motor function and behaviors suggestive of decreased fear and anxiety. They conclude that loss of Grhl3 in the brain leads to significant alterations in locomotor activity and decreased self-inhibition, and as such, these mice may serve as a novel model of human conditions of impulsive behavior or hyperactivity. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 775-788, 2017.

Targeted disruption of the orphan receptor Gpr151 does not alter pain-related behaviour despite a strong induction in dorsal root ganglion expression in a model of neuropathic pain

BACKGROUND

Gpr151 is an orphan GPCR whose function is unknown. The restricted pattern of neuronal expression in the habenula, dorsal horn of the spinal cord and dorsal root ganglion plus homology with the galanin family of receptors imply a role in nociception.

RESULTS

Real-time quantitative RT-PCR demonstrated a 49.9±2.9 fold highly significant (P<0.001) increase in Gpr151 mRNA expression in the dorsal root ganglion 7days after the spared nerve injury model of neuropathic pain. Measures of acute, inflammatory and neuropathic pain behaviours were not significantly different using separate groups of Gpr151 loss-of-function mutant mice and wild-type controls. Galanin at concentrations between 100nM and 10μM did not induce calcium signalling responses in ND7/23 cells transfected with Gpr151.

CONCLUSIONS

Our results indicate that despite the very large upregulation in the DRG after a nerve injury model of neuropathic pain, the Gpr151 orphan receptor does not appear to be involved in the modulation of pain-related behaviours. Further, galanin is unlikely to be an endogenous ligand for Gpr151.

Circuits Regulating Pleasure and Happiness: The Evolution of the Amygdalar-Hippocampal-Habenular Connectivity in Vertebrates

Appetitive-searching (reward-seeking) and distress-avoiding (misery-fleeing) behavior are essential for all free moving animals to stay alive and to have offspring. Therefore, even the oldest ocean-dwelling animal creatures, living about 560 million years ago and human ancestors, must have been capable of generating these behaviors. The current article describes the evolution of the forebrain with special reference to the development of the misery-fleeing system. Although, the earliest vertebrate ancestor already possessed a dorsal pallium, which corresponds to the human neocortex, the structure and function of the neocortex was acquired quite recently within the mammalian evolutionary line. Up to, and including, amphibians, the dorsal pallium can be considered to be an extension of the medial pallium, which later develops into the hippocampus. The ventral and lateral pallium largely go up into the corticoid part of the amygdala. The striatopallidum of these early vertebrates becomes extended amygdala, consisting of centromedial amygdala (striatum) connected with the bed nucleus of the stria terminalis (pallidum). This amygdaloid system gives output to hypothalamus and brainstem, but also a connection with the cerebral cortex exists, which in part was created after the development of the more recent cerebral neocortex. Apart from bidirectional connectivity with the hippocampal complex, this route can also be considered to be an output channel as the fornix connects the hippocampus with the medial septum, which is the most important input structure of the medial habenula. The medial habenula regulates the activity of midbrain structures adjusting the intensity of the misery-fleeing response. Within the bed nucleus of the stria terminalis the human homolog of the ancient lateral habenula-projecting globus pallidus may exist; this structure is important for the evaluation of efficacy of the reward-seeking response. The described organization offers a framework for the regulation of the stress response, including the medial habenula and the subgenual cingulate cortex, in which dysfunction may explain the major symptoms of mood and anxiety disorders.

An essential role of acetylcholine-glutamate synergy at habenular synapses in nicotine dependence

A great deal of interest has been focused recently on the habenula and its critical role in aversion, negative-reward and drug dependence. Using a conditional mouse model of the ACh-synthesizing enzyme choline acetyltransferase (Chat), we report that local elimination of acetylcholine (ACh) in medial habenula (MHb) neurons alters glutamate corelease and presynaptic facilitation. Electron microscopy and immuno-isolation analyses revealed colocalization of ACh and glutamate vesicular transporters in synaptic vesicles (SVs) in the central IPN. Glutamate reuptake in SVs prepared from the IPN was increased by ACh, indicating vesicular synergy. Mice lacking CHAT in habenular neurons were insensitive to nicotine-conditioned reward and withdrawal. These data demonstrate that ACh controls the quantal size and release frequency of glutamate at habenular synapses, and suggest that the synergistic functions of ACh and glutamate may be generally important for modulation of cholinergic circuit function and behavior.

Neuronal connectivity between habenular glutamate-kisspeptin1 co-expressing neurons and the raphe 5-HT system

The habenula, located on the dorsal thalamic surface, is an emotional and reward processing center. As in the mammalian brain, the zebrafish habenula is divided into dorsal (dHb) and ventral (vHb) subdivisions that project to the interpeduncular nucleus and median raphe (MR) respectively. Previously, we have shown that kisspeptin 1 (Kiss1) expressing in the vHb, regulates the serotonin (5‐HT) system in the MR. However, the connectivity between the Kiss1 neurons and the 5‐HT system remains unknown. To resolve this issue, we generated a specific antibody against zebrafish Kiss1 receptor (Kiss‐R1); using this primary antibody we found intense immunohistochemical labeling in the ventro‐anterior corner of the MR (vaMR) but not in 5‐HT neurons, suggesting the potential involvement of interneurons in 5‐HT modulation by Kiss1. Double‐fluorescence labeling showed that the majority of habenular Kiss1 neurons are glutamatergic. In the MR region, Kiss1 fibers were mainly seen in close association with glutamatergic neurons and only scarcely within GABAergic and 5‐HT neurons. Our findings indicate that the habenular Kiss1 neurons potentially modulate the 5‐HT system primarily through glutamatergic neurotransmission via as yet uncharacterized interneurons.

The neuropeptide kisspeptin (Kiss1) play a key role in vertebrate reproduction. We have previously shown modulatory role of habenular Kiss1 in the raphe serotonin (5‐HT) systems. This study proposed that the habenular Kiss1 neurons modulate the 5‐HT system primarily through glutamatergic neurotransmission, which provides an important insight for understanding of the modulation of 5‐HT system by the habenula‐raphe pathway.