Restrained rats and the observer effect in physiology

Abstract

Accurate measurement is at the core of science. Yet, the observer effect in quantum mechanics is a challenge to accurate measurement. The observer effect occurs when observing or measuring some parameter of a physical state controlled by quantum mechanics alters the state or value of that parameter. For example, when a scientist in the double-slit experiment determines which slit a photon is going through, the interference pattern produced by the wave nature of light is altered. An observer effect is also seen in physiology, although not at such a fundamental level. In physiology, scientists often alter the value of the parameter of interest by the methods used to measure that parameter. Core temperature measurement in mammals is an example of the observer effect. The traditional method of measuring core temperature has been to insert a temperature probe into the colon of an animal. In an insightful experiment, Poole & Stevenson (1977) demonstrated that the measurement of core temperature in this way causes an increase in core temperature. In a similar fashion, restraining rats in order to measure core temperature or some other variable can alter the parameters of interest (Nagasaka et al. 1980). Unlike the situation in quantum mechanics, where the observer effect appears to be an unavoidable aspect of reality, the observer effect in physiology can often be minimized by improvements in technology. In the field of temperature regulation, one technological improvement is the development of radiotelemetry methods of measuring core temperature. With these methods, both restraint and colonic probing can be eliminated and the impact of these factors on core temperature determined. For example, using radiotelemetry measures of core temperature, both Berkey et al. (1990) and Morley et al. (1990) found that the frequently reported increased core temperature of spontaneously hypertensive rats was by and large an artifact of the procedures used to determine core temperature. In the previous issue of Experimental Physiology, Aydin and colleagues (Aydin et al. 2011) report on another ‘Who knew?’ finding through the use of radiotelemetry core temperature and heart rate measurements in rats. The authors asked a straightforward question: does restraint interact with thermoneutral and near-thermoneutral environmental temperatures in the regulation of heart rate and core temperature? No one else had thought to ask this important question before. For these experiments, Aydin, Grace and Gordon implanted rats with radiotransmitters, adapted them to restraint for 4 days, and then measured core temperature and heart rate during exposure to environmental temperatures between 14 and 30°C either while unrestrained or while restrained in an acrylic nose-inhalation tube. These tubes are routinely used in toxicological studies of gasses and aerosols. As changes in core temperature and heart rate have been used as end-points in assessing the toxicology of various agents, the impact of restraint on these physiological variables is of considerable interest (Gordon et al. 2008). Furthermore, the authors make the important point that such studies are usually carried out at ‘room temperature’, which can vary with time of day, time of year, location and other factors. So, what did the authors find regarding interactions between restraint and changes in ‘room temperature’? The unrestrained rats showed a transient increase in core temperatures during the first 40 min of measurement that was not dependent on environmental temperature. Tail skin temperatures were measured with a standard thermocouple taped to the tail at the beginning and end of the exposure period. Tail skin temperatures decreased during exposure to the cooler temperatures and increased during exposure to the warmer temperatures, indicating that the rats were effectively regulating core temperature by altering tail blood flow and thus non-evaporative heat loss. These physiological responses to exposure to the different environmental temperatures were altered by restraining the rats. At environmental temperatures between 16 and 22°C, their core temperatures during the exposure period were similar to those of the unrestrained rats. However, at environmental temperatures of 24–30°C, core temperatures of the restrained rats continued to increase throughout the exposure period, and at an environmental temperature of 14°C the core temperature steadily declined after a very short period of increasing. The restraint-induced alterations in core temperature appeared to be related to differences in tail skin blood flow, although differences in conductive heat loss, which were not measured, may also have played a role. The authors calculated a heat loss index based on the ratio of the skin to environmental temperature gradient and the core to environmental temperature gradient. Based on this heat loss index, the authors concluded that the stress due to restraint led to increased vasoconstriction of the tail at temperatures below 28°C. Restraint stress also led to changes in heart rate responses to exposure. Cooler environmental temperatures led to more sustained increases in heart rate in both unrestrained and restrained rats, but heart rates in the restrained rats at the end of the exposure period were higher than in the unrestrained rats. Most importantly, the authors also concluded that if rats are unrestrained, they are able to maintain normal core temperature at all environmental temperatures between 14 and 30°C, which they indicate are the limits of normothermia for adult rats. Furthermore, they concluded that restraint significantly shifts these limits of normothermia to a much narrower range of 16–20°C. This means that the results of toxicology and other experiments using restrained rats that are carried out at the normal ‘room temperature’ of around 22°C might be impacted by the changes in temperature regulation that result from the rats being outside the upper limit of normothermia. In addition, modest day-to-day or season-to-season changes in ‘room temperature’, which can occur in many laboratories around the world, might be having a much greater impact on experimental results than previously suspected. Thus, we can thank Aydin and colleagues for reminding us of the importance of the observer effect and in helping to increase the reliability of future research in this area.