Repeated estradiol benzoate treatment protects against the lordosis-inhibitory effects of restraint and prevents effects of the antiprogestin, RU486.

The following experiment was designed to test two specific questions: (1) Does the antiprogestin, RU486, reduce emergence of lordosis behavior and/or proceptivity in rats given repeated treatment with 10μg estradiol benzoate (EB) and/or a single high dose (40μg) of EB? (2) Does RU486 accentuate the effects of a 5min restraint experience on sexual behaviors in rats given repeated treatment with estradiol benzoate (EB) and/or a high dose of EB? RU486 was used to determine if a high dose and/or repeated treatment with EB enhanced proceptivity and reduced the response to mild stress through an intracellular progesterone receptor-mediated process. Ovariectomized Fischer rats were injected with a single dose of 10 or 40μg estradiol benzoate (EB) or received 4consecutiveweeks of treatment with 10μg EB. Forty-eight hours after the last treatment with EB, rats were injected with 5mg/kg of the antiprogestin, RU486, or the RU486 vehicle. That afternoon, rats were monitored for sexual behaviors. Sexually-receptive rats were then restrained for 5min and again tested for sexual behaviors. A separate set of rats received 4consecutiveweeks of 10μg EB treatment before treatment with a higher (5mg/rat) dose of RU486. Lordosis to mount ratios, lordosis quality, proceptivity, and resistance were monitored. RU486 had no effect on the emergence of sexual behaviors but did accentuate the lordosis-inhibitory effect of restraint in rats given a single treatment with EB. Rats treated for 4consecutiveweeks with EB showed no effect of restraint and were unaffected by RU486. These findings lead to the suggestion that repeated EB initiates select behavioral effects that are not mimicked by acute EB treatment and that the intracellular progesterone receptor may not be involved.