Slowly inactivating outward currents in a cuticular mechanoreceptor neuron of the cockroach ( Periplaneta americana ). Torkkeli, Paivi H., and Andrew S. French. Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia, B3H 4H7, Canada.
APStracts 2:0147N, 1995.
SUMMARY AND CONCLUSIONS
1 . Although rapid adaptation is a widespread feature of sensory receptors, its ionic basis has not been clearly established in any touch receptor, because their small sizes have severely restricted the range of experiments that can be performed. In the cockroach tactile spine, intracellular voltage- clamp recordings are now possible. 2 . The basic electrophysiological properties of the cockroach femoral tactile spine neuron were studied using discontinuous (switching) single-electrode current- and voltage-clamp recordings. A slowly inactivating voltage-sensitive K + outward current was detected after the major inward currents were blocked with tetrodotoxin (TTX). 3 . The total outward current activated in less than 1 ? ms at voltages above 0 ? mV. At moderate depolarizations it did not inactivate, but at higher depolarizations an inactivation time constant of 260 ? ms was measured. Some recordings also revealed an additional, slower inactivation time constant of 2.5 s. 4 . More than half of the voltage sensitive K + outward current could be blocked with the Ca 2+ channel blockers Co 2+ and Cd 2+ . Tetraethylammonium chloride (TEA) also reduced the amplitude of the outward current to about half of its original amplitude. The actions of both blockers were reversible, and probably reflect overlapping blockades of two separate outward currents. 5 . The reversal potentials of the currents that remained after block with Co 2+ (-91.7 ? mV) or TEA (-86.8 ? mV) were both near the K + equilibrium potential expected for the tactile spine neuron. The voltage dependencies of activation of the Co 2+ - and TEA-resistant currents were both well fitted by Boltzmann distributions, giving values of half maximal activation (V 50 ) equal to -34.5 ? mV for the Co 2+ -resistant current and - 51.3 ? mV for the TEA-resistant current. 6 . Current-clamp recordings revealed that the TEA-sensitive K + current was the major component of action potential repolarization, but that it did not effect the frequency of action potentials evoked by steady depolarization. On the other hand, blockers of Ca 2+ -sensitive K + currents (Cd 2+ , Co 2+ or charybdotoxin) reduced adaptation and increased the frequency of action potentials significantly, but did not effect the duration or amplitude of individual action potentials. 

Received 15 February 1994; accepted in final form 2 May 1995.
APS Manuscript Number J111-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 16 May 1995.