Possible mechanism by which alpha-melanotropin advances the time of vaginal opening

The aim of this work was to determine whether the acute administration of alpha-melanotropin (alpha-MSH) advances the time of vaginal opening (VO) by modifying the secretion of luteinizing hormone (LH), prolactin, growth hormone (GH), progesterone (P), estradiol (E2) and/or testosterone (T). Female rats received a subcutaneous injection of estradiol benzoate (EB; 10 micrograms/100 g body weight) on day 26 and 27 postnatal (PN). On day 28 PN, a third dose of EB was given together with either intraperitoneal alpha-MSH (treated group) or 0.1 ml saline (control group). On day 32 PN, the animals were inspected for VO and groups of rats were decapitated at hourly intervals between 12:00 and 18:00 hours. Serum concentrations of the hormones listed above were measured by radioimmunoassay. Acute administration of alpha-MSH significantly advances (P < 0.001) the day of VO compared to the control group, as well as increased serum LH levels (P < 0.001) in the period under study, in comparison to controls, but had no significant effect on any of the others hormones analyzed. LH profiles were determined in more detail beginning on the day after alpha-MSH administration (day 29 PN) to VO (day 33 PN). Blood samples were collected from the cannulated external jugular vein at 20 minutes intervals daily between 14:00 and 18:00 hours. alpha-MSH treatment did not significantly modify plasma LH levels on days 29 and 30 PN, although treated animals showed a sustained increase in LH on day 31 PN. On this day, mean LH levels of treated animals was significantly (P < 0.05) higher than the control group.(ABSTRACT TRUNCATED AT 250 WORDS)