Temperature enhances exocytosis efficiency at the mouse inner hair cell ribbon synapse

Hearing relies on fast and sustained neurotransmitter release from inner hair cells (IHCs) onto the afferent auditory nerve fibres. The temperature dependence of Ca2+ current and transmitter release at the IHCs ribbon synapse has not been investigated thus far. To assess the influence of temperature on calcium-triggered exocytosis, patch-clamp recordings of voltage-gated L-type Ca2+ influx and exocytic membrane capacitance changes were performed at room (25 degrees C) and physiological (35-37 degrees C) temperatures. An increase in temperature within this range increased the L-type Ca2+ current amplitude of IHCs (Q(10) = 1.3) and accelerates the activation kinetics. Fast exocytosis, probed by 20 ms depolarization, was enhanced at physiological temperature with a Q(10) of 2.1. The amplitude of fast release was elevated disproportionately to the increase in Ca2+ influx. In contrast, the rate of sustained exocytosis (exocytic rate between 20 and 100 ms of depolarization) did not show a significant increase at physiological temperature. Altogether, these data indicate that the efficiency of fast exocytosis is higher at physiological temperature than at room temperature and suggest that the number of readily releasable vesicles available at the active zone is higher at physiological temperature.