We have investigated the effect of blocking alpha-MSH on the release of LH in rats at pro-oestrus. Continuous infusion of Pro-Leu-Gly-NH2 (PLG; 0.2 microgram/h), from the afternoon of dioestrus-2 until the day of oestrus, inhibited LH at pro-oestrus and the next ovulation on the day of oestrus. Plasma alpha-MSH levels decreased significantly in rats treated with PLG. This inhibitory effect on LH release was not observed in rats treated with a continuous infusion of a diluted antiserum against alpha-MSH (0.5 microliter/h, diluted 1/10) from the afternoon of dioestrus-2 until the day of oestrus. On the contrary, the administration of undiluted antiserum against alpha-MSH (0.5 microliter/h) blocked LH release and ovulation. The release of alpha-MSH into the blood was also significantly blocked by this treatment. When serum levels of progesterone were measured at 12.00 and 14.00 h in rats in which alpha-MSH release was blocked by PLG or concentrated antiserum, a significant decrease was observed when compared with the levels of progesterone in control rats. These results show that the blockade of alpha-MSH inhibits LH release and the next ovulation in rats treated with PLG or with concentrated antiserum against alpha-MSH, through a blockade of progesterone of adrenal origin.
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Caballero et al. (1993) studied this question.