Functional and Pathological Influence of Hypothermia on Spike Activity of Cortical Neurons

In sensorimotor cortical slices of guinea pig using local iontophoretic application of glutamate to the soma and dendrites it was found that a decrease of temperature of incubating fluid from 34 to 21°C - 22°C changes the somatic responses to the local injection of glutamate to the dendritic loci, while the responses to iontophoretic application of glutamate to the soma remain unchanged. Hypothermic changes in reactivity to dendritic stimulation start below 30°C and coincide with changes in the spontaneous activity of neurons, both in the direction of increasing and decreasing the frequency of firing in different nerve cells. On hypothermic decrease of spontaneous activity, the latencies of evoked dendritic responses on the soma became more longer, while on hypothermic increase of firing level, somatic spike responses to iontophoretic application of glutamate to dendritic loci appeared with shorter latencies. Hypothermic changes in the physiological parameters of neurons were accompanied by a drop in spike amplitude at the same temperature and with its further decrease. At the same time, there was a decrease of spike reaction to iontophoretic application of acetylcholine below 30°. It is proposed that the reason for hypothermic changes of neuronal activity is decreasing rate of M-cholinergic process at 27°C - 29°C which leads to opening K+ channels of neuronal membranes and hence to attenuation of conductive function of dendrites and to imbalance of K+ ion homeostasis. Peculiarities of hypothermic regulation of neuronal spike activity depend on individual functional properties of cortical neurons.