Infrared thermography for non-invasive measurement of social inequality aversion in rodents and potential usefulness for future animal-friendly studies

Abstract

Infrared thermography is a method that detects thermal radiation energy and can measure the body surface temperature of animals from a distance. While rectal temperature has traditionally been used to measure animals' core temperature, thermal imaging can avoid the stress and potential rise of body temperature deriving from handling of the animals. Additionally, being non-invasive and contactless, thermal imaging allows free movement of the animals. The validity of this technique as a psychophysiological method has been proven in a series of stress-induced hyperthermia (SIH) studies of mice under social inequality conditions. Restraint in a holder elicits SIH in mice. A restrained mouse surrounded by freely moving cage mates displays increased SIH suggesting that social inequality enhances the stress. Social inequality can be examined also in unrestrained mice, in particular through unequal distribution of food. In this protocol, a food-deprived mouse is given a small piece of cheese, while its cage mate is given a large piece of cheese. This inequity causes SIH, suggesting social inequality aversion in mice. Thus, social inequality in different situations similarly increased SIH. Importantly, in future studies infrared thermography could also be used to evaluate emotional arousal states different from stress (for example to assess reactivity to rewards or in social and sexual preference tests). Moreover, the technique could be used to investigate also cognitive arousal induced by novelty. Indeed, infrared thermography could be a particularly useful tool for animal-friendly studies of cognition and emotion in rodents.

Keywords: advantageous inequality; disadvantageous inequality; inequality aversion; infrared thermography; stress-induced hyperthermia.

Figure 1

Experimental settings for the assessment of social inequality aversion in mice. (A) Setting to test inequality aversion to restraint stress (Watanabe, 2015). A mouse holder for blood sampling (diameter, 3 cm; length, 10 cm) was used to restrain the mouse. All animals were placed in cages (20 × 15 × 13 cm). (B) Setting to test inequality aversion in food distribution (Watanabe, 2017). An arena made of transparent acrylic plastic was employed. A central partition, a transparent acrylic plastic barrier with small holes (diameter: 0.5 cm; distance between holes: 0.3 cm), divided the arena into two equally sized chambers. Each chamber measured 19 × 11 × 20 cm. One mouse received a small piece of cheese, while the other one received a large piece. (C) Setting to test disadvantageous inequality aversion in food distribution (Watanabe, 2019). The experimental chamber was a 20 × 25 × 20 cm acrylic box. A transparent cylindrical tube (diameter, 10 cm; height, 20 cm) made of acrylic and featuring several holes (diameter of 0.5 cm with a distance of 0.3 cm between each hole) was placed vertically in the center of the experimental chamber. The test mouse was then placed in the cylinder. Yellow triangles indicate pieces of cheese. (D) Setting for the simultaneous recording of behavioral preference and body temperature (Watanabe, 2017). The apparatus used for the behavioral tests was a conventional conditioned place preference apparatus (MED ENV3015) with three compartments: two lateral compartments (16 × 13 × 12 cm) and a central compartment (6 × 13 × 12 cm). The central compartment was connected to the two lateral compartments through guillotine doors. A box made of gray acrylic plate was placed in each lateral compartment, so that the external appearance of the lateral compartments was identical. In each box, a transparent acrylic partition was placed 5 cm from the end wall to create a separated stimulus area. In this setting preference between a cage mate eating cheese (left) and a piece of cheese without cage mate (right) was tested. The subject mouse was placed in the center.