Development of nitric oxide synthase-defined neurons in the sea urchin larval ciliary band and evidence for a chemosensory function during metamorphosis.

We previously reported that initiation of metamorphosis of larvae of Lytechinus pictus is negatively regulated by nitric oxide (NO) and cGMP. We have examined the expression of nitric oxide synthase (NOS) and cGMP in cells of the developing larva. A section of the post-oral ciliary band of feeding larvae includes neural cells defined by their expression of both NOS and the echinoderm neural-specific antibody 1E11. These neurons project processes to the pre-oral neuropile during larval development. Larvae regenerated this section of the ciliary band after its excision, complete with NOS-defined neurons that projected again to the pre-oral neuropile. Excision of ectoderm containing the post-oral ciliary band prevented a behavioral and morphogenetic response of competent larvae to biofilm, and delayed initiation of metamorphosis. Elevated cGMP levels were detected in several larval and juvenile cell types prior to metamorphosis. Treatment of larvae with ODQ, an inhibitor of soluble guanylate cyclase, decreased cGMP levels and induced metamorphosis while a generator of NO counteracted this effect, indicating inhibition of metamorphosis by NO operates via interaction with soluble guanylate cyclase. We discuss these observations, proposing that the NOS-defined neurons in the post-oral ciliary band have a chemosensory function during settlement and metamorphosis that involves morphologically specialized ectoderm and manipulation of fluid flow. We provide a tentative cellular model of how environmental signals may be transduced into a metamorphic response.