Temperature dependence of vesicular dynamics at excitatory synapses of rat hippocampus

Abstract

How vesicular dynamics parameters depend on temperature and how temperature affects the parameter change during prolonged high frequency stimulation was determined by fitting a model of vesicular storage and release to the amplitudes of the excitatory post-synaptic currents (EPSC) recorded from CA1 neurons in rat hippocampal slices. The temperature ranged from low (13 °C) to higher and more physiological temperature (34 °C). Fitting the model of vesicular storage and release to the EPSC amplitudes during a single pair of brief high-low frequency stimulation trains yields the estimates of all parameters of the vesicular dynamics, and with good precision. Both fractional release and replenishment rate decrease as the temperature rises. Change of the underlying 'basic' parameters (release coupling, replenishment coupling and readily releasable pool size), which the model-fitting also yields is complex. The replenishment coupling between the readily releasable pool (RRP) and resting pool increases with temperature (which renders the replenishment rate higher), but this is more than counterbalanced by greater RRP size (which renders the replenishment rate lower). Finally, during long, high frequency patterned stimulation that leads to significant synaptic depression, the replenishment rate decreases markedly and rapidly at low temperatures (<22 °C), but at high temperatures (>28 °C) the replenishment rate rises with stimulation, making synapses better able to maintain synaptic efficacy.

Keywords: Excitatory synapse; Hippocampus; Model fitting; Replenishment rate; Synaptic depression; Temperature.

Fig. 1

As the temperature increases, both fractional release and replenishment rate decrease. a Diagram and equivalent electrical circuit of a secretory cell with two vesicular pools, readily releasable and resting pool. b EPSC amplitudes with best model fits at 21 and 31 °C. The vertical sticks represent the amplitudes of individual EPSCs, whereas the continuous line is the model-fit of the EPSC amplitude changes. c, d Ten consecutive parameter estimates, from the same cell at both temperatures. e, f As the temperature increases, both the fractional release and replenishment rate decrease, with best-fitted lines (Fit_FR = −0.63 * T + 22.73; r = 0.68 and Fit_RR = −0.06 * T + 2.41; r = 0.47, respectively). Each circle is a different cell, and each represents the mean parameter estimate of >7 stimulation trains. Vertical bars are standard errors. Stimulation consisted of pairs of brief high (10 Hz, 5 s) and low (5 Hz, 5 s) frequency trains, repeated ≥7 times with 1 min for recovery

Fig. 2

Replenishment coupling and RRP size rise with temperature. a–c As the temperature increases, the release coupling (1/R₀) does not change (Fit_1/R0 = 0.0004 * T + 0.1908; r = 0.08), whereas the replenishment coupling (1/R₁) and the readily releasable pool size rise [Fit_1/R1 = 10^(0.03 * T − 3.18); r = 0.35 and Fit_C1 = 10^(0.10 * T − 4.50); r = 0.70, respectively]. Each circle is a different cell, and each represents the mean parameter estimate of >7 stimulation trains. Vertical bars are standard errors. Stimulation consisted of pairs of brief high (10 Hz, 5 s) and low (5 Hz, 5 s) frequency trains, repeated ≥7 times with 1 min for recovery

Fig. 3

Vesicular storage and release parameters change during long, high frequency patterned stimulation in a temperature dependent manner. a, b EPSC amplitudes together with the model fits at 16 and 25 °C. c Fractional release is lower at high temperature, but regardless of temperature it does not change with stimulation (Fit_FR16 = −0.01 * T + 7.58; r = 0.17 and Fit_FR25 = −0.0002 * T + 4.410; r = 0.01). d The replenishment rate decreases with stimulation, but this decrease is attenuated at higher temperature [Fit_RR16 = −0.1 + 1.2 * exp(−T/39.7); r = 0.96 and Fit_RR25 = 6936.5 − 6936.8 * exp(−T/3100000); r = 0.03]. Patterned stimulation was 10 Hz for 5 s followed by 5 Hz for 5 s, repeated 10 times

Fig. 4

The effect of temperature on the change of fractional release and replenishment rate during long, high frequency stimulation. a The slopes of the change of the fractional release versus stimulation time fits slightly increase with temperature (Fit_FR = 0.004 * T − 0.069; r = 0.35). b _1–2 The amplitudes, but not the time constants, of the replenishment rate versus stimulation time fits increase with temperature [Fit_RRAmp = −1.92 + 0.16 * exp(−T/8.30); r = 0.62 and Fit_RRTime = 14.1 * T − 183.4; r = 0.29, respectively]. Patterned stimulation consisted of 10 repeating trains of brief high (10 Hz, 5 s)–low (5 Hz, 5 s) frequencies. Each symbol represents a different cell

Fig. 5

The effect of temperature on the change of ‘basic’ parameters during stimulation. a Temperature does not significantly affect the release coupling versus stimulation time slopes (Fit_1/R0 = 0.06 * T + 1.73; r = 0.29). b _1–2 The amplitudes, but not the decay times, of the replenishment coupling versus stimulation time fits increase with temperature (Fit_1/R1Amp = 0.17 * T − 3.85; r = 0.46 and Fit_1/R1Time = −12.4 * T + 473.3; r = 0.24, respectively). c _1–2 The amplitudes, but not the time constants, of the RRP size versus stimulation time fits decrease with temperature [Fit_C1Amp = 3.7 − 0.2 * exp(−T/7.6); r = 0.63 and Fit_C1Time = 1.6 * T + 92.4; r = 0.03, respectively]. Patterned stimulation consisted of 10 repeating trains of brief high (10 Hz, 5 s)–low (5 Hz, 5 s) frequencies. Each symbol represents a different cell

Fig. 6

The temperature dependence of the ‘open system’ contribution to the synaptic depression during long, high frequency stimulation. a Relative depression [(A₀ − A₇₀)/A₀] decreases with temperature [the best-fitted exponential curve is: Fit_RD = 114.4 − 14.2 * exp(−T/16.1); r = 0.85]. b Relative depression that would have been observed if there is no ‘open system’ contribution, with best-fitted exponential curve [Fit_RD = 170.3 − 63.0 * exp(−T/40.0); r = 0.77]. c Contribution of the ‘open system’ to the total depression (the difference between a and b), with best-fitted exponential curve [Fit_RD = −0.709 − 0.002 * exp(−T/3.425); r = 0.61]. Patterned stimulation consisted of 10 repeating trains of brief high (10 Hz, 5 s)–low (5 Hz, 5 s) frequencies. Each symbol represents a different cell