Estrogen and androgen synergize in the regulation of various neuroendocrine functions. To determine a potential cellular basis of this synergism, we measured androgen receptor (AR) in the cytosol of 16 hypothalamic and limbic nuclei and subregions in castrated male rats and castrated rats treated with estradiol. Androgen receptor was measured by a previously validated in vitro binding assay using the synthetic androgen methyltrienolone ([3H]R1881). Male Sprague-Dawley rats (250â€“350 g) were castrated 2 weeks before the implantation of a 2.5-cm Silastic capsule filled with crystalline 17Î²-estradiol. Control rats were sham implanted. Estrogen treatment lasted for 1 week, after which time the animals were killed, their brains were frozen and sectioned, and individual nuclei and subregions were removed by a tissue punch technique. Tissue from six rats were combined for each determination. The highest levels of AR were found in the ventromedial nucleus (16.5 Â±1.4 fmol/mg protein), medial preoptic area (12.1 Â± 1.4 fmol/mg protein), bed nucleus of the stria terminalis (11.6 Â± 1.4 fmol/mg protein), lateral septum (11.4 Â± 1.4 fmol/mg protein), arcuate nucleus-median eminence (10.9 Â± 2.1 fmol/mg protein), and medial amygdala (10.3 Â± 0.9 fmol/mg protein). Estrogen treatment resulted in significant increases in AR in medial preoptic area (14.8 Â± 0.6 fmol/mg protein; P < 0.05) and medial amygdala (14.6 Â± 1.2 fmol/mg protein; P < 0.02). Subsequent studies using block-dissected hypothalamuspreoptic area, anterior pituitary, and prostate revealed significant estrogen-mediated elevations in AR in anterior pituitary cytosol [42.2 Â± 3.0 vs. 26.4 Â± 1.6 fmol/mg protein (control); P < 0.01], but not in hypothalamus-preoptic area or prostate cytosols. Estrogen treatment had no effect on AR affinity. The binding of [3H]R1881 was specific for AR and was not affected by the addition of radioinert progesterone to the incubation tube. Estimates of AR concentration were similar regardless of whether [3H]R1881 or [3H]dihydrotestosterone was used as the ligand. In this study, we describe the distribution of AR throughout the hypothalamus and limbic areas using biochemical techniques. In addition, we have identified some cellular events that may mediate the synergistic actions of estrogen and androgen on the neuroendocrine system.
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