The effects of aging on mu and delta opioid receptors in the spinal cord of Fischer-344 rats

The lordosis-inhibiting effects of the 5-HT1A receptor agonist, (+/-)8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), were examined in ovariectomized rats, hormone primed with 2.5, 7.5, or 25 micrograms estradiol benzoate plus 500 micrograms progesterone. 8-OH-DPAT (50, 100 or 200 ng per bilateral site) infused into the ventromedial nucleus of the hypothalamus (VMN), inhibited lordosis behavior in all hormone-treated conditions. However, animals primed with 2.5 micrograms estradiol benzoate were significantly more affected by the infusion than rats primed with 7.5 or 25 micrograms of the hormone. These findings strengthen prior speculations that 5-HT1A receptor function is modulated by estrogen.