Induction of precocious puberty in the female rat after chronic naloxone administration during the neonatal period: the opiate 'brake' on prepubertal gonadotrophin secretion.

Studies were undertaken using the opiate receptor antagonist naloxone to examine the hypothesis that endogenous opiates may have a restraining effect on prepubertal gonadotrophin secretion and may be involved in the maturation of the central nervous system mechanisms regulating the onset of puberty in the female rat. Naloxone (2.5 mg/kg) administered intraperitoneally every 6 h to female rats from day 1 to day 10 of postnatal life significantly (P less than 0.001) advanced the age of onset of puberty assessed in terms of the day of vaginal opening and first oestrus (32.3 +/- 0.2 vs 40.8 +/- 0.4 days in control saline-treated animals). Animals so treated with naloxone showed significantly (P less than 0.001) higher levels of FSH (761.4 +/- 87.6 vs 483.8 +/- 57.2 micrograms/l in control animals) and LH (562.8 +/- 57.4 vs 351.3 +/- 43.3 micrograms/l in control animals) at the first late pro-oestrus and a significantly (P less than 0.001) higher number of ova released at first oestrus (12.4 +/- 0.4 vs 8.1 +/- 0.3 in controls). Body weight at first oestrus was significantly (P less than 0.001) lower in the naloxone-treated animals, an indication that these animals were much younger. The weights (per 100 g body wt) of the ovaries and uteri at the first oestrus were significantly (P less than 0.01) higher in the naloxone-treated rats than in the controls. However, there were no significant differences in the weights of the adrenals and anterior pituitary glands between the two groups of animals.(ABSTRACT TRUNCATED AT 250 WORDS)