The Journal of Neurophysiology Vol. 87 No. 1 January 2002, pp. 222-228
Copyright ©2002 by the American Physiological Society

Role of Ca²⁺ in the Synchronization of Transmitter Release at Calyceal Synapses in the Auditory System of Rat

Department of Physiology, Faculty of Medicine, Kyoto University, Kyoto 606-8501, Japan

Chuhma, Nao and Harunori Ohmori. Role of Ca²⁺ in the Synchronization of Transmitter Release at Calyceal Synapses in the Auditory System of Rat. J. Neurophysiol. 87: 222-228, 2002. The synchronization of transmitter release in the synapse of the medial nucleus of the trapezoid body (MNTB) is achieved during early postnatal development as a consequence of elimination of delayed asynchronous releases and appears to reflect changes in the dynamics of Ca²⁺ entry and clearance. To examine the role of Ca²⁺ in regulating synchronization of transmitter release in the mature synapse (after postnatal day 9, P9), we perturbed Ca²⁺ dynamics systematically. Replacement of external Ca²⁺ (2 mM) with Sr²⁺ induced delayed asynchronous release following the major EPSC. We tried to reproduce asynchronous releases without using Sr²⁺ and instead by manipulating the time course and the size of Ca²⁺ transient in the presynaptic terminal, under the assumption that replacement of external Na⁺ with Li⁺ or application of eosin-Y would prolong the lifetime of Ca²⁺ transient by reducing the rate of Ca²⁺ extrusion from the terminal. With application of Li⁺, Ca²⁺ transient in the terminal was prolonged, the EPSC decay time course was prolonged, and the EPSC amplitude increased. However, these EPSCs were not followed by delayed asynchronous release. When Ca²⁺ influx was reduced, either by partial Ca²⁺ channel blockade with a low concentration of Cd²⁺ or -agatoxin IVA, a marked asynchronous release resulted. This was further enhanced by the combined application of Li⁺ or eosin-Y. These results suggest that cooperative increases of both Ca²⁺ influx and Ca²⁺ clearance capacities leading to a sharper Ca²⁺ spike in the presynaptic terminal underlie synchronized transmitter release in the presynaptic terminal of the MNTB.