Effects of cinnarizine on calcium and pressure-dependent potassium currents in guinea pig vestibular hair cells

Abstract

In vestibular hair cells, K+ currents induced by rises in hydrostatic pressure have recently been demonstrated. These currents are inhibited by charybdotoxin, a blocker of Ca2+-dependent K+ channels. On the other hand, cinnarizine is a blocker of voltage-gated Ca2+ currents in hair cells and is used as a drug in conditions with vestibular vertigo. Our aim was to test in patch-clamp experiments (conventional whole-cell mode) whether cinnarizine, by reducing Ca2+ influx, inhibited Ca2+ and pressure-sensitive K+ currents in vestibular type-II hair cells of guinea pigs. A quantitatively similar inhibition of K+ currents was evoked by extracellular Ca2+ removal, cinnarizine (0.5 microM), and the L-type Ca2+ channel blocker nifedipine (3 microM). Cinnarizine abrogated increases of K+ currents induced by increases in the hydrostatic pressure (from 0.2 to 0.5 cm H2O). At a higher concentration (1 microM), cinnarizine elicited K+ current inhibitions larger than those elicited by Ca2+ removal. Moreover, it reduced K+ currents in the absence of Ca2+, in contrast to nifedipine. However, charybdotoxin abolished these effects of cinnarizine. We thus conclude that cinnarizine inhibits, by two mechanisms, pressure-induced currents that are sensitive to charybdotoxin and Ca2+. It reduces Ca2+ influx and exerts a Ca2+-independent inhibition, with a lower IC50 than that required for Ca2+ channel blockade. These two actions may importantly contribute to its therapeutic effects.

Neuropharmacology of Vestibular System Disorders

“…From the pharmacological point of view, the participation of calcium channels in neurotransmitter release at synaptic terminals constitutes the primary therapeutic target of calcium channel blockers, although the calcium-activated potassium current may also be a significant target contributing to the action of these drugs [34]. Neurotransmitter release in hair cells is mediated by high voltage-activated calcium channels (Ca V 1.3) that can be blocked by dihydropyridines [4, 8, 11, 112].…”

“…Cinnarizine, besides its L-type calcium channel blocking action [8], acts as a blocker of the pressure-sensitive potassium channels. This may be relevant in the vestibule, since this last type of channel can be activated when inner ear endolymphatic hydrops develops [34, 35]. In addition, cinnarizine and flunarizine have antihistaminic action (acting on H 1 receptor) and potential (although very weak) nicotinic receptor antagonism.…”

Ultrasonic bath depth control and regulation in single cell recordings

“…Therefore, pressure-sensitive currents induced, e.g., by a hydrostatic pressure of 0.2 cm H 2 O should be minimized [4,5]. On the other hand, the setup allows also quite accurate manipulations of the bath depth after calibration of the ultrasonic signal.…”

“…Vestibular type II hair cells were isolated as described [4]. The cell capacitance was measured during continuous superfusion in patch-clamp experiments as in the case of cell-free electrodes.…”

“…Second, vestibular hair cells are extremely sensitive to pressure. Changes in the hydrostatic pressure as small as 0.2 cm H 2 O are sufficient to modulate a pressure-dependent K + current, I K,p [4,5]. Therefore, we wished to develop a bath system that allows superfusion of patch-clamped cells and avoids variations in the bath depth that would induce I K,p or change the pipette capacitance beyond the noise level of the capacitance measurements.…”