Contraction, Serotonin-Elicited Modulation, and Membrane Currents of Dissociated Fibers of Aplysia Buccal Muscle

Feeding muscles of the buccal mass of Aplysia are innervated by cholinergic and serotonergic neurons. Buccal muscle 15 contracts in response to acetylcholine (ACh). During feeding arousal, ACh-elicited contraction of muscle 15 is potentiated by serotonin (5-HT). This paper demonstrates a dissociated cell preparation of muscle 15 in which cellular mechanisms regulating contraction can be investigated. Dissociated muscle fibers contracted in response to both KCl and ACh. Serotonin (10-6 M) significantly potentiated the shortening caused by both KCl and ACh. Potentiation lasted at least 4 min, similar to potentiation in intact muscles. Four types of currents recorded by patch clamp methods are illustrated. With 540 mM KCl in the patch electrode, stretch-activated channels having a chord conductance of 150 pS are observed in on-cell patches. In whole cell configuration, ACh elicits inward current at a holding potential of -60 mV. With high potassium in the electrode, depolarization elicits an outward current. The voltage-dependent outward current is blocked with cesium in the electrode and 4-aminopyridine and tetraethylammonium outside the cells. The remaining voltage-dependent inward current is calcium dependent. The voltage-dependent inward and outward currents are activated within the range of depolarization produced by ACh and may therefore play roles in regulating contractile responses elicited by ACh.