Postnatal plasticity in a spinal nucleus of Mongolian gerbils in response to androgen

The spinal nucleus of the bulbocavernosus (SNB) is a sexually dimorphic motor pool located dorsolateral to the central canal in Mongolian gerbils (Meriones unguiculatus) that innervates two sexually dimorphic perineal muscles, the bulbocavenosus (BC) and levator ani (LA), and the sexually monomorphic external anal sphincter (AS). In most rodents the SNB is fully sexually differentiated shortly after birth. In gerbils, however, sexual differentiation of the SNB occurs postnatally and continues through puberty. Sex differences in the gerbil SNB appear to result from cell recruitment to the male SNB in response to testosterone rather than by death of cells, as in female rats. Motoneurons are recruited to the male gerbil SNB throughout the first month of life and again at puberty while the number remains static in female gerbils. To further understand this unusual motoneuron increase, we explored the response of the postnatal gerbil SNB-BC system to hormonal manipulation.;Prenatal treatment of females with androgens resulted in more and larger SNB motoneurons and heavier BC muscles than controls, though these increases were not enough to fully masculinize them. Postnatal treatment of females with androgen metabolites resulted in more SNB motoneurons compared with controls; however, the somal size was not altered. Males castrated at birth had fewer and smaller SNB motoneurons than controls, though these measures were still greater than those of females.;As in rats and mice, male and female gerbils are born with BC and LA muscles, though these muscles are absent in adult females. Treatment of females with testoterone propionate (TP) on postnatal day (PND) 1 or PND5 masculinized these muscles through at least PND10. Treatment of males with TP increased the weight and fiber size of BC muscles compared to controls. SNB motoneurons in males increased significantly in size and number with both age and TP treatment.;TP replacement in adulthood, 75 days after prepubertal castration, resulted in a fully masculinized SNB while other hormone treatments were less successful. New cells appeared within two days of TP treatment and connected to the BC muscle within 16 days. Preliminary evidence suggests that these new motoneurons are the result of mitosis in the spinal cord in response to TP.