Relationship between Brain and Body Temperature in Anesthetized Animals Measured by Ultralocal Thermometry

Temperature is one of the least studied biophysical characteristics of the brain, although both the rate of biochemical reactions and the electrical activity of nervous tissue directly depend on it. The purpose of this study was to analyze the relationship between brain and body temperatures in anesthetized animals. Simultaneous measurement of brain and body temperature (rectally) was carried out at ambient temperature controlled with a thermal mat. The temperature of the deep layers of the somatosensory cortex was measured by ultralocal thermometry using a diamond thermometer. Under anesthesia, the body and brain temperature dropped to 27 C (4 degrees above ambient temperature). When the thermal mat was turned on, the brain and body began to heat up synchronously. The brain initially lagged behind, but when the critical temperature was reached, it began to release heat in quantities exceeding the influx from the blood, reaching physiological values of 37C. A reverse experiment with decreasing the temperature of the thermal mat showed a similar picture: a synchronous start of the decrease, but with reverse dynamics. Thus, we can distinguish two phases of the brains reaction to external heating: passive - when neuronal activity is decreased, and active - after internal regulatory mechanisms are triggered, which, in its turn, slows down the temperature drop. In general, the data obtained in the present work indicate that the temperature of neural tissue is not linearly related to body temperature.