Voltage-Dependent Conductances in Primary Sensory Hair Cells. Abdesslam Chrachri and Roddy Williamson. The Marine Biological Association of the UK, Citadel Hill, Plymouth PL1 2PB, England1 and Dept. Biology, University of Plymouth, Plymouth, PL4 8AA,England.
APStracts 4:155N, 1997.
ABSTRACT
Cephalopods, such as sepia, squid and octopus, show a well developed and sophisticated control of balance particularly during prey capture and escape behaviours. There are two separate areas of sensory epithelium in cephalopod statocysts, a macula/statolith system which detects linear accelerations (gravity) and a crista/cupula system which detects rotational movements. The aim of this study is to characterize the ionic conductances in the basolateral membrane of primary sensory hair cells. These were studied using whole-cell patch clamp technique which allowed us to identify five ionic conductances in the isolated primary hair cells; an inward sodium current, an inward calcium current and three potassium outward currents. These outward currents were distinguishable on the basis of their voltage-dependence and pharmacological sensitivities. First, a transient outward current (IA) was elicited by depolarizing voltage steps from a holding potential of -60 mV, was inactivated by holding the cell at -40 mV and was blocked by 4-aminopyridine. A second, voltage-sensitive, outward current with a sustained time course was identified. This current was not blocked by 4-aminopyridine, nor inactivated at a holding potential of -40mV and could hence be separated from IA using these protocols. A third outward current that depended on Ca2+ entry for its activation was detected, this current was identified by its sensitivity to Ca2+ channel blockers such as Co2+ and Cd2+ and by the N-shaped profile of its current-voltage curve. Inward currents were studied using cesium aspartate solution in the pipette to block the outward currents. Two inward currents were observed in the primary sensory hair cells. A fast transient inward current, which is presumably responsible for spike generation. This inward current appeared as a rapidly activating inward current; this was strongly voltage-dependent. Three lines of evidence suggest that this fast transient inward current is a Na+ current (INa). First, it was blocked by TTX; second, it was also blocked by Na+-free saline; and third, it was inactivated when primary hair cells were held at a potential above -40 mV. The sustained inward current was not affected by TTX and was increased in amplitude 5 min after equimolar Ba2+ replaced Ca2+ as a charge carrier. This inward current was also blocked after external application of 2 mmol l-1 Co2+ or Cd2+. Furthermore, this current was significantly reduced in a dose- dependent manner by nifedipine, suggesting that it is an L-type Ca2+ current (ICa). 

Received 1 July 1996; accepted in final form 17 July 1997.
APS Manuscript Number J513-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 28 August 1997