CONICET | Buscador de Institutos y Recursos Humanos

In the Organ of Corti, the sensory epithelia of the of the mammalian auditory system, inner hair cells (IHCs) transduce sound stimuli into electrical signals that are conveyed to the central nervous system (CNS), while outer hair cells (OHCs), by virtue of their electromotility, participate in the amplification of sound. OHCs function is modulated by efferent cholinergic olivocochlear (OC) fibers projecting from the CNS. In the present work we studied the properties of the OC synapse onto OHCs. Briefly, synaptic activity was recorded in voltage-clamped OHCs from an excised apical turn of the mouse cochlea (10-12 postnatal days) during stimulation of OC fibers innervating them with a bipolar electrode placed in the modiolar region. Activation of efferent terminals by single shocks evoked inhibitory postsynaptic currents (IPSCs) with very low rate of success (quantal content: 0.14 ± 0.03, n=30 cells). Paired -pulse protocols showed that this synapse facilitates with maximum efficacy at pulse intervals of 10 ms (facilitation index = 2.1±0.4; n=8). Accordingly, trains of stimuli at different frequencies (10-100 Hz) produced increasing levels of transmitter release. This phenomenon, together with summation of synaptic currents, resulted in an increase of OHC responses proportional to the stimulus frequency (Imax/single-shock IPSC = 5.4±1.0; 8.5±3.7; 12.3±0,8; 15.6± 1,0 for 25, 50, 60 and 80 Hz, respectively, n=2-4). These results show that this synapse can facilitate at intervals that correspond to the physiological frequencies at which OC fibers fire. This property could be relevant in coding different degrees of activation of OC fibers in response to variable sound intensities.