Facilitating efferent inhibition of inner hair cells in the cochlea of the neonatal rat

Cholinergic brainstem neurones make inhibitory synapses on outer hair cells (OHCs) in the mature mammalian cochlea and on inner hair cells (IHCs) prior to the onset of hearing. We used electrical stimulation in an excised organ of Corti preparation to examine evoked release of acetylcholine (ACh) onto neonatal IHCs from these efferent fibres. Whole-cell voltage-clamp recording revealed that low frequency (0.25-1Hz) electrical stimulation produced evoked inhibitory postsynaptic currents (IPSCs) at a relatively high fraction of failures (65%) and with mean amplitudes of about -20pA at -90mV, corresponding to a quantum content of similar to 1. Evoked IPSCs had biphasic waveforms at -60mV, were blocked reversibly by alpha-bungarotoxin and strychnine and are most likely mediated by the alpha 9/alpha 10 acetylcholine receptor, with subsequent activation of calcium-dependent potassium (SK2) channels. Paired pulse stimulation with intervals of 10-100ms caused facilitation of 200-300% in the mean IPSC amplitude. A train of 10 pulses with an interpulse interval of 25ms produced increasingly larger IPSCs with maximum amplitudes greater than -100pA due to facilitation and summation throughout the train. Repetitive efferent stimulation at 5Hz or higher hyperpolarized IHCs by 5-10mV and could completely prevent the generation of calcium action potentials normally evoked by depolarizing current injection.