Voltage dependence of transmitter release at the IHC ribbon synapse

JUAN D. GOUTMAN; ELISABETH GLOWATZKI

Denver, CO, USA

Congreso; XXX ARO MidWinter Meeting; 2007

ARO

In the inner ear, hair cells transform sound signals into receptor potentials. The intensity of the sound signal translates into the size of the inner hair cell receptor potential and the rate of action potentials in the auditory nerve fiber response.  This relationship should be reflected in the transfer function at the inner hair cell (IHC) afferent synapse. We studied the voltage dependence of transmitter release at the IHC afferent synapse by performing simultaneous whole cell recordings from inner hair cells (IHCs) and corresponding afferent dendrites (AFs) in the postnatal rat organ of Corti (P9-P11) (n = xx). To exclude AMPA receptor desensitization recordings were done in 100 microM cyclothiazide.  IHCs were depolarized with 10 mV steps from -89 to +41 mV for 200 ms every 15 s, or in 2 mV steps from -49 to -29 mV (x to x repetitions per step).   IHC calcium currents were isolated pharmacologically and  the AF response was monitored at a holding potential of -84 mV.  At voltage steps negative to -49 mV, no synaptic activity was found. Around -49 mV, both, calcium current and AF response activated.  Between -49 and -29 mV, both calcium current and AF response increased linearly, indicating a linear calcium dependence of vesicle release. A linear calcium dependence was also obtained when the rate of EPSCs (last 100 ms) was measured. However, the average EPSC amplitude stayed constant throughout the voltage range tested, suggesting that the size of the EPSCs, which are thought to activate due to multivesicular release, is calcium-independent. A linear calcium dependence of transmitter release has also been found in the frog papilla (Keen & Hudspeth, 2006), and may serve for faithful coding of sound intensity.