Reduced Efficacy of 8-OH-DPAT's Inhibition of Lordosis Behavior by Prior Estrogen Treatment

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.