Immature female rats received implants containing 17β-estradiol on postnatal day 28 at 0900 h, followed 24 h later by either blank capsules or progesterone. Between 15001600 h on the day of progesterone (or blank capsule) implantation, these rats, a group of unoperated or sham controls, and a group of estrogen-progesterone-treated immature male rats were killed and perfused, and their brains processed for immunocy-tochemistry of c-fos antigen and LHRH. LHRH neurons consistently expressed c-fos after estrogen-progesterone treatment in females but not males; in only one of four females examined was c-fos induced after estrogen treatment. No fos was associated with LHRH neurons in the control groups. The LHRH neurons that expressed c-fos were located in the preoptic area and ante-rior hypothalamus; more rostral LHRH cells did not appear stimulated. These data demonstrate that gonadal steroids, administered in a paradigm that predictably produces timed stimulation of LH release, induce c-/os in LHRH neurons. The induction of c-fos in LHRH neurons provides a potentially useful and powerful tool for studying LHRH activation at the cellular level.
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