Dopamine-opiate interaction in the regulation of neostriatal and pallidal neuronal activity as assessed by opioid precursor peptides and glutamate decarboxylase messenger RNA expression.

Neostriatal GABAergic neurons projecting to the globus pallidus synthesize the opioid peptide enkephalin, while those innervating the substantia nigra pars reticulata and the entopeduncular nucleus synthesize dynorphin. The differential control exerted by dopamine on the activity of these two efferent projections concerns also the biosynthesis of these opioid peptides. Using in situ hybridization histochemistry, we investigated the role of opioid co-transmission in the regulation of neostriatal and pallidal activity. The expression of the messenger RNAs encoding glutamate decarboxylase-the biosynthetic enzyme of GABA-and the precursor peptides of enkephalin (preproenkephalin) and dynorphin (preprodynorphin) were measured in rats after a sustained blockade of opioid receptors by naloxone (s.c. implanted osmotic minipump, eight days, 3 mg/kg per h), and/or a subchronic blockade of D2 dopamine receptors by haloperidol (one week, 1.25 mg/kg s.c. twice a day). The density of mu opioid receptors in the neostriatum and globus pallidus was determined by autoradiography. Naloxone treatment resulted in a strong up-regulation of neostriatal and pallidal mu opioid receptors that was not affected by the concurrent administration of haloperidol. Haloperidol alone produced a moderate down-regulation of neostriatal and pallidal micro opioid receptors. Haloperidol strongly stimulated the expression of neostriatal preproenkephalin and preprodynorphin messenger RNAs. This effect was partially attenuated by naloxone, which alone produced moderate increases in preproenkephalin and preprodynorphin messenger RNA levels. In the neostriatum, naloxone did not affect either basal or haloperidol-stimulated glutamate decarboxylase messenger RNA expression. A strong reduction of glutamate decarboxylase messenger RNA expression was detected over pallidal neurons following either naloxone or haloperidol treatment, but concurrent administration of the two antagonists did not result in a further decrease. The amplitude of the variations of mu opioid receptor density and of preproenkephalin and preprodynorphin messenger RNA levels suggests that the regulation of neostriatal and pallidal micro opioid receptors is more susceptible to a direct opioid antagonism, while the biosynthesis of opioid peptides in the neostriatum is more dependent on the dopaminergic transmission. The down-regulation of mu opioid receptors following haloperidol represents probably an adaptive change to increased enkephalin biosynthesis and release. The haloperidol-induced increase in neostriatal preprodynorphin messenger RNA expression might result from an indirect, intermittent stimulation of neostriatal D1 receptors. The haloperidol-induced decrease of pallidal glutamate decarboxylase messenger RNA expression suggests, in keeping with the current functional model of the basal ganglia, that the activation of the striatopallidal projection produced by the interruption of neostriatal dopaminergic transmission reduces the GABAergic output of the globus pallidus. The reduction of pallidal glutamate decarboxylase messenger RNA expression following opioid receptor blockade indicates an indirect, excitatory influence of enkephalin upon globus pallidus neurons and, consequently, a functional antagonism between the two neuroactive substances (GABA and enkephalin) of the striatopallidal projection in the control of globus pallidus output. Through this antagonism enkephalin could partly attenuate the GABA-mediated effects of a dopaminergic denervation on pallidal neuronal activity.     

Further characterization of preproenkephalin mRNA-containing cells in the rodent globus pallidus

The globus pallidus (external pallidum of primates) is an essential nucleus within basal ganglia circuitry, in part because it receives at least one-half of striatal efferent projections. Neurons of the globus pallidus can be divided into subpopulations based on anatomical, physiological, and chemical features. Globus pallidus neurons project to several structures (the striatum, subthalamic nucleus, entopeduncular nucleus, and substantia nigra pars reticulata), have one of two alternative waveforms (positive/negative versus negative/positive), contain either the calcium binding protein parvalbumin or the neuropeptide precursor preproenkephalin mRNA and show differential immediate early gene responses to dopamine receptor agonists and antagonists. The objective of the present study was to characterize in greater detail the preproenkephalin mRNA-containing pallidal neurons using Sprague-Dawley rats. In situ hybridization for preproenkephalin mRNA was combined with immunocytochemical detection of: (i) the neuron-specific nuclear protein, NeuN, (ii) FluoroGold-labeled pallidostriatal and pallidosubthalamic cells, or (iii) Fos induced by either systemic combined D1-class/D2-class dopamine receptor agonists or a D2-class receptor antagonist. These experiments demonstrated that a substantial population (42%) of globus pallidus neurons contains preproenkephalin mRNA, and that globus pallidus neurons retrogradely labeled after FluoroGold injections into the striatum are more frequently preproenkephalinergic, compared to the population of pallidosubthalamic neurons. Furthermore, systemic administration of a D2 receptor antagonist, eticlopride, induced Fos immunoreactivity predominantly in globus pallidus neurons expressing preproenkephalin mRNA, while combined administration of D1 and D2 receptor agonists induced Fos predominantly in pallidal neurons lacking preproenkephalin mRNA.These results support the conclusion that preproenkephalin mRNA identifies one of the two major subpopulations of pallidal neurons. This preproenkephalin mRNA-expressing pallidal subpopulation preferentially targets the striatum and is more readily activated in its immediate early gene expression by D2 receptor antagonists than by dopamine receptor agonists. This projection provides a pallidal substrate for the dopaminergic regulation of striatal information processing.     

Metabolic activity of excitatory parafascicular and pedunculopontine inputs to the subthalamic nucleus in a rat model of Parkinson's disease

Using a combination of metabolic measurement and retrograde tracing, we show that the neurons in the pedunculopontine nucleus and parafascicular nucleus of the thalamus that project to the subthalamic nucleus are hyperactive after nigrostriatal dopaminergic denervation in rats. In Parkinson's disease, the loss of dopaminergic neurons induces a cascade of functional changes in the basal ganglia circuitry including a hyperactivity of the subthalamic nucleus. This hyperactivity is thought to be due to a diminution of the inhibitory pallidal influence. However, recent studies have suggested that other cerebral structures are involved in the subthalamic neuronal hyperactivity. This study was undertaken to identify these cerebral structures. Neurons projecting to the subthalamic nucleus were identified by retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, injected into the subthalamic nucleus of rats with 6-hydroxydopamine unilateral lesion of the substantia nigra pars compacta and sham-lesioned animals. Metabolic activity was determined in the same neurons using in situ hybridization for the first subunit of cytochrome oxidase messenger RNA, a metabolic marker, and image analysis. Horseradish peroxidase-labeled neurons were found in the globus pallidus, parafascicular and pedunculopontine nucleus and sometimes in raphe nuclei and the substantia nigra pars compacta. Measurement of metabolic activity was performed for the globus pallidus, the pedunculopontine and parafascicular nuclei. The expression level of the first subunit of cytochrome oxidase messenger RNA in neurons projecting to the subthalamic nucleus was 62% higher in parafascicular neurons and 123% higher in pedunculopontine neurons in 6-hydroxydopamine-lesioned rats, compared to sham-lesioned animals. An increase was also observed in the globus pallidus, but did not reach significance.Our results suggest that hyperactivity of subthalamic neurons could be due, at least in part, to an increase of excitatory input arising from the pedunculopontine and parafascicular nuclei. These data also suggest that the latter structures may play an important role in the physiopathology of Parkinson's disease.     

Separate neuronal populations of the rat globus pallidus projecting to the subthalamic nucleus, auditory cortex and pedunculopontine tegmental area.

The topographic arrangement of globus pallidus neurons sending axons to the subthalamic nucleus, auditory cortex and pedunculopontine tegmental nucleus was studied in the rat using retrograde fluorescent tracers. Neurons projecting to the subthalamic nucleus were localized in the rostral part of the globus pallidus, while neurons projecting to the auditory cortex and to the pedunculopontine tegmental nucleus were located in the caudal part. The two populations of pallidocortical and pallidotegmental neurons were also distributed in a separate manner within the caudal globus pallidus. The former neurons were large and located more ventromedially, whereas the latter were medium-sized and located more dorsolaterally. Using a retrograde fluorescent tracing technique combined with choline acetyltransferase immunofluorescence histochemistry, it was found that a vast majority of pallidocortical neurons expressed choline acetyltransferase immunoreactivity, and that pallidotegmental neurons rarely exhibited choline acetyltransferase immunoreactivity. A method of retrograde tracing with wheatgerm agglutinin conjugated with horseradish peroxidase associated to immunohistochemistry for glutamate decarboxylase confirmed the GABAergic nature of the pallidotegmental pathway. The present study revealed the independent nature of the globus pallidus neurons projecting to the subthalamic nucleus, auditory cortex and pedunculopontine tegmental nucleus. Within this cellular arrangement, the presence of functionally distinct neuronal populations at the caudal pallidal level was also identified, with large cholinergic cells innervating the neocortex and medium-sized GABAergic cells "feeding" the mesencephalic tegmentum.     

A horseradish peroxidase study of afferent connections of the globus pallidus in Macaca mulatta

Summary The high tonic discharge rates of globus pallidus neurons in awake monkeys suggest that these neurons may receive some potent excitatory input. Because most current electrophysiological evidence suggests that the major described pallidal afferent systems from the neostriatum are primarily inhibitory, we used retrograde transport of horseradish peroxidase (HRP) to identify possible additional sources of pallidal afferent fibers. The appropriate location was determined before HRP injection by mapping the characteristic high frequency discharge of single pallidal units in awake animals. In animals with injections confined to the internal pallidal segment, retrograde label was seen in neurons of the pedunculopontine nucleus, dorsal raphe nucleus, substantia nigra, caudate, putamen, subthalamic nucleus, parafascicular nucleus, zona incerta, medial and lateral subthalamic tegmentum, parabrachial nuclei, and locus coeruleus. An injection involving the external pallidal segment and the putamen as well resulted in additional labeling of cells in centromedian nucleus, pulvinar, and the ventromedial thalamus.Abbreviations AC anterior commissure - CG central grey - CM centromedian nucleus - CN caudate nucleus - DM dorsomedial nucleus - DR dorsal raphe nucleus - DSCP decussation of superior cerebellar peduncle - GPe globus pallidus, external segment - GPi globus pallidus, internal segment - LC locus coeruleus - LL lateral lemniscus - MG medial geniculate nucleus - ML medial lemniscus - NVI abducens nucleus - OT optic tract - Pbl lateral parabrachial nucleus - Pbm medial parabrachial nucleus - Pf parafascicular nucleus - PPN pedunculopontine nucleus - PuO oral pulvinar nucleus - RN red nucleus - SCP superior cerebellar peduncle - SI substantia innominata - SNc substantia nigra, pars compacta - SNr substantia nigra, pars reticulata - STN subthalamic nucleus - TMT mamillothalamic tract - VA ventral anterior nucleus - VLc ventral lateral nucleus, pars caudalis - VLm ventral lateral nucleus, pars medialis - VLo ventral lateral nucleus, pars oralis - VPI ventral posterior inferior nucleus - VPM ventral posterior medial nucleus - VPLc ventral posterior lateral nucleus, pars caudalis - ZI zona incerta     

Dopaminergic innervation of the rat globus pallidus characterized by microdialysis and immunohistochemistry

The present study examined the dopaminergic innervation of the rat globus pallidus by in vivo microdialysis and immunohistochemistry in more detail. Using tyrosine hydroxylase immunohistochemistry, two classes of dopaminergic fibers were distinguished morphologically in the globus pallidus. Unilateral infusion of 6-hydroxydopamine into the substantia nigra produced a loss of dopaminergic fiber density in the globus pallidus which was correlated with the nigral extent of the lesion. These findings are in line with the notion that a degenerative loss of nigral dopaminergic cell bodies might also affect the dopamine input of extrastriatal structures such as the globus pallidus. Using in vivo microdialysis, we tested whether dopamine measured in the globus pallidus is of neuronal origin. Perfusion of tetrodotoxin induced a strong and transient decrease of pallidal dopamine. The tetrodotoxin-sensitivity of pallidal dopamine demonstrates the functional significance of the nigropallidal dopaminergic innervation.     

Projections of the substantia nigra, ventral tegmental area and amygdala to the pallidum in dog brain

Using the method based on HRP retrograde axonal transport organization of projections of substantia nigra, tegmental ventral field and amygdala on pallidum was studied. Neuronal fibres from all dopaminergic portions of substantia nigra and tegmental ventral field were found to project on both structures of dog dorsal pallidum (globus pallidus and entopeduncular nucleus). Ventral pallidum receives projectional axons only from neurons of basal nucleus of amygdala and tegmental ventral field.     

GABAergic control of rat substantia nigra dopaminergic neurons: role of globus pallidus and substantia nigra pars reticulata

Dopaminergic neurons in vivo fire spontaneously in three distinct patterns or modes. It has previously been shown that the firing pattern of substantia nigra dopaminergic neurons can be differentially modulated by local application of GABA(A) and GABA(B) receptor antagonists. The GABA(A) antagonists, bicuculline or picrotoxin, greatly increase burst firing in dopaminergic neurons whereas GABA(B) antagonists cause a modest shift away from burst firing towards pacemaker-like firing. The three principal GABAergic inputs to nigral dopaminergic neurons arise from striatum, globus pallidus and from the axon collaterals of nigral pars reticulata projection neurons, each of which appear to act in vivo primarily on GABA(A) receptors (see preceding paper). In this study we attempted to determine on which afferent pathway(s) GABA(A) antagonists were acting to cause burst firing. Substantia nigra dopaminergic neurons were studied by single unit extracellular recordings in urethane anesthetized rats during pharmacologically induced inhibition and excitation of globus pallidus. Muscimol-induced inhibition of pallidal neurons produced an increase in the regularity of firing of nigral dopaminergic neurons together with a slight decrease in firing rate. Bicuculline-induced excitation of globus pallidus neurons produced a marked increase in burst firing together with a modest increase in firing rate. These changes in firing rate were in the opposite direction to what would be expected for a monosynaptic GABAergic pallidonigral input. Examination of the response of pars reticulata GABAergic neurons to similar manipulations of globus pallidus revealed that the firing rates of these neurons were much more sensitive to changes in globus pallidus neuron firing rate than dopaminergic neurons and that they responded in the opposite direction. Pallidal inhibition produced a dramatic increase in the firing rate of pars reticulata GABAergic neurons while pallidal excitation suppressed the spontaneous activity of pars reticulata GABAergic neurons. These data suggest that globus pallidus exerts significant control over the firing rate and pattern of substantia nigra dopaminergic neurons through a disynaptic pathway involving nigral pars reticulata GABAergic neurons and that at least one important way in which local application of bicuculline induces burst firing of dopaminergic neurons is by disinhibition of this tonic inhibitory input.     

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.     

Direct projections from the central amygdaloid nucleus to the globus pallidus and substantia nigra in the cat.

Employing both anterograde and retrograde axonal tracing, we investigated direct projections from the central amygdaloid nucleus to the basal ganglia in the cat. The anterograde axonal tracing of Phaseolus vulgaris-leucoagglutinin revealed that projection fibers from the central amygdaloid nucleus to the basal ganglia ended in the globus pallidus (the feline homolog to the external segment of the globus pallidus of primates) and substantia nigra. The amygdalopallidal fibers terminated chiefly in the medial most part of the globus pallidus at its caudal level. The amygdalonigral fibers terminated densely in the substantia nigra pars lateralis, and moderately in the dorsolateral part of the substantia nigra pars reticulata; none of them were found to end in the substantia nigra pars compacta. Both of the amygdalopallidal and amygdalonigral projections were ipsilateral. These neuronal connections were confirmed by retrograde axonal tracing of cholera toxin B subunit in the second set of the experiments: The cells of origin of the amygdalopallidal and amygdalonigral projections were located predominantly in the lateral part of the central amygdaloid nucleus, and additionally in the intercalated cell islands of the amygdala. Most of them were of small bipolar or multipolar type. The cells projecting to the globus pallidus were preferentially distributed at the rostral levels of the central nucleus and intercalated cell islands of the amygdaloid complex, while those projecting to the substantia nigra were mainly located at the caudal levels of these amygdaloid subdivisions. In the third set of the experiments, sequential double-antigen immunofluorescence histochemistry for transported cholera toxin B subunit and horseradish peroxidase showed that some single neurons in the lateral part of the central amygdaloid nucleus, particularly at its middle level, issued axon collaterals to both the globus pallidus and substantia nigra pars lateralis. The results of the present study indicate that the central amygdaloid nucleus sends projection fibers to the globus pallidus and substantia nigra possibly to exert a limbic influence upon forebrain motor mechanisms.     

Striatal, pallidal, and pars reticulata evoked inhibition of nigrostriatal dopaminergic neurons is mediated by GABA(A) receptors in vivo

Dopaminergic neurons express both GABA(A) and GABA(B) receptors and GABAergic inputs play a significant role in the afferent modulation of these neurons. Electrical stimulation of GABAergic pathways originating in neostriatum, globus pallidus or substantia nigra pars reticulata produces inhibition of dopaminergic neurons in vivo. Despite a number of prior studies, the identity of the GABAergic receptor subtype(s) mediating the inhibition evoked by electrical stimulation of neostriatum, globus pallidus, or the axon collaterals of the projection neurons from substantia nigra pars reticulata in vivo remain uncertain. Single-unit extracellular recordings were obtained from substantia nigra dopaminergic neurons in urethane anesthetized rats. The effects of local pressure application of the selective GABA(A) antagonists, bicuculline and picrotoxin, and the GABA(B) antagonists, saclofen and CGP-55845A, on the inhibition of dopaminergic neurons elicited by single-pulse electrical stimulation of striatum, globus pallidus, and the thalamic axon terminals of the substantia nigra pars reticulata projection neurons were recorded in vivo. Striatal, pallidal, and thalamic induced inhibition of dopaminergic neurons was always attenuated or completely abolished by local application of the GABA(A) antagonists. In contrast, the GABA(B) antagonists, saclofen or CGP-55845A, did not block or attenuate the stimulus-induced inhibition and at times even increased the magnitude and/or duration of the evoked inhibition. Train stimulation of globus pallidus and striatum also produced an inhibition of firing in dopaminergic neurons of longer duration. However this inhibition was largely insensitive to either GABA(A) or GABA(B) antagonists although the GABA(A) antagonists consistently blocked the early portion of the inhibitory period indicating the presence of a GABA(A) component. These data demonstrate that dopaminergic neurons of the substantia nigra pars compacta are inhibited by electrical stimulation of striatum, globus pallidus, and the projection neurons of substantia nigra pars reticulata in vivo. This inhibition appears to be mediated via the GABA(A) receptor subtype, and all three GABAergic afferents studied appear to possess inhibitory presynaptic GABA(B) autoreceptors that are active under physiological conditions in vivo.     

Striatal axons to the globus pallidus, entopeduncular nucleus and substantia nigra come mainly from separate cell populations in cat

The extent to which individual striatal neurons send collaterals to the globus pallidus, entopeduncular nucleus and substantia nigra in the cat brain was determined by double-retrograde tracing with rhodamine fluorescent latex microspheres in combination with either horseradish peroxidase or the fluorescent nuclear dye Diamidino Yellow. In each case, two of the three target nuclei were injected, each with a different tracer, until all three possible combinations of two had been obtained several times. In all cases in which the tracer encroaches upon a striatal target, there are cells labeled in the striatum of a size and shape that is consistent with the observation that they mainly belong to the category of medium striatal cells. Since the striatal projections to the globus pallidus, entopeduncular nucleus and substantia nigra are each topographically organized, the zones of cell-labeling within the striatum vary depending upon the portion of the target nucleus involved by the deposit. Thus, in many cases the fields of striatal cells containing one label overlap only slightly with those in which cells containing the other label occur. In other cases, however, there is extensive overlap of the striatal zones containing cells marked with either tracer. In all cases, very few double-labeled cells can be found, even where hundreds of cells labeled with either tracer are freshly intermingled. Doubly labeled cells occur somewhat more frequently in those cases where the tracers are placed in the entopeduncular nucleus and substantia nigra than in those with the other two combinations, suggesting that striatal axons branch more often to the entopeduncular nucleus and substantia nigra than to the globus pallidus and nigra or globus pallidus and entopeduncular nucleus. These findings confirm, that, in the cat as in the primate, the striatal axons to the substantia nigra arise from cells that are largely separate from the striatopallidal population, and further show that the axons to the globus pallidus and entopeduncular nucleus also emanate mainly from different cells.     

Fetal striatal neurons grafted into the ibotenate lesioned adult striatum: efferent projections and synaptic contacts in the host globus pallidus

The efferent projections to the host brain from intrastriatal grafts have been examined at the ultrastructural level. Cell suspensions of E14 rat fetal striatal tissue were implanted into the ibotenic acid lesioned caudate-putamen of adult rats. After survival times of at least five months, the anterograde neuronal tracer Phaseolus vulgaris leucoagglutinin was injected into the grafts. Consistent with our previous light microscopical analysis, anterogradely labelled fibres could be followed from the grafts into the host globus pallidus along the normal trajectory of the striatopallidal pathway in the internal capsule, and a few fibres also reached the entopeduncular nucleus and the substantia nigra. In the electron microscope, the graft-derived efferents were seen to be myelinated as they projected caudally along the fibre bundles of the internal capsule, and they were thus similar to the neostriatal projection in normal control animals. In the host globus pallidus, the graft-derived fibres ramified into a terminal network forming morphologically normal synaptic contacts with neuronal elements in the host globus pallidus. A total of 118 synaptic contacts were identified, all of them formed symmetric membrane specializations. The major postsynaptic targets were dendritic shafts (90%), with only a few contacts with spines or small shafts (8%) and perikarya (2%). The morphology of the synaptic contacts and their distribution on different postsynaptic targets were similar to that which was found in the globus pallidus after tracer-injections into the caudate-putamen of normal control animals. The results show that grafted fetal striatal neurons can grow along the myelinated territory of the internal capsule to reinstate a seemingly normal synaptic input to the previously denervated neurons in the host globus pallidus.     

Topography and functional role of dopaminergic projections from the ventral mesencephalic tegmentum to the ventral pallidum.

A dopaminergic projection from the ventral tegmental area to the ventral pallidum was identified in the rat using anterograde tract tracing and combined retrograde tracing-immunocytochemistry. The projection was found to be topographically organized such that fibers innervating the ventromedial ventral pallidum arose from neurons located along the midline nuclei of the ventral mesencephalon, including the nucleus interfascicularis and nucleus linearis caudalis. Ventral tegmental neurons situated more laterally, in the nucleus parabrachialis pigmentosus and nucleus paranigralis, projected to the ventromedial and dorsolateral ventral pallidum. The substantia nigra did not supply a major contribution to this projection. The proportion of ventral tegmental area dopaminergic neurons projecting to the ventral pallidum ranged from approximately 30% to 60%. The functional significance of the projection is indicated since intra-ventral pallidum microinjections of dopamine elicited a dose-dependent increase in locomotor activity. Furthermore, whereas pretreatment of the ventral pallidum with the GABAA agonist muscimol has been shown to attenuate opioid-induced locomotor activity elicited from the ventral pallidum, it did not attenuate the dopamine-induced motor response. Thus, while mu-opioids in the ventral pallidum may presynaptically regulate GABAergic efferents from the nucleus accumbens, it appears that the dopaminergic input directly influences the ventral pallidal output neuron which is involved in locomotion.     

Pallidal control of substantia nigra dopaminergic neuron firing pattern and its relation to extracellular neostriatal dopamine levels

The firing patterns of dopaminergic neurons in vivo are strongly modulated by afferent input. The principal GABAergic inputs to the dopaminergic neurons of the substantia nigra originate from neurons of the neostriatum, globus pallidus and substantia nigra pars reticulata. It has previously been shown that the firing pattern of nigral dopaminergic neurons can be manipulated by pharmacologically induced excitation or inhibition of the globus pallidus with relatively little effect on firing rate. We used this technique to explore the relation between the firing pattern of dopaminergic neurons and extracellular dopamine levels in the neostriatum in vivo. Specifically, we tested whether an increase in burst firing in dopaminergic neurons produced by increased pallidal activity led to increased extracellular dopamine levels in the neostriatum. Single unit extracellular recording combined with simultaneous microdialysis was used to measure the firing rates and patterns of dopaminergic neurons and extracellular striatal dopamine levels, respectively, during bicuculline-induced excitation of the globus pallidus. Pallidal excitation resulted in a marked increase in burst firing in dopaminergic neurons along with only a slight increase in firing rate, but produced a significant elevation (approximately 45%) in neostriatal dopamine levels. These data suggest that afferent-induced burst firing in dopaminergic neurons leads to an increase in extracellular dopamine levels in the neostriatum when compared with less bursty patterns with similar overall firing rates.     

Efferent fibers of the subthalamic nucleus in the monkey. A comparison of the efferent projections of the subthalamic nucleus,substantia nigra and globus pallidus

A study was made to determine the efferent projections of the subthalamic nucleus in the monkey. Because of the impossibility of producing lesions in this nucleus, not involving adjacent structures, lesions were produced by different stereotaxic approaches. Comparisons were made with degeneration resulting from localized lesions in substantia nigra and globus pallidus. Degeneration resulting from these lesions was studied in transverse and sagittal sections stained by the Nauta-Gygax method. Efferent fibers from the subthalamic nucleus pass through the internal capsule into the medial pallidal segment; a few fibers are distributed to the lateral pallidum. Some subthalamic efferent fibers pass to the contralateral globus pallidus via the dorsal supraoptic decussation, but none projection to the thalamus. Nigral efferent fibers project to parts of the ventral anterior (VAmc) and ventral lateral (VLm) thalamic nuclei. The medial pallidal segment gives fibers to: (1) ventral anterior (VA), ventral lateral (VLo) and centromedian (CM) thalamic nuclei, and (2) the pedunculopontine nucleus. The lateral pallidal segment projects exclusively to the subthalamic nucleus. Thalamic projections of the substania nigra and globus pallidus are distinctive. Subthalamic projections to the globus pallidus are more profuse than those of the substantia nigra. The following hypothesis is presented: Subthalamic dyskinesia, due to lesions in the subthalamic nucleus, is a consequence of removal of inhibitory influences acting upon the medial segment of the globus pallidus.     

Nigrostriatal lesion and dopamine agonists affect firing patterns of rodent entopeduncular nucleus neurons

Altered activity of the entopeduncular nucleus, the rodent homologue of the globus pallidus internal segment in primates, is thought to mediate behavioral consequences of midbrain dopamine depletion in rodents. Few studies, however, have examined dopaminergic modulation of spiking activity in this nucleus. This study characterizes changes in entopeduncular neuronal activity after nigrostriatal dopaminergic lesion and the effects of systemic treatment with selective D(1) (SKF 38393) and D(2) (quinpirole) agonists in lesioned rats. Extracellular single-unit recordings were performed in awake immobilized rats, either in neurologically intact animals (n = 42) or in animals that had received unilateral 6-hydroxydopamine infusion into the medial forebrain bundle several weeks previously (n = 35). Nigrostriatal lesion altered baseline activity of entopeduncular neurons in several ways. Interspike interval distributions had significantly decreased modes and significantly increased coefficient of variation, skewness and kurtosis; yet interspike interval mean (the inverse of firing rate) was not affected. Also, spectral analysis of autocorrelograms indicated that lesion significantly reduced the incidence of regular-spiking neurons and increased the incidence of neurons with 4-18 Hz oscillations. Dopamine agonist treatment reversed some lesion-induced effects: quinpirole reversed changes in interspike interval distribution mode and coefficient of variation, while combined quinpirole and SKF 38393 blocked the appearance of 4-18 Hz oscillations. However, no agonist treatment normalized all aspects of entopeduncular activity. Additionally, inhibition of firing rates by D(1) or combined D(1)/D(2) receptor activation indicated that dopamine agonists affected the overall level of entopeduncular activity in a manner similar to that found in the substantia nigra pars reticulata and globus pallidus internal segment after dopamine neuron lesion. These data demonstrate that lesion of the nigrostriatal tract leads to modifications of several aspects of firing pattern in the rodent entopeduncular nucleus and so expand on similar findings in the rodent substantia nigra pars reticulata and in the globus pallidus internal segment in humans and nonhuman primates. The results support the view that dysfunction in the basal ganglia after midbrain dopamine neuron loss relates more consistently to abnormal activity patterns than to net changes in firing rate in the basal ganglia output nuclei, while overall decreases in firing rate in these structures may play a more important role in adverse motor reactions to dopamine agonist treatments.     

Nucleus tegmenti pedunculopontinus: Efferent connections with special reference to the basal ganglia,studied in the rat by anterograde and retrograde transport of horseradish peroxidase

The efferent connections of the brain stem nucleus tegmenti pedunculopontinus were studied in the rat using the techniques of anterograde and retrograde transport of the enzyme horseradish peroxidase, laying particular emphasis on that part of pedunculopontinus which receives direct descending projections from the basal ganglia and related nuclei. In a preliminary series of experiments horseradish peroxidase was injected into either the entopeduncular nucleus or the subthalamic nucleus and, following anterograde transport of enzyme, terminal labelling was identified in nucleus tegmenti pedunculopontinus, surrounding the brachium conjunctivum in the caudal mesencephalon.In a subsequent series of experiments, horseradish peroxidase was injected into that region of nucleus tegmenti pedunculopontinus which receives entopeduncular and subthalamic efferents and its efferent projections were studied by anterograde transport of the enzyme. The results indicate that nucleus tegmenti pedunculopontinus gives rise to widely distributed efferent projections which terminate rostrally in mesencephalic, diencephalic and telencephalic structures and caudally in the pontine tegmentum. In the mesencephalon, terminal labelling was found in the pars compacta of the ipsilateral substantia nigra and sometimes in the adjoining ventral tegmental area. Labelling was also found in the ipsilateral half of the periaqueductal grey. In the diencephalon terminal labelling occurred bilaterally in the subthalamic nucleus and ipsilaterally in the intralaminar nuclei of the thalamus. Further rostrally, terminal labelling was particularly evident in the ipsilateral pallidal complex, especially in the caudal two-thirds of the entopeduncular nucleus and the ventral half of the caudal third of the globus pallidus. Caudal to pedunculopontine injection sites dense labelling was observed in the reticular formation of the pontine tegmentum.In a final series of experiments, confirmation of apparent pedunculopontine efferent projections was sought using the retrograde transport of horseradish peroxidase. Enzyme was injected into sites possibly receiving pedunculopontine efferents and the peribrachial area of the brain stem was examined for retrograde cell labelling. In this way, pedunculopontine projections were confirmed to the globus pallidus, entopeduncular nucleus, subthalamic nucleus, substantia nigra, parafascicular nucleus and pontine reticular formation. Injections into the globus pallidus or subthalamic nucleus gave rise to retrograde cell labelling bilaterally in pedunculopontinus. In addition, retrograde transport studies alone demonstrated projections from pedunculopontinus to the cerebral cortex and to the spinal cord.It is concluded that the nucleus tegmenti pedunculopontinus has reciprocal relationships with parts of the basal ganglia and some functionally related nuclei (in particular, the pallidal complex, subthalamic nucleus and substantia nigra). These connections support the view that nucleus tegmenti pedunculopontinus is likely to be involved in the subcortical regulation and mediation of basal ganglia influences upon the lower motor system. This suggests a potential role for pedunculopontine afferent and efferent pathways in the pathophysiology of basal ganglia related disorders of movement.     

Connectivity of striatal grafts implanted into the ibotenic acid-lesioned striatum--III. Efferent projecting graft neurons and their relation to host afferents within the grafts

Efferent projections of intrastriatally implanted striatal neurons have been studied using a combination of anterograde and retrograde axonal tracers. Adult rats subjected to a unilateral ibotenic acid lesion of the head of the caudate putamen received cell suspension grafts obtained from 14 15-day-old striatal primordia. Three and a half to 20 months after transplantation the rats received either intratransplant injections of the anterograde axonal tracer Phaseolus vulgaris leucoagglutinin or injections of fluorescent retrograde tracers. Fluoro-Gold and rhodamine-labelled latex beads, into the host globus pallidus and substantia nigra. Injections of Phaseolus vulgaris leucoagglutinin located entirely within the grafts labelled axons that ramified extensively within the tissue itself, as well as axons that extended caudally, across the graft host border, along the myelinated fascicles of the internal capsule to arborize in the medial parts of the host globus pallidus. A few axons also reached the entopeduncular nucleus. Injections of Fluoro-Gold into the host globus pallidus labelled large numbers of graft neurons, which had a prominent patchy distribution and were most abundant in the caudal portions of the grafts. Clear retrograde labelling was also seen after injection of Fluoro-Gold or rhodamine beads into the host substantia nigra, although the number of labelled graft neurons was 30-50 times lower than that seen after pallidal injections. Combined injections of Fluoro-Gold into the pallidus and rhodamine beads into the nigra showed that the vast majority of cells labelled from the nigra were also labelled by Fluoro-Gold from the pallidus. In some of the grafted and Fluoro-Gold-injected animals, the fetal donor tissue had been labelled with [3H]thymidine prior to transplantation. Many examples of neurons labelled with both [3H]thymidine and Fluoro-Gold were found after tracer injections into the host globus pallidus, and double-labelled neurons were identified also after Fluoro-Gold injections into the host substantia nigra. In several animals retrograde tracing was combined with labelling of host dopaminergic afferents (by tyrosine hydroxylase immunohistochemistry) and cortical afferents (by injections of Phaseolus vulgaris leucoagglutinin into the host frontal cortex). Comparison of adjacent sections revealed a striking overlap between the patches of Fluoro-Gold-labelled graft neurons (labelled from the host pallidum) and the dense patches of tyrosine hydroxylase-positive terminals. In addition, many of the Fluoro-Gold-labelled cell patches received a high density of cortical afferents labelled by Phaseolus vulgaris leucoagglutinin.(ABSTRACT TRUNCATED AT 400 WORDS)     

Chemical organization of projection neurons in the rat accumbens nucleus and olfactory tubercle

Projection neurons in the ventral striatum, the accumbens nucleus and olfactory tubercle, were examined by combining the retrograde tracing method and immunocytochemistry with antibodies against C-terminals of the preprodynorphin (PPD), preproenkephalin (PPE), preprotachykinin A (PPTA) and preprotachykinin B (PPTB). When the retrograde tracer was injected into the ventral pallidum, about 60% and 40% of retrogradely labeled neurons in the accumbens nucleus were immunoreactive for PPD and PPE, respectively. In contrast, all accumbens nucleus neurons projecting to the ventral mesencephalic regions including the substantia nigra and ventral tegmental area were immunopositive for PPD but not for PPE. Although no olfactory tubercle neurons projected fibers to the mesencephalic regions, 60% and 40% of olfactory tubercle neurons projecting to the ventrolateral portion of the ventral pallidum were immunoreactive for PPD and PPE, respectively, as were the accumbens nucleus neurons. About 70% of accumbens nucleus and olfactory tubercle neurons projecting to the ventral pallidum and all accumbens nucleus neurons projecting to the ventral mesencephalic regions showed PPTA immunoreactivity. A small population (2-12%) of accumbens neurons projecting to the ventral pallidum and mesencephalic regions displayed immunoreactivity for PPTB. Compared with the dorsal striatopallidal projection neurons that were reported to mostly express PPE, it was characteristic of the ventral striatum that only the smaller population (about 40%) of ventral striatopallidal projection neurons expressed PPE. This suggests that the ventral striatopallidal projection system is less specialized than the dorsal striatopallidal system in terms of peptide production, or that the ventral pallidum should be compared with a combined region of the globus pallidus and entopeduncular nucleus in the dorsal system.