EFFECTS OF HORMONAL TREATMENT AND HISTORY ON SCOPOLAMINE INHIBITION OF LORDOSIS

The muscarinic receptor blocker, scopolamine, inhibits the display of lordosis behavior in female rats but its effectiveness depends on horm onal. conditions. In these experiments, systemic administration of sco polamine (0.031-4 mg/kg) inhibited lordosis in ovariectomized rats bro ught into receptivity by treatment with a low dose of estradiol benzoa te (EB, 0.25 mu g for 3 days) with progesterone (P, 500 mu g for 1 day ), or a high dose of EB (25 mu g for 3 days) with and without P. Howev er, the effectiveness of scopolamine was reduced at the high dose of E B and with the addition of P. Furthermore, scopolamine failed to inhib it lordosis in females treated on a second week with the high dose of EB with or without P, unless an interval of at least 3 weeks separated the two sets of steroid treatments. The reduced effectiveness of scop olamine cannot be explained by peripheral mechanisms because its inhib itory effect on lordosis also was reduced following repeated hormonal exposure even when scopolamine was infused directly into the lateral v entricles.