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Autonomic Innervation of the Swimbladder in the Zebrafish, Danio rerio.  F. M. Smith1, J. L. Finney2, G. N. Robertson1 and R. P. Croll2    1Dept. of Anatomy and Neurobiology, and 2Dept. of Physiology and Biophysics, Dalhousie University, Halifax, NS, Canada.     We investigated the anatomy and patterns of autonomic innervation of the adult zebrafish swimbladder as the first step in analysis of the reflex control of buoyancy, and thus 3-dimensional orientation, in this organism.  The zebrafish swimbladder consists of an anterior and posterior chamber connected by a narrow neck, with the posterior chamber connected to the esophagus by a patent pneumatic duct.  Phalloidin labelling of F-actin showed striated and smooth muscle in the wall of the duct, and bands of smooth muscle along the lateral aspects of the walls of both chambers.  Antibodies against zn-12 and SV2, markers for zebrafish neurons and synaptic vesicles, respectively, were used to show the overall pattern of swimbladder innervation and synaptic terminal and bouton distribution.  Antibodies against choline acetyltransferase and tyrosine hydroxylase  were used to label the parasympathetic and sympathetic components, respectively, of this innervation.  We also examined immunoreactivity for the neuropeptides NPY, VIP and serotonin, which are associated with the peripheral nervous system.  These studies were supplemented with histochemical procedures for detecting catcholamines and acetylcholinesterase.  Results of labelling with the general neuronal markers showed that the swimbladder and associated vasculature were richly innervated by both intrinsic and extrinsic autonomic neurons.  Parasympathetic innervation (neuronal somata and fibres) appeared to be limited to the portion of the pneumatic duct proximal to the esophagus.  In contrast, sympathetic nerve fibres were associated with the vasculature and muscle in the walls of the duct and both chambers of the swimbladder.   Peptides and serotonin were detected in nerves associated with vessels in the duct and walls of both swimbladder chambers.  Autonomic reflexes involved in buoyancy control are thus capable of acting via multiple effectors located in all regions of the  zebrafish swimbladder. 

(Supported by Canadian Space Agency and Natural Sciences and Engineering  Research Council of Canada.)