The Journal of Neurophysiology Vol. 77 No. 3 March 1997, pp. 1496-1502
Copyright ©1997 The American Physiological Society

Serotonergic Modulation of a Voltage-Gated Calcium Current in Parapodial Swim Muscle From Aplysia brasiliana

P. J. Laurienti and J. E. Blankenship

Marine Biomedical Institute, University of Texas Medical Branch, Galveston, Texas 77555-1069

Laurienti, P. J. and J. E. Blankenship. Serotonergic modulation of a voltage-gated calcium current in parapodial swim muscle from Aplysia brasiliana. J. Neurophysiol. 77: 1496-1502, 1997. Here we describe the effects of serotonin (5-HT) on dissociated parapodial muscle fibers from Aplysia brasiliana. 5-HT has previously been implicated as a modulatory transmitter at the parapodial neuromuscular junction. Exogenously applied or endogenously released 5-HT increases the amplitude of motoneuron-induced excitatory junctional potentials and contractions in parapodial muscle. Exogenously applied 5 µM 5-HT increases the amplitude of a voltage-gated inward calcium current in isolated muscle fibers by an average of 42% in response to a voltage step from 70 to 10 mV. The amplitude of the inward current was increased at all voltages tested, with the peak increase occurring between 30 and 20 mV. The dihydropyridine calcium channel antagonist nifedipine (10 µM) blocked this effect of 5-HT. The data indicate that 5-HT increases a previously identified calcium current in parapodial muscle fibers that is similar to the vertebrate L-type current. Although several types of K⁺ channels exist in these fibers, including Ca²⁺-dependent K⁺ channels, the results suggest that 5-HT has little effect on these currents. Parapodial muscle contractions during swimming behavior occur in response to bursts of motoneuron action potentials that produce graded muscle depolarizations that occur over a 1- to 2-s period rather than being instantaneous or rapid responses as might be produced by one or two action potentials or a brief voltage step. With the use of 1-s voltage ramps, we attempted to mimic physiological depolarization and demonstrate that 5-HT is able to increase the amplitude of the inward calcium current. The data presented in this paper provide evidence that 5-HT increases the Ca²⁺ current, which may be one mechanism by which 5-HT modulates muscle contractions during swim behavior.

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