Immunoreactive patterns of S-2 and S-3 sacral roots: Neuroanatomic investigations in dogs

Purpose: The so called Law of Magendie and Bell implies the separation of t he spinal roots with regard to their functional potential: motoneurons are represented in the ventral roots and sensory fibers in the dorsal roots. Ho wever, it is surprising that, although the dog appears to be the most commo n animal for neurostimulation research, most studies on the topographical m apping of neuropeptides and other neuronal markers have been carried out on the central and peripheral nervous system of other mammalians. Our aim was to characterize the immunoreactive pattern of S-2 and S-3 sacral nerves re garding afferent and efferent nerve fibers in the both ventral and dorsal s pinal roots in dogs. Material and Methods: S-2 and S-3 sacral nerves from 5 male dogs were studi ed to localize various neuropeptides [Calcitonin Gene-Related Peptide (CGRP ), Substance P (SP), Vasoactive Intestinal Polypeptide (VIP) and Neuropepti de Y (NPY)]. In addition, nerve fibers were studied with regard to immunore activity of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d ) and acetylcholinesterase (AChE). Results: Immunoreactivity for all neuropeptides was detected win both S-2 a nd S-3. In the S-2 and S-3 dorsal root, a high density of CGRP and SP was o bserved, whereas, in the ventral root, single CGRP-positive axons were pres ent. Most of the CGRP and SP immunoreactive material in the dorsal root was seen around myelinated axons. The pattern of VIP showed a high density in the S-2 dorsal root and a notably lower density in the ventral root. S-3 sh owed a similar VIP distribution. In the S-2 dorsal root, a high density of NPY was observed; in the ventral root it was notably lower. NPY-immunoreact ivity of S-3 showed a lower distribution. NADPH-d was detected in both S-2 and S-3. Low density of immunoreactivity was observed in the S-2 ventral ro ot, but no NADPH-d-immunoreactive axons were found in the S-3 ventral root. The distribution pattern in S-3 was similar. AChE was detected in both S-2 and S-3. A moderate AChE-density, which almost exclusively marks efferent nerve fibers, was observed in both the S-2 and the S-3 ventral root, but no immunoreactive axons were found in the dorsal root. Conclusions: The present study describes the localization of four neuropept ides within the dorsal and ventral root of both S-2 and S-3 in the dog. The notably higher concentration of the neuropeptides, which almost exclusivel y are found in afferent nerve fibers, in the dorsal roots is consistent wit h the so called Law of Magendie and Bell. In addition, neuropeptides were p resent, although in a much lower density, in the ventral root, confirming p revious studies in which the presence of few afferent fibers was suggested. The finding of NADPH-d activity within in the sacral dorsal roots supports the hypothesis that nitric oxide may function as an important preganglioni c transmitter in the canine sacral afferent pathway.