Neuronal size in the spinal nucleus of the bulbocavernosus: direct modulation by androgen in rats with mosaic androgen insensitivity

Abstract

The motoneurons of the spinal nucleus of the bulbocavernosus (SNB) and its target muscles, the bulbocavernosus and levator ani, form a sexually dimorphic circuit that is developmentally dependent on androgen exposure and exhibits numerous structural and functional changes in response to androgen exposure in adulthood. Castration of male adult rats causes shrinkage of SNB somata, and testosterone replacement reverses this effect, but the site at which androgen is acting to cause this change is undetermined. We exploited the X-chromosome residency of the androgen receptor (AR) gene to generate androgenized female rats that were heterozygous for the testicular feminization mutant (tfm) AR mutation and that, as a consequence of ontogenetic random X-inactivation, expressed a blend of androgen-sensitive wild-type cells and tfm-affected androgen-insensitive cells in the SNB. Chronic testosterone treatment of adult mosaics increased soma sizes only in androgen-competent wild-type SNB cells. The size of tfm-affected SNB somata in the same animals did not differ from the size of either the wild-type or tfm-affected SNB neurons in control mosaics that did not receive androgen treatment in adulthood. Because the muscle targets of the SNB are known to be uniformly androgen-sensitive in tfm mosaics, this mosaic analysis provides unambiguous evidence that androgenic effects on motoneuron soma size are mediated locally in the SNB. It is possible that the neuronal AR plays a permissive role in coordinating the actions of androgen.

Fig. 1.

A, Varied androgen receptor distribution in the SNB of the tfm mosaic, as revealed by AR ICC without Nissl counterstain. Open arrowindicates a wild-type motoneuron exhibiting dense nuclear androgen receptor ICC label. Filled arrows indicatetfm-affected motoneurons, identifiable by their lack of nuclear androgen receptor label; the nucleoli are visible and mark the location of the nuclei. In wild-type animals, virtually all SNB motoneurons exhibit the wild-type pattern of dense nuclear AR immunolabeling. Scale bar, 50 μm. B, Nissl counterstain of AR ICC material permits measurement of the somata of SNB cells with (open arrow) or without (filled arrows) nuclear AR. Scale bar, 100 μm.

Fig. 2.

Effect of androgen treatment on SNB soma size intfm mosaics. Androgen-competent wild-type cells that were exposed to testosterone exhibited significantly enlarged somata relative to all other groups. Other groups did not differ from one another. See Results for details.