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Melanocortins May Stimulate Reproduction by Activating Orexin Neurons in the Dorsomedial Hypothalamus and Kisspeptin Neurons in the Preoptic Area of the Ewe

Department of Physiology, Monash University, Clayton, Victoria 3800, Australia

Address all correspondence and requests for reprints to: Prof. Iain Clarke, PhD, Department of Physiology, P.O. Box 13F, Monash University, Clayton, Victoria 3800, Australia. E-mail: iain.clarke{at}med.monash.edu.au.

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 µg/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 µg/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.