Electrical Properties of the Cockroach Fast Coxal Depressor Motoneuron Soma Depend upon the Different Characteristics of Individual Calcium Components.
JANETTE D. MILLS AND ROBERT M. PITMAN.
School of Biological and Medical Sciences, Gatty Marine Laboratory, University of St. Andrews, Fife KY16 8LB, United Kingdom.
APStracts 4:131N, 1997.
ABSTRACT
The 'fast' coxal depressor motoneurone (Df) of the cockroach is among the most extensively studied of insect neurones. It has been shown that the cell body of this neurone can exhibit active electrical properties, which may change over time or with chemical modulation. To further understand these electrical events and their modulation, inward currents in Df have been characterised under conditions in which outward currents have been suppressed.
The inward current activated at potentials positive to -60 mV and peaked between -10 and 0 mV when measured in barium saline and between 0 and +10 mV when measured in calcium saline. The inward current was insensitive to Ni2+ (100??M) but reduced by verapamil (50??M) and abolished by Cd2+ (1?mM).
Two components of ICa were identified by their sensitivity to either 50 ?M nifedipine or micromolar Cd2+. The nifedipine-sensitive component activated positive to -60 mV and peaked between -10 and 0 mV, whereas, the Cd2+-sensitive component activated positive to -40 mV and peaked between +10 and +20 mV.
Immediately after dissection, depolarization of Df evoked plateau potentials, while one to four hours after dissection, depolarization evoked action potentials. The plateau potentials were insensitive to 100 ?M Cd2+ but blocked by 50 ?M nifedipine, whereas, the spikes required a combination of nifedipine (50??M) and Cd2+ (100??M) for complete suppression, indicating that only one component of ICa contributes to the plateau potential, while both components contribute to action potentials.
Currents measured in calcium saline decayed faster than currents measured in barium saline. The inactivation characteristics were investigated using double-pulse voltage-clamp experiments. ICa showed a greater degree of inactivation and slower recovery from inactivation than did IBa. Current decay and the extent of inactivation were reduced after injection of the calcium-chelator BAPTA. This suggests that the calcium current of this neurone displays calcium-dependent inactivation. An additional mechanism, most probably voltage-dependent inactivation, also occurs since IBa even in neurones injected with BAPTA displayed some inactivation.
The inactivation characteristics may be important in determining activity displayed by Df. Indirect evidence suggests that intracellular calcium is high immediately after dissection. At this time, the calcium current may, therefore, be reduced due to calcium-dependent inactivation. This may, at least partly, explain why the cell does not spike shortly after dissection.
Received 4 February 1997; accepted in final form 7 July 1997.
APS Manuscript Number J103-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 24 July 1997