The effect of barium and some channel blockers on sensory discharge of the frog labyrinth posterior canal recorded at rest and during rotation.

The effect on the afferent synaptic transmission of Ba2+, Sr2+, tetraethylammonium (TEA) and 4-aminopyridine (4-AP) has been investigated in the isolated frog labyrinth by intracellularly recording the posterior canal resting and evoked receptor discharge. BaCl2 (0.3 mM) or SrCl2 (1.8 mM) substitution for normal external CaCl2 restored the afferent activity without affecting the membrane potential of the sensory fibres. On further increasing Ba2+ concentration (0.5-5 mM) a dose-dependent increase in the EPSP and spike discharges was observed in all the units examined. Ba2+ (1.8-4 mM) removed the depression of the sensory activity operated by CoCl2 (3 mM), while its facilitatory effect was completely antagonized by raising Ca2+ concentration (up to 10 mM). TEA (20 mM) elicited a clear-cut increase in the EPSP and spike discharges which, however, was less consistent than that produced by Ba2+ (1 mM). The increment in spike frequency produced by TEA and Ba2+ proved to be inversely related to the initial resting firing level of the different units. The 4-AP (4-20 mM) effect resulted in a decrease of the sensory activity, which was fully restored by TEA or Ba2+. In normal saline a linear relationship was found between the mean unit resting discharge and the respective excitatory peak response during sinusoidal rotation (0.1-0.3 Hz). This result suggest that the mechanical response is mainly determined by the unit resting level. Consistent evoked responses were obtained under TEA and Ba2+ treatment which proved to depend linearly on the new mean resting discharge of the different units. Conversely, a reduced evoked response was invariably observed in all the fibres tested in the presence of 4-AP. The present results suggest that Ba2+ and Sr2+ may substitute for Ca2+ in the transmitter release process at the cyto-neural junction, the ability of Ba2+ being even larger than that of Sr2+ and Ca2+ itself. The effects of TEA and 4-AP are discussed in the light of their possible interaction with the presynaptic K+-currents recently described in hair cells.