Mechanisms Underlying Developmental Speeding in AMPA-EPSC Decay Time at the Calyx of Held

Abstract

The time course of synaptic conductance is important in temporal precision of information processing in the neuronal network. The AMPA receptor (AMPAR)-mediated EPSCs at the calyx of Held become faster in decay time as animals mature. To clarify how desensitization and deactivation of AMPARs contribute to developmental speeding of EPSCs, we compared the decay time of quantal EPSCs (qEPSCs) with the deactivation and desensitization times of AMPAR currents induced in excised patches by fast glutamate application (AMPA patch currents). Both the deactivation and desensitization times of AMPA patch currents became markedly faster from postnatal day 7 (P7) to P14 and changed little thereafter. In individual neurons, throughout development (P7-P21), the time constants of deactivation and fast desensitization in AMPA patch currents were similar to each other and close to the qEPSC decay time constant. Cyclothiazide (CTZ) abolished the fast desensitization, prolonged deactivation of AMPA patch currents, and slowed the decay time of EPSCs. The effects of CTZ on AMPA patch currents were unchanged throughout development, whereas its effect on EPSCs became weaker as animals matured. In single-cell reverse transcription-PCR analysis, glutamate receptor subunit 4 (GluR4) flop increased from P7 to P14 and changed little thereafter. At P7, the GluR4 flop abundance had an inverse correlation with the qEPSC decay time. These results together suggest that both desensitization and deactivation of AMPARs are involved in the EPSC decay time, but the contribution of desensitization decreases during postnatal development at the calyx of Held.

The Origin of Quantal Size Variation: Vesicular Glutamate Concentration Plays a Significant Role

“…This modification reduced the time constant of desensitization during prolonged application of 10 mM glutamate to 1.9 ms (from 4.0 ms) in simulation (data not shown), which was similar to the experimentally measured value at calyces (Geiger et al, 1995;Koike-Tani et al, 2005). With this modified scheme, we repeated simulation as shown in Figure 7A-C. We found that the rise time decreased as the simulated mEPSC amplitude increased, which was almost identical to the results shown in Figure 7A-C (data not shown).…”

“…For the mean mEPSC measured with the series resistance compensation, the 10 -90% rise time was 0.21 Ϯ 0.01 ms (n ϭ 8 synapses, 2015 mEPSCs), the 20 -80% decay time was 1.61 Ϯ 0.30 (n ϭ 8 synapses), and the experimentally estimated peak number of AMPA receptors being activated by release of a vesicle is ϳ22 (Sahara and Takahashi, 2001). Furthermore, the time constants of AMPA receptor desensitization and deactivation are ϳ1-3 and ϳ1-1.5 ms, respectively, in the postsynaptic neurons of the calyx-type synapse at 7-to 11-d-old rats (Geiger et al, 1995;Koike-Tani et al, 2005). These time constants are similar to those (desensitization, 4.9 ms; deactivation, 1.1 ms) (data not shown) obtained by simulation using the AMPA receptor reaction scheme of Hausser and Roth (1997).…”

“…Second, only ϳ3% of spontaneous fusion events induced by high potassium application were found to have a small fusion pore at calyx-type synapses (He et al, 2006). Koike-Tani et al (2005) showed that postsynaptic receptors are composed of different subunits at different developmental stages. We could not fully exclude the possibility that some mEPSCs are mediated by different AMPA receptor subunits.…”

Synaptic Vesicles in Mature Calyx of Held Synapses Sense Higher Nanodomain Calcium Concentrations during Action Potential-Evoked Glutamate Release

“…In excised patches, time constants for AMPAR deactivation are close to the fast time constant for AMPAR desensitization, and both are similar to the mEPSC decay (Koike-Tani et al, 2005). If AMPAR desensitization would significantly shape the decay of AP-evoked EPSCs (Trussell and Fischbach, 1989), our deconvolution analysis would underestimate the duration of the release transient.…”

SNARE Force Synchronizes Synaptic Vesicle Fusion and Controls the Kinetics of Quantal Synaptic Transmission

“…AMPARs composed of the GluR2, -3 and -4 flop splice variants show faster desensitization than those containing flip variants (Mosbacher et al, 1994;Geiger et al, 1995;Koike et al, 2000;Koike-Tani et al, 2005). Therefore, we considered whether the altered mEPSC time course in 11 aa mutant cells could arise from a larger contribution of flip isoforms.…”