THE EFFECT OF BARIUM AND SOME K-CHANNEL BLOCKERS ON THE AFFERENT SYNAPTIC TRANSMISSION IN THE ISOLATED FROG LABYRINTH

The effect of Ba, TEA and 4-AP on the afferent synaptic transmission has been investigated in the isolated frog labyrinth by intracellularly recording the resting and the evoked receptor discharge from the posterior canal. Ba (1mM; Ca-free Ringer solution) and TEA (20mM) elicited a clear-cut increase in the spontaneous EPSP and spike discharges. On the other hand, 4-AP (20mM) application resulted in a decrease of the sensory activity. During sinusoidal rotation at 0.1 and 0.3 Hz consistent responses were observed in TEA and Ba solutions, while the mechanically evoked discharge was greatly reduced in the presence of 4-AP. Under the drug treatments, as well as in normal saline, the evoked responses proved to increase linearly with the resting discharge frequency. The present results suggest that: 1) the unit response is determined by its resting level; 2) Ba may substitute for Ca in the transmitter release process at the cytoneural junction, its ability being even greater than that of Ca itself. The effects of TEA and 4-AP are discussed in the light of their possible interaction with the presynaptic hair cell K-currents