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

Abstract

In rats, androgens in adulthood regulate the morphology of motoneurons in the spinal nucleus of the bulbocavernosus (SNB), including the size of their somata and the length of their dendrites. There are conflicting reports about whether androgens exert similar influences on SNB motoneurons in mice. We castrated or sham-operated C57BL6J mice at 90 days of age and, thirty days later, injected cholera toxin conjugated horseradish peroxidase into the bulbocavernosus muscle (to label SNB motoneurons) on one side, and into intrinsic foot muscles contralaterally (to label motoneurons of the retrodorsolateral nucleus (RDLN)). Castrated mice had significantly smaller SNB somas compared to sham operated mice while there were no differences in soma size of RDLN motoneurons. Dendritic length in C57BL6J mice, estimated in 3-dimensions, also decreased significantly after adult castration. In rats, androgens act directly through androgen receptors (AR) in SNB motoneurons to control soma size and nearly all their SNB motoneurons contain AR. Since SNB somata in C57BL6J mice shrank after adult castration, we used immunocytochemistry to characterize AR expression in SNB cells as well as motoneurons in the RDLN and dorsolateral nucleus (DLN). A pattern of labeling matched that seen previously in rats: the highest percentage of AR-immunoreactive motoneurons are in the SNB (98%), the lowest in the RDLN (25%), and an intermediate number in the DLN (78%). This pattern of AR labeling is consistent with the possibility that androgens also act directly on SNB motoneurons in mice to regulate soma size in mice.

The Kisspeptin Receptor GPR54 Is Required for Sexual Differentiation of the Brain and Behavior

“…Only those motoneurons in which the nucleus was visible were included in the counts. As reported previously in mice (Wagner and Clemens, 1989;Forger et al, 1997;Zuloaga et al, 2007), SNB motoneurons had a slightly more dispersed distribution than in rats, and many SNB cells extended ventrally and ventrolaterally along the graywhite border of the medial ventral horn.…”

“…In GPR54 KO males, both the number of SNB motoneurons (in adults) and anogenital distance (at 3 weeks of age) were significantly reduced relative to WT males, suggesting reduced androgen activity sometime in perinatal development. In mice, castration in adulthood moderately decreases SNB cell size and dendrite length (Zuloaga et al, 2007) and, in some strains, cell number (Wee and Clemens, 1987). However, differences in adult androgen levels cannot account for our genotypic differences in SNB number, because all animals were testosterone-replaced in adulthood for at least 4 -6 weeks before SNB analysis.…”

Social and hormonal triggers of neural plasticity in naked mole-rats

“…Each of the neural areas examined express high levels of androgen and/or estrogen receptors in rats and mice [e.g., 31-34] and are hormone-dependent. For example, motoneurons innervating the perineal muscles are highly androgen sensitive in other rodents and decrease to about half their size following gonadectomy of adult males [35-37]. We did not see a decrease in ON soma size after long term GDX in naked mole-rats and, if anything, these animals had the highest percentages of large cells in the study.…”

Sex difference in Onuf's nucleus homologue in the Asian musk shrew

“…Sex differences in motoneuron size, though also widespread in mammals and frequently influenced by androgens, are believed to arise through independent mechanisms and are likely a direct effect on the motoneuron rather than an indirect effect through muscle-derived trophic factors (Freeman and Breedlove, 1995; Watson et al, 2001; Zuloaga et al, 2007). This effect is not limited to motoneuron clusters innervating primarily sexually differentiated and especially androgen-sensitive muscles or to ones in which males have more motoneurons than females.…”

“…The homologous system in the guinea pig shows sex differences in motoneuron size but not number (Freeman and Breedlove, 1995). The RDLN motoneurons of rats (Leslie et al, 1991) but not mice (Zuloaga et al 2007) increase in size with adult androgen treatment. In the musk shrew, we saw the largest sex difference in motoneuron size in the DLN, a smaller sex difference in the VLN (whose target muscles are unknown) and none in the RDLN.…”