Activation of presynaptic GABAB receptors enables sustained synaptic transmission at high rate of stimulation at cholinergic olivocochlear-hair cell synapses

Abstract

Outer hair cells (OHCs) receive an efferent input from the medial olivocochlear system (MOC) whose activation turns down the gain of the auditory system. MOC efferent fibers also transiently innervate the inner hair cells (IHCs) since birth to the onset of hearing (around P12 in altricial rodents). Although acetylcholine (ACh) is the main neurotransmitter at these synapses, an abundant GABA innervation is also present. Electrical stimulation of efferent fibers triggers the release of ACh, but also produces the activation of presynaptic GABAB receptors, that in turn downregulates the amount of ACh release. GABA-mediated mechanism is through the inhibition of P/Q type Ca2+ channels. In this work we discuss the consequences of GABAB-mediated inhibition in the short-term plasticity of these synapses. Inhibitory synaptic currents (IPSC) were recorded in IHCs and OHCs of acutely isolated organs of Corti at P9-P14, while MOC fibers were electrically stimulated. Preliminary results suggest that by reducing the amount of ACh release, activation of GABAB receptors reduces synaptic depression and enables sustained transmission during high-frequency stimulation at these synapses.