Effects of naloxone on the serum luteinizing hormone level and the number of Fos-positive gonadotropin-releasing hormone neurons in immature female rats

To examine developmental changes in the number of gonadotropin-releasing ho rmone (GnRH) neurons activated by an opioid receptor antagonist in female r ats, blood sampling and double-labeled immunocytochemistry for Fos and GnRH were performed after the injection of naloxone (NAL) in immature (postnata l d16 and d30) and mature female rats. Three age groups of rats were perfus ed with 4% paraformaldehyde-PB 90 min after the subcutaneous injection of N AL (2.5 mg/kg) or saline. All tissue incubation and staining for double-lab eled immunocytochemistry were simultaneously performed. Although no signifi cant developmental change was observed in the total number of GnRH neurons (p > 0.05), NAL-induced increases in serum luteinizing hormone (LH) concent rations were much greater in the d16 group than those in the d30 and mature groups (p < 0.01). Conversely, Fos-positive GnRH neurons were rarely obser ved in d16, and some Fos-positive GnRH neurons were observed in the d30 gro up (p < 0.05 vs. saline) and the mature group (p < 0.01 vs. saline). These results suggest that opiatergic inhibitory system on GnRH neuron in immatur e female rats is different from that in mature female rats. (C) 2000 Elsevi er Science B.V. All rights reserved.