Characterization of commissural interneurons in the lumbar region of the neonatal rat spinal cord

Neurons with axons that extend to the contralateral side of the spinal cord -commissural interneurons (CINs)-coordinate left/right alternation during l ocomotion. Little is known about the organization of CINs in the mammalian spinal cord. To determine the numbers, distribution, dendritic morphologies , axonal trajectories, and termination patterns of CINs located in the lumb ar spinal cord of the neonatal rat, several different retrograde and antero grade axonal tracing paradigms were performed with fluorescent dextran amin es and the lipophilic tracer 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarb ocyanine perchlorate (DiI). CINs with ascending (aCINs) and descending (dCI Ns) axons were labeled independently. The aCINs and dCINs occupied differen t but overlapping domains within the transverse plane. The aCINs were clust ered into four recognizable groups, and the dCINs were clustered into two r ecognizable groups. All dCINs and most aCINs were located within the gray m atter, with somata ranging from 10-30 mu m in diameter and with large, mult ipolar dendritic trees. One group of aCINs was located outside the gray mat ter along the dorsal and dorsolateral margin and had dendrites that were ne arly confined to the dorsolateral surface. All CIN axons traversed the vent ral commissure at right angles to the midline. CIN axons coursed up to six or seven segments rostrally and/or caudally in the ventral and ventrolatera l white matter and gave off collaterals over a shorter range, predominantly to the ventral gray matter. These findings show that the lumbar spinal cor d of the neonatal rat contains substantial numbers of CINs with axon projec tions and collateral ranges spanning several segments and that CINs project ing rostrally vs. caudally have different distributions in the transverse p lane. The study provides an anatomical framework for future electrophysiolo gical studies of the spinal neuronal circuits underlying locomotion in mamm als. J. Comp. Neurol. 403:332-345, 1999. (C) 1999 Wiley-Liss, Inc.