Habenula: crossroad between the basal ganglia and the limbic system

How olfaction disorders can cause depression? The role of habenular degeneration

The removal of bilateral olfactory bulbs (OBs) can result in serious behavioral, neurochemical, neuroendocrine, and neuroimmune alterations in depressed patients. However, there is little information on how olfactory bulbectomy (OBX) leads to depression. Habenular nuclei and their connections are important in the regulation of psychomotor and psychosocial behaviors through afferent impulses of the olfactory system. Therefore, we investigated whether OB lesions lead to habenular degeneration. We used a sample of 50 rats (25 female and 25 male) for this study. Of these rats, five male and five female rats were taken as the control group. The remaining 40 rats (20 male and 20 female rats) constituted the study group, and frontal burr holes were performed at the OB level on these rats. OB cauterization was applied to 10 male and 10 female rats (n=10, 10; study group 1), mechanical OBX was applied to five male and five female rats (n=5, 5; study group 2), and no procedure was performed on the remaining 10 rats (n=5, 5). The psychomotor movements; pregnancy rates; and sexual, feeding, maternal, social, and grooming behaviors for both study groups were observed daily for 3 months. Their OBs, olfactory cortices, and habenular complexes were examined using stereological methods. All of the animals in the study groups, especially in the cauterization group, demonstrated anorexia, nutritional disorders, weight loss, psychomotor retardation, sexual aversion, decreased grooming behavior, and reduced social interaction similar to depression symptoms. As compared to the control group, the pregnancy rates, number of offspring per mother rat, and birth weights in the study groups were lower, whereas the number of stillbirths was higher. Gross anatomical examinations revealed that the OBs of all of the animals in the study groups were atrophied. Histopathological examinations detected prominent neuronal loss due to apoptosis in the habenular structures in the study groups. We detected a relationship between a decreased healthy neuronal density of the habenula and depressive symptomatology in rats with OBX. We suggest that olfaction disorders might cause neuropsychiatric disorders by affecting neuronal degeneration in habenular nuclei.

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

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

Defining the habenula in human neuroimaging studies

Recently there has been renewed interest in the habenula; a pair of small, highly evolutionarily conserved epithalamic nuclei adjacent to the medial dorsal (MD) nucleus of the thalamus. The habenula has been implicated in a range of behaviours including sleep, stress and pain, and studies in non-human primates have suggested a potentially important role in reinforcement processing, putatively via its effects on monoaminergic neurotransmission. Over the last decade, an increasing number of neuroimaging studies have reported functional responses in the human habenula using functional magnetic resonance imaging (fMRI). However, standard fMRI analysis approaches face several challenges in isolating signal from this structure because of its relatively small size, around 30 mm(3) in volume. In this paper we offer a set of guidelines for locating and manually tracing the habenula in humans using high-resolution T1-weighted structural images. We also offer recommendations for appropriate pre-processing and analysis of high-resolution functional magnetic resonance imaging (fMRI) data such that signal from the habenula can be accurately resolved from that in surrounding structures.

The neurobiology of depression and antidepressant action

We present a comprehensive overview of the neurobiology of unipolar major depression and antidepressant drug action, integrating data from affective neuroscience, neuro- and psychopharmacology, neuroendocrinology, neuroanatomy, and molecular biology. We suggest that the problem of depression comprises three sub-problems: first episodes in people with low vulnerability ('simple' depressions), which are strongly stress-dependent; an increase in vulnerability and autonomy from stress that develops over episodes of depression (kindling); and factors that confer vulnerability to a first episode (a depressive diathesis). We describe key processes in the onset of a 'simple' depression and show that kindling and depressive diatheses reproduce many of the neurobiological features of depression. We also review the neurobiological mechanisms of antidepressant drug action, and show that resistance to antidepressant treatment is associated with genetic and other factors that are largely similar to those implicated in vulnerability to depression. We discuss the implications of these conclusions for the understanding and treatment of depression, and make some strategic recommendations for future research.

Differential projections from the lateral habenula to the rostromedial tegmental nucleus and ventral tegmental area in the rat

The mesopontine rostromedial tegmental nucleus (RMTg) is a mostly γ-aminobutyric acid (GABA)ergic structure believed to be a node for signaling aversive events to dopamine (DA) neurons in the ventral tegmental area (VTA). The RMTg receives glutamatergic inputs from the lateral habenula (LHb) and sends substantial GABAergic projections to the VTA, which also receives direct projections from the LHb. To further specify the topography of LHb projections to the RMTg and VTA, small focal injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin were aimed at different subdivisions of the LHb. The subnuclear origin of LHb inputs to the VTA and RMTg was then confirmed by injections of the retrograde tracer cholera toxin subunit b into the VTA or RMTg. Furthermore, we compared the topographic position of retrogradely labeled neurons in the RMTg resulting from VTA injections with that of anterogradely labeled axons emerging from the LHb. As revealed by anterograde and retrograde tracing, LHb projections were organized in a strikingly topographic manner, with inputs to the RMTg mostly arising from the lateral division of the LHb (LHbL), whereas inputs to the VTA mainly emerged from the medial division of the LHb (LHbM). In the RMTg, profusely branched LHb axons were found in close register with VTA projecting neurons and were frequently apposed to the latter. Overall, our findings demonstrate that LHb inputs to the RMTg and VTA arise from different divisions of the LHb and provide direct evidence for a disynaptic pathway that links the LHbL to the VTA via the RMTg.

DREADDing the lateral habenula: a review of methodological approaches for studying lateral habenula function

The lateral habenula (LHb) is part of the habenular complex in the dorsal diencephalon. The LHb is an important regulator of several neurotransmitter systems in the midbrain; disturbances in this regulation may contribute to mood disorders, abnormalities in cognition, drive, and addiction. Owing to the critical role this nucleus plays in modulating activity of midbrain nuclei, there has been a rapid increase in studies targeting the LHb in the recent years. In this review, we describe studies using traditional approaches to elucidate the function of this brain region, such as lesion, electrical and chemical stimulation, electrophysiology and in vivo microdialysis. We have selected a variety of illustrative studies to discuss each of these methods. Next, we describe studies using methods that are based upon recent advances in molecular biology techniques including recent results from our laboratory using the Designer Receptor Exclusively Activated by Designer Drug (DREADD) technology. Using a Gi/o-coupled DREADD, we found that inhibition of the LHb reduces depression-like behavior in the forced swim test in a manner that suggests enhanced serotonergic activity. The emerging picture reveals that the LHb is likely to be a critical node in the network of subcortical nuclei that regulate aversive learning, motivation, stress responses, etc. We describe how recently developed methods have advanced the study of the LHb and are leading research of this brain region in promising new directions. This article is part of a Special Issue entitled Optogenetics (7th BRES).

Cellular Circadian Clocks in Mood Disorders

Bipolar disorder (BD) and major depressive disorder (MDD) are heritable neuropsychiatric disorders associated with disrupted circadian rhythms. The hypothesis that circadian clock dysfunction plays a causal role in these disorders has endured for decades but has been difficult to test and remains controversial. In the meantime, the discovery of clock genes and cellular clocks has revolutionized our understanding of circadian timing. Cellular circadian clocks are located in the suprachiasmatic nucleus (SCN), the brain’s primary circadian pacemaker, but also throughout the brain and peripheral tissues. In BD and MDD patients, defects have been found in SCN-dependent rhythms of body temperature and melatonin release. However, these are imperfect and indirect indicators of SCN function. Moreover, the SCN may not be particularly relevant to mood regulation, whereas the lateral habenula, ventral tegmentum, and hippocampus, which also contain cellular clocks, have established roles in this regard. Dysfunction in these non-SCN clocks could contribute directly to the pathophysiology of BD/MDD. We hypothesize that circadian clock dysfunction in non-SCN clocks is a trait marker of mood disorders, encoded by pathological genetic variants. Because network features of the SCN render it uniquely resistant to perturbation, previous studies of SCN outputs in mood disorders patients may have failed to detect genetic defects affecting non-SCN clocks, which include not only mood-regulating neurons in the brain but also peripheral cells accessible in human subjects. Therefore, reporters of rhythmic clock gene expression in cells from patients or mouse models could provide a direct assay of the molecular gears of the clock, in cellular clocks that are likely to be more representative than the SCN of mood-regulating neurons in patients. This approach, informed by the new insights and tools of modern chronobiology, will allow a more definitive test of the role of cellular circadian clocks in mood disorders.

Axonal branching patterns of ventral pallidal neurons in the rat

The ventral pallidum (VP) is a key component of the cortico-basal ganglia circuits that process motivational and emotional information, and also a crucial site for reward. Although the main targets of the two VP compartments, medial (VPm) and lateral (VPl) have already been established, the collateralization patterns of individual axons have not previously been investigated. Here we have fully traced eighty-four axons from VPm, VPl and the rostral extension of VP into the olfactory tubercle (VPr), using the anterograde tracer biotinylated dextran amine in the rat. Thirty to fifty percent of axons originating from VPm and VPr collateralized in the mediodorsal thalamic nucleus and lateral habenula, indicating a close association between the ventral basal ganglia-thalamo-cortical loop and the reward network at the single axon level. Additional collateralization of these axons in diverse components of the extended amygdala and corticopetal system supports a multisystem integration that may take place at the basal forebrain. Remarkably, we did not find evidence for a sharp segregation in the targets of axons arising from the two VP compartments, as VPl axons frequently collateralized in the caudal lateral hypothalamus and ventral tegmental area, the well-known targets of VPm, while VPm axons, in turn, also collateralized in typical VPl targets such as the subthalamic nucleus, substantia nigra pars compacta and reticulata, and retrorubral field. Nevertheless, VPl and VPm displayed collateralization patterns that paralleled those of dorsal pallidal components, confirming at the single axon level the parallel organization of functionally different basal ganglia loops.

Long-term course of brain-derived neurotrophic factor serum levels in a patient treated with deep brain stimulation of the lateral habenula

INTRODUCTION

According to the neurotrophin hypothesis, a brain-derived neurotrophic factor (BDNF) decrease has been postulated as a pivotal pathomechanism in affective disorder, and the treatment-associated increase in peripheral BDNF has been linked to therapeutic efficacy of antidepressant drugs and electroconvulsive therapy. However, in deep brain stimulation (DBS), a still experimental antidepressant treatment approach, this issue has not yet been investigated.

METHODS

We examine the long-term course of serum BDNF levels in a 64-year-old woman who is being treated with DBS of the lateral habenula for severe major depressive disorder.

RESULTS

Our main findings are a significant increase in BDNF serum levels following DBS of the lateral habenula and an inverse U-shaped correlation of depression scores and BDNF levels.

DISCUSSION

The data indicate that DBS, like other effective antidepressant treatments, may contribute to an increase in peripheral BDNF levels, which are thought to reflect central nervous DBS-induced neuroplastic changes. Moreover, our observations underscore the complex nature of disease-associated BDNF alterations. Their identification as either state or trait marker remains controversial and requires larger-scale longitudinal studies.

Prediction of SSRI treatment response in major depression based on serotonin transporter interplay between median raphe nucleus and projection areas

Recent mathematical models suggest restored serotonergic burst-firing to underlie the antidepressant effect of selective serotonin reuptake inhibitors (SSRI), resulting from down-regulated serotonin transporters (SERT) in terminal regions. This mechanism possibly depends on the interregional balance between SERTs in the raphe nuclei and in terminal regions before treatment. To evaluate these hypotheses on a systems level in humans in vivo, we investigated SERT availability and occupancy longitudinally in patients with major depressive disorder using positron emission tomography (PET) and the radioligand [11C]DASB. Measurements were performed before and after a single oral dose, as well as after three weeks (mean 24.73±3.3 days) of continuous oral treatment with either escitalopram (10 mg/day) or citalopram (20 mg/day). Data were analyzed using voxel-wise linear regression and ANOVA to evaluate SERT binding, occupancy and binding ratios (SERT binding of the entire brain compared to SERT binding in the dorsal and median raphe nuclei) in relation to treatment outcome. Regression analysis revealed that treatment response was predicted by pre-treatment SERT binding ratios, i.e., SERT binding in key regions of depression including bilateral habenula, amygdala-hippocampus complex and subgenual cingulate cortex in relation to SERT binding in the median but not dorsal raphe nucleus (p<0.05 FDR-corrected). Similar results were observed in the direct comparison of responders and non-responders. Our data provide a first proof-of-concept for recent modeling studies and further underlie the importance of the habenula and subgenual cingulate cortex in the etiology of and recovery from major depression. These findings may indicate a promising molecular predictor of treatment response and stimulate new treatment approaches based on regional differences in SERT binding.

Deep brain stimulation for treatment-resistant depression: efficacy, safety and mechanisms of action

Deep brain stimulation (DBS), a neuromodulation therapy that has been used successfully in the treatment of symptoms associated with movement disorders, has recently undergone clinical trials for individuals suffering from treatment-resistant depression (TRD). Although the small patient numbers and open label study design limit our ability to identify optimum targets and make definitive conclusions about treatment efficacy, a review of the published research demonstrates significant reductions in depressive symptomatology and high rates of remission in a severely treatment-resistant patient group. Despite these encouraging results, an incomplete understanding of the mechanisms of action underlying the therapeutic effects of DBS for TRD is highlighted, paralleling the incomplete understanding of the neuroanatomy of mood regulation and treatment resistance. Proposed mechanisms of action include short and long-term local effects of stimulation at the neuronal level, to modulation of neural network activity.

Shared features of S100B immunohistochemistry and cytochrome oxidase histochemistry in the ventroposterior thalamus and lateral habenula in neonatal rats

The ventroposterior thalamus and the habenular nuclei of the epithalamus are relevant to the monoaminergic system functionally and anatomically. The glia-derived S100B protein plays a critical role in the development of the nervous system including the monoaminergic systems. In this study, we performed an immunohistochemical study of glia-related proteins including S100B, serotonin transporter, and microtubule-associated protein 2, as well as cytochrome oxidase histochemistry in neonatal rats. Results showed the same findings for S100B immunohistochemistry between the ventroposterior thalamus and the lateral habenula at postnatal day 7: intense staining in cell bodies of astrocytes, diffusely spread immunoproduct in the intercellular space, and S100B-free areas as well as a strong reaction to cytochrome oxidase histochemistry. Further common features were the scarcity of glial fibrillary acidic protein-positive astrocytes and the few apoptotic cells observed. The results of the cytochrome oxidase reaction suggested that S100B is released actively into intercellular areas in restricted brain regions showing high neuronal activity at postnatal day 7. Pathology of the ventroposterior thalamus and the habenula is suggested in mental disorders, and S100B might be a key factor for investigations in these areas.

Molecular regionalization of the diencephalon

The anatomic complexity of the diencephalon depends on precise molecular and cellular regulative mechanisms orchestrated by regional morphogenetic organizers at the neural tube stage. In the diencephalon, like in other neural tube regions, dorsal and ventral signals codify positional information to specify ventro-dorsal regionalization. Retinoic acid, Fgf8, BMPs, and Wnts signals are the molecular factors acting upon the diencephalic epithelium to specify dorsal structures, while Shh is the main ventralizing signal. A central diencephalic organizer, the zona limitans intrathalamica (ZLI), appears after neurulation in the central diencephalic alar plate, establishing additional antero-posterior positional information inside diencephalic alar plate. Based on Shh expression, the ZLI acts as a morphogenetic center, which cooperates with other signals in thalamic specification and pattering in the alar plate of diencephalon. Indeed, Shh is expressed first in the basal plate extending dorsally through the ZLI epithelium as the development proceeds. Despite the importance of ZLI in diencephalic morphogenesis the mechanisms that regulate its development remain incompletely understood. Actually, controversial interpretations in different experimental models have been proposed. That is, experimental results have suggested that (i) the juxtaposition of the molecularly heterogeneous neuroepithelial areas, (ii) cell reorganization in the epithelium, and/or (iii) planar and vertical inductions in the neural epithelium, are required for ZLI specification and development. We will review some experimental data to approach the study of the molecular regulation of diencephalic regionalization, with special interest in the cellular mechanisms underlying planar inductions.

Patterning and compartment formation in the diencephalon

The diencephalon gives rise to structures that play an important role in connecting the anterior forebrain with the rest of the central nervous system. The thalamus is the major diencephalic derivative that functions as a relay station between the cortex and other lower order sensory systems. Almost two decades ago, neuromeric/prosomeric models were proposed describing the subdivision and potential segmentation of the diencephalon. Unlike the laminar structure of the cortex, the diencephalon is progressively divided into distinct functional compartments consisting principally of thalamus, epithalamus, pretectum, and hypothalamus. Neurons generated within these domains further aggregate to form clusters called nuclei, which form specific structural and functional units. We review the recent advances in understanding the genetic mechanisms that are involved in the patterning and compartment formation of the diencephalon.

Morphologic and immunohistochemical organization of the human habenular complex

The habenular complex (HbCpx) is a phylogenetically conserved brain structure located in the epithalamus of vertebrates. Despite its fundamental role in decision-making processes and the proposed link between habenular dysfunction and neuropsychiatric conditions, little is known about the structural and functional organization of the HbCpx in humans. The goal of this study was thus to provide a first systematic morphologic and immunohistochemical analysis of the human HbCpx to begin dissecting its nuclear and subnuclear organization. Our results confirmed that the human HbCpx is subdivided into medial (MHb) and lateral (LHb) nuclei, each showing a large degree of intranuclear morphologic heterogeneity. Analysis of serially stained sections using a combination of morphologic and immunohistochemical criteria allowed the distinction of five subnuclei in both the MHb and LHb. Overall, the observed subnuclear organization of the MHb in humans resembles the organization of subnuclei in the MHb of rats. The shape, relative size, and intranuclear organization of the LHb, however, show significant differences. The contribution of the LHb to the entire HbCpx is about five times larger in humans than in rats. Noteworthy, a dorsal domain of the LHb that contains afferent myelinated fibers from the stria medullaris and shows GABA-(B) -R(1) immunoreactive cells, appears substantially enlarged in humans when compared to rats. This feature seems to account for a large part of the relative growth in size of the LHb in humans and opens the intriguing possibility of an increased influence of limbic and striatal afferents into the LHb of humans.

Investigation of the neuroanatomical substrates of reward seeking following protracted abstinence in mice

Persistent vulnerability to relapse represents a major challenge in the treatment of drug addiction. The brain circuitry that underlies relapse-like behaviour can be investigated using animal models of drug seeking. As yet there have been no comprehensive brain mapping studies that have specifically examined the neuroanatomical substrates of cue-induced opiate seeking following abstinence in a mouse operant paradigm. The aim of this study was to compare the brain regions involved in sucrose vs. morphine seeking following protracted abstinence in mice. Male CD1 mice were trained to respond for either sucrose (10% w/v) or intravenous morphine (0.1 mg kg(-1) per infusion) in an operant paradigm in the presence of a discrete cue. Once stable responding was established, mice were subjected to abstinence in their home cages for 3 weeks and then perfused for tissue collection, or returned to the operant chambers to assess cue-induced reward seeking before being perfused for tissue collection. Brain tissue was processed for Fos immunohistochemistry and Fos expression was quantified in a range of brain nuclei. We identified unique patterns of neuronal activation for sucrose and morphine seeking mice as well as some overlap. Structures activated in both ‘relapse' groups included the anterior cingulate and orbitofrontal cortex, nucleus accumbens shell, bed nucleus of the stria terminalis, substantia nigra pars compacta, ventral tegmental area, hippocampus, periaqueductal grey, locus coeruleus and lateral habenula. Structures that were more activated in morphine seeking mice included the nucleus accumbens core, basolateral amygdala, substantia nigra pars reticulata, and the central nucleus of the amygdala. The dorsal raphe was the only structure examined that was specifically activated in sucrose seeking mice. Overall our findings support a cortico-striatal limbic circuit driving opiate seeking, and we have identified some additional circuitry potentially relevant to reward seeking following abstinence.

Mapping pain activation and connectivity of the human habenula

The habenula, located in the posterior thalamus, is implicated in a wide array of functions. Animal anatomical studies have indicated that the structure receives inputs from a number of brain regions (e.g., frontal areas, hypothalamic, basal ganglia) and sends efferent connections predominantly to the brain stem (e.g., periaqueductal gray, raphe, interpeduncular nucleus). The role of the habenula in pain and its anatomical connectivity are well-documented in animals but not in humans. In this study, for the first time, we show how high-field magnetic resonance imaging can be used to detect habenula activation to noxious heat. Functional maps revealed significant, localized, and bilateral habenula responses. During pain processing, functional connectivity analysis demonstrated significant functional correlations between the habenula and the periaqueductal gray and putamen. Probabilistic tractography was used to assess connectivity of afferent (e.g., putamen) and efferent (e.g., periaqueductal gray) pathways previously reported in animals. We believe that this study is the first report of habenula activation by experimental pain in humans. Since the habenula connects forebrain structures with brain stem structures, we suggest that the findings have important implications for understanding sensory and emotional processing in the brain during both acute and chronic pain.

The neurobiology of anhedonia and other reward-related deficits

Anhedonia, or markedly diminished interest or pleasure, is a hallmark symptom of major depression, schizophrenia and other neuropsychiatric disorders. Over the past three decades, the clinical definition of anhedonia has remained relatively unchanged, although cognitive psychology and behavioral neuroscience have expanded our understanding of other reward-related processes. Here, we review the neural bases of the construct of anhedonia that reflects deficits in hedonic capacity and also closely linked to the constructs of reward valuation, decision-making, anticipation and motivation. The neural circuits subserving these reward-related processes include the ventral striatum, prefrontal cortical regions, and afferent and efferent projections. An understanding of anhedonia and other reward-related constructs will facilitate the diagnosis and treatment of disorders that include reward deficits as key symptoms.

Distinct roles of three frontal cortical areas in reward-guided behavior

Functional magnetic resonance imaging was used to measure activity in three frontal cortical areas, the lateral orbitofrontal cortex (lOFC), medial orbitofrontal cortex (mOFC)/ventromedial frontal cortex (vmPFC), and anterior cingulate cortex (ACC), when expectations about type of reward, and not just reward presence or absence, could be learned. Two groups of human subjects learned 12 stimulus-response pairings. In one group (Consistent), correct performances of a given pairing were always reinforced with a specific reward outcome, whereas in the other group (Inconsistent), correct performances were reinforced with randomly selected rewards. The mOFC/vmPFC and lOFC were not distinguished by simple differences in relative preference for positive and negative outcomes. Instead lOFC activity reflected updating of reward-related associations specific to reward type; lOFC was active whenever informative outcomes allowed updating of reward-related associations, regardless of whether the outcomes were positive or negative, and the effects were greater when consistent stimulus-outcome and response-outcome mappings were present. A psychophysiological interaction analysis demonstrated changed coupling between lOFC and brain areas for visual object representation, such as perirhinal cortex, and reward-guided learning, such as the amygdala, ventral striatum, and habenula/mediodorsal thalamus. In contrast, mOFC/vmPFC activity reflected expected values of outcomes and occurrence of positive outcomes, regardless of consistency of outcome mappings. The third frontal cortical region, the ACC, reflected the use of reward type information to guide response selection. ACC activity reflected the probability of selecting the correct response, was greater when consistent outcome mappings were present, and was related to individual differences in propensity to select the correct response.

Modulation of striatal projection systems by dopamine

The basal ganglia are a chain of subcortical nuclei that facilitate action selection. Two striatal projection systems--so-called direct and indirect pathways--form the functional backbone of the basal ganglia circuit. Twenty years ago, investigators proposed that the striatum's ability to use dopamine (DA) rise and fall to control action selection was due to the segregation of D(1) and D(2) DA receptors in direct- and indirect-pathway spiny projection neurons. Although this hypothesis sparked a debate, the evidence that has accumulated since then clearly supports this model. Recent advances in the means of marking neural circuits with optical or molecular reporters have revealed a clear-cut dichotomy between these two cell types at the molecular, anatomical, and physiological levels. The contrast provided by these studies has provided new insights into how the striatum responds to fluctuations in DA signaling and how diseases that alter this signaling change striatal function.

Mechanistic insights into nicotine withdrawal

Smoking is responsible for over 400,000 premature deaths in the United States every year, making it the leading cause of preventable death. In addition, smoking-related illness leads to billions of dollars in healthcare expenditures and lost productivity annually. The public is increasingly aware that successfully abstaining from smoking at any age can add years to one's life and reduce many of the harmful effects of smoking. Although the majority of smokers desire to quit, only a small fraction of attempts to quit are actually successful. The symptoms associated with nicotine withdrawal are a primary deterrent to cessation and they need to be quelled to avoid early relapse. This review will focus on the neuroadaptations caused by chronic nicotine exposure and discuss how those changes lead to a withdrawal syndrome upon smoking cessation. Besides examining how nicotine usurps the endogenous reward system, we will discuss how the habenula is part of a circuit that plays a critical role in the aversive effects of high nicotine doses and nicotine withdrawal. We will also provide an updated summary of the role of various nicotinic receptor subtypes in the mechanisms of withdrawal. This growing knowledge provides mechanistic insights into current and future smoking cessation therapies.

Vasopressin innervation of the mouse (Mus musculus) brain and spinal cord

The neuropeptide vasopressin (AVP) has been implicated in the regulation of numerous physiological and behavioral processes. Although mice have become an important model for studying this regulation, there is no comprehensive description of AVP distribution in the mouse brain and spinal cord. With C57BL/6 mice, we used immunohistochemistry to corroborate the location of AVP-containing cells and to define the location of AVP-containing fibers throughout the mouse central nervous system. We describe AVP-immunoreactive (-ir) fibers in midbrain, hindbrain, and spinal cord areas, which have not previously been reported in mice, including innervation of the ventral tegmental area, dorsal and median raphe, lateral and medial parabrachial, solitary, ventrolateral periaqueductal gray, and interfascicular nuclei. We also provide a detailed description of AVP-ir innervation in heterogenous regions such as the amygdala, bed nucleus of the stria terminalis, and ventral forebrain. In general, our results suggest that, compared with other species, the mouse has a particularly robust and widespread distribution of AVP-ir fibers, which, as in other species, originates from a number of different cell groups in the telencephalon and diencephalon. Our data also highlight the robust nature of AVP innervation in specific regulatory nuclei, such as the ventral tegmental area and dorsal raphe nucleus among others, that are implicated in the regulation of many behaviors.

Amphetamine differentially modifies the expression of monoaminergic and GABAergic synaptic boutons and processes in lateral habenula, dorsal and ventral hippocampal formation

The habenular complex is thought to be associated with cognitive functions and indirectly connected with the hippocampal formation (HF). Thus the responses of the monoaminergic and GABAergic neurons were examined in both structures to the psychostimulant, amphetamine (Amph). Immunocytochemical analysis was performed on brain sections prepared from adult mice treated with a single or multiple (2 doses/day, 7 doses in total) injections of saline or Amph, 5mg/kg. The synaptic boutons were verified by immuno-electron microscopy specific for parvalbumin (PV), glutamic acid decarboxylase(67) (GAD(67)), aromatic amino acid decarboxylase (AADC) or dopamine-β-hydroxylase (DBH). In the lateral part of the lateral habenula (LHb), at 4h post-acute Amph, the densities of PV-positive boutons/processes and DBH-boutons were decreased by approximate 75% and 72% respectively, compared with corresponding saline-controls; however, at 4h post-repeated Amph exposure, PV was increased by 244%, and DBH unaltered. In the dorsal HF (DHF), at 4h post-repeated Amph exposure, GAD(67)-boutons and PV resembled controls in CA1 and CA3 pyramidal cell layers, whereas in the granule cell layer of dentate gyrus (DG), PV was increased by 112%, and GAD(67) unchanged. As shown by biochemical methods, at 4h post-repeated Amph, the decreased level of DHF GABA probably correlates with the immunocytochemical changes. In the ventral HF (VHF), at 4h post-repeated Amph treatment, PV and the enzymes of CA1 and DG were unaltered, while CA3 PV was decreased by 63%, and AADC-boutons increased 55%. Double immuno-electron microscopy revealed synaptic contacts between PV and GAD(67) containing presynaptic or postsynaptic elements, and between PV or GAD(67) and DBH or AADC. This ultrastructural evidence may support the functional significance of the Amph-induced differential changes, which could reflect Amph toxicity and distinct characteristics of the LHb, DHF and VHF.

Addiction circuitry in the human brain

A major challenge in understanding substance-use disorders lies in uncovering why some individuals become addicted when exposed to drugs, whereas others do not. Although genetic, developmental, and environmental factors are recognized as major contributors to a person's risk of becoming addicted, the neurobiological processes that underlie this vulnerability are still poorly understood. Imaging studies suggest that individual variations in key dopamine-modulated brain circuits, including circuits involved in reward, memory, executive function, and motivation, contribute to some of the differences in addiction vulnerability. A better understanding of the main circuits affected by chronic drug use and the influence of social stressors, developmental trajectories, and genetic background on these circuits is bound to lead to a better understanding of addiction and to more effective strategies for the prevention and treatment of substance-use disorders.

Discovering a new anatomical structure in the brain: implications for neuropsychiatry and therapy

Describing new brain structures may open research avenues and improve our knowledge of brain functions and of brain disorders. It may also provide new neuroanatomical targets for treatments. A detailed understanding of neuroanatomy is a prerequisite of understanding the pathomechanisms underlying psychiatric disorders on a molecular and cellular level. The tail of the ventral tegmental area (tVTA), also known as rostromedial tegmental nucleus (RMTg), is a recently described region which may be a major inhibitory control centre for the dopaminergic systems. These systems participates to behavioural functions and are implicated in the aetiology, symptoms or treatment of neurological or psychiatric diseases, such as Parkinson's, schizophrenia, mood disorders, attention-deficit hyperactivity-disorder (ADHD) and drug abuse. Belonging to the reticular formation, the tVTA may constitute a major inhibitory GABAergic input to these dopaminergic systems. Moreover, it is sensitive to drugs of abuse, to stimulant or arousing drugs, to aversive stimuli and it could also be the main relay between lateral habenula and VTA. First described in rats, and proposed as a component of the emotional motor system implicated in basic survival behaviours, tVTA appears as a relevant structure for molecular psychiatry, which should foster research to define and study this brain region in the human brain.

Serotonin-dependent depression in Parkinson's disease: a role for the subthalamic nucleus?

Depression is the most common neuropsychiatric co-morbidity in Parkinson's disease (PD). The underlying mechanism of depression in PD is complex and likely involves biological, psychosocial and therapeutic factors. The biological mechanism may involve changes in monoamine systems, in particular the serotonergic (5-hydroxytryptamine, 5-HT) system. It is well established that the 5-HT system is markedly affected in the Parkinsonian brain, with evidence including pathological loss of markers of 5-HT axons as well as cell bodies in the dorsal and median raphe nuclei of the midbrain. However, it remains unresolved whether alterations to the 5-HT system alone are sufficient to confer vulnerability to depression. Here we propose low 5-HT combined with altered network activity within the basal ganglia as critically involved in depression in PD. The latter hypothesis is derived from a number of recent findings that highlight the close interaction between the basal ganglia and the 5-HT system, not only in motor but also limbic functions. These findings include evidence that clinical depression is a side effect of deep brain stimulation (DBS) of the subthalamic nucleus (STN), a treatment option in advanced PD. Further, it has recently been demonstrated that STN DBS in animal models inhibits 5-HT neurotransmission, and that this change may underpin depressive-like side effects. This review provides an overview of 5-HT alterations in PD and a discussion of how these changes might combine with altered basal ganglia network activity to increase depression vulnerability.

Chronic deep brain stimulation of the lateral habenula nucleus in a rat model of depression

In the present study, we aim to determine the antidepressant effects of chronic deep brain stimulation (DBS) of the lateral habenula nucleus (LHb) in a rat model of depression and to explore the potential mechanism of DBS induced improvement of depressive symptoms. To establish the rat depression model, animals were repeatedly exposed to a set of chronic mild stressors for four consecutive weeks. The open-field and sucrose consumption tests were used as measures of depression. For DBS treatment, rats were stereotaxically implanted with electrodes into the LHb and stimulated over a course of 28 d. A separate positive control group was given pharmacotherapy with clomipramine hydrochloride. Open-field testing was used to determine behavioral changes following DBS treatment. Monoamine concentrations in blood and brain tissues were determined by fluorescence spectrophotometry. This study demonstrates that DBS of the LHb region significantly improved depressive-like symptoms in the rat model. These improvements manifested as elevated numbers of crossings and rearings during the open-field test in DBS-treated depressed rats compared to controls. In addition, concentrations of monoamines including norepinephrine (NE), dopamine (DA), and serotonin (5-HT) in blood serum and brain tissues were also increased by DBS of the LHb. Therefore, significant improvements in all outcomes were detected following chronic DBS treatment. These results indicate that long-term DBS treatment of the LHb region effectively improved depressive symptoms in rats, likely as a result of elevated monoamine levels. LHb DBS may therefore provide a valuable therapeutic strategy for the clinical treatment of depression.

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.

Emerging, reemerging, and forgotten brain areas of the reward circuit: Notes from the 2010 Motivational Neural Networks conference

On April 24-27, 2010, the Motivational Neuronal Networks meeting took place in Wrightsville Beach, North Carolina. The conference was devoted to "Emerging, re-emerging, and forgotten brain areas" of the reward circuit. A central feature of the conference was four scholarly discussions of cutting-edge topics related to the conference's theme. These discussions form the basis of the present review, which summarizes areas of consensus and controversy, and serves as a roadmap for the next several years of research.

Mapping vocalization-related immediate early gene expression in echolocating bats

Recent studies of spontaneously vocalizing primates, cetaceans, bats and rodents suggest these animals possess a limited but meaningful capacity to manipulate the timing and acoustic structure of their vocalizations, yet the neural substrate for even the simplest forms of vocal modulation in mammals remains unknown. Echolocating bats rapidly and routinely manipulate the acoustic structure of their outgoing vocalizations to improve echolocation efficiency, reflecting cognitive rather than limbic control of the vocal motor pathways. In this study, we used immunohistochemical localization of immediate early gene (c-fos) expression to map neural activity in the brains of spontaneously echolocating stationary Mexican free-tailed bats. Our results support the current model of vocal control obtained largely through microstimulation studies, but also provide evidence for the contributions of two novel regions, the dorsolateral caudate nucleus and mediodorsal thalamic nucleus, which together suggest a striatothalamic feedback loop may be involved in the control of echolocation pulse production. Additionally, we found evidence of a motivation pathway, including the lateral habenula, substantia nigra pars compacta, and raphe nuclei. These data provide novel insights into where and how mammalian vocalizations may be regulated by sensory, contextual and motivational cues.

The inhibitory influence of the lateral habenula on midbrain dopamine cells: ultrastructural evidence for indirect mediation via the rostromedial mesopontine tegmental nucleus

The lateral habenula (LHb) provides an important source of negative reinforcement signals to midbrain dopamine (DA) cells in the substantia nigra and ventral tegmental area (VTA). This profound and consistent inhibitory influence involves a disynaptic connection from glutamate neurons in the LHb to some population of γ-aminobutyric acid (GABA) cells that, in turn, innervates DA neurons. Previous studies demonstrated that the GABA cells intrinsic to the VTA receive insufficient synaptic input from the LHb to serve as the primary source of this intermediate connection. In this investigation, we sought ultrastructural evidence supporting the hypothesis that a newly identified region of the brainstem, the rostromedial mesopontine tegmental nucleus (RMTg), is a more likely candidate for inhibiting midbrain DA cells in response to LHb activation. Electron microscopic examination of rat brain sections containing dual immunoreactivity for an anterograde tracing agent and a phenotypic marker revealed that: 1) more than 55% of the synapses formed by LHb axons in the RMTg were onto GABA-labeled dendrites; 2) more than 80% of the synapses formed by RMTg axons in the VTA contacted dendrites immunoreactive for the DA synthetic enzyme tyrosine hydroxylase; and 3) nearly all RMTg axons formed symmetric synapses and contained postembedding immunoreactivity for GABA. These findings indicate that the newly identified RMTg region is an intermediate structure in a disynaptic pathway that connects the LHb to VTA DA neurons. The results have important implications for understanding mental disorders characterized by a dysregulation of reward circuitry involving LHb and DA cell populations.

Caudate nucleus signals for breaches of expectation in a movement observation paradigm

The striatum has been established as a carrier of reward-related prediction errors. This prediction error signal concerns the difference between how much reward was predicted and how much reward is gained. However, it remains to be established whether general breaches of expectation, i.e., perceptual prediction errors, are also implemented in the striatum. The current study used functional magnetic resonance imaging (fMRI) to investigate the role of caudate nucleus in breaches of expectation. Importantly, breaches were not related to the occurrence or absence of reward. Preceding the fMRI study, participants were trained to produce a sequence of whole-body movements according to auditory cues. In the fMRI session, they watched movies of a dancer producing the same sequences either according to the cue (88%) or not (12%). Caudate nucleus was activated for the prediction-violating movements. This activation was flanked by activity in posterior superior temporal sulcus, the temporo-parietal junction and adjacent angular gyrus, a network that may convey the deviating movement to caudate nucleus, while frontal areas may reflect adaptive adjustments of the current prediction. Alternative interpretations of caudate activity relating either to the saliency of breaches of expectation or to behavioral adaptation could be excluded by two control contrasts. The results foster the notion that neurons in the caudate nucleus code for a breach in expectation, and point toward a distributed network involved in detecting, signaling and adjusting behavior and expectations toward violated prediction.

Error-related functional connectivity of the habenula in humans

Error detection is critical to the shaping of goal-oriented behavior. Recent studies in non-human primates delineated a circuit involving the lateral habenula (LH) and ventral tegmental area (VTA) in error detection. Neurons in the LH increased activity, preceding decreased activity in the VTA, to a missing reward, indicating a feedforward signal from the LH to VTA. In the current study we used connectivity analyses to reveal this pathway in humans. In 59 adults performing a stop signal task during functional magnetic resonance imaging, we identified brain regions showing greater psychophysiological interaction with the habenula during stop error as compared to stop success trials. These regions included a cluster in the VTA/substantia nigra (SN), internal segment of globus pallidus, bilateral amygdala, and insula. Furthermore, using Granger causality and mediation analyses, we showed that the habenula Granger caused the VTA/SN, establishing the direction of this interaction, and that the habenula mediated the functional connectivity between the amygdala and VTA/SN during error processing. To our knowledge, these findings are the first to demonstrate a feedforward influence of the habenula on the VTA/SN during error detection in humans.

Variation in asymmetry of the habenular nucleus correlates with behavioural asymmetry in a cichlid fish

Asymmetries in brain and behaviour have been demonstrated in numerous species representing all major vertebrate taxa, and may be a universal feature of the vertebrate nervous system. While descriptions of lateralization at the behavioural and neuroanatomical level are widespread, examples of correlation between asymmetries in behaviour and neural structures remain relatively scarce. In the past few years, the habenular nucleus has emerged as a potential site for the neural basis of some lateralized behaviours. Here we investigate the relation between continuous individual variation in asymmetry of the habenulae and behaviour in the detour task in the convict cichlid (Amatitlania nigrofasciata). We found that both male and female convicts show a significant population-level bias towards relatively larger left habenulae. We also show that habenular asymmetry is correlated with behavioural lateralization in both males and females, but in opposite directions. This adds to previous studies showing both in convict cichlids and other vertebrates an interaction between sex and lateralized behaviour. The results of this study increase our understanding of the role of the habenula in lateralized behaviour and highlight the importance of a comparative approach to understanding the development and evolution of habenular asymmetry.

The arbitration-extension hypothesis: a hierarchical interpretation of the functional organization of the Basal Ganglia

Based on known anatomy and physiology, we present a hypothesis where the basal ganglia motor loop is hierarchically organized in two main subsystems: the arbitration system and the extension system. The arbitration system, comprised of the subthalamic nucleus, globus pallidus, and pedunculopontine nucleus, serves the role of selecting one out of several candidate actions as they are ascending from various brain stem motor regions and aggregated in the centromedian thalamus or descending from the extension system or from the cerebral cortex. This system is an action-input/action-output system whose winner-take-all mechanism finds the strongest response among several candidates to execute. This decision is communicated back to the brain stem by facilitating the desired action via cholinergic/glutamatergic projections and suppressing conflicting alternatives via GABAergic connections. The extension system, comprised of the striatum and, again, globus pallidus, can extend the repertoire of responses by learning to associate novel complex states to certain actions. This system is a state-input/action-output system, whose organization enables it to encode arbitrarily complex Boolean logic rules using striatal neurons that only fire given specific constellations of inputs (Boolean AND) and pallidal neurons that are silenced by any striatal input (Boolean OR). We demonstrate the capabilities of this hierarchical system by a computational model where a simulated generic “animal” interacts with an environment by selecting direction of movement based on combinations of sensory stimuli, some being appetitive, others aversive or neutral. While the arbitration system can autonomously handle conflicting actions proposed by brain stem motor nuclei, the extension system is required to execute learned actions not suggested by external motor centers. Being precise in the functional role of each component of the system, this hypothesis generates several readily testable predictions.

Mesolimbic effects of the antidepressant fluoxetine in Holtzman rats, a genetic strain with increased vulnerability to stress

This is the first metabolic mapping study of the effects of fluoxetine after learned helplessness training. Antidepressants are the most commonly prescribed medications, but the regions underlying treatment effects in affectively disordered brains are poorly understood. We hypothesized the antidepressant action of fluoxetine would produce adaptations in mesolimbic regions after 2 weeks of treatment. We used Holtzman rats, a genetic strain showing susceptibility to novelty-evoked hyperactivity and stress-evoked helplessness, to map regional brain metabolic effects caused by fluoxetine treatment. Animals underwent learned helplessness, and subsequently immobility time was scored in the forced swim test (FST). On the next day, animals began receiving 2 weeks of fluoxetine (5mg/kg/day) or vehicle and were retested in the FST at the end of drug treatment. Antidepressant behavioral effects of fluoxetine were analyzed using a ratio of immobility during pre- and post-treatment FST sessions. Brains were analyzed for regional metabolic activity using quantitative cytochrome oxidase histochemistry as in our previous study using congenitally helpless rats. Fluoxetine exerted a protective effect against FST-induced immobility behavior in Holtzman rats. Fluoxetine also caused a significant reduction in the mean regional metabolism of the nucleus accumbens shell and the ventral hippocampus as compared to vehicle-treated subjects. Additional networks affected by fluoxetine treatment included the prefrontal-cingulate cortex and brainstem nuclei linked to depression (e.g., habenula, dorsal raphe and interpeduncular nucleus). We concluded that corticolimbic regions such as the prefrontal-cingulate cortex, nucleus accumbens, ventral hippocampus and key brainstem nuclei represent important contributors to the neural network mediating fluoxetine antidepressant action.

Habenula volume in bipolar disorder and major depressive disorder: a high-resolution magnetic resonance imaging study

BACKGROUND

Increased activity of the habenula has been implicated in the etiology of major depressive disorder (MDD), in which reductions in habenula volume are present after death. We conducted the first magnetic resonance imaging analysis of habenula volume in MDD and bipolar disorder (BD).

METHODS

High-resolution images (resolution approximately .4 mm(3)) were acquired with a 3T scanner, and a pulse sequence was optimized for tissue contrast resolution. The habenula was manually segmented by one rater blind to diagnosis. Seventy-four healthy control subjects (HC) were compared with both medicated (lithium/divalproex, n = 15) and unmedicated, depressed BD (n = 22) patients; unmedicated, depressed MDD patients (n = 28); and unmedicated MDD patients in remission (n = 32).

RESULTS

The unmedicated BD patients displayed significantly smaller absolute (p < .01) and normalized (p < .05) habenula volumes than the HC subjects. In post hoc assessments analyzing men and women separately, the currently-depressed women with MDD had smaller absolute (p < .05) habenula volumes than the HC women. None of the other psychiatric groups differed significantly from the HC group.

CONCLUSIONS

We provide further evidence for the involvement of the habenula in affective illness but suggest that a reduction in volume might be more pronounced in unmedicated, depressed BD subjects and female currently depressed MDD subjects. The habenula plays major roles in the long-term modification of monoamine transmission and behavioral responses to stress and in the suppression of dopamine cell activity after the absence of an expected reward. A reduction in habenula volume might thus have functional consequences that contribute to the risk for developing affective disease.

Effects of drugs of abuse on putative rostromedial tegmental neurons, inhibitory afferents to midbrain dopamine cells

Recent findings have underlined the rostromedial tegmental nucleus (RMTg), a structure located caudally to the ventral tegmental area, as an important site involved in the mechanisms of aversion. RMTg contains γ-aminobutyric acid neurons responding to noxious stimuli, densely innervated by the lateral habenula and providing a major inhibitory projection to reward-encoding midbrain dopamine (DA) neurons. One of the key features of drug addiction is the perseverance of drug seeking in spite of negative and unpleasant consequences, likely mediated by response suppression within neural pathways mediating aversion. To investigate whether the RMTg has a function in the mechanisms of addicting drugs, we studied acute effects of morphine, cocaine, the cannabinoid agonist WIN55212-2 (WIN), and nicotine on putative RMTg neurons. We utilized single unit extracellular recordings in anesthetized rats and whole-cell patch-clamp recordings in brain slices to identify and characterize putative RMTg neurons and their responses to drugs of abuse. Morphine and WIN inhibited both firing rate in vivo and excitatory postsynaptic currents (EPSCs) evoked by stimulation of rostral afferents in vitro, whereas cocaine inhibited discharge activity without affecting EPSC amplitude. Conversely, nicotine robustly excited putative RMTg neurons and enhanced EPSCs, an effect mediated by α7-containing nicotinic acetylcholine receptors. Our results suggest that activity of RMTg neurons is profoundly influenced by drugs of abuse and, as important inhibitory afferents to midbrain DA neurons, they might take place in the complex interplay between the neural circuits mediating aversion and reward.

Serotonin and dopamine: unifying affective, activational, and decision functions

Serotonin, like dopamine (DA), has long been implicated in adaptive behavior, including decision making and reinforcement learning. However, although the two neuromodulators are tightly related and have a similar degree of functional importance, compared with DA, we have a much less specific understanding about the mechanisms by which serotonin affects behavior. Here, we draw on recent work on computational models of dopaminergic function to suggest a framework by which many of the seemingly diverse functions associated with both DA and serotonin-comprising both affective and activational ones, as well as a number of other functions not overtly related to either-can be seen as consequences of a single root mechanism.

The anatomy and physiology of the ocular motor system

Accurate diagnosis of abnormal eye movements depends upon knowledge of the purpose, properties, and neural substrate of distinct functional classes of eye movement. Here, we summarize current concepts of the anatomy of eye movement control. Our approach is bottom-up, starting with the extraocular muscles and their innervation by the cranial nerves. Second, we summarize the neural circuits in the pons underlying horizontal gaze control, and the midbrain connections that coordinate vertical and torsional movements. Third, the role of the cerebellum in governing and optimizing eye movements is presented. Fourth, each area of cerebral cortex contributing to eye movements is discussed. Last, descending projections from cerebral cortex, including basal ganglionic circuits that govern different components of gaze, and the superior colliculus, are summarized. At each stage of this review, the anatomical scheme is used to predict the effects of lesions on the control of eye movements, providing clinical-anatomical correlation.

Identification of brain nuclei implicated in cocaine-primed reinstatement of conditioned place preference: a behaviour dissociable from sensitization

Relapse prevention represents the primary therapeutic challenge in the treatment of drug addiction. As with humans, drug-seeking behaviour can be precipitated in laboratory animals by exposure to a small dose of the drug (prime). The aim of this study was to identify brain nuclei implicated in the cocaine-primed reinstatement of a conditioned place preference (CPP). Thus, a group of mice were conditioned to cocaine, had this place preference extinguished and were then tested for primed reinstatement of the original place preference. There was no correlation between the extent of drug-seeking upon reinstatement and the extent of behavioural sensitization, the extent of original CPP or the extinction profile of mice, suggesting a dissociation of these components of addictive behaviour with a drug-primed reinstatement. Expression of the protein product of the neuronal activity marker c-fos was assessed in a number of brain regions of mice that exhibited reinstatement (R mice) versus those which did not (NR mice). Reinstatement generally conferred greater Fos expression in cortical and limbic structures previously implicated in drug-seeking behaviour, though a number of regions not typically associated with drug-seeking were also activated. In addition, positive correlations were found between neural activation of a number of brain regions and reinstatement behaviour. The most significant result was the activation of the lateral habenula and its positive correlation with reinstatement behaviour. The findings of this study question the relationship between primed reinstatement of a previously extinguished place preference for cocaine and behavioural sensitization. They also implicate activation patterns of discrete brain nuclei as differentiators between reinstating and non-reinstating mice.

High field FMRI reveals thalamocortical integration of segregated cognitive and emotional processing in mediodorsal and intralaminar thalamic nuclei

Thalamocortical loops, connecting functionally segregated, higher order cortical regions, and basal ganglia, have been proposed not only for well described motor and sensory regions, but also for limbic and prefrontal areas relevant for affective and cognitive processes. These functions are, however, more specific to humans, rendering most invasive neuroanatomical approaches impossible and interspecies translations difficult. In contrast, non-invasive imaging of functional neuroanatomy using fMRI allows for the development of elaborate task paradigms capable of testing the specific functionalities proposed for these circuits. Until recently, spatial resolution largely limited the anatomical definition of functional clusters at the level of distinct thalamic nuclei. Since their anatomical distinction seems crucial not only for the segregation of cognitive and limbic loops but also for the detection of their functional interaction during cognitive–emotional integration, we applied high resolution fMRI on 7 Tesla. Using an event-related design, we could isolate thalamic effects for preceding attention as well as experience of erotic stimuli. We could demonstrate specific thalamic effects of general emotional arousal in mediodorsal nucleus and effects specific to preceding attention and expectancy in intralaminar centromedian/parafascicular complex. These thalamic effects were paralleled by specific coactivations in the head of caudate nucleus as well as segregated portions of rostral or caudal cingulate cortex and anterior insula supporting distinct thalamo–striato–cortical loops. In addition to predescribed effects of sexual arousal in hypothalamus and ventral striatum, high resolution fMRI could extent this network to paraventricular thalamus encompassing laterodorsal and parataenial nuclei. We could lend evidence to segregated subcortical loops which integrate cognitive and emotional aspects of basic human behavior such as sexual processing.

Toward a neurobiology of delusions

Delusions are the false and often incorrigible beliefs that can cause severe suffering in mental illness. We cannot yet explain them in terms of underlying neurobiological abnormalities. However, by drawing on recent advances in the biological, computational and psychological processes of reinforcement learning, memory, and perception it may be feasible to account for delusions in terms of cognition and brain function. The account focuses on a particular parameter, prediction error--the mismatch between expectation and experience--that provides a computational mechanism common to cortical hierarchies, fronto-striatal circuits and the amygdala as well as parietal cortices. We suggest that delusions result from aberrations in how brain circuits specify hierarchical predictions, and how they compute and respond to prediction errors. Defects in these fundamental brain mechanisms can vitiate perception, memory, bodily agency and social learning such that individuals with delusions experience an internal and external world that healthy individuals would find difficult to comprehend. The present model attempts to provide a framework through which we can build a mechanistic and translational understanding of these puzzling symptoms.

Overexpression of CB2 cannabinoid receptors results in neuroprotection against behavioral and neurochemical alterations induced by intracaudate administration of 6-hydroxydopamine

The role of CB2 cannabinoid receptors in the behavioral and neurochemical changes induced by intracaudate administration of 6-hydroxydopamine (6-OHDA) was evaluated. 6-OHDA (12 μg/4 μL) or its vehicle was injected in the caudate-putamen (CPu) of mice overexpressing the CB2 cannabinoid receptor (CB2xP) and wild type (WT) mice. Motor impairment, emotional behavior, and cognitive alterations were evaluated. Tyrosine hydroxylase (TH), glial fibrillary acidic protein (GFAP), and ionized calcium-binding adapter molecule 1 (Iba-1) were measured by immunocytochemistry in the CPu and/or substantia nigra (SN) of CB2xP mice and WT mice. Oxidative/nitrosative and neuroinflammatory parameters were also measured in the CPu and cortex of 6-OHDA-treated and sham-treated mice. 6-OHDA-treated CB2xP mice presented significantly less motor deterioration than 6-OHDA-treated WT mice. Immunocytochemical analysis of tyrosine hydroxylase in the SN and CPu revealed significantly fewer lesions in CB2xP mice than in WT mice. GFAP and Iba-1 immunostaining revealed less astrocyte and microglia recruitment to the treated area of the CPu in CB2xP mice. Malonyldialdehyde (MDA) concentrations were lower in the striatum and cerebral cortex of sham-treated CB2xP mice than in sham-treated WT mice. The administration of 6-OHDA increased MDA levels in both WT mice and CB2xP mice; it increased the oxidized (GSSG)/reduced (GSH) glutathione ratio in the striatum in WT mice alone compared with matched sham-treated controls. The results revealed that overexpression of CB2 cannabinoid receptors decreased the extent of motor impairment and dopaminergic neuronal loss, reduced the recruitment of astrocytes and microglia to the lesion, and decreased the level of various oxidative parameters. These results suggest that CB2 receptors offer neuroprotection against dopaminergic injury.

Electrical stimulation of the lateral habenula produces an inhibitory effect on sucrose self-administration

The lateral habenula (LHb) plays a role in prediction of negative reinforcement, punishment and aversive responses. In the current study, we examined the role that the LHb plays in regulation of negative reward responses and aversion. First, we tested the effect of intervention in LHb activity on sucrose reinforcing behavior. An electrode was implanted into the LHb and rats were trained to self-administer sucrose (20%; 16 days) until at least three days of stable performance were achieved (as represented by the number of active lever presses in self-administration cages). Rats subsequently received deep brain stimulation (DBS) of the LHb, which significantly reduced sucrose self-administration levels. In contrast, lesion of the LHb increased sucrose-seeking behavior, as demonstrated by a delayed extinction response to substitution of sucrose with water. Furthermore, in a modified non-rewarding conditioned-place-preference paradigm, DBS of the LHb led to aversion to the context associated with stimulation of this brain region. We postulate that electrical stimulation of the LHb attenuates positive reward-associated reinforcement by natural substances.