Supraspinal projections to the ventromedial lumbar spinal cord in adult male rats.

In the present study, the fluorescent tract tracing compound Fluorogold was used to study the afferents of the SNB (spinal nucleus of the bulbocavernosus), which is found in the ventromedial spinal grey and innervates penile muscles of the male rat. Fluorogold was iontophoretically injected into the SNB, which was located by recording antidromic activation of the motoneurons after stimulating the bulbocavernosus muscle. Retrogradely labeled cells were found in laminae I, V-IX, and area X of the lumbar spinal cord, suggesting segmental input to the SNB. Supraspinally, the greatest number of labeled cells were in the medulla oblongata, particularly in the lateral vestibular nucleus, gigantocellular reticular nucleus, and ventral and alpha divisions of the gigantocellular reticular nucleus. Labeled cells were also observed in the medullary raphe nuclei, the ventral medullary nucleus, and the spinal vestibular nucleus. In the pons, labeled cells were observed in the nucleus locus coeruleus, nucleus subcoeruleus, and caudal pontine reticular nucleus. No labeled cells were present in the cerebellum, rostral pons, mesencephalon, and cerebral cortex. The most rostral occurrence of labeled cells was in the medial parvicellular division of the hypothalamic paraventricular nucleus. These potential afferents to the SNB identified in male rats imply that the inputs to motoneurons that innervate sex-specific muscles involved in male reproductive behavior may be similar to the inputs to lumbar motoneurons described in the female rat that innervate muscles involved in female sexual behavior.