FOS expression in gonadotropin-releasing hormone neurons: Enhancement by steroid treatment and mating

Expression of the protooncoprotein FOS is now widely believed to be a marker for neuronal activation. In female rats, a steroid-induced LH surge is accompanied by an increase in FOS-positive GnRH neurons, especially in the region of the organum vasculosum of the lamina terminalis. The present study, conducted in mice, has examined the effects of both steroid hormone treatment and sexual behavior on the expression of FOS in GnRH neurons and their distribution in the central nervous system. Thirty-three ovariectomized mice, each bearing a sc priming capsule of 17β-estradiol, were divided into five groups, four of which were treated sequentially with estradiol benzoate (1 μg) and progesterone (500 μg). In females maintained on 17β-estradiol only and killed between 1400-1530 h, only 1.3 ± 0.7% of GnRH neurons contained FOS, while treatment with estradiol benzoate/progesterone increased FOS expression significantly to 31.7 ± 8.5% in the same time period. In animals killed at 1530-1700 h, FOS expression declined in the absence of a male (13.8 ± 2.2%) or when the male present in the cage displayed some sexual behavior but did not ejaculate (13.0 ± 8.6%). Interestingly, the expression of FOS was maintained at a high level (42.3 ± 11.4%) into the late afternoon in females paired with a reproductively successful (ejaculating) male. There was a positive correlation (r² = 0.65; P < 0.01) between the level of LH and the number of FOS-positive GnRH neurons. Hence, the expression of FOS in GnRH neurons was enhanced by both a steroid regimen leading to a LH surge and an intense level of mating behavior. Mapping of the GnRH neurons indicates that in animals with the highest level of FOS expression, FOS-positive GnRH neurons were not confined to the region of the organum vasculosum of the lamina terminalis, but were found more widely distributed along the entire rostro-caudal axis of these cells.