Serotonin has both excitatory and inhibitory modulatory effects on feeding muscles in Aplysia.

Serotonin (10(-8) M) produced opposite long-lasting (up to 10 min) effects on acetylcholine-elicited contractions of different buccal mass muscles of Aplysia. Contractions of the dorsal extrinsic muscle and accessory radula closer muscle were enhanced by serotonin; whereas contractions of the ventral extrinsic muscle were inhibited by serotonin. The effect of higher concentrations of serotonin on dorsal and ventral extrinsic muscles was in the same direction as at 10(-8) M but was greater in both magnitude and duration. The phase of feeding-protraction or retraction--during which a muscle is active--is not correlated with the direction of modulation produced by serotonin.