Lesion of the habenular efferent pathway produces anxiety and locomotor hyperactivity in rats: a comparison of the effects of neonatal and adult lesions

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.

Expression of endogenous epitope-tagged GPR4 in the mouse brain

GPR4 is a proton-sensing G protein-coupled receptor implicated in many peripheral and central physiological processes. GPR4 expression has previously been assessed only via detection of the cognate transcript or indirectly, by use of fluorescent reporters. In this work, CRISPR/Cas9 knock-in technology was used to encode a hemagglutinin (HA) epitope tag within the endogenous locus of Gpr4 and visualize GPR4-HA in the mouse central nervous system using a specific, well characterized HA antibody; GPR4 expression was further verified by complementary Gpr4 mRNA detection. HA immunoreactivity was found in a limited set of brain regions, including in the retrotrapezoid nucleus (RTN), serotonergic raphe nuclei, medial habenula, lateral septum, and several thalamic nuclei. GPR4 expression was not restricted to cells of a specific neurochemical identity as it was observed in excitatory, inhibitory, and aminergic neuronal cell groups. HA immunoreactivity was not detected in brain vascular endothelium, despite clear expression of Gpr4 mRNA in endothelial cells. In the RTN, GPR4 expression was detected at the soma and in proximal dendrites along blood vessels and the ventral surface of the brainstem; HA immunoreactivity was not detected in RTN projections to two known target regions. This localization of GPR4 protein in mouse brain neurons corroborates putative sites of expression where its function has been previously implicated (e.g., CO2-regulated breathing by RTN), and provides a guide for where GPR4 could contribute to other CO2/H+ modulated brain functions. Finally, GPR4-HA animals provide a useful reagent for further study of GPR4 in other physiological processes outside of the brain.Significance Statement GPR4 is a proton-sensing G-protein coupled receptor whose expression is necessary for a number of diverse physiological processes including acid-base sensing in the kidney, immune function, and cancer progression. In the brain, GPR4 has been implicated in the hypercapnic ventilatory response mediated by brainstem neurons. While knockout studies in animals have clearly demonstrated its necessity for normal physiology, descriptions of GPR4 expression have been limited due to a lack of specific antibodies for use in mouse models. In this paper, we implemented a CRISPR/Cas9 knock-in approach to incorporate the coding sequence for a small epitope tag into the locus of GPR4. Using these mice, we were able to describe GPR4 protein expression directly for the first time.

Central and peripheral actions of nicotine that influence blood glucose homeostasis and the development of diabetes

Cigarette smoking has long been recognized as a risk factor for type 2 diabetes (T2D), although the precise causal mechanisms underlying this relationship remain poorly understood. Recent evidence suggests that nicotine, the primary reinforcing component in tobacco, may play a pivotal role in connecting cigarette smoking and T2D. Extensive research conducted in both humans and animals has demonstrated that nicotine can elevate blood glucose levels, disrupt glucose homeostasis, and induce insulin resistance. The review aims to elucidate the genetic variants of nicotinic acetylcholine receptors associated with diabetes risk and provide a comprehensive overview of the available data on the mechanisms through which nicotine influences blood glucose homeostasis and the development of diabetes. Here we emphasize the central and peripheral actions of nicotine on the release of glucoregulatory hormones, as well as its effects on glucose tolerance and insulin sensitivity. Notably, the central actions of nicotine within the brain, which encompass both insulin-dependent and independent mechanisms, are highlighted as potential targets for intervention strategies in diabetes management.

Neuropeptide Y in the medial habenula alleviates migraine-like behaviors through the Y1 receptor

BACKGROUND

Migraine is a highly disabling health burden with multiple symptoms; however, it remains undertreated because of an inadequate understanding of its neural mechanisms. Neuropeptide Y (NPY) has been demonstrated to be involved in the modulation of pain and emotion, and may play a role in migraine pathophysiology. Changes in NPY levels have been found in patients with migraine, but whether and how these changes contribute to migraine is unknown. Therefore, the purpose of this study was to investigate the role of NPY in migraine-like phenotypes.

METHODS

Here, we used intraperitoneal injection of glyceryl trinitrate (GTN, 10 mg/kg) as a migraine mouse model, which was verified by light-aversive test, von Frey test, and elevated plus maze test. We then performed whole-brain imaging with NPY-GFP mice to explore the critical regions where NPY was changed by GTN treatment. Next, we microinjected NPY into the medial habenula (MHb), and further infused Y1 or Y2 receptor agonists into the MHb, respectively, to detect the effects of NPY in GTN-induced migraine-like behaviors.

RESULTS

GTN effectively triggered allodynia, photophobia, and anxiety-like behaviors in mice. After that, we found a decreased level of GFP⁺ cells in the MHb of GTN-treated mice. Microinjection of NPY attenuated GTN-induced allodynia and anxiety without affecting photophobia. Furthermore, we found that activation of Y1-but not Y2-receptors attenuated GTN-induced allodynia and anxiety.

CONCLUSIONS

Taken together, our data support that the NPY signaling in the MHb produces analgesic and anxiolytic effects through the Y1 receptor. These findings may provide new insights into novel therapeutic targets for the treatment of migraine.

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

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

Water deprivation induces hypoactivity in rats independently of oxytocin receptor signaling at the central amygdala

INTRODUCTION

Vasopressin (AVP) and oxytocin (OXT) are neuropeptides produced by magnocellular neurons (MCNs) of the hypothalamus and secreted through neurohypophysis to defend mammals against dehydration. It was recently demonstrated that MCNs also project to limbic structures, modulating several behavioral responses.

METHODS AND RESULTS

We found that 24 h of water deprivation (WD) or salt loading (SL) did not change exploration or anxiety-like behaviors in the elevated plus maze (EPM) test. However, rats deprived of water for 48 h showed reduced exploration of open field and the closed arms of EPM, indicating hypoactivity during night time. We evaluated mRNA expression of glutamate decarboxylase 1 (Gad1), vesicular glutamate transporter 2 (Slc17a6), AVP (Avpr1a) and OXT (Oxtr) receptors in the lateral habenula (LHb), basolateral (BLA) and central (CeA) amygdala after 48 h of WD or SL. WD, but not SL, increased Oxtr mRNA expression in the CeA. Bilateral pharmacological inhibition of OXTR function in the CeA with the OXTR antagonist L-371,257 was performed to evaluate its possible role in regulating the EPM exploration or water intake induced by WD. The blockade of OXTR in the CeA did not reverse the hypoactivity response in the EPM, nor did it change water intake induced in 48-h water-deprived rats.

DISCUSSION

We found that WD modulates exploratory activity in rats, but this response is not mediated by oxytocin receptor signaling to the CeA, despite the upregulated Oxtr mRNA expression in that structure after WD for 48 h.

Functional brain-wide network mapping during acute stress exposure in rats: Interaction between the lateral habenula and cortical, amygdalar, hypothalamic and monoaminergic regions

Upon stress exposure, a broad network of structures comes into play in order to provide adequate responses and restore homeostasis. It has been known for decades that the main structures engaged during the stress response are the medial prefrontal cortex, the amygdala, the hippocampus, the hypothalamus, the monoaminergic systems (noradrenaline, dopamine and serotonin) and the periaqueductal gray. The lateral habenula (LHb) is an epithalamic structure directly connected to prefrontal cortical areas and to the amygdala, whereas it functionally interacts with the hippocampus. Also, it is a main modulator of monoaminergic systems. The LHb is activated upon exposure to basically all types of stressors, suggesting it is also involved in the stress response. However, it remains unknown if and how the LHb functionally interacts with the broad stress response network. In the current study we performed in rats a restraint stress procedure followed by immunohistochemical staining of the c-Fos protein throughout the brain. Using graph theory-based functional connectivity analyses, we confirm the principal hubs of the stress network (e.g., prefrontal cortex, amygdala and periventricular hypothalamus) and show that the LHb is engaged during stress exposure in close interaction with the medial prefrontal cortex, the lateral septum and the medial habenula. In addition, we performed DREADD-induced LHb inactivation during the same restraint paradigm in order to explore its consequences on the stress response network. This last experiment gave contrasting results as the DREADD ligand alone, clozapine-N-oxide, was able to modify the network.

An overview of the behavioral, neurobiological and morphological effects of topiramate in rats exposed to chronic unpredictable mild stress

The environmental psychological stress causes depressive disorders. Stress causes many neurobiological, neurodegenerative changes in brain. Topiramate (TPM) is used in the treatment of epilepsy and psychiatric diseases. However, there are conflicting findings that TPM disrupts cognitive functions. We aimed to investigate the effects of TPM on depression, anxiety, learning and memory as well as neurobiological, morphological changes in rats exposed to chronic unpredictable mild stress (CUMS). After CUMS was formed by random application of nine mild stressors for 45 days, TPM (at doses of 0.1, 1, 10, 100 mg/kg) was administered for 21 days. Sucrose preference, locomotor activity, forced swimming, elevated plus maze and Morris water maze tests were performed. Corticosterone, BDNF (Brain-derived neurotrophic factor) and glutamate levels and volumes of hippocampus were evaluated. Body weights of the rats were measured. Immobilization time increased in CUMS, CUMS + TPM0.1 in forced swimming test and time spent in platform quadrant increased in Control + TPM1, CUMS, CUMS + TPM0.1, CUMS + TPM1 in Morris water maze test. Control + TPM1 decreased distance to platform in Morris water maze while CUMS + TPM100 increased. Learning is impaired in CUMS + TPM100 while it is improved in Control + TPM1. BDNF levels increased in CUMS and glutamate levels increased in CUMS, CUMS + TPM10. Body weight decreased in CUMS, CUMS + TPM0.1, CUMS + TPM1, CUMS + TPM100. Hippocampus volumes increased in CUMS. In conclusion, CUMS improved cognition and this finding was supported by the increase of BDNF levels and volume of hippocampus. TPM 1 mg/kg improved cognition in non-stressed rats. TPM 0.1 and 1 mg/kg improved while TPM 100 mg/kg impaired memory in rats exposed to stress.

Habenula orphan G-protein coupled receptors in the pathophysiology of fear and anxiety

The phasic emotion, fear, and the tonic emotion, anxiety, have been conventionally inspected in clinical frameworks to epitomize memory acquisition, storage, and retrieval. However, inappropriate expression of learned fear in a safe environment and its resistance to suppression is a cardinal feature of various fear-related disorders. A significant body of literature suggests the involvement of extra-amygdala circuitry in fear disorders. Consistent with this view, the present review underlies incentives for the association between the habenula and fear memory. G protein-coupled receptors (GPCRs) are important to understand the molecular mechanisms central to fear learning due to their neuromodulatory role. The efficacy of a pharmacological strategy aimed at exploiting habenular-GPCR desensitization machinery can serve as a therapeutic target combating the pathophysiology of fear disorders. Originating from this milieu, the conserved nature of orphan GPCRs in the brain, with some having the highest expression in the habenula can lead to recent endeavors in understanding its functionality in fear circuitry.

Blockade of pre-synaptic and post-synaptic GABAB receptors in the lateral habenula produces different effects on anxiety-like behaviors in 6-hydroxydopamine hemiparkinsonian rats

Although the output of the lateral habenula (LHb) controls the activity of midbrain dopaminergic and serotonergic systems, which are implicated in the pathophysiology of anxiety, it is not known how blockade of GABA_B receptors in the region affects anxiety-like behaviors, particularly in Parkinson's disease-related anxiety. In this study, unilateral 6-hydroxydopamine lesions of the substantia nigra pars compacta in rats induced anxiety-like behaviors, led to hyperactivity of LHb neurons and decreased the level of extracellular dopamine (DA) in the basolateral amygdala (BLA) compared to sham-lesioned rats. Intra-LHb injection of pre-synaptic GABA_B receptor antagonist CGP36216 produced anxiolytic-like effects, while the injection of post-synaptic GABA_B receptor antagonist CGP35348 induced anxiety-like responses in both groups. Further, intra-LHb injection of CGP36216 decreased the firing rate of the neurons, and increased the GABA/glutamate ratio in the LHb and release of DA and serotonin (5-HT) in the BLA; conversely, CGP35348 increased the firing rate of the neurons and decreased the GABA/glutamate ratio and release of DA and 5-HT in sham-lesioned and the lesioned rats. However, the doses of the antagonists producing these behavioral effects in the lesioned rats were lower than those in sham-lesioned rats, and the duration of action of the antagonists on the firing rate of the neurons and release of the neurotransmitters was prolonged in the lesioned rats. Collectively, these findings suggest that pre-synaptic and post-synaptic GABA_B receptors in the LHb are involved in the regulation of anxiety-like behaviors, and degeneration of the nigrostriatal pathway up-regulates function and/or expression of these receptors.

Brain Iron Accumulation and the Formation of Calcifications After Developmental Zika Virus Infection

Intracranial calcifications (ICC) are the most common neuropathological finding in the brains of children exposed in utero to the Zika virus (ZIKV). Using a mouse model of developmental ZIKV infection, we reported widespread calcifications in the brains of susceptible mice that correlated in multiple ways with the behavioral deficits observed. Here, we examined the time course of ICC development and the role of iron deposition in this process, in 3 genetically distinct inbred strains of mice. Brain iron deposits were evident by Perls' staining at 2 weeks post infection, becoming increasingly dense and coinciding with calcium buildup and the formation of ICCs. A regional analysis of the brains of susceptible mice (C57BL/6J and 129S1/SvImJ strains) revealed the presence of iron initially in regions containing many ZIKV-immunoreactive cells, but then spreading to regions containing few infected cells, most notably the thalamus and the fasciculus retroflexus. Microglial activation was widespread initially and later delineated the sites of ICC formation. Behavioral tests conducted at 5-6 weeks of age revealed greater deficits in mice with the most extensive iron deposition and calcification of subcortical regions, such as thalamus. These findings point to iron deposition as a key factor in the development of ICCs after developmental ZIKV infection.

Modulation of brain stimulation reward and locomotor activity by ionotropic glutamate receptors of the tail of the ventral tegmental area

The rostromedial tegmental nucleus also referred to as the tail of the ventral tegmental area (tVTA) contains a cluster of gamma-aminobutyric acid (GABA)ergic neurons that receive dense glutamatergic afferents from the lateral habenula (LHb), and project to dopamine (DA) neurons of the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). In light of previous evidence implicating glutamate transmission in the regulation of midbrain DA neuronal activity, we first assessed the impact of intra-tVTA microinjection of NBQX (0.8 nmol/side) and PPPA (0.825 nmol/side), respectively AMPA and NMDA receptor antagonists, on reward induced by intracranial self-stimulation (ICSS) and on locomotor activity. Since the tVTA contains a large concentration of mu opioid receptors, additional measures were obtained following microinjection of endomorphin-1 (EM-1, 1 nmol/side) to confirm tVTA placements. Then, using small interfering RNAs (siRNAs), we tested the effect of tVTA downregulation of the GluN1 subunit of the NMDA receptor on reward and locomotor activity. Results show that NBQX, PPPA and EM-1 all enhance reward and locomotor activity, effects that were of different magnitude in rostral and intermediate parts of the tVTA. On the other hand, a reduction in GluN1 subunits used a marked decrease in operant responding for ICSS, but failed to alter ICSS reward and the reward-enhancing effect of PPPA. Our results support a role for the tVTA as a main inhibitory component of DA-dependent behavioral measures, and suggest that tVTA NMDA receptors that modulate reward are most likely expressed on tVTA afferent terminals.

An Adult Brain Atlas Reveals Broad Neuroanatomical Changes in Independently Evolved Populations of Mexican Cavefish

A shift in environmental conditions impacts the evolution of complex developmental and behavioral traits. The Mexican cavefish, Astyanax mexicanus, is a powerful model for examining the evolution of development, physiology, and behavior because multiple cavefish populations can be compared to an extant, ancestral-like surface population of the same species. Many behaviors have diverged in cave populations of A. mexicanus, and previous studies have shown that cavefish have a loss of sleep, reduced stress, an absence of social behaviors, and hyperphagia. Despite these findings, surprisingly little is known about the changes in neuroanatomy that underlie these behavioral phenotypes. Here, we use serial sectioning to generate brain atlases of surface fish and three independent cavefish populations. Volumetric reconstruction of serial-sectioned brains confirms convergent evolution on reduced optic tectum volume in all cavefish populations tested. In addition, we quantified volumes of specific neuroanatomical loci within several brain regions that have previously been implicated in behavioral regulation, including the hypothalamus, thalamus, and habenula. These analyses reveal an enlargement of the hypothalamus in all cavefish populations relative to surface fish, as well as subnuclei-specific differences within the thalamus and prethalamus. Taken together, these analyses support the notion that changes in environmental conditions are accompanied by neuroanatomical changes in brain structures associated with behavior. This atlas provides a resource for comparative neuroanatomy of additional brain regions and the opportunity to associate brain anatomy with evolved changes in behavior.

Mouse Strain and Sex-Dependent Differences in Long-term Behavioral Abnormalities and Neuropathologies after Developmental Zika Infection

Exposure of the developing fetus to Zika virus (ZIKV) results in a set of brain abnormalities described as the congenital Zika syndrome. Although microcephaly is the most obvious outcome, neuropathologies, such as intracranial calcifications and polymicrogyria, can occur in the absence of microcephaly. Moreover, the full impact of exposure on motor, social, and cognitive skills during development remains uncharacterized. We examined the long-term neurobehavioral consequences of neonatal ZIKV exposure in four genetically divergent inbred mouse strains (C57BL/6J, 129S1/SvImJ, FVB/NJ, and DBA/2J). Male and female mice were infected on postnatal day 1, considered comparable with exposure late in the second trimester of humans. We demonstrate strain differences in early susceptibility to the virus and the time course of glial reaction in the brain. These changes were associated with strain- and sex-dependent differences in long-term behavioral abnormalities that include hyperactivity, impulsiveness, and motor incoordination. In addition, the adult brains of susceptible mice exhibited widespread calcifications that may underlie the behavioral deficits observed. Characterization of the neuropathological sequelae of developmental exposure to the Zika virus in different immunocompetent mouse strains provides a foundation for identifying genetic and immune factors that contribute to long-term neurobehavioral consequences in susceptible individuals.SIGNIFICANCE STATEMENT Developmental Zika virus (ZIKV) infection is now known to cause brain abnormalities in infants that do not display microcephaly at birth, and the full impact of these more subtle neuropathologies has yet to be determined. We demonstrate in a mouse model that long-lasting behavioral aberrations occur after developmental ZIKV exposure. We compare four divergent mouse strains and find that the effects of Zika infection differ greatly between strains, in terms of behavioral changes, sex differences, and the intracranial calcifications that develop in the brains of susceptible mice. These findings provide a foundation for identifying susceptibility factors that lead to the development of abnormal behaviors secondary to ZIKV infection early in life.

Neonatal septal lesions prevent behavioral defeminization caused by neonatal treatment with estradiol in female rats

Male rats rarely show lordosis, a female sexual behavior, because of strong inhibition of the behavior in the lateral septum. Because neonatal treatment with estradiol (E₂) in female rats decreases lordosis, it is believed that the lateral septum is a target of E₂ action to defeminize or masculinize the lordosis-inhibiting system. Here, we tested the hypothesis that disruption of the lateral septum before E₂ treatment prevents the effect of neonatal E₂ on lordosis. Female rats that underwent radiofrequency-induced septal lesions or sham operation on postnatal day 4 (PD4, day of birth = PD1) were subcutaneously injected with E₂ or sesame oil vehicle alone on PD5. Vaginal opening and smears were checked. After sexual maturation, lordosis tests were performed. The effects of neonatal septal lesions on lordosis in male rats were also observed. Sham-operated and E₂-treated female rats showed a reduction in lordosis and irregular estrous cycles. Conversely, septal lesioned and E₂-treated females exhibited higher levels of lordosis, although their estrous cycles were irregular. These results suggest that neonatal septal lesions prevent females from being behaviorally defeminized by neonatal E₂. Additionally, neonatally septal lesioned males displayed higher levels of lordosis than sham-operated males. These results suggest that E₂, which is produced by the aromatization of testicular testosterone in the neonatal period, acts on the lateral septum to organize the lordosis-inhibiting system.

Lesions of lateral habenula attenuate win-stay but not lose-shift responses in a competitive choice task

Multiple neural systems contribute to choice adaptation following reinforcement. Recent evidence suggests that the lateral habenula (LHb) plays a key role in such adaptations, particularly when reinforcements are worse than expected. Here, we investigated the effects of bilateral LHb lesions on responding in a binary choice task with no discriminatory cues. LHb lesions in rats decreased win-stay responses but surprisingly left lose-shift responses intact. This same dissociated effect was also observed after systemic administration of d-amphetamine in a separate cohort of animals. These results suggest that at least some behavioural responses triggered by reward omission do not depend on an intact LHb.

Cbln2 and Cbln4 are expressed in distinct medial habenula-interpeduncular projections and contribute to different behavioral outputs

Cerebellins are important neurexin ligands that remain incompletely understood. Two critical questions in particular remain unanswered: do different cerebellins perform distinct functions, and do these functions act in the initial establishment of synapses or in rendering nascent synapses capable of normal synaptic transmission? Here we show that in mice, Cbln2 and Cbln4 are expressed in the medial habenula (MHb) nucleus in different types of neurons that project to distinct target neurons in the interpeduncular nucleus. Conditional genetic deletion of Cbln2 in the MHb impaired synaptic transmission at Cbln2⁺ synapses in the interpeduncular neurons within 3 wk, but decreased synapse numbers only after 3 mo, suggesting a functional, but not a structural, requirement for Cbln2 in synapses formed by Cbln2-expressing neurons. In contrast, genetic deletions of Cbln4 in the MHb had no major effect on synaptic transmission or synapse numbers in interpeduncular target neurons. Nevertheless, MHb ablation of both Cbln2 and Cbln4 significantly impaired behavioral responses in mice, but affected different types of behaviors. Specifically, Cbln2 MHb deletions decreased spatial learning, as measured in the water T-maze, whereas Cbln4 MHb deletions increased anxiety levels, as monitored in the open field test and elevated plus maze. Thus, Cbln2 and Cbln4 are expressed in distinct MHb neurons that contribute to different behaviors.

Non-motor Behavioral Alterations of PGC-1α-Deficient Mice - A Peculiar Phenotype With Slight Male Preponderance and No Apparent Progression

Dysfunction of peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1α) has been linked to various neurodegenerative and neuropsychiatric disorders; however, reports on psychic behavioral alterations on PGC-1α-deficient animals are sparse. The present study revisited prior observations of anxiety-related, depression-related, and hippocampal memory-related observations having been made on different PGC-1α-deficient murine strains, in a large-scale analysis on whole-body full-length (FL-)PGC-1α-deficient mice. The examinations were performed on animals covering a wide age range enrolled from both sexes, and included paradigms such as the open-field, elevated plus maze, light-dark box, tail suspension test, and spatial recognition two-trial Y-maze. The findings revealed no signs of previously reported anxiety-like behavior, but revealed an unexpected phenotype with decreased anxiety behavior consistent throughout different paradigms, with slight male preponderance. This was associated with despair-like anhedonic behavior, consistent with that reported previously, but did not associate with either peripheral or brain alterations in kynurenic acid synthesis, which was previously proposed. Though male FL-PGC-1α-deficient mice tended to perform poorer in the hippocampus-based spatial learning paradigm, the genotype overall was not associated with impairment in spatial memory, contradicting with prior observations. None of the observed alterations deteriorated with age, similarly to motor alterations as reported previously. The most likely contributors of this peculiar phenotype are discussed, with clinicopathological correlations drawn. Being the first to address these behavioral domains within the same PGC-1α-deficient strain, our findings extend the knowledge about the complex in vivo effect of PGC-1α dysfunction and add important notes to research in the field of PGC-1α in connection with neuropsychiatric disorders.

The tail of the ventral tegmental area in behavioral processes and in the effect of psychostimulants and drugs of abuse

The tail of the ventral tegmental area (tVTA) is a recently identified structure that exerts a major inhibitory drive onto midbrain dopamine (DA) neurons. Also referred to as the rostromedial tegmental nucleus (RMTg), the tVTA is a cluster of gamma-aminobutyric acid (GABA)ergic neurons that starts within the posterior end of the VTA, where it is restricted dorsolateral to the caudal part of the interpeduncular nucleus, and extends into the pons. First identified in the rat, the tVTA has been described in many species, including mice and monkeys, as a region exhibiting similar anatomical and behavioral properties; it receives strong excitatory inputs from the lateral habenula (LHb), conveys negative reward-related information, and inhibits midbrain DA neuron activity. As an important inhibitory afferent to midbrain DA neurons, the tVTA is also implicated in drug abuse and in the complex interplay between reward and aversion processes. The overarching goal of this review is to provide the current state of knowledge on the anatomy and connectivity of the tVTA and to discuss recent evidence implicating this structure in reward-related processes and in the effect of psychostimulants and drugs of abuse.

The lateral habenula interacts with the hypothalamo-pituitary adrenal axis response upon stressful cognitive demand in rats

The lateral habenula (LHb) is involved in emotional and cognitive behaviors. Recently, we have shown in rats that blockade of excitatory inputs to the LHb not only induced deficits of memory retrieval in the water maze, but also altered swim strategies (i.e., induced excessive thigmotaxis). The latter observation, although consistent with the occurrence of memory deficits, could also possibly be the consequence of an excessive level of stress, further suggesting a role for the LHb in the stress response in our behavioral paradigm. To test this hypothesis we performed in rats intra-LHb infusion of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 267 ng/side in 0.3 μL), or vehicle, and assessed the responsiveness of the hypothalamo-pituitary adrenal (HPA) axis to environmental stressful or non-stressful situations. We have measured plasma corticosterone (CORT) concentrations at different time points before and following intra-LHb infusion of CNQX - or of the same volume of vehicle - in three conditions: during the probe test of a water maze experiment; in an anxiety test, the elevated plus maze; and in a home cage condition. Whereas there were no differences in the home cage condition and in the elevated plus maze, in the water maze experiment we observed that CNQX-treated rats presented, along with memory deficits, a higher level of blood CORT than vehicle-treated rats. These results suggest that perturbations of the modulation of the HPA axis are consecutive to the alteration of LHb function, whether it is the result of a defective direct control of the LHb over the HPA axis, or the consequence of memory deficits.

Development and connectivity of the habenular nuclei

Accumulating evidence has reinforced that the habenular region of the vertebrate dorsal forebrain is an essential integrating center, and a region strongly implicated in neurological disorders and addiction. Despite the important and diverse neuromodulatory roles the habenular nuclei play, their development has been understudied. The emphasis of this review is on the dorsal habenular nuclei of zebrafish, homologous to the medial nuclei of mammals, as recent work has revealed new information about the signaling pathways that regulate their formation. Additionally, the zebrafish dorsal habenulae have become a valuable model for probing how left-right differences are established in a vertebrate brain. Sonic hedgehog, fibroblast growth factors and Wingless-INT proteins are all involved in the generation of progenitor cells and ultimately, along with Notch signaling, influence habenular neurogenesis and left-right asymmetry. Intriguingly, a genetic network has emerged that leads to the differentiation of dorsal habenular neurons and, through localized chemokine signaling, directs the posterior outgrowth of their newly emerging axons towards their postsynaptic target, the midbrain interpeduncular nucleus.

The Lateral Habenula Circuitry: Reward Processing and Cognitive Control

There has been a growing interest in understanding the role of the lateral habenula (LHb) in reward processing, affect regulation, and goal-directed behaviors. The LHb gets major inputs from the habenula-projecting globus pallidus and the mPFC, sending its efferents to the dopaminergic VTA and SNc, serotonergic dorsal raphe nuclei, and the GABAergic rostromedial tegmental nucleus. Recent studies have made advances in our understanding of the LHb circuit organization, yet the precise mechanisms of its involvement in complex behaviors are largely unknown. To begin to address this unresolved question, we present here emerging cross-species perspectives with a goal to provide a more refined understanding of the role of the LHb circuits in reward and cognition. We begin by highlighting recent findings from rodent experiments using optogenetics, electrophysiology, molecular, pharmacology, and tracing techniques that reveal diverse neural phenotypes in the LHb circuits that may underlie previously undescribed behavioral functions. We then discuss results from electrophysiological studies in macaques that suggest that the LHb cooperates with the anterior cingulate cortex to monitor action outcomes and signal behavioral adjustment. Finally, we provide an integrated summary of cross-species findings and discuss how further research on the connectivity, neural signaling, and physiology of the LHb circuits can deepen our understanding of the role of the LHb in normal and maladaptive behaviors associated with mental illnesses and drug abuse.

Role of the lateral habenula in memory through online processing of information

Our memory abilities, whether they involve short-term working memory or long-term episodic or procedural memories, are essential for our well-being, our capacity to adapt to constraints of our environment and survival. Therefore, several key brain regions and neurotransmitter systems are engaged in the processing of sensory information to either maintain such information in working memory so that it will quickly be used, and/or participate in the elaboration and storage of enduring traces useful for longer periods of time. Animal research has recently attracted attention on the lateral habenula which, as shown in rodents and non-human primates, seems to process information stemming in the main regions involved in memory processing, e.g., the medial prefrontal cortex, the hippocampus, the amygdala, the septal region, the basal ganglia, and participates in the control of key memory-related neurotransmitters systems, i.e., dopamine, serotonin, acetylcholine. Recently, the lateral habenula has been involved in working and spatial reference memories, in rodents, likely by participating in online processing of contextual information. In addition, several behavioral studies strongly suggest that it is also involved in the processing of the emotional valance of incoming information in order to adapt to particularly stressful situations. Therefore, the lateral habenula appears like a key region at the interface between cognition and emotion to participate in the selection of appropriate behaviors.

The habenula in psychiatric disorders: More than three decades of translational investigation

The habenula is an epithalamic structure located at the center of the dorsal diencephalic conduction system, a pathway involved in linking forebrain to midbrain regions. Composed of a medial and lateral subdivisions, the habenula receives inputs from the limbic system and basal ganglia mainly through the stria medullaris (SM), and projects to midbrain regions through the fasciculus retroflexus (FR). An increasing number of studies have implicated this structure in psychiatric disorders associated with dysregulated reward circuitry function, notably mood disorders, schizophrenia, and substance use disorder. However, despite significant progress in research, the mechanisms underlying the relationship between the habenula and the pathophysiology of psychiatric disorders are far from being fully understood, and still need further investigation. This review provides a closer look at key findings from animal and human studies illustrating the role of the habenula in mood disorders, schizophrenia, and substance use disorder, and discusses the clinical potential of using this structure as a therapeutic target.

Anxiety and Nicotine Dependence: Emerging Role of the Habenulo-Interpeduncular Axis

While innovative modern neuroscience approaches have aided in discerning brain circuitry underlying negative emotional behaviors including fear and anxiety responses, how these circuits are recruited in normal and pathological conditions remains poorly understood. Recently, genetic tools that selectively manipulate single neuronal populations have uncovered an understudied circuit, the medial habenula (mHb)-interpeduncular (IPN) axis, that modulates basal negative emotional responses. Interestingly, the mHb-IPN pathway also represents an essential circuit that signals heightened anxiety induced by nicotine withdrawal. Insights into how this circuit interconnects with regions more classically associated with anxiety, and how chronic nicotine exposure induces neuroadaptations resulting in an anxiogenic state, may thereby provide novel strategies and molecular targets for therapies that facilitate smoking cessation, as well as for anxiety relief.

The habenula as a critical node in chronic stress-related anxiety

The habenula is activated in response to stressful and aversive events, resulting in exploratory inhibition. Although possible mechanisms for habenula activation have been proposed, the effects of chronic stress on the habenular structure have never been studied. Herein, we assessed changes in volume, cell density and dendritic structure of habenular cells after chronic stress exposure using stereological and 3D morphological analysis. This study shows for the first time that there is a hemispherical asymmetry in the medial habenula (MHb) of the adult rat, with the right MHb containing more neurons than its left counterpart. Additionally, it shows that chronic stress induces a bilateral atrophy of both the MHb and the lateral habenula (LHb). This atrophy was accompanied by a reduction of the number of neurons in the right MHb and the number of glial cells in the bilateral LHb, but not by changes in the dendritic arbors of multipolar neurons. Importantly, these structural changes were correlated with elevated levels of serum corticosterone and increased anxious-like behavior in stressed animals. To further assess the role of the habenula in stress-related anxiety, bilateral lesions of the LHb were performed; interestingly, in lesioned animals the chronic stress protocol did not trigger increases in circulating corticosterone or anxious-like behavior. This study highlights the role of the habenula in the stress responses and how its sub-regions are structurally impacted by chronic stress with physiological and behavioral consequences.

Role for the Rostromedial Tegmental Nucleus in Signaling the Aversive Properties of Alcohol

BACKGROUND

While the rewarding effects of alcohol contribute significantly to its addictive potential, it is becoming increasingly appreciated that alcohol's aversive properties also play an important role in the propensity to drink. Despite this, the neurobiological mechanism for alcohol's aversive actions is not well understood. The rostromedial tegmental nucleus (RMTg) was recently characterized for its involvement in aversive signaling and has been shown to encode the aversive properties of cocaine, yet its involvement in alcohol's aversive actions have not been elucidated.

METHODS

Adult male and female Long-Evans rats underwent conditioned taste aversion (CTA) procedures where exposure to a novel saccharin solution was paired with intraperitoneal administration of saline, lithium chloride (LiCl), or ethanol (EtOH). Control rats underwent the same paradigm except that drug and saccharin exposure were explicitly unpaired. Saccharin consumption was measured on test day in the absence of drug administration, and rats were sacrificed 90 to 105 minutes following access to saccharin. Brains were subsequently harvested and processed for cFos immunohistochemistry. The number of cFos-labeled neurons was counted in the RMTg and the lateral habenula (LHb)-a region that sends prominent glutamatergic input to the RMTg.

RESULTS

In rats that received paired drug and saccharin exposure, EtOH and LiCl induced significant CTA compared to saline to a similar degree in males and females. Both EtOH- and LiCl-induced CTA significantly enhanced cFos expression in the RMTg and LHb but not the hippocampus. Similar to behavioral measures, no significant effect of sex on CTA-induced cFos expression was observed. cFos expression in both the RMTg and LHb was significantly correlated with CTA magnitude with greater cFos being associated with more pronounced CTA. In addition, cFos expression in the RMTg was positively correlated with LHb cFos.

CONCLUSIONS

These data suggest that the RMTg and LHb are involved in the expression of CTA and are consistent with previous work implicating the RMTg in aversive signaling. Furthermore, increased cFos expression in the RMTg following EtOH-induced CTA suggests that this region plays a role in signaling alcohol's aversive properties.

Pharmacological modulation of lateral habenular dopamine D2 receptors alters the anxiogenic response to cocaine in a runway model of drug self-administration

Cocaine has long been known to produce an initial "high" followed by an aversive/anxiogenic "crash". While much is known about the neurobiology of cocaine's positive/rewarding effects, the mechanisms that give rise to the drug's negative/anxiogenic actions remain unclear. Recent research has implicated the lateral habenula (LHb) in the encoding of aversive events including the anxiogenic response to cocaine. Of particular interest in this regard are the reciprocal connections between the LHb and the ventral tegmental area (VTA). VTA-DA neurons innervate different subsets of LHb cells that in turn feedback upon and modulate VTA neuronal activity. Here we examined the impact of D2 receptor activation and inhibition on the anxiogenic response to cocaine using a runway model of self-administration that is sensitive to the dual and opposing effects of the drug. Male rats ran a straight alley for IV cocaine (1.0mg/kg) following bilateral intra-LHb infusions of the D2 receptor antagonist, cis-flupenthixol (0, 7.5 or 15μg/side) or the D2 agonist, sumanirole (0, 5 or 10μg/side). Vehicle-pretreated controls developed approach-avoidance conflict behaviors about goal-box entry reflective of the dual positive and negative effects of cocaine. These behaviors were significantly diminished during LHb-D2 receptor antagonism and increased by the LHb D2 receptor agonist. These results demonstrate that activity at the D2 receptor in the lateral habenula serves to modulate the anxiogenic response to cocaine.

High-fructose diet during periadolescent development increases depressive-like behavior and remodels the hypothalamic transcriptome in male rats

Fructose consumption, which promotes insulin resistance, hypertension, and dyslipidemia, has increased by over 25% since the 1970s. In addition to metabolic dysregulation, fructose ingestion stimulates the hypothalamic-pituitary-adrenal (HPA) axis leading to elevations in glucocorticoids. Adolescents are the greatest consumers of fructose, and adolescence is a critical period for maturation of the HPA axis. Repeated consumption of high levels of fructose during adolescence has the potential to promote long-term dysregulation of the stress response. Therefore, we determined the extent to which consumption of a diet high in fructose affected behavior, serum corticosterone, and hypothalamic gene expression using a whole-transcriptomics approach. In addition, we examined the potential of a high-fructose diet to interact with exposure to chronic adolescent stress. Male Wistar rats fed the periadolescent high-fructose diet showed increased anxiety-like behavior in the elevated plus maze and depressive-like behavior in the forced swim test in adulthood, irrespective of stress history. Periadolescent fructose-fed rats also exhibited elevated basal corticosterone concentrations relative to their chow-fed peers. These behavioral and hormonal responses to the high-fructose diet did not occur in rats fed fructose during adulthood only. Finally, rats fed the high-fructose diet throughout development underwent marked hypothalamic transcript expression remodeling, with 966 genes (5.6%) significantly altered and a pronounced enrichment of significantly altered transcripts in several pathways relating to regulation of the HPA axis. Collectively, the data presented herein indicate that diet, specifically one high in fructose, has the potential to alter behavior, HPA axis function, and the hypothalamic transcriptome in male rats.

Increased CRF signalling in a ventral tegmental area-interpeduncular nucleus-medial habenula circuit induces anxiety during nicotine withdrawal

Increased anxiety is a prominent withdrawal symptom in abstinent smokers, yet the neuroanatomical and molecular bases underlying it are unclear. Here we show that withdrawal-induced anxiety increases activity of neurons in the interpeduncular intermediate (IPI), a subregion of the interpeduncular nucleus (IPN). IPI activation during nicotine withdrawal was mediated by increased corticotropin releasing factor (CRF) receptor-1 expression and signalling, which modulated glutamatergic input from the medial habenula (MHb). Pharmacological blockade of IPN CRF1 receptors or optogenetic silencing of MHb input reduced IPI activation and alleviated withdrawal-induced anxiety; whereas IPN CRF infusion in mice increased anxiety. We identified a mesointerpeduncular circuit, consisting of ventral tegmental area (VTA) dopaminergic neurons projecting to the IPN, as a potential source of CRF. Knockdown of CRF synthesis in the VTA prevented IPI activation and anxiety during nicotine withdrawal. These data indicate that increased CRF receptor signalling within a VTA-IPN-MHb circuit triggers anxiety during nicotine withdrawal.

Effects of lorazepam and WAY-200070 in larval zebrafish light/dark choice test

Zebrafish larvae spend more time in brightly illuminated area when placed in a light/dark testing environment. Here we report that the anxiolytic drugs lorazepam and diazepam increased the time larval fish spent in the dark compartment in the light/dark test. Lorazepam did not affect the visual induced optokinetic response of larval fish. Gene expression levels of c-fos and crh were significantly increased in the hypothalamus of fish larvae underwent light/dark choice behavior, whilst lorazepam treatment alleviated the increased c-fos and crh expressions. Furthermore, we found estrogen receptor β gene expression level was increased in fish larvae underwent light/dark choice. We next examined effects of estrogen receptor modulators (estradiol, BPA, PHTPP, and WAY-200070) in the light/dark test. We identified WAY-200070, a highly selective ERβ agonist significantly altered the light/dark choice behavior of zebrafish larvae. Further investigation showed WAY-200070 treatment caused a reduction of crh expression level in the hypothalamus, suggesting activation of ERβ signaling attenuate the stress response. Interestingly, WAY-200070 treatment caused marked increase of c-fos expression in the habenula of fish larvae underwent behavior test. These results suggest WAY-200070 activation of ERβ mediated signaling may regulate anxiety related behavior in zebrafish through modulation of neuronal activity in habenula.

Reward and adversity processing circuits, their competition and interactions with dopamine and serotonin signaling

We propose that dorsal anterior cingulate cortex (dACC), anterior insula (AI) and adjacent caudolateral orbitofrontal cortex (clOFC), project to lateral habenula (LHb) and D2 loop of ventral striatum (VS), forming a functional adversity processing circuit, directed towards inhibitory avoidance and self-control. This circuit learns what is bad or harmful to us, evaluates and predicts risks - to stop us from selecting and going/moving for the bad or suboptimal choices that decrease our well-being and survival chances. Proposed role of dACC is to generate a WARNING signal when things are going (or might end) bad or wrong to prevent negative consequences: pain, harm, loss or failure. The AI signals about bad, low, noxious and aversive qualities, which might make us sick or cause discomfort. These cortical adversity processing regions activate directly and indirectly (via D2 loop of VS) the LHb, which then inhibits dopamine and serotonin release (and is reciprocally inhibited by VTA/SNc, DRN) to avoid choosing and doing things leading to harm or loss, but also to make us feel worse, even down when overstimulated. We propose that dopamine attenuates output of the adversity processing circuit, thus decreasing inhibitory avoidance and self-control, while serotonin attenuates dACC, AI, clOFC, D1 loop of VS, LHb, amygdala and pain pathway. Thus, by reciprocal inhibition, by causing dopamine and serotonin suppression - and by being suppressed by them, the adversity processing circuit competes with reward processing circuit for control of choice behaviour and affective states. We propose stimulating effect of dopamine and calming inhibitory effect of serotonin on the active avoidance circuit involving amygdala, linked to threat processing, anger, fear, self-defense and violence. We describe causes and roles of dopamine and serotonin signaling in health and in mental dysfunctions. We add new idea on ventral ACC role in signaling that we are doing well and inducing serotonin, when we gain/reach safety, comfort, valuable resources (social or biological rewards), affection and achieve goals.

The Role of Habenula in Motivation and Reward

Located centrally along the dorsal diencephalic system, the habenula is divided into two structures: the medial and the lateral portions. It serves as an important relay between the forebrain and several hindbrain sites. In the last few years, a huge attention has been devoted to this structure, especially the lateral habenula (LHb), which seems to play an important role in emotion, motivation, and reward. Recent studies using techniques such as electrophysiology and neuroimaging have shown that the LHb is involved in motivational control of behavior. Its dysfunction is often associated with depression, schizophrenia, and mood disorder. This review focuses on providing a neuroanatomical and behavioral overview of some of the research previously done on the LHb. First, we describe the anatomical structure of the habenula and we explain how it is involved in reward and motivation. Then, we will discuss how this structure is linked to the limbic system, to finally provide a comparison between several studies that have used electrolytic lesions.

A cholinergic hypothesis of the unconscious in affective disorders

The interactions between distinct pharmacological systems are proposed as a key dynamic in the formation of unconscious memories underlying rumination and mood disorder, but also reflect the plastic capacity of neural networks that can aid recovery. An inverse and reciprocal relationship is postulated between cholinergic and monoaminergic receptor subtypes. M1-type muscarinic receptor transduction facilitates encoding of unconscious, prepotent behavioral repertoires at the core of affective disorders and ADHD. Behavioral adaptation to new contingencies is mediated by the classic prototype receptor: 5-HT1A (Gi/o) and its modulation of M1-plasticity. Reversal of learning is dependent on increased phasic activation of midbrain monoaminergic nuclei and is a function of hippocampal theta. Acquired hippocampal dysfunction due to abnormal activation of the hypothalamic-pituitary-adrenal (HPA) axis predicts deficits in hippocampal-dependent memory and executive function and further impairments to cognitive inhibition. Encoding of explicit memories is mediated by Gq/11 and Gs signaling of monoamines only. A role is proposed for the phasic activation of the basal forebrain cholinergic nucleus by cortical projections from the complex consisting of the insula and claustrum. Although controversial, recent studies suggest a common ontogenetic origin of the two structures and a functional coupling. Lesions of the region result in loss of motivational behavior and familiarity based judgements. A major hypothesis of the paper is that these lost faculties result indirectly, from reduced cholinergic tone.

Maturation-dependent behavioral deficits and cell injury in developing animals during the subacute postictal period

Prolonged early-life seizures are associated with disruptions of affective and cognitive function. Postictal disturbances, temporary functional deficits that persist for hours to days after seizures, have not yet been thoroughly characterized. Here, we used kainic acid (KA) to induce status epilepticus (SE) in immature rats at three developmental stages (postnatal day (P) 15, 21, or 30) and subsequently assessed spatial learning and memory in a Barnes maze, exploratory behavior in an open field, and the spatiotemporal distribution of cell injury during the first 7-10 days of the postictal period. At 1 day post-SE, P15-SE rats showed no deficit in the Barnes maze but were hyperexploratory in an open field compared with their littermate controls. In contrast, P21- and P30-SE rats exhibited markedly impaired performance in the Barnes maze and exhibited significantly reduced open field exploration suggestive of anxiety-like behavior. These behavioral changes were transient in P15 rats but more persistent in P21 and enduring in P30 rats after KA-SE. The time course of behavioral deficits in P21 and P30 rats was temporally correlated with the presence of neuronal injury in the lateral septal nuclei, amygdala, and ventral subiculum/CA1, regions involved in modulation of the hypothalamic-pituitary-adrenal stress response.

Inactivation of the lateral habenula reduces anxiogenic behavior and cocaine seeking under conditions of heightened stress

Recent anatomical and functional studies have renewed interest in the lateral habenula (LHb), a critical brain region that works in an opponent manner to modulate aversive and appetitive processes. In particular, increased LHb activation is believed to drive anxiogenic states during stressful conditions. Here, we reversibly inactivated the LHb with GABA receptor agonists (baclofen/muscimol) in rats prior to testing in an open field, elevated plus maze, and defensive burying task in the presence or absence of yohimbine, a noradrenergic α2-receptor antagonist that acts as an anxiogenic stressor. In a second set of experiments using a cocaine self-administration and reinstatement model, we inactivated the LHb during extinction responding and cue-induced reinstatement of cocaine seeking in the presence or absence of yohimbine pretreatment. Inactivation of the LHb after yohimbine treatment attenuated anxiogenic behavior by increasing time spent in the open arms and reducing the time spent burying. Inactivation of the LHb also reduced cocaine seeking when cue-induced reinstatement occurred in the presence of yohimbine, but did not affect extinction responding or cue-induced reinstatement by itself. These data demonstrate that the LHb critically regulates states of heightened anxiety during both unconditioned behavior and conditioned appetitive processes.

Inversion of left-right asymmetry alters performance of Xenopus tadpoles in nonlateralized cognitive tasks

Left-right behavioural biases are well documented across the animal kingdom, and handedness has long been associated with cognitive performance. However, the relationship between body laterality and cognitive ability is poorly understood. The embryonic pathways dictating normal left-right patterning have been molecularly dissected in model vertebrates, and numerous genetic and pharmacological treatments now facilitate experimental randomization or reversal of the left-right axis in these animals. Several recent studies showed a link between brain asymmetry and strongly lateralized behaviours such as eye use preference. However, links between laterality of the body and performance on cognitive tasks utilizing nonlateralized cues remain unknown. Xenopus tadpoles are an established model for the study of early left-right patterning, and protocols were recently developed to quantitatively evaluate learning and memory in these animals. Using an automated testing and training platform, we tested wild-type, left-right-randomized and left-right-reversed tadpoles for their ability to learn colour cues in an automated assay. Our results indicate that animals with either randomization or reversal of somatic left-right patterning learned more slowly than wild-type siblings, although all groups were able to reach the same performance optimum given enough training sessions. These results are the first analysis of the link between body laterality and learning of nonlateralized cues, and they position the Xenopus tadpole as an attractive and tractable model for future studies of the links between asymmetry of the body, lateralization of the brain and behaviour.

Distinct roles of segregated transmission of the septo-habenular pathway in anxiety and fear

The posterior septum consisting of the triangular septum (TS) and the bed nucleus of the anterior commissure (BAC) is predominantly linked with the medial habenula (MHb) and has been implicated in the control of anxiety and fear responses. However, its anatomical and functional linkage has largely remained elusive. We established a transgenic mouse model in which the TS and BAC projection neurons were visualized by GFP fluorescence and selectively eliminated by immunotoxin-mediated cell targeting. The linkage between the TS/BAC and the MHb constitutes two parallel pathways composed of the TS-ventral MHb, the core part of the interpeduncular nucleus (IPN), and the BAC-dorsal MHb, the peripheral part of the IPN. Ablation of the TS and BAC projection neurons selectively impaired anxiety and enhanced fear responses and learning, respectively. Inputs from the TS and BAC to the MHb are thus segregated by two parallel pathways and play specialized roles in controlling emotional behaviors.

Genetic dissection of medial habenula-interpeduncular nucleus pathway function in mice

The habenular complex linking forebrain and midbrain structures is subdivided into the medial (mHb) and the lateral nuclei (lHb). The mHb is characterized by the expression of specific nicotinic acetylcholine receptor isoforms and the release of acetylcholine to the interpeduncular nucleus (IPN), the sole output region of the mHb. The specific function of this circuit, however, is poorly understood. Here we generated transgenic mice in which mHb cells were selectively ablated postnatally. These lesions led to large reductions in acetylcholine levels within the IPN. The mutant mice exhibited abnormalities in a wide range of behavioral domains. They tended to be hyperactive during the early night period and were maladapted when repeatedly exposed to new environments. Mutant mice also showed a high rate of premature responses in the 5-choice serial reaction time task (5-CSRTT), indicating impulsive and compulsive behavior. Additionally, mice also exhibited delay and effort aversion in a decision-making test, deficits in spatial memory, a subtle increase in anxiety levels, and attenuated sensorimotor gating. IntelliCage studies under social housing conditions confirmed hyperactivity, environmental maladaptation, and impulsive/compulsive behavior, delay discounting, deficits in long-term spatial memory, and reduced flexibility in complex learning paradigms. In 5-CSRTT and adaptation tasks, systemic administration of nicotine slowed down nose-poke reaction and enhanced adaptation in control but not mutant mice. These findings demonstrate that the mHb–IPN pathway plays a crucial role in inhibitory control and cognition-dependent executive functions.

The aqueous crude extract of Montanoa frutescens produces anxiolytic-like effects similarly to diazepam in Wistar rats: involvement of GABAA receptor

ETHNOPHARMACOLOGICAL RELEVANCE

Cihuapatli is the Nahuatl name assigned to some medicinal plants grouped in the genus Montanoa, where Montanoa frutescens (Family: Asteraceae, Tribe: Heliantheae) is included. The crude extract from these plants has been used for centuries in the Mexican traditional medicine as a remedy for reproductive impairments and mood disorders. Experimental studies have systematically corroborated the traditional use of cihuapatli on reproductive impairments and sexual motivation, however, the effect on mood and "nervous" disorders, remains to be explored.

MATERIALS AND METHODS

The anxiolytic-like effect of aqueous crude extract of M. frutescens (25, 50 and 75 mg/kg) was investigated in male Wistar rats evaluated in the elevated plus-maze and compared with several doses of diazepam (1, 2 and 4 mg/kg) as a reference anxiolytic drug. Picrotoxin (1 mg/kg), a noncompetitive antagonist of the GABA(A) receptor, was used in experimental procedures to evaluate if this receptor could be involved in the anxiolytic-like effects produced by M. frutescens. To discard hypoactivity, hyperactivity, or no changes associated with treatments, which could interfere with the behavioral activity in the elevated plus-maze, rats were subjected to the open field test.

RESULTS

M. frutescens at 50 mg/kg showed anxiolytic-like activity similarly to 2 mg/kg of diazepam, without disrupts in general motor activity. The anxiolytic-like effect of M. frutescens detected in the elevated plus-maze was blocked by picrotoxin, indicating that GABA(A) receptors are involved in the modulation of this effect.

CONCLUSIONS

The results corroborate the use of M. frutescens in folk Mexican ethnomedicine as a potential anxiolytic agent and suggest that this effect is mediated by the GABA(A) receptors. Additionally, some sedative effects with high doses of M. frutescens were detected in the present study.

Unmasking the mysteries of the habenula in pain and analgesia

The habenula is a small bilateral structure in the posterior-medial aspect of the dorsal thalamus that has been implicated in a remarkably wide range of behaviors including olfaction, ingestion, mating, endocrine and reward function, pain and analgesia. Afferent connections from forebrain structures send inputs to the lateral and medial habenula where efferents are mainly projected to brainstem regions that include well-known pain modulatory regions such as the periaqueductal gray and raphe nuclei. A convergence of preclinical data implicates the region in multiple behaviors that may be considered part of the pain experience including a putative role in pain modulation, affective, and motivational processes. The habenula seems to play a role as an evaluator, acting as a major point of convergence where external stimuli is received, evaluated, and redirected for motivation of appropriate behavioral response. Here, we review the role of the habenula in pain and analgesia, consider its potential role in chronic pain, and review more recent clinical and functional imaging data of the habenula from animals and humans. Even through the habenula is a small brain structure, advances in structural and functional imaging in humans should allow for further advancement of our understanding of its role in pain and analgesia.

Electrical stimulation of the primate lateral habenula suppresses saccadic eye movement through a learning mechanism

The lateral habenula (LHb) is a brain structure which represents negative motivational value. Neurons in the LHb are excited by unpleasant events such as reward omission and aversive stimuli, and transmit these signals to midbrain dopamine neurons which are involved in learning and motivation. However, it remains unclear whether these phasic changes in LHb neuronal activity actually influence animal behavior. To answer this question, we artificially activated the LHb by electrical stimulation while monkeys were performing a visually guided saccade task. In one block of trials, saccades to one fixed direction (e.g., right direction) were followed by electrical stimulation of the LHb while saccades to the other direction (e.g., left direction) were not. The direction-stimulation contingency was reversed in the next block. We found that the post-saccadic stimulation of the LHb increased the latencies of saccades in subsequent trials. Notably, the increase of the latency occurred gradually as the saccade was repeatedly followed by the stimulation, suggesting that the effect of the post-saccadic stimulation was accumulated across trials. LHb stimulation starting before saccades, on the other hand, had no effect on saccade latency. Together with previous studies showing LHb activation by reward omission and aversive stimuli, the present stimulation experiment suggests that LHb activity contributes to learning to suppress actions which lead to unpleasant events.

A role for the habenula in the regulation of locomotor activity cycles

Although much is known about the regulation of the circadian rest-activity cycle by the hypothalamic suprachiasmatic nucleus in nocturnal rodents, little is known about the neural substrates that regulate the temporal organization of nocturnal activity within the active phase. In this report, data are presented in Syrian hamsters to implicate the habenula - believed to be involved in motivation, reward and motor control--as a candidate site for such a role. First, by examining hamsters during the day and night and by introducing a 'novel' running wheel in order to induce daytime motor activity, we showed that immunoreactive c-Fos expression in the lateral and medial habenula is related to motor activity/arousal. Second, by transecting the habenula's major efferent pathway (fasciculus retroflexus), we showed that the interruption of habenula neural output alters the daily amount of motor activity, lengthens the period of the circadian rest-activity rhythm and disrupts the species-typical pattern of nocturnal motor activity, measured as either wheel-running behavior or general locomotor activity. Instead of the usual pattern of night-time locomotion, characterized by a prolonged bout of elevated activity in the early night followed by shorter sporadic bouts or the cessation of activity altogether, lesioned animals exhibited a more homogeneous, undifferentiated temporal profile extending across the night. These data suggest a previously unrecognized function of the habenula whereby it regulates the temporal pattern of activity occurring within a circadian rest-activity window set by the suprachiasmatic nucleus.

Neurocircuitry of the nicotinic cholinergic system

Continuing to discover how the brain works is one of the great challenges ahead of us. Although understanding the brain anatomy and its functional organization provided a first and indispensable foundation, it became clear that a static view was insufficient. To understand the complexity of neuronal communication, it is necessary to examine the chemical nature of the neurotransmission and, using the example of the acetylcholine receptors, follow the different layers of networks that can be distinguished. The natural alkaloid nicotine contained in tobacco leaves acts as an agonist with a subclass of acetylcholine receptors, and provides an interesting tool to approach brain functions. Analysis of the nicotinic acetylcholine receptors, which are ligand gated channels, revealed that these receptors are expressed at different critical locations on the neurons including the synaptic boutons, neurites, cell bodies, and even on the axons. These receptors can modulate the activity at the microcircuit synaptic level, in the cell processing of information, and, by acting on the velocity of action potential, the synchrony of communication between brain areas. These actions at multiple levels of brain organization provide an example of the complexity of brain neurocircuitry and an illustration of the relevance of this knowledge for psychiatry.

Evidence for structural abnormalities of the human habenular complex in affective disorders but not in schizophrenia

BACKGROUND

The habenular complex is composed of important relay nuclei linking the limbic forebrain to the midbrain and brain stem nuclei. Based on clinical observations, experiments with animals and theoretical considerations, it has been speculated that this brain area might be involved in psychiatric diseases (i.e. schizophrenia and depression). However, evidence in favour of this hypothesis is still lacking because the human habenular complex has rarely been studied with regard to mental illness.

METHOD

We examined habenular volumes in post-mortem brains of 17 schizophrenia patients, 14 patients with depression (six patients with major depression and eight patients with bipolar depression) and 13 matched controls. We further determined the neuronal density, cell number and cell area of the medial habenular nuclei of the same cohorts using a counting box and a computer-assisted instrument.

RESULTS

Significantly reduced habenular volumes of the medial and lateral habenula were estimated in depressive patients in comparison to normal controls and schizophrenia patients. We also found a reduction in neuronal cell number and cell area in depressive patients for the right side compared to controls and schizophrenia patients. No such changes were seen in schizophrenia.

CONCLUSIONS

Our anatomical data argue against prominent structural alterations of the habenular nuclei in schizophrenia but demonstrate robust alterations in depressive patients. We are currently applying immunohistochemical markers to better characterize neuronal subpopulations of this brain region in schizophrenia and depression.

Lack of effect of habenula lesion on heroin self-administration in rats

We examined the effects of bilateral electric lesion of the habenula (Hb) on the acquisition and maintenance of heroin self-administration. The rats were trained to self-administer heroin (0.05 mg/kg/infusion) under FR1 schedule in daily 4h sessions. A progressive ratio schedule (PR3-4) was used to evaluate the relative motivational value of heroin reinforcement. Compared with the controls, neither pre-training nor post-training of Hb lesions had any effects on the total amount of infusions and motivational value of heroin reward. However, pre-training Hb lesion caused transient active and inactive nose-poke responding in the early phase of training, suggesting increased locomotor exploration. The results suggest that Hb might not play an important role in mediating the acute reinforcing effect of heroin.