Immunohistochemical distribution of substance P, serotonin, and methionine enkephalin in sexually dimorphic nuclei of the rat lumbar spinal cord.

The purpose of the present study was to identify chemically some potential inputs to lumbar motoneurons of the rat in the spinal nucleus of the bulbocavernosus, ventral motor pool, dorsolateral nucleus, and retrodorsolateral nucleus. Substance P-like immunoreactivity and serotonin-like immunoreactivity were found in all four motor nuclei, with dense immunoreactive profiles surrounding motoneurons and their processes. Enkephalin-like immunoreactivity was restricted to the sexually dimorphic nuclei, the spinal nucleus of the bulbocavernosus, and the dorsolateral nucleus. Within the spinal nucleus of the bulbocavernosus, enkephalin-like immunoreactive profiles were apposed to the processes of motoneurons but not their somata. In contrast, enkephalin-like immunoreactivity surrounded motoneuron somata in the medial part but not the lateral part of the dorsolateral nucleus, in the location of motoneurons projecting to the ischiocavernosus muscle. Moreover, the density of serotonin-like immunoreactivity was also greater in the medial part of the dorsolateral nucleus. On the basis of the chemo-architecture and the connections of the dorsolateral nucleus, we suggest the division of this motor column into a medial part composed of ischiocavernosus motoneurons surrounded by enkephalin- and serotonin-like immunoreactivity and a lateral part that contains neurons that project to the sphincter urethrae muscle. Total spinal transection severely depleted both serotonin-like and substance P-like material in the lumbar ventral horn. No changes in the distribution of enkephalin-like immunoreactivity were observed following this lesion. It is therefore suggested that in the ventral horn, substance P- and serotonin-like material are derived from supraspinal tracts, whereas enkephalin-like material is derived from intrinsic nerve cell bodies of the spinal cord.