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1 The Cochlear Neurotransmission Laboratory, Center for Hearing and Balance, Department of Otolaryngology–Head and Neck Surgery, Johns Hopkins University School of Medicine, Baltimore, MD, USA
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 (0.25–1 Hz) electrical stimulation produced evoked inhibitory postsynaptic currents (IPSCs) at a relatively high fraction of failures (65%) and with mean amplitudes of about –20 pA at –90 mV, corresponding to a quantum content of 
1. Evoked IPSCs had biphasic waveforms at –60 mV, were blocked reversibly by 
-bungarotoxin and strychnine and are most likely mediated by the 9/10 acetylcholine receptor, with subsequent activation of calcium-dependent potassium (SK2) channels. Paired pulse stimulation with intervals of 10–100 ms caused facilitation of 200–300% in the mean IPSC amplitude. A train of 10 pulses with an interpulse interval of 25 ms produced increasingly larger IPSCs with maximum amplitudes greater than –100 pA due to facilitation and summation throughout the train. Repetitive efferent stimulation at 5 Hz or higher hyperpolarized IHCs by 5–10 mV and could completely prevent the generation of calcium action potentials normally evoked by depolarizing current injection.
(Received 24 March 2005;
accepted after revision 4 May 2005;
first published online 5 May 2005)
Corresponding author E. Glowatzki: 521 Traylor Building, Johns Hopkins University School of Medicine, 720 Rutland Avenue, Baltimore, MD 21205-2195, USA. Email: eglowatz{at}bme.jhu.edu
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