Influence of alpha-melanotropin on LH release in the rat

The effects of MSH on spontaneous and induced ovulation as well as on serum LH levels were studied. Rats which received progesterone on D1 and D3 were injected i.p. with 30 microgram of alpha-MSH or saline solution and examined for ovulation on the following day. The injection of alpha-MSH induced ovulation in 90% of the animals; saline solution, on the contrary, induced an ovulatory response only in 10% of the cases. Rats injected with beta-MSH yielded similar results to those injected with saline solution, indicating a possible specific alhpa-MSH action. The replacement of the second progesterone injection by alpha-MSH failed to increase to ovulation indicating that alpha-MSH synergizes or alters progesterone action. It would seem that alpha-MSH stimulates ovulation-potentiating progesterone effects on the CNS. LH effectively induced ovulation in rats where spontaneous ovulation was blocked by an injection of progesterone on D1. The i.p. injection of 30 microgram alpha-MSH lowered the amount of LH necessary to induce ovulation. Rats which received alpha-MSH and LRH showed a marked increase in serum LH levels compared with rats where alpha-MSH had been replaced by saline solution. Collectively the information provided in this paper is consistent with the view that the release of MSH observed in the morning of PE would signal the discharge of LH from the pituitary gland occurring in the afternoon of PE.