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

Development of the Epithalamus in Alligator mississippiensis

The epithalamus is present in all vertebrates where it is a central part of the dorsal diencephalic conduction system whose functions are critical for survival. The epithalamus consists of both nuclei and tracts. Studies on the development of the epithalamus in amniotes (reptiles, birds, and mammals) based on cytoarchitecture have commonly been part of a larger report on the embryogenesis of the diencephalon. Of these, observations on the epithalamus of reptiles are few with limited descriptions and figures. The present analysis fills this gap in knowledge by examining the development of the epithalamus in one group of reptiles, Alligator mississippiensis, using stains for cells and fibers. The time of origin and subsequent development of the nuclei and the tracts that course through the epithalamus were determined. These data provide a basis for future studies and for comparisons with other amniotes.

Role of phospholipase Cη1 in lateral habenula astrocytes in depressive-like behavior in mice

Phospholipase C (PLC) enzymes play crucial roles in intracellular calcium-signaling transduction. Several brain PLC subtypes have been extensively studied, implicating them in psychiatric disorders such as depression, epilepsy and schizophrenia. However, the role of the recently identified PLCη remains largely unknown. We found that PLCη1 is prominently expressed in lateral habenula (LHb) astrocytes. Here, to investigate its physiological role, we generated astrocyte-specific PLCη1 conditional knockout (cKO) mice (Plch1f/f; Aldh1l1-CreERT2). In these cKO mice, we observed a reduction in cellular morphological complexity metrics, such as total process length, as well as a decrease in the passive membrane conductance of LHb astrocytes. Additionally, neuronal function was impacted by the cKO, as the synaptic efficacy and firing rates of LHb neurons increased, while extrasynaptic long-term depression was impaired. Both tonic α-amino-3-hydroxy-5-methyl-4-isoxazolepdlropionic acid receptor/N-methyl-D-aspartate receptor (AMPAR/NMDAR) currents and extracellular glutamate levels were reduced. Interestingly, chemogenetic activation of astrocytes restored the reduced tonic AMPAR/NMDAR currents in cKO mice. Furthermore, LHb astrocyte-specific deletion of PLCη1 via AAV-GFAP-Cre injection induced depressive-like behaviors in mice, which were reversed by chemogenetic activation of LHb astrocytes. Finally, we found that restraint stress exposure decreased Plch1 mRNA expression in the LHb. These findings suggest that PLCη1 could be a potential therapeutic target for depression and highlight the critical role of astrocytes in the etiology of neuropsychiatric disorders.

A hypothalamic circuit underlying the dynamic control of social homeostasis

Social grouping increases survival in many species, including humans1,2. By contrast, social isolation generates an aversive state ('loneliness') that motivates social seeking and heightens social interaction upon reunion3-5. The observed rebound in social interaction triggered by isolation suggests a homeostatic process underlying the control of social need, similar to physiological drives such as hunger, thirst or sleep3,6. In this study, we assessed social responses in several mouse strains, among which FVB/NJ mice emerged as highly, and C57BL/6J mice as moderately, sensitive to social isolation. Using both strains, we uncovered two previously uncharacterized neuronal populations in the hypothalamic preoptic nucleus that are activated during either social isolation or social rebound and orchestrate the behaviour display of social need and social satiety, respectively. We identified direct connectivity between these two populations and with brain areas associated with social behaviour, emotional state, reward and physiological needs and showed that mice require touch to assess the presence of others and fulfil their social need. These data show a brain-wide neural system underlying social homeostasis and provide significant mechanistic insights into the nature and function of circuits controlling instinctive social need and for the understanding of healthy and diseased brain states associated with social context.

Mixed representations of choice direction and outcome by GABA/glutamate cotransmitting neurons in the entopeduncular nucleus

The basal ganglia (BG) are an evolutionarily conserved and phylogenetically old set of sub-cortical nuclei that guide action selection, evaluation, and reinforcement. The entopeduncular nucleus (EP) is a major BG output nucleus that contains a population of GABA/glutamate cotransmitting neurons (EPSst+) that specifically target the lateral habenula (LHb) and whose function in behavior remains mysterious. Here, we use a probabilistic switching task that requires an animal to maintain flexible relationships between action selection and evaluation to examine when and how GABA/glutamate cotransmitting neurons contribute to behavior. We find that EPSst+ neurons are strongly engaged during this task and show bidirectional changes in activity during the choice and outcome periods of a trial. We then tested the effects of either permanently blocking cotransmission or modifying the GABA/glutamate ratio on behavior in well-trained animals. Neither manipulation produced detectable changes in behavior despite significant changes in synaptic transmission in the LHb, demonstrating that the outputs of these neurons are not required for ongoing action-outcome updating in a probabilistic switching task.

Days-old zebrafish rapidly learn to recognize threatening agents through noradrenergic and forebrain circuits

Animals need to rapidly learn to recognize and avoid predators. This ability may be especially important for young animals due to their increased vulnerability. It is unknown whether, and how, nascent vertebrates are capable of such rapid learning. Here, we used a robotic predator-prey interaction assay to show that 1 week after fertilization-a developmental stage where they have approximately 1% the number of neurons of adults-zebrafish larvae rapidly and robustly learn to recognize a stationary object as a threat after the object pursues the fish for ∼1 min. Larvae continue to avoid the threatening object after it stops moving and can learn to distinguish threatening from non-threatening objects of a different color. Whole-brain functional imaging revealed the multi-timescale activity of noradrenergic neurons and forebrain circuits that encoded the threat. Chemogenetic ablation of those populations prevented the learning. Thus, a noradrenergic and forebrain multiregional network underlies the ability of young vertebrates to rapidly learn to recognize potential predators within their first week of life.

The Lateral Habenula Is Necessary for Maternal Behavior in the Naturally Parturient Primiparous Mouse Dam

Mammalian parenting is an unusually demanding commitment. How has the reward system been co-opted to ensure parental care? Previous work has implicated the lateral habenula (LHb), an epithalamic nucleus, as a potential intersection of parenting behavior and reward. Here, we examine the role of the LHb in the maternal behavior of naturally parturient primiparous mouse dams. We show that kainic acid lesions of the LHb induced a severe maternal neglect phenotype in dams toward their biological pups. Next, we demonstrate that chronic chemogenetic inactivation of the LHb using inhibitory DREADDs impaired acquisition and performance of various maternal behaviors, such as pup retrieval and nesting. We present a random intercept model suggesting LHb inactivation prevents the acquisition of pup retrieval, a novel maternal behavior in primiparous mouse dams, and decreases nest building performance, an already-established behavior, in primiparous mouse dams. Lastly, we examine the spatial histology of kainic acid-treated dams with a random intercept model, which suggests the role of LHb in maternal behavior may be preferentially localized at the posterior aspect of this structure. Together, these findings serve to establish the LHb as required for maternal behavior in the mouse dam, thereby complementing previous findings implicating the LHb in parental behavior using pup-sensitized virgin female mice.

STZ-induced hyperglycemia differentially influences mitochondrial distribution and morphology in the habenulointerpeduncular circuit

Introduction

Diabetes is a metabolic disorder of glucose homeostasis that is a significant risk factor for neurodegenerative diseases, such as Alzheimer's disease, as well as mood disorders, which often precede neurodegenerative conditions. We examined the medial habenulainterpeduncular nucleus (MHb-IPN), as this circuit plays crucial roles in mood regulation, has been linked to the development of diabetes after smoking, and is rich in cholinergic neurons, which are affected in other brain areas in Alzheimer's disease.

Methods

This study aimed to investigate the impact of streptozotocin (STZ)-induced hyperglycemia, a type 1 diabetes model, on mitochondrial and lipid homeostasis in 4% paraformaldehyde-fixed sections from the MHb and IPN of C57BL/6 J male mice, using a recently developed automated pipeline for mitochondrial analysis in confocal images. We examined different time points after STZ-induced diabetes onset to determine how the brain responded to chronic hyperglycemia, with the limitation that mitochondria and lipids were not examined with respect to cell type or intracellular location.

Results

Mitochondrial distribution and morphology differentially responded to hyperglycemia depending on time and brain area. Six weeks after STZ treatment, mitochondria in the ventral MHb and dorsal IPN increased in number and exhibited altered morphology, but no changes were observed in the lateral habenula (LHb) or ventral IPN. Strikingly, mitochondrial numbers returned to normal dynamics at 12 weeks. Both blood glucose level and glycated hemoglobin (HbA1C) correlated with mitochondrial dynamics in ventral MHb, whereas only HbA1C correlated in the IPN. We also examined lipid homeostasis using BODIPY staining for neutral lipids in this model given that diabetes is associated with disrupted lipid homeostasis. BODIPY staining intensity was unchanged in the vMHb of STZ-treated mice but increased in the IPN and VTA and decreased in the LHb at 12 weeks. Interestingly, areas that demonstrated changes in mitochondria had little change in lipid staining and vice versa.

Discussion

This study is the first to describe the specific impacts of diabetes on mitochondria in the MHb-IPN circuit and suggests that the cholinergic MHb is uniquely sensitive to diabetesinduced hyperglycemia. Further studies are needed to understand the functional and behavioral implications of these findings.

Substantial projections from the lateral division of the lateral habenula to the dorsal raphe nucleus and from the lateral habenula to the contralateral ventral tegmental area

The lateral habenular nucleus (LHb) projects to the dorsal raphe nucleus (DRN) and ventral tegmental area (VTA). Prior studies have reported that the medial division of the LHb (LHb-m) mainly projects to the DRN, while the LHb mainly projects to the ipsilateral VTA; however, due to only a few studies of projection ratio analysis, the degree of projection of minor and major pathways remains unclear, and the potential significance of minor pathways may be overlooked. After injecting the retrograde tracer into the mice DRN, the proportion of labeled neurons was 63.50 % in the LHb-m and 36.50 % in the lateral division of the LHb (LHb-l). The proportion distributions of labeled neurons were 26.90 % in the anterior LHb-m and 73.10 % in the posterior LHb-m. When the retrograde tracer was injected into the VTA, the percentage of labeled neurons was 64.85 % in the ipsilateral LHb and 35.15 % in the contralateral LHb. The number of cells projecting from anterior and posterior ipsilateral LHb-m to the VTA varied among the individual injection sites. Our quantitative analysis with multiple subjects showed that the projections from LHb-l to DRN and from LHb to contralateral VTA were indeed minor pathways, however, they were substantial. These results provide an anatomical basis for the potential importance of minor pathways projected from the LHb to the VTA and DRN and for the difference in the densities of neurons projecting from the anterior and posterior LHb-m to the DRN.

Interpretable spatially aware dimension reduction of spatial transcriptomics with STAMP

Spatial transcriptomics produces high-dimensional gene expression measurements with spatial context. Obtaining a biologically meaningful low-dimensional representation of such data is crucial for effective interpretation and downstream analysis. Here, we present Spatial Transcriptomics Analysis with topic Modeling to uncover spatial Patterns (STAMP), an interpretable spatially aware dimension reduction method built on a deep generative model that returns biologically relevant, low-dimensional spatial topics and associated gene modules. STAMP can analyze data ranging from a single section to multiple sections and from different technologies to time-series data, returning topics matching known biological domains and associated gene modules containing established markers highly ranked within. In a lung cancer sample, STAMP delineated cell states with supporting markers at a higher resolution than the original annotation and uncovered cancer-associated fibroblasts concentrated on the tumor edge's exterior. In time-series data of mouse embryonic development, STAMP disentangled the erythro-myeloid hematopoiesis and hepatocytes developmental trajectories within the liver. STAMP is highly scalable and can handle more than 500,000 cells.

Mixed representations of choice direction and outcome by GABA/glutamate cotransmitting neurons in the entopeduncular nucleus

The basal ganglia (BG) are an evolutionarily conserved and phylogenetically old set of sub-cortical nuclei that guide action selection, evaluation, and reinforcement. The entopeduncular nucleus (EP) is a major BG output nucleus that contains a population of GABA/glutamate cotransmitting neurons (EP Sst+ ) that specifically target the lateral habenula (LHb) and whose function in behavior remains mysterious. Here we use a probabilistic switching task that requires an animal to maintain flexible relationships between action selection and evaluation to examine when and how GABA/glutamate cotransmitting neurons contribute to behavior. We find that EP Sst+ neurons are strongly engaged during this task and show bidirectional changes in activity during the choice and outcome periods of a trial. We then tested the effects of either permanently blocking cotransmission or modifying the GABA/glutamate ratio on behavior in well-trained animals. Neither manipulation produced detectable changes in behavior despite significant changes in synaptic transmission in the LHb, demonstrating that the outputs of these neurons are not required for on-going action-outcome updating in a probabilistic switching task.

Nighttime-specific differential gene expression in suprachiasmatic nucleus and habenula is associated with resilience to chronic social stress

The molecular mechanisms that link stress and biological rhythms still remain unclear. The habenula (Hb) is a key brain region involved in regulating diverse types of emotion-related behaviours while the suprachiasmatic nucleus (SCN) is the body's central clock. To investigate the effects of chronic social stress on transcription patterns, we performed gene expression analysis in the Hb and SCN of stress-naïve and stress-exposed mice. Our analysis revealed a large number of differentially expressed genes and enrichment of synaptic and cell signalling pathways between resilient and stress-naïve mice at zeitgeber 16 (ZT16) in both the Hb and SCN. This transcriptomic signature was nighttime-specific and observed only in stress-resilient mice. In contrast, there were relatively few differences between the stress-susceptible and stress-naïve groups across time points. Our results reinforce the functional link between circadian gene expression patterns and differential responses to stress, thereby highlighting the importance of temporal expression patterns in homoeostatic stress responses.

Dysregulation of kappa opioid receptor neuromodulation of lateral habenula synaptic function following a repetitive mild traumatic brain injury

Mild traumatic brain injury (mTBI) increases the risk of affective disorders, anxiety and substance use disorder. The lateral habenula (LHb) plays an important role in pathophysiology of psychiatric disorders. Recently, we demonstrated a causal link between mTBI-induced LHb hyperactivity due to excitation/inhibition (E/I) imbalance and motivational deficits in male mice using a repetitive closed head injury mTBI model. A major neuromodulatory system that is responsive to traumatic brain injuries, influences affective states and also modulates LHb activity is the dynorphin/kappa opioid receptor (Dyn/KOR) system. However, the effects of mTBI on KOR neuromodulation of LHb function are unknown. Here, we first used retrograde tracing in male and female Cre mouse lines and identified several major KOR-expressing and two prominent Dyn-expressing inputs projecting to the mouse LHb, highlighting the medial prefrontal cortex (mPFC) and the ventromedial nucleus of the hypothalamus (VMH) as the main LHb-projecting Dyn inputs that regulate KOR signaling to the LHb. We then functionally evaluated the effects of in vitro KOR modulation of spontaneous synaptic activity within the LHb of male and female sham and mTBI mice at 4 week post-injury. We observed sex-specific differences in spontaneous release of glutamate and GABA from presynaptic terminals onto LHb neurons with higher levels of presynaptic glutamate and GABA release in females compared to male mice. However, KOR effects on the spontaneous E/I ratios and synaptic drive ratio within the LHb did not differ between male and female sham and mTBI mice. KOR activation generally suppressed spontaneous glutamatergic transmission without altering GABAergic transmission, resulting in a significant but sex-similar reduction in net spontaneous E/I and synaptic drive ratios in LHb neurons of sham mice. Following mTBI, while responses to KOR activation at LHb glutamatergic synapses remained intact, LHb GABAergic synapses acquired an additional sensitivity to KOR-mediated inhibition where we observed a reduction in GABA release probability in response to KOR stimulation in LHb neurons of mTBI mice. Further analysis of percent change in spontaneous synaptic ratios induced by KOR activation revealed that independent of sex mTBI switches KOR-driven synaptic inhibition of LHb neurons (normally observed in sham mice) in a subset of mTBI mice toward synaptic excitation resulting in mTBI-induced divergence of KOR actions within the LHb. Overall, we uncovered the sources of major Dyn/KOR-expressing synaptic inputs projecting to the mouse LHb. We demonstrate that an engagement of intra-LHb Dyn/KOR signaling provides a global KOR-driven synaptic inhibition within the mouse LHb independent of sex. The additional engagement of KOR-mediated action on LHb GABAergic transmission by mTBI could contribute to the E/I imbalance after mTBI, with Dyn/KOR signaling serving as a disinhibitory mechanism for LHb neurons of a subset of mTBI mice.

Serotonin release in the habenula during emotional contagion promotes resilience

Negative emotional contagion-witnessing others in distress-affects an individual's emotional responsivity. However, whether it shapes coping strategies when facing future threats remains unknown. We found that mice that briefly observe a conspecific being harmed become resilient, withstanding behavioral despair after an adverse experience. Photometric recordings during negative emotional contagion revealed increased serotonin (5-HT) release in the lateral habenula. Whereas 5-HT and emotional contagion reduced habenular burst firing, limiting 5-HT synthesis prevented burst plasticity. Enhancing raphe-to-habenula 5-HT was sufficient to recapitulate resilience. In contrast, reducing 5-HT release in the habenula made witnessing a conspecific in distress ineffective to promote the resilient phenotype after adversity. These findings reveal that 5-HT supports vicarious emotions and leads to resilience by tuning definite patterns of habenular neuronal activity.

A conserved cell-type gradient across the human mediodorsal and paraventricular thalamus

The mediodorsal thalamus (MD) and adjacent midline nuclei are important for cognition and mental illness, but their cellular composition is not well defined. Using single-nucleus and spatial transcriptomics, we identified a conserved excitatory neuron gradient, with distinct spatial mapping of individual clusters. One end of the gradient was expanded in human MD compared to mice, which may be related to the expansion of granular prefrontal cortex in hominids. Moreover, neurons preferentially mapping onto the parvocellular division MD were associated with genetic risk for schizophrenia and bipolar disorder. Midbrain-derived inhibitory interneurons were enriched in human MD and implicated in genetic risk for major depressive disorder.

The behavioral relevance of a modular organization in the lateral habenula

Behavioral strategies for survival rely on the updates the brain continuously makes based on the surrounding environment. External stimuli-neutral, positive, and negative-relay core information to the brain, where a complex anatomical network rapidly organizes actions, including approach or escape, and regulates emotions. Human neuroimaging and physiology in nonhuman primates, rodents, and teleosts suggest a pivotal role of the lateral habenula in translating external information into survival behaviors. Here, we review the literature describing how discrete habenular modules-reflecting the molecular signatures, anatomical connectivity, and functional components-are recruited by environmental stimuli and cooperate to prompt specific behavioral outcomes. We argue that integration of these findings in the context of valence processing for reinforcing or discouraging behaviors is necessary, offering a compelling model to guide future work.

A potentiation of REM sleep-active neurons in the lateral habenula may be responsible for the sleep disturbance in depression

Psychiatric disorders with dysfunction of the lateral habenula (LHb) show sleep disturbance, especially a disinhibition of rapid eye movement (REM) sleep in major depression. However, the role of LHb in physiological sleep control and how LHb contributes to sleep disturbance in major depression remain elusive. Here, we found that functional manipulations of LHb glutamatergic neurons bidirectionally modulated both non-REM (NREM) sleep and REM sleep. Activity recording revealed heterogeneous activity patterns of LHb neurons across sleep/wakefulness cycles, but LHb neurons were preferentially active during REM sleep. Using an activity-dependent tagging method, we selectively labeled a population of REM sleep-active LHb neurons and demonstrated that these neurons specifically promoted REM sleep. Neural circuit studies showed that LHb neurons regulated REM sleep via projections to the ventral tegmental area but not to the rostromedial tegmental nucleus. Furthermore, we found that the increased REM sleep in a depression mouse model was associated with a potentiation of REM sleep-active LHb neurons, including an increased proportion, elevated spike firing, and altered activity mode. Importantly, inhibition of REM sleep-active LHb neurons not only attenuated the increased REM sleep but also alleviated depressive-like behaviors in a depression mouse model. Thus, our results demonstrated that REM sleep-active LHb neurons selectively promoted REM sleep, and a potentiation of these neurons contributed to depression-associated sleep disturbance.

aKNNO: single-cell and spatial transcriptomics clustering with an optimized adaptive k-nearest neighbor graph

Typical clustering methods for single-cell and spatial transcriptomics struggle to identify rare cell types, while approaches tailored to detect rare cell types gain this ability at the cost of poorer performance for grouping abundant ones. Here, we develop aKNNO to simultaneously identify abundant and rare cell types based on an adaptive k-nearest neighbor graph with optimization. Benchmarking on 38 simulated and 20 single-cell and spatial transcriptomics datasets demonstrates that aKNNO identifies both abundant and rare cell types more accurately than general and specialized methods. Using only gene expression aKNNO maps abundant and rare cells more precisely compared to integrative approaches.

Burst firing in Output-Defined Parallel Habenula Circuit Underlies the Antidepressant Effects of Bright Light Treatment

Research highlights the significance of increased bursting in lateral habenula (LHb) neurons in depression and as a focal point for bright light treatment (BLT). However, the precise spike patterns of LHb neurons projecting to different brain regions during depression, their roles in depression development, and BLT's therapeutic action remain elusive. Here, LHb neurons are found projecting to the dorsal raphe nucleus (DRN), ventral tegmental area (VTA), and median raphe nucleus (MnR) exhibit increased bursting following aversive stimuli exposure, correlating with distinct depressive symptoms. Enhanced bursting in DRN-projecting LHb neurons is pivotal for anhedonia and anxiety, while concurrent bursting in LHb neurons projecting to the DRN, VTA, and MnR is essential for despair. Remarkably, reducing bursting in distinct LHb neuron subpopulations underlies the therapeutic effects of BLT on specific depressive behaviors. These findings provide valuable insights into the mechanisms of depression and the antidepressant action of BLT.

Transcriptomic analysis of the human habenula in schizophrenia

Pathophysiology of many neuropsychiatric disorders, including schizophrenia (SCZD), is linked to habenula (Hb) function. While pharmacotherapies and deep brain stimulation targeting the Hb are emerging as promising therapeutic treatments, little is known about the cell type-specific transcriptomic organization of the human Hb or how it is altered in SCZD. Here we define the molecular neuroanatomy of the human Hb and identify transcriptomic changes in individuals with SCZD compared to neurotypical controls. Utilizing Hb-enriched postmortem human brain tissue, we performed single nucleus RNA-sequencing (snRNA-seq; n=7 neurotypical donors) and identified 17 molecularly defined Hb cell types across 16,437 nuclei, including 3 medial and 7 lateral Hb populations, several of which were conserved between rodents and humans. Single molecule fluorescent in situ hybridization (smFISH; n=3 neurotypical donors) validated snRNA-seq Hb cell types and mapped their spatial locations. Bulk RNA-sequencing and cell type deconvolution in Hb-enriched tissue from 35 individuals with SCZD and 33 neurotypical controls yielded 45 SCZD-associated differentially expressed genes (DEGs, FDR < 0.05), with 32 (71%) unique to Hb-enriched tissue. eQTL analysis identified 717 independent SNP-gene pairs (FDR < 0.05), where either the SNP is a SCZD risk variant (16 pairs) or the gene is a SCZD DEG (7 pairs). eQTL and SCZD risk colocalization analysis identified 16 colocalized genes. These results identify topographically organized cell types with distinct molecular signatures in the human Hb and demonstrate unique genetic changes associated with SCZD, thereby providing novel molecular insights into the role of Hb in neuropsychiatric disorders.

One Sentence Summary

Transcriptomic analysis of the human habenula and identification of molecular changes associated with schizophrenia risk and illness state.

Multiple cholinergic receptor subtypes coordinate dual modulation of acetylcholine on anterior and posterior paraventricular thalamic neurons

Paraventricular thalamus (PVT) plays important roles in the regulation of emotion and motivation through connecting many brain structures including the midbrain and the limbic system. Although acetylcholine (ACh) neurons of the midbrain were reported to send projections to PVT, little is known about how cholinergic signaling regulates PVT neurons. Here, we used both RNAscope and slice patch-clamp recordings to characterize cholinergic receptor expression and ACh modulation of PVT neurons in mice. We found ACh excited a majority of anterior PVT (aPVT) neurons but predominantly inhibited posterior PVT (pPVT) neurons. Compared to pPVT with more inhibitory M2 receptors, aPVT expressed higher levels of all excitatory receptor subtypes including nicotinic α4, α7, and muscarinic M1 and M3. The ACh-induced excitation was mimicked by nicotine and antagonized by selective blockers for α4β2 and α7 nicotinic ACh receptor (nAChR) subtypes as well as selective antagonists for M1 and M3 muscarinic ACh receptors (mAChR). The ACh-induced inhibition was attenuated by selective M2 and M4 mAChR receptor antagonists. Furthermore, we found ACh increased the frequency of excitatory postsynaptic currents (EPSCs) on a majority of aPVT neurons but decreased EPSC frequency on a larger number of pPVT neurons. In addition, ACh caused an acute increase followed by a lasting reduction in inhibitory postsynaptic currents (IPSCs) on PVT neurons of both subregions. Together, these data suggest that multiple AChR subtypes coordinate a differential modulation of ACh on aPVT and pPVT neurons.

Optogenetic activation of the lateral habenulaD1R-ventral tegmental area circuit induces depression-like behavior in mice

A number of different receptors are distributed in glutamatergic neurons of the lateral habenula (LHb). These glutamatergic neurons are involved in different neural pathways, which may identify how the LHb regulates various physiological functions. However, the role of dopamine D1 receptor (D1R)-expressing habenular neurons projecting to the ventral tegmental area (VTA) (LHbD1R-VTA) remains not well understood. In the current study, to determine the activity of D1R-expressing neurons in LHb, D1R-Cre mice were used to establish the chronic restraint stress (CRS) depression model. Adeno-associated virus was injected into bilateral LHb in D1R-Cre mice to examine whether optogenetic activation of the LHb D1R-expressing neurons and their projections could induce depression-like behavior. Optical fibers were implanted in the LHb and VTA, respectively. To investigate whether optogenetic inhibition of the LHbD1R-VTA circuit could produce antidepressant-like effects, the adeno-associated virus was injected into the bilateral LHb in the D1R-Cre CRS model, and optical fibers were implanted in the bilateral VTA. The D1R-expressing neuronal activity in the LHb was increased in the CRS depression model. Optogenetic activation of the D1R-expressing neurons in LHb induced behavioral despair and anhedonia, which could also be induced by activation of the LHbD1R-VTA axons. Conversely, optogenetic inhibition of the LHbD1R-VTA circuit improved behavioral despair and anhedonia in the CRS depression model. D1R-expressing glutamatergic neurons in the LHb and their projections to the VTA are involved in the occurrence and regulation of depressive-like behavior.

The lateral habenula: A hub for value-guided behavior

The habenula is an evolutionarily highly conserved diencephalic brain region divided into two major parts, medial and lateral. Over the past two decades, studies of the lateral habenula (LHb), in particular, have identified key functions in value-guided behavior in health and disease. In this review, we focus on recent insights into LHb connectivity and its functional relevance for different types of aversive and appetitive value-guided behavior. First, we give an overview of the anatomical organization of the LHb and its main cellular composition. Next, we elaborate on how distinct LHb neuronal subpopulations encode aversive and appetitive stimuli and on their involvement in more complex decision-making processes. Finally, we scrutinize the afferent and efferent connections of the LHb and discuss their functional implications for LHb-dependent behavior. A deepened understanding of distinct LHb circuit components will substantially contribute to our knowledge of value-guided behavior.

Whole-brain connections of glutamatergic neurons in the mouse lateral habenula in both sexes

BACKGROUND

The lateral habenula (LHb) is an epithalamus nucleus that is evolutionarily conserved and involved in various physiological functions, such as encoding value signals, integrating emotional information, and regulating related behaviors. The cells in the LHb are predominantly glutamatergic and have heterogeneous functions in response to different stimuli. The circuitry connections of the LHb glutamatergic neurons play a crucial role in integrating a wide range of events. However, the circuitry connections of LHb glutamatergic neurons in both sexes have not been thoroughly investigated.

METHODS

In this study, we injected Cre-dependent retrograde trace virus and anterograde synaptophysin-labeling virus into the LHb of adult male and female Vglut2-ires-Cre mice, respectively. We then quantitatively analyzed the input and output of the LHb glutamatergic connections in both the ipsilateral and contralateral whole brain.

RESULTS

Our findings showed that the inputs to LHbvGlut2 neurons come from more than 30 brain subregions, including the cortex, striatum, pallidum, thalamus, hypothalamus, midbrain, pons, medulla, and cerebellum with no significant differences between males and females. The outputs of LHbvGlut2 neurons targeted eight large brain regions, primarily focusing on the midbrain and pons nuclei, with distinct features in presynaptic bouton across different brain subregions. While correlation and cluster analysis revealed differences in input and collateral projection features, the input-output connection pattern of LHbvGlut2 neurons in both sexes was highly similar.

CONCLUSIONS

This study provides a systematic and comprehensive analysis of the input and output connections of LHbvGlut2 neurons in male and female mice, shedding light on the anatomical architecture of these specific cell types in the mouse LHb. This structural understanding can help guide further investigations into the complex functions of the LHb.

Input-output specific orchestration of aversive valence in lateral habenula during stress dynamics

Stress has been considered as a major risk factor for depressive disorders, triggering depression onset via inducing persistent dysfunctions in specialized brain regions and neural circuits. Among various regions across the brain, the lateral habenula (LHb) serves as a critical hub for processing aversive information during the dynamic process of stress accumulation, thus having been implicated in the pathogenesis of depression. LHb neurons integrate aversive valence conveyed by distinct upstream inputs, many of which selectively innervate the medial part (LHbM) or lateral part (LHbL) of LHb. LHb subregions also separately assign aversive valence via dissociable projections to the downstream targets in the midbrain which provides feedback loops. Despite these strides, the spatiotemporal dynamics of LHb-centric neural circuits remain elusive during the progression of depression-like state under stress. In this review, we attempt to describe a framework in which LHb orchestrates aversive valence via the input-output specific neuronal architecture. Notably, a physiological form of Hebbian plasticity in LHb under multiple stressors has been unveiled to incubate neuronal hyperactivity in an input-specific manner, which causally encodes chronic stress experience and drives depression onset. Collectively, the recent progress and future efforts in elucidating LHb circuits shed light on early interventions and circuit-specific antidepressant therapies.

Energy balance drives diurnal and nocturnal brain transcriptome rhythms

Plasticity in daily timing of activity has been observed in many species, yet the underlying mechanisms driving nocturnality and diurnality are unknown. By regulating how much wheel-running activity will be rewarded with a food pellet, we can manipulate energy balance and switch mice to be nocturnal or diurnal. Here, we present the rhythmic transcriptome of 21 tissues, including 17 brain regions, sampled every 4 h over a 24-h period from nocturnal and diurnal male CBA/CaJ mice. Rhythmic gene expression across tissues comprised different sets of genes with minimal overlap between nocturnal and diurnal mice. We show that non-clock genes in the suprachiasmatic nucleus (SCN) change, and the habenula was most affected. Our results indicate that adaptive flexibility in daily timing of behavior is supported by gene expression dynamics in many tissues and brain regions, especially in the habenula, which suggests a crucial role for the observed nocturnal-diurnal switch.

Relapse to cocaine seeking is regulated by medial habenula NR4A2/NURR1 in mice

Drugs of abuse can persistently change the reward circuit in ways that contribute to relapse behavior, partly via mechanisms that regulate chromatin structure and function. Nuclear orphan receptor subfamily4 groupA member2 (NR4A2, also known as NURR1) is an important effector of histone deacetylase 3 (HDAC3)-dependent mechanisms in persistent memory processes and is highly expressed in the medial habenula (MHb), a region that regulates nicotine-associated behaviors. Here, expressing the Nr4a2 dominant negative (Nurr2c) in the MHb blocks reinstatement of cocaine seeking in mice. We use single-nucleus transcriptomics to characterize the molecular cascade following Nr4a2 manipulation, revealing changes in transcriptional networks related to addiction, neuroplasticity, and GABAergic and glutamatergic signaling. The network controlled by NR4A2 is characterized using a transcription factor regulatory network inference algorithm. These results identify the MHb as a pivotal regulator of relapse behavior and demonstrate the importance of NR4A2 as a key mechanism driving the MHb component of relapse.

The lateral habenula is required for maternal behavior in the mouse dam

Mammalian parenting is an unusually demanding commitment. How did evolution co-opt the reward system to ensure parental care? Previous work has implicated the lateral habenula (LHb), an epithalamic nucleus, as a potential intersection of parenting behavior and reward. Here, we examine the role of the LHb in the maternal behavior of naturally parturient mouse dams. We show that kainic acid lesions of the LHb induced a severe maternal neglect phenotype in dams towards their biological pups. Next, we demonstrate that through chronic chemogenetic inactivation of the LHb using DREADDs impaired acquisition and performance of various maternal behaviors, such as pup retrieval and nesting. We present a random intercepts model suggesting LHb-inactivation prevents the acquisition of the novel pup retrieval maternal behavior and decreases nest building performance, an already-established behavior, in primiparous mouse dams. Lastly, we examine the spatial histology of kainic-acid treated dams with a random intercepts model, which suggests that the role of LHb in maternal behavior may be preferentially localized at the posterior aspect of this structure. Together, these findings serve to establish the LHb as required for maternal behavior in the mouse dam, thereby complementing previous findings implicating the LHb in parental behavior using pup-sensitized virgin female mice.

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.

The Neuroanatomy of the Habenular Complex and Its Role in the Regulation of Affective Behaviors

The habenular complex is a diencephalic structure divided into the medial and lateral divisions that lie within the epithalamus of most vertebrates. This brain structure, whose activities are mainly regulated via inputs/outputs from and to the stria medullaris and the fasciculus retroflexus, plays a significant role in the modulation of anti-reward behaviors in both the rodent and human brain. Such anti-reward circuits are regulated by dopaminergic and serotonergic projections with several other subcortical and cortical regions; therefore, it is plausible that impairment to this key subcortical structure or its connections contributes to the pathogenesis of affective disorders. Current literature reveals the existence of structural changes in the habenula complex in individuals afflicted by such disorders; however, there is a need for more comprehensive investigations to elucidate the underlying neuroanatomical connections that underpin disease development. In this review article, we aim to provide a comprehensive view of the neuroanatomical differences between the rodent and human habenular complex, the main circuitries, and provide an update on the emerging roles of this understudied subcortical structure in the control of affective behaviors, with special emphasis to morbid conditions of the affective sphere.

Stress-induced translation of KCNB1 contributes to the enhanced synaptic transmission of the lateral habenula

The lateral habenula (LHb) is a well-established brain region involved in depressive disorders. Synaptic transmission of the LHb neurons is known to be enhanced by stress exposure; however, little is known about genetic modulators within the LHb that respond to stress. Using recently developed molecular profiling methods by phosphorylated ribosome capture, we obtained transcriptome profiles of stress responsive LHb neurons during acute physical stress. Among such genes, we found that KCNB1 (Kv2.1 channel), a delayed rectifier and voltage-gated potassium channel, exhibited increased expression following acute stress exposure. To determine the roles of KCNB1 on LHb neurons during stress, we injected short hairpin RNA (shRNA) against the kcnb1 gene to block its expression prior to stress exposure. We observed that the knockdown of KCNB1 altered the basal firing pattern of LHb neurons. Although KCNB1 blockade did not rescue despair-like behaviors in acute learned helplessness (aLH) animals, we found that KCNB1 knockdown prevented the enhancement of synaptic strength in LHb neuron after stress exposure. This study suggests that KCNB1 may contribute to shape stress responses by regulating basal firing patterns and neurotransmission intensity of LHb neurons.

Plasticity of neuronal dynamics in the lateral habenula for cue-punishment associative learning

The brain's ability to associate threats with external stimuli is vital to execute essential behaviours including avoidance. Disruption of this process contributes instead to the emergence of pathological traits which are common in addiction and depression. However, the mechanisms and neural dynamics at the single-cell resolution underlying the encoding of associative learning remain elusive. Here, employing a Pavlovian discrimination task in mice we investigate how neuronal populations in the lateral habenula (LHb), a subcortical nucleus whose excitation underlies negative affect, encode the association between conditioned stimuli and a punishment (unconditioned stimulus). Large population single-unit recordings in the LHb reveal both excitatory and inhibitory responses to aversive stimuli. Additionally, local optical inhibition prevents the formation of cue discrimination during associative learning, demonstrating a critical role of LHb activity in this process. Accordingly, longitudinal in vivo two-photon imaging tracking LHb calcium neuronal dynamics during conditioning reveals an upward or downward shift of individual neurons' CS-evoked responses. While recordings in acute slices indicate strengthening of synaptic excitation after conditioning, support vector machine algorithms suggest that postsynaptic dynamics to punishment-predictive cues represent behavioral cue discrimination. To examine the presynaptic signaling in LHb participating in learning we monitored neurotransmitter dynamics with genetically-encoded indicators in behaving mice. While glutamate, GABA, and serotonin release in LHb remain stable across associative learning, we observe enhanced acetylcholine signaling developing throughout conditioning. In summary, converging presynaptic and postsynaptic mechanisms in the LHb underlie the transformation of neutral cues in valued signals supporting cue discrimination during learning.

Synaptic and circuit functions of multitransmitter neurons in the mammalian brain

Neurons in the mammalian brain are not limited to releasing a single neurotransmitter but often release multiple neurotransmitters onto postsynaptic cells. Here, we review recent findings of multitransmitter neurons found throughout the mammalian central nervous system. We highlight recent technological innovations that have made the identification of new multitransmitter neurons and the study of their synaptic properties possible. We also focus on mechanisms and molecular constituents required for neurotransmitter corelease at the axon terminal and synaptic vesicle, as well as some possible functions of multitransmitter neurons in diverse brain circuits. We expect that these approaches will lead to new insights into the mechanism and function of multitransmitter neurons, their role in circuits, and their contribution to normal and pathological brain function.

Spatial atlas of the mouse central nervous system at molecular resolution

Spatially charting molecular cell types at single-cell resolution across the 3D volume is critical for illustrating the molecular basis of brain anatomy and functions. Single-cell RNA sequencing has profiled molecular cell types in the mouse brain1,2, but cannot capture their spatial organization. Here we used an in situ sequencing method, STARmap PLUS3,4, to profile 1,022 genes in 3D at a voxel size of 194 × 194 × 345 nm3, mapping 1.09 million high-quality cells across the adult mouse brain and spinal cord. We developed computational pipelines to segment, cluster and annotate 230 molecular cell types by single-cell gene expression and 106 molecular tissue regions by spatial niche gene expression. Joint analysis of molecular cell types and molecular tissue regions enabled a systematic molecular spatial cell-type nomenclature and identification of tissue architectures that were undefined in established brain anatomy. To create a transcriptome-wide spatial atlas, we integrated STARmap PLUS measurements with a published single-cell RNA-sequencing atlas1, imputing single-cell expression profiles of 11,844 genes. Finally, we delineated viral tropisms of a brain-wide transgene delivery tool, AAV-PHP.eB5,6. Together, this annotated dataset provides a single-cell resource that integrates the molecular spatial atlas, brain anatomy and the accessibility to genetic manipulation of the mammalian central nervous system.

Single-Nucleus Transcriptional Profiling of GAD2-Positive Neurons From Mouse Lateral Habenula Reveals Distinct Expression of Neurotransmission- and Depression-Related Genes

Background

Glutamatergic projection neurons of the lateral habenula (LHb) drive behavioral state modulation by regulating the activity of midbrain monoaminergic neurons. Identifying circuit mechanisms that modulate LHb output is of interest for understanding control of motivated behaviors.

Methods

A small population of neurons within the medial subnucleus of the mouse LHb express the GABAergic (gamma-aminobutyric acidergic)-synthesizing enzyme GAD2, and they can inhibit nearby LHb projection neurons; however, these neurons lack markers of classic inhibitory interneurons, and they coexpress the vesicular glutamate transporter VGLUT2. To determine the molecular phenotype of these neurons, we genetically tagged the nuclei of GAD2-positive cells and used fluorescence-activated nuclear sorting to isolate and enrich these nuclei for single-nucleus RNA sequencing.

Results

Our data confirm that GAD2+/VGLUT2+ neurons intrinsic to the LHb coexpress markers of both glutamatergic and GABAergic transmission and that they are transcriptionally distinct from either GABAergic interneurons or habenular glutamatergic neurons. We identify gene expression programs within these cells that show sex-specific differences in expression and that are implicated in major depressive disorder, which has been linked to LHb hyperactivity. Finally, we identify the Ntng2 gene encoding the cell adhesion protein netrin-G2 as a marker of LHb GAD2+/VGLUT2+ neurons and a gene product that may contribute to their target projections.

Conclusions

These data show the value of using genetic enrichment of rare cell types for transcriptome studies, and they advance understanding of the molecular composition of a functionally important class of GAD2+ neurons in the LHb.

Feed-forward Activation of Habenula Cholinergic Neurons by Local Acetylcholine

While the functional and behavioral role of the medial habenula (MHb) is still emerging, recent data indicate an involvement of this nuclei in regulating mood, aversion, and addiction. Unique to the MHb is a large cluster of cholinergic neurons that project to the interpeduncular nucleus and densely express acetylcholine receptors (AChRs) suggesting that the activity of these cholinergic neurons may be regulated by ACh itself. Whether endogenous ACh from within the habenula regulates cholinergic neuron activity has not been demonstrated. Supporting a role for ACh in modulating MHb activity, acetylcholinesterase inhibitors increased the firing rate of MHb cholinergic neurons in mouse habenula slices, an effect blocked by AChR antagonists and mediated by ACh which was detected via expressing fluorescent ACh sensors in MHb in vivo. To test if cholinergic afferents innervate MHb cholinergic neurons, we used anterograde and retrograde viral tracing to identify cholinergic inputs. Surprisingly, tracing experiments failed to detect cholinergic inputs into the MHb, including from the septum, suggesting that MHb cholinergic neurons may release ACh within the MHb to drive cholinergic activity. To test this hypothesis, we expressed channelrhodopsin in a portion of MHb cholinergic neurons while recording from non-opsin-expressing neurons. Light pulses progressively increased activity of MHb cholinergic neurons indicating feed-forward activation driven by MHb ACh release. These data indicate MHb cholinergic neurons may utilize a unique feed-forward mechanism to synchronize and increase activity by releasing local ACh.

Critical role of lateral habenula circuits in the control of stress-induced palatable food consumption

Chronic stress fuels the consumption of palatable food and can enhance obesity development. While stress- and feeding-controlling pathways have been identified, how stress-induced feeding is orchestrated remains unknown. Here, we identify lateral habenula (LHb) Npy1r-expressing neurons as the critical node for promoting hedonic feeding under stress, since lack of Npy1r in these neurons alleviates the obesifying effects caused by combined stress and high fat feeding (HFDS) in mice. Mechanistically, this is due to a circuit originating from central amygdala NPY neurons, with the upregulation of NPY induced by HFDS initiating a dual inhibitory effect via Npy1r signaling onto LHb and lateral hypothalamus neurons, thereby reducing the homeostatic satiety effect through action on the downstream ventral tegmental area. Together, these results identify LHb-Npy1r neurons as a critical node to adapt the response to chronic stress by driving palatable food intake in an attempt to overcome the negative valence of stress.

Tet2 acts in the lateral habenula to regulate social preference in mice

The lateral habenula (LHb) has been considered a moderator of social behaviors. However, it remains unknown how LHb regulates social interaction. Here, we show that the hydroxymethylase Tet2 is highly expressed in the LHb. Tet2 conditional knockout (cKO) mice exhibit impaired social preference; however, replenishing Tet2 in the LHb rescues social preference impairment in Tet2 cKO mice. Tet2 cKO alters DNA hydroxymethylation (5hmC) modifications in genes that are related to neuronal functions, as is confirmed by miniature two-photon microscopy data. Further, Tet2 knockdown in the glutamatergic neurons of LHb causes impaired social behaviors, but the inhibition of glutamatergic excitability restores social preference. Mechanistically, we identify that Tet2 deficiency reduces 5hmC modifications on the Sh3rf2 promoter and Sh3rf2 mRNA expression. Interestingly, Sh3rf2 overexpression in the LHb rescues social preference in Tet2 cKO mice. Therefore, Tet2 in the LHb may be a potential therapeutic target for social behavior deficit-related disorders such as autism.

A Hypothalamic Circuit Underlying the Dynamic Control of Social Homeostasis

Social grouping increases survival in many species, including humans1,2. By contrast, social isolation generates an aversive state (loneliness) that motivates social seeking and heightens social interaction upon reunion3-5. The observed rebound in social interaction triggered by isolation suggests a homeostatic process underlying the control of social drive, similar to that observed for physiological needs such as hunger, thirst or sleep3,6. In this study, we assessed social responses in multiple mouse strains and identified the FVB/NJ line as exquisitely sensitive to social isolation. Using FVB/NJ mice, we uncovered two previously uncharacterized neuronal populations in the hypothalamic preoptic nucleus that are activated during social isolation and social rebound and that orchestrate the behavior display of social need and social satiety, respectively. We identified direct connectivity between these two populations of opposite function and with brain areas associated with social behavior, emotional state, reward, and physiological needs, and showed that animals require touch to assess the presence of others and fulfill their social need, thus revealing a brain-wide neural system underlying social homeostasis. These findings offer mechanistic insight into the nature and function of circuits controlling instinctive social need and for the understanding of healthy and diseased brain states associated with social context.

Probing the orphan receptors: Tools and directions

The endogenous ligands activating a large fraction of the G Protein Coupled Receptor (GPCR) family members have yet to be identified. These receptors are commonly labeled as orphans (oGPCRs), and because of the absence of available pharmacological tools they are currently understudied. Nonetheless, genome wide association studies, together with research using animal models identified many physiological functions regulated by oGPCRs. Similarly, mutations in some oGPCRs have been associated with rare genetic disorders or with an increased risk of developing pathologies. The once underestimated pharmacological potential of targeting oGPCRs is increasingly being exploited by the development of novel tools to understand their biology and by drug discovery endeavors aimed at identifying new modulators of their activity. Here, we summarize recent advancements in the field of oGPCRs and future directions.

A neural substrate for negative affect dictates female parental behavior

Parental behaviors secure the well-being of newborns and concomitantly limit negative affective states in adults, which emerge when coping with neonatal distress becomes challenging. Whether negative-affect-related neuronal circuits orchestrate parental actions is unknown. Here, we identify parental signatures in lateral habenula neurons receiving bed nucleus of stria terminalis innervation (^BNSTLHb). We find that LHb neurons of virgin female mice increase their activity following pup distress vocalization and are necessary for pup-call-driven aversive behaviors. LHb activity rises during pup retrieval, a behavior worsened by LHb inactivation. Intersectional cell identification and transcriptional profiling associate ^BNSTLHb cells to parenting and outline a gene expression in female virgins similar to that in mothers but different from that in non-parental virgin male mice. Finally, tracking and manipulating ^BNSTLHb cell activity demonstrates their specificity for encoding negative affect and pup retrieval. Thus, a negative affect neural circuit processes newborn distress signals and may limit them by guiding female parenting.

Single-nucleus transcriptional profiling of GAD2-positive neurons from mouse lateral habenula reveals distinct expression of neurotransmission- and depression-related genes

Glutamatergic projection neurons of the lateral habenula (LHb) drive behavioral state modulation by regulating the activity of midbrain monoaminergic neurons. Identifying circuit mechanisms that modulate LHb output is of interest for understanding control of motivated behaviors. A small population of neurons within the medial subnucleus of the mouse LHb express the GABAergic synthesizing enzyme GAD2, and they can inhibit nearby LHb projection neurons; however, these neurons lack markers of classic inhibitory interneurons and they co-express the vesicular glutamate transporter VGLUT2. To determine the molecular phenotype of these neurons, we genetically tagged the nuclei of GAD2-positive cells and used fluorescence-activated nuclear sorting to isolate and enrich these nuclei for single nuclear RNA sequencing (FANS-snRNAseq). Our data confirm that GAD2+/VGLUT2+ neurons intrinsic to the LHb co-express markers of both glutamatergic and GABAergic transmission and that they are transcriptionally distinct from either GABAergic interneurons or habenular glutamatergic neurons. We identify gene expression programs within these cells that show sex-specific differences in expression and that are implicated in major depressive disorder (MDD), which has been linked to LHb hyperactivity. Finally, we identify the Ntng2 gene encoding the cell adhesion protein Netrin-G2 as a marker of LHb GAD2+/VGLUT+ neurons and a gene product that may contribute to their target projections. These data show the value of using genetic enrichment of rare cell types for transcriptome studies, and they advance understanding of the molecular composition of a functionally important class of GAD2+ neurons in the LHb.

Hedgehog-interacting protein acts in the habenula to regulate nicotine intake

Hedgehog-interacting protein (HHIP) sequesters Hedgehog ligands to repress Smoothened (SMO)-mediated recruitment of the GLI family of transcription factors. Allelic variation in HHIP confers risk of chronic obstructive pulmonary disease and other smoking-related lung diseases, but underlying mechanisms are unclear. Using single-cell and cell-type-specific translational profiling, we show that HHIP expression is highly enriched in medial habenula (MHb) neurons, particularly MHb cholinergic neurons that regulate aversive behavioral responses to nicotine. HHIP deficiency dysregulated the expression of genes involved in cholinergic signaling in the MHb and disrupted the function of nicotinic acetylcholine receptors (nAChRs) through a PTCH-1/cholesterol-dependent mechanism. Further, CRISPR/Cas9-mediated genomic cleavage of the Hhip gene in MHb neurons enhanced the motivational properties of nicotine in mice. These findings suggest that HHIP influences vulnerability to smoking-related lung diseases in part by regulating the actions of nicotine on habenular aversion circuits.

Diversification of habenular organization and asymmetries in teleosts: Insights from the Atlantic salmon and European eel

Habenulae asymmetries are widespread across vertebrates and analyses in zebrafish, the reference model organism for this process, have provided insight into their molecular nature, their mechanisms of formation and their important roles in the integration of environmental and internal cues with a variety of organismal adaptive responses. However, the generality of the characteristics identified in this species remains an open question, even on a relatively short evolutionary scale, in teleosts. To address this question, we have characterized the broad organization of habenulae in the Atlantic salmon and quantified the asymmetries in each of the identified subdomains. Our results show that a highly conserved partitioning into a dorsal and a ventral component is retained in the Atlantic salmon and that asymmetries are mainly observed in the former as in zebrafish. A remarkable difference is that a prominent left-restricted pax6 positive nucleus is observed in the Atlantic salmon, but undetectable in zebrafish. This nucleus is not observed outside teleosts, and harbors a complex presence/absence pattern in this group, retaining its location and cytoarchitectonic organization in an elopomorph, the European eel. These findings suggest an ancient origin and high evolvability of this trait in the taxon. Taken together, our data raise novel questions about the variability of asymmetries across teleosts and their biological significance depending on ecological contexts.

Cell-type-specific population dynamics of diverse reward computations

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

The limitations of investigating appetite through circuit manipulations: are we biting off more than we can chew?

Disordered eating can underpin a number of debilitating and prevalent chronic diseases, such as obesity. Broader advances in psychopharmacology and biology have motivated some neuroscientists to address diet-induced obesity through reductionist, pre-clinical eating investigations on the rodent brain. Specifically, chemogenetic and optogenetic methods developed in the 21st century allow neuroscientists to perform in vivo, region-specific/projection-specific/promoter-specific circuit manipulations and immediately assess the impact of these manipulations on rodent feeding. These studies are able to rigorously conclude whether a specific neuronal population regulates feeding behaviour in the hope of eventually developing a mechanistic neuroanatomical map of appetite regulation. However, an artificially stimulated/inhibited rodent neuronal population that changes feeding behaviour does not necessarily represent a pharmacological target for treating eating disorders in humans. Chemogenetic/optogenetic findings must therefore be triangulated with the array of theories that contribute to our understanding of appetite. The objective of this review is to provide a wide-ranging discussion of the limitations of chemogenetic/optogenetic circuit manipulation experiments in rodents that are used to investigate appetite. Stepping into and outside of medical science epistemologies, this paper draws on philosophy of science, nutrition, addiction biology and neurophilosophy to prompt more integrative, transdisciplinary interpretations of chemogenetic/optogenetic appetite data. Through discussing the various technical and epistemological limitations of these data, we provide both an overview of chemogenetics and optogenetics accessible to non-neuroscientist obesity researchers, as well as a resource for neuroscientists to expand the number of lenses through which they interpret their circuit manipulation findings.

Encore: Behavioural animal models of stress, depression and mood disorders

Animal studies over the past two decades have led to extensive advances in our understanding of pathogenesis of depressive and mood disorders. Among these, rodent behavioural models proved to be of highest informative value. Here, we present a comprehensive overview of the most popular behavioural models with respect to physiological, circuit, and molecular biological correlates. Behavioural stress paradigms and behavioural tests are assessed in terms of outcomes, strengths, weaknesses, and translational value, especially in the domain of pharmacological studies.

Intersectional mapping of multi-transmitter neurons and other cell types in the brain

Recent developments in intersectional strategies have greatly advanced our ability to precisely target brain cell types based on unique co-expression patterns. To accelerate the application of intersectional genetics, we perform a brain-wide characterization of 13 Flp and tTA mouse driver lines and selected seven for further analysis based on expression of vesicular neurotransmitter transporters. Using selective Cre driver lines, we created more than 10 Cre/tTA combinational lines for cell type targeting and circuit analysis. We then used VGLUT-Cre/VGAT-Flp combinational lines to identify and map 30 brain regions containing neurons that co-express vesicular glutamate and gamma-aminobutyric acid (GABA) transporters, followed by tracing their projections with intersectional viral vectors. Focusing on the lateral habenula (LHb) as a target, we identified glutamatergic, GABAergic, or co-glutamatergic/GABAergic innervations from ∼40 brain regions. These data provide an important resource for the future application of intersectional strategies and expand our understanding of the neuronal subtypes in the brain.

Molecular signatures and cellular diversity during mouse habenula development

The habenula plays a key role in various motivated and pathological behaviors and is composed of molecularly distinct neuron subtypes. Despite progress in identifying mature habenula neuron subtypes, how these subtypes develop and organize into functional brain circuits remains largely unknown. Here, we performed single-cell transcriptional profiling of mouse habenular neurons at critical developmental stages, instructed by detailed three-dimensional anatomical data. Our data reveal cellular and molecular trajectories during embryonic and postnatal development, leading to different habenular subtypes. Further, based on this analysis, our work establishes the distinctive functional properties and projection target of a subtype of Cartpt⁺ habenula neurons. Finally, we show how comparison of single-cell transcriptional profiles and GWAS data links specific developing habenular subtypes to psychiatric disease. Together, our study begins to dissect the mechanisms underlying habenula neuron subtype-specific development and creates a framework for further interrogation of habenular development in normal and disease states.

Advancements in the Quest to Map, Monitor, and Manipulate Neural Circuitry

Neural circuits and the cells that comprise them represent the functional units of the brain. Circuits relay and process sensory information, maintain homeostasis, drive behaviors, and facilitate cognitive functions such as learning and memory. Creating a functionally-precise map of the mammalian brain requires anatomically tracing neural circuits, monitoring their activity patterns, and manipulating their activity to infer function. Advancements in cell-type-specific genetic tools allow interrogation of neural circuits with increased precision. This review provides a broad overview of recombination-based and activity-driven genetic targeting approaches, contemporary viral tracing strategies, electrophysiological recording methods, newly developed calcium, and voltage indicators, and neurotransmitter/neuropeptide biosensors currently being used to investigate circuit architecture and function. Finally, it discusses methods for acute or chronic manipulation of neural activity, including genetically-targeted cellular ablation, optogenetics, chemogenetics, and over-expression of ion channels. With this ever-evolving genetic toolbox, scientists are continuing to probe neural circuits with increasing resolution, elucidating the structure and function of the incredibly complex mammalian brain.