The presence and possible role of a galanin-like peptide in the mudpuppy heart

Abstract

A correlated histochemical and pharmacological study was undertaken to establish the presence, origin, and possible function of nerve fibers containing a galanin-like peptide in the mudpuppy ( Necturus maculosus) heart. Whole mount preparations of septum-sinus venosus or atria and sections of ventricular muscle were prepared for immunocytochemistry. Galanin-immunoreactive fibers were found coursing diffusely across the septum-sinus venosus to form complex networks over cardiac muscle strands. Individual atrial muscle strands were densely innervated by galanin-immunoreactive fibers and galaninimmunoreactive fibers were also observed in the epicardial and myocardial layers of the ventricle. Most of the parasympathetic postganglionic neurons in the cardiac ganglion and many of the small intensely fluorescent-like cells exhibited galanin immunoreactivity. Galanin-immunoreactive fibers were present in the nerve trunks connecting clusters of parasympathetic postganglionic neurons. Close associations between galanin-positive fibers and individual parasympathetic postganglionic neurons were also observed. The presence of the galanin-immunoreactive fibers was similar in preparations taken from animals pretreated with 6-hydroxydopamine to that seen in preparations taken from control animals, indicating that the galanin-positive fibers were not sympathetic postganglionic axons. Moreover, the galaninimmunoreactive nerve fibers were separate from fibers containing substance P and/or calcitonin gene-related peptide that have previously been shown to be processes of afferent fibers. In twitch-tension experiments, galanin in the range 1 × 10 −7 to 1 × 10 −6M caused cardioinhibition of spontaneously beating isolated septal-sinus venosus preparations. Galanin also produced a concentrationdependent 1 × 10 −7 to 1 × 10 −6M decrease in the twitch-tension development of electrically stimulated atrial or ventricular preparations. Local application of galanin produced hyperpolarization of cardiac muscle fibers in both isolated septal-sinus venosus preparations and atrial preparations. The response of individual parasympathetic ganglion cells to local application of galanin varied between neurons; some neurons were depolarized whereas others were hyperpolarized. We conclude that a galanin-like peptide is contained in both the parasympathetic postganglionic neurons and small intensely fluorescent-like cells and their processes. Further, we hypothesize that in the case of the parasympathetic postganglionic neurons, the galanin-like peptide may work in conjunction with acetylcholine to regulate cardiac activity.