THE ROLE OF ATP IN THE EFFERENT CONTROL OF THE FROG LABYRINTH RECEPTOR DISCHARGE

In the isolated labyrinth electrical stimulation of the anterior-horizontal nerves activates, via an axon-reflex, the posterior canal efferent systems inducing either facilitation of transmitter release in a subpopulation of hair cells or inhibition of release in others. While the cholinergic nature of efferent inhibition is well-established, the nature of efferent facilitation is far from demonstrated. Recently possible roles of ATP in hair cell functioning have been suggested; in this context the discharge af afferent units from the postetior canal was examined in the presence of external ATP (10-4-10-3 M) or ATP-ys (10-4 M). The resting rate of the facilitated units consistently increased in the presence of the drugs, which conversely did not affect either the resting discharge of the inhibited units or the time-course of inhibition. We have previously demonstrated, by noise analysis procedure, that the hair cells release quanta asynchronously giving rise to single mEPSPs that extensively overlapp at high frequency (Rossi M. al., J.Physiol, in press). Using this procedure the mEPSP rate as well as their peak amplitude and time interval distributions were determined in control solution and after bath application of ATP or ATP-ys. The effect of ATP was dose-dependent and ensued rapidly: it was also transient and within 1-3 minutes the inEPSP rate declined to the control level. In these fibres efferent facilitation, tested after the effect of ATP or ATP-ys had vanished, was dramaticalty reduced in comparison with the facilitation observed in control solution. During ATP action mEPSPs peak amplitudes were lognormally distributed and their time intervals exponentially distributed, as in control conditions. Results suggest that ATP acts at the basal pole of the hair cells mimicking the facilitatory transmitter action and that only the rate of transmitter release, but not its basic mechanism, is affected by ATP.