Inhibition of gonadotropin secretion induced by cholecystokinin implants in the medial preoptic area by the dopamine receptor blocker, pimozide, in the rat.

We have shown that the implantation of cholecystokinin octapeptide sulfated form (CCK-8-S) or dopamine (DA) in the medial preoptic area (MPO) in ovariectomized estrogen-primed rats induced an enhancement of the afternoon rise in LH secretion. The present study was done to examine the possible functional interaction of both substances in the brain. Two groups of ovariectomized estradiol (5 micrograms, s.c.)-primed rats were administered intraperitoneally either pimozide (1 mg/kg BW; pimozide-treated group) or 0.1 M tartaric acid as the control solution (pimozide-untreated group). CCK-8-S or DA was implanted in the MPO through chronically implanted guide cannulae and the effects on serum LH and FSH concentrations were compared between both groups. An empty cannula was used as the control. Implantation of either CCK-8-S or DA in the pimozide-untreated group induced marked enhancements of afternoon rises in LH and FSH levels, which normally occurred, but with a lesser magnitude in the animals with the empty cannula. In the pimozide-treated group, however, neither CCK-8-S nor DA induced changes in the LH and FSH levels. It was evident that both effects of CCK-8-S and DA on LH and FSH secretion were blocked by pretreatment with the DA receptor blocker. These data indicate that DA receptors are required to be intact when CCK-8-S implants manifest their action in stimulating gonadotropin secretion.