ESTROGEN-RECEPTOR IMMUNOSTAINING IN THE PREOPTIC AREA AND MEDIAL BASAL HYPOTHALAMUS OF ESTRADIOL BENZOATE-TREATED AND PRAZOSIN-TREATED FEMALE GUINEA-PIGS

Evidence has accumulated showing that the alpha1-adrenergic antagonist prazosin decreases nuclear estrogen binding in the hypothalamus of th e guinea-pig. In this study we asked if prazosin treatment alters estr ogen receptor (ER) protein content as reflected by changes in ER-immun oreactivity. The monoclonal rat antibody H222 directed against ER was used to detect ER-immunoreactive (ER-ir) cells in eight specific preop tic and hypothalamic brain regions of ovariectomized Hartley strain gu inea-pigs treated with estradiol benzoate and 1.0 mg/kg prazosin or ve hicle. Immunocytochemical parameters which provided optimum conditions for detection of even modest changes in ER-immunoreactivity were firs t established. Then, using these optimum conditions, we compared 1) th e mean number of ER-ir profiles, 2) the mean density of ER-ir staining , and 3) the distribution of ER-ir staining density readings, between conditions within each of the eight brain regions. No differences in a ny of these measures were found between prazosin- and vehicle-treated females. We also compared the percentage of ER-ir nuclear profiles acr oss the eight cell groups investigated in estradiol benzoate- and vehi cle-treated females. The medial preoptic area had by far the highest p ercentage (48%) of ER-ir profiles (P < 0.05) compared to all seven oth er brain regions (23% to 32% ER-ir cells). Our data, showing that ER-i mmunoreactivity is not reduced (6 h) after prazosin treatment, suggest s that mechanisms other than alterations in ER protein should be consi dered when interpreting the effects of prazosin on the retention of es tradiol by nuclear or cytosolic extracts of hypothalamic lysates.