Measurement of 'Core' Temperature in the Rat

Journal/Book: Reprinted from Nature Vol. 210 No. 5038 pp. 854-855 May 21 1966. 1966;

Abstract: Department of Pharmacology School of Medicine University of California Los Angeles. IN investigations involving changes in body temperature in the rate1-3 we have noted discrepancies in the control levels and changes in response to given doses of drugs such as morphine chlorpromazine and reserpine between our own data and those quoted for comparable situations in the literature. Although a wide variety of techniques for measuring body temperature have been used the most commonly used method in the conscious animal is the intermittent or chronic insertion of a thermo-sensitive device into the rectum. Few authors state the distance to which the thermometer is inserted but where the measurement is given it is generally in the rat of the order of 3·0-5·0 cm. Using a thermistor probe 3·5 mm in diameter and a tele-thermometer with which the temperature can be read to 0·1° C the temperature was recorded at various distances from the anal sphincter of six rate weighing between 200 and 300 g (Fig. 1)(without Fig.). The record assumes a plateau between 6 and 8 cm to give a mean of 38·0 ± 0·1° C and it can be seen that the readings fall off rapidly at distances of less than 5 cm. If the readings were to be taken by repeated introduction of the probe to a distance of around 3 cm a slight variation in position could produce a considerable error in the readings and this could easily be of greater magnitude than the changes being investigated. A more serious source of error is apparent from Fig. 2 where the fall in body temperature at the time of maximum effect of a systemic dose of 35 mg/kg of morphine sulphate is recorded at different distances in four animals. Measurements at leas than 5 cm would indicate a considerable variation in response and possibly obscure significant effects. With a rat weighing from 200 to 300 g the maximum distance to which the probe can be inserted is approximately 8·0 cm and post-mortem examination shows that at this distance the tip of the thermistor is lying behind the liver and close to the diaphragm. ... ___MH

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