L-arginine via nitric oxide is an inhibitory feedback modulator of Aplysia feeding

An increase in L-arginine hemolymph concentration acts as a postingestion signal inhibiting Aplysia feeding. At physiological concentrations (a 10- M increase over background), the inhibitory effect of L-arginine is too weak to block feeding in hungry animals. However, a 10- M increase in L-arginine concentration acts along with another inhibitory stimulus, the sustained presence of food odor, to inhibit feeding after a period of access to food. A physiological concentration of L-arginine also blocked the excitatory effect of a stimulus enhancing feeding, pheromones secreted by mating conspecifics. High concentrations of L-arginine (2.5 mM) alone also inhibited ad libitum feeding. L-arginine is the substrate from which nitric oxide synthase (NOS) produces nitric oxide (NO). Both an NO donor and a 10- M increase in L-arginine inhibited biting in response to a weak food stimulus. Treatment with NOS inhibitors initiated food-finding and biting in the absence of food, indicating that food initiates feeding against a background of tonic nitrergic inhibition. Increased feeding in response to blocking NOS is accompanied by firing of the metacerebral (MCC) neuron, a monitor of food arousal. The excitatory effect on the MCC of blocking NOS is indirect. The data suggest that L-arginine acts by amplifying NO synthesis, which acts as a background stimulus inhib- iting feeding. Background modulation of neural activity and behavior by NO may also be present in other systems, but such modulation may be difficult to identify because its effects are evident only in the context of additional stimuli modulating behavior.