Serotonin and forskolin enhance both magnitude of contraction and relaxation rate of Aplysia dorsal extrinsic muscle independently of acetylcholine receptor

Hexamethonium bromide (Hex. Br.) blocks acetylcholine (ACh) elicited contractions but not electrically elicited contractions of isolated preparations of Aplysia californica dorsal extrinsic muscle. Serotonin (5-hydroxytryptamine, 5-HT) enhances both magnitude and relaxation rate of ACh and electrically elicited contractions. In the presence of Hex. Br., 5-HT still exhibits its modulatory effects on electrically elicited contractions. Forskolin enhances both magnitude and relaxation rate of ACh and electrically elicited contractions. Forskolin (10(-5) M) increases the cyclic AMP content of the accessory radula closer and dorsal extrinsic muscles.