Melanocortins may stimulate reproduction by activating orexin neurons in the dorsomedial hypothalamus and kisspeptin neurons in the preoptic area of the ewe.
To further test the hypothesis that melanocortins stimulate the reproductive axis, we treated ewes with melanocortin agonist (MTII) in the luteal phase of the estrous cycle and during seasonal anestrus. Lateral ventricular infusion of MTII (10 microg/h) during the luteal phase increased LH secretion. Retrograde neuronal tracing in the brain showed few proopiomelanocortin or kisspeptin cells in the arcuate nucleus, but more than 70% of kisspeptin cells in the dorsolateral preoptic area (POA), projecting to the ventromedial POA in which GnRH cells are located. MTII infusion (20 h) was repeated in luteal phase ewes and brains were harvested to measure gene expression of preproorexin and kisspeptin. Expression of orexin in the dorsomedial hypothalamus and kisspeptin in the POA was up-regulated by MTII treatment and Kiss1 in the arcuate nucleus was down-regulated. Seasonally anestrous ewes were progesterone primed and then treated (lateral ventricular) with MTII (10 microg/h) or vehicle for 30 h, and blood samples were collected every 2 h from 4 h before infusion until 6 h afterward to monitor acute response in terms of LH levels. A rise in basal LH levels was seen, but samples collected around the time of the predicted LH surge did not indicate that an ovulatory event occurred. We conclude that melanocortins are positive regulators of the reproductive neuroendocrine system, but treatment with melanocortins does not fully overcome seasonal acyclicity. The stimulatory effect of melanocortin in the luteal phase of the estrous cycle may be via the activation of kisspeptin cells in the POA and/or orexin cells in the dorsomedial hypothalamus.
Backholer
Kathryn
K
Department of Physiology, Monash University, Clayton, Victoria 3800, Australia.
Smith
Jeremy
J
Clarke
Iain J
IJ
D000818:Animals; D001111:Arcuate Nucleus of Hypothalamus / Q000166:cytology / Q000378:metabolism; D030762:Estrous Cycle; D005260:Female; D015870:Gene Expression / Q000187:drug effects; D007031:Hypothalamus / Q000166:cytology / Q000378:metabolism; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism; D007986:Luteinizing Hormone / Q000097:blood; D053486:Melanocortins / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology*; D009474:Neurons / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D009479:Neuropeptides / Q000235:genetics / Q000378:metabolism; D000068797:Orexins; D010456:Peptides, Cyclic / Q000494:pharmacology; D011301:Preoptic Area / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011863:Radioimmunoassay; D012098:Reproduction / Q000187:drug effects*; D012756:Sheep; D025521:Tumor Suppressor Proteins / Q000235:genetics / Q000378:metabolism; D000521:alpha-MSH / Q000031:analogs & derivatives / Q000494:pharmacology
D047908:Intracellular Signaling Peptides and Proteins; D053486:Melanocortins; D009479:Neuropeptides; D000068797:Orexins; D010456:Peptides, Cyclic; D012333:RNA, Messenger; D025521:Tumor Suppressor Proteins; C079282:melanotan-II; D000521:alpha-MSH; D007986:Luteinizing Hormone