Diurnal oscillation in glial fibrillary acidic protein in a perisuprachiasmatic area and its relationship to the luteinizing hormone surge in the female rat.

It is well known that the reproductive cycle in the female rat is closely associated with the circadian rhythms of motor activity and that this phenomenon requires the presence of estrogens. Estrogens induce plastic changes in neural connectivity and these changes could be the result of glial modifications. We have measured glial fibrillary acidic protein (GFAP) immunoreactivity in order to localize the area in which the coupling of the circadian rhythms to the generation of the luteinizing hormone (LH) surge may occur. As circadian rhythms are driven by the suprachiasmatic nucleus (SCN), GFAP immunoreactivity was measured in 5 areas of the SCN and surrounding regions. It was measured at two times during daylight (10.00 and 17.00 h) in ovariectomized (OVX) females implanted with Silastic capsules containing either estradiol benzoate (EB) or oil (control). Differences between morning and afternoon GFAP immunoreactivity were observed in a peri-SCN area, dorsal to the SCN and close to the 3rd ventricle, in estrogen-treated as well as in control OVX females. However, this difference increased in the subgroup of EB-treated females which displayed the strongest LH rhythmicity. These results suggest that the peri-SCN area could be an important locus for synaptic changes linking circadian rhythms with the estrogen-induced LH surge.