Presynaptic Ca2+Requirements and Developmental Regulation of Posttetanic Potentiation at the Calyx of Held

Authors:
  • XLXuelin Lou
    Xuelin Lou
  • RSRalf Schneggenburger
    Ralf Schneggenburger
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Abstract
Large excitatory synapses in the auditory system, such as the calyx of Held, faithfully transmit trains of action potentials up to a frequency of a few hundred hertz, and these synapses are thought to display a limited repertoire of synaptic plasticity. Here, we show that brief trains of 100 Hz stimulation induce posttetanic potentiation (PTP) of transmitter release at the calyx of Held. In young rats [postnatal day 4 (P4) to P6], PTP could be induced with shorter 100 Hz trains compared with older age groups (P8-P10 and P12-P14), but the maximal amount of PTP was similar, with ∼200% of control EPSC amplitude. The size of the readily releasable pool of vesicles was not increased significantly during PTP. Bath application of the membrane-permeable Ca2+chelator EGTA-AM suppressed PTP, indicating a role for presynaptic Ca2+in PTP at the calyx of Held. Presynaptic Ca2+imaging showed that the intracellular Ca2+concentration, [Ca2+]i, was increased by 40-120 nmat the peak of PTP, and this “residual” [Ca2+]idecayed in parallel with PTP, with time constants in the range of 10-60 s. During whole-cell recording of the presynaptic calyx of Held, PTP was absent, and the decay of residual [Ca2+]iwas strongly accelerated. The data show that the calyx of Held expresses a mechanism of transmitter release potentiation in which a small, sustained elevation of basal [Ca2+]iincreases the transmitter release probability after trains of high-frequency stimulation.

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