Retrograde HRP tracing combined with a pharmacohistochemical method for GABA transaminase for the identification of presumptive GABAergic projections to the habenula

The behavioral relevance of a modular organization in the lateral habenula

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

ω-3 Polyunsaturated Fatty Acids Facilitate the Repair of Peripheral Nerve Defects with Chemically Extracted Acellular Allograft in Rats

Acellular allograft (ACA) improves the repair and reconstruction of long peripheral nerve defects. ω-3 Polyunsaturated fatty acids (PUFAs) carry a neuroprotective potential, and their effects on ACA bridging were elucidated. Thirty rats with long gap sciatic nerve defects (15 mm long) were randomly divided into three groups (n = 10): ACA, ACA + PUFAs, and autograft (AU). Limb condition, wet weight of tibialis anterior muscle (TAM), nerve electrophysiology, S-100, horseradish peroxidase (HRP), and percentage of splenic CD4+ and CD8 + T-lymphocytes were evaluated for 12 weeks after the operation. Rats in the AU and ACA + PUFA groups showed superior condition in affected limbs compared to the ACA group. At 12 wk after surgery, the wet weight of TAM in the ACA + PUFA group was higher than that in the ACA group (0.4519 ± 0.1185 vs. 0.3049 ± 0.1272; P < 0.01) but lower than that in the AU group (0.4519 ± 0.1185, 0.5628 ± 0.0092; P < 0.05). In all the three groups, sole irritation elicited withdrawal reflex, and S-100 staining was detected in plantar skin. Moreover, horseradish peroxidase staining was overt in both the ventral horn and dorsal root ganglion of the spinal cord. Nerve conduction velocity (m/s), amplitude of action potential (mV), or somatosensory evoked potentials in ACA + PUFAs (28.81 ± 1.04, 2.20 ± 0.27, 6.98 ± 0.29) were significantly different from that in the AU (35.71 ± 1.28, 1.81 ± 0.19, 8.15 ± 0.52; P < 0.05) and ACA (20.03 ± 1.94, 2.95 ± 0.36, 5.22 ± 0.53; P < 0.01) groups. The percentages of splenic CD4+ and CD8+ cells were similar among the three groups. Omega-3 PUFAs improve the bridging effect of ACA on long gap peripheral nerve defects by promoting neuroprotection without arousing an immune response.

Inhibition of the dorsomedial hypothalamus substantially decreases brown adipose tissue sympathetic discharge induced by activation of the lateral habenula

The lateral habenula (LHb) is an evolutionarily ancient nucleus that plays an important role in the detection of salient/adverse environmental events. We have previously shown that the LHb is involved in brown adipose tissue (BAT) thermogenesis elicited by stressful situations, and that the medullary raphé, a key lower brainstem sympathetic control centre, mediates BAT thermogenesis elicited by stimulating the LHb. Since there are no direct projections from the LHb to the medullary raphé, it is plausible that the dorsomedial hypothalamus (DMH), a brain region known to be important for thermoregulatory responses to stress, is involved in this thermogenic pathway. In this study we aimed to test this possibility. In anaesthetized Sprague-Dawley rats, we recorded electrical discharges directly from sympathetic fibres that innervate BAT, as well as BAT temperature. Injections of bicuculline (1 nmol in 100 nl), a neuronal activator by disinhibition, into the LHb increased BAT sympathetic nerve discharge by 4.9 ± 1.4dBμV (n = 7, P < 0.05) and BAT temperature by 1.0 ± 0.1 °C (n = 7, P < 0.01). Subsequent injections of muscimol (0.25 nmol in 100 nl), a neuronal inhibitor, into the DMH promptly reduced BAT sympathetic nerve discharge by 4.7 ± 1.3 dBμV (n = 7, P < 0.05) and BAT temperature by 0.3 ± 0.1 °C (n = 7, P < 0.05). Injections of a mixture of the ionotropic glutamate receptor antagonists, DL-2-Amino-5-phosphonopentanoic acid (AP5) and 6-cyano-7-nitroquinoxaline-2,3-dioneis (CNQX) into the DMH, after activation of the LHb, also significantly decreased BAT sympathetic nerve discharge and BAT temperature. These results suggest that, for sympathetically-mediated BAT thermogenesis, the DMH is part of the neural circuitry linking the LHb with the medullary raphé.

Comparison of GABA, Somatostatin, and Corticotrophin-Releasing Hormone Expression in Axon Terminals That Target the Parabrachial Nucleus

Several forebrain areas have been shown to project to the parabrachial nucleus (PBN) and exert inhibitory and excitatory influences on taste processing. Some sources of descending input such as the central nucleus of the amygdala (CeA) might utilize somatostatin (Sst) and/or corticotrophin-releasing hormone (Crh) to influence taste processing in the PBN (Panguluri S, Saggu S, Lundy R. 2009. Comparison of somatostatin and corticotrophin-releasing hormone immunoreactivity in forebrain neurons projecting to taste-responsive and non-responsive regions of the parabrachial nucleus in rat. Brain Res 1298:57-69; Magableh A, Lundy R. 2014. Somatostatin and corticotrophin releasing hormone cell types are a major source of descending input from the forebrain to the parabrachial nucleus in mice. Chem Senses 39:673-682). Since the predominate effect of CeA stimulation on PBN taste-evoked responses is inhibition, this study used transgenic reporter lines (Sst/TdTomato and Crh/TdTomato) and electron microscopy to assess Sst/gamma aminobutyric acid (GABA) and Crh/GABA coexpression in axon terminals within the PBN. Robust expression of Sst and Crh axon terminals was observed in the PBN. The majority of Sst-positive axon terminals were positive for GABA expression, while the majority of Crh terminals were not. The results indicate that Sst-expressing neurons, but not Crh neurons, are a source of GABAergic input to the PBN. To assess whether the CeA is a source of GABAergic input to the PBN, the CeA of Sst-cre mice was injected with cre-dependent enhanced yellow fluorescent protein (EYFP) virus and PBN tissue processed for GABA and EYFP expression. Again, the majority of EYFP Sst-positive axon terminals in the PBN coexpressed GABA. Together, the present results suggest that CeA neurons marked by Sst expression represent a major extrinsic source of GABAergic input to the PBN and this could underlie the predominate inhibitory effect of CeA stimulation on taste-evoked responses in the PBN.

Untangling the dorsal diencephalic conduction system: a review of structure and function of the stria medullaris, habenula and fasciculus retroflexus

The often-overlooked dorsal diencephalic conduction system (DDCS) is a highly conserved pathway linking the basal forebrain and the monoaminergic brainstem. It consists of three key structures; the stria medullaris, the habenula and the fasciculus retroflexus. The first component of the DDCS, the stria medullaris, is a discrete bilateral tract composed of fibers from the basal forebrain that terminate in the triangular eminence of the stalk of the pineal gland, known as the habenula. The habenula acts as a relay hub where incoming signals from the stria medullaris are processed and subsequently relayed to the midbrain and hindbrain monoaminergic nuclei through the fasciculus retroflexus. As a result of its wide-ranging connections, the DDCS has recently been implicated in a wide range of behaviors related to reward processing, aversion and motivation. As such, an understanding of the structure and connections of the DDCS may help illuminate the pathophysiology of neuropsychiatric disorders such as depression, addiction and pain. This is the first review of all three components of the DDCS, the stria medullaris, the habenula and the fasciculus retroflexus, with particular focus on their anatomy, function and development.

Circuits and functions of the lateral habenula in health and in disease

The past decade has witnessed exponentially growing interest in the lateral habenula (LHb) owing to new discoveries relating to its critical role in regulating negatively motivated behaviour and its implication in major depression. The LHb, sometimes referred to as the brain's 'antireward centre', receives inputs from diverse limbic forebrain and basal ganglia structures, and targets essentially all midbrain neuromodulatory systems, including the noradrenergic, serotonergic and dopaminergic systems. Its unique anatomical position enables the LHb to act as a hub that integrates value-based, sensory and experience-dependent information to regulate various motivational, cognitive and motor processes. Dysfunction of the LHb may contribute to the pathophysiology of several psychiatric disorders, especially major depression. Recently, exciting progress has been made in identifying the molecular and cellular mechanisms in the LHb that underlie negative emotional state in animal models of drug withdrawal and major depression. A future challenge is to translate these advances into effective clinical treatments.

Lateral Preoptic Control of the Lateral Habenula through Convergent Glutamate and GABA Transmission

The lateral habenula (LHb) is a brain structure that participates in cognitive and emotional processing and has been implicated in several mental disorders. Although one of the largest inputs to the LHb originates in the lateral preoptic area (LPO), little is known about how the LPO participates in the regulation of LHb function. Here, we provide evidence that the LPO exerts bivalent control over the LHb through the convergent transmission of LPO glutamate and γ-aminobutyric acid (GABA) onto single LHb neurons. In vivo, both LPO-glutamatergic and LPO-GABAergic inputs to the LHb are activated by aversive stimuli, and their predictive cues yet produce opposing behaviors when stimulated independently. These results support a model wherein the balanced response of converging LPO-glutamate and LPO-GABA are necessary for a normal response to noxious stimuli, and an imbalance in LPO→LHb glutamate or GABA results in the type of aberrant processing that may underlie mental disorders.

The dorsal diencephalic conduction system in reward processing: Spotlight on the anatomy and functions of the habenular complex

The dorsal diencephalic conduction system (DDC) is a highly conserved pathway in vertebrates that provides a route for the neural information to flow from forebrain to midbrain structures. It contains the bilaterally paired habenular nuclei along with two fiber tracts, the stria medullaris and the fasciculus retroflexus. The habenula is the principal player in mediating the dialogue between forebrain and midbrain regions, and functional abnormalities in this structure have often been attributed to pathologies like mood disorders and substance use disorder. Following Matsumoto and Hikosaka seminal work on the lateral habenula as a source of negative reward signals, the last decade has witnessed a great surge of interest in the role of the DDC in reward-related processes. However, despite significant progress in research, much work remains to unfold the behavioral functions of this intriguing, yet complex, pathway. This review describes the current state of knowledge on the DDC with respect to its anatomy, connectivity, and functions in reward and aversion processes.

Altered Volume and Functional Connectivity of the Habenula in Schizophrenia

The pathogenesis of schizophrenia (SCH) is associated with the dysfunction of monoamine neurotransmitters, the synthesis and release of which are mainly regulated by a key structure, the habenular (Hb) nucleus. However, little is known regarding whether SCH is associated with structural or functional alterations in the Hb. In this study, we combined structural and resting-state functional magnetic resonance imaging to investigate the changes in volume and functional connectivity of the Hb in 15 patients with SCH vs. 16 age- and gender-matched healthy controls (HCs). Morphologically, the absolute volume of the bilateral Hb was significantly lower in the SCH patients than in the HCs. Functionally, the bilateral Hb showed significantly enhanced functional connectivity with the left medial prefrontal cortex (mPFC) in the SCH patients. Additionally, the SCH patients exhibited increased functional connectivity of the left Hb with the left lingual gyrus and right inferior frontal gyrus (IFG). A further exploratory analysis revealed that the SCH patients showed increased functional connectivity between the right Hb and several subcortical regions related to dopaminergic pathways, including the left ventral striatum, caudate and putamen. Finally, the increased functional connectivity of the right Hb with the mPFC was positively correlated with the Brief Psychiatric Rating Scale (BPRS) scores in the patients. Together, these results suggest that the altered volume and functional connectivity of the Hb may be involved in the pathogenesis of SCH and thus that the Hb may serve as a potential target in developing new therapeutic strategies in SCH.

Review of the cytology and connections of the lateral habenula, an avatar of adaptive behaving

The cytology and connections of the lateral habenula (LHb) are reviewed. The habenula is first introduced, after which the cytology of the LHb is discussed mainly with reference to cell types, general topography and descriptions of subnuclei. An overview of LHb afferent connections is given followed by some details about the projections to LHb from a number of structures. An overview of lateral habenula efferent connections is given followed by some details about the projections from LHb to a number of structures. In considering the afferent and efferent connections of the LHb some attention is given to the relative validity of regarding it as a bi-partite structure featuring 'limbic' and 'pallidal' parts. The paper ends with some concluding remarks about the relative place of the LHb in adaptive behaving.

An Evolutionary Hypothesis of Binary Opposition in Functional Incompatibility about Habenular Asymmetry in Vertebrates

Many vertebrates have asymmetrical circuits in the nervous system. There are two types of circuit asymmetry. Asymmetrical circuits in sensory and/or motor systems are usually related to lateralized behaviors. It has been hypothesized that spatial asymmetry in the environment and/or social interactions has led to the evolution of asymmetrical circuits by natural selection. There are also asymmetrical circuits that are not related to lateralized behaviors. These circuits lie outside of the sensory and motor systems. A typical example is found in the habenula (Hb), which has long been known to be asymmetrical in many vertebrates, but has no remarkable relationship to lateralized behaviors. Instead, the Hb is a hub wherein information conveyed to the unilateral Hb is relayed to diverging bilateral nuclei, which is unlikely to lead to lateralized behavior. Until now, there has been no hypothesis regarding the evolution of Hb asymmetry. Here, we propose a new hypothesis that binary opposition in functional incompatibility applies selection pressure on the habenular circuit and leads to asymmetry. Segregation of the incompatible functions on either side of the habenula is likely to enhance information processing ability via creating shorter circuits and reducing the cost of circuit duplication, resulting in benefits for survival. In zebrafish and mice, different evolutionary strategies are thought to be involved in Hb asymmetry. In zebrafish, which use a strategy of structurally fixed asymmetry, the asymmetrical dorsal Hb leads to constant behavioral choices in binary opposition. In contrast, in mice, which use a strategy of functionally flexible lateralization, the symmetrical lateral Hb is functionally lateralized. This makes it possible to process complicated information and to come to variable behavioral choices, depending on the specific situation. These strategies are thought to be selected for and preserved by evolution under selection pressures of rigidity and flexibility of sociability in zebrafish and mice, respectively, as they are beneficial for survival. This hypothesis is highly valuable because it explains how the Hb evolved differently in terms of asymmetry and lateralization among different species. In addition, one can propose possible experiments for the verification of this hypothesis in future research.

Ethanol potentiates both GABAergic and glutamatergic signaling in the lateral habenula

Ethanol's aversive property may limit it's use, but the underlying mechanisms are no well-understood. Emerging evidence suggests a critical role for the lateral habenula (LHb) in the aversive response to various drugs, including ethanol. We previously showed that ethanol enhances glutamatergic transmission and stimulates LHb neurons. GABAergic transmission, a major target of ethanol in many brain regions, also tightly regulates LHb activity. This study assessed the action of ethanol on LHb GABAergic transmission in rat brain slices. Application of ethanol accelerated spontaneous action potential firing of LHb neurons, and LHb activity was increased by the GABA_A receptor antagonist gabazine, and ethanol-induced acceleration of LHb firing was further increased by gabazine. Additionally, ethanol potentiated GABAergic transmission (inhibitory postsynaptic currents, IPSCs) with an EC₅₀ of 1.5 mM. Ethanol-induced potentiation of IPSCs was increased by a GABA_B receptor antagonist; it was mimicked by dopamine, dopamine receptor agonists, and dopamine reuptake blocker, and was completely prevented by reserpine, which depletes store of catecholamine. Moreover, ethanol-induced potentiation of IPSCs involved cAMP signaling. Finally, ethanol enhanced simultaneously glutamatergic and GABAergic transmissions to the majority of LHb neurons: the potentiation of the former being greater than that of the latter, the net effect was increased firing. Since LHb excitation may contribute to aversion, ethanol-induced potentiation of GABAergic inhibition tends to reduce aversion.

Lateral habenula integration of proactive and retroactive information mediates behavioral flexibility

The lateral habenula (LHb) is known to play an important role in signaling aversive or adverse events that have happened or are predicted by cues under Pavlovian conditions. In rodents, it is also required for behavioral flexibility when changes in reward outcomes signal that strategies should be changed. It is not known whether the LHb also controls appetitive behaviors when an animal is able to utilize external cues proactively to guide upcoming decisions. In order to test this, male Long-Evans rats were trained to switch between two arms of a figure eight maze based on the tone presented prior to the choice. Importantly, the tones were switched every three to six trials so rats were able establish a response pattern before being required to switch. This caused rats to rely on both proactive (tones) and retroactive information (reward feedback) to guide behavior. Inactivation of the LHb with the GABA agonists baclofen and muscimol impaired overall performance by increasing both errors when the tones are switched (switch errors) as well as on subsequent trials (perseverative errors) indicating that both proactive and retroactive information are utilized by the LHb to guide behavioral flexibility. Once a correct choice was made in a given block, LHb inactivated rats did not make more errors than controls. A control study revealed that the LHb is not required for tone or reward magnitude discrimination per se. These results demonstrate for the first time that the LHb contributes to behavioral flexibility through utilizing both proactive and retroactive information when performing appetitive tasks.

Ongoing behavioral state information signaled in the lateral habenula guides choice flexibility in freely moving rats

The lateral habenula (LHb) plays a role in a wide variety of behaviors ranging from maternal care, to sleep, to various forms of cognition. One prominent theory with ample supporting evidence is that the LHb serves to relay basal ganglia and limbic signals about negative outcomes to midbrain monoaminergic systems. This makes it likely that the LHb is critically involved in behavioral flexibility as all of these systems have been shown to contribute when flexible behavior is required. Behavioral flexibility is commonly examined across species and is impaired in various neuropsychiatric conditions including autism, depression, addiction, and schizophrenia; conditions in which the LHb is thought to play a role. Therefore, a thorough examination of the role of the LHb in behavioral flexibility serves multiple functions including understanding possible connections with neuropsychiatric illnesses and additional insight into its role in cognition in general. Here, we assess the LHb’s role in behavioral flexibility through comparisons of the roles its afferent and efferent pathways are known to play. Additionally, we provide new evidence supporting the LHb contributions to behavioral flexibility through organization of specific goal directed actions under cognitively demanding conditions. Specifically, in the first experiment, a majority of neurons recorded from the LHb were found to correlate with velocity on a spatial navigation task and did not change significantly when reward outcomes were manipulated. Additionally, measurements of local field potential (LFP) in the theta band revealed significant changes in power relative to velocity and reward location. In a second set of experiments, inactivation of the LHb with the gamma-aminobutyric acid (GABA) agonists baclofen and muscimol led to an impairment in a spatial/response based repeated probabilistic reversal learning task. Control experiments revealed that this impairment was likely due to the demands of repeated switching behaviors as rats were unimpaired on initial discrimination acquisition or retention of probabilistic learning. Taken together, these novel findings compliment other work discussed supporting a role for the LHb in action selection when cognitive or emotional demands are increased. Finally, we discuss future mechanisms by which a superior understanding of the LHb can be obtained through additional examination of behavioral flexibility tasks.

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

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

The role of lateral habenula-dorsal raphe nucleus circuits in higher brain functions and psychiatric illness

Serotonergic neurons in the dorsal raphe nucleus (DRN) play an important role in regulation of many physiological functions. The lateral nucleus of the habenular complex (LHb) is closely connected to the DRN both morphologically and functionally. The LHb is a key regulator of the activity of DRN serotonergic neurons, and it also receives reciprocal input from the DRN. The LHb is also a major way-station that receives limbic system input via the stria medullaris and provides output to the DRN and thereby indirectly connects a number of other brain regions to the DRN. The complex interactions of the LHb and DRN contribute to the regulation of numerous important behavioral and physiological mechanisms, including those regulating cognition, reward, pain sensitivity and patterns of sleep and waking. Disruption of these functions is characteristic of major psychiatric illnesses, so there has been a great deal of interest in how disturbed LHb-DRN interactions may contribute to the symptoms of these illnesses. This review summarizes recent research related to the roles of the LHb-DRN system in regulation of higher brain functions and the possible role of disturbed LHb-DRN function in the pathogenesis of psychiatric disorders, especially depression.

Electrophysiological characterization of entopeduncular nucleus neurons in anesthetized and freely moving rats

The EntoPeduncular nucleus (EP), which is homologous to the internal segment of the Globus Pallidus (GPi) in primates, is one of the two basal ganglia (BG) output nuclei. Despite their importance in cortico-BG information processing, EP neurons have rarely been investigated in rats and there is no available electrophysiological characterization of EP neurons in vivo. We recorded and analyzed the activity of EP neurons in freely moving as well as anesthetized rats, and compared their activity patterns. Examination of neuronal firing statistics during wakefulness suggested that similar to neurons recorded in the primate GPi, EP neurons are a single population characterized by Poisson-like firing. Under isoflurane anesthesia the firing rate of EP neurons decreased substantially and their coefficient of variation and relative duration of quiescence periods increased. Investigation of the relationship between firing rate and depth of anesthesia revealed two distinct neuronal groups: one that decreased its firing rate with the increase in anesthesia level, and a second group where the firing rate was independent of anesthesia level. Post-hoc examination of the firing properties of the two groups showed that they were statistically distinct. These results may thus help reconcile in vitro studies in rats and primates which have reported two distinct neuronal populations, and in vivo studies in behaving primates indicating one homogeneous population. Our data support the existence of two distinct neuronal populations in the rat EP that can be distinguished by their characteristic firing response to anesthesia.

Synaptic and cellular profile of neurons in the lateral habenula

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

Hyperactivation of the habenula as a link between depression and sleep disturbance

Depression occurs frequently with sleep disturbance such as insomnia. Sleep in depression is associated with disinhibition of the rapid eye movement (REM) sleep. Despite the coincidence of the depression and sleep disturbance, neural substrate for depressive behaviors and sleep regulation remains unknown. Habenula is an epithalamic structure regulating the activities of monoaminergic neurons in the brain stem. Since the imaging studies showed blood flow increase in the habenula of depressive patients, hyperactivation of the habenula has been implicated in the pathophysiology of the depression. Recent electrophysiological studies reported a novel role of the habenular structure in regulation of REM sleep. In this article, we propose possible cellular mechanisms which could elicit the hyperactivation of the habenular neurons and a hypothesis that dysfunction in the habenular circuit causes the behavioral and sleep disturbance in depression. Analysis of the animals with hyperactivated habenula would open the door to understand roles of the habenula in the heterogeneous symptoms such as reduced motor behavior and altered REM sleep in depression.

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

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

Independent circuits in the basal ganglia for the evaluation and selection of actions

The basal ganglia are critical for selecting actions and evaluating their outcome. Although the circuitry for selection is well understood, how these nuclei evaluate the outcome of actions is unknown. Here, we show in lamprey that a separate evaluation circuit, which regulates the habenula-projecting globus pallidus (GPh) neurons, exists within the basal ganglia. The GPh neurons are glutamatergic and can drive the activity of the lateral habenula, which, in turn, provides an indirect inhibitory influence on midbrain dopamine neurons. We show that GPh neurons receive inhibitory input from the striosomal compartment of the striatum. The striosomal input can reduce the excitatory drive to the lateral habenula and, consequently, decrease the inhibition onto the dopaminergic system. Dopaminergic neurons, in turn, provide feedback that inhibits the GPh. In addition, GPh neurons receive direct projections from the pallium (cortex in mammals), which can increase the GPh activity to drive the lateral habenula to increase the inhibition of the neuromodulatory systems. This circuitry, thus, differs markedly from the "direct" and "indirect" pathways that regulate the pallidal (e.g., globus pallidus) output nuclei involved in the control of motion. Our results show that a distinct reward-evaluation circuit exists within the basal ganglia, in parallel to the direct and indirect pathways, which select actions. Our results suggest that these circuits are part of the fundamental blueprint that all vertebrates use to select actions and evaluate their outcome.

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.

Input to the lateral habenula from the basal ganglia is excitatory, aversive, and suppressed by serotonin

The lateral habenula (LHb) has recently been identified as a key regulator of the reward system by driving inhibition onto dopaminergic neurons. However, the nature and potential modulation of the major input to the LHb originating from the basal ganglia are poorly understood. Although the output of the basal ganglia is thought to be primarily inhibitory, here we show that transmission from the basal ganglia to the LHb is excitatory, glutamatergic, and suppressed by serotonin. Behaviorally, activation of this pathway is aversive, consistent with its role as an "antireward" signal. Our demonstration of an excitatory projection from the basal ganglia to the LHb explains how LHb-projecting basal ganglia neurons can have similar encoding properties as LHb neurons themselves. Our results also provide a link between antireward excitatory synapses and serotonin, a neuromodulator implicated in depression.

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.

Brain processing of the mammary pheromone in newborn rabbits

Chemosignals strongly contribute to social interactions in mammals, including mother-young relationships. In the European rabbit, a volatile compound emitted by lactating females in milk, the 2-methylbut-2-enal, has been isolated. Carrying the properties of a pheromone, in particular the spontaneous ability to release critical sucking-related movements in newborns, it has been called the mammary pheromone (MP). Lesion of the vomeronasal organ and preliminary 2-deoxyglucose data suggested that the MP could be processed by the main olfactory system. However, the neuronal substrate that sustains the MP-induced response of neonates remained unknown. Here, we evaluated Fos expression in 4-day-old-rabbits exposed to the MP (in comparison with control neonates exposed to non-relevant odorant, no odorant or unmanipulated pups) both at the level of the olfactory bulb and central brain regions. Evidence of high and widespread Fos immunoreactivity in the main olfactory bulb appear in MP pups while the accessory olfactory bulb exhibits a negligible staining. However, no obvious bulbar pattern of Fos expression is observed, when in contrast a certain pattern emerges with the neutral odorant. Compared to this latter, the MP exposure increases Fos expression in the anterior piriform cortex, the organum vasculosum of the lamina terminalis and the habenula, with a tendency in the lateral preoptic region. For the first time, a pheromone essential for mother-young interaction is thus highlighted for its processing by the main olfactory system, the whole olfactory bulb, and by brain regions involved in osmoregulation, thirst and motivation-guided motor responses.

The habenula prevents helpless behavior in larval zebrafish

Animals quickly learn to avoid predictable danger. However, if pre-exposed to a strong stressor, they do not display avoidance even if this causes continued contact with painful stimuli [1, 2]. In rodents, lesioning the habenula, an epithalamic structure that regulates the monoaminergic system, has been reported to reduce avoidance deficits caused by inescapable shock [3]. This is consistent with findings that inability to overcome a stressor is accompanied by an increase in serotonin levels [4]. However, other studies conclude that habenula lesions cause avoidance deficits [5, 6]. These contradictory results may be caused by lesions affecting unintended regions [6]. To clarify the role of the habenula, we used larval zebrafish, whose transparency and amenability to genetic manipulation enables more precise disruption of cells. We show that larval zebrafish learn to avoid a light that has been paired with a mild shock but fail to do so when pre-exposed to inescapable shock. Photobleaching of habenula afferents expressing the photosensitizer KillerRed causes a similar failure in avoidance. Expression of tetanus toxin in dorsal habenula neurons is sufficient to prevent avoidance. We suggest that this region may signal the ability to control a stressor, and that its disruption could contribute to anxiety disorders.

Neuronal nicotinic acetylcholine receptors as pharmacotherapeutic targets for the treatment of alcohol use disorders

Alcohol use disorders (AUDs) are complex, and developing effective treatments will require the combination of novel medications and cognitive behavioral therapy approaches. Epidemiological studies have shown there is a high correlation between alcohol consumption and tobacco use, and the prevalence of smoking in alcoholics is as high as 80% compared to about 30% for the general population. Both preclinical and clinical data provide evidence that nicotine administration increases alcohol intake and non-specific nicotinic receptor antagonists reduce alcohol-mediated behaviors. As nicotine interacts specifically with the neuronal nicotinic acetylcholine receptor (nAChR) system, this suggests that nAChRs play an important role in the behavioral effects of alcohol. In this review, we discuss the importance of nAChRs for the treatment of AUDs and argue that the use of FDA approved nAChR ligands, such as varenicline and mecamylamine, approved as smoking cessation aids may prove to be valuable treatments for AUDs. We also address the importance of combining effective medications with behavioral therapy for the treatment of alcohol dependent individuals.

NOvelty-related motivation of anticipation and exploration by dopamine (NOMAD): implications for healthy aging

Studies in humans and animals show that dopaminergic neuromodulation originating from the substantia nigra/ventral tegmental area (SN/VTA) of the midbrain enhances hippocampal synaptic plasticity for novel events and has a motivationally energizing effect on actions through striatal mechanisms. In this review, we discuss how these mechanisms of dopaminergic neuromodulation connect to the behavioural and functional consequences that age-related structural degeneration of the SN/VTA exerts on declarative memory. We propose a framework called 'NOvelty-related Motivation of Anticipation and exploration by Dopamine' (NOMAD) which captures existing links between novelty, dopamine, long-term memory, plasticity, energization and their relation to aging. We propose that maximizing the use of this mechanism by maintaining mobility and exploration of novel environments could be a potential mechanism to slow age-related decline of memory.

Functional imaging of the human dopaminergic midbrain

Invasive recording of dopamine neurons in the substantia nigra and ventral tegmental area (SN/VTA) of behaving animals suggests a role for these neurons in reward learning and novelty processing. In humans, functional magnetic resonance imaging (fMRI) is currently the only non-invasive event-related method to measure SN/VTA activity, but it is debated to what extent fMRI enables inference about dopaminergic responses within the SN/VTA. We consider the anatomical and functional parcellation of the primate SN/VTA and find that its homogeneity suggests little variation in the regional specificity of fMRI signals for reward-related dopaminergic responses. Hence, these responses seem to be well captured by the compound fMRI signal from the SN/VTA, which seems quantitatively related to dopamine release in positron emission tomography (PET). We outline how systematic investigation of the functional parcellation of the SN/VTA in animals, new developments in fMRI analysis and combined PET-fMRI studies can narrow the gap between fMRI and dopaminergic neurotransmission.

Differential tonic influence of lateral habenula on prefrontal cortex and nucleus accumbens dopamine release

Conditions of increased cognitive or emotional demand activate dopamine release in a regionally selective manner. Whereas the brief millisecond response of dopamine neurons to salient stimuli suggests that dopamine's influence on behaviour may be limited to signalling certain cues, the prolonged availability of dopamine in regions such as the prefrontal cortex and nucleus accumbens is consistent with the well described role of dopamine in maintaining motivation states, associative learning and working memory. The behaviourally elicited terminal release of dopamine is generally attributed to increased excitatory drive on dopamine neurons. Our findings here, however, indicate that this increase may involve active removal of a tonic inhibitory control on dopamine neurons exerted by the lateral habenula (LHb). Inhibition of LHb in behaving animals transiently increased dopamine release in the prefrontal cortex, nucleus accumbens and dorsolateral striatum. The inhibitory influence was more pronounced in the nucleus accumbens and striatum than in the prefrontal cortex. This pattern of regional dopamine activation after LHb inhibition mimicked conditions of reward availability but not increased cognitive demand. Electrical or chemical stimulation of LHb produced minimal reduction of extracellular dopamine, suggesting that in an awake brain the inhibition associated with tonic LHb activity represents a near-maximal influence on dopamine neurotransmission. These data indicate that LHb may be critical for functional differences in dopamine neurons by preferentially modulating dopamine neurons that project to the nucleus accumbens over those neurons that primarily project to the prefrontal cortex.

Neuroendocrine and behavioral responses to weak electric fields in adult sea lampreys (Petromyzon marinus)

We characterized the behavioral and neuroendocrine responses of adult sea lampreys (Petromyzon marinus) to weak electric fields. Adult sea lampreys, captured during upstream spawning migration, exhibited limited active behaviors during exposure to weak electric fields and spent the most time attached to the wall of the testing arena near the cathode (-). For adult male sea lampreys, exposure to weak electric fields resulted in increased lamprey (l) GnRH-I mRNA expression but decreased lGnRH-I immunoreactivities in the forebrain, and decreased Jun (a neuronal activation marker) mRNA levels in the brain stem. Similar effects were not observed in the brains of female sea lampreys after weak electric field stimulation. The influence of electroreception on forebrain lGnRH suggests that electroreception may modulate the reproductive systems in adult male sea lampreys. The changes in Jun expression may be associated with swimming inhibition during weak electric field stimulation. The results for adult sea lampreys are the opposite of those obtained using parasitic-stage sea lampreys, which displayed increased activity during and after cathodal stimulation. Our results demonstrate that adult sea lampreys are sensitive to weak electric fields, which may play a role in reproduction. They also suggest that electrical stimuli mediate different behaviors in feeding-stage and spawning-stage sea lampreys.

Forebrain neurons that project to the gustatory parabrachial nucleus in rat lack glutamic acid decarboxylase

Evidence suggests that GABA might mediate the inhibitory influence of centrifugal inputs on taste-evoked responses in the parabrachial nucleus (PBN). Previous studies show that activation of the gustatory cortex (GC), bed nucleus of the stria terminalis (BNST), central nucleus of the amygdala (CeA), and lateral hypothalamus (LH) inhibits PBN taste responses, GABAergic neurons are present in these forebrain regions, and GABA reduces the input resistance of PBN neurons. The present study investigated the expression of glutamic acid decarboxylase immunoreactivity (GAD_67 ir) in GC, BNST, CeA, and LH neurons that project to the PBN in rats. After anesthesia (50 mg/kg ip Nembutal), injections of the retrograde tracer Fluorogold (FG) were made in the physiologically defined gustatory PBN. Brain tissue containing the above forebrain structures was processed and examined for FG and GAD_67 ir. Similar to previous studies, each forebrain site contained retrogradely labeled neurons. Our results suggest further that the major source of input to the PBN taste region is the CeA (608 total cells) followed by GC (257 cells), LH (106 cells), and BNST (92 cells). This suggests a differential contribution to centrifugal control of PBN taste processing. We further show that despite the presence of GAD_67 neurons in each forebrain area, colocalization was extremely rare, occurring only in 3 out of 1,063 FG-labeled cells. If we assume that the influence of centrifugal input is mediated by direct projections to the gustatory region of the PBN, then GABAergic forebrain neurons apparently are not part of this descending pathway.

A conductor hidden in the orchestra? Role of the habenular complex in monoamine transmission and cognition

Influences of the habenular complex on electrophysiological and neurochemical aspects of brain functioning are well known. However, its role in cognition has been sparsely investigated until recently. The habenular complex, composed of medial and lateral subdivisions, is a node linking the forebrain with midbrain and hindbrain structures. The lateral habenula is the principal actor in this direct dialogue, while the medial habenula mostly conveys information to the interpeduncular nucleus before this modulates further regions. Here we describe neuroanatomical and physiological aspects of the habenular complex, and its role in cognitive processes, including new behavioral, electrophysiological and imaging findings. Habenular complex lesions result in deficits in learning, memory and attention, some of which decline during repeated testing, while others become worse, consistent with multiple roles in cognition. The habenular complex is particularly responsive to feedback about errors. Electrophysiological studies indicate a role in metaplasticity, the modulation of neuroplasticity. These studies thus reveal important roles of the habenular complex in learning, memory and attention.

The structural organization of connections between hypothalamus and cerebral cortex

Motivated behavior requires coordinated somatic, autonomic, and endocrine responses, and may be divided into initiation, procurement, and consummatory phases (Swanson, L.W. and Mogenson, G.J., Neural mechanisms for the functional coupling of autonomic, endocrine and somatomotor responses in adaptative behavior, Brain Res. Rev., 3 (1981) 1-34). Obviously, such behavior may involve the entire central nervous system, although it is important to identify circuitry or systems that mediate the behavior directed toward specific goal objects. This problem has recently been clarified by the identification of hypothalamic subsystems important for the execution of instinctive behaviors related to ingestion, reproduction, and defense. These subsystems are modulated by sensory (reflex), central control (e.g., circadian), and voluntary (cortical) inputs. The latter are dominated by inputs from the ventral temporal lobe and medial prefrontal region, which are both direct and via associated parts of the basal nuclei (ganglia). Hypothalamic output is characterized by descending projections to brainstem and spinal motor systems, and by projections back to the cerebral cortex, which are both direct and via a continuous rostromedial part of the dorsal thalamus. This thalamic region includes the anterior, medial, and midline groups, which in turn innervate a continuous ring of cortex that includes the hippocampal formation and the cingulate, prefrontal, and insular regions. Parts of this thalamic region also innervate the ventral striatum, which receives a massive input from the cortical rings as well.

Interactive effects of stimulation of D1 and D2 dopamine receptors on Fos expression in the lateral habenula

We have previously shown that systemic administration of non-selective dopamine agonists results in a pronounced expression of the proto-oncoprotein Fos within the lateral habenula. In the current study we examined the effects of selective D1 and D2 dopamine receptor agonists on habenular Fos expression. Rats were injected with various doses of the selective D2 agonist quinpirole (0, 0.62 or 2.5 mg/kg) either alone or in combination with various doses of the selective full D1 agonist A-77636 (0, 0.75 or 3.0 mg/kg). The selective agonists, by themselves, induced only small increases in Fos-like immunoreactivity within the lateral habenula, but combinations of the two drugs resulted in a very robust response. These findings indicate that D1 and D2 receptor agonists interact to induce Fos expression within the habenula and that the nature of this interaction differs from that reported in the striatum and the globus pallidus.

Afferent and efferent connections of the habenula in the rainbow trout (Oncorhynchus mykiss): an indocarbocyanine dye (DiI) study

The habenula is a conserved structure in the brain of vertebrates. With the aim of further understanding of the evolution of the habenular system in vertebrates, we studied the afferent and efferent connections of the habenula of the rainbow trout. Experiments included application of the carbocyanine dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) into the habenula, telencephalon, pineal organ, posterior tubercle, and interpeduncular nucleus (IPN). The results obtained reveal a consistent pattern of habenular connections. Most afferents originate from three nuclei, one extending from the preoptic region to the rostral thalamus (the entopeduncular nucleus), the second located in the region of the hypothalamus-posterior tubercle and consisting of large bipolar cells (tuberculohabenular nucleus), and the third in the preoptic region (preoptic nucleus). A few large neurons of the locus coeruleus appeared to be labeled in some cases. The trout habenula also receives pineal and parapineal projections. Small labeled glial cells were observed in the thalamus around the fasciculus retroflexus and, sometimes, around the IPN. The most conspicuous efferents coursed in the fasciculus retroflexus to the IPN, the isthmal raphe, and the central gray. The existence of olfactohabenular or habenulotelencephalic projections is discussed.

Dopamine agonists and stress produce different patterns of Fos-like immunoreactivity in the lateral habenula

In rats treated systemically with either amphetamine, amfonelic acid or apomorphine, large numbers of cells displaying Fos-like immunoreactivity (FLI) could be seen in the lateral zone of the lateral habenula. The induction of FLI by amphetamine could be blocked either by pretreatment with haloperidol or by 6-hydroxydopamine lesions of ascending dopamine fibers at the level of the lateral hypothalamus. In contrast, a variety of stressors selectively induced FLI in the most medial portion of the lateral habenula. These findings support the concept of a functional differentiation of the medial and lateral regions of the lateral habenula and provide further evidence for involvement of the habenula in the circuitry of the basal ganglia.

The striatum and the globus pallidus send convergent synaptic inputs onto single cells in the entopeduncular nucleus of the rat: a double anterograde labelling study combined with postembedding immunocytochemistry for GABA

The entopeduncular nucleus is one of the major output stations of the basal ganglia. In order to better understand the role of this structure in information flow through the basal ganglia, experiments have been performed in the rat to examine the chemical nature, morphology, and synaptology of the projections from the globus pallidus and striatum to the entopeduncular nucleus. In order to examine the morphology and synaptology of pallidoentopeduncular terminals and striatoentopeduncular terminals, rats were subjected to a double anterograde labelling study. The globus pallidus was injected with Phaseolus vulgaris-leucoagglutinin (PHA-L), and on the same side of the brain, the striatum was injected with biocytin. The entopeduncular nuclei of these animals were then examined for anterogradely labelled pallidal and striatal terminals. Rich plexuses of PHA-L-labelled pallidal terminals and biocytin-labelled striatal terminals were identified throughout the entopeduncular nucleus. At the electron microscopic level, the pallidal boutons were classified as two types. The majority (Type 1), were large boutons that formed symmetrical synapses with the dendrites and perikarya of neurones in the entopeduncular nucleus. Type 2 PHA-L-labelled terminals were much rarer, slightly smaller, and formed asymmetrical synapses. It is suggested that the Type 2 boutons are not derived from the globus pallidus but from the subthalamic nucleus. The biocytin-labelled terminals from the striatum had the typical morphological features of striatal terminals and formed symmetrical synapses. The distribution of the postsynaptic targets of the pallidal terminals and the striatal terminals differed in that the pallidal terminals preferentially made synaptic contact with the more proximal regions of the neurones in the entopeduncular nucleus, whereas the striatal terminals were located more distally on the dendritic trees. Examination in the electron microscope of areas where there was an overlap of the two sets of anterogradely labelled boutons revealed that terminals from the globus pallidus and the striatum made convergent synaptic contact with the perikarya and dendrites of individual neurones in the entopeduncular nucleus. In order to examine the chemical nature of the input to the entopeduncular nucleus from the globus pallidus and the striatum, ultrathin sections were immunostained by the postembedding method to reveal endogenous GABA. Three classes of GABA-containing terminals were identified; two of them formed symmetrical synapses and one rare type formed asymmetrical synapses. The combination of the GABA immunocytochemistry and anterograde labelling revealed that both the striatal and pallidal afferents that make symmetrical synapses with neurones in the entopeduncular nucleus, including those involved in convergent inputs, are GABAergic.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of lesions of the habenular nuclei on the development of sensitization to the behavioral activational effects of repeatedly administered morphine in the rat

The effects of lesions of the habenular nuclei on the development of sensitization to the behavioral activational effects of morphine (MOR), administered repeatedly either systemically or directly into the ventral tegmental area (VTA) were examined. Lesions of the habenular nuclei blocked the early-appearing sedative effects and enhanced the later-appearing locomotor activational effects seen after systemic injections of MOR (10 mg/kg, i.p.). Habenular lesions did not potentiate the development of sensitization to the locomotor-activational effects seen with the repeated, systemic administration of MOR. The bilateral injection of MOR (5.0 micrograms/0.5 microliter/side) directly into the VTA of animals with habenular lesions resulted in the performance of stereotyped behaviors that appeared as early as the second MOR exposure and remained at high levels with repeated MOR treatment. The stereotyped behavior shown by lesioned animals did not appear to interfere with the acute locomotor activational effects of intra-VTA MOR nor the development of sensitization to these effects when it was administered repeatedly. These results are in agreement with previous research suggesting that by disinhibiting the dopamine (DA) systems, habenular lesions enhance the acute behavioral activational effects of MOR. The results also suggest that the habenular nuclei do not control the changes in the response of the DA systems underlying the development of sensitization to the locomotor-activating effects of MOR when administered repeatedly.

Demonstration of a neuronal projection from the entopeduncular nucleus to the substantia nigra of the rat

The two neuronal tracers, Phaseolus vulgaris leucoagglutinin (PHA-L) and Cholera toxin subunit B (CHB) were used in order to study a possible neuronal projection from the entopeduncular nucleus to the ventral mesencephalon in the rat. Both tracers were identified in brain sections by means of immunohistochemistry. After injection of PHA-L in the entopeduncular nucleus PHA-L-positive nerve fibers observed in the mesencephalon were moderate in number and mostly restricted to the mediodorsal part of the substantia nigra, pars reticulata. In addition, a low number of PHA-L-immunoreactive nerve fibers was found in the substantia nigra, pars compacta, the rostral part of the ventral tegmental area, and in the deep mesencephalic nucleus. In agreement with these observations, several labeled neurons were observed in the ipsilateral entopeduncular nucleus after injections of CHB in the substantia nigra. These results indicate the presence of a direct neuronal projection between the two major output channels of the basal ganglia.

Relevance of the habenular complex to neuropsychiatry: a review and hypothesis

Since the initial observation by Brown (1914) that electrical stimulation applied to the habenular efferent bundle in the chimpanzee evoked a pattern of respiration which closely resembled the act of laughter, the habenular complex has remained a mysterious structure. The anatomy of the habenular complex is well delineated (Jones, 1985) forming a major component of the dorsal diencephalic conduction system. Data derived mainly from animal experimentation over the past decade point to the fact that the habenular complex functions as an important link between the limbic forebrain and the midbrain-extrapyramidal motor system. The elucidation of the functions of the habenular complex may thus significantly increase the current insight into the understanding of the interaction between behavioral and motor functions. Clearly, such information would be of great relevance for further understanding of neuropsychiatric disorders such as schizophrenia, Parkinson's disease, Tardive dyskinesia, and Tourette's syndrome in which behavioral and motor impairments are interfaced. This review summarizes anatomical, functional, and pharmacological aspects of the habenular complex and discusses its potential contribution to the pathophysiology of selected neuropsychiatric and movement disorders.

Intracellular study of rat entopeduncular nucleus neurons in an in vitro slice preparation: response to subthalamic stimulation

Responses of rat entopeduncular nucleus (EP) neurons after stimulation of the subthalamic nucleus (STh) and the morphology of the EP neurons were studied using brain slice preparations. EP neurons were classified into two types based on their electrophysiological properties as reported previously. Of 87 EP neurons, 72 were Type I and the rest were Type II. Synaptic responses to STh stimulation were different in these two cell types. STh stimulation evoked excitatory postsynaptic potentials (EPSPs) followed by strong inhibitory postsynaptic potentials (IPSPs) in Type I neurons and EPSPs without strong IPSPs in Type II neurons. The EPSPs were considered to be monosynaptic because no large change in the latency (1.7 +/- 0.5 ms) resulted by alteration of stimulus intensity. The EPSPs were reversibly suppressed by kynurenic acid in a dose-dependent manner. Bath application of (+)-tubocurarine (10-50 microM) had no effect on EPSPs or IPSPs. Bath application of bicuculline methiodide (50-100 microM) markedly suppressed IPSPs evoked by STh stimulation and at the same time increased the amplitude and duration of EPSPs without affecting the latency. In the presence of bicuculline methiodide, EPSPs could induce plateau potentials and slow action potentials. Some type I and Type II neurons were intracellularly labeled by biocytin. Type I neurons were located throughout the EP but Type II neurons were located mainly in the dorsal portion of the EP. Medium sized somata of both Type I and Type II neurons were spine-free and fusiform or round in shape. They had 3-4 thick primary dendrites with diameters of 2-5 micron that branched into thin secondary dendrites.(ABSTRACT TRUNCATED AT 250 WORDS)

A direct neuronal projection from the entopeduncular nucleus to the globus pallidus. A PHA-L anterograde tracing study in the rat

The efferent projections of the rat entopeduncular nucleus were examined by use of the anterogradely transported lectin Phaseolus vulgaris-leucoagglutinin (PHA-L). Injections of PHA-L into different parts of the entopeduncular nucleus resulted in a moderate number of labeled nerve fibers in the ipsilateral globus pallidus. The fibers displayed a heterogeneous morphology: some were of small caliber with few delicate varicosities, others were of medium caliber with several more bulbous nerve terminals. Restricted injections in the dorsal and ventral parts of the entopeduncular nucleus, respectively, showed that the dorsal part of the entopeduncular nucleus projects to the dorsal and rostral parts of the dorsal pallidum and the ventral part to the ventral and caudal parts.

Intracellular study of rat entopeduncular nucleus neurons in an in vitro slice preparation: electrical membrane properties

Electrical properties of rat entopeduncular nucleus (EP) neurons were studied in vitro using slice preparations. Of 108 EP neurons recorded, 104 were classified into two types based on their membrane properties. Type I neurons (n = 86) possessed: (1) a strong, time-dependent anomalous rectification that was sensitive to Cs+; (2) a weak spike adaptation; and (3) a strong rebound excitation with a low threshold Ca-spike and fast spikes. Many Type I neurons displayed spontaneous repetitive firing. Some of them generated spontaneous Ca-dependent plateau potentials with fast spikes upon application of tetraethylammonium bromide. Type II neurons (n = 18) had: (1) no apparent rectification; (2) a strong spike adaptation; and (3) a ramp-shaped repolarization, similar to the A-current, at the offset of a hyperpolarizing pulse. Features common to both types included: (1) a similar range of the input resistance; (2) capability of generating high threshold Ca-spikes; and (3) generation of postactive hyperpolarizations (i.e. Ca-activated K-conductance). The great majority (Type I) of rat EP neurons share similar electrical properties. A minority of neurons (Type II) behave differently from Type I neurons and share similar properties among themselves.

Collateralization and GAD immunoreactivity of descending pallidal efferents

The first phase of this study involves injecting a different fluorescent retrograde axonal tracer into the subthalamic nucleus, the substantia nigra, and the mesopontine tegmentum. Multiple labeled cells are found within the caudal third of the globus pallidus. The entopeduncular nucleus and adjacent basal forebrain structures such as the substantia innominata, lateral hypothalamus, bed nucleus of the stria terminalis, and central nucleus of the amygdala all exhibit some dye containing cell, although multiple labeled cells are rare. The second phase of this study involves injecting a different fluorescent retrograde tracer into either the substantia nigra, or the mesopontine tegmentum, and subsequent processing of the tissue for glutamic acid decarboxylase (GAD) immunocytochemistry. Many dye and antibody double-labeled cells could be found within the entopeduncular nucleus and the caudal third of the globus pallidus. This is in contrast to the surrounding basal forebrain regions with brainstem efferents which were rarely GAD-positive. This study indicates that the collateral pattern and immunocytochemistry of globus pallidus neurons with descending efferents are distinct from other basal forebrain neurons having similar efferents. These results also extend previous findings and suggest that the neuron of the pallidal complex are heterogeneous with respect to their patterns of projections. In particular, the present findings question previous assumptions concerning the homology of pallidal segments between primate and rodent species.

Ultrastructural differences between pallidohabenular terminals and pallidothalamic terminals

Pallidal terminals in the cat lateral habenular nucleus and the ventral anterior and ventral lateral nuclear complex of the thalamus have been examined electron microscopically. A significant difference was obtained after a quantitative analysis of size and shape parameters of synaptic vesicles in the identified terminals: synaptic vesicles in the pallidohabenular terminals had smaller mean elongation index and larger mean vesicle profile area than those in the pallidothalamic terminals.

The efferent projections of the rat lateral habenular nucleus revealed by the PHA-L anterograde tracing method

The efferent connections of the rat lateral habenular nucleus (LHb) were demonstrated using anterograde transport of the lectin Phaseolus vulgaris leucoagglutinin (PHA-L). Following PHA-L injections into the LHb, neuronal somata located in the lateral two thirds of the LHb were labeled with PHA-L. Individual axonal fibers and terminal specializations were clearly visible. This permitted detailed mapping of both efferent fiber pathways and terminal distributions. Previous reports on fiber pathways were substantially confirmed and several new findings were revealed. (1) Major rostrally oriented fibers enter the medial forebrain bundle via 3 routes which initially branch from the fasciculus retroflexus: the mediodorsal thalamic nucleus and ventromedial thalamic nucleus; the zona incerta and fields of Forel; and the ventral tegmental area of Tsai. (2) A major decussation to the contralateral thalamic nuclei occurs in the central medial thalamic nucleus. (3) Caudally directed fibers follow two courses: one to the deep mesencephalic nucleus, central grey, dorsal raphe nucleus and the deep layers of the superior colliculus; and the other to the median raphe nucleus, oral pontine reticular formation and raphe pontis nucleus. The present results offer more detailed information concerning the dorsal diencephalic system.

Non-cholinergic basal forebrain neurons project to the contralateral basal forebrain in the rat

Following injections of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) or the fluorescent tracer fluoro-gold into the magnocellular preoptic area and the horizontal limb of the diagonal band, retrogradely labelled neurons were found in the homotopic region of the contralateral basal forebrain. Labelled fibers apparently arising from these neurons travelled in the stria medullaris and the habenular commissure to terminate in the contralateral basal forebrain. Although the neurons retrogradely labelled with fluoro-gold in the contralateral basal forebrain were similar in size to choline acetyltransferase (ChAT)-immunoreactive neurons, and were intermingled with them, none was ChAT-positive. WGA-HRP injections into the nucleus basalis magnocellularis did not result in retrograde labelling in the contralateral basal forebrain. These findings suggest that non-cholinergic neurons may serve as a direct link between the two sides of selective magnocellular basal forebrain regions.

Distribution of GABA-immunoreactive neurons in the thalamus of the squirrel monkey (Saimiri sciureus)

A light microscopic study of the cellular localization of GABA in the thalamus of the squirrel monkey (Saimiri sciureus) was undertaken by means of the indirect peroxidase-antiperoxidase method using a highly purified antiserum directed against GABA-glutaraldehyde-lysyl-protein conjugate. GABA-immunoreactive cell bodies and axon terminals were visualized in all thalamic nuclei in the squirrel monkey but their relative density varied from one nucleus to the other. At the level of the anterior nuclear group, GABA-positive cells and terminals abounded in the anterodorsal nucleus but were much less numerous in the anteromedial and anteroventral nuclei. In the nuclei of the ventral group, GABA-immunoreactive cells were found to be smaller and less numerous than nonimmunoreactive neurons. In the ventral anterior nucleus, GABA-positive neuronal profiles formed typical clusters, whereas they were more uniformly distributed in the posterior nuclei of the ventral group. In the intralaminar nuclei, GABA-immunoreactive cells and terminals abounded in the dorsal portion of the paracentral and centrolateral nuclei, whereas more caudally, GABA-positive terminals pervaded the entire parafascicular nucleus. In the mediodorsal nucleus, GABA-positive cell bodies and axon terminals formed typical clusters of various sizes scattered within the lateral parvocellular portion of the nucleus, while GABA-immunoreactive neuronal profiles were less numerous and more uniformly distributed in the medial portion of this structure. In the nuclei of the posterior group, GABA-immunoreactive neuronal profiles were uniformly distributed except in the pulvinar where they abounded in the inferior and oral parts but were scarce in the medial part. In the dorsal lateral geniculate nucleus, the magnocellular layers received the most massive GABA-positive innervation and contained the largest number of GABA-immunoreactive cell bodies. In the ventral lateral geniculate nucleus, GABA-positive cells occurred only ventrolaterally while GABA-immunoreactive terminals pervaded the entire structure. In the medial geniculate nucleus, GABA-immunoreactive cell bodies and terminals abounded particularly within the ventromedial third of the structure. In the habenula, a few GABA-immunoreactive cell bodies and numerous GABA-positive terminals were scattered throughout the lateral habenular nucleus, whereas only a few GABA-immunoreactive terminals surrounded the closely packed unreactive cells in the medial habenular nucleus. In contrast to other thalamic nuclei all neurons in the reticular nucleus displayed GABA immunoreactivity.(ABSTRACT TRUNCATED AT 400 WORDS)