The effects of intraventricular injection of beta-endorphin on initial estrogen action to induce lordosis behavior.

Ovariectomized female rats subcutaneously (SC) injected or intracerebrally implanted with estradiol benzoate (EB), and given progesterone SC were used as experimental animals to assess the effects of the beta-endorphin (beta-EP) neuronal system on lordosis behavior. In intraventricular (IV) injection of beta-EP at the onset of sc EB priming, the lordosis behavior was significantly (p < 0.001) facilitated. In contrast, the lordosis behavior was significantly (p < 0.001) inhibited by IV injection of naloxone, an opioid receptor antagonist. beta-EP facilitation of lordosis was observed exclusively within the initial stage of estrogen action. The behavior was significantly (p < 0.001) facilitated by IV injection of beta-EP given with an intracerebral implantation of crystalline EB into the septal-preoptic regions. However, the lordosis behavior was significantly (p < 0.001) inhibited by beta-EP when EB was implanted into the ventromedial hypothalamus. Animals receiving EB implants into the mesencephalic reticular formation were not affected by beta-EP. The present study suggests that the beta-EP neuronal system stimulates sexual receptivity through an action on the central nervous system in relation to the site of estrogen-initial activation to induce the lordosis reflex. The sites of beta-EP action may be the estrogen receptive septal-preoptic and hypothalamic regions; the former for facilitatory effect and the latter for inhibitory effect.