The effect of estrogen treatment on scopolamine inhibition of lordosis.

Previous evidence indicates that the cholinergic muscarinic antagonist, scopolamine, inhibits lordosis in female rats. In the experiments reported here, the effects of various doses and repeated administrations of estrogen on the scopolamine inhibition of lordosis were examined. In the first experiment, intraperitoneal injections of scopolamine (1 mg/rat) completely inhibited lordosis in ovariectomized rats primed with low doses of estradiol benzoate (0.25 or 0.5 micrograms for 3 days) and progesterone (500 micrograms). However, scopolamine was significantly less effective in inhibiting lordosis in females primed with a higher dose of estradiol benzoate (25 micrograms for 3 days) and progesterone (500 micrograms). When hormone priming was repeated on subsequent weeks, scopolamine continued to inhibit lordosis in females that received 0.25 micrograms estradiol benzoate but was less effective in females primed with 0.5 micrograms. Scopolamine failed to inhibit lordosis in females treated with 25 micrograms estradiol benzoate on these later tests. In the second experiment, various doses of scopolamine (1, 2, or 4 mg/rat) were administered intraperitoneally to females primed with the highest dose of estradiol benzoate (25 micrograms) and progesterone (500 micrograms). Lordosis was inhibited equally by all scopolamine doses during the first week. As in the first experiment, scopolamine failed to inhibit lordosis at all doses on subsequent weeks of testing. These results indicate that the ability of scopolamine to inhibit lordosis is reduced by increasing the dose or the number of estrogen exposures. Because higher doses of scopolamine failed to restore its inhibitory effect on lordosis an upregulation of muscarinic receptors by estrogen cannot account for the reduced effectiveness of scopolamine.