抄録
Quantal packets of transmitter do not normally saturate postsynaptic AMPA receptors at the calyx of Held, a fast relay synapse in the auditory brainstem, of mature rats (postnatal day (P) 13-29; Ishikawa et al., 2002; Yamashita et al., 2003). However, this may not be the case at the immature calyx of Held (P7), where an increase in extracellular Ca2+ concentration does not increase the amplitude of evoked EPSCs. To test this possibility, we infused transmitter L-glutamate into the nerve terminal through whole-cell patch pipettes, and examined its effect on quantal and evoked EPSCs at the immature calyx of Held (P7-9). The endogenous glutamate concentration in the presynaptic terminal, estimated by matching quantal size between calyceal terminals with or without loading L-glutamate, was 1-10 mM, similar to the mature calyx of Held. Infusion of 100 mM L-glutamate into the calyceal terminal increased the mean amplitude of quantal EPSCs, but had no effect on the amplitude of evoked EPSCs. When the release probability was lowered by reducing extracellular Ca2+ concentration, decay time of evoked EPSCs became faster, indicating that multiple quanta are released at a single release site. We conclude that a single quantal packet of transmitter does not saturate postsynaptic AMPA receptors, whereas simultaneously released multiple packets of transmitter overlap and eventually saturate AMPA receptors at the immature calyx of Held. [Jpn J Physiol 55 Suppl:S148 (2005)]
