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

JUAN D. GOUTMAN

San Diego, California

Congreso; 37th Annual MidWinter Meeting of the Association for Research in Otolaryngology; 2014

Association for Research in Otolaryngology

Background At the onset of an acoustic stimulus, spike rate in auditory nerve neurons increases abruptly and then decays into a steady-state phase, of up to hundreds of hertz. This rate can be maintained as long as the stimulus duration, determining a high demand of activity at the inner hair cell (IHC) ribbon synapse. In this work, we investigated how synaptic responses recovered after prolonged stimulation of the IHC. Also, we sought for causes of the decay in synaptic responses. Methods Explants of the organ of corti from neonatal rats were used for experiments (apical region). Simultaneous pre- and post-synaptic patch-clamp recordings at the IHC ribbon synapse were performed. The intracellular calcium concentration was measured with ratiometric fluorescent indicators, introduced in the IHC through the recording electrode. Fluorescence intensity was detected with a CCD camera. The series of differential equations corresponding to a model of the IHC ribbon synapse were solved numerically. Results We observed a fast recovery process with an average rate per second of 0.25 of the onset responses when using pulse at -20 mV. A faster rate was observed after pulses at -30/-40 mV (0.48 1/s). Depletion of synaptic vesicles was consistently higher as presynaptic Vm increased, and so was the global [Ca2+]i at the IHC, as expected for larger calcium currents. Therefore, recovery rate would not depend on presynaptic calcium concentration, but on vesicle availability. This conclusion was further confirmed by evaluating recovery at individual time points after depleting pulses of different Vm. Deconvolution analysis of ensemble responses at different IHC Vm indicated an onset component, followed by a steady state release that is < 10% of peak. Synaptic depression occurred regardless of the number of vesicles released at the onset. Simulations with a ribbon synapse model did not show equally adapting release rates. Instead, moderate or low peak release rates were followed by sustained componentes. Conclusion Fast recovery of synaptic responses is observed after prolonged stimulations at the IHC ribbon synapse. This process would ensure sustained activity for long periods of time. Less intense stimuli allow for faster recovery, possibly due to remaining vesicles in reserve pools. Unlike shown before, calcium would not modulate recovery. Synaptic depression at sub-maximal stimuli suggest that vesicles are not the sole limiting factor, but other mechanisms may apply.