The basic elements of the NO/cGMP signaling pathway have been identified in the nervous
systems of animals from nearly all of the major phyla. In crustaceans, the NO/cGMP
pathway is associated with certain fundamental neuronal processes, including sensory
integration and the organization and production of motor behavior. Here I review the
evidence for NO synthesis and action in crustacean neural networks, with an emphasis
on the rhythmic motor circuits of the crab stomatogastric ganglion (STG). In the STG,
NO appears to be released as an orthograde transmitter from descending projection
neurons. NO's receptor, a cytopasmic isoform of guanylate cyclase (sGCI, is expressed
in a subset of the cells that participate in the gastric mill and pyloric central
pattern generating networks. In spontaneously-active, in vitro preparations of the
STG, pharmacological inhibitors of the NO/cGMP pathway cause the two rhythmic motor
patterns to collapse into a single conjoint rhythm. Parallel motor output is restored
when the ganglion is returned to normal saline. Although precise mechanisms have yet
to be determined, these data suggest that NO and cGMP play an important role in the
functional organization of STG networks. The STG, as well as other crustacean models,
provides a promising context for studying the physiological and behavioral aspects
of NO-mediated signaling in the nervous system.
