Sustained antidepressant effect of ketamine through NMDAR trapping in the LHb.

Ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist1, has revolutionized the treatment of depression because of its potent, rapid and sustained antidepressant effects2-4. Although the elimination half-life of ketamine is only 13 min in mice5, its antidepressant activities can last for at least 24 h6-9. This large discrepancy poses an interesting basic biological question and has strong clinical implications. Here we demonstrate that after a single systemic injection, ketamine continues to suppress burst firing and block NMDARs in the lateral habenula (LHb) for up to 24 h. This long inhibition of NMDARs is not due to endocytosis but depends on the use-dependent trapping of ketamine in NMDARs. The rate of untrapping is regulated by neural activity. Harnessing the dynamic equilibrium of ketamine-NMDAR interactions by activating the LHb and opening local NMDARs at different plasma ketamine concentrations, we were able to either shorten or prolong the antidepressant effects of ketamine in vivo. These results provide new insights into the causal mechanisms of the sustained antidepressant effects of ketamine. The ability to modulate the duration of ketamine action based on the biophysical properties of ketamine-NMDAR interactions opens up new opportunities for the therapeutic use of ketamine.

Astroglial Kir4.1 in the lateral habenula drives neuronal bursts in depression.

Enhanced bursting activity of neurons in the lateral habenula (LHb) is essential in driving depression-like behaviours, but the cause of this increase has been unknown. Here, using a high-throughput quantitative proteomic screen, we show that an astroglial potassium channel (Kir4.1) is upregulated in the LHb in rat models of depression. Kir4.1 in the LHb shows a distinct pattern of expression on astrocytic membrane processes that wrap tightly around the neuronal soma. Electrophysiology and modelling data show that the level of Kir4.1 on astrocytes tightly regulates the degree of membrane hyperpolarization and the amount of bursting activity of LHb neurons. Astrocyte-specific gain and loss of Kir4.1 in the LHb bidirectionally regulates neuronal bursting and depression-like symptoms. Together, these results show that a glia-neuron interaction at the perisomatic space of LHb is involved in setting the neuronal firing mode in models of a major psychiatric disease. Kir4.1 in the LHb might have potential as a target for treating clinical depression.

Ketamine blocks bursting in the lateral habenula to rapidly relieve depression.

The N-methyl-d-aspartate receptor (NMDAR) antagonist ketamine has attracted enormous interest in mental health research owing to its rapid antidepressant actions, but its mechanism of action has remained elusive. Here we show that blockade of NMDAR-dependent bursting activity in the 'anti-reward center', the lateral habenula (LHb), mediates the rapid antidepressant actions of ketamine in rat and mouse models of depression. LHb neurons show a significant increase in burst activity and theta-band synchronization in depressive-like animals, which is reversed by ketamine. Burst-evoking photostimulation of LHb drives behavioural despair and anhedonia. Pharmacology and modelling experiments reveal that LHb bursting requires both NMDARs and low-voltage-sensitive T-type calcium channels (T-VSCCs). Furthermore, local blockade of NMDAR or T-VSCCs in the LHb is sufficient to induce rapid antidepressant effects. Our results suggest a simple model whereby ketamine quickly elevates mood by blocking NMDAR-dependent bursting activity of LHb neurons to disinhibit downstream monoaminergic reward centres, and provide a framework for developing new rapid-acting antidepressants.

Entopeduncular Nucleus Projections to the Lateral Habenula Contribute to Cocaine Avoidance.

The aversive properties associated with drugs of abuse influence both the development of addiction and relapse. Cocaine produces strong aversive effects after rewarding effects wear off, accompanied by increased firing in the lateral habenula (LHb) that contributes to downstream activation of the rostromedial tegmental nucleus (RMTg). However, the sources of this LHb activation are unknown, as the LHb receives many excitatory inputs whose contributions to cocaine aversion remain uncharacterized. Using cFos activation and in vivo electrophysiology in male rats, we demonstrated that the rostral entopeduncular nucleus (rEPN) was the most responsive region to cocaine among LHb afferents examined and that single cocaine infusions induced biphasic responses in rEPN neurons, with inhibition during cocaine's initial rewarding phase transitioning to excitation during cocaine's delayed aversive phase. Furthermore, rEPN lesions reduced cocaine-induced cFos activation by 2-fold in the LHb and by a smaller proportion in the RMTg, while inactivation of the rEPN or the rEPN-LHb pathway attenuated cocaine avoidance behaviors measured by an operant runway task and by conditioned place aversion (CPA). These data show an essential but not exclusive role of rEPN and its projections to the LHb in processing the aversive effects of cocaine, which could serve as a novel target for addiction vulnerability.SIGNIFICANCE STATEMENT Cocaine produces well-known rewarding effects but also strong aversive effects that influence addiction propensity, but whose mechanisms are poorly understood. We had previously reported that the lateral habenula (LHb) is activated by cocaine and contributes to cocaine's aversive effects, and the current findings show that the rostral entopeduncular nucleus (rEPN) is a major contributor to this LHb activation and to conditioned avoidance of cocaine. These findings show a critical, though not exclusive, rEPN role in cocaine's aversive effects, and shed light on the development of addiction.

Acetaldehyde Excitation of Lateral Habenular Neurons via Multiple Cellular Mechanisms.

Acetaldehyde (ACD), the first metabolite of ethanol, is implicated in several of ethanol's actions, including the reinforcing and aversive effects. The neuronal mechanisms underlying ACD's aversive effect, however, are poorly understood. The lateral habenula (LHb), a regulator of midbrain monoaminergic centers, is activated by negative valence events. Although the LHb has been linked to the aversive responses of several abused drugs, including ethanol, little is known about ACD. We, therefore, assessed ACD's action on LHb neurons in rats. The results showed that intraperitoneal injection of ACD increased cFos protein expression within the LHb and that intra-LHb infusion of ACD induced conditioned place aversion in male rats. Furthermore, electrophysiological recording in brain slices of male and female rats showed that bath application of ACD facilitated spontaneous firing and glutamatergic transmission. This effect of ACD was potentiated by an aldehyde dehydrogenase (ALDH) inhibitor, disulfiram (DS), but attenuated by the antagonists of dopamine (DA) receptor (DAR) subtype 1 (SCH23390) and subtype 2 (raclopride), and partly abolished by the pretreatment of DA or DA reuptake blocker (GBR12935; GBR). Moreover, application of ACD initiated a depolarizing inward current (IACD) and enhanced the hyperpolarizing-activated currents in LHb neurons. Bath application of Rp-cAMPs, a selective cAMP-PKA inhibitor, attenuated ACD-induced potentiation of EPSCs and IACD Finally, bath application of ZD7288, a selective blocker of hyperpolarization-activated cyclic nucleotide-gated channels, attenuated ACD-induced potentiation of firing, EPSCs, and IACD These results show that ACD exerts its aversive property by exciting LHb neurons via multiple cellular mechanisms, and new treatments targeting the LHb may be beneficial for alcoholism.SIGNIFICANCE STATEMENT Acetaldehyde (ACD) has been considered aversive peripherally and rewarding centrally. However, whether ACD has a central aversive property is unclear. Here, we report that ACD excites the lateral habenula (LHb), a brain region associated with aversion and negative valence, through multiple cellular and molecular mechanisms. Intra-LHb ACD produces significant conditioned place aversion. These results suggest that ACD's actions on the LHb neurons might contribute to its central aversive property and new treatments targeting the LHb may be beneficial for alcoholism.

Potentiation of glutamatergic synaptic transmission onto lateral habenula neurons following early life stress and intravenous morphine self-administration in rats.

Early life stress presents an important risk factor for drug addiction and comorbid depression and anxiety through persistent effects on the mesolimbic dopamine pathways. Using an early life stress model for child neglect (a single 24 h episode of maternal deprivation, MD) in rats, recent published works from our lab show that MD induces dysfunction in the ventral tegmental area and its negative controller, the lateral habenula (LHb). MD-induced potentiation of glutamatergic synaptic transmission onto LHb neurons shifts the coordination of excitation/inhibition (E/I) balance towards excitation, resulting in an increase in the overall spontaneous neuronal activity with elevation in bursting and tonic firing, and in the intrinsic excitability of LHb neurons in early adolescent male rats. Here, we explored how MD affects intravenous morphine self-administration (MSA) acquisition and sucrose preference as well as glutamatergic synaptic function in LHb neurons of adult male rats self-administering morphine. We found that MD-induced increases in LHb neuronal and glutamatergic synaptic activity and E/I ratio persisted into adulthood. Moreover, MD significantly reduced morphine intake, triggered anhedonia-like behaviour in the sucrose preference test and was associated with persistent glutamatergic potentiation 24 h after the last MSA session. MSA also altered the decay time kinetics of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (AMPAR) currents in LHb neurons of control rats during this time period. Our data highlight that early life stress-induced glutamatergic plasticity in LHb may dampen the positive reinforcing and motivational properties of both natural rewards and opioids, and may contribute to the development of anhedonia and dysphoric states associated with opioids.

Protein profiling in the habenula after chronic (-)-menthol exposure in mice.

The identification of proteins that are altered following nicotine/tobacco exposure can facilitate and positively impact the investigation of related diseases. In this report, we investigated the effects of chronic (-)-menthol exposure in 14 murine brain regions for changes in total ß2 subunit protein levels and changes in epibatidine binding levels using immunoblotting and radioligand binding assays. We identified the habenula as a region of interest due to the region's marked decreases in ß2 subunit and nAChR levels in response to chronic (-)-menthol alone. Thus, we further examined the habenula, a brain region associated with both the reward and withdrawal components of addiction, for additional protein level alterations using mass spectrometry. A total of 552 proteins with altered levels were identified after chronic (-)-menthol exposure. Enriched in the proteins with altered levels after (-)-menthol exposure were proteins associated with signaling, immune systems, RNA regulation, and protein transport. The continuation and expansion of the brain region-specific protein profiling in response to (-)-menthol will provide a better understanding of how this common flavorant in tobacco and e-liquid products may affect addiction and general health.

Genetic inactivation of the sigma-1 chaperone protein results in decreased expression of the R2 subunit of the GABA-B receptor and increased susceptibility to seizures.

There is a growing body of evidence demonstrating the significant involvement of the sigma-1 chaperone protein in the modulation of seizures. Several sigma-1 receptor (Sig1R) ligands have been demonstrated to regulate the seizure threshold in acute and chronic seizure models. However, the mechanism by which Sig1R modulates the excitatory and inhibitory pathways in the brain has not been elucidated. The aim of this study was to compare the susceptibility to seizures of wild type (WT) and Sig1R knockout (Sig1R-/-) mice in intravenous pentylenetetrazol (PTZ) and (+)-bicuculline (BIC) infusion-induced acute seizure and Sig1R antagonist NE-100-induced seizure models. To determine possible molecular mechanisms, we used quantitative PCR, Western blotting and immunohistochemistry to assess the possible involvement of several seizure-related genes and proteins. Peripheral tissue contractile response of WT and Sig1R-/- mice was studied in an isolated vasa deferentia model. The most important finding was the significantly decreased expression of the R2 subunit of the GABA-B receptor in the hippocampus and habenula of Sig1R-/- mice. Our results demonstrated that Sig1R-/- mice have decreased thresholds for PTZ- and BIC-induced tonic seizures. In the NE-100-induced seizure model, Sig1R-/- animals demonstrated lower seizure scores, shorter durations and increased latency times of seizures compared to WT mice. Sig1R-independent activities of NE-100 included downregulation of the gene expression of iNOS and GABA-A γ2 and inhibition of KCl-induced depolarization in both WT and Sig1R-/- animals. In conclusion, the results of this study indicate that the lack of Sig1R resulted in decreased expression of the R2 subunit of the GABA-B receptor and increased susceptibility to seizures. Our results confirm that Sig1R is a significant molecular target for seizure modulation and warrants further investigation for the development of novel anti-seizure drugs.

Activation and Blockade of Serotonin-4 Receptors in the Lateral Habenula Produce Antidepressant Effects in the Hemiparkinsonian Rat.

BACKGROUND: The 5-hydroxytryptamine (5-HT) neurotransmitter system and lateral habenula (LHb) are involved in the regulation of depression, while the mechanisms remain to be clarified. OBJECTIVES: The effects and possible mecha-nism underlying activation or blockade of 5-HT4 receptors (5-HT4Rs) in the LHb in depression were investigated by behavioral and neurochemical methods based on a Parkinson's disease (PD) rat model. METHOD: 6-Hydroxydopamine (6-OHDA) was injected unilaterally into the substantia nigra pars compacta to establish the PD rat model. The depressive-like behaviors were measured by the forced swimming test (FST) and sucrose preference test (SPT). The concentrations of dopamine (DA), noradrenaline (NA) and 5-HT in the related brain regions were measured by a neurochemical method. RESULTS: The 6-OHDA lesions increased the immobility time in the FST and decreased the sucrose consumption in the SPT, suggesting the induction of depressive-like behaviors. Intra-LHb injection of BIMU-8 (5-HT4R agonist) or GR113808 (5-HT4R antagonist) produced antidepressant effects in the lesioned rats. Intra-LHb injection of BIMU-8 significantly increased the DA levels in the medial prefrontal cortex (mPFC) and ventral hippocampus (vHip), increased the 5-HT level in the mPFC and decreased the NA level in the vHip only in the lesioned rats, while intra-LHb injection of GR113808 changed DA, NA and 5-HT levels in the mPFC, LHb and vHip in both sham and the lesioned rats. CONCLUSIONS: All these results suggest that activation or blockade of the LHb 5-HT4Rs produce antidepressant effects in the 6-OHDA-lesioned rats, which are related to the changes of monoamines in the limbic and limbic-related regions.

Effect of chemogenetic inhibition of lateral habenula neuronal activity on cocaine- and food-seeking behaviors in the rat.

A major problem in the treatment of cocaine addiction is high rates of relapse. Relapse is often provoked by acute reexposure to cocaine-associated cues or to cocaine itself. The lateral habenula (LHb), an epithalamic nucleus, regulates midbrain dopaminergic systems that are known to be involved in cocaine taking and seeking behaviors. However, the role of this nucleus in cocaine self-administration and reinstatement of cocaine seeking has not been entirely parsed out. We used an operant self-administration and reinstatement procedure to explore the effect of Designer Receptors Exclusively Activated by Designer Drug (DREADD)-induced transient inhibition of LHb neurons on cocaine taking and seeking. Firstly, rats were injected with adeno-associated viral vectors expressing hM4 Di (a Gi/o -coupled DREADD) into the LHb, trained to self-administer cocaine (0.75 mg/kg/infusion), and the effect of clozapine-N-oxide (an inert ligand that activates DREADDs) was assessed on cocaine self-administration. Secondly, rats were injected with hM4 Di into the LHb, trained to self-administer cocaine; the operant response was extinguished, and cue- and cocaine priming-induced reinstatement was assessed. Thirdly, we tested the generality of the effect of inhibiting LHb neurons by assessing the effect of this manipulation on food-taking and seeking. hM4 Di -induced inhibition of LHb neurons increased cocaine but not food self-administration. In contrast, this manipulation decreased reinstatement of cocaine, but not food-seeking. Taken together, our data suggest that hM4 Di - induced LHb inhibition specifically mediates taking and seeking behaviors reinforced by cocaine but not by natural reinforcers. Further, our data indicate a dissociation in the role of LHb neurons on cocaine self-administration versus reinstatement of cocaine seeking.

Lateral Habenula 5-HT2C Receptor Function Is Altered by Acute and Chronic Nicotine Exposures.

Serotonin (5-HT) is important in some nicotine actions in the CNS. Among all the 5-HT receptors (5-HTRs), the 5-HT2CR has emerged as a promising drug target for smoking cessation. The 5-HT2CRs within the lateral habenula (LHb) may be crucial for nicotine addiction. Here we showed that after acute nicotine tartrate (2 mg/kg, i.p.) exposure, the 5-HT2CR agonist Ro 60-0175 (5-640 µg/kg, i.v.) increased the electrical activity of 42% of the LHb recorded neurons in vivo in rats. Conversely, after chronic nicotine treatment (6 mg/kg/day, i.p., for 14 days), Ro 60-0175 was incapable of affecting the LHb neuronal discharge. Moreover, acute nicotine exposure increased the 5-HT2CR-immunoreactive (IR) area while decreasing the number of 5-HT2CR-IR neurons in the LHb. On the other hand, chronic nicotine increased both the 5-HT2CR-IR area and 5-HT2CR-IR LHb neurons in the LHb. Western blot analysis confirmed these findings and further revealed an increase of 5-HT2CR expression in the medial prefrontal cortex after chronic nicotine exposure not detected by the immunohistochemistry. Altogether, these data show that acute and chronic nicotine exposure differentially affect the central 5-HT2CR function mainly in the LHb and this may be relevant in nicotine addiction and its treatment.

Chronic Nicotine Exposure Alters the Neurophysiology of Habenulo-Interpeduncular Circuitry.

Antagonism of nicotinic acetylcholine receptors (nAChRs) in the medial habenula (MHb) or interpeduncular nucleus (IPN) triggers withdrawal-like behaviors in mice chronically exposed to nicotine, implying that nicotine dependence involves the sensitization of nicotinic signaling. Identification of receptor and/or neurophysiological mechanisms underlying this sensitization is important, as it could promote novel therapeutic strategies to reduce tobacco use. Using an approach involving photoactivatable nicotine, we previously demonstrated that chronic nicotine (cNIC) potently enhances nAChR function in dendrites of MHb neurons. However, whether cNIC modulates downstream components of the habenulo-interpeduncular (Hb-IP) circuit is unknown. In this study, cNIC-mediated changes to Hb-IP nAChR function were examined in mouse (male and female) brain slices using molecular, electrophysiological, and optical techniques. cNIC enhanced action potential firing and modified spike waveform characteristics in MHb neurons. Nicotine uncaging revealed nAChR functional enhancement by cNIC on proximal axonal membranes. Similarly, nAChR-driven glutamate release from MHb axons was enhanced by cNIC. In IPN, the target structure of MHb axons, neuronal morphology, and nAChR expression is complex, with stronger nAChR function in the rostral subnucleus [rostral IPN (IPR)]. As in MHb, cNIC induced strong upregulation of nAChR function in IPN neurons. This, coupled with cNIC-enhanced nicotine-stimulated glutamate release, was associated with stronger depolarization responses to brief (1 ms) nicotine uncaging adjacent to IPR neurons. Together, these results indicate that chronic exposure to nicotine dramatically alters nicotinic cholinergic signaling and cell excitability in Hb-IP circuits, a key pathway involved in nicotine dependence.SIGNIFICANCE STATEMENT This study uncovers several neuropharmacological alterations following chronic exposure to nicotine in a key brain circuit involved in nicotine dependence. These results suggest that smokers or regular users of electronic nicotine delivery systems (i.e., "e-cigarettes") likely undergo sensitization of cholinergic circuitry in the Hb-IP system. Reducing the activity of Hb-IP nAChRs, either volitionally during smoking cessation or inadvertently via receptor desensitization during nicotine intake, may be a key trigger of withdrawal in nicotine dependence. Escalation of nicotine intake in smokers, or tolerance, may involve stimulation of these sensitized cholinergic pathways. Smoking cessation therapeutics are only marginally effective, and by identifying cellular/receptor mechanisms of nicotine dependence, our results take a step toward improved therapeutic approaches for this disorder.

Involvement of the Dorsal Hippocampus 5-HT1A Receptors in the Regulation of Depressive-Like Behaviors in Hemiparkinsonian Rats.

BACKGROUND: Depression is one of the most common neuropsychiatric disturbances in Parkinson's disease (PD), but its pathophysiology is not definite. Lines of evidence have indicated that the hippocampus and serotonin 1A (5-HT1A) receptors are related to the regulation of depression. OBJECTIVE: The purpose of the present study was to observe the effect of 5-HT1A receptors in the dorsal hippocampus (dHIP) on PD-related depression in rats. METHODS: Unilateral 6-hydroxydopamine lesioning of the medial forebrain bundle (MFB) was used to establish the hemiparkinsonian rat model. The effects of intra-dHIP injection of the 5-HT1A receptor -agonist 8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT) or antagonist WAY-100635 on depressive-like behaviors were observed in sucrose preference and forced swim tests in control and lesioned rats. Monoamine levels including dopamine (DA), 5-HT, and noradrenaline (NA) in depression-related brain regions were determined by a neurochemical method in all groups. RESULTS: Behavioral results showed that MFB lesions induced depressive-like behaviors. Intra-dHIP injection of 8-OH-DPAT produced antidepressant effects, while WAY-100635 induced or increased the depressive-like behaviors in both control and the lesioned rats. Neurochemical results found that intra-dHIP injection of 8-OH-DPAT significantly increased DA and 5-HT levels in the medial prefrontal cortex (mPFC), lateral habenula (LHb), ventral hippocampus and amygdala in the lesioned group and decreased NA levels in the mPFC and LHb in the control group. Moreover, after injection of WAY-100635, NA levels in all these regions of the lesioned group were significantly increased. CONCLUSIONS: These findings suggest that hippocampal 5-HT1A receptors regulate depression and PD-related depression by neurochemical mechanisms.

Retrograde inhibition by a specific subset of interpeduncular α5 nicotinic neurons regulates nicotine preference.

Repeated exposure to drugs of abuse can produce adaptive changes that lead to the establishment of dependence. It has been shown that allelic variation in the α5 nicotinic acetylcholine receptor (nAChR) gene CHRNA5 is associated with higher risk of tobacco dependence. In the brain, α5-containing nAChRs are expressed at very high levels in the interpeduncular nucleus (IPN). Here we identified two nonoverlapping α5 + cell populations (α5- Amigo1 and α5- Epyc ) in mouse IPN that respond differentially to nicotine. Chronic nicotine treatment altered the translational profile of more than 1,000 genes in α5- Amigo1 neurons, including neuronal nitric oxide synthase (Nos1) and somatostatin (Sst). In contrast, expression of few genes was altered in the α5- Epyc population. We show that both nitric oxide and SST suppress optically evoked neurotransmitter release from the terminals of habenular (Hb) neurons in IPN. Moreover, in vivo silencing of neurotransmitter release from the α5- Amigo1 but not from the α5- Epyc population eliminates nicotine reward, measured using place preference. This loss of nicotine reward was mimicked by shRNA-mediated knockdown of Nos1 in the IPN. These findings reveal a proaddiction adaptive response to chronic nicotine in which nitric oxide and SST are released by a specific α5+ neuronal population to provide retrograde inhibition of the Hb-IPN circuit and thereby enhance the motivational properties of nicotine.

Acute and Chronic Nicotine Exposures Differentially Affect Central Serotonin 2A Receptor Function: Focus on the Lateral Habenula.

Nicotine addiction is a serious public health problem causing millions of deaths worldwide. Serotonin (5-hydroxytryptamine; 5-HT) is involved in central nervous system (CNS) nicotine effects, and it has been suggested as a promising pharmacological target for smoking cessation. In this regard, what is particularly interesting are the 5-HT2A receptors (5-HT2ARs) and the lateral habenula (LHb), a central area in nicotine addiction that we showed to be under a strong 5-HT2AR-modulation. Single-cell extracellular recording of LHb neurons was used to study the 5-HT2AR function by intravenously administrating the potent agonist TCB-2. Acute nicotine (2 mg/kg, intraperitoneal, i.p.) and chronic nicotine (6 mg/kg/day for 14 days) differently affected both the 5-HT2AR-immuno reactive (IR) neuron number and the 5-HT2AR immunostaining area in the different brain areas studied. After acute nicotine, TCB-2 cumulative doses (5-640 µg/kg, intravenous, i.v.) bidirectionally affected the activity of 74% of LHb recorded neurons. After chronic nicotine treatment, TCB-2 was only capable of decreasing the LHb firing rate. The expression of 5-HT2AR under acute and chronic nicotine exposure was studied in the LHb and in other brain areas involved in nicotine effects in rats by using immunohistochemistry. These data reveal that acute and chronic nicotine differentially affect the 5-HT2AR function in different brain areas and this might be relevant in nicotine addiction and its treatment.

Total Recall: Lateral Habenula and Psychedelics in the Study of Depression and Comorbid Brain Disorders.

Depression impacts the lives and daily activities of millions globally. Research into the neurobiology of lateral habenula circuitry and the use of psychedelics for treating depressive states has emerged in the last decade as new directions to devise interventional strategies and therapies. Several clinical trials using deep brain stimulation of the habenula, or using ketamine, and psychedelics that target the serotonergic system such as psilocybin are also underway. The promising early results in these fields require cautious optimism as further evidence from experiments conducted in animal systems in ecologically relevant settings, and a larger number of human studies with improved spatiotemporal neuroimaging, accumulates. Designing optimal methods of intervention will also be aided by an improvement in our understanding of the common genetic and molecular factors underlying disorders comorbid with depression, as well as the characterization of psychedelic-induced changes at a molecular level. Advances in the use of cerebral organoids offers a new approach for rapid progress towards these goals. Here, we review developments in these fast-moving areas of research and discuss potential future directions.

Neurons in ventral tegmental area tonically inhibit sympathetic outflow to brown adipose tissue: possible mediation of thermogenic signals from lateral habenula.

The lateral habenula (LHb), a nucleus involved in the response to salient, especially adverse, environmental events, is implicated in brown adipose tissue (BAT) thermogenesis caused by these events. LHb-elicited thermogenesis involves a neural pathway to the lower brain stem sympathetic control center in the medullary raphé. There are no direct connections from the LHb to the medullary raphé. LHb-mediated behavioral responses involve inhibitory control over the dopamine neurons in the ventral tegmental area (VTA), mediated via an excitatory drive from the LHb to GABAergic neurons in the tail of the VTA. We hypothesized that inhibition of the VTA is also involved in LHb-mediated BAT thermogenesis. To test this hypothesis, inhibition of neurons in the VTA with muscimol increased BAT sympathetic nerve discharge by 22.0 ± 9.2 dBµV ( n = 24, P < 0.0001) and BAT temperature by 1.2 ± 0.1°C ( P < 0.001). This response was abolished by inhibition of the medullary raphé neurons with muscimol. BAT thermogenesis initiated with focal injections of bicuculline in the LHb was reversed by subsequent blockade of GABAA receptors in the VTA with bicuculline. These results suggest that, at least in anesthetized rats, neurons in the VTA tonically inhibit BAT thermogenesis via a link, presently unknown, to the medullary raphé. Removal of this VTA-initiated inhibition is an important mechanism whereby LHb neurons activate BAT thermogenesis.

Elevation of Transient Receptor Potential Vanilloid 1 Function in the Lateral Habenula Mediates Aversive Behaviors in Alcohol-withdrawn Rats.

WHAT WE ALREADY KNOW ABOUT THIS TOPIC: Chronic alcohol use and withdrawal leads to increased pain perception, anxiety, and depression. These aberrant behaviors are accompanied by increased excitatory glutamatergic transmission to, and activity of, the lateral habenula neurons.Vanilloid type 1, or TRPV1, channels are expressed in the habenula and they facilitate glutamatergic transmission. Whether TRPV1 channel plays a role in habenula hyperactivity is not clear. WHAT THIS ARTICLE TELLS US THAT IS NEW: Glutamatergic transmission in the lateral habenula was inhibited by TRPV1 channel antagonists. In vivo, local administration of TRPV1 antagonists into the lateral habenula attenuated hyperalgesia, anxiety, and relapse-like drinking in rats who chronically consumed alcohol.The data suggest that enhanced TRPV1 channel function during withdrawal may contribute to aberrant behavior that promotes relapse alcohol consumption. BACKGROUND: Recent rat studies indicate that alcohol withdrawal can trigger a negative emotional state including anxiety- and depression-like behaviors and hyperalgesia, as well as elevated glutamatergic transmission and activity in lateral habenula neurons. TRPV1, a vanilloid receptor expressed in the habenula, is involved in pain, alcohol dependence, and glutamatergic transmission. The authors therefore hypothesized that TRPV1 contributes to the changes in both the behavioral phenotypes and the habenula activity in alcohol-withdrawn rats. METHODS: Adult male Long-Evans rats (n = 110 and 280 for electrophysiology and behaviors, respectively), randomly assigned into the alcohol and water (Naïve) groups, were trained to consume either alcohol or water-only using an intermittent-access procedure. Slice electrophysiology was used to measure spontaneous excitatory postsynaptic currents and firing of lateral habenula neurons. The primary outcome was the change in alcohol-related behaviors and lateral habenula activity induced by pharmacologic manipulation of TRPV1 activity. RESULTS: The basal frequency of spontaneous excitatory postsynaptic currents and firing of lateral habenula neurons in alcohol-withdrawn rats was significantly increased. The TRPV1 antagonist capsazepine (10 µM) induced a stronger inhibition on spontaneous excitatory postsynaptic currents (mean ± SD; by 26.1 ± 27.9% [n = 11] vs. 6.7 ± 18.6% [n = 17], P = 0.027) and firing (by 23.4 ± 17.6% [n = 9] vs. 11.9 ± 16.3% [n = 12], P = 0.025) in Withdrawn rats than Naive rats. By contrast, the TRPV1 agonist capsaicin (3 µM) produced a weaker potentiation in Withdrawn than Naïve rats (spontaneous excitatory postsynaptic currents: by 203.6 ± 124.7% [n = 20] vs. 415.2 ± 424.3% [n = 15], P < 0.001; firing: 38.1 ± 14.7% [n = 11] vs. 73.9 ± 41.9% [n = 11], P < 0.001). Conversely, capsaicin's actions in Naïve but not in Withdrawn rats were significantly attenuated by the pretreatment of TRPV1 endogenous agonist N-Oleoyldopamine. In Withdrawn rats, intra-habenula infusion of TRPV1 antagonists attenuated hyperalgesia and anxiety-like behaviors, decreased alcohol consumption upon resuming drinking, and elicited a conditioned place preference. CONCLUSIONS: Enhanced TRPV1 function may contribute to increased glutamatergic transmission and activity of lateral habenula neurons, resulting in the aberrant behaviors during ethanol withdrawal.

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.

ErbB4 Receptors in the Medial Habenula Regulate Contextual Fear Memory.

The Medial Habenular (MHb) and the Lateral Habenular nuclei are 2 main parts of the habenular complex (Hb). Recent studies showed that MHb plays an important role in memory, and in the expression of ErbB4. However, the expression of MHb ErbB4 receptor and its role in fear memory is not well understood. In this study, western blotting and quantitative real-time polymerase chain reaction were used to assess the protein and mRNA levels of ErbB4 in the process of contextual fear conditioning. A pharmacological approach was used to block and stimulate the ErbB4 receptor. Contextual fear conditioning tests induced a significant increase on the expression of ErbB4 at various times in the Hb and the MHb. Moreover, the blockade and stimulation of MHb ErbB4 receptors did not affect the fear formation but impaired and improved the contextual-dependent fear expression. Furthermore, in vitro electrophysiological recordings showed that the blockade of the MHb ErbB4 receptor reduced the presynaptic gamma-amino butyric acid release. ErbB4 is a susceptible gene for schizophrenia and the above findings may provide new insights into the mechanisms of fear-related responses.