enkephalin--ala(2)-mephe(4)-gly(5)- and estradiol-3-benzoate

The mu-opioid receptor (MOR), a G-protein-coupled receptor, is internalized after endogenous agonist binding. Although receptor activation and internalization are separate events, internalization is a good assay for activation because endogenous opioid peptides all induce internalization. Estrogen treatment of ovariectomized rats induces MOR internalization, providing a neurochemical signature of estrogen activation of the medial preoptic nucleus. MOR activation appears to be the mechanism via which estrogen acts in the medial preoptic area to prevent the display of female reproductive behavior during the first 20-24 hr after estrogen treatment. Naltrexone, an alkaloid universal opioid receptor antagonist, prevented MOR internalization, suggesting that estrogen induces the release of endogenous opioid peptides that in turn activate the MOR. Enkephalins and beta-endorphin are nonselective endogenous MOR ligands. The most selective endogenous MOR ligands are the endomorphins. Infusions of selective MOR agonists, H-Tyr-d-Ala-Gly-N-Met-Phe-glycinol-enkephalin (DAMGO) or endomorphin-1, into the medial preoptic nucleus attenuated lordosis, and their effects were blocked with the MOR antagonist H-d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP). Infusion of endomorphin-1 internalized MOR. To determine whether progestin also acts via the MOR system to facilitate reproductive behavior, ovariectomized rats were primed with 17beta-estradiol and progesterone. Progestin facilitation of lordosis was correlated with a reduction of estrogen-induced MOR internalization. Progestin reversed estrogen-induced MOR internalization, suggesting that progesterone blocked estrogen-induced endogenous opioid release, relieving estrogen inhibition and facilitating lordosis. These results indicate a central role of MOR in the mediation of sex steroid activation of the CNS to regulate female reproductive behavior.

Topics: Analgesics, Opioid; Animals; Cell Count; Dose-Response Relationship, Drug; Drug Administration Schedule; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Estradiol; Estrogens; Female; Immunohistochemistry; Male; Naltrexone; Narcotic Antagonists; Oligopeptides; Ovariectomy; Posture; Preoptic Area; Progesterone; Rats; Rats, Long-Evans; Receptors, Opioid, mu; Sexual Behavior, Animal; Somatostatin

Endogenous opioid systems in the hypothalamus inhibit gonadotropin-releasing hormone (GnRH) secretion, and a reduction in this inhibitory input (disinhibition) is thought to be part of the neural mechanism of the preovulatory GnRH/luteinizing hormone (LH) surge. We showed previously that intracerebroventricular infusion of the highly specific opioid mu-receptor agonist DAMGO delayed the oestrogen-induced LH surge in ovariectomized (OVX) ewes, whereas both delta- and kappa-agonists were ineffective. The aim of the present study was to establish the anatomical site of this effect. The most likely hypothalamic sites of action are the medial preoptic area (MPOA), where most GnRH perikarya are located in sheep, and/or the median eminence (ME), where GnRH fibres terminate on hypophysial portal blood vessels. Conscious, unrestrained OVX ewes with permanent bilateral guide tubes implanted into either the MPOA or the mediobasal hypothalamus (MBH), close to the ME, were injected (i.m.) with oestradiol benzoate (EB) 50 microg (t = 0 h). In this model, EB elicits a time-delayed surge in LH secretion after 13-19 h. Jugular venous blood was sampled at half-hourly intervals from -2 to 0 h, and from 10 to 26 h. From 12 to 20 h, bilateral infusions of either the highly specific opioid mu-agonist DAMGO (40 nmol/h bilaterally) or saline were given into the MPOA or MBH at 2.5 microl/h. Guide tube placements were confirmed histologically. The mean (+/- SEM) time to the onset of the LH surge was significantly (p < 0.01) increased in the animals (n = 9) that received DAMGO infusion into the MPOA (20.5 +/- 1.4 vs. 15.7 +/- 0.6 h in the saline-infused controls). The effect was clearly apparent in 6/9 of the DAMGO-infused animals. The mean (+/- SEM) time to LH surge onset was also significantly (p < 0.01) increased in the animals (n = 8) that received DAMGO infusion into the MBH (19.7 +/- 1.2 vs. 14.3 +/- 0.5 h). In this case, the effect was clearly apparent in 4/8 of the DAMGO-infused animals. We conclude that bilateral infusion of DAMGO into either the MPOA or the MBH can delay the EB-induced LH surge in OVX ewes. These data provide further evidence for dual hypothalamic sites of opioid regulation of GnRH secretion, and are consistent with the hypothesis that disinhibition from opioid tone at both the MPOA and MBH/ME is permissive of the preovulatory GnRH/LH surge.

Topics: Animals; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalins; Estradiol; Female; Gonadotropin-Releasing Hormone; Hypothalamus, Middle; Kinetics; Luteinizing Hormone; Ovariectomy; Preoptic Area; Receptors, Opioid, mu; Sheep