Lateral habenular neurons projecting to reward-processing monoaminergic nuclei express hyperpolarization-activated cyclic nucleotid-gated cation channels

Effects of Repetitive Mild Traumatic Brain Injury on Corticotropin-Releasing Factor Modulation of Lateral Habenula Excitability and Motivated Behavior

Mild traumatic brain injury (mTBI) is a significant health burden due to mTBI-related chronic debilitating cognitive and psychiatric morbidities. Recent evidence from our laboratory suggests a possible dysregulation within reward/motivational circuit function at the level of a subcortical structure, the lateral habenula (LHb), where we demonstrated a causal role for hyperactive LHb in mTBI-induced motivational deficits in self-care grooming behavior in young adult male mice when exposed to mTBI injury during late adolescence (at ∼8 weeks old). Here we extended this observation by further characterizing neurobehavioral effects of this repetitive closed head injury model of mTBI in both young adult male and female mice on LHb excitability, corticotropin releasing factor (CRF) modulation of LHb activity, and behavioral responses of motivation to self-care behavior, and approach versus avoidance behavior in the presence of a social- or threat-related stimulus. We show that mTBI increases LHb spontaneous tonic activity in female mice similar to what we previously observed in male mice as well as promoting LHb neuronal hyperexcitability and hyperpolarization-induced LHb bursting in both male and female mice. Interestingly, mTBI only increases LHb intrinsic excitability in male mice coincident with higher levels of the hyperpolarization-activated cation currents (HCN/Ih) and reduces levels of the M-type potassium currents while potentiating M-currents without altering intrinsic excitability in LHb neurons of female mice. Since persistent dysregulation of brain CRF systems is suggested to contribute to chronic psychiatric morbidities and that LHb neurons are highly responsive to CRF, we then tested whether LHb CRF subsystem becomes engaged following mTBI. We found that in vitro inhibition of CRF receptor type 1 (CRFR1) within the LHb normalizes mTBI-induced enhancement of LHb tonic activity and hyperexcitability in both sexes, suggesting that an augmented intra-LHb CRF-CRFR1-mediated signaling contributes to the overall LHb hyperactivity following mTBI. Behaviorally, mTBI diminishes motivation for self-care grooming in female mice as in male mice. mTBI also alters defensive behaviors in the looming shadow task by shifting the innate defensive behaviors towards more passive action-locking rather than escape behaviors in response to an aerial threat in both male and female mice as well as prolonging the latency to escape responses in female mice. While, this model of mTBI reduces social preference in male mice, it induces higher social novelty seeking during the novel social encounters in both male and female mice. Overall, our study provides further translational validity for the use of this preclinical model of mTBI for investigation of mTBI-related reward circuit dysfunction and mood/motivation-related behavioral deficits in both sexes while uncovering a few sexually dimorphic neurobehavioral effects of this model that may differentially affect young males and females when exposed to this type of mTBI injury during late adolescence.

Missing Puzzle Pieces in Dementia Research: HCN Channels and Theta Oscillations

Increasing evidence indicates a role of hyperpolarization activated cation (HCN) channels in controlling the resting membrane potential, pacemaker activity, memory formation, sleep, and arousal. Their disfunction may be associated with the development of epilepsy and age-related memory decline. Neuronal hyperexcitability involved in epileptogenesis and EEG desynchronization occur in the course of dementia in human Alzheimer's Disease (AD) and animal models, nevertheless the underlying ionic and cellular mechanisms of these effects are not well understood. Some suggest that theta rhythms involved in memory formation could be used as a marker of memory disturbances in the course of neurogenerative diseases, including AD. This review focusses on the interplay between hyperpolarization HCN channels, theta oscillations, memory formation and their role(s) in dementias, including AD. While individually, each of these factors have been linked to each other with strong supportive evidence, we hope here to expand this linkage to a more inclusive picture. Thus, HCN channels could provide a molecular target for developing new therapeutic agents for preventing and/or treating dementia.

Chronic social stress blunts core body temperature and molecular rhythms of Rbm3 and Cirbp in mouse lateral habenula

Chronic social stress in mice causes behavioural and physiological changes that result in perturbed rhythms of body temperature, activity and sleep-wake cycle. To further understand the link between mood disorders and temperature rhythmicity in mice that are resilient or susceptible to stress, we measured core body temperature (Tcore) before and after exposure to chronic social defeat stress (CSDS). We found that Tcore amplitudes of stress-resilient and susceptible mice are dampened during exposure to CSDS. However, following CSDS, resilient mice recovered temperature amplitude faster than susceptible mice. Furthermore, the interdaily stability (IS) of temperature rhythms was fragmented in stress-exposed mice during CSDS, which recovered to control levels following stress. There were minimal changes in locomotor activity after stress exposure which correlates with regular rhythmic expression of Prok2 - an output signal of the suprachiasmatic nucleus. We also determined that expression of thermosensitive genes Rbm3 and Cirbp in the lateral habenula (LHb) were blunted 1 day after CSDS. Rhythmic expression of these genes recovered 10 days later. Overall, we show that CSDS blunts Tcore and thermosensitive gene rhythms. Tcore rhythm recovery is faster in stress-resilient mice, but Rbm3 and Cirbp recovery is uniform across the phenotypes.

Chronic social stress blunts core body temperature and molecular rhythms ofRbm3andCirbpin mouse lateral habenula.. [DOI: 10.1101/2023.01.02.522528]

Chronic social stress in mice causes behavioral and physiological changes that result in perturbed rhythms of body temperature, activity and sleep-wake cycle. To further understand the link between mood disorders and temperature rhythmicity in mice that are resilient or susceptible to stress, we measured core body temperature (Tcore) before and after exposure to chronic social defeat stress (CSDS). We found that Tcore amplitudes of stress-resilient and susceptible mice are dampened during exposure to CSDS. However, following CSDS, resilient mice recovered temperature amplitude faster than susceptible mice. Furthermore, the interdaily stability (IS) of temperature rhythms was fragmented in stress-exposed mice during CSDS, which recovered to control levels following stress. There were minimal changes in locomotor activity after stress exposure which correlates with regular rhythmic expression ofProk2- an output signal of the suprachiasmatic nucleus. We also determined that expression of thermosensitive genesRbm3andCirbpin the lateral habenula (LHb) were blunted 1-day after CSDS. Rhythmic expression of these genes recovered 10 days later. Overall, we show that CSDS blunts Tcore and thermosensitive gene rhythms. Tcore rhythm recovery is faster in stress-resilient mice, butRbm3andCirbprecovery is uniform across the phenotypes.

A novel giant non-cholinergic striatal interneuron restricted to the ventrolateral striatum coexpresses Kv3.3 potassium channel, parvalbumin, and the vesicular GABA transporter

The striatum is the main input structure of the basal ganglia. Distinct striatal subfields are involved in voluntary movement generation and cognitive and emotional tasks, but little is known about the morphological and molecular differences of striatal subregions. The ventrolateral subfield of the striatum (VLS) is the orofacial projection field of the sensorimotor cortex and is involved in the development of orofacial dyskinesias, involuntary chewing-like movements that often accompany long-term neuroleptic treatment. The biological basis for this particular vulnerability of the VLS is not known. Potassium channels are known to be strategically localized within the striatum. In search of possible molecular correlates of the specific vulnerability of the VLS, we analyzed the expression of voltage-gated potassium channels in rodent and primate brains using qPCR, in situ hybridization, and immunocytochemical single and double staining. Here we describe a novel, giant, non-cholinergic interneuron within the VLS. This neuron coexpresses the vesicular GABA transporter, the calcium-binding protein parvalbumin (PV), and the Kv3.3 potassium channel subunit. This novel neuron is much larger than PV neurons in other striatal regions, displays characteristic electrophysiological properties, and, most importantly, is restricted to the VLS. Consequently, the giant striatal Kv3.3-expressing PV neuron may link compromised Kv3 channel function and VLS-based orofacial dyskinesias.

Functional connectome of arousal and motor brainstem nuclei in living humans by 7 Tesla resting-state fMRI

Brainstem nuclei play a pivotal role in many functions, such as arousal and motor control. Nevertheless, the connectivity of arousal and motor brainstem nuclei is understudied in living humans due to the limited sensitivity and spatial resolution of conventional imaging, and to the lack of atlases of these deep tiny regions of the brain. For a holistic comprehension of sleep, arousal and associated motor processes, we investigated in 20 healthy subjects the resting-state functional connectivity of 18 arousal and motor brainstem nuclei in living humans. To do so, we used high spatial-resolution 7 Tesla resting-state fMRI, as well as a recently developed in-vivo probabilistic atlas of these nuclei in stereotactic space. Further, we verified the translatability of our brainstem connectome approach to conventional (e.g. 3 Tesla) fMRI. Arousal brainstem nuclei displayed high interconnectivity, as well as connectivity to the thalamus, hypothalamus, basal forebrain and frontal cortex, in line with animal studies and as expected for arousal regions. Motor brainstem nuclei showed expected connectivity to the cerebellum, basal ganglia and motor cortex, as well as high interconnectivity. Comparison of 3 Tesla to 7 Tesla connectivity results indicated good translatability of our brainstem connectome approach to conventional fMRI, especially for cortical and subcortical (non-brainstem) targets and to a lesser extent for brainstem targets. The functional connectome of 18 arousal and motor brainstem nuclei with the rest of the brain might provide a better understanding of arousal, sleep and accompanying motor function in living humans in health and disease.

Lamotrigine Attenuates Neuronal Excitability, Depresses GABA Synaptic Inhibition, and Modulates Theta Rhythms in Rat Hippocampus

Theta oscillations generated in hippocampal (HPC) and cortical neuronal networks are involved in various aspects of brain function, including sensorimotor integration, movement planning, memory formation and attention. Disruptions of theta rhythms are present in individuals with brain disorders, including epilepsy and Alzheimer’s disease. Theta rhythm generation involves a specific interplay between cellular (ion channel) and network (synaptic) mechanisms. HCN channels are theta modulators, and several medications are known to enhance their activity. We investigated how different doses of lamotrigine (LTG), an HCN channel modulator, and antiepileptic and neuroprotective agent, would affect HPC theta rhythms in acute HPC slices (in vitro) and anaesthetized rats (in vivo). Whole-cell patch clamp recordings revealed that LTG decreased GABA_A-fast transmission in CA3 cells, in vitro. In addition, LTG directly depressed CA3 and CA1 pyramidal neuron excitability. These effects were partially blocked by ZD 7288, a selective HCN blocker, and are consistent with decreased excitability associated with antiepileptic actions. Lamotrigine depressed HPC theta oscillations in vitro, also consistent with its neuronal depressant effects. In contrast, it exerted an opposite, enhancing effect, on theta recorded in vivo. The contradictory in vivo and in vitro results indicate that LTG increases ascending theta activating medial septum/entorhinal synaptic inputs that over-power the depressant effects seen in HPC neurons. These results provide new insights into LTG actions and indicate an opportunity to develop more precise therapeutics for the treatment of dementias, memory disorders and epilepsy.

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 (I _ACD) 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 I _ACD Finally, bath application of ZD7288, a selective blocker of hyperpolarization-activated cyclic nucleotide-gated channels, attenuated ACD-induced potentiation of firing, EPSCs, and I _ACD 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.

Reproducibility of automated habenula segmentation via deep learning in major depressive disorder and normal controls with 7 Tesla MRI

The habenula is one of the most important brain regions for investigating the etiology of psychiatric diseases such as major depressive disorder (MDD). However, the habenula is challenging to delineate with the naked human eye in brain imaging due to its low contrast and tiny size, and the manual segmentation results vary greatly depending on the observer. Therefore, there is a great need for automatic quantitative analytic methods of the habenula for psychiatric research purposes. Here we propose an automated segmentation and volume estimation method for the habenula in 7 Tesla magnetic resonance imaging based on a deep learning-based semantic segmentation network. The proposed method, using the data of 69 participants (33 patients with MDD and 36 normal controls), achieved an average precision, recall, and dice similarity coefficient of 0.869, 0.865, and 0.852, respectively, in the automated segmentation task. Moreover, the intra-class correlation coefficient reached 0.870 in the volume estimation task. This study demonstrates that this deep learning-based method can provide accurate and quantitative analytic results of the habenula. By providing rapid and quantitative information on the habenula, we expect our proposed method will aid future psychiatric disease studies.

Early life stress dysregulates kappa opioid receptor signaling within the lateral habenula

The lateral habenula (LHb) is an epithalamic brain region associated with value-based decision making and stress evasion through its modulation of dopamine (DA)-mediated reward circuitry. Specifically, increased activity of the LHb is associated with drug addiction, schizophrenia and stress-related disorders such as depression, anxiety and posttraumatic stress disorder. Dynorphin (Dyn)/Kappa opioid receptor (KOR) signaling is a mediator of stress response in reward circuitry. Previously, we have shown that maternal deprivation (MD), a severe early life stress, increases LHb spontaneous neuronal activity and intrinsic excitability while blunting the response of LHb neurons to extrahypothalamic corticotropin-releasing factor (CRF) signaling, another stress mediator. CRF pathways also interact with Dyn/KOR signaling. Surprisingly, there has been little study of direct KOR regulation of the LHb despite its distinct role in stress, reward and aversion processing. To test the functional role of Dyn/KOR signaling in the LHb, we utilized ex-vivo electrophysiology combined with pharmacological tools in rat LHb slices. We show that activation of KORs by a KOR agonist (U50,488) exerted differential effects on the excitability of two distinct sub-populations of LHb neurons that differed in their expression of hyperpolarization-activated cation currents (HCN, Ih). Specifically, KOR stimulation increased neuronal excitability in LHb neurons with large Ih currents (Ih+) while decreasing neuronal excitability in small/negative Ih (Ih-) neurons. We found that an intact fast-synaptic transmission was required for the effects of U50,488 on the excitability of both Ih- and Ih+ LHb neuronal subpopulations. While AMPAR-, GABAAR-, or NMDAR-mediated synaptic transmission alone was sufficient to mediate the effects of U50,488 on excitability of Ih- neurons, either GABAAR- or NMDAR-mediated synaptic transmission could mediate these effects in Ih+ neurons. Consistently, KOR activation also altered both glutamatergic and GABAergic synaptic transmission where stimulation of presynaptic KORs uniformly suppressed glutamate release onto LHb neurons while primarily decreased or in some cases increased GABA release. We also found that MD significantly increased immunolabeled Dyn (the endogenous KOR agonist) labeling in neuronal fibers in LHb while significantly decreasing mRNA levels of KORs in LHb tissues compared to those from non-maternally deprived (non-MD) control rats. Moreover, the U50,488-mediated increase in LHb neuronal firing observed in non-MD rats was absent following MD. Altogether, this is the first demonstration of the existence of functional Dyn/KOR signaling in the LHb that can be modulated in response to severe early life stressors such as MD.

Early Life Stress- and Drug-Induced Histone Modifications Within the Ventral Tegmental Area

Psychiatric illnesses are a major public health concern due to their prevalence and heterogeneity of symptom presentation resulting from a lack of efficacious treatments. Although dysregulated dopamine (DA) signaling has been observed in a myriad of psychiatric conditions, different pathophysiological mechanisms have been implicated which impede the development of adequate treatments that work across all patient populations. The ventral tegmental area (VTA), a major source of DA neurons in the brain reward pathway, has been shown to have altered activity that contributes to reward dysregulation in mental illnesses and drug addiction. It has now become better appreciated that epigenetic mechanisms contribute to VTA DA dysfunction, such as through histone modifications, which dynamically regulate transcription rates of critical genes important in synaptic plasticity underlying learning and memory. Here, we provide a focused review on differential histone modifications within the VTA observed in both humans and animal models, as well as their relevance to disease-based phenotypes, specifically focusing on epigenetic dysregulation of histones in the VTA associated with early life stress (ELS) and drugs of abuse. Locus- and cell-type-specific targeting of individual histone modifications at specific genes within the VTA presents novel therapeutic targets which can result in greater efficacy and better long-term health outcomes in susceptible individuals that are at increased risk for substance use and psychiatric disorders.

Transcriptional and Spatial Resolution of Cell Types in the Mammalian Habenula

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

A resting-state study of volumetric and functional connectivity of the habenular nucleus in treatment-resistant depression patients

OBJECTIVE

To investigate the volumetric and functional connectivity of the habenular nucleus in treatment-resistant depression (TRD) patients using the resting-state functional magnetic resonance imaging (rs-fMRI) approach.

METHODS

A total of 15 TRD patients, who visited the Mental Health Institute of the First Hospital Affiliated with Jilin University between August 2014 and March 2015, along with 15 normal subjects, were enrolled into this study for structural and functional imaging. Functional connectivity analysis was performed using bilateral habenular nuclei as the region of interest in contrast to whole-brain voxels.

RESULTS

No significant difference of absolute volume was found in bilateral habenular nuclei between TRD patients and healthy controls, or after controlling for individual total intracranial volume. However, functional connectivity analysis showed increased connectivity between the right habenular nucleus with the medial superior frontal gyrus, anterior cingulate cortex and medial orbitofrontal gyrus, and decreased connectivity with the corpus callosum in the TRD group. For the left habenular nucleus seed, the brain region with increased functional connectivity in the inferior temporal gyrus and decreased functional connectivity in the insular was found in the TRD patients.

CONCLUSION

Abnormal functional connectivity was present between the habenular nucleus and the default mode network in TRD patients. Dysfunction in habenular nucleus-related circuitry for processing negative emotion might form the pathological basis for TRD. Significant asymmetric functional connectivity was also found between bilateral habenular nuclei in TRD patients. Such asymmetry suggests potentially divergent strategy for intervention on bilateral habenular nucleus regions in the future management of depression.

Circadian neurons in the lateral habenula: Clocking motivated behaviors

The main circadian clock in mammals is located in the hypothalamic suprachiasmatic nucleus (SCN), however, central timing mechanisms are also present in other brain structures beyond the SCN. The lateral habenula (LHb), known for its important role in the regulation of the monoaminergic system, contains such a circadian clock whose molecular and cellular mechanisms as well as functional role are not well known. However, since monoaminergic systems show circadian activity, it is possible that the LHb-clock's role is to modulate the rhythmic activity of the dopamine, serotonin and norephinephrine systems, and associated behaviors. Moreover, the LHb is involved in different pathological states such as depression, addiction and schizophrenia, states in which sleep and circadian alterations have been reported. Thus, perturbations of circadian activity in the LHb might, in part, be a cause of these rhythmic alterations in psychiatric ailments. In this review the current state of the LHb clock and its possible implications in the control of monoaminergic systems rhythms, motivated behaviors (e.g., feeding, drug intake) and depression (with circadian disruptions and altered motivation) will be discussed.

Habenula volume increases with disease severity in unmedicated major depressive disorder as revealed by 7T MRI

The habenula is a paired epithalamic structure involved in the pathogenesis of major depressive disorder (MDD). Evidence comes from its impact on the regulation of serotonergic and dopaminergic neurons, the role in emotional processing and studies on animal models of depression. The present study investigated habenula volumes in 20 unmedicated and 20 medicated MDD patients and 20 healthy controls for the first time by applying a triplanar segmentation algorithm on 7 Tesla magnetic resonance (MR) whole-brain T1 maps. The hypothesis of a right-side decrease of habenula volumes in the MDD patients was tested, and the relationship between volumetric abnormalities and disease severity was exploratively investigated. Absolute and relative total and hemispheric habenula volumes did not differ significantly between the three groups. In the patients with short duration of disease for which medication effects could be ruled out, significant correlations were found between bilateral habenula volumes and HAMD-17- and BDI-II-related severities. In the medicated patients, this positive relationship disappeared. Our findings suggest an involvement of habenula pathology in the beginning of MDD, while general effects independent of severity or stage of disease did not occur. Our findings warrant future combined tractographic and functional investigation using ultra-high-resolution in vivo MR imaging.

Serotonin stimulates lateral habenula via activation of the post-synaptic serotonin 2/3 receptors and transient receptor potential channels

There is growing interest on the role of the lateral habenula (LHb) in depression, because it closely and bilaterally connects with the serotoninergic raphe nuclei. The LHb sends glutamate efferents to the raphe nuclei, while it receives serotoninergic afferents, and expresses a high density of serotonin (5-HT) receptors. Recent studies suggest that 5-HT receptors exist both in the presynaptic and postsynaptic sites of LHb neurons, and activation of these receptors may have different effects on the activity of LHb neurons. The current study focused on the effect of 5-HT on the postsynaptic membrane. We found that 5-HT initiated a depolarizing inward current (I((5-HTi))) and accelerated spontaneous firing in ∼80% of LHb neurons in rat brain slices. I((5-HTi)) was also induced by the 5-HT uptake blocker citalopram, indicating activity of endogenous 5-HT. I((5-HTi)) was diminished by 5-HT(2/3) receptor antagonists (ritanserin, SB-200646 or ondansetron), and activated by the selective 5-HT(2/3) agonists 1-(3-Chlorophenyl) piperazine hydrochloride or 1-(3-Chlorophenyl) biguanide hydrochloride. Furthermore, I((5-HTi)) was attenuated by 2-Aminoethyl diphenylborinate, a blocker of transient receptor potential channels, and an IP3 receptor inhibitor, indicating the involvement of transient receptor potential channels. These results demonstrate that the reciprocal connection between the LHb and the 5-HT system highlights a key role for 5-HT stimulation of LHb neurons that may be important in the pathogenesis of depression.

Correlating habenular subnuclei in rat and mouse by using topographic, morphological, and cytochemical criteria

The mammalian habenulae consist of medial (MHb) and lateral (LHb) nuclear complexes. Especially the LHb has received much interest because it has been recognized as the potential center of an "anti-reward system." Subnuclear organization and connectivity of the LHb are well known. In contrast, criteria to classify habenular neurons into distinct groups with potentially different biological functions are missing, most likely as a result of the lack of appropriate marker proteins. Actually, a huge amount of data concerning the localization of more than 20,000 mouse protein genes is provided in the Allen Brain Atlas. Unfortunately, the immediate use of this information is prohibited by the fact that the subnuclear organization of the habenular complexes in mouse is not known so far. The present report, therefore, uses topographic, structural, and cytochemical information from the rat to recognize corresponding areas within the mouse habenulae. Taking advantage of the fact that the Klüver-Barrera technique allows simultaneous observation of neuronal cell bodies and myelinated fibers, we were able to correlate subnuclear areas in the mouse habenula to subnuclei, which had been rigorously identified by several criteria in the rat. Our data suggest that the topographic localization of habenular subnuclei is rather similar between mouse and rat and that they may be homologous in these two species. Consequently, our data may allow using the Allen Brain Atlas as a source of basal information, which should be helpful to select candidate molecular markers for functionally different neurons in the mouse and potentially in higher mammalian species.

Activation of the habenula complex evokes autonomic physiological responses similar to those associated with emotional stress

Neurons in the lateral habenula (LHb) discharge when an animal anticipates an aversive outcome or when an expected reward is not forthcoming, contributing to the behavioral response to aversive situations. So far, there is little information as to whether the LHb also contributes to autonomic physiological responses, including increases in body temperature (emotional hyperthermia) that are integrated with defensive behaviors. Vasoconstriction in cutaneous vascular bed and heat production in brown adipose tissue (BAT) both contribute to emotional hyperthermia. Our present study determines whether stimulation of the LHb elicits constriction of the tail artery and BAT thermogenesis in anesthetized Sprague–Dawley rats. Disinhibition of neurons in LHb with focal microinjections of bicuculline (1 nmol in 100 nl, bilaterally) acutely increased BAT temperature (+0.6 ± 0.1°C, n = 9 rats, P < 0.01) and reduced tail artery blood flow (by 88 ± 4%, n = 9 rats, P < 0.01). Falls in mesenteric blood flow, simultaneously recorded, were much less intense. The pattern of BAT thermogenesis and cutaneous vasoconstriction elicited by stimulating the habenula is similar to the pattern observed during stress-induced emotional hyperthermia, suggesting that the habenula may be important in this response.

Lateral habenula and the rostromedial tegmental nucleus innervate neurochemically distinct subdivisions of the dorsal raphe nucleus in the rat

The lateral habenula (LHb) is an epithalamic structure differentiated in a medial (LHbM) and a lateral division (LHbL). Together with the rostromedial tegmental nucleus (RMTg), the LHb has been implicated in the processing of aversive stimuli and inhibitory control of monoamine nuclei. The inhibitory LHb influence on midbrain dopamine neurons has been shown to be mainly mediated by the RMTg, a mostly GABAergic nucleus that receives a dominant input from the LHbL. Interestingly, the RMTg also projects to the dorsal raphe nucleus (DR), which also receives direct LHb projections. To compare the organization and transmitter phenotype of LHb projections to the DR, direct and indirect via the RMTg, we first placed injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin into the LHb or the RMTg. We then confirmed our findings by retrograde tracing and investigated a possible GABAergic phenotype of DR-projecting RMTg neurons by combining retrograde tracing with in situ hybridization for GAD67. We found only moderate direct LHb projections to the DR, which mainly emerged from the LHbM and were predominantly directed to the serotonin-rich caudal DR. In contrast, RMTg projections to the DR were more robust, emerged from RMTg neurons enriched in GAD67 mRNA, and were focally directed to a distinctive DR subdivision immunohistochemically characterized as poor in serotonin and enriched in presumptive glutamatergic neurons. Thus, besides its well-acknowledged role as a GABAergic control center for the ventral tegmental area (VTA)-nigra complex, our findings indicate that the RMTg is also a major GABAergic relay between the LHb and the DR.

Transcriptomic-anatomic analysis of the mouse habenula uncovers a high molecular heterogeneity among neurons in the lateral complex, while gene expression in the medial complex largely obeys subnuclear boundaries

The mammalian habenula with its medial and lateral complexes has gained much interest in recent years, while knowledge on the detailed biological functions of these nuclei is still scarce. Novel strategies to differentiate and identify habenular cell types are required. Such attempts have largely failed, most likely due to the lack of appropriate molecular markers. One important tool to approach this dilemma is available in form of the Allen Brain Atlas (ABA), which provides detailed expression patterns of many genes in the mouse brain. In the present report, ABA tools in combination with visual inspection of ISH images were used to detect transcripts, which are strongly expressed in medial (MHb) and lateral (LHb) habenular complexes. Against our expectations, most transcripts were differentially distributed throughout the LHb, disregarding boundaries of subnuclear areas. Nine distinct distribution patterns were recognized. Yet, several transcripts could not be attributed to one of these, suggesting a high diversity of neuron types in the LHb. In the MHb, in contrast, many transcripts tended to obey subnuclear boundaries. The differential distribution of others like Adcyap1, Chrna3, or Trp53i11 suggests the presence of a novel subfield adjacent to the region of the MHbVm, which now is termed intermediate field of the ventral MHb. In addition, the localizations of Amigo2, Adcyap1, and a couple of other transcripts suggest a lateral extension of the MHb, which is here, termed HbX area. Apparently, this area is composed of intermingled MHb and LHb neurons and may allow functional interaction between the both habenular complexes.

Differences in the expression pattern of HCN isoforms among mammalian tissues: sources and implications

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels play a critical role in a broad range of cell types, but the expression of the various HCN isoforms is still poorly understood. In the present study we have compared the expression of HCN isoforms in rat excitable and non-excitable tissues at both the mRNA and protein levels. Real-time PCR and Western blot analysis revealed distinct expression patterns of the four HCN isoforms in brain, heart, pituitary and kidney, with inconsistent mRNA-protein expression correlation. The HCN2 was the most abundant mRNA transcript (95.6, 78.0 and 59.0 % in kidney heart and pituitary, respectively) except in the brain (42.0 %) whereas HCN4 was the most abundant protein isoform. Our results suggest that HCN channels are mostly produced by the HCN4 isoform in heart, which contrasts with the sharp differences in the isoform stoichiometry in pituitary (15 HCN4:2 HCN2:1 HCN1:1 HCN3), kidney (24 HCN4:2 HCN3:1 HCN2:1 HCN1) and brain (3 HCN4:2 HCN2:1 HCN1:1 HCN3). Moreover, deviations of the electrophoretic molecular weight (MW) of the HCN isoforms relative to the theoretical MW were observed, suggesting that N-glycosylation and enzymatic proteolysis influences HCN channel surface expression. We hypothesize that selective cleavage of HCN channels by membrane bound metalloendopeptidases could account for the multiplicity of properties of native HCN channels in different tissues.

Synaptic and cellular profile of neurons in the lateral habenula

The lateral habenula (LHb) is emerging as a crucial structure capable of conveying rewarding and aversive information. Recent evidence indicates that a rapid increase in the activity of LHb neurons drives negative states and avoidance. Furthermore, the hyperexcitability of neurons in the LHb, especially those projecting to the midbrain, may represent an important cellular correlate for neuropsychiatric disorders like depression and drug addiction. Despite the recent insights regarding the implications of the LHb in the context of reward and aversion, the exact nature of the synaptic and cellular players regulating LHb neuronal functions remains largely unknown. Here we focus on the synaptic and cellular physiology of LHb neurons. First, we discuss the properties of excitatory transmission and the implications of glutamate receptors for long-term synaptic plasticity; second, we review the features of GABAergic transmission onto LHb neurons; and finally, we describe the contribution that neuromodulators such as dopamine (DA) and serotonin may have for LHb neuronal physiology. We relate these findings to the role that the LHb can play in processing aversive and rewarding stimuli, both in health and disease states.

The lateral habenular nucleus mediates signal transduction from the insular cortex in OSA rats

PURPOSE

The insular cortex (Ic) and habenular nuclei (Hb) of the limbic system are associated with human and animal dyspnea by regulating 5-hydroxytryptamine (5-HT) release in the raphe nuclei (RN). However, the Hb are composed of the medial habenular nucleus (MHb) and the lateral habenular nucleus (LHb). Therefore, it is still unclear whether the Ic signal is conducted through the MHb or LHb. This study aimed to investigate the role of the Hb and Ic in obstructive sleep apnea syndrome (OSA) and the functional relationship between these two structures.

METHODS

We monitored multiple indicators, including the respiration movement curve, neuronal activity in the MHb and LHb, and arterial blood gas, when stimulating the anterior Ic of Wistar rats. We compared the results with the control group (stimulating the surrounding cortex).

RESULTS

Electrical stimulation of the Ic in the rat brain caused respiratory disturbances, apnea, reduced blood pH, and aggravated base deficit (more negative base excess value) compared to control animals (p < 0.05). It also reduced the spontaneous firing of the MHb neurons but increased that of the LHb neurons. Electrical stimulation of the Ic induces apnea in rats in a similar manner to human OSA. The Ic and the Hb are functionally linked. Stimulation of the Ic inhibits the MHb, but activates the LHb. It induces OSA-like symptoms by enhancing LHb-mediated inhibition of the RN.

CONCLUSIONS

This study illustrates the mechanism by which an animal model of OSA is created by stimulating the Ic and promotes understanding of OSA pathogenesis.

Dopamine D4 receptor excitation of lateral habenula neurons via multiple cellular mechanisms

Glutamatergic lateral habenula (LHb) output communicates negative motivational valence to ventral tegmental area (VTA) dopamine (DA) neurons via activation of the rostromedial tegmental nucleus (RMTg). However, the LHb also receives a poorly understood DA input from the VTA, which we hypothesized constitutes an important feedback loop regulating DA responses to stimuli. Using whole-cell electrophysiology in rat brain slices, we find that DA initiates a depolarizing inward current (I(DAi)) and increases spontaneous firing in 32% of LHb neurons. I(DAi) was also observed upon application of amphetamine or the DA uptake blockers cocaine or GBR12935, indicating involvement of endogenous DA. I(DAi) was blocked by D4 receptor (D4R) antagonists (L745,870 or L741,742), and mimicked by a selective D4R agonist (A412997). I(DAi) was associated with increased whole-cell conductance and was blocked by Cs+ or a selective blocker of hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channel, ZD7288. I(DAi) was also associated with a depolarizing shift in half-activation voltage for the hyperpolarization-activated cation current (Ih) mediated by HCN channels. Recordings from LHb neurons containing fluorescent retrograde tracers revealed that I(DAi) was observed only in cells projecting to the RMTg and not the VTA. In parallel with direct depolarization, DA also strongly increased synaptic glutamate release and reduced synaptic GABA release onto LHb cells. These results demonstrate that DA can excite glutamatergic LHb output to RMTg via multiple cellular mechanisms. Since the RMTg strongly inhibits midbrain DA neurons, activation of LHb output to RMTg by DA represents a negative feedback loop that may dampen DA neuron output following activation.

Reexposure to nicotine during withdrawal increases the pacemaking activity of cholinergic habenular neurons

The discovery of genetic variants in the cholinergic receptor nicotinic CHRNA5-CHRNA3-CHRNB4 gene cluster associated with heavy smoking and higher relapse risk has led to the identification of the midbrain habenula-interpeduncular axis as a critical relay circuit in the control of nicotine dependence. Although clear roles for α3, β4, and α5 receptors in nicotine aversion and withdrawal have been established, the cellular and molecular mechanisms that participate in signaling nicotine use and contribute to relapse have not been identified. Here, using translating ribosome affinity purification (TRAP) profiling, electrophysiology, and behavior, we demonstrate that cholinergic neurons, but not peptidergic neurons, of the medial habenula (MHb) display spontaneous tonic firing of 2-10 Hz generated by hyperpolarization-activated cyclic nucleotide-gated (HCN) pacemaker channels and that infusion of the HCN pacemaker antagonist ZD7288 in the habenula precipitates somatic and affective signs of withdrawal. Further, we show that a strong, α3β4-dependent increase in firing frequency is observed in these pacemaker neurons upon acute exposure to nicotine. No change in the basal or nicotine-induced firing was observed in cholinergic MHb neurons from mice chronically treated with nicotine. We observe, however, that, during withdrawal, reexposure to nicotine doubles the frequency of pacemaking activity in these neurons. These findings demonstrate that the pacemaking mechanism of cholinergic MHb neurons controls withdrawal, suggesting that the heightened nicotine sensitivity of these neurons during withdrawal may contribute to smoking relapse.

A glutamatergic projection from the lateral hypothalamus targets VTA-projecting neurons in the lateral habenula of the rat

Homeostasis describes the fundamental biological ability of individuals to maintain stable internal conditions in a changing environment. Homeostatic reactions include internal adjustments as well as behavioral responses. In vertebrates, behavioral responses are induced by the reward system. This system originates in the ventral tegmental area (VTA) and leads to increased dopamine levels in the forebrain whenever activated. A major inhibitor of VTA activity is the lateral habenula (LHb). This epithalamic structure is able to almost completely suppress dopamine release, either directly or via the rostromedial tegmental nucleus (RMTg), when rewarding expectations are not met. A major input to the LHb arises from the lateral hypothalamic area (LHA), an important regulator of the homeostatic system. Currently, little is known about the effects of the strong hypothalamic projection on the activity of LHb neurons. In the present study, we analyze neurotransmitters and cellular targets of the LHA-LHb projection in the rat. Therefore, anterograde tracing from the LHA was combined with the visualization of neurotransmitters in the LHb. These experiments revealed a mainly glutamatergic projection, probably exerting excitatory effects on the targeted LHb cells. These cellular targets were analyzed in a second step. Anterograde tracing from the LHA in combination with retrograde tracing from the VTA/RMTg region revealed that LHb neurons projecting to the VTA/RMTg region are densely targeted by the LHA projection. Visualization of synaptophysin at these contact sites indicates that the contact sites indeed are synapses. Taken together, the present study describes a strong mainly glutamatergic projection from the LHA that targets VTA/RMTg-projecting neurons in the LHb. These findings emphasize the potential role of the LHb as direct link between homeostatic areas and reward circuitries, which may be important for the control of homeostatic behaviors.