Modulation of transmitter release at the IHC ribbon synapse by presynaptic resting potential

JUAN D. GOUTMAN; ELISABETH GLOWATZKI

Phoenix, AZ, EE. UU.

Congreso; XXXI ARO MidWinter Meeting; 2008

Synaptic transmission at the inner hair cell (IHC) afferent synapse requires high precision in order to code for sound signals. Here we describe a mechanism by which precision of transmitter release at this synapse might be improved. Simultaneous whole cell patch clamp recordings where performed from IHCs and contacting afferent dendrites in excised organs of Corti from postnatal rats (P9-P11). The afferent synaptic response was studied while manipulating IHC membrane potential and calcium influx. Using this approach, we previously have characterized the voltage dependence of release with a maximum at ~ -30 mV. In response to steady IHC depolarizations, the afferent fiber response showed synaptic depression (Goutman and Glowatzki, 2007). When IHCs were depolarized from a hyperpolarized potential, -89 mV, to -29 mV for a brief period of time (2 ms), a high failure rate for release was found. Successful responses showed a wide range of EPSC amplitudes and also long (~4 ms) and varying delays from the onset of the presynaptic depolarization. If a conditioning step to an intermediate potential (~-50 mV)  was applied to the IHC before the test pulse, a significant increase in the probability of release was observed. Additionally, the delay of the response was shorter (~2 ms) and less variable. Similar results were found when conditioning was induced with a paired pulse protocol. These results suggest that release at the IHC ribbon synapse can be modulated by the presynaptic membrane potential in a way that resembles facilitation in the CNS. Under physiological conditions, the IHC may have a resting potential positive enough to allow for continuous calcium influx and transmitter release, resulting in spontaneous activity in auditory nerve fibers. Our results show that at this IHC resting potential the synapse may be conditioned and therefore activate release with shorter delays and less variability.