NATURAL FLUCTUATION AND GONADAL HORMONE REGULATION OF ASTROCYTE IMMUNOREACTIVITY IN DENTATE GYRUS

The number and the surface density of cells immunoreactive for the specific astrocytic marker glial fibrillary acidic protein (GFAP), were evaluated in both the hilus of the dentate gyrus and the granular layer of the vermis of the cerebellar cortex of adult female rats during the different phases of the estrous cycle, after ovariectomy and after the pharmacological administration of estradiol and/or progesterone to ovariectomized rats. Although no significant differences were detected in the number of immunoreactive cells among the different experimental groups studied, their surface density showed significant changes in the hilus of the dentate gyrus. The surface density of immunoreactive cells was increased in the afternoon of proestrus and on the morning of estrus compared to the morning of proestrus, diestrus, and metestrus, was decreased after ovariectomy, and showed a dose-dependent increase in ovariectomized rats injected with 17beta estradiol (1, 10, or 300 mug/rat), alone or in combination with progesterone (500 mug/rat). In contrast, it was not affected by the administration of 17alpha estradiol (300 mug/rat). The surface density of immunoreactive cells was significantly increased over control values by 5 h after the injection of 17beta estradiol (300 mug/rat) and as early as 1 h after the administration of progesterone. The separate injection of either 17beta estradiol or progesterone had smaller effects on the surface density of immunoreactive cells than did the administration of both hormones together. The surface density of GFAP-immunoreactive cells reached maximal values by 24 h after the administration of 17beta estradiol and/or progesterone and returned to control levels by 48 h after the combined injection of progesterone and 17beta estradiol, while in the rats that were injected with only one of the two hormones, the surface density of immunoreactive cells remained over control values for at least 9 days. No such hormonal effects on GFAP-immunoreactive cells were observed in the cerebellar cortex.