Season Primes the Brain in an Arctic Hibernator to Facilitate Entrance into Torpor Mediated by Adenosine A₁ Receptors

Abstract

Torpor in hibernating mammals defines the nadir in mammalian metabolic demand and body temperature that accommodates seasonal periods of reduced energy availability. The mechanism of metabolic suppression during torpor onset is unknown, although the CNS is a key regulator of torpor. Seasonal hibernators, such as the arctic ground squirrel (AGS), display torpor only during the winter, hibernation season. The seasonal character of hibernation thus provides a clue to its regulation. In the present study, we delivered adenosine receptor agonists and antagonists into the lateral ventricle of AGSs at different times of the year while monitoring the rate of O₂ consumption and core body temperature as indicators of torpor. The A₁ antagonist cyclopentyltheophylline reversed spontaneous entrance into torpor. The adenosine A₁ receptor agonist N⁶-cyclohexyladenosine (CHA) induced torpor in six of six AGSs tested during the mid-hibernation season, two of six AGSs tested early in the hibernation season, and none of the six AGSs tested during the summer, off-season. CHA-induced torpor within the hibernation season was specific to A₁AR activation; the A₃AR agonist 2-Cl-IB MECA failed to induce torpor, and the A_2aR antagonist MSX-3 failed to reverse spontaneous onset of torpor. CHA-induced torpor was similar to spontaneous entrance into torpor. These results show that metabolic suppression during torpor onset is regulated within the CNS via A₁AR activation and requires a seasonal switch in the sensitivity of purinergic signaling.