Striatal Output Neurons Research Articles

Relationship between EEG potentials and intracellular activity of striatal and cortico-striatal neurons: an in vivo study under different anesthetics.

The functions of the basal ganglia are achieved through excitation of striatal output neurons (SONs) by converging cortical glutamergic afferents. We assessed the relationship between different patterns of activity in cortico-striatal (C-S) cells and the electrical behavior of SONs in vivo. Intracellular activities of rat C-S neurons in the orofacial motor cortex and of SONs, located in the projection field of this cortical region, were recorded under different anesthetics, which generate various temporal patterns of cortical activity. A surface electroencephalogram (EEG) of the orofacial motor cortex was simultaneously performed with intracellular recordings and EEG waves were used as correlates of a coherent synaptic activity in cortical neurons. Under barbiturate anesthesia C-S neurons showed rhythmic (5--7 Hz) supra-threshold depolarizations in phase with large amplitude EEG waves. The correlative activity of SONs was characterized by large amplitude oscillation-like synaptic depolarizations that could trigger action potentials. Under ketamine-xylazine anesthesia C-S neurons exhibited a step-like behavior consisting of depolarizing plateaus (up states), leading to multiple spike discharges, interrupted by hyperpolarizing periods (down states). The related activity of SONs was step-like membrane potential fluctuations with firing confined to the early part of the striatal up state. In C-S neurons and SONs up states coincided with slow recurrent EEG waves (approximately 1 Hz). Finally, under neurolept-analgesia an apparently disorganized EEG activity was associated with a lack of rhythmic discharge in C-S neurons. This uncorrelated activity in C-S neurons resulted in an absence of spontaneous firing as well as of large amplitude synaptic depolarizations in SONs. In the present study we demonstrate that SONs shape their input-output relationship by filtering out uncorrelated synaptic activity and that a minimal synchronization in the cortico-striatal afferents is required to produce significant synaptic depolarization in SONs.

Alterations in GAD67, dynorphin and enkephalin mRNA in striatal output neurons following priming in the 6-OHDA model of Parkinson's disease.

In the 6-hydroxydopamine (6-OHDA) rat model of Parkinson's disease, administration of a dopaminergic agonist sensitizes rats to a subsequent administration of dopaminergic drugs given days apart (priming). In situ hybridization was used to evaluate changes on striatal gene expression of rats primed three days previously with either L-dopa, SKF38393 or quinpirole. Double labeling was used to identify the neuronal population in which such alterations occurred. GAD67 and enkephalin mRNA were increased by the lesion whereas dynorphin mRNA was decreased as compared to the intact striatum. Priming with L-dopa and SKF38393 significantly increased GAD67 mRNA in the lesioned striatum and reversed dynorphin mRNA reduction, as compared to drug-naive rats, whereas quinpirole failed to produce any effect. Enkephalin mRNA was not affected by priming. Results suggest that 6-OHDA lesion-induced adaptive changes on striatal gene expression are modified by priming. Priming brings striatal output neurons to a higher level of activity, which may explain the sensitized behavioral response observed following a dopaminergic agonist challenge. These changes are in relation to the different types of dopamine agonists utilized and suggest that modifications in gene expression induced by priming might be predictive of the dyskinetic potential of a drug.

Role of a striatal slowly inactivating potassium current in short-term facilitation of corticostriatal inputs: a computer simulation study.

Striatal output neurons (SONs) integrate glutamatergic synaptic inputs originating from the cerebral cortex. In vivo electrophysiological data have shown that a prior depolarization of SONs induced a short-term (</=1 sec) increase in their membrane excitability, which facilitated the ability of corticostriatal synaptic potentials to induce firing. Here we propose, using a computational model of SONs, that the use-dependent, short-term increase in the responsiveness of SONs mainly results from the slow kinetics of a voltage-dependent, slowly inactivating potassium A-current. This mechanism confers on SONs a form of intrinsic short-term memory that optimizes the synaptic input-output relationship as a function of their past activation.

The ciliary neurotrophic factor and its receptor, CNTFRα

Ciliary neurotrophic factor (CNTF) is expressed in glial cells within the central and peripheral nervous systems. CNTF stimulates gene expression, cell survival or differentiation in a variety of neuronal cell types such as sensory, sympathetic, ciliary and motor neurons. In addition, effects of CNTF on oligodendrocytes as well as denervated and intact skeletal muscle have been documented. CNTF itself lacks a classical signal peptide sequence of a secreted protein, but is thought to convey its cytoprotective effects after release from adult glial cells by some mechanism induced by injury. Interestingly, mice that are homozygous for an inactivated CNTF gene develop normally and initially thrive. Only later in adulthood do they exhibit a mild loss of motor neurons with resulting muscle weakness, leading to the suggestion that CNTF is not essential for neural development, but instead acts in response to injury or other stresses. The CNTF receptor complex is most closely related to, and shares subunits with the receptor complexes for interleukin-6 and leukemia inhibitory factor. The specificity conferring α subunit of the CNTF complex (CNTFRα), is extremely well conserved across species, and has a distribution localized predominantly to the nervous system and skeletal muscle. CNTFRα lacks a conventional transmembrane domain and is thought to be anchored to the cell membrane by a glycosyl-phosphatidylinositol linkage. Mice lacking CNTFRα die perinatally, perhaps indicating the existence of a second developmentally important CNTF-like ligand. Signal transduction by CNTF requires that it bind first to CNTFRα, permitting the recruitment of gp130 and LIFRβ, forming a tripartite receptor complex. CNTF-induced heterodimerization of the β receptor subunits leads to tyrosine phosphorylation (through constitutively associated JAKs), and the activated receptor provides docking sites for SH2-containing signaling molecules, such as STAT proteins. Activated STATs dimerize and translocate to the nucleus to bind specific DNA sequences, resulting in enhanced transcription of responsive genes. The neuroprotective effects of CNTF have been demonstrated in a number of in vitro cell models as well as in vivo in mutant mouse strains which exhibit motor neuron degeneration. Intracerebral administration of CNTF and CNTF analogs has also been shown to protect striatal output neurons in rodent and primate models of Huntington's disease. Treatment of humans and animals with CNTF is also known to induce weight loss characterized by a preferential loss of body fat. When administered systemically, CNTF activates downstream signaling molecules such as STAT-3 in areas of the hypothalamus which regulate food intake. In addition to its neuronal actions, CNTF and analogs have been shown to act on non-neuronal cells such as glia, hepatocytes, skeletal muscle, embryonic stem cells and bone marrow stromal cells.

Brain-derived neurotrophic factor-mediated protection of striatal neurons in an excitotoxic rat model of Huntington's disease, as demonstrated by adenoviral gene transfer.

Huntington's disease (HD) is a genetic disorder leading to the degeneration of striatal GABA-ergic output neurons. No treatment is currently available for this devastating disorder, although several neurotrophic factors, including brain-derived neurotrophic factor (BDNF), have been shown to be beneficial for striatal neuron survival. We analyzed the effect of adenovirus-mediated transfer of the BDNF gene in a model of HD. Using a stereological procedure, three groups of rats were given an intrastriatal injection of adenovirus encoding BDNF, beta-galactosidase, or sham surgery. Two weeks after treatment, the animals were lesioned with quinolinic acid (QUIN), a toxin that induces striatal neuron death by an excitotoxic process. One month after the lesion, histological study revealed that striatal neurons were protected only in rats treated with the BDNF adenovirus. Volume measurements showed that the QUIN-induced lesions were 55% smaller in the BDNF adenovirus-treated group than in the beta-galactosidase adenovirus-treated group (p < 0.05), and the sham-treated group (p < 0.05). To determine the survival of striatal GABA-ergic output neurons after the QUIN-induced lesion, we immunostained brain sections with DARPP-32, an antibody specific for striatal output neurons. Prior treatment with the BDNF adenovirus resulted in a cell survival of 64%, whereas that after beta-galactosidase treatment was 46% (p < 0.05), showing that the BDNF adenovirus protected the striatal neurons. These results indicate that transfer of the BDNF gene is of therapeutic value for Huntington's disease.

In vivo induction of striatal long-term potentiation by low-frequency stimulation of the cerebral cortex

Both long-term depression and long-term potentiation have been described at corticostriatal synapses. These long-lasting changes in synaptic strength were classically induced by high-frequency (100 Hz) electrical stimulations of cortical afferents. The purpose of the present study was to test the ability of corticostriatal connections to express use-dependent modifications after cortical stimulation applied at the frequency of synchronization of corticostriatal inputs observed in our in vivo preparation, i.e. the barbiturate-anesthetized rat. For this study we used an identified monosynaptic corticostriatal pathway, between the orofacial motor cortex and its target region in the striatum. Intracellular recording of striatal output neurons showed spontaneous large-amplitude oscillation-like depolarizations exhibiting a strong periodicity with a narrow frequency band at 5 Hz. Using the focal electroencephalogram of the cortical region projecting to the recorded cells, we found that membrane potential oscillations in striatal neurons were in phase with episodes of spontaneous cortical spindle waves. To determine directly the pattern of activity of corticostriatal neurons, we performed intracellular recordings of electrophysiologically identified corticostriatal neurons simultaneously with the corresponding surface electroencephalogram. We found that corticostriatal cells ( n=7) exhibited periods of spontaneous 5-Hz discharges in phase with the cortical spindle waves. Therefore, we have tested the effect of repetitive cortical stimulations at this low frequency (5 Hz, 500–1000 pulses) on the corticostriatal synaptic efficacy. In 62% of cases (eight of 13 neurons tested), this conditioning was able to produce long-term potentiation in the corticostriatal synaptic efficacy. The mean increase of excitatory postsynaptic potential amplitude ranged from 13.3% to 172% (mean=67.3%, n=8). These results provide additional support for physiological long-term potentiation at corticostriatal connections. Furthermore, this study demonstrates that corticostriatal long-term potentiation can be induced by synchronization at low frequency of cortical afferents. Our data support the concept that the striatal output neuron may operate as a coincidence detector of converging cortical information.

Thalamic inputs to striatal interneurons in monkeys: synaptic organization and co-localization of calcium binding proteins

Recent studies indicate that extrinsic inputs from sensorimotor regions of the cerebral cortex and the centromedian intralaminar thalamic nucleus terminate preferentially upon specific subpopulations of striatal output neurons in monkeys. The objective of the present study was to verify whether this specificity of innervation also characterizes the synaptic interactions between thalamic inputs from the centromedian nucleus and the four major populations of striatal interneurons. This was achieved by double labelling techniques at the electron microscope level, combining the anterograde transport of biotinylated-dextran amine with the immunostaining for specific markers of striatal interneurons (somatostatin, parvalbumin, choline acetyltransferase and calretinin). Injections of biotinylated-dextran amine in the centromedian nucleus led to dense bands of anterograde labelling which, in double immunostained sections, largely overlapped with the four populations of interneurons in the post-commissural region of the putamen. In the electron microscope, biotinylated-dextran amine-containing terminals formed asymmetric axo-dendritic synapses with somatostatin-, parvalbumin-, and choline acetyltransferase-containing elements. However, synapses between anterogradely labelled terminals and calretinin-positive neurons were not found. In sections processed to localize biotinylated-dextran amine and parvalbumin or calretinin, double-labelled terminals (biotinylated-dextran amine/parvalbumin and biotinylated-dextran amine/calretinin), morphologically similar to thalamostriatal boutons, were found in the striatum indicating that calcium binding proteins may be expressed by thalamostriatal neurons. To test this possibility, we combined the retrograde transport of lectin-conjugated horseradish peroxidase from the putamen with parvalbumin and calretinin immunostaining and found that, indeed, most of the retrogradely labelled cells in the centromedian nucleus displayed parvalbumin and calretinin immunoreactivity. Moreover, co-localization studies revealed that calretinin and parvalbumin co-exist in single neurons of the centromedian nucleus. In conclusion, striatal interneurons immunoreactive for somatostatin, parvalbumin and choline acetyltransferase, but not those containing calretinin, receive strong inputs from the centromedian nucleus in monkeys. Moreover, our findings indicate that parvalbumin and calretinin co-exist in individual thalamostriatal neurons. In combination with our previous data, these results suggest that thalamic information may be conveyed to striatal projection neurons both, directly via excitatory synaptic inputs, or indirectly via striatal interneurons. The relative importance of those direct and indirect thalamic influences upon the activity of striatal output neurons remains to be established.

Selective effects of partial and severe lesions of the serotonergic systems on Met-enkephalin and substance P neurons in rat basal ganglia

The effects of partial (80%) vs. severe (> 95%) depletion of serotonin (5-HT) on peptide expression in basal ganglia were examined using immunocytochemical and in situ hybridization histochemical approaches. Topographical analysis of the changes in Met-enkephalin (Met-enk) and substance P (SP) levels were performed on the rat striatum, globus pallidus and substantia nigra 3 weeks after injecting 3 µl (partial lesion) or 6 µ l (severe lesion) 5,7-dihydroxytryptamine (6.6 µ g/μl) into the anterior raphe nuclei. Both kinds of lesion led to significant increases (39–42%) in Met-enk immunoreactivity in the striatum; a corresponding increase (21%) was detected in the globus pallidus only after severe 5-HT depletion. Only the severe lesion increased the SP immunoreactivity in the striatum (32%) and substantia nigra (26%). Neither striatal preproenkephalin nor preprotachykinin levels showed significant differences with the control values. These results suggest that the neuronal accumulation of Met-enk or SP may be attributable to post-transcriptional events, such as a blockade of the peptide release, and that 5-HT may, thus, exert a facilitatory influence on the striatal output neurons. The results obtained after partial lesion indicate a preferential sensitivity of striatal Met-enk vs. SP containing terminals to the 5-HT denervation. These differences are illustrated in selective regional changes in peptide labeling. These data point to some balance exerted by the serotonergic and dopaminergic inputs on these neuronal populations.

Glial cell line-derived neurotrophic factor attenuates the excitotoxin-induced behavioral and neurochemical deficits in a rodent model of Huntington's disease

The present study determined the effects of intraventricularly administered glial cell line-derived neurotrophic factor on the behavioral and neurochemical sequelae of unilateral excitotoxic lesions of the striatum. Distinct asymmetrical rotational behavior in response to peripheral administration of amphetamine (5 mg/kg) was noted one and two weeks following injections of quinolinic acid (200 nmol) into two sites in the left striatum. In rats given a single intraventricular injection of glial cell line-derived neurotrophic factor (10-1000 μg) 30 min before the toxin, amphetamine-induced rotational behavior was significantly attenuated. Analysis of Nissl-stained coronal sections showed marked neuronal loss in the striatum ipsilateral to the quinolinic acid injections, which was at least partially prevented by glial cell line-derived neurotrophic factor. D 1 and D 2 dopamine binding sites in the striatum, the majority of which are localized to subpopulations of GABAergic neurons, were decreased to a similar extent by quinolinic acid. Moreover, the reduction was attenuated by glial cell line-derived neurotrophic factor treatment to a similar degree, suggesting that the two subpopulations of GABAergic striatal output neurons are equally vulnerable to excitotoxic damage. Concomitant changes in neurotransmitter function as a result of the lesion were also observed: [ 3H]GABA uptake into striatal target tissues (globus pallidus and substantia nigra) was considerably reduced in the lesioned compared to the contralateral unlesioned tissues, as were [ 3H]choline and [ 3H]dopamine uptake into striatal synaptosomes. Similarly, striatal choline acetyltransferase activity was decreased by the lesion. Decrements in neuropeptide levels of similar magnitude were evident ipsilateral to the lesion; substance P, met-enkephalin and dynorphin A contents in the globus pallidus and substantia nigra were significantly reduced. Striatal somatostatin and neuropeptide Y levels were not altered. All of the neurochemical deficits induced by striatal quinolinic acid lesions were attenuated by intraventicular delivery of glial cell line-derived neurotrophic factor. Continuous intraventricular infusion of this trophic factor (10 μg/day) over a two-week period did not afford notable improvement compared to the single injection of 10 μg. In contrast, continuous infusion of brain-derived neurotrophic factor (10 μg/day) directly into the striatum did not affect any of the neurochemical parameters studied. However, neurotrophin-3 (10 μg/day) delivery into the striatum significantly increased [ 3H]GABA uptake, but only modestly affected [ 3H]choline uptake. The results indicate that glial cell line-derived neurotrophic factor counteracts neuronal damage induced by a striatal excitotoxic insult and support its potential use as a treatment for central nervous system disorders that may be a consequence of excitotoxic processes, such as Huntington's disease.

Homozygotes and heterozygotes for ciliary neurotrophic factor null alleles do not show earlier onset of Huntington's disease.

The CAG repeat number on Huntington's disease (HD) chromosomes accounts for about 60% of the variance in the age at onset of HD. However, distinct familial factors may also influence the age at onset. HD is associated with loss of medium-sized GABA-ergic striatal output neurons. Intracerebral administration of human ciliary neurotrophic factor (CNTF) protects striatal output neurons in excitotoxic rodent and primate models of HD induced by intrastriatal quinolinic acid injection. We have examined the effect of a common null mutation in the human CNTF gene on the age of onset of HD using a multiple regression approach that takes into account the CAG repeat number on HD chromosomes. We failed to detect an earlier onset of HD in nine homozygotes and 71 heterozygotes with this CNTF mutation compared with 203 homozygotes with wild-type alleles.

In vivo activity-dependent plasticity at cortico-striatal connections: evidence for physiological long-term potentiation.

The purpose of the present study was to investigate in vivo the activity-dependent plasticity of glutamatergic cortico-striatal synapses. Electrical stimuli were applied in the facial motor cortex and intracellular recordings were performed in the ipsilateral striatal projection field of this cortical area. Recorded cells exhibited the typical intrinsic membrane properties of striatal output neurons and were identified morphologically as medium spiny type I neurons. Subthreshold cortical tetanization produced either short-term posttetanic potentiation or short-term depression of cortically-evoked excitatory postsynaptic potentials. When coupled with a postsynaptic depolarization leading the membrane potential to a suprathreshold level, the tetanus induced long-term potentiation (LTP) of cortico-striatal synaptic transmission. Induction of striatal LTP was prevented by intracellular injection of a calcium chelator suggesting that this synaptic plasticity involves an increase of postsynaptic free calcium concentration. Contrasting with previous in vitro studies our findings demonstrate that LTP constitutes the normal form of use-dependent plasticity at cortico-striatal synapses. Since excitation of striatal neurons produces a disinhibition of premotor networks, LTP at excitatory striatal inputs should favor the initiation of movements and therefore could be critical for the functions of basal ganglia in motor learning.

Chronic administration of dl-allyl-glycine into the neostriatum, disorganises the firing modes of the nigral dopaminergic neurons in the rat

Nigral dopaminergic (DA) neurons have been reported to fire according to three modes: very regular (pacemaker 42%) irregular (random 46%) and bursty (12%). The switch from simple spiking mode (pacemaker or random) to bursty firing would correspond to an increase in DA release necessary for the performance of a new motor act. As nigral DA cells are impinged upon by a high percentage of GABAergic afferents we blocked striatal GABAergic output neurons by chronic administration into the neostriatum of allyl-glycine, a glutamic acid decarboxylase (GAD) inhibitor. After treatment, rats presented hyperkinesia and hypertonia on the injected side and recordings showed a drastic change in the percentage distribution of nigral DA cell discharge patterns; 85% were `random', 12% `pacemaker' and 3% bursty. Such a disturbance, by impeding adapted DA release, may account for the hyperkinetic and dystonic disorders observed.

Protective effect of encapsulated cells producing neurotrophic factor CNTF in a monkey model of Huntington's disease.

Huntington's disease is a genetic disorder that results from degeneration of striatal neurons, particularly those containing GABA (gamma-aminobutyric acid). There is no effective treatment for preventing or slowing this neuronal degeneration. Ciliary neurotrophic factor (CNTF) is a trophic factor for striatal neurons and therefore a potential therapeutic agent for Huntington's disease. Here we evaluate CNTF as a neuroprotective agent in a nonhuman primate model of Huntington's disease. We gave cynomolgus monkeys intrastriatal implants of polymer-encapsulated baby hamster kidney fibroblasts that had been genetically modified to secrete human CNTF. One week later, monkeys received unilateral injections of quinolinic acid into the previously implanted striatum to reproduce the neuropathology seen in Huntington's disease. Human CNTF was found to exert a neuroprotective effect on several populations of striatal cells, including GABAergic, cholinergic and diaphorase-positive neurons which were all destined to die following administration of quinolinic acid. Human CNTF also prevented the retrograde atrophy of layer V neurons in motor cortex and exerted a significant protective effect on the GABAergic innervation of the two important target fields of the striatal output neurons (the globus pallidus and pars reticulata of the substantia nigra). Our results show that human CNTF has a trophic influence on degenerating striatal neurons as well as on critical non-striatal regions such as the cerebral cortex, supporting the idea that human CNTF may help to prevent the degeneration of vulnerable striatal populations and cortical-striatal basal ganglia circuits in Huntington's disease.

Dose-related effects of continuous levodopa infusion in rats with unilateral lesions of the substantia nigra

Preclinical studies in rats have demonstrated markedly different effects of intermittent and continuous levodopa administration on many biochemical and functional parameters yet the dose regimens employed have not been fully evaluated. In this study, rats with unilateral 6-hydroxydopamine nigral lesions were administered levodopa (0–1200 mg/kg/day) and benserazide (25 mg/kg/day) subcutaneously via osmotic minipump and studied 20–22 h later for rotational behavior, striatal dopamine concentration, and regional cerebral glucose utilization (RCGU). Levodopa infusion at 100 mg/kg/day resulted in minimal rotation and minimal striatal dopamine replacement but did increase RCGU in the subthalamic nucleus and decrease RCGU in the lateral habenula, consistent with a selective inhibition of the striatopallidal GABAergic (indirect striatal output) pathway. Levodopa infusion at 100 mg/kg/day did not significantly increase RCGU in the entopeduncular nucleus (EP) and substantia nigra pars reticulata (SNr), as does the acute injection of levodopa (25–50 mg/kg), indicating that this levodopa dose elicits only part of the spectrum of metabolic effects elicited by acute levodopa injection. Higher doses of levodopa (400–1200 mg/kg/day) resulted in moderate rates of rotation, dose-dependent increases in striatal dopamine, and increased RCGU in the EP and SNr, consistent with activation of the striatonigral GABAergic (direct striatal output) pathway. In the EP and SNr, the two major output nuclei of the basal ganglia, levodopa infusion at 800 and 1200 mg/kg/day reproduced the metabolic effects elicited by acute injection of levodopa. These results demonstrate, for the first time, dose-dependent effects of levodopa on distinct populations of striatal output neurons which may be relevant to the pathogenesis of levodopa-induced dyskinesias in Parkinson's disease. The minimal dopamine replacement and partial functional effects elicited by levodopa infusion at 100 mg/kg/day indicate the need for caution in the interpretation of prior studies of continuous levodopa infusion which employed this dose.

Glutamatergic regulation of striatal peptide gene expression in rats.

The mRNA levels encoding enkephalin and substance P were measured in the rat striatum following cortical ablation, blockade of N-methyl-D-aspartate (NMDA) receptors or inhibition of glutamate release by lamotrigine. Unilateral ablation of the cerebral cortex resulted in a decrease of substance P mRNA levels particularly in the rostral dorsolateral and dorsomedial striatum ipsilateral to the lesion. There was a similar trend for a reduction in levels of enkephalin mRNA. Continuous, intrastriatal infusion of the competitive NMDA receptor antagonist, 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid, (CPP, 0.12 and 1.2microg/day) decreased both enkephalin mRNA and substance P mRNA in dose-dependent manner evenly throughout the striatum adjacent to the infusion site. Following subchronic administration of the presumed glutamate release inhibitor, lamotrigine (5 and 20mg/kg IP) there was no significant alterations in either enkephalin mRNA or substance P mRNA levels in the striatum. Both enkephalin mRNA and substance P mRNA expression in the rat striatum appear tonically stimulated through postsynaptic NMDA receptor mediated mechanisms. This contrasts with differential dopaminergic modulation of peptides in striatal output neurons.

Selective putaminal excitotoxic lesions in non-human primates model the movement disorder of Huntington disease.

While dyskinetic movements have been reported in primates with unilateral excitotoxic lesions following stimulation by dopaminergic agonists, the presence and intensity of the dyskinetic syndromes have varied extensively with size and location of lesion. With the intent of producing a more reliable behavioral model of Huntington disease, anatomically-defined lesions of limited size were produced by magnetic resonance imaging-guided stereotaxic injection of quinolinic acid in specific regions within the caudate and putamen of rhesus monkeys. The location and extent of the lesions were verified by magnetic resonance imaging as well as quantitative positron emission tomography imaging with the dopamine D1 specific receptor ligand SCH 39166 as a marker for striatal output neurons. The quality, frequency and duration of dyskinetic movements were assessed and quantified before and after administration of 0.5 mg/kg apomorphine in multiple test sessions over several months. Selective unilateral lesions in the posterior putamen, but not in the anterior putamen or the head of the caudate, produced marked dystonia and dyskinesia after apomorphine administration. While combined unilateral lesions of the caudate and posterior putamen produced dyskinesia similar to selective posterior putaminal lesions, combined unilateral lesions of the anterior and posterior putamen did not elicit dyskenesia. On the basis of these results, one monkey received a bilateral selective lesion in the posterior putamen. This animal remained healthy and exhibited marked spontaneous Huntington-like chorea spontaneously in the first 48 h after lesioning and persistent apomorphine-induced dyskinesia thereafter. We conclude that bilateral selective excitotoxic lesions of the posterior putamen provide an improved model of the movement disorder of Huntington disease.

GABAergic modulation of striatal peptide expression in rats and the alterations induced by dopamine antagonist treatment

GABAergic modulation of enkephalin, substance P and glutamic acid decarboxylase (GAD67) gene expression and the alterations induced by dopamine receptor blockade were studied in the rat striatum. Following subchronic treatment with the GABA-A agonist muscimol, the GABA-B agonist baclofen or the GABA transaminase inhibitor γ-vinyl GABA there were no significant changes in striatal peptide and GAD67 gene expression. Following repeated administration of the D-2 antagonists, eticlopride and haloperidol, there was an increase in enkephalin and GAD67 mRNA levels and parallel decrease in that of substance P. These were unaffected by co-administration of γ-vinyl GABA. The D-1 antagonist, SCH 23390 administered alone or together with γ-vinyl GABA did not alter peptide or GAD67 mRNA levels. It seems that pharmacological stimulation of GABA receptors has little effect on enkephalin, substance P or GAD67 mRNA expression in striatal output neurons.

Output architecture of the primate putamen

The basal ganglia act through direct and indirect striatopallidal output pathways that have different effects on cortical activity. This division has been proposed to underlie the fundamental distinction between hyperkinetic and hypokinetic movement disorders such as Parkinson's disease and Huntington's disease. Evidence to date does not favor a relationship between this dual organization and the division of the striatum into striosome and matrix compartments. However, the possibility has been raised that the division of striatopallidal output paths reflects a compartmentalization of the matrix itself into clusters of different striatopallidal projection neurons. We directly tested this hypothesis in squirrel monkeys by comparing the distributions of striatal output neurons retrogradely labeled from the two pallidal segments. Striatopallidal neurons labeled by small injections confined to either the external pallidum (GPe) or the internal pallidum (GPi) formed small clusters ("matrisomes") in the matrix compartment of the putamen. However, contrary to previous predictions, labeled matrisomes projecting to GPe were not systematically separated from those projecting to GPi. They could overlap extensively, and within individual matrisomes GPe-projecting neurons and GPi-projecting neurons were extensively intermixed. Double-retrograde labeling analysis in single sections demonstrated that only 2.1 +/- 2.7% of labeled striatal neurons were doubly labeled from both GPe and GPi--a number not significantly different from zero. GPe-projecting and GPi-projecting neurons in the putamen also differed sharply in their expression of enkephalin-like immunoreactivity: 71.3 +/- 7.6% of the neurons labeled by GPe injections were enkephalin positive, in contrast to 10.0 +/- 3.6% of the neurons labeled by GPi injections. These results suggest that in the primate, populations of striatopallidal output neurons are grouped in clusters in the matrix, but that individual neurons in any given cluster project either to GPe or to GPi. Matrisomal clustering may thus coordinate signals sent into the direct and indirect pathways of the basal ganglia from distributed populations of projection neurons in the striatum.

Striatal c-fos Induction by Cocaine or Apomorphine Occurs Preferentially in Output Neurons Projecting to the Substantia Nigra in the Rat.

Fluorogold or rhodamine-labelled latex beads were injected in the substantia nigra (SN) or the globus pallidus (GP) in order retrogradely to label striatal output neurons that project to the two target structures. Ten days later, striatal c-fos was induced by systemic administration of cocaine (five normal rats; 25 mg/kg cocaine i.p. 2 h before killing) or apomorphine (five unilaterally dopamine-denervated rats; 0.25 mg/kg apomorphine s. c. 2 h before killing), and detection of the Fos protein in the striatum was achieved by immunofluorescence. Sections through the caudate-putamen that displayed good labelling from both SN and GP were selected for a quantitative analysis: the number of retrogradely labelled cells that exhibited Fos immunoreactivity, as well as the total number of retrogradely labelled cells located within a grid (0.16 mm2 in size) were counted manually at 25 x magnification. Cocaine induced a proportionally higher c-fos expression in striato-nigral compared to striato-pallidal neurons, whereas apomorphine activated Fos almost exclusively in striato-nigral neurons. The present findings are consistent with the idea that striatal c-fos induction by dopaminergic agents is primarily mediated by an interaction with D1-receptors, which are thought to be selectively localized on neurons projecting to SN.

Dopaminergic transplants normalize amphetamine- and apomorphine-induced Fos expression in the 6-hydroxydopamine-lesioned striatum

Dopamine receptor-mediated Fos protein expression in the striatum has been used to monitor dopamine receptor activation at the cellular level after dopaminergic denervation and reinnervation by fetal nigrat transplants. The pattern of striatal Fos expression after systemic administration of either the dopamine receptor agonist, apomorphine, or the dopamine-releasing agent, amphetamine, was studied in rats which had received cell suspension grafts of fetal ventrat mesencephalic neurons into the striatum after a complete 6-hydroxydopamine lesion of mesostriatal dopaminergic projection. Grafted animals, and normal and lesioned controls were killed 2 h after administration of either d-amphetamine (5 mg/kg, i.p.) or apomorphine (0.25 mg/kg, s.c.). Fos protein was detected immunohistochemically, and the density of Fos-immunoreactive cell nuclei was measured in 12 selected areas of caudate-putamen, nucleus accumbens and globus pallidus by computerized image analysis. Consistent with previous studies, amphetamine induced high Fos expression in the medial and dorsal parts of the intact caudate-putamen and significantly lower expression in the denervated caudate-putamen. A significant difference between lesioned and intact striata was present also in globus pallidus, but not in nucleus accumbens. In grafted rats, amphetamine-induced Fos activation was restored to normal or supranormal levels in the anterior and centrat caudate-putamen (i.e. close to the graft deposits), whereas in the tail of caudate-putamen Fos expression was significantly lower than normal. The side-to-side difference in globus pallidus seen in lesioned rats was no longer present in the grafted animals. Apomorphine led to high Fos activation throughout the dopamine-depleted caudate-putamen, whereas only very few immunopositive cells were observed in the intact caudate-putamen. Also in globus pallidus and nucleus accumbens, a significantly higher number of Fos-immunoreactive cells was detected on the denervated side. In the grafted rats, apomorphine-induced Fos activation was similar to normal in all striatal areas sampled, as well as in the globus pallidus. The graft-induced effect extended over a considerably larger area than that covered by the graft-derived tyrosine hydroxylase-immunoreactive innervation. These findings indicate that fetal ventrat mesencephalic transplants normalize dopamine receptor mediated function in the 6-hydroxydopamine-lesioned caudate-putamen and nucleus accumbens, as well as in a primary target of the striatal output neurons, the globus pallidus. The results support the idea that dopamine released from the grafted neurons, both under baseline conditions and after amphetamine administration, exerts functional effects over a larger volume of the striatum than that reached by the graft-derived fibers.