ANDROGEN RECEPTORS AND ESTROGEN-RECEPTORS ARE COLOCALIZED IN MALE-RATHYPOTHALAMIC AND LIMBIC NEURONS THAT EXPRESS FOS IMMUNOREACTIVITY INDUCED BY MATING

Conversion of testosterone into estradiol is important for male rat se xual behavior, and both steroids probably contribute to mating. The di stributions of neurons containing androgen receptors (AR) and estrogen receptors (ER) overlap, and many AR-immunoreactive (AR-ir) neurons ex press Fos immunoreactivity (Fos-ir) induced by mating. Because mating- induced Fos-ir in the male rat occurs mainly in AR-ir neurons, and bec ause both steroids are important for mating, we hypothesized that (i) AR-ir and ER-ir are colocalized and that (ii) some of these neurons ar e activated during mating. We examined, in adjacent sections from the medial preoptic area (MPN) through the central tegmental field (CTF), the expression of ER-ir in: (i) AR-ir-containing neurons, and (ii) Fos -ir-expressive neurons. PG21 anti-AR, OA-11-824 anti-c-fos, H222 or 1D 5 anti-ER primary antibodies were visualized, respectively, with cyani ne-conjugated, fluorescein-or cyanine-conjugated, and fluorescein-conj ugated secondary antibodies in male rats which were killed 1 h after e jaculating with a receptive female. In MPN, bed nucleus of the stria t erminalis (BNST), and medial amygdala (MEA), 80-90% of ER-ir labeling occurred in AR-ir-positive neurons but only about 30 % of AR-ir neuron s were ER-ir-positive. No ER-ir was found in the CTF. This suggests th e presence of three types of brain neurons sensitive to gonadal steroi d hormones: neurons sensitive to androgens only, neurons sensitive to both androgens and estrogens, and neurons sensitive to estrogens only. About 50% of ER-ir labeling occurred in cells expressing mating-induc ed Fos-ir but only about 30% of Fos-ir neurons were ER-ir-positive. Th ese findings suggest that, in the MPN, at least two different neuronal populations are activated during mating: the first contains AR-ir onl y and the second contains AR-ir and ER-ir. In the BNST and MEA, at lea st three hormonally sensitive populations are activated during mating: the two described above plus a third population which expresses ER-ir only.