Figure 2. Sensitivity to the torpor-inducing effects of the A1AR agonist CHA increases as the hibernation season progresses. a, CHA during the off-season, when animals were not displaying spontaneous torpor, induced a slight decrease in V̇O2 and Tb in all six AGSs tested. b, Early in the hibernation season after all animals showed evidence of spontaneous torpor, CHA induced a torpor-like response in two of six animals tested. c, In the remaining four animals, the same dose of the drug did not induce torpor. d, By the middle of the hibernation season (midseason), the same dose of CHA induced torpor in all six AGSs tested. e, Spontaneous torpor in one AGS. f, Pentobarbital, regardless of season, induced a response similar to CHA during the off-season (n = 3). (The time scale on the x-axis in c applies to d and e and is a continuous 30 h.) g–l, Detail of the first 4.5 h of a–f illustrates that CHA-induced torpor resembles spontaneous torpor where a rapid drop in metabolism is followed by a slow gradual decrease in Tb. g, During the off-season CHA induces a rapid drop in Tb that begins before and at the same rate as the decline in O2 consumption. h, j, k, When CHA induces torpor (h, j) and when animals spontaneously enter torpor (k), Tb declines more slowly than O2 consumption. g, i, l, When CHA fails to induce torpor (g, i) and after pentobarbital (l), Tb and O2 consumption decline at similar rates. Data shown are means ± SEM.