Involvement of the thalamus in the reciprocal regulation of the two nigrostriatal dopaminergic pathways

The effects of various sagittal sections on the asymmetric changes in ³H]dopamine release from nerve terminals and dendrites of the two nigrostriatal dopaminergic pathways induced by the application of α-methylparatyrosine ord-amphetamine in the left substantia nigra were examined in halothane-anaesthetized cats implanted with push-pull cannulae in both caudate nuclei and both substantiae nigrae. ³H]tyrosine was continuously delivered to each push-pull cannula and ³H]dopamine was estimated in serial fractions of superfusates. In cats without a lesion, α-methylparatyrosine (10^?4M) reduced the release of ³H]dopamine in the left substantia nigra and induced an opposite effect in the left caudate nucleus. In contrast, ³H]dopamine release was enhanced in the right substantia nigra and reduced in the right caudate nucleus. Opposite asymmetric responses were observed in the four structures during application ofd-amphetamine (10^?6M) into the left substantia nigra. The sagittal section of the mesencephalic decussations was without effect on the changes in ³H]dopamine release induced either by α-methylparatyrosine ord-amphetamine. The section of the corpus callosum and of the commissura anterior potentiated, whereas the section of the thalamic massa intermedia prevented thed-amphetamine-induced reduction of ³H]dopamine release in the left caudate nucleus. This could indicate that neuronal circuits under the control of contralateral structures may contribute to the regulation of the activity of ipsilateral dopaminergic neurons. Only the section of the thalamic massa intermedia prevented the contralateral responses evoked by the unilateral nigral application of α-methylparatyrosine ord-amphetamine.These results suggest that nigrothalamic neurons and thalamic nuclei are involved in the reciprocal regulation of both nigrostriatal dopaminergic pathways.