GABA in the thalamic motor nuclei modulates dopamine release from the two dopaminergic nigrostriatal pathways in the cat.

Halothane anesthetized cats were implanted with five push-pull cannulae and the release of (3H)dopamine (3H-DA), newly synthetized from (3H)tyrosine, was measured in both caudate nuclei (CN) and substantiae nigrae (SN). GABA was applied by means of one of the cannula in the ventralis medialis (VM) and the adjacent part of the ventralis lateralis (VL) nuclei of the thalamus. Autoradiographic studies performed with (14C)-GABA showed that the diffusion of the amino-acid in our experimental conditions was restricted to a 4.2 mm3 sphere at the tip of the push-pull cannula. Multi-unit activity was recorded both at the site of GABA injection and in the contralateral VM or VL nucleus. During and after a 30-min application of GABA (10(-5) M) in the VM-VL nuclei, neuronal firing was markedly enhanced locally and decreased in the homologous structures, while (3H)DA release increased in both CN and the contralateral SN. No effect was observed in the ipsilateral SN. These results show that thalamic motor nuclei could play a role in the regulation of the activity of the two dopaminergic nigrostriatal pathways and will be discussed in the light of previous data indicating a role of the thalamus in the bilateral regulation of dopaminergic transmission in the basal ganglia.