In urethane anesthetized rats, the iontophoretic administration of dop
amine (DA) induced an inhibition of flash-evoked activity in the major
ity of geniculate cells investigated. Excitatory effects of DA also we
re found in some neurons of the dorsal lateral geniculate nucleus. The
observed excitatory effects of DA were blocked selectively by D2 rece
ptor antagonists, and the majority of inhibitory effects could be bloc
ked by D1 receptor antagonists. In some neurons, the D2 receptor antag
onist also blocked the DA-induced inhibition. Nine of 33 neurons teste
d responded differently to DA according to the amount ejected: with lo
wer iontophoretic currents they increased their rates of discharge, wh
ereas higher DA ejecting currents resulted in a suppression of their a
ctivity. 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 D
2 agonists could be reversed by the receptor-specific dopamine antagon
ists. Presumed local circuit interneurons appeared to be involved in m
ediation of some inhibitory effects of DA, since the D2-induced inhibi
tions could be abolished by the GABA(A) antagonist, bicuculline. The m
ajority of cells also was affected by DA antagonists given alone; thes
e cells' responses to light usually were of an inhibitory nature. The
results show that like other monoamines, DA also is involved in certai
n aspects of visual processing at the level of the thalamus. (C) 1996
Wiley-Liss, Inc.