Nitric Oxide Synthase Activity in the Molluscan CNS

: Putative nitric oxide synthase (NOS) activity was assayed in molluscan CNS through histochemical localization of NADPH‐diaphorase and through measurement of l‐arginine/l‐citrulline conversion. Several hundreds of NADPH‐dependent diaphorase‐positive neurons stained consistently darkly in the nervous system of the predatory opisthobranch Pleurobranchaea californica, whereas stained neurons were relatively sparse and/or light in the other opisthobranchs (Philine, Aplysia, Tritonia, Flabellina, Cadlina, Armina, Coriphella, and Doriopsilla sp.) and cephalopods (Sepia and Rossia sp.). l‐Arginine/l‐citrulline conversion was β‐NADPH dependent, insensitive to removal of Ca2+, inhibited by the calmodulin blocker trifluoperazine, and inhibited by the competitive NOS inhibitor N‐nitro‐l‐arginine methyl ester (l‐NAME) but not d‐NAME. Inhibitors of arginase [l‐valine and (+)‐S‐2‐amino‐5‐iodoacetamidopentanoic acid)] did not affect l‐citrulline production in the CNS. NOS activity was largely associated with the particulate fraction and appeared to be a novel, constitutive Ca2+‐independent isoform. Enzymatic conversion of l‐arginine/l‐citrulline in Pleurobranchaea and Aplysia CNS was 4.0 and 9.8%, respectively, of that of rat cerebellum. l‐Citrulline formation in gill and muscle of Pleurobranchaea was not significant. The localization of relatively high NOS activity in neuron somata in the CNS of Pleurobranchaea is markedly different from the other opisthobranchs, all of which are grazers. Potentially, this is related to the animal's opportunistic predatory lifestyle.