BRAIN TEMPERATURE AND HIPPOCAMPAL FUNCTION

Even though homeothermic animals regulate the body temperature, fluctu ations up to 2-3 degrees C may occur during physiological conditions. In many species, including the rat, a similar variation can be measure d in the brain temperature. Such changes are expressed throughout the brain with a preserved gradient between the warmer basal and cooler do rsal parts. In spite of these recordable physiological changes, spatia l learning is quite robust, in that it occurs at brain temperatures be tween 30 and 39 degrees C. Even drastic cooling (to below 15 degrees C ) fails to affect consolidation or storage of information when the ani mal is tested after rewarming. The physiological temperature fluctuati ons have significant consequences for electrophysiological responses i n the brain. Various bioelectrical signals are more sensitive during w arming, axonal conduction is speeded up, and stimulus-elicited transmi tter release becomes faster and more synchronized. Action potentials h ave shorter rise and decay times in warm conditions, and the amplitude becomes slightly smaller. Population responses are differently affect ed by these changes. Dentate field potentials in response to stimulati on of perforant-path fibers appear with shorter latency in warm condit ions, and the rate of rise in the field EPSP is increased. Paradoxical ly, the amplitude of the population spike is reduced. This is due to a combination of reduced amplitude of individual action potentials and reduced efficiency of the summation of groups of action potentials. Du e to the large effects of temperature on hippocampal field potentials, it is mandatory that brain temperature changes are monitored and/or c ontrolled whenever such responses are recorded in freely moving and an esthetized animals. (C) 1995 Wiley-Liss, Inc.