The effects of temperature on vesicular supply and release in autaptic cultures of rat and mouse hippocampal neurons

Membrane fusion plays a central role in the synaptic vesicle cycle. While many of the pre- and post-fusion events have been investigated at room temperature, few researchers have investigated these processes at more physiologically relevant temperatures. We have used autaptic cultures of hippocampal neurons to investigate changes in the size and refilling rate of the readily releasable pool (RRP) of synaptic vesicles brought about by an increase in temperature from 25 to 35 degreesC. We have also examined temperature-dependent changes in spontaneous and action potential (AP)evoked release as well as the fraction of the RRP that is released during an AP. Although we found a threefold increase in the refilling rate of the RRP at the higher temperature, there was no apparent change in the size of the RRP with increased temperature. Moreover, we observed a slight but significant decrease in the quanta released during an AP. This increased refilling rate and decreased release probability resulted in a reduction of both the degree and time course of synaptic depression during high frequency stimulation at the higher temperature. This reduction in synaptic depression was accompanied by an increased maintenance of the synchronous component of release during high frequency stimulation. These findings indicate that the dynamics of vesicular supply and release in hippocampal neurons at room temperature are significantly different at near physiological temperatures and could affect our present understanding of the way in which individual neurons and networks of neurons process information.