Postnatal development of the spinal nucleus of the bulbocavernosus in the Mongolian gerbil: Effects of peripubertal gonadal hormones on anatomy, androgen receptor, and neurotrophin receptor expression

The spinal nucleus of the bulbocavernosus (SNB) is an example of a sexually dimorphic structure within the central nervous system. The SNB is comprised of a pool of motoneurons within the lumbosacral spinal cord. The SNB innervates the bulbocavernosus muscle (BC), the levator ani muscle (LA), and the anal sphincter (AS). During development, the SNB undergoes a pattern of cellular proliferation, migration, and cell death by which male animals maintain a larger motoneuron population than do females of the same species. Unlike other rodents, male gerbils do not develop a visible BC muscle until the onset of puberty. Without an apparent target organ it was not known if the SNB contained a full complement of mature motoneurons. Studies to examine the development of the gerbil SNB suggest that a peripubertal organization of the SNB-BC system occurs in response to androgen. This peripubertal organization takes the form of an increase in motoneuron size and apparent number.;This study also examines the normal ontogeny of the SNB by analyzing thionin stained sections of male and female gerbils from the day of birth, postnatal day (PND) 1 through PND 25. At all ages examined, males had larger and more SNB motoneurons than did females. An increase in the number of motoneurons was seen throughout postnatal development. Additionally, the effects of gonadal steroids on the prepubertal SNB were studied. To examine the peripubertal effects of gonadal hormones on SNB development, prepubertal (PND25) male gerbils were castrated and treated with gonadal steroids. Only androgen could maintain SNB motoneuron number, size, calcitonin gene-related peptide (CGRP) immunoreactivity, and BC/LA muscle size compared to intact adult male gerbils. This indicates a dependence upon androgen for masculinization of the SNB-BC system. Therefore, neonatal animals were examined for the expression of androgen receptor (AR) mRNA as well as p75 neurotrophin receptor mRNA. The p75 neurotrophin receptor has been shown to regulate motoneuron survivability, size, and CGRP expression. Neonatal gerbils show AR mRNA expression preceding p75 mRNA expression. Immunocytochemical examination of AR and p75 protein expression under the peripubertal gonadal hormone experimental paradigm indicated a dependence upon androgen for AR expression, however, p75 protein expression was not affected regardless of gonadal hormone exposure. Comparison of these findings to that of another sexually dimorphic spinal nucleus, the dorsolateral nucleus, and to a nonsexually dimorphic spinal nucleus, the ventral motor pool, indicates that the SNB has a unique developmental mechanism scheme.