The Journal of Neurophysiology Vol. 78 No. 5 November 1997, pp. 2455-2466
Copyright ©1997 The American Physiological Society

Electrical Properties of a Cockroach Motor Neuron Soma Depend on Different Characteristics of Individual Ca 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

Mills, Janette D. and Robert M. Pitman. Electrical properties of a cockroach motor neuron soma depend on different characteristics of individual Ca components. J. Neurophysiol. 78: 2455-2466, 1997. The "fast" coxal depressor motor neuron (D_f) of the cockroach is among the most extensively studied of insect neurons. It has been shown that the cell body of this neuron 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 D_f have been characterized 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 µM) but reduced by verapamil (50 µM) and abolished by Cd²⁺ (1 mM). Two components of I_Ca were identified by their sensitivity to either 50 µM 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 D_f evoked plateau potentials, whereas 1-4 h after dissection, depolarization evoked action potentials. The plateau potentials were insensitive to 100 µM Cd²⁺ but blocked by 50 µM nifedipine, whereas the spikes required a combination of nifedipine (50 µM) and Cd²⁺ (100 µM) for complete suppression, indicating that only one component of I_Ca contributes to the plateau potential, whereas both components contribute to action potentials. Currents measured in calcium saline decayed faster than currents measured in barium saline. The inactivation characteristics were investigated with the use of double-pulse voltage-clamp experiments. I_Ca showed a greater degree of inactivation and slower recovery from inactivation than did I_Ba. Current decay and the extent of inactivation were reduced after injection of the calcium-chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid (BAPTA). This suggests that the calcium current of this neuron displays calcium-dependent inactivation. An additional mechanism, most probably voltage-dependent inactivation, also occurs because I_Ba, even in neurons injected with BAPTA, displayed some inactivation. The inactivation characteristics may be important in determining activity displayed by D_f. 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.

This article has been cited by other articles:

				A. Husch, S. Hess, and P. Kloppenburg Functional Parameters of Voltage-Activated Ca2+ Currents From Olfactory Interneurons in the Antennal Lobe of Periplaneta americana J Neurophysiol, January 1, 2008; 99(1): 320 - 332. [Abstract] [Full Text] [PDF]

				B. Ch. Ludwar, S. Westmark, A. Buschges, and J. Schmidt Modulation of Membrane Potential in Mesothoracic Moto- and Interneurons During Stick Insect Front-Leg Walking J Neurophysiol, October 1, 2005; 94(4): 2772 - 2784. [Abstract] [Full Text] [PDF]

				Y. Li and D. J. Bennett Persistent Sodium and Calcium Currents Cause Plateau Potentials in Motoneurons of Chronic Spinal Rats J Neurophysiol, August 1, 2003; 90(2): 857 - 869. [Abstract] [Full Text] [PDF]

				J. Haag and A. Borst Spatial Distribution and Characteristics of Voltage-Gated Calcium Signals Within Visual Interneurons J Neurophysiol, February 1, 2000; 83(2): 1039 - 1051. [Abstract] [Full Text] [PDF]

				M. Jeziorski, R. Greenberg, and P. Anderson The molecular biology of invertebrate voltage-gated Ca(2+) channels J. Exp. Biol., January 3, 2000; 203(5): 841 - 856. [Abstract] [PDF]

				P. Benquet, J. L. Guen, G. Dayanithi, Y. Pichon, and F. Tiaho omega -AgaIVA-Sensitive (P/Q-type) and -Resistant (R-type) High-Voltage-Activated Ba2+ Currents in Embryonic Cockroach Brain Neurons J Neurophysiol, November 1, 1999; 82(5): 2284 - 2293. [Abstract] [Full Text] [PDF]

				V. Morisset and F. Nagy Ionic Basis for Plateau Potentials in Deep Dorsal Horn Neurons of the Rat Spinal Cord J. Neurosci., September 1, 1999; 19(17): 7309 - 7316. [Abstract] [Full Text] [PDF]

				J. D. Mills and R. M. Pitman Contribution of Potassium Conductances to a Time-Dependent Transition in Electrical Properties of a Cockroach Motoneuron Soma J Neurophysiol, May 1, 1999; 81(5): 2253 - 2266. [Abstract] [Full Text] [PDF]

				R. Hewes Voltage-dependent ionic currents in the ventromedial eclosion hormone neurons of Manduca sexta J. Exp. Biol., January 9, 1999; 202(17): 2371 - 2383. [Abstract] [PDF]

				H. Le Corronc, B. Hue, and R. M. Pitman Ionic Mechanisms Underlying Depolarizing Responses of an Identified Insect Motor Neuron to Short Periods of Hypoxia J Neurophysiol, January 1, 1999; 81(1): 307 - 318. [Abstract] [Full Text] [PDF]

				C. Amat, B. Lapied, A. S. French, and B. Hue Na+-Dependent Neuritic Spikes Initiate Ca2+-Dependent Somatic Plateau Action Potentials in Insect Dorsal Paired Median Neurons J Neurophysiol, November 1, 1998; 80(5): 2718 - 2726. [Abstract] [Full Text] [PDF]