Temperature modulation of slow and fast cortical rhythms.

In the local cortical network, spontaneous emergent activity self-organizes in rhythmic patterns. These rhythms include a slow one (<1 Hz), consisting in alternation of up and down states, and also faster rhythms (10-80 Hz) generated during up states. Varying the temperature in the bath between 26 and 41 degrees C resulted in a strong modulation of the emergent network activity. Up states became shorter for warmer temperatures and longer with cooling, whereas down states were shortest at physiological (36-37 degrees C) temperature. The firing rate during up states was robustly modulated by temperature, increasing with higher temperatures. The sparse firing rate during down states hardly varied with temperature, thus resulting in a progressive merging of up and down states for temperatures around 30 degrees C. Below 30 degrees C and down to 26 degrees C the firing lost rhythmicity, becoming progressively continuous. The slope of the down-to-up transitions, which reflects the speed of recruitment of the local network, was progressively steeper for higher temperatures, whereas wave-propagation speed exhibited only a moderate increase. Fast rhythms were particularly sensitive to temperature. Broadband high-frequency fluctuations in the local field potential were maximal for recordings at 36-38 degrees C. Overall, we found that maintaining cortical slices at physiological temperature is critical for the generated activity to be analogous to that in vivo. We also demonstrate that changes in activity with temperature were not secondary to oxygenation changes. Temperature variation sets the in vitro cortical network at different functional regimes, allowing the exploration of network activity generation and control mechanisms.