Acceleration of AMPA receptor kinetics underlies temperature-dependent changes in synaptic strength at the rat calyx of Held.

It is well established that synaptic transmission declines at temperatures below physiological, but many in vitro studies are conducted at lower temperatures. Recent evidence suggests that temperature-dependent changes in presynaptic mechanisms remain in overall equilibrium and have little effect on transmitter release at low transmission frequencies. Our objective was to examine the postsynaptic effects of temperature. Whole-cell patch-clamp recordings from principal neurons in the medial nucleus of the trapezoid body showed that a rise from 25 degrees C to 35 degrees C increased miniature EPSC (mEPSC) amplitude from -33 +/- 2.3 to -46 +/- 5.7 pA (n=6) and accelerated mEPSC kinetics. Evoked EPSC amplitude increased from -3.14 +/- 0.59 to -4.15 +/- 0.73 nA with the fast decay time constant accelerating from 0.75 +/- 0.09 ms at 25 degrees C to 0.56 +/- 0.08 ms at 35 degrees C. Direct application of glutamate produced currents which similarly increased in amplitude from -0.76 +/- 0.10 nA at 25 degrees C to -1.11 +/- 0.19 nA 35 degrees C. Kinetic modelling of fast AMPA receptors showed that a temperature-dependent scaling of all reaction rate constants by a single multiplicative factor (Q10=2.4) drives AMPA channels with multiple subconductances into the higher-conducting states at higher temperature. Furthermore, Monte Carlo simulation and deconvolution analysis of transmission at the calyx of Held showed that this acceleration of the receptor kinetics explained the temperature dependence of both the mEPSC and evoked EPSC. We propose that acceleration in postsynaptic AMPA receptor kinetics, rather than altered presynaptic release, is the primary mechanism by which temperature changes alter synaptic responses at low frequencies.