Characterization of a P-type calcium current in a crayfish motoneuron and its selective modulation by impulse activity.
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 approximately 10 mV in the I-V relationship. Application of the P-type Ca2+ channel antagonist omega-agatoxin IVA (omega-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 omega-conotoxin GVIA produced a reduction in the Ba2+ current. The voltage-dependent activation of this P-type (omega-AgTX-IVA-sensitive) Ca2+ channel was similar to previously identified P-type channels, but different from that of the non-P-type (omega-AgTX-IVA-resistant) Ca2+ channels. When Ca2+ currents were measured 6-7 h after an increase in impulse activity (5-Hz stimulation for 45-60 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, Ca(2+)-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.
Hong
S J
SJ
Department of Biological Sciences, University at Albany, State University of New York 12222, USA.
Lnenicka
G A
GA
D000818:Animals; D003400:Astacoidea / Q000502:physiology*; D001464:Barium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D004594:Electrophysiology; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009046:Motor Neurons / Q000187:drug effects / Q000502:physiology*; D018408:Patch-Clamp Techniques; D012604:Scorpion Venoms / Q000494:pharmacology; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D012604:Scorpion Venoms; C087603:agitoxin 2; D001464:Barium