Differential distribution of nerve fibers immunoreactive for substance P and calcitonin gene-related peptide in the superficial and deep muscle layers of the dorsum of the rat

We compared the distribution of substance P (SP) and calcitonin gene-related peptide (CGRP) fibers of the superficial muscle layer (trapezius muscle), median muscle layer (rhomboideus muscle), and deep muscle layer (longissimus and spinalis muscles) of the dorsum of the rat. SP- and CGRP-immunoreactive fibers were seen along the walls of various types of blood vessels and within nerve bundles in skeletal muscles of all layers. Coexistence of SP and CGRP was evident in nerve fibers along the blood vessel walls. The total number of CGRP varicosities per millimeter square of muscle surface area was evaluated quantitatively, and CGRP varicosities were found to be significantly more numerous in the superficial muscle layer than in the deeper ones. After capsaicin treatment, most of the SP and CGRP fibers along the blood vessel walls were eliminated. These results suggest that sensory nerve fibers containing SP and CGRP are distributed more abundantly in the superficial muscle layer than in the deeper ones and that they might be involved in the regulation of local blood flow. The finding of SP- and CGRP-immunoreactive nerve fibers along the blood vessel walls connecting the trapezius muscle and the hypodermis raises the possibility that sensory stimuli to the skin affect the local blood flow of superficial muscle through collaterals of cutaneous fibers.