Distribution and Overlap of Fos and Androgen Receptor Protein in the Brainstem of Male Rats: Implications for Reproductive Behavior

Appetitive and consumatory sexual behaviors are critically dependant upon gonadal steroids, the medial preoptic area, and functional androgen receptors (AR). Utilizing the expression of the protein product of the immediate-early gene c-fos as a marker of behavior-dependent neural activation, anatomical sites involved in reproductive behavior have previously been colocalized with AR in regions of the forebrain, midbrain, and lumbar spinal cord. Because brainstem pontomedullary afferents express AR mRNA and preferentially synapse onto sex-activated cells, it is probable that areas in the medulla and pons are involved in the display of reproductive behavior, possibly orchestrated via androgen receptor dependant activation. To more fully assess brainstem contributions to reproductive behaviour, AR receptor and Fos immunoreactivity (AR-ir and Fos-ir, respectively) were compared in the pons and medulla of copulating (cop), noncopulating (non-cop), and socially isolated (si) male rats, allowing the identification of potential targets of androgenic influence on copulation. Copulation-selective Fos-ir was detected in auditory processing areas of the dorsal cochlear nucleus in the medulla, and the nucleus of the trapezoid body in the pons. In the medullary linear nucleus, the cops and non-cops did not differ in the amount of Fos, however, both contained more Fos when compared to the si animals. Equivocal patterns of Fos expression were detected in the intermediate reticular nuclei, and the posterodorsal and reticulo-tegmental areas. Contrary to studies localizing AR mRNA by in situ hybridization, AR-ir was not detected in auditory or somatosensory areas, and except for the prepositus nucleus, areas carrying vestibular information were AR negative, as were sites containing cranial nerve motoneurons. In contrast, nuclei of the solitary tract displayed dense AR-ir, as did several sites in the tegmental and central gray areas. Consistent with previous reports implicating inhibitory supraspinal ventral medullary nuclei in sexual behavior, the raphe pallidus, the lateral paragigantocellular and the ventral gigantocellular reticular nuclei contained both AR-ir and copulation-induced Fos-