I. Liste

Sprouting of the serotonergic afferents into striatum after selective lesion of the dopaminergic system by MPTP in adult mice

Neonatal destruction of the nigrostrial dopaminergic (DA) system with 6-hydroxydopamine leads to serotonergic (5-HT) hyperinnervation of the striatum. However, it is not clear whether this occurs in adult animals. We investigated whether serotonergic sprouting occurs in adult mice subjected to bilateral lesion of the DA system by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The effects of the lesion were evaluated using a new rotarod test and immunohistochemistry. One hundred days after the last MPTP lesion, there was a clear bilateral serotonergic hyperinnervation throughout the striatum. Additionally, those mice showing the highest reductions in striatal tyrosine hydroxylase (TH) immunoreactivity and in rotarod performance showed the highest density of serotonergic innervation (116% increase). The functional consequences of this process in Parkinsons disease and secondary parkinsonism remain to be clarified.

Treadmill running induces striatal Fos expression via NMDA glutamate and dopamine receptors.

Abstract Several non-physiological stimuli (i.e. pharmacological or electrical stimuli) have been shown to induce Fos expression in striatal neurons. In this work, striatal Fos (i.e. Fos-like) expression was studied after physiological stimulation, i.e. motor activity (treadmill running at 36 m/min for 20 min). In rats killed 2 h after the treadmill session, Fos expression was observed in the medial region of the rostral and central striatum, and in the dorsal region of the caudal striatum. Fos expression was prevented by pretreatment with the non-competitive N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK-801 (0.1 mg/kg) or the D1 dopamine receptor antagonist SCH-23390 (0.1 mg/kg), but not by pretreatment with the D2 receptor antagonist eticlopride (0.5 mg/kg). Thirty-six hours after 6-hydroxydopamine lesion, a considerable reduction in treadmill-induced Fos expression was observed in both sides; however, Fos expression in the lesioned striatum was higher than in the contralateral intact striatum. Several weeks after unilateral 6-hydroxydopamine lesion of the nigrostriatal system, treadmill-induced Fos expression was significantly, but not totally, reduced in the lesioned striatum. Corticostriatal deafferentation also led to considerable reduction in treadmill-induced Fos expression. The present results indicate that exercise induces striatal Fos expression and that, under physiological stimulation, concurrent activation of D1 and NMDA receptors is necessary for such expression to occur. Reduction of Fos expression is practically absolute after acute blockage of these receptors, but not after lesions, possibly due partially to compensatory changes.

Time course of striatal changes induced by 6-hydroxydopamine lesion of the nigrostriatal pathway, as studied by combined evaluation of rotational behaviour and striatal Fos expression

Changes taking place after unilateral 6-hydroxydopamine lesion of the dopaminergic nigrostriatal system have been studied by performing spontaneous, amphetamine-induced and apomorphine-induced rotational behaviour testing and tyrosine hydroxylase (TH) and Fos protein immunohistochemistry in the same rats. Apomorphine at a low dosage (0.25 mg/kg) induced contraversive rotation and supersensitive striatal Fos expression that were detected 24–48 h post-lesion and gradually increased in magnitude. Twenty-four hours after lesion, both high (5 mg/kg) and low doses (0.5 mg/kg) of D-amphetamine induced contraversive rotation and intense striatal Fos activation on the denervated side; however, only the higher dose induced Fos on the normal side. Two, 3 and 4 days after lesion, 0.5 mg/kg amphetamine induced contraversive rotation, but 5 mg/kg induced transitory contraversive rotation which switched to ipsiversive. In the normal striatum, only high doses of amphetamine induced Fos, but Fos induction in the denervated striatum was similar with both doses: areas showing severely decreased TH immunoreactivity still showed considerable Fos immunoreactivity, and some areas still showing TH immunoreactivity had higher Fos density than in the normal side. Seven and 14 days after lesion the loss of TH immunoreactivity and apomorphine-induced supersensitive Fos expression were more evenly distributed, and amphetamine induced only ipsiversive rotation and a low density of Fos-positive nuclei in the denervated striatum. These results indicate that the severe and progressive loss of dopaminergic terminals is counteracted by an early and rapidly progressing dopamine supersensitivity, together with a higher susceptibility to drug-induced dopamine release. This explains the apparently paradoxical contraversive rotation induced by amphetamine during the first week post-lesion. However, experiments involving successive drug injections indicated that only the first amphetamine injection releases dopamine from the lesioned terminals.

Cortical stimulation induces Fos expression in striatal neurons via NMDA glutamate and dopamine receptors

Cortical electrical stimulation has been shown to induce dense and widespread Fos expression throughout the ipsilateral and contralateral striatum. This raises interest for studying the mechanisms underlying the regulation of striatal neuron activity by cortical afferents, and for elucidating the interactions with other systems. However, the receptors mediating cortical-stimulation-induced expression of Fos in striatal neurons have not been identified. This was studied in the work reported here by stimulating the cortex after administration of glutamate or dopamine receptor antagonists, or after 6-hydroxydopamine (6-OHDA) lesion of the nigrostriatal dopaminergic system. Pretreatment with the non-competitive N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK-801 led to a marked reduction in the stimulation-induced density of Fos-immunoreactive nuclei in both the medial (about 80% reduction) and lateral (about 50-60% reduction) striatum. Preadministration of the D1-selective dopamine antagonist SCH-23390 alone or in combination with the D2-selective dopamine antagonist eticlopride led to a reduction in the stimulation-induced density of Fos-positive nuclei of about 60-65% in the lateral striatum, but no significant change in the medial region. The effects of 6-OHDA lesion were less pronounced, and the stimulation-induced density of Fos-immunoreactive nuclei decreased by only about 25% in the lateral region. These results indicate that both dopamine and NMDA glutamate receptors are involved in the induction of Fos by cortical stimulation, and support the hypothesis that cortex-dopamine interactions in the lateral striatum may be functionally different from those in the medial striatum.

INTERACTION BETWEEN THE SEROTONERGIC, DOPAMINERGIC, AND GLUTAMATERGIC SYSTEMS IN FENFLURAMINE-INDUCED FOS EXPRESSION IN STRIATAL NEURONS

Fenfluramine (FE) is a halogenated amphetamine derivative used in the treatment of obesity and thought to induce serotonin (5‐HT) release from nerve terminals and to reduce re‐uptake. However, other pathways may also be involved. In this work, the effects of FE on the major striatal afferent systems, and the possible interactions of these systems in FE‐induced striatal expression of Fos, were studied by lesion of the serotonergic and/or dopaminergic system and administration of NMDA glutamate (MK‐801) or D1 dopamine (SCH‐23390) receptor antagonists. Both the D1 and NMDA receptor antagonists suppressed Fos expression in response to FE almost entirely. FE‐induced Fos expression was also dramatically reduced 24 h after 6‐hydroxydopamine (6‐OHDA) lesion of the dopaminergic system. However, the reduction was not so marked after chronic 6‐OHDA lesion, probably due to compensatory changes. Chronic (5,7‐dihydroxytryptamine injection, 4 weeks before) or acute (p‐chlorophenylalanine injection) lesion of the serotonergic system led to a marked reduction in Fos expression in response to FE (decrease of about 50%). After simultaneous chronic lesion of both serotonergic and dopaminergic systems, a considerable number of Fos‐positive nuclei were still observed (decrease of about 70% in the dorsal and dorsomedial regions). The FE‐induced expression of Fos was almost totally suppressed (decrease of about 95% in the dorsal and dorsomedial regions) after simultaneous acute lesion. Our results indicate that FE‐induced striatal expression of Fos is due in large measure to DA release and dopaminergic stimulation of D1 receptors. However, concurrent stimulation of NMDA glutamate receptors also appears to be essential, and 5‐HT release (although not indispensable) doubles striatal Fos expression. Synapse 28:71–82, 1998.

Time course of striatal, pallidal and thalamic α1, α2 and β2/3 GABAA receptor subunit changes induced by unilateral 6-OHDA lesion of the nigrostriatal pathway

Abstract Immunocytochemical techniques were used to investigate the distribution and abundance of GABAA receptor subunits (α1, α2 and β2/3) in the brains of unilaterally 6-OHDA-lesioned rats. Three and 7 days after lesion, the α2-subunit was significantly more abundant in the lesion-ipsilateral striatum than in the lesion-contralateral striatum; by 4 weeks after lesion, however, no significant between-side differences were observed. Three and 7 days after lesion, the α1-subunit was significantly less abundant in the lesion-ipsilateral globus pallidus than in the lesion-contralateral side; again, this difference disappeared within 4 weeks of lesion. Similarly, α1 was initially less abundant in several relay thalamic nuclei on the lesioned side while α2 was initially more abundant in intralaminar thalamic nuclei on the lesioned side. There were no significant between-side changes for the β2/3-subunits. Comparison of non-lesioned and 6-OHDA-lesioned rats revealed significant differences in brain areas which also showed differences on comparison of the lesioned and non-lesioned sides of 6-OHDA-lesioned rats. These results suggest that there is an early adaptation to the lesion, achieved through changes in GABAA receptor abundance. That some of these changes are no longer apparent after 4 weeks is due not only to partial reversion of the changes in the lesioned side but also to compensatory changes in the non-lesioned side.

Locomotor-activity-induced changes in striatal levels of preprotachykinin and preproenkephalin mRNA. Regulation by the dopaminergic and glutamatergic systems

The mechanisms by which dopaminergic and glutamatergic inputs interact to regulate striatal neuropeptide expression during physiological motor activity are poorly understood. In this work, striatal expression of preprotachykinin (PPT) and preproenkephalin (PPE) mRNA was studied by in situ hybridization in rats killed 2 h after treadmill running (36 m/min for 20 min). Treadmill running induced a significant increase in the levels of both PPT (60% increase) and PPE (90% increase) mRNA in the striatum of normal rats. The increase in the level of PPT mRNA was blocked in rats previously subjected to nigrostriatal deafferentation (i.e., 6-hydroxydopamine lesion) or pretreated with D1-receptor antagonist SCH-23390 (0.1 mg/kg), the D2-receptor antagonist eticlopride (0.5 mg/kg), or the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK-801 (0.1 mg/kg). The running-induced increase in the level of PPE mRNA was blocked in rats pretreated with SCH-23390 or MK-801. Rats subjected to nigrostriatal deafferentation or pretreated with eticlopride showed an increase in PPE mRNA levels (around 150% and 40% increase, respectively), that was enhanced by running (around 230% and 160% increase, respectively). These results suggest that locomotor activity increases, in a NMDA receptor dependent fashion, the excitatory influence of the corticostriatal glutamatergic system on the two populations of striatal projection neurons, as reflected by increases in the levels of PPT and PPE mRNA. The results obtained after dopamine depletion or injection of dopamine receptor antagonists suggest that a concomitant increase in dopamine release may enhance PPT mRNA level in striatonigral neurons via D1 receptors, and reduce PPE mRNA level in striatopallidal neurons via D2 receptors. Additionally, levels of dopamine and glutamate may be regulated by other complex indirect mechanisms.

GABAA receptorα1-subunit-immunopositive neurons in the rat striatum

Abstract The morphological characteristics and distribution of striatal neurons expressing the GABA A receptor α-subunit were investigated following immunolabeling with an at-subunit-specific antibody. α 1 -Immunopositive striatal neurons are relatively scarce. Those located in the dorsal striatum are small and have rounded somata and numerous dendrite varicosities. α 1 -Immunoreactive neurons with these characteristics were likewise observed in the ventral striatum, which also contained large pyramidal neurons with smooth dendrites and polygonal neurons with spiny dendrites. A fourth neuron type (oval neurons with dendrites oriented in bipolar fashion) was found in the lateral striatum. All four neuron types were observed in the rostral, central and caudal striatum. Double-labeling experiments using an antibody specific for DARPP-32 (a cell-level indicator of dopamine D1-like receptors) and the α 1 antibody showed a complete lack of colocalization of these two markers in striatal neurons.

Fenfluramine-induced increase in preproenkephalin mRNA levels in the striatum: Interaction between the serotonergic, glutamatergic, and dopaminergic systems

Fenfluramine (FE) is a halogenated amphetamine derivative that has been used in the treatment of obesity. It has been suggested that the effects of FE on the striatum are mediated by serotonergic mechanisms. However, several major afferent systems may be involved, and administration of FE may be useful to study interactions between these systems. In this work, the effects of FE on striatopallidal neurons and the possible involvement of the major striatal afferent systems were studied in rats by determination of FE‐induced changes in striatal levels of preproenkephalin (PPE) mRNA using in situ hybridization. Injection of FE induced a significant increase (60%) in striatal levels of PPE mRNA. This increase was blocked by pretreatment with the D1 dopamine receptor antagonist SCH‐23390 or with the NMDA glutamate receptor antagonist MK‐801, or by lesion of the serotonergic system with 5,7‐dihydroxytryptamine or p‐chlorophenylalanine. In 6‐hydroxydopamine lesioned rats, the lesion‐induced increase in PPE mRNA levels was not affected by injection of FE, but was reduced by simultaneous serotonergic deafferentation. The results suggest that the serotonergic, glutamatergic, and dopaminergic system interact to increase striatal PPE mRNA levels after FE administration. Synapse 35:182–191, 2000.

Intrathalamic Implants of GABA-Releasing Polymer Matrices Reduce Motor Impairments in Rats with Excitotoxically Lesioned Striata

Current models of basal ganglia disorders suggest that the choreoathetosis is the end result of reduced GABAergic inhibition of the motor thalamus. GABA-releasing polymer matrices or control matrices without GABA were implanted either unilaterally or bilaterally in the vicinity of the ventromedial thalamic nucleus of normal rats and of rats with unilateral or bilateral excitotoxic striatal lesions (rat model of Huntingtons disease), to study the effects of these GABA-releasing matrices on amphetamine-induced rotational behavior (unilateral implants in unilaterally lesioned rats) and on overnight spontaneous locomotor activity (bilateral implants in bilaterally lesioned rats). Unilateral implants led to a reduction (about 25%) in motor asymmetry; the response was transitory, probably because of the exhaustion of GABA release by the matrix. Some rats showed a more marked and permanent reduction of motor asymmetry, but this was probably due to lesion of the ventromedial nucleus or its thalamocortical projection. Bilateral implants of GABA-releasing matrices (but not control matrices) led to a marked (about 65%) but again transitory reduction in the locomotor hyperactivity induced by bilateral striatal lesion. These results suggest that implantation of a GABA-releasing source may be an effective alternative to intrathalamic fetal-tissue grafts or lesions as an experimental approach to the treatment of hyperkinetic movement disorders.