Patterns of dye coupling in lumbar motor nuclei of the rat

Gap junctions exist on motoneurons of the spinal nucleus of the bulbocavernosus (SNB) and the dorsolateral nucleus (DLN), both sexually dimorphic motor nuclei in the lumbar spinal cord of the rat. In addition, messenger RNA for gap junction proteins is expressed in motoneurons of the retrodorsolateral nucleus (RDLN), a nondimorphic spinal motor nucleus that innervates a muscle of the foot. Gap junctions on SNB and DLN motoneurons are androgen sensitive; the number and size of gap junctions decrease following castration, a change that can be reversed with exogenous testosterone replacement. In contrast, RDLN gap junction mRNA levels remain constant throughout hormone manipulation. In this study, dye coupling was used to examine patterns of gap junction-mediated connectivity in these three lumbar spinal motor nuclei. Injection of dye into single motoneurons resulted in spatially extensive labeling of neighboring cells in all three nuclei; significantly more coupling was observed in the sexually dimorphic nuclei than in the RDLN. Dye-coupled clusters of cells included motoneurons and interneurons; coupling was bilateral in the SNB. Treatment with oleamide, a gap junction blocker, completely attenuated labeling. In all nuclei, androgen manipulation did not alter the number, identity, or distribution of coupled cells. Thus, sexually dimorphic nuclei in the spinal cord exhibit greater dye coupling than do nondimorphic populations, and the patterns of connectivity are insensitive to androgen despite modification of their number and size.