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 throug h measurement of L-arginine/L-citrulline conversion. Several hundreds of NADPH-dependent diaphorase-positive neurons stained consistently da rkly in the nervous system of the predatory opisthobranch Pleurobranch aea californica, whereas stained neurons were relatively sparse and/or light in the other opisthobranchs (Philine, Aplysia, Tritonia, Flabel lina, Cadlina, Armina, Coriphella, and Doriopsilla sp.) and cephalopod s (Sepia and Rossia sp.). L-Arginine/L-citrulline conversion was beta- NADPH dependent, insensitive to removal of Ca2+, inhibited by the calm odulin blocker trifluoperazine, and inhibited by the competitive NOS i nhibitor N-nitro-L-arginine methyl ester (L-NAME) but not D-NAME. Inhi bitors of arginase [L-valine and (+)-S-2-amino-5-iodoaceiamidopentanoi c acid)] did not affect L-citrulline production in the CNS. NOS activi ty was largely associated with the particulate fraction and appeared t o be a novel, constitutive Ca2+-independent isoform. Enzymatic convers ion of L-arginine/L-citrulline in Pleurobranchaea and Aplysia CNS was 4.0 and 9.8%, respectively, of that of rat cerebellum. L-Citrulline fo rmation 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 opisthobra nchs, all of which are grazers. Potentially, this is related to the an imal's opportunistic predatory lifestyle.