Effects of unilateral electrical stimulation of various thalamic nuclei on the release of dopamine from dendrites and nerve terminals of neurons of the two nigrostriatal dopaminergic pathways.

The role of several motor and intralaminar thalamic nuclei in the regulation of dopamine release from terminals and dendrites of the nigrostriatal dopaminergic neurons was investigated in halothane-anaesthetized cats. For this purpose, the effects of the unilateral electrical stimulation of various thalamic nuclei on the release of newly synthesized [3H]dopamine were simultaneously determined in both substantiae nigrae and caudate nuclei using the push-pull cannula method. The electrical stimulation of the motor nuclei was the only one to induce asymmetric changes in the four structures since [3H]dopamine release was enhanced in the ipsilateral caudate nucleus and reduced in the contralateral structure while opposite responses were observed in the corresponding substantiae nigrae. A reduction of [3H]dopamine release occurred in the four structures or only in the contralateral substantia nigra and caudate nucleus following the stimulation of the parafascicularis nucleus and the adjacent posterior part of the nucleus centrum medianum or of the nucleus centralis lateralis and the adjacent paralaminar part of the nucleus medialis dorsalis, respectively. The stimulation of the anterior part of the nucleus centrum medianum, which in contrast to other thalamic nuclei examined, receives few nigral inputs, selectively enhanced [3H]dopamine release in the contralateral substantia nigra. No significant changes in [3H]dopamine release were seen either in the substantiae nigrae or in the caudate nuclei following the stimulation of midline thalamic nuclei. These results indicate that the motor and intralaminar thalamic nuclei exert multiple and selective influences on the release of dopamine from terminals and/or dendrites of the dopaminergic neurons. They also further support a role of thalamic nuclei in the transfer of information from one substantia nigra to the contralateral dopaminergic neurons. The possible involvement of connections between paired thalamic nuclei was underlined by the observations of evoked potentials in contralateral homologous nuclei following unilateral stimulation of motor, or some intralaminar, nuclei. The present report provides new insights on the mechanisms contributing to the reciprocal and/or bilateral regulations of nigrostriatal dopaminergic pathways.