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pictures may be missing from this format of the document.*** Synopsis: Several neuroactive compounds have been implicated as playing roles in the circuitry that controls larval metamorphosis in marine molluscs. For the caenogastropod Ilyanassa obsoleta, results of neuroanatomical studies suggest that the production of nitric oxide (NO) increases throughout the planktonic stage and that NO production is necessary for the maintenance of the larval state, especially as it becomes metamorphically competent. Bath application or injection of exogenous serotonin (5HT) can initiate metamorphosis in competent larvae, and exogenous NO can inhibit such serotonergically-induced metamorphosis. Inhibition of endogenous nitric oxide synthase (NOS) can also trigger larval metamorphosis. The production of endogenous NO appears to decrease concurrently with the initiation of metamorphosis, but the specific interactions between serotonergic and nitrergic neurons are unknown. Evidence in support of NO acting to up-regulate the enzyme guanylyl cyclase (GC) is still equivocal. Thus, we do not yet know if NO exerts its effects through the actions of cyclic 3',5'-guanosine monophosphate (cGMP) or by a cGMP-independent mechanism. The ubiquity of nitrergic signal-ling and its significance for developing molluscan embryos and larvae are still the subject of speculation and require further investigation.

Production of nitric oxide (NO), an evolutionarily conserved, intercellular signaling molecule, appears to be required for the maintenance of the larval state in the gastropod mollusc Ilyanassa obsoleta. Pharmacological inactivation of endogenous nitric oxide synthase (NOS), the enzyme that generates NO, can trigger metamorphosis in physiologically competent larvae of this species. Neuropils in the brains of these competent larvae display histochemical reactivity for NADPH diaphorase (NADPHd), an indication of neuronal NOS activity. The intensity of NADPHd staining is greatest in the neuropil of the apical ganglion (AG), a region of the brain that contains the apical sensory organ and that innervates the bilobed ciliated velum, the larval swimming and feeding organ. Once metamorphosis is initiated, the intensity of NADPHd staining in the AG and presumably, concomitant NO production, decline. The AG is finally lost by the end of larval metamorphosis, some 4 days after induction. To determine if the neurons of the AG are a source of larval NO, we conducted immunocytochemical studies on larval Ilyanassa with commercially available antibodies to mammalian neuronal NOS. We localized NOS-like immunoreactivity (NOS-IR) to 3 populations of cells in competent larvae: somata of the AG and putative sensory neurons in the edge of the mantle and foot. Immunocytochemistry on pre-competent larvae demonstrated that numbers of NOS-IR cells in the AG increase throughout the planktonic larval stage.