The effects of bilateral VMN infusion with the
5-HT1A receptor agonist, (+/-)-8-hydroxy-2-(di-n-propylamino)
tetralin (8-
OH-DPAT; 200, 1000, or 2000 ng), on
lordosis behavior were examined in hormonally primed ovariectomized rats. When rats were given a single injection with 25 microg
estradiol benzoate followed 48 h later with 500 microg
progesterone, inhibition of
lordosis behavior was evident at all doses of
8-OH-DPAT. However, when rats were treated with 25 microg
estradiol benzoate followed 7 days later with a second injection of 25 microg
estradiol benzoate and then
progesterone, none of the doses of
8-OH-DPAT effectively inhibited
lordosis behavior. In some rats,
cannulae were located near the most rostral portion of the VMN. In these rats, there was no effect of the second
estrogen treatment on the response to
8-OH-DPAT. Therefore, a second experiment was performed to specifically evaluate the effects of two
estradiol benzoate treatments on the response to bilateral
8-OH-DPAT infusion in the rostral VMN. In contrast to the reduced effectiveness of the
8-OH-DPAT infusion in the mid to caudal VMN in rats given two
injections with
estradiol benzoate, 2000 ng
8-OH-DPAT continued to effectively inhibit
lordosis behavior following the
5-HT1A receptor agonist's infusion into the more rostral areas. These findings are discussed in relation to earlier studies in which the potency, but not the efficacy, of
8-OH-DPAT was reduced following systemic treatment with the
5-HT1A receptor agonist.