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. Key words: mosaic; androgen; spinal nucleus of the bulbocavernosus; tfm mutation; soma size; steroid receptorsMost vertebrate motoneurons possess androgen receptors (ARs) (Pfaff, 1968;Sar and Stumpf, 1977;Kelley, 1986;Simerly et al., 1990), making them attractive models of steroidal effects on neuronal structure and function. The spinal nucleus of the bulbocavernosus (SNB), a cluster of motoneurons located in the dorsomedial aspect of lumbar segments 5 and 6, innervates the sexually dimorphic perineal muscles bulbocavernosus and levator ani (BC/LA), which participate in copulatory behavior. Adult male rats have more and larger SNB motoneurons than do females Arnold, 1980, 1981;Schroder, 1980). A perinatal surge of testosterone (T) in males rescues the BC/LA musculature from involution (Cihak et al., 1970) and inhibits apoptosis in the SNB (Nordeen et al., 1985). Exposing females to exogenous androgens during this critical period permanently masculinizes the SNB-BC/LA system (Breedlove and Arnold, 1983). The SNB critical period developmentally precedes AR expression in SNB cells (Jordan et al., 1991), and systemic androgen can rescue genetically androgen-insensitive SNB motoneurons provided that their BC/LA targets remain androgencompetent (Freeman et al., 1996). Furthermore, androgen is unable to rescue the SNB if the BC/LA muscles are treated with anti-androgen (Fishman and Breedlove, 1992) or extirpated (Kurz et al., 1992), indicating that androgen acts indirectly, via actions in the BC/LA, to rescu...

Androgen-sensitivity of somata and dendrites of spinal nucleus of the bulbocavernosus (SNB) motoneurons in male C57BL6J mice

“…Given that adult SNB motoneurons in C57BL6J mice contain ARs and that the size of their cell bodies also depends on adult levels of androgen, androgen may also act directly via ARs in SNB motoneurons to regulate _soma size in C57BL6J mice, as it does in rats (Watson et al, 2001). We also found that the percentage of AR-IR motoneurons in C57BL6J mice was highest in the SNB, lowest in the RDLN and intermediate in the DLN, matching the rank order of AR expression levels previously described for rats .…”

“…Castration in adulthood reduces SNB soma size and BC/LA muscle size in rats and also in some strains of mice, while androgen treatment reverses this effect (Breedlove & Arnold, 1981;Forger and Breedlove, 1986;Wee & Clemens, 1987). In rats, androgens act directly through androgen receptors in SNB motoneurons to regulate soma size (Watson et al, 2001). The length of SNB dendrites also decreases after castration of adult rats (Kurz et al, 1986).…”

Steroid Receptor Coactivator-1 Is Not Required for Androgen-Mediated Sexual Differentiation of Spinal Motoneurons

“…The SNB is comprised of 2 distinct groups of motoneurons: the group that responds morphologically to androgen projects to the sexually dimorphic BC/LA muscles, whereas the group that does not respond morphologically to androgens projects to the sexually monomorphic external anal sphincter [19]. Although it is clear that androgens act via ARs in SNB motoneurons to increase the size of their somata [20], both androgen-responsive and androgen-unresponsive subpopulations of SNB motoneurons express AR [18]. Taken together, these results indicate that AR expression within motoneurons is necessary but not sufficient for androgen to increase the size of SNB motoneurons.…”

Androgen regulation of axon growth and neurite extension in motoneurons

“…The fact that androgens increase cell size and process outgrowth in MN hybrid cells, but only in those expressing AR, provides evidence that androgens are capable of acting directly on motoneurons to affect these properties in a cell autonomous fashion in vitro (Brooks et al, 1998). In accordance, a series of experiments using rats with mosaic androgen insensitivity (in which a portion of motoneurons do not express functional AR) has demonstrated that androgen can regulate motoneurons in a cell autonomous manner in vivo as well (Monks et al, 1999;Monks and Watson, 2001;Watson et al, 2001).…”