NADPH-Diaphorase activity changes during gangliogenesis and metamorphosis in the gastropod mollusc Ilyanassa obsoleta

Gaseous nitric oxide (NO) is produced through the action of the enzyme nitric oxide synthase (NOS) and acts as a neurotransmitter (Jacklet and Gruhn, 1994b; Elphick et al., 1995a; Jacklet, 1995) in the nervous systems of adult gastropod molluscs. By comparison, little or no information appears to exist about the ontogeny of molluscan NOS‐containing neurons. NADPH‐diaphorase (NADPHd) has been determined biochemically and histochemically to colocalize with NOS immunoreactivity in neurons; NOS is an isoform of NADPHd (Dawson et al., 1991; Hope et al., 1991). We used NADPHd histochemistry to map the distribution of NOS activity in the nervous systems of larvae, including metamorphosing individuals, and juveniles of the marine snail Ilyanassa obsoleta. Several ganglionic neuropils displayed reaction product throughout development. The most intense NADPHd staining occurred in the neuropil of the apical ganglion, a specialized larval structure. Intermediate staining levels occurred in neuropils of the cerebral, pedal, and pleural ganglia. Larval buccal and intestinal ganglia showed little reaction product, with slight increases arising in metamorphically competent larvae. NADPHd activity conspicuously decreased in the central nervous systems of metamorphosing larvae. The osphradial ganglion, which was present in young larvae, showed only weak NADPHd activity. Our results provide evidence for the existence of a nitrergic signalling system in molluscan larvae and juveniles. © 1996 Wiley‐Liss, Inc.