The effects of cooling and rewarming on the neuronal activity of pyramidal neurons in guinea pig hippocampal slices

To investigate the reversibility of neuronal functions during deep and mild hypothermia, we have examined changes in membrane properties of pyramidal neurons of the CA3 region of hippocampal slices during cooling and rewarming (8 similar to 37 degreesC) of the perfusion medium. Hypothermia reduced the excitatory postsynaptic potential (EPSP) slope in a temperature dependent manner, but the EPSP amplitude was enhanced transiently between 30 and 25 degreesC. In observing spikes generated by either orthodromic stimulation or by direct intracellular current injection, the critical threshold for spike generation was decreased transiently at a temperature of 30 degreesC. In addition, the numbers of spikes were increased transiently regardless of the progressive prolongation of spike duration and latency with cooling. The resting membrane potential was stable from 37 to 20 degreesC. However, this potential showed a depolarizing shin at 15 degreesC. The neuronal activities, including membrane properties, recovered fully when the temperature was raised to 35 degreesC even from a low of 15 degreesC. In addition, field population spikes (PS) recorded in the pyramidal cell layer showed a complete reversibility after long-term severe hypothermia (8 degreesC). These results suggest that synaptic function, neuronal excitability and membrane properties maintain reversibility during deep hypothermia, as well as in mild hypothermia. (C) 2001 Elsevier Science B.V. All rights reserved.