Common structural organization of two output nuclei of primate basal ganglia

The GABA uptake inhibitor beta-alanine reduces pilocarpine-induced tremor and increases extracellular GABA in substantia nigra pars reticulata as measured by microdialysis

Substantia nigra pars reticulata (SNr) is a major output nucleus of the basal ganglia that receives GABAergic projections from neostriatum and globus pallidus. Previous research has shown that local pharmacological manipulations of GABA in SNr can influence tremulous jaw movements in rats. Tremulous jaw movements are defined as rapid vertical deflections of the lower jaw that resemble chewing but are not directed at a particular stimulus, and evidence indicates that these movements share many characteristics with parkinsonian tremor in humans. In order to investigate the role of GABA in motor functions related to tremor, the present study tested the GABA uptake blocker beta-alanine for its ability to reduce pilocarpine-induced tremulous jaw movements. In a parallel experiment, the effect of an active dose of beta-alanine on dialysate levels of GABA in SNr was assessed using microdialysis methods. GABA levels in dialysis samples were measured using high performance liquid chromatography with electrochemical detection. beta-Alanine (250-500 mg/kg) significantly reduced tremulous jaw movements induced by pilocarpine (4.0 mg/kg). Moreover, systemic administration of beta-alanine at a dose that reduced tremulous jaw movements (500 mg/kg) resulted in a substantial increase in extracellular levels of GABA in SNr compared to the pre-injection baseline. Thus, the present results are consistent with the hypothesis that GABAergic tone in SNr plays a role in the regulation of tremulous jaw movements. This research may lead to a better understanding of how parkinsonian symptoms are modulated by SNr GABA mechanisms.

Pharmacological and biochemical aspects of GABAergic neurotransmission: pathological and neuropsychobiological relationships

1. The GABAergic neurotransmission has been implicated in the modulation of many neural networks in forebrain, midbrain and hindbrain, as well as, in several neurological disorders. 2. The complete comprehension of GABA system neurochemical properties and the search for approaches in identifying new targets for the treatment of neural diseases related to GABAergic pathway are of the extreme relevance. 3. The present review will be focused on the pharmacology and biochemistry of the GABA metabolism, GABA receptors and transporters. In addition, the pathological and psychobiological implications related to GABAergic neurotransmission will be considered.

Substantia nigra pars reticulata GABA is involved in the regulation of operant lever pressing: pharmacological and microdialysis studies

Substantia nigra pars reticulata (SNr) is an important mesencephalic nucleus that functions as a relay area for basal ganglia output. SNr receives GABAergic inputs from the neostriatum and globus pallidus, and in turn sends projections to a variety of motor areas. Although a large number of studies have focused upon the behavioral functions of basal ganglia dopamine, much less is known about the behavioral functions of SNr GABA. The present studies were undertaken to investigate the role of SNr GABA in lever pressing behavior. In the first experiment, the GABA(A) antagonist bicuculline was infused locally into SNr to determine if blockade of GABA receptors interfered with the performance of lever pressing on a fixed ratio 5 schedule. SNr injections of bicuculline produced a dose-related suppression of operant responding. Analysis of interresponse time bins showed that SNr bicuculline produced a response slowing characterized by a relative reduction in the number of fast interresponse times, and an increase in the relative number of pauses. In an additional experiment, microdialysis methods were used to determine if extracellular GABA is elevated during the performance of fixed ratio five lever pressing. During the 30 min lever pressing session, extracellular GABA showed a significant and substantial increase relative to baseline levels. These data support the hypothesis that SNr GABA is involved in the regulation of motor output, and indicate that GABA release in this structure is increased during behavioral stimulation.

Tremulous jaw movements in rats: a model of parkinsonian tremor

Several pharmacological and neurochemical conditions in rats induce 'vacuous' or 'tremulous' jaw movements. Although the clinical significance of these movements has been a subject of some debate, considerable evidence indicates that the non-directed, chewing-like movements induced by cholinomimetics, dopamine antagonists and dopamine depletions have many of the characteristics of parkinsonian tremor. These movements occur within the 3-7 Hz peak frequency range that is characteristic of parkinsonian tremor. Tremulous jaw movements are induced by many of the conditions that are associated with parkinsonism, and suppressed by several different antiparkinsonian drugs, including scopolamine, benztropine, L-DOPA, apomorphine, bromocriptine, amantadine and clozapine. Striatal cholinergic and dopaminergic mechanisms are involved in the generation of tremulous jaw movements, and substantia nigra pars reticulata appears to be a major basal ganglia output region through which the jaw movements are regulated. Future research on the neurochemical and anatomical characteristics of tremulous jaw movements could yield important insights into the brain mechanisms that generate tremulous movements.

Involvement of pallidal and nigral GABA mechanisms in the generation of tremulous jaw movements in rats

Four experiments were conducted to investigate the role of pallidal and nigral GABA in the generation of tremulous jaw movements in rats. In these experiments, tremulous jaw movements were induced by i.p. injections of the anticholinesterase tacrine (5.0 mg/kg). Previous work has shown that the tremulous jaw movements induced by cholinomimetics and dopamine depletion are dependent upon striatal mechanisms. Thus, the present study investigated potential striatal output pathways that could be involved in the generation of these movements. Because there are GABAergic projections from neostriatum to entopeduncular nucleus (medial globus pallidus) and substantia nigra pars reticulata, the GABA agonist muscimol was injected directly into these structures to study the effects of GABA stimulation on tacrine-induced jaw movements. Injections of muscimol into the entopeduncular nucleus (25-100 ng) failed to have any significant effects on tacrine-induced vacuous jaw movements. However, injections of muscimol (12.5-50 ng) into the substantia nigra pars reticulata blocked the jaw movements induced by tacrine. In the third experiment, it was again demonstrated that 25.0 ng of muscimol injected directly into the substantia nigra pars reticulata blocked the jaw movements induced by tacrine; in addition, it was shown that injections of this dose 2.0 mm dorsal to the substantia nigra pars reticulata failed to affect tacrine-induced tremulous jaw movements. It was shown in the fourth experiment that injections of muscimol into a more medial portion of the substantia nigra pars reticulata also reduced tacrine-induced tremulous jaw movements. These results indicate that stimulation of GABA(A) receptors in substantia nigra pars reticulata can block tacrine-induced tremulous jaw movements. This finding is consistent with the notion that striatonigral GABA projections are involved in the generation of tremulous jaw movements. It is also possible that striatonigral GABA mechanisms are involved in human clinical phenomena such as parkinsonian tremor.

Topographic distribution and collateral projections of the two major populations of nigrothalamic neurons. A retrograde labeling study in the rat

The principal target nuclei of nigrothalamic projections in the rat are the mediodorsal (MD) and ventromedial (VM) nuclei. The present study examined the patterns of distribution and collateral projections of the two major groups of nigrothalamic neurons, i.e., nigro-MD and nigro-VM neurons. Retrograde fluorescent labeling with Fluoro-Gold was used to examine whether the distribution areas of nigro-MD and nigro-VM neurons might be overlapped with or segregated from each other in the substantia nigra pars reticulata. A clear tendency was observed that nigro-MD neurons were distributed more ventrally than nigro-VM neurons. It was further examined by retrograde fluorescent double labeling with Fluoro-Gold and Fluoro-Ruby whether or not these nigrothalamic neurons might provide axon collaterals to the superior colliculus or the pontine reticular formation. The nigro-MD neurons were found to send axon collaterals to the superior colliculus more frequently than the nigro-VM neurons. Additionally, a small number of nigrothalamic neurons were found to send axon collaterals to the pontine reticular formation. The functional significance of the two major populations of nigrothalamic neurons was discussed on the basis of their collateral projections to the superior colliculus or the pontine reticular formation.

Surgery for Parkinson's disease

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