Changes in plasma, pituitary and brain alpha-MSH content in rats from birth to sexual maturity.

Immunoreactive alpha-MSH was measured in plasma, pituitary and brain of male and female rats on the day of birth and at intervals afterwards up to 55-70 days of age. Plasma alpha-MSH concentrations on the day of birth were 528 +/- 111 pg/ml and 406 +/-167 pg/ml in female and male rats, respectively. Plasma alpha MSH concentrations then fell and remained low until the onset of sexual maturity when they again rose reaching 406 +/- 38 pg/ml in 70 day old females and 312 +/- 46 pg/ml in 55 day old males. Pituitary alpha-MSH concentrations also changed with age and in male rats generally reflected the changes in plasma alpha-MSH concentrations. In females, on the other hand, pituitary alpha-MSH concentrations showed a gradual increase with age. Concentrations of alpha-MSH in the hypothalamus and brain increased with age and as with plasma and pituitary reached peak values in sexually mature animals. These findings are consistent with the idea that both pituitary and brain alpha-MSH have a role in sexually mature rats. Although alpha-MSH may have a role in sexual behaviour there was no evidence of any change in brain alpha-MSH throughout the estrous cycle.