In female rats, estrogen has been reported to enhance cocaine sensitization. Here we investigated the effect of estrogen and cocaine treatments on GABA(B)-stimulated [(35)S]GTPgammaS binding. Ovariectomized rats without (OVX) and with estrogen treatment (OVX-EB) were pretreated with saline or cocaine (15 mg/kg, i.p.) for 5 days and after 1 week of withdrawal challenged with cocaine. One hour after the final injection, animals were sacrificed, brains immediately frozen, and stored at -70 degrees C for subsequent cryosectioning. In vitro functional autoradiography was performed using baclofen (300 microM), a GABA(B) receptor agonist, to stimulate [(35)S]GTPgammaS binding in tissue sections at the level of the ventral tegmental area (VTA). OVX-EB rats showed lower levels of [(35)S]GTPgammaS binding in the VTA (-15%) and entorhinal cortex (EC) (-60%). The effect of cocaine on GABA(B)-mediated G-protein activation varied with the presence of estrogen. Repeated cocaine administration reduced [(35)S]GTPgammaS binding in the VTA and EC of OVX rats and increased it in OVX-EB. Thus, our data suggest that estrogen reduces GABA(B)-mediated G-protein activation in female rats. The results also show that estrogen strongly influences cocaine-induced alterations in GABA(B) function in the VTA and EC of female rats.