Inhibitors of nitric oxide synthase induce larval settlement and metamorphosis of the polychaete annelid Capitella teleta

Abstract

The neurotransmitter nitric oxide (NO) has been implicated in the inhibitory control of metamorphosis of some marine gastropods, echinoderms, and ascidians. We have explored whether or not metamorphosis of metatrochophore larvae of the polychaete annelid Capitella teleta is also regulated by NO. Immunohistochemical analysis revealed a bilateral group of three large nitric oxide synthase (NOS) immunoreactive cells that lie dorsal to the pharynx and extend ventral processes toward the pharynx in the region of the dorsal pharyngeal pad. Smaller NOS-immunoreactive cells were distributed widely throughout the body but concentrated in the prostomium and pygidium. Histological analysis for NO, using the NO detector diaminofluorescein-FM, showed that NO concentration was high in the larval midgut, although diffuse amounts of NO were detected throughout the body. Inhibitors of NOS, including s-methylisothiourea sulfate, aminoguanidine hemisulfate, 7-nitroindazole, and N-methyl-L-arginine all induced settlement and metamorphosis of the Capitella larvae in a concentration-dependent manner. The NO donor nitroprusside prevented the induction of settlement and metamorphosis induced by the NOS inhibitor N-methyl-L-arginine, but did not prevent settlement and metamorphosis induced by a marine sediment extract, exogenous serotonin, or by the serotonin reuptake inhibitor fluoxetine. Pre-incubating larvae with the serotonin receptor antagonist ketanserin also inhibited settlement and metamorphosis in response to NOS inhibitors. These results suggest that endogenous production of NO maintains the larval state in C. teleta, and that endogenous serotonin stimulates metamorphosis in a way similar to that described previously for larvae from other major invertebrate phyla.