Modification of the effects of glutamate by nitric oxide (NO) in a pattern-generating network

Previous studies have shown that nitric oxide (NO) transforms the responses of various neurons to glutamate, though it remained unclear whether this mechanism is involved in the formation of behavior. We therefore studied the buccal generator of the feeding rhythm of the mollusk Lymnaea stagnalis (pond snail). In this organism, glutamate and NO are synthesized by defined neurons; glutamate is the neurotransmitter for the second phase of the standard triphasic feeding rhythm. Motoneuron B4 was used for monitoring. Studies using isolated CNS preparations showed that in some cases glutamate evoked hyperpolarization of B4 and terminated rhythmic network activity (n=17; group 1), while in other cases glutamate evoked depolarization of B4 and activation of a non-standard biphasic rhythm (n=12; group 2). In group 1, the NO donor nitroprusside lifted the inhibitory effect of glutamate (n=13), with transformation into an excitatory effect in nine cases. In group 2, the NO acceptor PTIO transformed the excitatory effect of glutamate into an inhibitory effect (n=7). These results provide evidence that: 1) the responses of the central generator of the buccal motor rhythm to glutamate depend on the NO level, and 2) this regulatory mechanism can modify feeding behavior.