Behavioral effects of dopamine agonists and antagonists: Influence of estrous cycle, ovariectomy, and estrogen replacement in rats

The influence of the hormonal condition on the reactivity of central dopami ne (DA) receptors was studied in male and in intact and ovariectomized (OVX ) female rats. They were injected with selective DA agonists, acting either on D-1 (SKF 38393, 2.5 or 10 mg/kg) or D-2 receptors (PPHT, 31.3 or 125 mu g/kg), or with selective DA antagonists, acting either on D-1 (SCH 23390, 6.25 or 25 mu g/kg), or D-2 receptors (sulpiride, 10 or 40 mg/kg). The acqu isition of an avoidance conditioning response (CAR) and the performance of some spontaneous motor behaviors were tested. Both D-1 and D-2 agonists and antagonists impaired the acquisition of CARs in diestrous, OVX, and male r ats. Nevertheless, the effects of these drugs during estrus and in estradio l-primed OVX rats were different according to the drug and the dose injecte d. Whereas SKF 38393 failed to induce significative changes, PPHT and low d oses of SCH 23390 and sulpiride improved the acquisition of CARS in those g roups. The effects on conditioning were not accompanied with equivalent cha nges in spontaneous motor activity. Estradiol level fluctuations that occur in female rats within the estrous cycle or in OVX rats primed with estradi ol would be responsive of changes in the response to DA agents. Although th e reactivity of central DA systems is differentially affected by the hormon al condition of the rat, the precise mechanism of this modulatory action re mains unknown. (C) 1998 Elsevier Science Inc.