The Journal of Neurophysiology Vol. 77 No. 1 January 1997, pp. 76-85
Copyright ©1997 The American Physiological Society

Characterization of a P-Type Calcium Current in a Crayfish Motoneuron and Its Selective Modulation by Impulse Activity

Sungwon J. Hong and Gregory A. Lnenicka

Neurobiology Research Center, Department of Biological Sciences, University at Albany, State University of New York, Albany, New York 12222

Hong, Sungwon J. and Gregory A. Lnenicka. Characterization of a P-type calcium current in a crayfish motoneuron and its selective modulation by impulse activity. J. Neurophysiol. 77: 76-85, 1997. Previous studies have demonstrated that the voltage-dependent Ca²⁺ current recorded from the cell body of the crayfish abdominal motoneuron, F3, undergoes a long-term reduction as a result of increased impulse activity. The properties of the Ca²⁺ channels undergoing this long-term change were examined with the use of two-electrode voltage-clamp techniques. The Ca²⁺ current was activated at 50 to 40 mV and its amplitude was maximal at 0 mV (135.0 ± 25.8 nA, mean ± SE, n = 14). The current-voltage relationship and the greater sensitivity of the Ca²⁺ channel to Cd²⁺ than Ni²⁺ indicated that Ca²⁺ influx occurs through high-voltage-activated (HVA) Ca²⁺ channels. Loose-patch recordings demonstrated that the Ca²⁺ current was generated by the membrane of the cell body. When Ba²⁺ was substituted for extracellular Ca²⁺, there was a 40% increase in the amplitude of the inward current and a negative shift of ~10 mV in the I-V relationship. Application of the P-type Ca²⁺ channel antagonist -agatoxin IVA (-AgTX IVA) produced a significant 33% (n = 6) reduction in the peak amplitude of the Ba²⁺ current, whereas neither the L-type Ca²⁺ channel antagonist nifedipine nor the N-type channel antagonist -conotoxin GVIA produced a reduction in the Ba²⁺ current. The voltage-dependent activation of this P-type (-AgTX-IVA-sensitive) Ca²⁺ channel was similar to previously identified P-type channels, but different from that of the non-P-type (-AgTX-IVA-resistant) Ca²⁺ channels. When Ca²⁺ currents were measured6-7 h after an increase in impulse activity (5-Hz stimulation for45-60 min), there was a 43% reduction in the amplitude of the P-type current, but no significant changes in the non-P-type current amplitude. These results demonstrate that at least two subtypes of HVA Ca²⁺ channels contribute to the macroscopic Ca²⁺ current observed in the cell body of this crayfish phasic motoneuron: one belongs to the previously described P-type Ca²⁺ channel and the other(s) does not belong to the N-, L-, or P-type Ca²⁺ channel. The long-term, Ca²⁺-dependent reduction in Ca²⁺ current previously demonstrated in motoneuron F3 is produced by the selective reduction of this P-type Ca²⁺ current. This activity-dependent reduction in the P-type Ca²⁺ current is likely involved in the long-term depression of transmitter release observed at the neuromuscular synapses of this motoneuron.

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