Intensity coding at the inner hair cell ribbon synapse is supported by a highly efficient recovery mechanism

JUAN D. GOUTMAN

Huerta Grande, Córdoba

Congreso; XXVIII Congreso Anual de la Sociedad Argentina de Investigación en Neurociencia; 2013

Sociedad Argentina de Investigación en Neurociencia

The auditory nerve represents the output of the cochlea and encodes intensity of acoustic stimuli in its firing rate. Sounds of increasing intensity produce a higher rate of action potentials. Inner hair cells (IHC) of the cochlea are the mammalian phono-receptors, transducing sound energy into graded changes in its membrane potentials (Vm). The IHC ribbon synapse is responsible for converting these changes in Vm into rates of action potentials in neurons of the auditory nerve. In this work, we investigated the time course of recovery of synaptic responses after 1 sec pulses at different IHC Vm. We observed a constant recovery process with an average rate per second of 0.25 of the initial response, when using pulses at -20 mV. A faster rate was observed after pulses at -30 mV (0.48 s-1). We also observed a higher intracellular calcium concentration in IHC with more positive Vm, as expected. Depletion of synaptic vesicles was consistently higher as presynaptic Vm increased. Therefore, recovery rate would depend upon the number of available release sites, but not on the presynaptic calcium concentration. This conclusion was further confirmed by evaluating recovery at individual time points after depleting pulses of different Vm. It can be calculated that recovery rate of the initial response matches with maximal release rate (~60 EPSCs/s). Increasing the extracellular calcium concentration (from 1.3 to 1.5 ? 2.0 mM) produced a higher release rate for submaximal pulses only.