The Journal of Neurophysiology Vol. 78 No. 6 December 1997, pp. 3125-3132
Copyright ©1997 The American Physiological Society

Voltage-Dependent Conductances in Cephalopod Primary Sensory Hair Cells

Abdesslam Chrachri¹ and Roddy Williamson^{1, 2}

¹ The Marine Biological Association of the United Kingdom, The Laboratory, Citadel Hill, Plymouth PL1 2PB; and ² Department of Biology, University of Plymouth, Plymouth PL4 8AA, United Kingdom

Chrachri, Abdesslam and Roddy Williamson. Voltage-dependent conductances in primary sensory hair cells. J. Neurophysiol. 78: 3125-3132, 1997. Cephalopods, such as sepia, squid, and octopus, show a well-developed and sophisticated control of balance particularly during prey capture and escape behaviors. 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 a 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 (I_A) 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 40 mV and hence could be separated from I_A 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 (I_Na). First, it was blocked by tetrodotoxin (TTX); second, it also was blocked by Na⁺-free saline; and third, it was inactivated when primary hair cells were held at a potential more than 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 also was blocked after external application of 2 mmol/l Co²⁺ or Cd²⁺. Furthermore, this current was reduced significantly in a dose-dependent manner by nifedipine, suggesting that it is an L-type Ca²⁺ current (I_Ca).

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