Distribution and morphology of cholinergic cells in the branchial epithelium of zebrafish (Danio rerio).

Acetylcholine is an excitatory neurotransmitter important for oxygen sensing in mammals. A cholinergic mechanism in the fish gill has been implicated in the hyperventilatory response to acute hypoxia; however, the identity and distribution of acetylcholine-containing cells in the gills is poorly defined. We test the hypothesis that cholinergic cells are present in the gill filament epithelium in zebrafish (Danio rerio), a model vertebrate for which oxygen chemoreceptors are well characterized, and that these cells would receive nervous innervation. Using immunohistochemistry and confocal microscopy, we observed 10.2 ± 0.6 cells immunoreactive for the vesicular acetylcholine transporter (VAChT) on the efferent aspect of each gill filament, where a high density of serotonergic oxygen-sensitive neuroepithelial cells (NECs) were located. VAChT-positive cells of the efferent epithelium were positioned within 10 μm of NECs. On the afferent aspect of the gill filaments, VAChT-positive cells were greater in number (30.8 ± 3.1 per filament). On the efferent and afferent filament aspects, VAChT-positive cells did not contain serotonin, but did express choline acetyltransferase (ChAT), the enzyme that synthesizes ACh, and were often closely apposed to nerve fibers labeled with the neuronal marker, zn-12. We conclude that cholinergic cells in the zebrafish gills were present in the primary epithelium of gill filaments, and formed contacts with nerve fibers. These studies provide morphological evidence for the presence of a cholinergic system in the zebrafish gill. Such a pathway may contribute to the reflex hyperventilatory response during hypoxia.