Estrogen receptor immunoreactivity in rat brain: rapid effects of estradiol injection

The existence of cytoplasmic estrogen receptor immunoreactivity (ER-IR) has been reported in the brain using three different antibodies raised against different epitopes on the ER protein. With each antibody, the darkest reaction product was seen within cell nuclei, but cytoplasmic ER-IR was also observed with each antibody in most neuroanatomical areas. We have reported previously that an injection of estradiol causes a rapid decrease in ER-IR in ovariectomized guinea pigs when the H 222 estrogen receptor antibody was used. In extending this work to rats, we used three different ER antibodies to determine if this decrease in ER-IR is likely to be due to down-regulation of the receptor by estradiol or a decrease in the ability of some antibodies to bind to their particular epitopes on the receptor. Estradiol injection in ovariectomized rats caused a rapid (within 20 min), nearly total loss of cytoplasmic ER-IR when the H 222 antibody was used to visualize the receptor. This decrease did not appear to be due to movement of the receptors to the cell nucleus, as cell nuclear ER-IR also decreased. When other antibodies were used, the extent of loss and the pattern of immunostaining were greatly influenced by the particular antibody. Extensive loss of cytoplasmic and cell nuclear ER-IR was seen when an antiserum against the hinge region of the receptor was used. However, when an antiserum against the N-terminus of the ER was used, a decrease was seen in cytoplasmic ER-IR, but little or no decrease was observed in cell nuclear ER-IR. Because loss of cell nuclear ER-IR was not seen with all of the antibodies, these results suggest that the dramatic decrease in cell nuclear ER-IR seen immediately after estradiol injection is due at least in part to a conformational change in the receptor. This, in turn, may impede association with particular antibodies during the immunocytochemical procedure.