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

Expression of HDAC3-Y298H Point Mutant in Medial Habenula Cholinergic Neurons Has No Effect on Cocaine-Induced Behaviors

Histone deacetylase 3 (HDAC3) is one of the most highly expressed HDACs in the brain shown to be a negative regulator of long-term memory formation. HDAC3 has also been shown to be involved in cocaine-associated behaviors, demonstrated by manipulations in the nucleus accumbens. Previous studies have demonstrated that expression of a dominant negative of a key HDAC3 target gene, nuclear receptor subfamily 4 group A member 2 (NR4A2), in cholinergic neurons of the medial habenula (MHb) blocked reinstatement of cocaine-induced conditioned place preference (CPP) as well as cue-induced intravenous self-administration (IVSA). Together, these findings suggested that HDAC3 would also be important for MHb-dependent reinstatement of CPP and IVSA, which we examined in this study. Contrary to our hypothesis, our results found that expression of an HDAC3 deacetylase dead point mutant within the cholinergic neurons of the mouse MHb had no effect on reinstatement or other cocaine-induced behaviors.

Cocaine Blocks Cholinergic Activity in the Medial Habenula Prior to But Not After Induced Preference for the Drug

Descending projections from the medial habenula potently influence brainstem systems associated with reward and mood. Relatedly, the ventral, cholinergic segment of the nucleus has been linked to nicotine and cocaine addiction. Here we report that cocaine has no effect on baseline firing in the ventral medial habenula but entirely blocks the self-sustained activity initiated by endogenous acetylcholine. This effect was not altered by antagonists to dopamine receptors and thus presumably reflects a direct action on cholinergic receptors. Remarkably, cocaine had no effect on endogenous cholinergic activity in mice that had been extinguished from an induced cocaine preference. In all, the drug has potent effects, albeit through an exotic mode of action, on the medial habenula and these are eliminated by prior experience with the drug. These results describe a novel target for cocaine that is plausibly related to the psychological effects of the drug, and an unexpected consequence of earlier use.

Modulating the Pronociceptive Effect of Sleep Deprivation: A Possible Role for Cholinergic Neurons in the Medial Habenula

Sleep deprivation has been shown to exacerbate pain sensitivity and may contribute to the onset of chronic pain, yet the precise neural mechanisms underlying this association remain elusive. In our study, we explored the contribution of cholinergic neurons within the medial habenula (MHb) to hyperalgesia induced by sleep deprivation in rats. Our findings indicate that the activity of MHb cholinergic neurons diminishes during sleep deprivation and that chemogenetic stimulation of these neurons can mitigate the results. Interestingly, we did not find a direct response of MHb cholinergic neurons to pain stimulation. Further investigation identified the interpeduncular nucleus (IPN) and the paraventricular nucleus of the thalamus (PVT) as key players in the pro-nociceptive effect of sleep deprivation. Stimulating the pathways connecting the MHb to the IPN and PVT alleviated the hyperalgesia. These results underscore the important role of MHb cholinergic neurons in modulating pain sensitivity linked to sleep deprivation, highlighting potential neural targets for mitigating sleep deprivation-induced hyperalgesia.

The putative role of the habenula in animal migration

BACKGROUND

When an addicted animal seeks a specific substance, it is based on the perception of internal and external cues that strongly motivate to pursue the acquisition of that compound. In essence, a similar process acts out when an animal leaves its present area to begin its circannual migration. This review article examines the existence of scientific evidence for possible relatedness of migration and addiction by influencing Dorsal Diencephalic Conduction System (DDCS) including the habenula.

METHODS

For this review especially the databases of Pubmed and Embase were frequently and non-systematically searched.

RESULTS

The mechanisms of bird migration have been thoroughly investigated. Especially the mechanism of the circannual biorhythm and its associated endocrine regulation has been well elucidated. A typical behavior called "Zugunruhe" marks the moment of leaving in migratory birds. The role of magnetoreception in navigation has also been clarified in recent years. However, how bird migration is regulated at the neuronal level in the forebrain is not well understood. Among mammals, marine mammals are most similar to birds. They use terrestrial magnetic field when navigating and often bridge long distances between breeding and foraging areas. Population migration is further often seen among the large hoofed mammals in different parts of the world. Importantly, learning processes and social interactions with conspecifics play a major role in these ungulates. Considering the evolutionary development of the forebrain in vertebrates, it can be postulated that the DDCS plays a central role in regulating the readiness and intensity of essential (emotional) behaviors. There is manifold evidence that this DDCS plays an important role in relapse to abuse after prolonged periods of abstinence from addictive behavior. It is also possible that the DDCS plays a role in navigation.

CONCLUSIONS

The role of the DDCS in the neurobiological regulation of bird migration has hardly been investigated. The involvement of this system in relapse to addiction in mammals might suggest to change this. It is recommended that particularly during "Zugunruhe" the role of neuronal regulation via the DDCS will be further investigated.

Neuronal Excitability in the Medial Habenula and Ventral Tegmental Area Is Differentially Modulated by Nicotine Dosage and Menthol in a Sex-Specific Manner

The medial habenula (MHb) has been identified as the limiting factor for nicotine intake and facilitating nicotine withdrawal. However, few studies have assessed MHb neuronal excitability in response to nicotine, and, currently, a gap in knowledge is present for finding behavioral correlates to neuronal excitability in the region. Moreover, no study to date has evaluated sex or nicotine dosage as factors of excitability in the MHb. Here, we utilized an e-vape self-administration (EVSA) model to determine differences between sexes with different nicotine dosages ± menthol. Following this paradigm, we employed patch-clamp electrophysiology to assess key metrics of MHb neuronal excitability in relation to behavioral endpoints. We observed female mice self-administered significantly more than males, regardless of dosage. We also observed a direct correlation between self-administration behavior and MHb excitability with low-dose nicotine + menthol in males. Conversely, a high dose of nicotine ± menthol yields an inverse correlation between excitability and self-administration behavior in males only. In addition, intrinsic excitability in the ventral tegmental area (VTA) does not track with the amount of nicotine self-administered. Rather, they correlate to the active/inactive discrimination of mice. Using fast-scan cyclic voltammetry, we also observed that dopamine release dynamics are linked to reinforcement-related behavior in males and motivation-related behaviors in females. These results point to a sex-specific difference in the activity of the MHb and VTA leading to distinct differences in self-administration behavior. His could lend evidence to clinical observations of smoking and nicotine-use behavior differing between males and females.

Synthesis of α3β4 Nicotinic Acetylcholine Receptor Modulators Derived from Aristoquinoline That Reduce Reinstatement of Cocaine-Seeking Behavior

Growing evidence suggests that inhibition of the α3β4 nicotinic acetylcholine receptor (nAChR) represents a promising therapeutic strategy to treat cocaine use disorder. Recently, aristoquinoline (1), an alkaloid from Aristotelia chilensis, was identified as an α3β4-selective nAChR inhibitor. Here, we prepared 22 derivatives of 1 and evaluated their ability to inhibit the α3β4 nAChR. These studies revealed structure-activity trends and several compounds with increased potency compared to 1 with few off-target liabilities. Additional mechanistic studies indicated that these compounds inhibit the α3β4 nAChR noncompetitively, but do not act as channel blockers, suggesting they are negative allosteric modulators. Finally, using a cocaine-primed reinstatement paradigm, we demonstrated that 1 significantly attenuates drug-seeking behavior in an animal model of cocaine relapse. The results from these studies further support a role for the α3β4 nAChR in the addictive properties of cocaine and highlight the possible utility of aristoquinoline derivatives in treating cocaine use disorder.

Evaluation of eGFP expression in the ChAT-eGFP transgenic mouse brain

BACKGROUND

A historically definitive marker for cholinergic neurons is choline acetyltransferase (ChAT), a synthesizing enzyme for acetylcholine, (ACh), which can be found in high concentrations in cholinergic neurons, both in the central and peripheral nervous systems. ChAT, is produced in the body of the neuron, transported to the nerve terminal (where its concentration is highest), and catalyzes the transfer of an acetyl group from the coenzyme acetyl-CoA to choline, yielding ACh. The creation of bacterial artificial chromosome (BAC) transgenic mice that express promoter-specific fluorescent reporter proteins (green fluorescent protein-[GFP]) provided an enormous advantage for neuroscience. Both in vivo and in vitro experimental methods benefited from the transgenic visualization of cholinergic neurons. Mice were created by adding a BAC clone into the ChAT locus, in which enhanced GFP (eGFP) is inserted into exon 3 at the ChAT initiation codon, robustly and supposedly selectively expressing eGFP in all cholinergic neurons and fibers in the central and peripheral nervous systems as well as in non-neuronal cells.

METHODS

This project systematically compared the exact distribution of the ChAT-eGFP expressing neurons in the brain with the expression of ChAT by immunohistochemistry using mapping and also made comparisons with in situ hybridization (ISH).

RESULTS

We qualitatively described the distribution of ChAT-eGFP neurons in the mouse brain by comparing it with the distribution of immunoreactive neurons and ISH data, paying special attention to areas where the expression did not overlap, such as the cortex, striatum, thalamus and hypothalamus. We found a complete overlap between the transgenic expression of eGFP and the immunohistochemical staining in the areas of the cholinergic basal forebrain. However, in the cortex and hippocampus, we found small neurons that were only labeled with the antibody and not expressed eGFP or vice versa. Most importantly, we found no transgenic expression of eGFP in the lateral dorsal, ventral and dorsomedial tegmental nuclei cholinergic cells.

CONCLUSION

While the majority of the forebrain ChAT expression was aligned in the transgenic animals with immunohistochemistry, other areas of interest, such as the brainstem should be considered before choosing this particular transgenic mouse line.

Sleep-mediated regulation of reward circuits: implications in substance use disorders

Our modern society suffers from both pervasive sleep loss and substance abuse-what may be the indications for sleep on substance use disorders (SUDs), and could sleep contribute to the individual variations in SUDs? Decades of research in sleep as well as in motivated behaviors have laid the foundation for us to begin to answer these questions. This review is intended to critically summarize the circuit, cellular, and molecular mechanisms by which sleep influences reward function, and to reveal critical challenges for future studies. The review also suggests that improving sleep quality may serve as complementary therapeutics for treating SUDs, and that formulating sleep metrics may be useful for predicting individual susceptibility to SUDs and other reward-associated psychiatric diseases.

Habenula bibliometrics: Thematic development and research fronts of a resurgent field

The habenula (Hb) is a small structure of the posterior diencephalon that is highly conserved across vertebrates but nonetheless has attracted relatively little research attention until the past two decades. The resurgent interest is motivated by neurobehavioral studies demonstrating critical functions in a broad spectrum of motivational and cognitive processes, including functions relevant to psychiatric diseases. The Hb is widely conceived as an "anti-reward" center that acts by regulating brain monoaminergic systems. However, there is still no general conceptual framework for habenula research, and no study has focused on uncovering potentially significant but overlooked topics that may advance our understanding of physiological functions or suggest potential clinical applications of Hb-targeted interventions. Using science mapping tools, we quantitatively and qualitatively analyzed the relevant publications retrieved from the Web of Science Core Collection (WoSCC) database from 2002 to 2021. Herein we present an overview of habenula-related publications, reveal primary research trends, and prioritize some key research fronts by complementary bibliometric analysis. High-priority research fronts include Ventral Pallidum, Nucleus Accumbens, Nicotine and MHb, GLT-1, Zebrafish, and GCaMP, Ketamine, Deep Brain Stimulation, and GPR139. The high intrinsic heterogeneity of the Hb, extensive connectivity with both hindbrain and forebrain structures, and emerging associations with all three dimensions of mental disorders (internalizing, externalizing, and psychosis) suggest that the Hb may be the neuronal substrate for a common psychopathology factor shared by all mental illnesses termed the p factor. A future challenge is to explore the therapeutic potential of habenular modulation at circuit, cellular, and molecular levels.

Circadian Influences on the Habenula and Their Potential Contribution to Neuropsychiatric Disorders

The neural circadian system consists of the master circadian clock in the hypothalamic suprachiasmatic nuclei (SCN) communicating time of day cues to the rest of the body including other brain areas that also rhythmically express circadian clock genes. Over the past 16 years, evidence has emerged to indicate that the habenula of the epithalamus is a candidate extra-SCN circadian oscillator. When isolated from the SCN, the habenula sustains rhythms in clock gene expression and neuronal activity, with the lateral habenula expressing more robust rhythms than the adjacent medial habenula. The lateral habenula is responsive to putative SCN output factors as well as light information conveyed to the perihabenula area. Neuronal activity in the lateral habenula is altered in depression and intriguingly disruptions in circadian rhythms can elevate risk of developing mental health disorders including depression. In this review, we will principally focus on how circadian and light signals affect the lateral habenula and evaluate the possibility that alteration in these influences contribute to mental health disorders.

Cocaine induces paradigm-specific changes to the transcriptome within the ventral tegmental area

During the initial stages of drug use, cocaine-induced neuroadaptations within the ventral tegmental area (VTA) are critical for drug-associated cue learning and drug reinforcement processes. These neuroadaptations occur, in part, from alterations to the transcriptome. Although cocaine-induced transcriptional mechanisms within the VTA have been examined, various regimens and paradigms have been employed to examine candidate target genes. In order to identify key genes and biological processes regulating cocaine-induced processes, we employed genome-wide RNA-sequencing to analyze transcriptional profiles within the VTA from male mice that underwent one of four commonly used paradigms: acute home cage injections of cocaine, chronic home cage injections of cocaine, cocaine-conditioning, or intravenous-self administration of cocaine. We found that cocaine alters distinct sets of VTA genes within each exposure paradigm. Using behavioral measures from cocaine self-administering mice, we also found several genes whose expression patterns corelate with cocaine intake. In addition to overall gene expression levels, we identified several predicted upstream regulators of cocaine-induced transcription shared across all paradigms. Although distinct gene sets were altered across cocaine exposure paradigms, we found, from Gene Ontology (GO) term analysis, that biological processes important for energy regulation and synaptic plasticity were affected across all cocaine paradigms. Coexpression analysis also identified gene networks that are altered by cocaine. These data indicate that cocaine alters networks enriched with glial cell markers of the VTA that are involved in gene regulation and synaptic processes. Our analyses demonstrate that transcriptional changes within the VTA depend on the route, dose and context of cocaine exposure, and highlight several biological processes affected by cocaine. Overall, these findings provide a unique resource of gene expression data for future studies examining novel cocaine gene targets that regulate drug-associated behaviors.

An optimized procedure for robust volitional cocaine intake in mice

Substance use disorder (SUD) is a behavioral disorder characterized by volitional drug consumption. Mouse models of SUD allow for the use of molecular, genetic, and circuit-level tools, providing enormous potential for defining the underlying mechanisms of this disorder. However, the relevance of results depends on the validity of the mouse models used. Self-administration models have long been the preferred preclinical model for SUD as they allow for volitional drug consumption, thus providing strong face validity. While previous work has defined the parameters that influence intravenous cocaine self-administration in other species-such as rats and primates-many of these parameters have not been explicitly assessed in mice. In a series of experiments, we showed that commonly used mouse models of self-administration, where behavior is maintained on a fixed-ratio schedule of reinforcement, show similar levels of responding in the presence and absence of drug delivery-demonstrating that it is impossible to determine when drug consumption is and is not volitional. To address these issues, we have developed a novel mouse self-administration procedure where animals do not need to be pretrained on sucrose and behavior is maintained on a variable-ratio schedule of reinforcement. This procedure increases rates of reinforcement behavior, increases levels of drug intake, and results in clearer delineation between drug-reinforced and saline conditions. Together, these data highlight a major issue with fixed-ratio models in mice that complicates subsequent analysis and provide a simple approach to minimize these confounds with variable-ratio schedules of reinforcement. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

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.

Chronic sleep fragmentation enhances habenula cholinergic neural activity

Sleep is essential to emotional health. Sleep disturbance, particularly REM sleep disturbance, profoundly impacts emotion regulation, but the underlying neural mechanisms remain elusive. Here we show that chronic REM sleep disturbance, achieved in mice by chronic sleep fragmentation (SF), enhanced neural activity in the medial habenula (mHb), a brain region increasingly implicated in negative affect. Specifically, after a 5-day SF procedure that selectively fragmented REM sleep, cholinergic output neurons (ChNs) in the mHb exhibited increased spontaneous firing rate and enhanced firing regularity in brain slices. The SF-induced firing changes remained intact upon inhibition of glutamate, GABA, acetylcholine, and histamine receptors, suggesting cell-autonomous mechanisms independent of synaptic transmissions. Moreover, the SF-induced hyperactivity was not because of enhanced intrinsic membrane excitability, but was accompanied by depolarized resting membrane potential in mHb ChNs. Furthermore, inhibition of TASK-3 (KCNK9) channels, a subtype of two-pore domain K⁺ channels, mimicked the SF effects by increasing the firing rate and regularity, as well as depolarizing the resting membrane potential in mHb ChNs in control-sleep mice. These effects of TASK-3 inhibition were absent in SF mice, suggesting reduced TASK-3 activity following SF. By contrast, inhibition of small-conductance Ca²⁺-activated K⁺ (SK) channels did not produce similar effects. Thus, SF compromised TASK-3 function in mHb ChNs, which likely led to depolarized resting membrane potential and increased spontaneous firing. These results not only demonstrate that selective REM sleep disturbance leads to hyperactivity of mHb ChNs, but also identify a key molecular substrate through which REM sleep disturbance may alter affect regulation.

HDAC3 Activity within the Nucleus Accumbens Regulates Cocaine-Induced Plasticity and Behavior in a Cell-Type-Specific Manner

Epigenetic mechanisms regulate processes of neuroplasticity critical to cocaine-induced behaviors. This includes the Class I histone deacetylase (HDAC) HDAC3, known to act as a negative regulator of cocaine-associated memory formation within the nucleus accumbens (NAc). Despite this, it remains unknown how cocaine alters HDAC3-dependent mechanisms. Here, we profiled HDAC3 expression and activity in total NAc mouse tissue following cocaine exposure. Although chronic cocaine did not affect expression of Hdac3 within the NAc, chronic cocaine did affect promoter-specific changes in HDAC3 and H4K8Ac occupancy. These changes in promoter occupancy correlated with cocaine-induced changes in expression of plasticity-related genes. To causally determine whether cocaine-induced plasticity is mediated by HDAC3's deacetylase activity, we overexpressed a deacetylase-dead HDAC3 point mutant (HDAC3-Y298H-v5) within the NAc of adult male mice. We found that disrupting HDAC3's enzymatic activity altered selective changes in gene expression and synaptic plasticity following cocaine exposure, despite having no effects on cocaine-induced behaviors. In further assessing HDAC3's role within the NAc, we observed that chronic cocaine increases Hdac3 expression in Drd1 but not Drd2-cells of the NAc. Moreover, we discovered that HDAC3 acts selectively within D1R cell-types to regulate cocaine-associated memory formation and cocaine-seeking. Overall, these results suggest that cocaine induces cell-type-specific changes in epigenetic mechanisms to promote plasticity important for driving cocaine-related behaviors.

SIGNIFICANCE STATEMENT Drugs of abuse alter molecular mechanisms throughout the reward circuitry that can lead to persistent drug-associated behaviors. Epigenetic regulators are critical drivers of drug-induced changes in gene expression. Here, we demonstrate that the activity of an epigenetic enzyme promotes neuroplasticity within the nucleus accumbens (NAc) critical to cocaine action. In addition, we demonstrate that these changes in epigenetic activity drive cocaine-seeking behaviors in a cell-type-specific manner. These findings are key in understanding and targeting cocaine's impact of neural circuitry and behavior.

A neuroscientist's guide to transgenic mice and other genetic tools

The past decade has produced an explosion in the number and variety of genetic tools available to neuroscientists, resulting in an unprecedented ability to precisely manipulate the genome and epigenome in behaving animals. However, no single resource exists that describes all of the tools available to neuroscientists. Here, we review the genetic, transgenic, and viral techniques that are currently available to probe the complex relationship between genes and cognition. Topics covered include types of traditional transgenic mouse models (knockout, knock-in, reporter lines), inducible systems (Cre-loxP, Tet-On, Tet-Off) and cell- and circuit-specific systems (TetTag, TRAP, DIO-DREADD). Additionally, we provide details on virus-mediated and siRNA/shRNA approaches, as well as a comprehensive discussion of the myriad manipulations that can be made using the CRISPR-Cas9 system, including single base pair editing and spatially- and temporally-regulated gene-specific transcriptional control. Collectively, this review will serve as a guide to assist neuroscientists in identifying and choosing the appropriate genetic tools available to study the complex relationship between the brain and behavior.

Increased cocaine self-administration in rats lacking the serotonin transporter: a role for glutamatergic signaling in the habenula

Serotonin (5-HT) and the habenula (Hb) contribute to motivational and emotional states such as depression and drug abuse. The dorsal raphe nucleus, where 5-HT neurons originate, and the Hb are anatomically and reciprocally interconnected. Evidence exists that 5-HT influences Hb glutamatergic transmission. Using serotonin transporter knockout (SERT^-/- ) rats, which show depression-like behavior and increased cocaine intake, we investigated the effect of SERT reduction on expression of genes involved in glutamate neurotransmission under both baseline conditions as well as after short-access or long-access cocaine (ShA and LgA, respectively) intake. In cocaine-naïve animals, SERT removal led to reduced baseline Hb mRNA levels of critical determinants of glutamate transmission, such as SLC1A2, the main glutamate transporter and N-methyl-D-aspartate (Grin1, Grin2A and Grin2B) as well as α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (Gria1 and Gria2) receptor subunits, with no changes in the scaffolding protein Dlg4. In response to ShA and LgA cocaine intake, SLC1A2 and Grin1 mRNA levels decreased in SERT^+/+ rats to levels equal of those of SERT^-/- rats. Our data reveal that increased extracellular levels of 5-HT modulate glutamate neurotransmission in the Hb, serving as critical neurobiological substrate for vulnerability to cocaine addiction.

Nicotine excites VIP interneurons to disinhibit pyramidal neurons in auditory cortex

Nicotine activates nicotinic acetylcholine receptors and improves cognitive and sensory function, in part by its actions in cortical regions. Physiological studies show that nicotine amplifies stimulus-evoked responses in sensory cortex, potentially contributing to enhancement of sensory processing. However, the role of specific cell types and circuits in the nicotinic modulation of sensory cortex remains unclear. Here, we performed whole-cell recordings from pyramidal (Pyr) neurons and inhibitory interneurons expressing parvalbumin (PV), somatostatin (SOM), and vasoactive intestinal peptide (VIP) in mouse auditory cortex, in vitro. Bath application of nicotine strongly depolarized and excited VIP neurons, weakly depolarized Pyr neurons, and had no effect on the membrane potential of SOM or PV neurons. The use of receptor antagonists showed that nicotine's effects on VIP and Pyr neurons were direct and indirect, respectively. Nicotine also enhanced the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in Pyr, VIP, and SOM, but not PV, cells. Using Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), we show that chemogenetic inhibition of VIP neurons prevents nicotine's effects on Pyr neurons. Since VIP cells preferentially contact other inhibitory interneurons, we suggest that nicotine drives VIP cell firing to disinhibit Pyr cell somata, potentially making Pyr cells more responsive to auditory stimuli. In parallel, activation of VIP cells also directly inhibits Pyr neurons, likely altering integration of other synaptic inputs. These cellular and synaptic mechanisms likely contribute to nicotine's beneficial effects on cognitive and sensory function.

New Insights Into Cholinergic Neuron Diversity

Cholinergic neurons comprise a small population of cells in the striatum but have fundamental roles in fine tuning brain function, and in the etiology of neurological and psychiatric disorders such as Parkinson’s disease (PD) or schizophrenia. The process of developmental cell specification underlying neuronal identity and function is an area of great current interest. There has been significant progress in identifying the developmental origins, commonalities in molecular markers, and physiological properties of the cholinergic neurons. Currently, we are aware of a number of key factors that promote cholinergic fate during development. However, the extent of cholinergic cell diversity is still largely underestimated. New insights into the biological basis of their specification indicate that cholinergic neurons may be far more diverse than previously thought. This review article, highlights the physiological features and the synaptic properties that segregate cholinergic cell subtypes. It provides an accurate picture of cholinergic cell diversity underlying their organization and function in neuronal networks. This review article, also discusses current challenges in deciphering the logic of the cholinergic cell heterogeneity that plays a fundamental role in the control of neural processes in health and disease.

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.

HDAC3-Mediated Repression of the Nr4a Family Contributes to Age-Related Impairments in Long-Term Memory

Aging is accompanied by cognitive deficits, including impairments in long-term memory formation. Understanding the molecular mechanisms that support preserved cognitive function in aged animals is a critical step toward identifying novel therapeutic targets that could improve memory in aging individuals. One potential mechanism is the Nr4a family of genes, a group of CREB-dependent nuclear orphan receptors that have previously been shown to be important for hippocampal memory formation. Here, using a cross-species approach, we tested the role of Nr4a1 and Nr4a2 in age-related memory impairments. Using a rat model designed to identify individual differences in age-related memory impairments, we first identified Nr4a2 as a key gene that fails to be induced by learning in cognitively impaired male aged rats. Next, using a mouse model that allows for genetic manipulations, we determined that histone deacetylase 3 (HDAC3) negatively regulates Nr4a2 in the aged male and female hippocampus. Finally, we show that overexpression of Nr4a1, Nr4a2, or both transcripts in the male mouse dorsal hippocampus can ameliorate age-related impairments in object location memory. Together, our results suggest that Nr4a2 may be a key mechanism that promotes preserved cognitive function in old age, with HDAC3-mediated repression of Nr4a2 contributing to age-related cognitive decline. More broadly, these results indicate that therapeutic strategies to promote Nr4a gene expression or function may be an effective strategy to improve cognitive function in old age.SIGNIFICANCE STATEMENT Aging is accompanied by memory impairments, although there is a great deal of variability in the severity of these impairments. Identifying molecular mechanisms that promote preserved memory or participate in cognitive reserve in old age is important to develop strategies that promote healthy cognitive aging. Here, we show that learning-induced expression of the CREB-regulated nuclear receptor gene Nr4a2 is selectively impaired in aged rats with memory impairments. Further, we show that Nr4a2 is regulated by histone deacetylase HDAC3 in the aged mouse hippocampus. Finally, we demonstrate that hippocampal overexpression of either Nr4a2 or its family member, Nr4a1, can ameliorate age-related memory impairments. This suggests that promoting Nr4a expression may be a novel strategy to improve memory in aging individuals.

Epigenetic regulation of immediate-early gene Nr4a2/Nurr1 in the medial habenula during reinstatement of cocaine-associated behavior

Propensity to relapse following long periods of abstinence is a key feature of substance use disorder. Drugs of abuse, such as cocaine, cause long-term changes in the neural circuitry regulating reward, motivation, and memory processes through dysregulation of various molecular mechanisms, including epigenetic regulation of activity-dependent gene expression. Underlying drug-induced changes to neural circuit function are the molecular mechanisms regulating activity-dependent gene expression. Of note, histone acetyltransferases and histone deacetylases (HDACs), powerful epigenetic regulators of gene expression, are dysregulated following both acute and chronic cocaine exposure and are linked to cocaine-induced changes in neural circuit function. To better understand the effect of drug-induced changes on epigenetic function and behavior, we investigated HDAC3-mediated regulation of Nr4a2/Nurr1 in the medial habenula, an understudied pathway in cocaine-associated behaviors. Nr4a2, a transcription factor critical in cocaine-associated behaviors and necessary for MHb development, is enriched in the cholinergic cell-population of the MHb; yet, the role of NR4A2 within the MHb in the adult brain remains elusive. Here, we evaluated whether epigenetic regulation of Nr4a2 in the MHb has a role in reinstatement of cocaine-associated behaviors. We found that HDAC3 disengages from Nr4a2 in the MHb in response to cocaine-primed reinstatement. Whereas enhancing HDAC3 function in the MHb had no effect on reinstatement, we found, using a dominant-negative splice variant (NURR2C), that loss of NR4A2 function in the MHb blocked reinstatement behaviors. These results show for the first time that regulation of NR4A2 function in the MHb is critical in relapse-like behaviors.

Central GLP-1 receptors: Novel molecular targets for cocaine use disorder

Given that the search for effective pharmacotherapies for cocaine use disorder has, thus far, been fruitless, there remains a critical need for conceptually innovative approaches toward identifying new medications to treat this disease. A better understanding of the neurocircuits and neurobiological mechanisms underlying cocaine taking and seeking may identify molecular substrates that could serve as targets for novel pharmacotherapies to treat cocaine use disorder. Recent preclinical evidence suggests that glucagon-like peptide-1 (GLP-1) receptor agonists could be re-purposed to treat cocaine craving-induced relapse. This review endeavors to comprehensively summarize the current literature investigating the efficacy of GLP-1 receptor agonists in reducing the rewarding and reinforcing effects of cocaine in animal models of cocaine use disorder. The role of central endogenous GLP-1 circuits in voluntary cocaine taking and seeking is also discussed. Behavioral, neurochemical, electrophysiological and molecular biology studies indicate that central GLP-1 receptor activation functionally modulates the mesolimbic reward system and decreases addiction-like phenotypes in rodents. Overall, an emerging preclinical literature provides compelling evidence to advance GLP-1 receptor agonists into clinical trials testing the efficacy of these medications in preventing cocaine craving-induced relapse.

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.

Activity-Dependent Epigenetic Remodeling in Cocaine Use Disorder

Substance use disorder (SUD) is a behavioral disorder characterized by cycles of abstinence, drug seeking, and relapse. SUD is characterized by aberrant learning processes which develop after repeated exposure to drugs of abuse. At the core of this phenotype is the persistence of symptoms, such as craving and relapse to drug seeking, long after the cessation of drug use. The neural basis of these behavioral changes has been linked to dysfunction in neural circuits across the brain; however, the molecular drivers that allow for these changes to persist beyond the lifespan of any individual protein remain opaque. Epigenetic adaptations - where DNA is modified to increase or decrease the probability of gene expression at key genes - have been identified as a mechanism underlying the long-lasting nature of drug-seeking behavior. Thus, to understand SUD, it is critical to define the interplay between neuronal activation and longer-term changes in transcription and epigenetic remodeling and define their role in addictive behaviors. In this review, we discuss the current understanding of drug-induced changes to circuit function, recent discoveries in epigenetic mechanisms that mediate these changes, and, ultimately, how these adaptations drive the persistent nature of relapse, with emphasis on adaptations in models of cocaine use disorder. Understanding the complex interplay between epigenetic gene regulation and circuit activity will be critical in elucidating the neural mechanisms underlying SUD. This, with the advent of novel genetic-based techniques, will allow for the generation of novel therapeutic avenues to improve treatment outcomes in SUD.

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.

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.