Thermal sensitivity of hypoxic responses in neocortical brain slices.

Electrophysiological responses to transient hypoxia were studied in neocortical brain slices from adult gerbils. Evoked responses and direct current (DC) potentials were recorded in layer III of the parietal cortex under normoxic and hypoxic conditions. The excitatory synaptic component of the evoked waveform was identified by its sensitivity to calcium and 6,7-dinitroquinoxaline-2,3-dione (DNQX). Under normoxic conditions, hypothermia reduced excitatory synaptic responses in a temperature-dependent manner. Under hypoxic conditions, hypothermia prolonged the delays to synaptic loss and hypoxic depolarization in a temperature-dependent manner. Synaptic recovery following a fixed period under hypoxic depolarization was greatly enhanced when hypoxia was administered at reduced temperature. The findings demonstrate that evoked responses are reduced under hypothermic conditions, but that these responses are sustained for a longer period of time during hypoxia. The data suggest that hypothermia protects against hypoxic damage to excitatory synaptic mechanisms in the neocortex both by prolonging the delay to hypoxic depolarization, and by extending the period of hypoxic depolarization that can be tolerated.