Procedure of rectal temperature measurement affects brain, muscle, skin, and body temperatures and modulates the effects of intravenous cocaine

Abstract

Rectal probe thermometry is commonly used to measure body core temperature in rodents because of its ease of use. Although previous studies suggest that rectal measurement is stressful and results in long-lasting elevations in body temperatures, we evaluated how this procedure affects brain, muscle, skin, and core temperatures measured with chronically implanted thermocouple electrodes in rats. Our data suggest that the procedure of rectal measurement results in powerful locomotor activation, rapid and strong increases in brain, muscle, and deep body temperatures, as well as a biphasic, down-up fluctuation in skin temperature, matching the response pattern observed during tail-pinch, a representative stressful procedure. This response, moreover, did not habituate after repeated day-to-day testing. Repeated rectal probe insertions also modified temperature responses induced by intravenous cocaine. Under quiet resting conditions, cocaine moderately increased brain, muscle, and deep body temperatures. However, during repeated rectal measurements, which increased temperatures, cocaine induced both hyperthermic and hypothermic responses. Direct comparisons revealed that body temperatures measured by a rectal probe are typically lower (approximately 0.6 degrees C) and more variable than body temperatures recorded by chronically implanted electrodes; the difference is smaller at low and greater at high basal temperatures. Because of this difference and temperature increases induced by the rectal probe per se, cocaine had no significant effect on rectal temperatures compared to control animals exposed to repeated rectal probes. Therefore, although rectal temperature measurements provide a decent correlation with directly measured deep body temperatures, the arousing influence of this procedure may drastically modulate the effects of other arousing stimuli and drugs.

Fig 1

Changes in NAcc, skin, muscle and body temperatures (A, absolute and B, relative) as well as locomotion (C) induced by rectal probe insertion and tail-pinch in naive (Day 1) and experienced (Day 5) rats. Filled symbols show values significantly different from the last pre-stimulus value. Each group represents an average of 18 tests performed in 9 rats.

Fig. 2

Relationships between basal NAcc temperatures and their relative changes induced by rectal probe and tail-pinch on Day 1 and Day 5. Each graph shows two regression lines and coefficients of correlation (r; **, p<0.01) for rectal probe and tail-pinch.

Fig. 3

Changes in NAcc, muscle, skin and body temperatures (A, absolute, B, relative with respect to baseline preceding the first probe) as well as locomotion (C) during repeated rectal probe insertion. Each line represents an average of data obtained in 6 rats. Filled symbols show values significantly different from either pre-first probe baseline (A) or values preceding each rectal probe (C).

Fig. 4

Changes in NAcc, skin, muscle and body temperatures (A, absolute values; B, relative change) as well as locomotion (C) induced by iv cocaine (0.75 mg/kg) with (right panel) and without (left panel) additional rectal temperature measurement. Filled symbols indicate values significantly different from baseline (left panel and first rectal probe in right panel) or pre-cocaine values (right panel). Vertical solid line shows the moment of cocaine injection and vertical hatched lines on the right graphs show the moments of rectal probes (n=6).

Fig. 5

Changes in NAcc temperature induced by iv cocaine depending pre-injection NAcc temperature. “Control” represents the effects of cocaine in quiet resting conditions and “rectal” represents cocaine-induced changes during repeated rectal probes. In both groups, the change was dependent upon baseline (r=-0.49 and -0.73). Solid lines are regression lines. Two vertical lines show mean basal temperatures in each group.

Fig. 6

A. Changes in body temperature induced by iv cocaine in quiet resting conditions (control) and during repeated rectal probes (rectal). Filled symbols mark values significantly different vs. baseline and pre-injection (p<0.05). B. Changes in rectal temperatures evaluated following repeated tests and shown as a relative change vs. the first value. In “cocaine” group, cocaine was injected at 10^th min after the first rectal probe. In both cases, the effect was significant (cocaine: ANOVA F_15,95=6.05, p<0.01; control: ANOVA F_15,95=3.02, p<0.05), but between-group difference was absent.

Fig. 7

Relationships between body and rectal temperatures. A shows absolute temperatures and B shows body-rectal difference. In both cases, there was significant correlation between body and rectal temperatures (see regression lines and coefficients of correlation) and the difference grew at higher temperatures. Interrupted hatched lines show the line of no effect and horizontal interrupted lines determine the degree of difference (0.5, 1.0 and 1.5°C).