STUDY OF FREQUENCY DEPENDENCE OF EPSP RATE AT THE CYTONEURAL JUNCTION OF FROG LABYRINTH

The frequency response of the cytoneural junction was studied in the frog labyrinth posterior canal by recording EPSPs in response to sinusoidal rotation at various frequencies (0.02 to 1.0 Hz) in the presence of tetrodotoxin. Peak accelerations ranged 3 to 63 deg/s2, but were kept constant (15-18 deg/s2) in some units by changing rotation amplitudes and frequencies. EPSP waveform was, evaluated from power spectra and EPSP rate was determined by means of a fluctuation analysis procedure devised to study responses as fast as the changes in EPSP rate during rotation at 1 Hz. At all rotation frequencies the responses were asymmetric, the increase in EPSP rate for excitatory acceleration being 1.2-2.5 larger than the decrease during the corresponding inhibitory acceleration. Asymmetrical sinusoids well fit all responses. Frequency responses were obtained for each unit in terms of GAIN (change in EPSP rate vs. acceleration), PHASE (phase lead vs. angular velocity) and ASYMMETRY (ratio between positive and negative responses). At constant acceleration the relation of gain vs. frequency was described by a low-pass filter function (corner frequency 0.05-0.14 Hz in the various units). This is in agreement with the behaviour predicted by the cupula-endolymph system. With changing peak accelerations the responses, normalized to peak acceleration, displayed the same behaviour (corner frequencies 0.05-0.19 Hz); phase leads were generally in accord with this same prediction. Asymmetry in the response did not display any clear-cut frequency dependence. In conclusion these data, together with previous observations for rotations with variable amplitude at 0.1 Hz (J.Gen.Physiol 94:303-327,1989), indicate that most of the static and dynamic properties so far described for the spike discharge at the cytoneural junction directly derive from the properties of neurotransmitter release by the presynaptic cell.