Effects of dopamine on neurons of the lateral geniculate nucleus: an iontophoretic study

In urethane anesthetized rats, the iontophoretic administration of dopamine (DA) induced an inhibition of flash-evoked activity in the majority of geniculate cells investigated. Excitatory effects of DA also were found in some neurons of the dorsal lateral geniculate nucleus. The observed excitatory effects of DA were blocked selectively by D2 receptor antagonists, and the majority of inhibitory effects could be blocked by D1 receptor antagonists. In some neurons, the D2 receptor antagonist also blocked the DA-induced inhibition. Nine of 33 neurons tested responded differently to DA according to the amount ejected: with lower iontophoretic currents they increased their rates of discharge, whereas higher DA ejecting currents resulted in a suppression of their activity. Iontophoretic administration of a D1 agonist (SKF 38393) for the most part induced a decrease in baseline activity, whereas the D2 agonist (quinpirole) frequently induced an increase. These effects of agonists were dose-dependent and reproducible. Effects of the D1 and D2 agonists could be reversed by the receptor-specific dopamine antagonists. Presumed local circuit interneurons appeared to be involved in mediation of some inhibitory effects of DA, since the D2-induced inhibitions could be abolished by the GABAA antagonist, bicuculline. The majority of cells also was affected by DA antagonists given alone; these cells' responses to light usually were of an inhibitory nature. The results show that like other monoamines, DA also is involved in certain aspects of visual processing at the level of the thalamus.