Organization and plasticity of GABAergic neurons in some extrapyramidal nuclei of the rat

Synaptic reorganisation in the rat striatum after dopaminergic deafferentation: an ultrastructural study using glutamate decarboxylase immunocytochemistry

The ultrastructure of GABAergic and non-GABAergic synapses in the adult rat neostriatum was examined 6-8 months after unilateral removal of the nigrostriatal dopaminergic pathway by 6-hydroxydopamine injection into the medial forebrain bundle. GABAergic profiles were identified by preembedding glutamate decarboxylase (GAD) immunocytochemistry performed on parasagittal vibratome sections. In three representative fields of the striatum, the nature and number of boutons and their postsynaptic partners were determined and the differences between the striata ipsi- and contralateral to the lesion analyzed. The percentage of GAD-immunoreactive boutons was increased from 23% on the intact side to 28% on the lesioned side. In addition, the GABAergic boutons underwent significantly more multiple contacts with several independent postsynaptic profiles, preferentially with dendritic spines. This could reflect a lesion-induced sprouting of local GABAergic axon terminals. On the other hand, although the vast majority of GABAergic boutons underwent synaptic contacts with dendrites (77% vs. 80%), the number of boutons per dendrite or per dendritic circumference remained unchanged. Thus, the higher frequency of GABAergic boutons may simply reflect the loss of the dopaminergic nerve endings, without a heterosynaptic replacement by GABAergic boutons. The deafferentation also induced structural changes of the postsynaptic profiles. Some dendritic spines had a shortened neck; others were completely integrated in the dendrite which now contained a spine apparatus and was contacted by boutons with the features of axospinous synapses. The spine retraction resulted in a quantitative decrease in the number of spines. Analysis of the synaptic curvature revealed that only spines with a flat contact zone were lost. Concurrently, the number of dendrites was increased, of the GAD-containing in particular, suggesting that the denrites of GABAergic interneurons tend to elongate and/or arborize. Taken together, the results of the present study show that the dopaminergic denervation caused a remodeling of the postsynaptic neurons. The relative increase of the number of GABAergic boutons and their synaptic contacts suggests that an altered wiring of the intrinsic GABAergic system contributes to the changes in the striatal output activity.

AMPA/kainic acid glutamate receptor antagonism in the zona incerta dorsal to the subthalamic nucleus inhibits amphetamine-induced stereotypy bur not locomotor activity

The effect of 6,7-dinitroquinoxaline-2,3-dione (DNQX), an alpha-amino-3- hydroxy-5-methyl-4-isoxazole-propionate (AMPA)/kainic acid glutamate receptor antagonist, injected into the zona incerta (ZI) was investigated to determine whether the behavioral responses to systemic amphetamine involve AMPA/kainic acid receptors in this brain region. Rats were injected bilaterally in the ZI with either vehicle or DNQX (1 microgram/0.5 microliter) and immediately given a systemic injection of D-amphetamine (0.5, 1.0 or 10.0 mg/kg, s.c.). Locomotor activity was recorded for 1 h. DNQX did not significantly affect hypermotility stimulated with 0.5 and 1.0 mg/kg amphetamine, but markedly increased the level of locomotor activity elicited by the higher dose, 10 mg/kg. To test the hypothesis that the enhanced locomotor response to high dose amphetamine was due to an inhibition of stereotyped behavior, the effect of DNQX in the ZI on amphetamine and apomorphine-induced stereotypy was investigated. DNQX significantly inhibited stereotypy induced by amphetamine (10 mg/kg) and apomorphine (1 mg/kg), with the onset of inhibition of amphetamine-induced stereotypy corresponding to the onset of enhanced locomotor activity. Ibotenic acid lesions of the ZI produced similar results, having an insignificant effect on locomotor activity stimulated by low dose amphetamine (1 mg/kg) and an attenuation of apomorphine-induced stereotypy which was of a magnitude comparable to that produced by DNQX. Thus, the AMPA/kainic acid subtypes of glutamate receptors in the ZI may be involved in the regulation of motor function mediated via striatal output but not mesolimbically generated locomotor activity.

Methylazoxymethanol (MAM)-induced brain lesion and oral dyskinesia in rats

The effects of methylazoxymethanol (MAM)-induced brain lesions on vacuous chewing movements (VCM) were examined in rats given chronic haloperidol treatment (0.1 or 1 mg/kg/day) for 18 months. At the end of the experiment striatal, pallidal, and nigral activities of glutamate decarboxylase (GAD) were measured. MAM-lesioned rats had an elevated rate of VCMs compared to unlesioned controls. This effect was stable during the whole 18-month experiment. In unlesioned control rats chronic haloperidol produced a gradual increase in VCM rates, but this effect was not further exacerbated in MAM-lesioned animals. After chronic haloperidol treatment with the higher dose (1 mg/kg/day) GAD activity was reduced in substantia nigra (-20%), globus pallidus (-35%), and striatum (-26%) of unlesioned rats. MAM caused a reduction of GAD activity in substantia nigra (-29%) and globus pallidus (-29%). Chronic haloperidol did not influence these effects of MAM-induced lesion. The present results show that a MAM-induced brain lesion, in contrast to cortical ablations, cannot be used to amplify the haloperidol-induced VCM increase or influence the nigral GAD activity in a rat model for tardive dyskinesia.

Cerebral blood flow, glucose utilization, and electrocorticograms following common carotid artery occlusion in gerbils

We designed this study to elucidate the relations between cerebral function and glucose metabolism during the early stage of ischemia. We induced focal cerebral ischemia in 28 gerbils by occluding the common carotid artery. We recorded electrocorticograms in 34 gerbils by positioning bipolar electrodes between the anterior and middle cerebral arteries. We related the electrocorticograms to local cerebral glucose utilization measured with [14C]2-deoxyglucose in half the gerbils. A characteristic pattern (a zone of markedly decreased [14C]2-deoxyglucose uptake surrounded by a narrow band of greatly increased uptake) was observed on the autoradiogram in nine of the 14 experimental gerbils (64%). An electrocorticogram recorded from such a band of increased uptake was characterized by transient suppression of electrical activity followed by partial or complete recovery, and local cerebral blood flow in gerbils showing this electrocorticographic type were variable (15.0-43.3 ml/100 g/min). An electrocorticogram recorded from the ischemic core and inner border of this band, even when [14C]2-deoxyglucose uptake was relatively high, was characterized by the complete disappearance of electrical activity just after occlusion; cerebral blood flow in gerbils that showed this electrocorticographic type were consistently less than 15.0 ml/100 g/min. Our investigation suggests that the transient disappearance of electrocorticographic activity in the periphery of ischemia, which has relatively high residual blood flow, may relate to the heterogeneity of glucose consumption during the early stage of ischemia.

Benzodiazepine receptors in normal human brain, in Parkinson's disease and in progressive supranuclear palsy

Benzodiazepine receptors have been characterized in human brain. They have been localized mainly in the cerebral cortex and a synaptosomal enrichment was observed after brain fractionation by differential centrifugation. Benzodiazepine receptors were studied in Parkinson's disease and in progressive supranuclear palsy (PSP). In both diseases, the [3H]flunitrazepam specific binding was unchanged when compared to control groups (Bmax and KD values) except in the caudate nucleus of parkinsonian patients where an increase of the specific binding was observed. The subcellular distribution profile of benzodiazepine receptors in Parkinson's disease was similar to that of controls. gamma-Aminobutyric acid (GABA) still enhanced the [3H]flunitrazepam-specific binding (increase of binding affinity), indicating that the functional link between GABA and benzodiazepine receptors remained intact in Parkinson's disease. The present results suggest that benzodiazepine receptors in human striatum are localized on neuronal elements which do not degenerate in Parkinson's disease and PSP.

Prenatal ontogeny of the GABAergic system in the rat brain: an immunocytochemical study

Prenatal development of the GABAergic system in the rat brain has been studied using an antiserum to GABA-glutaraldehyde-hemocyanin conjugates, specific for GABAergic neurons. The gamma-aminobutyric acid (GABA) system has been found to differentiate very early relative to other transmitter-identified neurons, such that by embryonic day 13 a well developed fiber network exists in the brainstem, mesencephalon and diencephalon, including a large projection in the posterior commissure and adjacent areas on the surface of the mesencephalon and tectum. Although no cell bodies are visible at this time, it appears that these fibers originate from the caudal brainstem and spinal cord. GABAergic cell bodies begin to appear on embryonic day 14 in the lateral cortical anlage. By embryonic day 16, they are also visible in the basal forebrain and in all regions of cortex where they are located in three zones: in layer I, below the cortical plate, and in the intermediate zone. Also contained in the outer part of layer I is a dense fiber plexus which stains intensely for GABA. These fibers may be part of the first contingent of cortical afferents to invade the telencephalic vesicle, an event which is thought to be a stimulus for the beginning of neuronal differentiation in this region. By E18, two bands of immunoreactivity are visible in layer I, which probably contain both cell bodies and fibers. The trajectories taken by growing GABAergic fibers in the brainstem, mesencephalon and diencephalon at embryonic day 13 and at subsequent stages of development are coincident with regions of both monoaminergic and peptidergic differentiation and appear to correspond to recently reported patterns of benzodiazepine receptors which appear slightly later. The early differentiation of the GABAergic system could indicate a trophic role for GABA in early brain development, possibly involving receptors for this neurotransmitter or related substances.

Elevated gamma-aminobutyric acid level in striatal but not extrastriatal brain regions in Parkinson's disease: correlation with striatal dopamine loss

We measured the concentration of gamma-aminobutyric acid (GABA), glutamic acid, and o-phosphoethanolamine in autopsied brain of 9 patients who died with idiopathic Parkinson's disease and 10 control subjects. In the control striatum GABA showed an uneven rostrocaudal distribution pattern with rostral subdivisions containing about 40 to 50% higher levels. When compared with controls, GABA concentrations in Parkinson's disease striatum were generally elevated. The GABA elevation was most pronounced in the caudal subdivision of the putamen; this striatal subdivision also showed the most severe dopamine loss. We observed in the caudal putamen a significant negative correlation between the (elevated) GABA and (reduced) dopamine levels (the latter expressed as the sum of dopamine plus 3-methoxytyramine). Milder nonsignificant elevations of GABA levels were observed in intermediate and rostral putamen followed by the caudate head subdivisions. GABA levels were normal in all extrastriatal brain areas examined. Striatal glutamic acid levels were markedly elevated in 3 of the 9 patients with Parkinson's disease. We suggest that the altered GABA metabolism in the striatum, especially the putamen, is consequent to the nigrostriatal deficiency in this disorder. This secondary change in striatal GABA function is likely to contribute to the basal ganglia dysfunction produced by the striatal dopamine loss and thus may be related to certain aspects of parkinsonian symptomatology.

Striatal GABAergic neuronal activity is not reduced in Parkinson's disease

The content of gamma-aminobutyric acid (GABA) and the activities of glutamic acid decarboxylase (GAD) and tyrosine hydroxylase (TH) were measured in whole putamen obtained at autopsy from 13 patients dying with idiopathic Parkinson's disease and 13 appropriate control subjects. Mean GABA content was significantly elevated (by 28%) in the putamen of the Parkinson's disease patients. TH activity was markedly reduced, while there was no significant reduction of GAD activity in the putamen of these patients. GABA content was also measured in both sides of the striatum in rats which had received unilateral injections of 6-hydroxydopamine (6-OHDA) in the vicinity of the axons of the nigrostriatal projection. Mean GABA content was found significantly elevated (by 33%) in the ipsilateral striatum. Loss of dopaminergic nigrostriatal neurons, in both human Parkinson's disease and in the rat 6-OHDA model, is accompanied by increased striatal GABA content. The assumption that GABAergic neurotransmission is reduced in the striatum in Parkinson's disease may not be correct.