Molecular mechanisms of androgen receptor expression

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Motoneurons of the spinal nucleus of the bulbocavernosus (SNB) of male rats innervate the perineal striated muscles bulbocavernosus and levator ani that attach to the penis. They have an important role in copulatory behavior, which is sensitive to alterations in circulating levels of androgen. Regulation of androgen receptor expression seems to be one of the important processes for functions of the androgen receptor (AR)-containing neurons under the influence of androgen. To clarify regulatory mechanisms of AR expression, we examined androgenic regulation of AR immunoreactivity in the SNB motoneurons in adult male rats by immunohistochemistry using the polyclonal antibody, PG21. In intact controls, intense AR immunoreactivity was confined to the cell nucleus. Androgen withdrawal significantly reduced both the intensity of AR immunoreactivity in the nuclei and number of AR immunoreactive nuclei of the SNB motoneurons within 1 day of castration. AR immunostaining in the nucleus was completely eliminated 5 days following castration. These changes were prevented by replacement of testosterone propionate (TP). The number of AR immunoreactive nuclei recovered to about half of the control levels within 20 min or 1 hr of TP administration to the castrated males, although the intensity of AR immunoreactivity was very weak. Both the intensity of AR immunoreactivity and number of AR immunoreactive nuclei recovered to the control levels 2 or 6 hr after TP injection. In addition, AR immunoreactivity of SNB motoneurons was examined in young and old male rats. In old animals, both the intensity of AR immunoreactivity in the nuclei and number of AR immunoreactive nuclei of the SNB motoneurons were significantly reduced. Plasma levels of testosterone in old animals were significantly smaller than those in young ones. These results suggest that androgen causes a significant up-regulation in AR expression of SNB motoneurons.