The open field assay is influenced by room temperature and by drugs that affect core body temperature

The open field assay is used to study anxiety-related traits and anxiolytic drugs in rodents. This assay entails measuring locomotor activity and time spent in the center of a chamber that is maintained at ambient room temperature. However, the ambient temperature in most laboratories varies daily and seasonally and can differ substantially between buildings. Here, we sought to evaluate how varying ambient temperature and core body temperature (CBT) affected open field locomotor activity and center time of wild-type (WT) and Trpm8 knock-out (Trpm8-/-) mice. TRPM8 is an ion channel that detects cool temperatures and is activated by cooling agents, including icilin and menthol. We found that the cooling agent icilin increased CBT and profoundly reduced distance traveled and center time of WT mice relative to vehicle controls. Likewise, cooling the ambient temperature to 4{degrees}C reduced distance traveled and center time of WT mice relative to Trpm8-/- mice. Conversely, the TRPM8 antagonist (M8-B) reduced CBT and increased distance traveled and center time of WT mice when tested at 4{degrees}C. Predictably, the TRPM8 antagonist (M8-B) had no effect on CBT or open field behavior of Trpm8-/- mice. The anxiolytic diazepam reduced CBT in both WT and Trpm8-/- mice. When tested at 4{degrees}C, diazepam increased distance traveled and center time in WT mice but did not alter open field behavior of Trpm8-/- mice. Our study shows that environmental temperature and drugs that affect CBT can influence locomotor behavior and center time in the open field assay, highlighting temperature (ambient and core) as sources of environmental and physiologic variability in this commonly used behavioral assay.