Ontogeny of androgen receptor expression in spinal nucleus of the bulbocavernosus motoneurons and their target muscles in male mice.

The spinal nucleus of the bulbocavernosus (SNB) in rodents is a neuromuscular system consisting of lumbar motoneurons and the perineal muscles they innervate, the bulbocavernosus and levator ani. This system is present prenatally in both males and females but degenerates postnatally in females because of the lack of perinatal androgens. Whether androgens act on the motoneurons or muscles in the SNB system to promote survival is a longstanding question. Evidence in rats suggests androgens act primarily on the muscles in development, given that the muscles express androgen receptor (AR) before the critical period of androgen-dependent cell rescue, whereas motoneurons develop AR after this period. We now report, based on a novel AR-reporter mouse model, that AR is expressed in the bulbocavernosus muscles of C57/BL6(J) mice as early as embryonic day 15, while, based on AR-immunocytochemistry, SNB motoneurons do not express AR until postnatal day 4. These results indicate that the ontogeny of AR expression in the mouse SNB system resembles that found in rats, suggesting that androgens may also act on perineal muscles in mice to rescue the SNB system.