Characterization of a P-type calcium current in a crayfish motoneuron and
its selective modulation by impulse activity.
Hong, Sungwon J. and Gregory A. Lnenicka.
Neurobiology Research Center, Department of Biological Sciences, University
at Albany, State University of New York Albany, NY 12222.
APStracts 3:0190N, 1996.
Previous studies have demonstrated that the voltage-dependent Ca 2+ 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 Ca 2+ channels undergoing this long-term change were
examined using two-electrode voltage-clamp techniques. The Ca 2+ current was
activated at _50 to _40 mV and its amplitude was maximal at 0 mV (_135.0 +/-
25.8 nA, n=14). The current-voltage (I-V) relationship and the greater
sensitivity of the Ca 2+ channel to Cd 2+ than Ni 2+ indicated that Ca 2+
influx occurs through high-voltage activated (HVA) Ca 2+ channels. Loose-patch
recordings demonstrated that the Ca 2+ current was generated by the membrane
of the cell body. When Ba 2+ was substituted for extracellular Ca 2+ , 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 Ca
2+ channel antagonist, _-agatoxin IVA (_-AgTX IVA), produced a significant 33%
(n=6) reduction in the peak amplitude of the Ba 2+ current (IC 50 : 183.5 +/-
45.4 nM), whereas neither the L-type Ca 2+ channel antagonist, nifedipine, nor
the N-type channel antagonist, _-conotoxin GVIA (_-CgTX GVIA), produced a
reduction in the Ba 2+ current. The voltage-dependent activation of this P-
type (_-AgTX IVA-sensitive) Ca 2+ channel was similar to previously identified
P-type channels, but different from that of the non-P-type (_-AgTX IVA-
resistant) Ca 2+ channels. When Ca 2+ currents were measured 6 to 7 hours
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 Ca 2+ channels contribute to the macroscopic Ca 2+
current observed in the cell body of this crayfish phasic motoneuron: one
belongs to the previously described P-type Ca 2+ channel and the other(s) does
not belong to the N, L, or P-type Ca 2+ channel. The long-term, Ca 2+ -
dependent reduction in Ca 2+ current previously demonstrated in motoneuron F-3
is produced by the selective reduction of this P-type Ca 2+ current. This
activity-dependent reduction in the P-type Ca 2+ current is likely involved in
the long-term depression of transmitter release observed at the neuromuscular
synapses of this motoneuron.
Received 10 April 1996; accepted in final form 28 August 1996.
APS Manuscript Number J384-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 7 October 1996