Seasonal and steroid-dependent effects on the modulation of LH secretion in the ewe by intracerebroventricularly administered beta-endorphin or naloxone.
The natural opioid ligand, beta-endorphin, and the opioid antagonist, naloxone, were administered intracerebroventricularly (i.c.v.) to evaluate effects on LH secretion in ovariectomized ewes and in ovariectomized ewes treated with oestradiol-17 beta plus progesterone either during the breeding season or the anoestrous season. Ovary-intact ewes were also studied during the follicular phase of the oestrous cycle. Jugular blood samples were taken at 10-min intervals for 8 h and either saline (20-50 microliters), 100 micrograms naloxone or 10 micrograms beta-endorphin were injected i.c.v. after 4 h. In addition, luteal phase ewes were injected i.c.v. with 25 micrograms beta-endorphin(1-27), a purported endogenous opioid antagonist. In ovariectomized ewes, irrespective of season, saline and naloxone did not affect LH secretion, but beta-endorphin decreased the plasma LH concentrations, by reducing LH pulse frequency. The effect of beta-endorphin was blocked by administering naloxone 30 min beforehand. Treating ovariectomized ewes with oestradiol-17 beta plus progesterone during the breeding season reduced plasma LH concentrations from 6-8 micrograms/l to less than 1 microgram/l. In these ewes, saline did not alter LH secretion, but naloxone increased LH pulse frequency and the plasma concentrations of LH within 15-20 min. During anoestrus, the combination of oestradiol-17 beta plus progesterone to ovariectomized ewes reduced the plasma LH concentrations from 3-5 micrograms/l to undetectable levels, and neither saline nor naloxone affected LH secretion. During the follicular phase of the oestrous cycle, naloxone enhanced LH pulse frequency, which resulted in increased plasma LH concentrations; saline had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)
Horton
R J
RJ
Medical Research Centre, Prince Henry's Hospital, Melbourne, Australia.
Francis
H
H
Clarke
I J
IJ
D000818:Animals; D004958:Estradiol / Q000502:physiology; D004971:Estrus; D005260:Female; D007276:Injections, Intraventricular; D007986:Luteinizing Hormone / Q000097:blood*; D009270:Naloxone / Q000008:administration & dosage / Q000494:pharmacology*; D010052:Ovariectomy; D011374:Progesterone / Q000502:physiology; D012756:Sheep; D001615:beta-Endorphin / Q000008:administration & dosage / Q000494:pharmacology*
D009270:Naloxone; D011374:Progesterone; D004958:Estradiol; D001615:beta-Endorphin; D007986:Luteinizing Hormone