Distribution of CGRP, substance P, VIP and somatostatin immunoreactive nerve fibers in the internal gills of larvae of the bullfrog, Rana catesbeiana.

CGRP, substance P (SP), VIP and somatostatin were demonstrated in the internal gills of larval bullfrogs using the immunohistochemical peroxidase-antiperoxidase method. Three groups of immunoreactive fibers were distinguished by their distribution area. The first group includes CGRP, SP and VIP immunoreactive fibers running in the connective tissue of the gill tufts. These fibers were associated with the capillaries. Some CGRP fibers were distributed similarly to SP fibers, and the density of VIP immunoreactive fibers was low. The second group includes CGRP, SP, VIP and somatostatin fibers distributed in the connective tissue surrounding the ceratobranchialia. The third group includes CGRP, SP and somatostatin fibers located in the branchial muscle. Their terminals appeared to be neuromuscular junctions. No immunoreactivity of leu- or met-enkephalins was found in the internal gills. From these findings, the first group of fibers is presumed to be involved in modulating ion transport of the internal gills. The second group of fibers except for the somatostatin fibers is thought to be continuations of the first group of fibers. The third group of fibers may possibly be involved in the modulation of transmissions at the neuromuscular junction. It is unclear whether somatostatin terminals are also involved in this.