Long-term castration effects motoneuron size but not number in the spinal nucleus of the bulbocavernosus in the adult male Mongolian gerbil

Abstract

The spinal nucleus of the bulbocavernosus (SNB) is a sexually dimorphic group of motoneurons in Rexed’s Lamina X of the lumbosacral spinal cord of the Mongolian gerbil. The SNB innervates the perineal musculature, the bulbocavernosus (BC), levator ani (LA), and external anal sphincter (EAS). Recent studies demonstrated a peripubertal component to the masculinization of the gerbil SNB with an apparent increase in both motoneuron size and number after puberty as measured with a Nissl stain. However, these studies could not determine if the apparent change in motoneuron number were due to the long-term castration involved in the methodology or due to a loss of motoneuron size beyond the point of being recognizable as motoneurons. Therefore, the current study was undertaken to examine this possibility by repeating the experimental protocol from previous peripubertal studies, on adult male gerbils with the addition of retrograde tract-tracing. Adult male gerbils were castrated at postnatal day (PND) 150 and given subcutaneous implants of testosterone proprionate (TP), dihydrotestosterone (DHT), estradiol benzoate (EB), EB and DHT, no steroid, or left intact. At PND215, the animals were injected with fluorogold (FG), a retrograde tract-tracer. At PND220, the animals were aldehyde perfused transcardially. The spinal cords were sectioned and alternate sections processed for either thionin-stain or FG visualization. Results indicated that long-term castration had no effect on SNB motoneuron number but did decrease SNB motoneuron size. TP, and to a lesser extent DHT, treatment could prevent the reduction in motoneuron size, however, EB could not. This study indicates that the maintenance of the adult male gerbil SNB–BC system is androgen, not estrogen, dependent and that long-term castration does not reduce motoneuron size to the point where they can no longer be distinguished.