The photoreceptors of Drosophila express a nitric oxide–sensitive guanylate cyclase during the first half of metamorphosis, when postsynaptic elements in the optic lobe are being selected. Throughout this period, the optic lobes show NADPH-diaphorase activity and stain with an antibody to nitric oxide synthase (NOS). The NOS inhibitor L-NAME, the NO scavenger PTIO, the sGC inhibitor ODQ, and methylene blue, which inhibits NOS and guanylate cyclase, each caused the disorganization of retinal projections and extension of photoreceptor axons beyond their normal synaptic layers in vitro. The disruptive effects of L-NAME were prevented with the addition of 8-bromo-cGMP. These results suggest NO and cGMP act to stabilize retinal growth cones at the start of synaptic assembly.
