Intracerebroventricular administration of melanin-concentrating hormone suppresses pulsatile luteinizing hormone release in the female rat.

Melanin-concentrating hormone (MCH) has been reported to be involved in the regulation of feeding behaviour in rats and mice. Because many neuropeptides that influence ingestive behaviour also regulate reproductive function, the present study was designed to determine if central administration of MCH changes pulsatile secretion of luteinizing hormone (LH) in the rats. Wistar-Imamichi strain female rats were ovariectomized and implanted with oestradiol to produce a moderate inhibitory feedback effect on LH release. The effects of i. c.v. injections of MCH on LH release were examined in freely moving animals. Blood samples were collected every 6 min for 3 h through an indwelling cannula. After 1 h of sampling, MCH (0.1, 1 or 10 microg/animal) or vehicle (saline) was injected into the third cerebroventricle. Because MCH is also reported to affect the hypothalamo-pituitary-adrenal (HPA) axis, which in turn, can influence reproductive function, plasma corticosterone concentrations were determined in the same animals at 30-min intervals during the first and last hours and every 12 min during the second hour of the 3-h sampling period. When expressed as per cent changes, mean plasma LH concentrations after MCH administration were significantly lower in the animals injected with all doses of the peptide compared with vehicle-treated animals; LH pulse frequency was significantly lowered by 1 microg of MCH. Per cent changes in mean plasma corticosterone levels were not significantly affected by MCH administration. These results in oestradiol-treated ovariectomized rats indicate that central MCH is capable of inhibiting pulsatile LH secretion. We have previously shown that 48-h fasting suppresses pulsatile LH release in the presence of oestrogen. Take together, these results raise the possibility that MCH could play a role in mediating the suppression of LH secretion during periods of reduced nutrition.