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 r ats. When rats were given a single injection with 25 mu g estradiol benzoat e followed 48 h later with 500 mu g progesterone, inhibition of lordosis be havior was evident at all doses of 8-OH-DPAT. However, when rats were treat ed with 25 mu g estradiol benzoate followed 7 days later with a second inje ction of 25 mu g estradiol benzoate and then progesterone, none of the dose s of 8-OH-DPAT effectively inhibited lordosis behavior. In some rats, cannu lae were located near the most rostral portion of the VMN. In these rats, t here was no effect of the second estrogen treatment on the response to 8-OH -DPAT. Therefore, a second experiment was performed to specifically evaluat e the effects of two estradiol benzoate treatments on the response to bilat eral 8-OH-DPAT infusion in the rostral VMN. In contrast to the reduced effe ctiveness 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 effe ctively inhibit lordosis behavior following the 5-HT1A receptor agonist's i nfusion into the more rostral areas. These findings are discussed in relati on to earlier studies in which the potency, but not the efficacy, of 8-OHDP AT was reduced following systemic treatment with the 5-HT1A receptor agonis t. (C) 1999 Academic Press.