Circannual Rhythm in Body Temperature, Torpor, and Sensitivity to A1 Adenosine Receptor Agonist in Arctic Ground Squirrels

Abstract

A1 adenosine receptor (A1AR) activation within the central nervous system induces torpor, but in obligate hibernators such as the arctic ground squirrel (AGS; Urocitellus parryii), A1AR stimulation induces torpor only during the hibernation season, suggesting a seasonal increase in sensitivity to A1AR signaling. The purpose of this research was to investigate the relationship between body temperature (Tb) and sensitivity to an adenosine A1 receptor agonist in AGS. We tested the hypothesis that increased sensitivity in A1AR signaling would lead to lower Tb in euthermic animals during the hibernation season when compared with the summer season. We further predicted that if a decrease in euthermic Tb reflects increased sensitivity to A1AR activation, then it should likewise predict spontaneous torpor. We used subcutaneous IPTT-300 transponders to monitor Tb in AGS housed under constant ambient conditions (12:12 L:D, 18 °C) for up to 16 months. These animals displayed an obvious rhythm in euthermic Tb that cycled with a period of approximately 8 months. Synchrony in the Tb rhythm within the group was lost after several months of constant L:D conditions; however, individual rhythms in Tb continued to show clear sine wave–like waxing and waning. AGS displayed spontaneous torpor only during troughs in euthermic Tb. To assess sensitivity to A1AR activation, AGS were administered the A1AR agonist N6-cyclohexyladenosine (CHA, 0.1 mg/kg, ip), and subcutaneous Tb was monitored. AGS administered CHA during a seasonal minimum in euthermic Tb showed a greater drug-induced decrease in Tb (1.6 ± 0.3 °C) than did AGS administered CHA during a peak in euthermic Tb (0.4 ± 0.3 °C). These results provide evidence for a circannual rhythm in Tb that is associated with increased sensitivity to A1AR signaling and correlates with the onset of torpor.