NADPH-diaphorase reactivity in the Mauthner cells of the swordtail fish, Xiphophorus helleri

The presence of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPHDH) in fixed tissue was histochemically demonstrated in the Mauthner cells of the teleost fish Xiphophorus helleri. This is the first detection of the enzyme in these giant neurons (which are restricted to fishes and amphibians) of a gnathostomate vertebrate. NADPHDH reactivity in fixed tissue is thought to reflect the activity of nitric oxide synthase. Thus, nitric oxide, a gaseous intercellular messenger, is probably synthesized in the Mauthner cells.