Delayed suppression of serum luteinizing hormone after naloxone treatment in neonatal female rats.

In female neonatal rats, opiate receptor blockade markedly raises serum luteinizing hormone (LH) levels. The LH effect of acute treatment with opiate antagonists is apparently brief in older rats; however, age-related differences in antagonist pharmacokinetics may result in different LH response patterns. The duration of LH response to naloxone (NAL) and naltrexone (NTX) was examined in 5 day-old (d.o.) female rats and compared to the duration of analgesia blockade. The rise in serum LH following opiate receptor blockade in 5 d.o. rats was of similar duration to that previously observed in older animals and much briefer than blockade of analgesia. Furthermore, neonatal rats exhibited a delayed suppression of LH 6 hr following NAL, but not NTX, treatment. Stimulation and later suppression of LH were still observed after five repetitive NAL treatments at 6 hr intervals.