Acute administration of alpha-melanotropin exerts a stimulatory control on puberty

The present experiments were undertaken to throw light on the physiological role played by alpha-MSH in the advent of female puberty. The serum and pituitary alpha-MSH concentrations were determined in prepubertal rats 5, 15, 25, 30, 33 days old and at vaginal opening. The serum levels showed a significant increase on day 30 and the pituitary alpha-MSH levels increased steadily from day 5 to day 33. To determine if a relation exists between the increase in alpha-MSH levels and the peak of LH prior to vaginal opening, prepubertal rats of the same age range as above received an injection of alpha-MSH or saline solution and were sacrificed 30 min later. The peptide increased LH serum levels only at 30 days of age. To examine the hypothesis that alpha-MSH was involved in determining the timing of puberty, prepubertal rats pretreated with estradiol benzoate received an injection of alpha-MSH or saline solution on day 28. This treatment advanced the time of vaginal opening by 2 days in the experimental animals. With the same experimental procedure, an increase in serum levels of LH and progesterone was also observed at 29 days of age. The action of alpha-MSH on GnRH was analysed by incubating median eminence from prepubertal rats 20, 25 and 30 days of age, with the peptide, alpha-MSH increased GnRH release only in 25-day-old rats. The above results show that acute treatment with alpha-MSH induces modifications in the hormones related to puberty and hence allows us to include alpha-MSH in the chain of neuroendocrine events involved in reproductive maturation.