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Timing and Efficacy of Ca 2+ Channel Activation in Hippocampal Mossy Fiber Boutons

2002; Society for Neuroscience; Volume: 22; Issue: 24 Linguagem: Inglês

10.1523/jneurosci.22-24-10593.2002

1529-2401

Josef Bischofberger, Jörg R. P. Geiger, Péter Jónás,

Neural dynamics and brain function

The presynaptic Ca 2+ signal is a key determinant of transmitter release at chemical synapses. In cortical synaptic terminals, however, little is known about the kinetic properties of the presynaptic Ca 2+ channels. To investigate the timing and magnitude of the presynaptic Ca 2+ inflow, we performed whole-cell patch-clamp recordings from mossy fiber boutons (MFBs) in rat hippocampus. MFBs showed large high-voltage-activated Ca 2+ currents, with a maximal amplitude of ∼100 pA at a membrane potential of 0 mV. Both activation and deactivation were fast, with time constants in the submillisecond range at a temperature of ∼23°C. An MFB action potential (AP) applied as a voltage-clamp command evoked a transient Ca 2+ current with an average amplitude of ∼170 pA and a half-duration of 580 μsec. A prepulse to +40 mV had only minimal effects on the AP-evoked Ca 2+ current, indicating that presynaptic APs open the voltage-gated Ca 2+ channels very effectively. On the basis of the experimental data, we developed a kinetic model with four closed states and one open state, linked by voltage-dependent rate constants. Simulations of the Ca 2+ current could reproduce the experimental data, including the large amplitude and rapid time course of the current evoked by MFB APs. Furthermore, the simulations indicate that the shape of the presynaptic AP and the gating kinetics of the Ca 2+ channels are tuned to produce a maximal Ca 2+ influx during a minimal period of time. The precise timing and high efficacy of Ca 2+ channel activation at this cortical glutamatergic synapse may be important for synchronous transmitter release and temporal information processing.