Choline acetyltransferase-immunoreactive neurons in the rat entopeduncular nucleus

The habenula-targeting neurons in the mouse entopeduncular nucleus contain not only somatostatin-positive neurons but also nitric oxide synthase-positive neurons

The entopeduncular nucleus (EPN) in rodents is one of the two major output nuclei of the basal ganglia and corresponds to the internal segment of the globus pallidus in primates. Previous studies have shown that the EPN contains three types of neurons that project to different targets, namely, parvalbumin (PV)-, somatostatin (SOM)-, and choline acetyltransferase-positive neurons. However, we have recently reported that neurons lacking immunoreactivities for these substances are present in the EPN. Here, we demonstrate that 27.7% of all EPN neurons showed immunoreactivity for nitric oxide synthase (NOS). Among them, NOS-only positive and NOS/SOM double-positive neurons accounted for 20.1% and 6.8%, respectively, whereas NOS/PV double-positive neurons were rarely observed. NOS-containing neurons were distributed in a shell region surrounding the thalamus-targeting, PV-rich core region of the EPN, especially in the ventromedial part of the shell. The retrograde tracer fluoro-gold (FG) was injected into several target regions of EPN neurons. Among FG-labeled EPN neurons after injection into the lateral habenula (LHb), NOS-only positive, NOS/SOM double-positive, and SOM-only positive neurons accounted for 25.7%, 15.2%, and 59.1%, respectively. We conclude that NOS-positive neurons are the second major population of LHb-targeting EPN neurons, suggesting their possible involvement in behaviors in response to aversive stimuli.

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.

Future perspectives on neural mechanisms underlying rhythm and music based neurorehabilitation in Parkinson's disease

Parkinson's disease (PD) is characterized primarily by a dysfunctional basal ganglia (BG) system, producing motor and non-motor symptoms. A significant number of studies have demonstrated that rhythmic auditory stimulation can improve gait and other motor behaviors in PD that are not well managed by the conventional therapy. As music, being highly complex stimulus, can modulate brain activity/function in distributed areas of brain, the therapeutic properties of music potentially extend to alleviate non-motor symptoms of PD. Despite the clinical, behavioral evidence and promises of rhythm and music based interventions, the neural substrates underlying the effectiveness are poorly understood. The goal of this review is to appraise the current state of knowledge in order to direct further neuroimaging studies that help to determine the therapeutic effects of rhythm and music based interventions for motor and non-motor symptoms of PD.

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.

Lateral Habenula Involvement in Impulsive Cocaine Seeking

The lateral habenula (LHb) is a brain structure receiving inputs from limbic forebrain areas and innervating major midbrain monoaminergic nuclei. Evidence indicates LHb involvement in sleep control, reward-based decision making, avoidance of punishment, and responses to stress. Additional work has established that the LHb mediates negative feedback in response to aversive events. As a hallmark of drug addiction is the inability to limit drug use despite negative consequences, we hypothesize that LHb dysfunction may have a role in the lack of control over drug seeking. Here we examine the effects of LHb inactivation in control over drug seeking in several cocaine self-administration (SA) paradigms in rats. We find that inhibition of the LHb with GABAergic agonists did not alter cocaine SA under progressive ratio or seeking/taking chained reinforcement schedules, or during punishment-induced suppression of cocaine-reinforced responding. In contrast, LHb inhibition increased cocaine seeking when the drug was not available in rats trained to discriminate its presence using an environmental cue. This effect of LHb inhibition was selective for cocaine, as it did not impair responding for sucrose reinforcement. The effect of LHb injection of GABA agonists was mimicked by intra-LHb muscarinic cholinergic (mACh) antagonist injection, and activation of mACh receptors excited a majority of LHb neurons in in vitro electrophysiology experiments. These results indicate that the LHb participates in the suppression of impulsive responding for cocaine through the activation of a cholinergic circuit, and they suggest that LHb dysfunction may contribute to impaired impulse control associated with drug addiction.

High-frequency stimulation of the subthalamic nucleus modulates neuronal activity in the lateral habenula nucleus

High-frequency stimulation (HFS) of the subthalamic nucleus (STN) is often used to treat movement disability in advanced Parkinson's disease, but some patients experience debilitating psychiatric effects including depression. Interestingly, HFS of the STN modulates 5-HT neurons in the dorsal raphe nucleus (DRN) which are linked to depression, but the neural substrate of this effect is unknown. Here, we tested the effect of STN stimulation on neuronal activity in the lateral habenula nucleus (LHb), an important source of input to DRN 5-HT neurons and also a key controller of emotive behaviours. LHb neurons were monitored in anaesthetized rats using single-unit extracellular recording, and localization within the LHb was confirmed by juxtacellular labelling. HFS of the STN (130 Hz) evoked rapid changes in the firing rate of the majority of LHb neurons tested (38 of 68). Some LHb neurons (19/68) were activated by HFS, while others (19/68), distinguished by a higher basal firing rate, were inhibited. LHb neurons that project to the DRN were identified using antidromic activation and collision testing (n = 17 neurons). Some of these neurons (5/17) were also excited by HFS of the STN, and others (7/17) were inhibited although this was only a statistical trend. In summary, HFS of the STN modulated the firing of LHb neurons, including those projecting to the DRN. The data identify that the STN impacts on the LHb-DRN pathway. Moreover, this pathway may be part of the circuitry mediating the psychiatric effects of STN stimulation experienced by patients with Parkinson's disease.

Basal ganglia circuit loops, dopamine and motivation: A review and enquiry

Dopamine neurons located in the midbrain play a role in motivation that regulates approach behavior (approach motivation). In addition, activation and inactivation of dopamine neurons regulate mood and induce reward and aversion, respectively. Accumulating evidence suggests that such motivational role of dopamine neurons is not limited to those located in the ventral tegmental area, but also in the substantia nigra. The present paper reviews previous rodent work concerning dopamine's role in approach motivation and the connectivity of dopamine neurons, and proposes two working models: One concerns the relationship between extracellular dopamine concentration and approach motivation. High, moderate and low concentrations of extracellular dopamine induce euphoric, seeking and aversive states, respectively. The other concerns circuit loops involving the cerebral cortex, basal ganglia, thalamus, epithalamus, and midbrain through which dopaminergic activity alters approach motivation. These models should help to generate hypothesis-driven research and provide insights for understanding altered states associated with drugs of abuse and affective disorders.

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.

Evolutionary conservation of the habenular nuclei and their circuitry controlling the dopamine and 5-hydroxytryptophan (5-HT) systems

The medial (MHb) and lateral (LHb) habenulae are a small group of nuclei that regulate the activity of monoaminergic neurons. Disruptions to these nuclei lead to deficits in a range of cognitive and motor functions from sleep to decision making. Interestingly, the habenular nuclei are present in all vertebrates, suggesting that they provide a common neural mechanism to influence these diverse functions. To unravel conserved habenula circuitry and approach an understanding of their basic function, we investigated the organization of these nuclei in the lamprey, one of the phylogenetically oldest vertebrates. Based on connectivity and molecular expression, we show that the MHb and LHb circuitry is conserved in the lamprey. As in mammals, separate populations of neurons in the LHb homolog project directly or indirectly to dopamine and serotonin neurons through a nucleus homologous to the GABAergic rostromedial mesopontine tegmental nucleus and directly to histamine neurons. The pallidal and hypothalamic inputs to the LHb homolog are also conserved. In contrast to other species, the habenula projecting pallidal nucleus is topographically distinct from the dorsal pallidum, the homolog of the globus pallidus interna. The efferents of the MHb homolog selectively target the interpeduncular nucleus. The MHb afferents arise from sensory (medial olfactory bulb, parapineal, and pretectum) and not limbic areas, as they do in mammals; consequently, the "context" in which this circuitry is recruited may have changed during evolution. Our results indicate that the habenular nuclei provide a common vertebrate circuitry to adapt behavior in response to rewards, stress, and other motivating factors.

The globus pallidus sends reward-related signals to the lateral habenula

As a major output station of the basal ganglia, the globus pallidus internal segment (GPi) projects to the thalamus and brainstem nuclei thereby controlling motor behavior. A less well known fact is that the GPi also projects to the lateral habenula (LHb) which is often associated with the limbic system. Using the monkey performing a saccade task with positionally biased reward outcomes, we found that antidromically identified LHb-projecting neurons were distributed mainly in the dorsal and ventral borders of the GPi and that their activity was strongly modulated by expected reward outcomes. A majority of them were excited by the no-reward-predicting target and inhibited by the reward-predicting target. These reward-dependent modulations were similar to those in LHb neurons but started earlier than those in LHb neurons. These results suggest that GPi may initiate reward-related signals through its effects on the LHb, which then influences the dopaminergic and serotonergic systems.

Toward an anatomy of disappointment: reward-related signals from the globus pallidus

The globus pallidus (internal segment, GPi) is traditionally regarded as part of the motor system. In this issue of Neuron, Hong and Hikosaka report on a little known projection from the monkey GPi to the lateral habenula that is modulated by reward. This adds an important branch to the brain's reward circuitry.

The presence and absence of prosencephalic cell groups relaying striatal information to the medial and lateral thalamus in tenrec

Although there are remarkable differences regarding the output organization of basal ganglia between mammals and non-mammals, mammalian species with poorly differentiated brain have scarcely been investigated in this respect. The aim of the present study was to identify the pallidal neurons giving rise to thalamic projections in the Madagascar lesser hedgehog tenrec (Afrotheria). Following tracer injections into the thalamus, retrogradely labelled neurons were found in the depth of the olfactory tubercle (particularly the hilus of the Callejal islands and the insula magna), in subdivisions of the diagonal band complex, the peripeduncular region and the thalamic reticular nucleus. No labelled cells were seen in the globus pallidus. Pallidal neurons were tentatively identified on the basis of their striatal afferents revealed hodologically using anterograde axonal tracer substances and immunohistochemically with antibodies against enkephalin and substance P. The data showed that the tenrec's medial thalamus received prominent projections from ventral pallidal cells as well as from a few neurons within and ventral to the cerebral peduncle. The only regions projecting to the lateral thalamus appeared to be the thalamic reticular nucleus (RTh) and the dorsal peripeduncular nucleus (PpD). On the basis of immunohistochemical data and the topography of its thalamic projections, the PpD was considered to be an equivalent to the pregeniculate nucleus in other mammals. There was no evidence of entopeduncular (internal pallidal) neurons being present within the RTh/PpD complex, neuropils of which did not stain for enkephalin and substance P. The ventrolateral portion of RTh, the only region eventually receiving a striatal input, projected to the caudolateral rather than the rostrolateral thalamus. Thus, the striatopallidal output organization in the tenrec appeared similar, in many respects, to the output organization in non-mammals. This paper considers the failure to identify entopeduncular neurons projecting to the rostrolateral thalamus in a mammal with a little differentiated cerebral cortex, and also stresses the discrepancy between this absence and the presence of a distinct external pallidal segment (globus pallidus).

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.

Manganese-enhanced MRI in a rat model of Parkinson's disease

PURPOSE

To measure intra- and inter-hemispheric connectivity within the basal ganglia (BG) nuclei in healthy and in unilateral 6-hydroxydopamine (6-OHDA) Parkinson disease rat model in order to test the BG interhemispheric connectivity hypothesis.

MATERIAL AND METHODS

The manganese-enhanced MRI (MEMRI) method with direct injection of manganese chloride into the entopeduncular (EP), substantia nigra (SN), and the Habenula nuclei in unilateral 6-OHDA (N = 22) and sham-operated (N = 16) rat groups was used. MEMRI measurements were applied before, 3, 24, and 48 hours post-manganese injection. Signal enhancements in T1-weighted images were compared between groups.

RESULTS

Manganese injection into the EP nucleus resulted with bihemispheric signal enhancements in the habenular complex (Hab) at both groups with stronger enhancements in the 6-OHDA group. It also exhibited lower sensorimotor cortex signal enhancement in the 6-OHDA rat group. SN manganese injection caused enhanced anteroventral thalamic and habenular nuclei signals in the 6-OHDA rat group. Manganese habenula injection revealed enhanced interpeduncular (IP) and raphe nuclei signals of the 6-OHDA rat group.

CONCLUSION

Modulations in the effective intra- and interhemispheric BG connectivity in unilateral 6-OHDA Parkinson's disease (PD) rat model support the BG interhemispheric connectivity hypothesis and suggest a linkage between the dopaminergic and serotonergic systems in PD, in line with clinical symptoms.

Functional recovery of cholinergic basal forebrain neurons under disease conditions: old problems, new solutions?

Recognition of the involvement of cholinergic neurons in the modulation of cognitive functions and their severe dysfunction in neurodegenerative disorders, such as Alzheimer's disease, initiated immense research efforts aimed at unveiling the anatomical organization and cellular characteristics of the basal forebrain (BFB) cholinergic system. Concomitant with our unfolding knowledge about the structural and functional complexity of the BFB cholinergic projection system, multiple pharmacological strategies were introduced to rescue cholinergic nerve cells from noxious attacks; however, a therapeutic breakthrough is still awaited. In this review, we collected recent findings that significantly contributed to our better understanding of cholinergic functions under disease conditions, and to the design of effective means to restore lost or damaged cholinergic functions. To this end, we first provide a brief survey of the neuroanatomical organization of BFB nuclei with emphasis on major evolutionary differences among mammalian species, in particular rodents and primates, and discuss limitations of the translation of experimental data to human therapeutic applications. Subsequently, we summarize the involvement of cholinergic dysfunction in the pathogenesis of severe neurological conditions, including stroke, traumatic brain injury, virus encephalitis and Alzheimer's disease, and emphasize the critical role of pro-inflammatory cytokines as common mediators of cholinergic neuronal damage. Moreover, we review leading functional concepts on the limited recovery of cholinergic neurons and their impaired plastic re-modeling, as well as on the hampered interplay of the ascending cholinergic and monoaminergic projection systems under neurodegenerative conditions. In addition, recent advances in the dynamic labeling of living cholinergic neurons by fluorochromated antibodies, referred to as in vivo labeling, and novel neuroimaging approaches as potential diagnostic tools of progressive cholinergic decline are surveyed. Finally, the potential of cell replacement strategies using embryonic and adult stem cells, and multipotent neural progenitors, as a means to recover damaged cholinergic functions, is discussed.

Neurochemical changes in the entopeduncular nucleus and increased oral behavior in rats treated subchronically with clozapine or haloperidol

The purpose of the present experiment was to test the possibility that atypical antipsychotics and classical antipsychotics differentially regulate specific neurochemical processes within the entopeduncular nucleus. For these experiments, rats were administered clozapine (25 mg/kg), haloperidol (1 mg/kg), or Tween-80 (control) daily for 21 days. Dopamine D(1)-receptor binding was assessed with in vitro receptor autoradiographic methods and the mRNAs corresponding to the two forms of glutamate decarboxylase (glutamate decarboxylase-65 and glutamate decarboxylase-67) were analyzed using in situ hybridization histochemical methods. In addition, vacuous chewing movements (VCM) were measured throughout the drug administration period as a functional indicator of drug action and changes in striatal dopamine D(2)-receptor binding were measured as a positive control for D(2)-receptor antagonist properties of haloperidol and clozapine. In agreement with previous reports, haloperidol increased D(2)-receptor binding throughout the striatum while clozapine had a more limited impact on D(2)-receptors. Behavioral analysis revealed that both haloperidol and clozapine enhanced the display of vacuous chewing movements to a similar extent but with a different postinjection latency. In the entopeduncular nucleus, clozapine increased D(1)-receptor binding compared to controls while haloperidol was without effect. With respect to the regulation of GAD mRNAs, haloperidol increased glutamate decarboxylase-65 and glutamate decarboxylase-67 mRNA levels throughout the entopeduncular nucleus. The effects of clozapine were restricted to increases in glutamate decarboxylase-65 mRNA. These studies show that clozapine and haloperidol, both of which increase the occurrence of VCM, differentially modulate the neurochemistry of the entopeduncular nucleus.