The distribution and colocalization of neuropeptides in fish cardiac neurons.

Most if not all intracardiac nerve cell bodies in four species of teleost fish and a primitive air breathing fish contained immunoreactivity (IR) to vasoactive intestinal peptide (VIP). Intracardiac nerve cell bodies contained no other neuropeptide although galanin (GAL)-, substance P (SP)- and calcitonin gene-related peptide (CGRP)-IR were detected in cardiac axons. Varicose VIP-IR axons were observed in close association to the cardiac muscle in the sinus venosus and atrium, but not in the ventricle. Slightly less than half the total number of VIP-IR axons also contained colocalised GAL-IR. A smaller number of varicose axons containing colocalised SP- and CGRP-IR were also present in the sinus venous and atrium. In addition, a subpopulation of CGRP-IR axons present in the sinus venosus and atrium did not contain SP-IR. SP-IR axons lacking CGRP-IR formed boutons around the axon hillock and soma of the majority of VIP-IR nerve cell bodies. Associated with a small number of VIP/-ganglion cells were VIP/- boutons. No neuropeptides were observed in the ventricle of any species of fish studied here. These results suggest that a VIP-like peptide is localised in the cholinergic postganglionic parasympathetic neurons. Associated with some of these neurons are nerve boutons containing either SP alone or VIP alone. In addition, the fish heart is innervated by extrinsic nerve fibres containing: GAL/VIP; CGRP alone; and CGRP/SP.