Synaptic transmission in the neocortex during reversible cooling

We studied the effects of reversible cooling on synaptic transmission in sl ices of rat visual cortex. Cooling had marked monotonic effects on the temp oral properties of synaptic transmission. It increased the latency of excit atory postsynaptic potentials and prolonged their time-course. Effects were non-monotonic on other properties, such as amplitude of excitatory postsyn aptic potentials and generation of spikes. The amplitude of excitatory post synaptic potentials increased, decreased, or remain unchanged while cooling down to about 20 degrees C, but thereafter it declined gradually in all ce lls studied. The effect of moderate cooling on spike generation was increas ed excitability, most probably due to the ease with which a depolarized mem brane potential could be brought to spike threshold by a sufficiently stron g excitatory postsynaptic potential. Stimuli that were subthreshold above 3 0 degrees C could readily generate spikes at room temperature. Only at well below 10 degrees C could action potentials be completely suppressed. Paire d-pulse facilitation was less at lower temperatures, indicating that synapt ic dynamics an different at room temperature as compared with physiological temperatures. These results have important implications for extrapolating in vitro data o btained at room temperatures to higher temperatures. The data also emphasiz e that inactivation by cooling might be a useful tool for studying interact ions between brain regions, but the data recorded within the cooled area do not allow reliable conclusions to be drawn about neural operations at norm al temperatures. (C) 2000 IBRO. Published by Elsevier Science Ltd. All righ ts reserved.