CONICET | Buscador de Institutos y Recursos Humanos

In the Organ of Corti, the sensory epithelia of the mammalian auditory system, the inner hair cells (IHCs) transduce sound stimuli into electrical signals that are conveyed to the central nervous system (CNS). Outer hair cells (OHCs), by virtue of their electromotility, participate in the amplification of sound stimuli. In the adult cochlea, the function of OHCs is modulated by efferent cholinergic olivocochlear (OC) fibers projecting from the CNS. The firing rate of OC fibers varies according to sound stimulation (noise, tones, monoaural, binaural, etc.) and it increases with sound intensity (Brown et al., 1998). It has been shown that changes in the firing rates of these fibers result in different degrees of inhibition of the auditory function exerted by the OC system (Galambos 1956, Gifford & Guinan 1987, Wiederhold & Kiang 1970). In the present work we used the technique developed by Goutman et al. (2005) to study 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 with a bipolar electrode placed in the modiolar region. Activation of efferent terminals by single shocks evoked inhibitory postsynaptic currents with a very low rate of success (P = 0.26±0.05, n=18 OHCs). 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 OHCs). Accordingly, trains of 5-10 stimuli at different frequencies (10-100 Hz) produced increasing levels of transmitter release proportional to the stimulus frequency (Imax normalized to first stimulus: 4.2 ± 0.3; 7.5 ± 0.8; 12.4 ± 0.7, 21.6 ± 5.9 for 25, 50, 60 and 80 Hz respectively, n = 3-11 OHCs). This increase in transmitter release was attributed both to summation and facilitation of synaptic responses. These results show that this synapse can facilitate at intervals that correlate with the physiological frequencies at which OC fibers fire. This property could be relevant for encoding different degrees of OC inhibition in response to variable sound stimulation.