Distribution of capsaicin-sensitive substance P- and calcitonin gene-related peptide-immunoreactive nerves in bovine respiratory tract

The distribution of nerve fibers immunoreactive for substance P (SP) and ca lcitonin gene-related peptide (CGRP) was examined by means of immunohistoch emical methods in the respiratory tract from nose to lung of normal and cap saicin-treated cattle. SP- and CGRP-immunoreactive (IR) nerve fibers with v aricosities were detected in all portions. They were more numerous in calve s than in cows. They were abundant in the nasal and laryngeal mucosae and t racheal bronchus, and few in number in the lung. SP- and CGRP-IR nerve fibe rs were mainly seen in the epithelium, in connective tissue beneath the epi thelium and around blood vessels, and in the glands throughout the respirat ory tract. In contrast, SP- and CGRP-IR nerve fibers were sparse in the smo oth muscle layer. Capsaicin treatment of neonates caused a remarkable reduc tion in the number of SP- and CGRP-IR nerve fibers in the respiratory tract of calves. Double immunofluorescence experiments showed the colocalization of SP and CGRP in most of the nerve fibers. The present findings suggest t hat SP- and CGRP-TR nerve fibers are involved in the regulation of the bovi ne respiratory tract, and that capsaicin-sensitive SP- and CGRP-IR nerve fi bers are sensory neurons of the bovine respiratory tract.