Does nitric oxide act in the ventromedial preoptic area to mediate oestrogen negative feedback in the seasonally anoestrous ewe?
Seasonal anoestrus in the ewe results from enhanced oestrogen negative feedback. Recent data have implicated the ventromedial preoptic area (vmPOA) as an important site of oestrogen action. This study addressed whether NO acts within the vmPOA to inhibit LH during seasonal anoestrus. In Experiment 1, microimplants containing Nomega-nitro-l-arginine methyl ester (l-NAME, NOS inhibitor), S-methyl thiocitrulline (SMTC, neural NOS (nNOS) inhibitor) or empty implants (control) were administered during mid-anoestrus to the vmPOA. l-NAME, but not SMTC, significantly increased LH pulse frequency. For Experiment 2, ewes in late anoestrus were administered 7-nitroindazole (7NI; nNOS inhibitor), l-NAME, SMTC, or empty implants. 7NI, but not l-NAME or SMTC, increased LH pulse frequency. In Experiment 3, the effects of microimplants and microinjections of l-NAME were compared in mid-anoestrus. Microinjections of l-NAME (300 nl at 10 microg/microl) increased LH pulse frequency, but microimplants did not. In late anoestrus, similar microinjections were ineffective. Taken together, the results of Experiments 1-3 suggested that NO inhibition may be stronger during the middle than at the end of seasonal anoestrus. To test this hypothesis, ewes in Experiment 4 received microinjection of l-NAME or vehicle thrice during the non-breeding season; none of the treatments increased LH pulse frequency. These results indicate that NO plays a role in the vmPOA in suppressing LH secretion during seasonal anoestrus because NOS inhibitors were consistently stimulatory when LH pulse frequency was low. However, the inconsistent and modest effects of these inhibitors suggest that NO actions in this area cannot completely account for the effects of inhibitory photoperiod.
McManus
Christina J
CJ
Department of Physiology and Pharmacology, West Virginia University, PO Box 9229, Morgantown, West Virginia 26506-9229, USA.
Valent
Miroslav
M
Hardy
Steven L
SL
Goodman
Robert L
RL
D000780:Anestrus / Q000378:metabolism*; D000818:Animals; D002956:Citrulline / Q000031:analogs & derivatives / Q000494:pharmacology; D004343:Drug Implants; D004967:Estrogens / Q000378:metabolism*; D025461:Feedback, Physiological; D005260:Female; D007986:Luteinizing Hormone / Q000097:blood / Q000378:metabolism; D008845:Microinjections; D019331:NG-Nitroarginine Methyl Ester / Q000008:administration & dosage; D009569:Nitric Oxide / Q000032:analysis / Q000378:metabolism*; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors; D052250:Nitric Oxide Synthase Type III / Q000037:antagonists & inhibitors; D011301:Preoptic Area / Q000737:chemistry / Q000378:metabolism*; D012621:Seasons; D012756:Sheep / Q000502:physiology*; D013268:Stimulation, Chemical; D013890:Thiourea / Q000031:analogs & derivatives / Q000494:pharmacology
D004343:Drug Implants; D004967:Estrogens; C086715:thiocitrulline; D002956:Citrulline; D009569:Nitric Oxide; D007986:Luteinizing Hormone; D019001:Nitric Oxide Synthase; D052250:Nitric Oxide Synthase Type III; D013890:Thiourea; D019331:NG-Nitroarginine Methyl Ester