| Summary: | In teleost fishes the neurochemicals involved in sensing and responding to hypoxia are unresolved. Serotonergic branchial neuroepithelial cells (NECs) are putative O2 chemoreceptors believed to be homologous to the neural crest (NC) derived APUD (amine-precursor uptake and decarboxylation) pulmonary NECs and carotid body type-1 glomus cells. Branchial NECs contain serotonin (5-HT), thought to be central to the induction of the hypoxic cardioventilatory reflexes. However, application of 5-HT in vivo does not elicit cardioventilatory reflexes similar to those elicited by hypoxia. But previous in vitro neural recordings from glossopharyngeal (IX) afferents innervating O2 chemoreceptors in the trout gill show the same discharge response to hypoxic conditions as does that of acetylcholine (ACh) application. This evidence strongly supports the cholinergic hypothesis of chemoreceptor impulse origin rather than a serotonergic-induced impulse origin model. We therefore hypothesized that NECs contain ACh among other neurochemicals in cells belonging to the APUD series. Although serotonergic branchial NECs did not colocalize with ACh using immunohistochemical methods, several populations of ACh and/or tyrosine hydroxylase (TH) (catecholaminergic) positive, dopamine (DA) negative, cells were found throughout the second gill arch of the channel catfish Ictalurus punctatus. In addition, the NC derivation marker zn-12 labelled the HNK-1-like epitope (Human natural killer) expressed by lamellar pillar cells’ collagen column-associated pillar cell adhesion molecules (CC-PCAMs), evidence confirming their hypothesized NC origin.
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