A delayed role for nitric oxide-sensitive guanylate cyclases in a migratory population of embryonic neurons

Neuronal differentiation requires a coordinated intracellular response to d iverse extracellular stimuli, but the role of specific signaling mechanisms in regulating this process is still poorly understood. Soluble guanylate c yclases (sGCs), which can be stimulated by diffusible free radical gasses s uch as nitric oxide (NO) and carbon monoxide (CO) to produce the intracellu lar messenger cGMP, have recently been found to be expressed within a varie ty of embryonic neurons and implicated in the control of both neuronal moti lity and differentiation. Using the enteric nervous system (ENS) of the mot h, Manduca sexta, we examined the role of NO and NO-sensitive sGCs during t he migration and differentiation of an identified set of migratory neurons (the EP cells). Shortly after the onset of their migration, a subset of EP cells began to express NO-sensitive sGC activity (visualized with an anti-c GMP antiserum). Unlike many neurons in the central nervous system, the expr ession of sGC activity in the EP cells was not transient but persisted thro ughout subsequent periods of axon elongation and terminal branch formation on the gut musculature. In contrast, nitric oxide synthase activity (visual ized using NADPH-diaphorase histochemistry) was undetectable in the vicinit y of the EP cells until the period of synapse formation. Manipulations desi gned to alter sGC and NOS activity in an in vivo embryonic culture preparat ion had no discernible effect on either the migration or axonal outgrowth o f the EP cells. In contrast, inhibition of both of these enzymes resulted i n a significant reduction in terminal synaptic branch formation within the postmigratory neurons. These results indicate that while NO-sensitive sGC a ctivity is expressed precociously within the EP cells during their initial migratory dispersal, a role for this signaling pathway can only be demonstr ated well after migration is complete, coincident with the formation of mat ure synaptic connections. (C) 1998 Academic Press.