Suppressive action of orexin A on pulsatile luteinizing hormone secretion is potentiated by a low dose of estrogen in ovariectomized rats.
Orexins are hypothalamic neuropeptides which stimulate luteinizing hormone (LH) secretion in estrogen- and progesterone-treated ovariectomized (OVX) rats and suppress it in OVX rats not treated with estrogen, suggesting a modulation by estrogen of the response to orexins. We examined the effects of orexin A on pulsatile LH secretion in OVX rats treated with a very small dose of estrogen so as to maintain the pulsatile secretion of LH. The estrogen treatment was done 24 h before the blood sampling by subcutaneously implanting a silicone tube (id = 1.5 mm, od = 2.5 mm, length = 25 mm) containing 17beta-estradiol (E(2)) dissolved in sesame oil at 20 microg/ml. In OVX rats treated with sesame oil as a control, the intracerebroventricular (icv) injection of orexin A (0.3 nmol, dissolved in 3 microl artificial cerebrospinal fluid) had no significant effect on the parameters of pulsatile LH secretion, i.e., pulse frequency and pulse amplitude, although it caused a small but statistically significant decrease in overall mean LH concentrations within 1 h. In OVX rats treated with E(2), the icv injection of orexin A significantly suppressed the pulsatile LH secretion; the frequency decreased for more than 2 h, inducing a rapid decline in overall mean LH concentrations. In view of the finding that a much higher dose of orexin A suppresses pulsatile LH secretion in OVX rats not treated with E(2), we suggest that the suppressive action of orexin A on pulsatile LH secretion is potentiated by estrogen.
Furuta
Miyako
M
Department of Physiology, Yokohama City University School of Medicine, 3-9 Fukuura, Kanazawa-ku, Yokohama, 236-0004 Japan.
Funabashi
Toshiya
T
Kimura
Fukuko
F
D000818:Animals; D002352:Carrier Proteins / Q000008:administration & dosage / Q000494:pharmacology*; D004343:Drug Implants; D004357:Drug Synergism; D004958:Estradiol / Q000008:administration & dosage / Q000494:pharmacology*; D005260:Female; D007276:Injections, Intraventricular; D047908:Intracellular Signaling Peptides and Proteins; D007986:Luteinizing Hormone / Q000097:blood / Q000378:metabolism*; D009479:Neuropeptides / Q000008:administration & dosage / Q000494:pharmacology*; D000068797:Orexins; D010052:Ovariectomy; D010507:Periodicity; D051381:Rats; D017208:Rats, Wistar
D002352:Carrier Proteins; D004343:Drug Implants; D047908:Intracellular Signaling Peptides and Proteins; D009479:Neuropeptides; D000068797:Orexins; D004958:Estradiol; D007986:Luteinizing Hormone