Effect of dopamine system activation on substantia nigra pars reticulata output neurons: variable single-unit responses in normal rats and inhibition in 6-hydroxydopamine-lesioned rats.

Previous single-unit recording studies have revealed that randomly selected pars reticulata neurons respond in a highly variable and complex fashion to intravenous administration of the dopamine agonist, apomorphine. The current studies were undertaken (1) to assess whether the variable pattern of responses of reticulata neurons to intravenous apomorphine correlates with their sites of projection and (2) to determine how reticulata responses to apomorphine might be altered by the presence of striatal dopaminergic supersensitivity. Extracellular, single-unit recording studies were conducted in anesthetized, paralyzed rats. Pars reticulata neurons were identified by antidromic activation from either the ventromedial nucleus of the thalamus or superior colliculus. Neurons of both subpopulations exhibited similar, highly variable changes in firing rate during the 10-min period immediately following intravenous injection of 320 micrograms/kg of apomorphine, a dose of the drug considered sufficient to stimulate striatal postsynaptic dopamine receptors. These responses, which were not qualitatively different from those previously observed among reticulata cells not distinguished on the basis of projection site, could be reversed by subsequent administration of dopamine antagonist drugs. In contrast to the variable responses in normal animals, the same dose of apomorphine caused a rapid and usually total inhibition of pars reticulata cell firing in rats which received 6-hydroxydopamine lesions of the nigrostriatal dopamine pathway 6 to 8 weeks prior to recording experiments. These inhibitions of firing could also be reversed by administration of dopamine antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)