Effects of treatment with the opioid antagonists naloxone and naltrexone on LH secretion and on sexual maturation in immature female rats.

The effects on sexual maturation of the opioid receptor antagonists naloxone and naltrexone were studied in the female rat. Neonatal treatment (days 1-10) with either naloxone (2.5 mg/kg at 6-h intervals) or naltrexone (20 or 50 mg/kg per day) did not advance sexual maturation as judged by age and body weight at vaginal opening and first ovulation. After treatment with naltrexone (20 mg/kg) first ovulation occurred 2.3 days earlier than in saline-treated control rats but this could be attributed to a growth-stimulating effect of naltrexone; the effect was not observed with 50 mg/kg. An effect of neonatal treatment with naloxone on serum LH levels was seen at 23 days of age (155 +/- 36 (S.E.M.) vs 14 +/- 4 micrograms LH/l in controls, P less than 0.01), but not at 29 or 33 days of age, at 2 days before first ovulation nor at first pro-oestrus. There were no differences in the number of ova released at first oestrus, nor in the length of the first cycle. Neonatal treatment with naltrexone (50 mg/kg per day) did not alter the response to treatment with human chorionic gonadotrophin at 28-31 days of age: ovulation of a mean of 7.3 ova was induced on day 32 in both naltrexone- and saline-treated rats. Naltrexone treatment (four daily injections of 20 mg/kg at 2-h intervals) at 28-32 days of age advanced first ovulation by 4.4 days in about 40% of the rats.(ABSTRACT TRUNCATED AT 250 WORDS)