Coexistence of Neuropeptides in the Amphibian Carotid Labyrinth

Abstract

The amphibian carotid labyrinth has arterial chemoreceptor and baroreceptor functions analogous to those of the mammalian carotid body and carotid sinus (Ishii et al, 1966). In addition, it has been suggested that the carotid labyrinth functions in controlling the blood flow to the internal and external carotid arteries (Kusakabe et al, 1987). We have observed several neuropeptide-containing nerve fibers in the carotid labyrinth, and suggested that the peptidergic innervation may participate in the function of the labyrinth (Kusakabe et al, 1991). In the carotid labyrinth, the coexistence of substance P (SP) with calcitonin gene-related peptide (CGRP) was first supposed from two adjacent sections (Kusakabe et al, 1991). Later, this speculation was clarified using the double-labelling method with an individual filter system (Kusakabe et al, 1993, 1994). It is now necessary to consider whether this coexistence is in a single axon within a bundle, or whether the two immunoreactivities originate from separate neurons in the same bundle. To clarify this, the precise coexistence of SP, CGRP, vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY), and galanin (GAL) was examined using a combination of double immunofluorescence labelling and a multiple dye filter system.