Motoneurons innervating guinea pig perineal muscles are sexually dimorphic in size but not number.

Sexual differentiation occurs prenatally in guinea pigs but extends into the postnatal period in rats. Steroids affect the development of two motoneuron nuclei of the rat lumbar spinal cord that innervate sexually dimorphic perineal muscles. The spinal nucleus of the bulbocavernosus (SNB) innervates the bulbocavernosus (BC) and levator ani (LA) muscles while the dorsolateral nucleus (DLN) innervates the ischiocavernosus (IC). In male rats, perinatal testosterone prevents degeneration of these muscles and results in a sex difference in both motoneuron size and number in adulthood. For comparative purposes, we examined the guinea pig motoneurons innervating these muscles, as well as those innervating the retractor penis (RP) and retractor clitoris (RC), muscles that have no counterpart in rats. Injections of horseradish peroxidase localized the BC/LA and IC motoneurons of guinea pigs to discrete columns in spinal levels L6 and S1, with the BC/LA motoneurons occupying a more medial position. The RP/RC motoneurons were found in L5. Motoneuronal soma area was larger in males in all examined motor pools, as was nuclear area of BC/LA and IC motoneurons. Although raw counts suggested a sex difference in cell number in the motor columns containing BC/LA and IC motoneurons, either of two different correction procedures for split nuclei error eliminated the sex difference in cell number, emphasizing the importance of such corrections when comparing neurons of different size.