Genetically Distinct Parallel Pathways in the Entopeduncular Nucleus for Limbic and Sensorimotor Output of the Basal Ganglia.

The basal ganglia (BG) integrate inputs from diverse sensorimotor, limbic, and associative regions to guide action-selection and goal-directed behaviors. The entopeduncular nucleus (EP) is a major BG output nucleus and has been suggested to channel signals from distinct BG nuclei to target regions involved in diverse functions. Here we use single-cell transcriptional and molecular analyses to demonstrate that the EP contains at least three classes of projection neurons-glutamate/GABA co-releasing somatostatin neurons, glutamatergic parvalbumin neurons, and GABAergic parvalbumin neurons. These classes comprise functionally and anatomically distinct output pathways that differentially affect EP target regions, such as the lateral habenula (LHb) and thalamus. Furthermore, LHb- and thalamic-projecting EP neurons are differentially innervated by subclasses of striatal and pallidal neurons. Therefore, we identify previously unknown subdivisions within the EP and reveal the existence of cascading, molecularly distinct projections through striatum and globus pallidus to EP targets within epithalamus and thalamus.

Cholinergic mechanisms of high-frequency stimulation in entopeduncular nucleus.

Chronic, high-frequency (>100 Hz) electrical stimulation, known as deep brain stimulation (DBS), of the internal segment of the globus pallidus (GPi) is a highly effective therapy for Parkinson's disease (PD) and dystonia. Despite some understanding of how it works acutely in PD models, there remain questions about its mechanisms of action. Several hypotheses have been proposed, such as depolarization blockade, activation of inhibitory synapses, depletion of neurotransmitters, and/or disruption/alteration of network oscillations. In this study we investigated the cellular mechanisms of high-frequency stimulation (HFS) in entopeduncular nucleus (EP; rat equivalent of GPi) neurons using whole cell patch-clamp recordings. We found that HFS applied inside the EP nucleus induced a prolonged afterdepolarization that was dependent on stimulation frequency, pulse duration, and current amplitude. The high frequencies (>100 Hz) and pulse widths (>0.15 ms) used clinically for dystonia DBS could reliably induce these afterdepolarizations, which persisted under blockade of ionotropic glutamate (kynurenic acid, 2 mM), GABAA (picrotoxin, 50 µM), GABAB (CGP 55845, 1 µM), and acetylcholine nicotinic receptors (DHßE, 2 µM). However, this effect was blocked by atropine (2 µM; nonselective muscarinic antagonist) or tetrodotoxin (0.5 µM). Finally, the muscarinic-dependent afterdepolarizations were sensitive to Ca(2+)-sensitive nonspecific cationic (CAN) channel blockade. Hence, these data suggest that muscarinic receptor activation during HFS can lead to feedforward excitation through the opening of CAN channels. This study for the first time describes a cholinergic mechanism of HFS in EP neurons and provides new insight into the underlying mechanisms of DBS.

Inhibitory short-term plasticity modulates neuronal activity in the rat entopeduncular nucleus in vitro.

The entopeduncular nucleus (EP) is one of the basal ganglia output nuclei integrating synaptic information from several pathways within the basal ganglia. The firing of EP neurons is modulated by two streams of inhibitory synaptic transmission, the direct pathway from the striatum and the indirect pathway from the globus pallidus. These two inhibitory pathways continuously modulate the firing of EP neurons. However, the link between these synaptic inputs to neuronal firing in the EP is unclear. To investigate this input-output transformation we performed whole-cell and perforated-patch recordings from single neurons in the entopeduncular nucleus in rat brain slices during repetitive stimulation of the striatum and the globus pallidus at frequencies within the in vivo activity range of these neurons. These recordings, supplemented by compartmental modelling, showed that GABAergic synapses from the striatum, converging on EP dendrites, display short-term facilitation and that somatic or proximal GABAergic synapses from the globus pallidus show short-term depression. Activation of striatal synapses during low presynaptic activity decreased postsynaptic firing rate by continuously increasing the inter-spike interval. Conversely, activation of pallidal synapses significantly affected postsynaptic firing during high presynaptic activity. Our data thus suggest that low-frequency striatal output may be encoded as progressive phase shifts in downstream nuclei of the basal ganglia while high-frequency pallidal output may continuously modulate EP firing.

A single-neuron tracing study of arkypallidal and prototypic neurons in healthy rats.

The external globus pallidus (GP) is known as a relay nucleus of the indirect pathway of the basal ganglia. Recent studies in dopamine-depleted and healthy rats indicate that the GP comprises two main types of pallidofugal neurons: the so-called "prototypic" and "arkypallidal" neurons. However, the reconstruction of complete arkypallidal neurons in healthy rats has not been reported. Here we visualized the entire axonal arborization of four single arkypallidal neurons and six single prototypic neurons in rat brain using labeling with a viral vector expressing membrane-targeted green fluorescent protein and examined the distribution of axon boutons in the target nuclei. Results revealed that not only the arkypallidal neurons but nearly all of the prototypic neurons projected to the striatum with numerous axon varicosities. Thus, the striatum is a major target nucleus for pallidal neurons. Arkypallidal and prototypic GP neurons located in the calbindin-positive and calbindin-negative regions mainly projected to the corresponding positive and negative regions in the striatum. Because the GP and striatum calbindin staining patterns reflect the topographic organization of the striatopallidal projection, the striatal neurons in the sensorimotor and associative regions constitute the reciprocal connection with the GP neurons in the corresponding regions.

High-Frequency Stimulation of the Rat Entopeduncular Nucleus Does Not Provide Functional or Morphological Neuroprotection from 6-Hydroxydopamine.

Deep brain stimulation (DBS) is the most common neurosurgical treatment for Parkinson's disease (PD). Whereas the globus pallidus interna (GPi) has been less commonly targeted than the subthalamic nucleus (STN), a recent clinical trial suggests that GPi DBS may provide better outcomes for patients with psychiatric comorbidities. Several laboratories have demonstrated that DBS of the STN provides neuroprotection of substantia nigra pars compacta (SNpc) dopamine neurons in preclinical neurotoxin models of PD and increases brain-derived neurotrophic factor (BDNF). However, whether DBS of the entopeduncular nucleus (EP), the homologous structure to the GPi in the rat, has similar neuroprotective potential in preclinical models has not been investigated. We investigated the impact of EP DBS on forelimb use asymmetry and SNpc degeneration induced by 6-hydroxydopamine (6-OHDA) and on BDNF levels. EP DBS in male rats received unilateral, intrastriatal 6-OHDA and ACTIVE or INACTIVE stimulation continuously for two weeks. Outcome measures included quantification of contralateral forelimb use, stereological assessment of SNpc neurons and BDNF levels. EP DBS 1) did not ameliorate forelimb impairments induced by 6-OHDA, 2) did not provide neuroprotection for SNpc neurons and 3) did not significantly increase BDNF levels in any of the structures examined. These results are in sharp contrast to the functional improvement, neuroprotection and BDNF-enhancing effects of STN DBS under identical experimental parameters in the rat. The lack of functional response to EP DBS suggests that stimulation of the rat EP may not represent an accurate model of clinical GPi stimulation.

Immunohistochemical study on the neuronal diversity and three-dimensional organization of the mouse entopeduncular nucleus.

The entopeduncular nucleus (EPN) is one of the major output nuclei of the basal ganglia in rodents. Previous studies have divided it into rostral and caudal halves, with the former containing somatostatin (SOM)-immunoreactive neurons and the latter dominated by parvalbumin (PV)-containing neurons, respectively. However, it is unclear whether this simple rostrocaudal segmentation is appropriate, and the possibility of the existence of other neuronal populations remains to be investigated. In this study the cytoarchitecture of the mouse EPN was analyzed immunohistochemically. Substance P (SP)-immunoreactivity determined the extent of the EPN, which was 800 µm-long along the rostrocaudal axis. PV-positive neurons were concentrated in the caudal two-thirds of this range. PV-negative neurons were abundant in the rostral half but were further located caudally around the PV neuron-rich core. PV(+)/SOM(-) and PV(-)/SOM(+) neurons constituted 28.6% and 45.7% of EPN neurons, respectively, whereas the remaining population (25.7%) exhibited neither immunoreactivity. Eleven percent of EPN neurons lacked immunoreactivity for glutamic acid decarboxylase, indicating their non-GABAergic nature. Three-dimensional reconstruction revealed that PV-rich/SP-poor core was surrounded by PV-poor/SP-rich shell region. Therefore, presumptive thalamus-targeting PV neurons are outnumbered by other populations, and the regional heterogeneity shown here might be related to functionally distinct pathways through the basal ganglia.

Spontaneous neural activity of the anterodorsal lobe and entopeduncular nucleus in adult zebrafish: a putative homologue of hippocampal sharp waves.

Spontaneous neural activity is instrumental in the formation and maintenance of neural circuits that govern behavior. In mammals, spontaneous activity is observed in the spinal cord, brainstem, diencephalon, and neocortex, and has been most extensively studied in the hippocampus. Using whole-brain in vitro recordings we establish the presence of spontaneous activity in two regions of the zebrafish telenchephalon: the entopeduncular nucleus (EN) and the anterodorsal lobe (ADL). The ADL is part of the lateral telencephalic pallium, an area hypothesized to be functionally equivalent to the mammalian hippocampus. In contrast, the EN has been hypothesized to be equivalent to the mammalian basal ganglia. The observed spontaneous activity is GABA modulated, sensitive to glutamate and chloride transporter antagonists, and is abolished by sodium pump blockers; moreover, the spontaneous activity in the ADL is a slow multiband event (∼100 ms) characterized by an embedded fast ripple wave (∼150-180 Hz). Thus, the spontaneous activity in the ADL shares physiological features of hippocampal sharp waves in rodents. We suggest that this spontaneous activity is important for the formation and maintenance of neural circuits in zebrafish and argue that applying techniques unique to the fish may open novel routes to understand the function of spontaneous activity in mammals.

Axonal failure during high frequency stimulation of rat subthalamic nucleus.

Deep brain stimulation (DBS) has been established as an effective surgical therapy for advanced Parkinson's disease (PD) and gains increasing acceptance for otherwise intractable neuropsychiatric diseases such as major depression or obsessive­compulsive disorders. In PD, DBS targets predominantly the subthalamic nucleus (STN) and relieves motor deficits only at high frequency (>100 Hz). In contrast to the well-documented clinical efficacy of DBS, its underlying principle remains enigmatic spawning a broad and, in part, contradictory spectrum of suggested synaptic and non-synaptic mechanisms within and outside STN. Here we focused on a crucial, but largely neglected issue in this controversy, namely the axonal propagation of DBS within and away from STN. In rat brain slices preserving STN projections to substantia nigra (SN) and entopeduncular nucleus (EP, the rodent equivalent of internal globus pallidus), STN-DBS disrupted synaptic excitation onto target neurons through an unexpected failure of axonal signalling. The rapid onset and, upon termination of DBS, recovery of this effect was highly reminiscent of the time course of DBS in the clinical setting. We propose that DBS-induced suppression of axonal projections from and to STN serves to shield basal ganglia circuitry from pathological activity arising in or amplified by this nucleus.

A novel stereotaxic apparatus for neuronal recordings in awake head-restrained rats.

A novel technique for neuronal recordings in awake head-restrained animals is presented. Our setup allows (1) daily repeat microelectrode studies in rats without use of anesthesia, (2) excellent stabilization of head using an eight point fixation, (3) painless head-restraint of the animal, (4) accurate stereotaxic localization during multiple sessions of recording, (5) to considerably reduced surgical time, (6) quick repositioning during chronic recording sessions and (7) high quality stabilized neuronal recordings during periods of rest and active movements.

Anterograde axonal transport of AAV2-GDNF in rat basal ganglia.

We elucidated the effects of parkinsonian degeneration on trafficking of AAV2-GDNF in the nigro-striatum (nigro-ST) of unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rats. Vector infused into striatum (ST) was transported to substantia nigra (SN), both pars compacta (SNc), and pars reticulata (SNr). In the lesioned hemisphere, glial cell line-derived neurotrophic factor (GDNF) immunoreactivity was only found in SNr consistent with elimination of SNc dopaminergic (DA) neurons by 6-OHDA. Further analysis showed that striatal delivery of AAV2-GDNF resulted in GDNF expression in globus pallidus (GP), entopeduncular nucleus (EPN), and subthalamic nucleus (STN) in both lesioned and unlesioned hemispheres. Injection of vector into SN, covering both SNc and SNr, resulted in striatal expression of GDNF in the unlesioned hemisphere but not in the lesioned hemisphere. No expression was seen in GP or EPN. We conclude that adeno-associated virus serotype 2 (AAV2) is transported throughout the nigro-ST exclusively by anterograde transport. This transport phenomenon directs GDNF expression throughout the basal ganglia in regions that are adversely affected in Parkinson's disease (PD) in addition to SNc. Delivery of vector to SN, however, does not direct expression of GDNF in ST, EPN, or GP. On this basis, we believe that striatal delivery of AAV2-GDNF is the preferred course of action for trophic rescue of DA function.

Topography and chemoarchitecture of the striatum and pallidum in a monotreme, the short-beaked echidna (Tachyglossus aculeatus).

The topography and chemoarchitecture of the striatum and pallidum in a monotreme, the short-beaked echidna (Tachyglossus aculeatus) have been studied using Nissl staining in conjunction with myelin staining, enzyme reactivity to acetylcholinesterase and NADPH diaphorase, and immunoreactivity to parvalbumin, calbindin, calretinin, tyrosine hydroxylase, neuropeptide Y, and neurofilament protein (SMI-32 antibody). All those components of the striatum and pallidum found in eutherian mammals could also be identified in the echidna's brain, with broad chemoarchitectural similarities to those regions in eutherian brains also apparent. There was a clear chemoarchitectural gradient visible with parvalbumin immunoreactivity of neurons and fibers, suggesting a subdivision of the echidna caudatoputamen into weakly reactive rostrodorsomedial and strongly reactive caudoventrolateral components. This may, in turn, relate to subdivision into associative versus sensorimotor CPu and reflect homology to the caudate and putamen of primates. Moreover, the chemoarchitecture of the echidna striatum suggested the presence of striosome-matrix architecture. The morphology of identified neuronal groups (i.e., parvalbumin, calbindin, and neuropeptide Y immunoreactive) in the echidna striatum and pallidum showed many similarities to those seen in eutherians, although the pattern of distribution of calbindin immunoreactive neurons was more uniform in the caudatoputamen of the echidna than in therians. These observations indicate that the same broad features of striatal and pallidal organization apply across all mammals and suggest that these common features may have arisen before the divergence of the monotreme and therian lineages.

Histogenetic compartments of the mouse centromedial and extended amygdala based on gene expression patterns during development.

The amygdala controls emotional and social behavior and regulates instinctive reflexes such as defense and reproduction by way of descending projections to the hypothalamus and brainstem. The descending amygdalar projections are suggested to show a cortico-striato-pallidal organization similar to that of the basal ganglia (Swanson [2000] Brain Res 886:113-164). To test this model we investigated the embryological origin and molecular properties of the mouse centromedial and extended amygdalar subdivisions, which constitute major sources of descending projections. We analyzed the distribution of key regulatory genes that show restricted expression patterns within the subpallium (Dlx5, Nkx2.1, Lhx6, Lhx7/8, Lhx9, Shh, and Gbx1), as well as genes considered markers for specific subpallial neuronal subpopulations. Our results indicate that most of the centromedial and extended amygdala is formed by cells derived from multiple subpallial subdivisions. Contrary to a previous suggestion, only the central--but not the medial--amygdala derives from the lateral ganglionic eminence and has striatal-like features. The medial amygdala and a large part of the extended amygdala (including the bed nucleus of the stria terminalis) consist of subdivisions or cell groups that derive from subpallial, pallial (ventral pallium), or extratelencephalic progenitor domains. The subpallial part includes derivatives from the medial ganglionic eminence, the anterior peduncular area, and possibly a novel subdivision, called here commissural preoptic area, located at the base of the septum and related to the anterior commissure. Our study provides a molecular and morphological foundation for understanding the complex embryonic origins and adult organization of the centromedial and extended amygdala.

[The organization of diencephalic zona incerta projections to the structures of the pallidum in dog brain].

The detailed organization of the projections of the individual sectors of the zona incerta of the diencephalon to the functionally diverse pallidal structures in dog brain was investigated by the method of antero- and retrograde axonal transport of horseradish peroxidase. It was found that the neurons of the caudal incertal sector innervated the globus pallidus and the nucleus entopeduncularis which predominantly received the innervation from the motor structures. The projections from single neurons of the dorsal and ventral incertal sectors are directed to the same pallidal structures. No connections of the zona incerta with the limbic ventral pallidum were established.

High frequency stimulation or elevated K+ depresses neuronal activity in the rat entopeduncular nucleus.

High frequency stimulation (HFS) is applied to many brain regions to treat a variety of neurological disorders/diseases, yet the mechanism(s) underlying its effects remains unclear. While some studies showed that HFS inhibits the stimulated nucleus, others report excitation. In this in vitro study, we stimulated the rat globus pallidus interna (entopeduncular nucleus, EP), a commonly stimulated area for Parkinson's disease, to investigate the effect of HFS-induced elevation of extracellular potassium (K(+)(e)) on rat EP neuronal activity. Whole-cell patch-clamp recordings and [K(+)](e) measurements were obtained in rat EP brain slices before, during and after HFS. After HFS (150 Hz, 10 s), [K(+)](e) increased from 2.5-9.6+/-1.4 mM, the resting membrane potential of EP neurons depolarized by 11.1+/-2.5 mV, spiking activity was significantly depressed, and input resistance decreased by 25+/-6%. The GABA(A) receptor blocker, gabazine, did not prevent these effects. The bath perfusion of 6 or 10 mM K(+), with or without synaptic blockers, mimicked the HFS-mediated effects: inhibition of spike activity, a 20+/-9% decrease in input resistance and a 17.4+/-3.0 mV depolarization. This depolarization exceeded predicted values of elevated [K(+)](e) on the resting membrane potential. A depolarization block did not fully account for the K(+)-induced inhibition of EP neuronal activity. Taken together, our results show that HFS-induced elevation of [K(+)](e) decreased EP neuronal activity by the activation of an ion conductance resulting in membrane depolarization, independent of synaptic involvement. These findings could explain the inhibitory effects of HFS on neurons of the stimulated nucleus.

Analysis of the morphological substrate for information processing in the pallidal nuclear complex of the dog brain in terms of the organizational characteristics of its afferent projections.

Axonal transport of retrograde markers was used to study the distribution of the afferent projections of the nuclei of the pallidal complex (the globus pallidus, the entopeduncular nucleus, and the ventral pallidum) from functionally diverse cortical and subcortical structures (cortical fields, substantia nigra, ventral tegmental field, and thalamus) in the dog brain. The results were used to analyze the morphological aspects both of the functional heterogeneity of the pallidum and integrative information processing, which underlie the mechanisms of adaptive behavior.

Effects of dynorphin on rat entopeduncular nucleus neurons in vitro.

The entopeduncular nucleus (EP) receives dense neostriatal afferent axons that contain dynorphin (DYN, an endogenous kappa-receptor agonist), in addition to GABA and substance P. To examine the role of DYN in the EP, whole-cell recordings were performed in rat brain slice preparations. Based on the physiological and morphological characteristics, all the neurons recorded were similar to the Type-I EP neuron described in a previous study. The kappa-receptor agonist dynorphin A (1-13) (DYN13) hyperpolarized and decreased the input resistance of approximately one-quarter of the EP neurons examined. The hyperpolarization was due to an increase in potassium conductance since current-voltage relationship curves obtained before and after DYN13 application crossed at the potassium equilibrium potential. In the presence of the glutamate blocker 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide and 3-(2-carboxypiperzin-4-yl)-propyl-1-phosphonic acid in artificial cerebrospinal fluid, stimulation of the globus pallidus evoked bicuculline-sensitive multi-component GABAergic responses in EP neurons. Application of DYN13 equally reduced the amplitudes of the short-latency response, conceivably evoked by pallido-EP axons, and the medium-latency response, conceivably evoked by striato-EP axons. These effects were reversed by bath application of a non-selective opioid antagonist naloxone or by a kappa-opioid receptor-selective antagonist nor-binaltorphimine dihydrochloride (nor-BNI), but not by the partial differential -antagonist naltrindole or the mu-antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2). DYN13 also reduced the frequency of tetrodotoxin-insensitive miniature-inhibitory postsynaptic potential (mIPSPs) without changing their amplitude distributions. The decrease of the frequency of mIPSPs was reversible upon washing and was also completely blocked by nor-BNI. The results of the present study on the EP indicated that DYN released from striatal axons might exert at least three different effects on these target nuclei. Firstly, DYN might provide negative feedback regulation of striatal GABAergic outputs at their termination sites. Secondly, DYN released from the striatal terminals might diffuse to the pallidal terminals, regulating their GABA release. Thirdly, DYN might exert a direct inhibition of EP neurons. Thus, DYN released from striatal axons might control the activity of EP neurons by reducing the GABAergic transmission and also by hyperpolarizing postsynaptic membrane.

Amphetamine-induced Fos expression is evident in gamma-aminobutyric acid neurons in the globus pallidus and entopeduncular nucleus in rats treated with intrastriatal c-fos antisense oligodeoxynucleotides.

Double immunostaining for Fos and gamma-aminobutyric acid (GABA) was used in a previously established animal model of striatal dysfunction to examine whether GABA-immunoreactive neurons in the globus pallidus (GP) and entopeduncular nucleus (EP) are activated to express Fos immunoreactivity by intraperitoneal injection of amphetamine. Striatal efferent activity was suppressed by intrastriatal infusions of antisense oligodeoxynucleotide targeted to the messenger RNA of the immediate early gene, c-fos. This suppression produced robust rotational behavior and expression of Fos in the ipsilateral GP and EP following amphetamine challenge. The expression of Fos in the ipsilateral GP and EP following amphetamine challenge is not observed in naïve or control antisense-treated animals. Quantitative analysis revealed that a majority of the amphetamine-activated (Fos-immunoreactive) neurons in the GP and EP express GABA. The present results suggest that inhibitory GABAergic projection neurons within these two nuclei are regulated by inhibitory striatal output and suggests that decreased inhibitory striatal output may contribute to the motor dysfunction observed in patients with Huntington's disease.

The neuronal structure of the globus pallidus in the rabbit--Nissl and Golgi studies.

The studies were carried out on the telencephalons of 12 adult rabbits. Two types of neurons were distinguished: 1. Large neurons (perikarya 18-40 microm), which have from 2 to 6 thick, long primary dendrites. Their perikarya have a polygonal, triangular and fusiform shape. The large neurons in the centre of GP have radiated dendritic trees, whereas the dendritic field of the cells along the borders of GP has an elongated shape. The dendritic arbour is not homogeneous. The dendrites may be covered with spindle-shaped dendritic swellings, bead-like processes, not numerous spines or they may be smooth as well. The dendritic branches form thin, beaded dendritic processes, that arise from any part of the dendritic tree, as well as "complex terminal endings" which have various types of appendages on their terminal portions. An axon emerges from a thick conical elongation either from the cell body or one of the dendritic trunks. These neurons are the most numerous in the investigated material. 2. Small nerve cells have been infrequent in our material. Their cell bodies are rounded or polygonal. From the perikarya there arise 2-4 thin dendritic trunks, which may have irregular swellings and few spines. The dendrites spread out in all directions, making the dendritic field round or oval in shape. Generally most axons of the small cells have not been impregnated. However, a few of them have a thin axon with a conical elongation, which emerges from the cell body and bifurcates into beaded processes.

Neostriatal and globus pallidus stimulation induced inhibitory postsynaptic potentials in entopeduncular neurons in rat brain slice preparations.

Recent anatomical studies revealed that the entopeduncular nucleus of the rat receives GABAergic inputs from both the neostriatum and the globus pallidus. The present study was undertaken to reveal the physiological features of these inputs using the intracellular recording method in rat brain slice preparations. Most of the entopeduncular nucleus neurons generated repetitive firing without spike accommodation with intracellular current stimulation and thus were classified as Type-I. A small number of neurons were classified as Type-II since they generated spikes with pronounced accommodation. Most of the Type-I, but none of Type-II, entopeduncular nucleus neurons exhibited monosynaptic GABAergic inhibitory postsynaptic potentials (IPSPs) after stimulation of the neostriatum and the globus pallidus. Neostriatal stimulation induced long latency IPSPs while pallidal stimulation induced long latency IPSPs compounded with short latency IPSPs. The IPSPs were mediated by GABA(A) receptors. The unitary IPSPs to striatal stimulation were small while those to pallidal stimulation were large in amplitude and able to reset ongoing rhythmic firing. The short latency IPSPs induced by pallidal stimulation reversed at a somatic membrane potential that was a few millivolts more depolarized than the long latency IPSPs, suggesting that the striatal inputs were evoked in more distal portions of the neurons than the pallidal inputs. Repetitive activation of these inputs resulted in a poor amplitude summation but a prolongation of the duration of the IPSPs. The results of the present study indicate that the pallidal projection to the entopeduncular nucleus is physiologically significant and that the neostriatum and the globus pallidus play important roles in controlling the activity of the entopeduncular nucleus, although in different ways.

Distribution of the major gamma-aminobutyric acid(A) receptor subunits in the basal ganglia and associated limbic brain areas of the adult rat.

Within the basal ganglia, gamma-aminobutyric acid (GABA) exerts a fundamental role as neurotransmitter of local circuit and projection neurons. Its fast hyperpolarizing action is mediated through GABA(A) receptors. These ligand-gated chloride channels are assembled from five subunits, which derive from multiple genes. Using immunocytochemistry, we investigated the distribution of 12 major GABA(A) receptor subunits (alpha1-5, beta1-3, gamma1-3, and delta) in the basal ganglia and associated limbic brain areas of the rat. Immunoreactivity for an additional subunit (subunit alpha6) was not observed. The striatum, the nucleus accumbens, and the olfactory tubercle displayed strong, diffuse staining for the subunits alpha2, alpha4, beta3, and delta presumably located on dendrites of the principal medium spiny neurons. Subunit alpha1-, beta2-, and gamma2-immunoreactivities were apparently mostly restricted to interneurons of these areas. In contrast, the globus pallidus, the entopeduncular nucleus, the ventral pallidum, the subthalamic nucleus, and the substantia nigra pars reticulata revealed dense networks of presumable dendrites of resident projection neurons, which were darkly labeled for subunit alpha1-, beta2-, and gamma2-immunoreactivities. The globus pallidus, ventral pallidum, entopeduncular nucleus, and substantia nigra pars reticulata, all areas receiving innervations from the striatum, displayed strong subunit gamma1-immunoreactivity compared to other brain areas. In the substantia nigra pars compacta and in the ventral tegmental area, numerous presumptive dopaminergic neurons were labeled for subunits alpha3, gamma3, and/or delta. This highly heterogeneous distribution of individual GABA(A) receptor subunits suggests the existence of differently assembled, and presumably also functionally different, GABA(A) receptors within individual nuclei of the basal ganglia and associated limbic brain areas.