Low-Voltage Activated Calcium Current Does not Regulate the Firing Behavior in Paired Mechanosensory Neurons with Different Adaptation Properties. Shin-Ichi Sekizawa, Andrew S. French and Päivi H.Torkkeli. Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia, B3H 4H7, Canada.
APStracts 6:0523N, 1999.
Low-voltage activated Ca2+-currents (LVA-ICa) are believed to perform several roles in neurons such as lowering the threshold for action potentials, promoting burst firing and oscillatory behavior, and enhancing synaptic excitation. They may also allow rapid increases in intracellular Ca2+-concentration. We discovered LVA-ICa in both members of paired mechanoreceptor neurons in a spider, where one neuron adapts rapidly (Type A) and the other slowly (Type B) in response to a step stimulus. To learn if ICa contributed to the difference in adaptation behavior we studied the kinetics of ICa from isolated somata under single-electrode voltage-clamp and tested its physiological function under current-clamp. LVA-ICa was large enough to fire single action potentials when all other voltage-activated currents were blocked, but we found no evidence that it regulated firing behavior. LVA-ICa did not lower the action potential threshold or affect firing frequency. Previous experiments have failed to find Ca2+-activated K+-current (IK(Ca)) in the somata of these neurons, so it is also unlikely that LVA-ICa interacts with IK(Ca) to produce oscillatory behavior. We conclude that LVA-Ca2+-channels in the somata, and possible in the dendrites, of these neurons open in response to the depolarization caused by receptor current and by the voltage-activated Na+-current (INa) that produces action potential(s). However, the role of the increased intracellular Ca2+-concentration in neuronal function remains enigmatic.
Received 2 August 1999; accepted in final form 14 October 1999.
APS Manuscript Number J648-9.
Article publication pending Journal of Neurophysiology.
ISSN 1080-4757 Copyright 1999 The American Physiological Society.
Published in APStracts on 21 December 1999