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 hormonal conditions. In these experiments, systemic administration of scopolamine (0.031-4 mg/kg) inhibited lordosis in ovariectomized rats brought into receptivity by treatment with a low dose of estradiol benzoate (EB, 0.25 micrograms for 3 days) with progesterone (P, 500 micrograms for 1 day), or a high dose of EB (25 micrograms for 3 days) with and without P. However, the effectiveness of scopolamine was reduced at the high dose of EB and with the addition of P. Furthermore, scopolamine failed to inhibit 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 scopolamine cannot be explained by peripheral mechanisms because its inhibitory effect on lordosis also was reduced following repeated hormonal exposure even when scopolamine was infused directly into the lateral ventricles.