Functional role of serotonergic neuromodulation in Aplysia.

The serotonergic metacerebral cell (MCC) of the mollusk Aplysia produces slow synaptic potentials in motor neurons of the buccal muscle, and increases the rate of ongoing rhythmic burst output of the buccal ganglion. In addition, the MCC acts peripherally to enhance the strength of buccal muscle contractions that are produced by firing of motor neurons. The potentiation of contraction is not associated with any detectable changes of resting membrane potential of muscle cells. Although MCC activity produces a small enhancement of excitatory junctional potentials, several experiments clearly indicate that the MCC has a direct potentiating effect on excitation-contraction coupling. The data suggest that potentiation of contraction might be mediated by cAMP. For example, activity of the MCC enchances the rate of accumulation of cAMP in buccal muscle, application of phosphodiesterase resistant analogs of cAMP potentiates muscle contraction, and a phosphodiesterase inhibitor enhances the effect of MCC stimulation. Recordings from free-moving animals indicate that the MCC becomes activated by exposure of the animal to food stimuli, and that the activation parallels the presence of a food-arousal state. Food-arousal is characterized by enhanced strength and increased frequency of biting responses. Both these effects can result from activity of the MCC. Thus, in this system, modulatory synaptic actions function to provide the substrate for a type behavioral modulation.