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Oxygen-sensing in the fish gill : a comparative and immunohistochemical study of putative oxygen-sensing cells Coolidge, Emily Hayes

Abstract

Numerous studies have determined approximate locations of chemoreceptive cells in freshwater fish, with the majority suggesting the gills as the primary oxygen-sensing site. Using immunohistochemistry, I investigated the distribution of neurotransmitters and markers for neuroepithelial cells (NECs), the putative oxygen sensing cells, in gills of freshwater fish with varying degrees of hypoxia tolerance. I compared the patterns of immunoreactivity of NEC markers (anti-serotonin and anti-synaptic vesicle marker) in two distantly related species with different hypoxia tolerances (Onchoryncus mykiss and Carassuis auratus) as well as two species from the same genus (Hoplias) that possess distinct differences in habitat and hypoxia tolerance. To speculate on the role of other potential chemosensory neurotransmitters in the gill, I also examined the distribution of catecholamines, acetylcholine, and adenosine. I found that all fish exhibited a cluster of innervated, serotonergic NECs at the filament tip acting as putative chemoreceptors, consistent with the ability of all fish to respond to aquatic hypoxia, while unique distributions of serotonergic NECs associated with the efferent filament artery correlated with physiological responses to internal hypoxia. These differences in serotonergic NEC distribution appeared to be adaptations related to hypoxia tolerance. Catecholaminergic cells were distinctly different from serotonergic cells, but often colocalized with acetylcholine. Cells containing both acetylcholine and catecholamine were located neighboring serotonergic cells. I propose that the non-innervated serotonergic cells of the lamellae, which are unique to hypoxia tolerant fish, play a chemosensory role in oxygen-sensing and function as neuromodulators acting directly on local vasculature or neighboring acetylcholinecatecholamine- containing cells.

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