Body temperature is highly regulated in mammals. However, thermal biology in smaller mammals (such as mice) is different from that in larger mammals (such as adult humans). For example, when mice are singly housed at room temperature, about half of caloric intake is burned to maintain body temperature (referred to as cold-induced thermogenesis), while humans require little cold-induced thermogenesis. Upon fasting, mice can reduce their body temperature by >10 C, while humans with extreme starvation lower body temperature by only 0.2 C. We are exploring the use of body temperature as an indicator of the perceived metabolic status of the mouse. For example, what is the effect on body temperature of a genetic manipulation or drug treatment? What genetic manipulations or drug treatments cause dissociation of body temperature from nutritional status? What are the neurotransmitters and neural mechanisms involved? Mice are also an ideal model system to study hypothermia, as the central regulatory mechanisms are likely conserved across mammals, but the mice show much greater changes than larger mammals. Thus, mice are a more sensitive species that can suggest studies that might be productively undertaken in larger individuals such as adult humans. We are interested in the neural control of body temperature and hypothermia, and in understanding pharmacologic inducers of hypothermia. Progress in FY2019 includes the following: Extracellular adenosine is a danger/injury signal that initiates protective physiology, such as hypothermia. We previously showed that adenosine can cause hypothermia via any one of the four adenosine receptors (ARs), A1, A2A, A2B, and A3. We generated mice lacking all four ARs (quad knockout QKO), to enable investigation of the AR dependence of physiologic processes, focusing on body temperature. The QKO mice demonstrate that ARs are not required for growth, metabolism, breeding, and body temperature regulation (diurnal variation, response to stress, and torpor). However, the mice showed decreased survival starting at about 15 weeks of age. While adenosine agonists cause profound hypothermia via each AR, adenosine did not cause hypothermia (or bradycardia or hypotension) in QKO mice, indicating that AR-independent signals do not contribute to adenosine-induced hypothermia. The hypothermia elicited by adenosine kinase inhibition (with A134974), inosine, or uridine also required ARs, as each was abolished in the QKO mice. The proposed mechanism for uridine-induced hypothermia is inhibition of adenosine transport by uridine, increasing local extracellular adenosine levels. In contrast, adenosine monophosphate (AMP)induced hypothermia was attenuated in QKO mice, demonstrating roles for both AR-dependent and AR-independent mechanisms in this process. The physiology of the QKO mice appears to be the sum of the individual knockout mice, without clear evidence for synergy, indicating that the actions of the four ARs are generally complementary. The phenotype of the QKO mice suggests that, while extracellular adenosine is a signal of stress, damage, and/or danger, it is less important for baseline regulation of body temperature.
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