Membrane properties and spike generation in rat visual cortical cells during reversible cooling

1. We studied the effects of reversible cooling between 35 and 7 degrees C on membrane properties and spike generation of cells in slices of rat visua l cortex. 2. Cooling led to a depolarization of the neurones and an increase of the i nput resistance, thus bringing the cells closer to spiking threshold. Excit ability, measured with intracellular current steps, increased with cooling. 3. Synaptic stimuli were most efficient in producing spikes at room tempera ture, but strong stimulation could evoke spikes even below 10 degrees C. 4. Spike width and total area increased with cooling, and spike amplitude w as maximal between 12 and 20 degrees C. Repetitive firing was enhanced in s ome cells by cooling to 20-25 degrees C, but was always suppressed at lower temperatures. 5. With cooling, passive potassium conductance decreased and the voltage-ga ted potassium current had a higher activation threshold and lower amplitude . At the same time, neither passive sodium conductance nor the activation t hreshold of voltage-dependent sodium channels changed. Therefore changing t he temperature modifies the ratio between potassium and sodium conductances , and thus alters basic membrane properties. 6. Data from two cells recorded in slices of cat visual cortex suggest a si milar temperature dependence of the membrane properties of neocortical neur ones to that described above in the rat. 7. These results provide a framework for comparison of the data recorded at different temperatures, but also show the limitations of extending the con clusions drawn from in vitro data obtained at room temperature to physiolog ical temperatures. Further, when cooling is used as an inactivation tool in vivo, it should be taken into account that the mechanism of inactivation i s a depolarization block. Only a region cooled below 10 degrees C is reliab ly silenced, but it is always surrounded by a domain of hyperexcitable cell s.