Circannual Rhythm in Body Temperature, Torpor, and Sensitivity to A 1 Adenosine Receptor Agonist in Arctic Ground Squirrels

A 1 adenosine receptor (A 1 AR) activation within the central nervous system induces torpor, but in obligate hibernators such as the arctic ground squirrel (AGS; Urocitellus parryii), A 1 AR stimulation induces torpor only during the hibernation season, suggesting a seasonal increase in sensitivity to A 1 AR signaling. The purpose of this research was to investigate the relationship between body temperature (T b ) and sensitivity to an adenosine A 1 receptor agonist in AGS. We tested the hypothesis that increased sensitivity in A 1 AR signaling would lead to lower T b in euthermic animals during the hibernation season when compared with the summer season. We further predicted that if a decrease in euthermic T b reflects increased sensitivity to A 1 AR activation, then it should likewise predict spontaneous torpor. We used subcutaneous IPTT-300 transponders to monitor T b 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 T b that cycled with a period of approximately 8 months. Synchrony in the T b rhythm within the group was lost after several months of constant L:D conditions; however, individual rhythms in T b continued to show clear sine wave–like waxing and waning. AGS displayed spontaneous torpor only during troughs in euthermic T b . To assess sensitivity to A 1 AR activation, AGS were administered the A 1 AR agonist N 6 -cyclohexyladenosine (CHA, 0.1 mg/kg, ip), and subcutaneous T b was monitored. AGS administered CHA during a seasonal minimum in euthermic T b showed a greater drug-induced decrease in T b (1.6 ± 0.3 °C) than did AGS administered CHA during a peak in euthermic T b (0.4 ± 0.3 °C). These results provide evidence for a circannual rhythm in T b that is associated with increased sensitivity to A 1 AR signaling and correlates with the onset of torpor.