Involvement of the thalamus in the asymmetric effects of unilateral sensory stimuli on the two nigrostriatal dopaminergic pathways in the cat

The effects of unilateral sensory stimuli on dopamine (DA) release from nerve terminals and dendrites of the two nigrostriatal dopaminergic pathways were estimated in halothane-anaesthetized cats without or with sagittal transections. In control animals, the electrical stimulation of the right forelimb enhanced DA release in the right caudate nucleus (CN) and decreased DA release in the right substantia nigra (SN). Opposite effects were observed in the contralateral structures. Sagittal transections of the corpus callosum and commissura anterior, or of the mesencephalic decussations or of the thalamic massa intermedia were made to investigate mechanisms involved in the reciprocal regulation of the two dopaminergic pathways. These sections were without effect on the spontaneous release of DA from nerve terminals and dendrites. The transection of the thalamic massa intermedia was the only one which interrupted the asymmetric changes in DA release induced by unilateral sensory stimuli; an increased dendritic release of DA was only seen in the left SN but it was significantly less pronounced than that observed in control cats. The other transections did not prevent the asymmetric changes in DA release evoked by the sensory stimulation. However, the mesencephalic sagittal transection significantly reduced the stimulatory effect on DA release induced in the left SN. These results suggest that the thalamus is involved in the transfer of information implicated in the reciprocal regulation of the two dopaminergic pathways. In the light of electrophysiological data, the role of nigrothalamic neurones in this phenomenon is discussed.