Effects of nigral application of muscimol on release of [3H] gamma-aminobutyrate and on multiunit activity in various cat thalamic nuclei.

The release of [3H] gamma-aminobutyrate (GABA) neosynthesized from [3H]glutamine was estimated in one substantia nigra and in the ipsilateral thalamus of halothane-anesthetized cats by perfusing a [3H]glutamine-enriched physiological medium through a push-pull cannula implanted in the two structures under investigation. After two hours of superfusion, muscimol (10(-6)M) was delivered through the nigral push-pull cannula for 50-60 min and local- and distal-evoked changes of [3H]GABA release were analyzed. In some experiments, changes of global neuronal activity induced by muscimol application were recorded in different thalamic nuclei, using a bipolar electrode. In a few of the above experiments, biochemical and electrophysiological determinations were simultaneously performed in the substantia nigra and the thalamus. The nigral application of muscimol (10(-6)M) induced locally an activation of the substantia nigra reticulata cells, as well as an increase in release of [3H]GABA. Distally, in the thalamus, two types of biochemical and electrophysiological responses were observed according to the localization of the tip of the push-pull cannula or the electrode. (1) An increased release of [3H]GABA and a depression of the global multi-unit cellular activity were obtained in the ventralis medialis-ventralis lateralis, the centralis lateralis and the paracentralis nuclei. These effects could reflect an activation of the GABAergic nigrothalamic neurons projecting to these different thalamic nuclei. (2) In contrast, in the medialis dorsalis paralamellar zone adjacent to the intralaminar nuclei of the thalamus, a decrease of [3H]GABA release and an activation of the multi-unit activity were obtained. These latter results may suggest either a polysynaptic response or the non-GABAergic nature of the nigrothalamic neurons afferent to the medialis dorsalis paralamellar zone.