ESTROGENS MODULATE THE RESPONSIVENESS OF INVIVO RECORDED STRIATAL NEURONS TO IONTOPHORETIC APPLICATION OF DOPAMINE IN RATS - ROLE OF D1 AND D2 RECEPTOR ACTIVATION

Estrogens are known to affect the functioning of the extrapyramidal motor system. Their actions concern both pre- and postsynaptic components of the dopaminergic nigrostriatal neurotransmission. Postsynaptically, estradiol alters the electrophysiological responses of striatal neurons to dopamine application. The aim of this study was to determine the respective roles of D1 and D2 receptor subtypes in this modification of dopamine action. Eighty-three spontaneously firing caudate neurons of ovariectomized female rats were extracellularly recorded during iontophoretic ejection of dopamine or dopaminergic agonists or antagonists. Between 5 and 10 h after a single 17-beta-estradiol injection (60-mu-g/kg), the predominant effect of dopamine was an activation of firing (63%), and the distribution of responses significantly differed from controls and from neurons recorded less than 5 h after estradiol injection (P < 0.001) on which dopamine mainly elicited inhibitory effects (71% and 63%, respectively). The excitatory effects of dopamine on this subset of spontaneously firing striatal neurons were blocked by the D1 antagonist SCH 23390 (56%), whereas the inhibitory actions were antagonized by sulpiride (62%), whose isolated application often had an excitatory effect (54%). The distributions of the responses to specific D1 or D2 agonists were not altered by estrogen treatment: the predominant effect of the D2 agonist RU 24213 was in all groups a reduction of firing rate (54%), and the D1 agonist SKF 38393 mainly induced either a decrease in mean firing rate (21%) or a biphasic effect consisting on an excitation-inhibition sequence (53%). These results suggest that estradiol does not qualitatively alter the coupling of D1 or D2 receptors to their electrophysiological effectors, but rather quantitatively changes the ratio between the D1 and D2 receptor-mediated components of dopamine actions, reinforcing the D1 and/or attenuating the D2 receptor-mediated component.