Season primes the brain in an arctic hibernator to facilitate entrance into torpor mediated by adenosine A(1) 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 O(2) consumption and core body temperature as indicators of torpor. The A(1) antagonist cyclopentyltheophylline reversed spontaneous entrance into torpor. The adenosine A(1) receptor agonist N(6)-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 A(1)AR activation; the A(3)AR agonist 2-Cl-IB MECA failed to induce torpor, and the A(2a)R 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 A(1)AR activation and requires a seasonal switch in the sensitivity of purinergic signaling.

Figure 1.

Onset of torpor requires A₁AR activation. a, An increase in the rate of O₂ consumption (V̇_O2) and an increase in T_b to euthermic levels occurred in all animals tested following administration of CPT (3 nmol, i.c.v.) during onset of spontaneous torpor. This indicates that A₁AR activation is necessary for torpor onset. b, Vehicle had no effect in any of the animals tested. Results are shown as means and SEM; n = 6 AGSs.

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 V̇_O2 and T_b 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 T_b. g, During the off-season CHA induces a rapid drop in T_b 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), T_b declines more slowly than O₂ consumption. g, i, l, When CHA fails to induce torpor (g, i) and after pentobarbital (l), T_b and O₂ consumption decline at similar rates. Data shown are means ± SEM.

Figure 3.

None of the vehicles tested produced a notable effect on T_b or V̇_O2. a–d, Vehicle (0.01 m phosphate buffer, i.c.v., for CHA; a–c); and saline (i.p., for pentobarbital; d) failed to produce any notable change in T_b or V̇_O2. Data shown are means and SEM; n = 6 AGSs.

Figure 4.

CHA-induced and spontaneous torpor is specific to A₁AR. a, The selective A₃AR agonist 2-Cl-IB-MECA (3 nmol, i.c.v.) failed to induce torpor in any of the animals tested, while a subsequent injection of CHA (0.5 nmol, i.c.v.) induced torpor (n = 3). Top trace is T_b; bottom trace is V̇_O2. b, MSX-3 (3 nmol, i.c.v.), a water-soluble prodrug of the A₂AR antagonist MSX-2, failed to reverse onset of spontaneous torpor (n = 3). Data shown are means and SEM.

Figure 5.

Enhanced purinergic signaling turns on the seasonal switch to hibernate in arctic ground squirrels. Schematic diagram modified from the two-switch model of Serkova et al. (2007) illustrates how seasonal sensitization of purinergic signaling primes the brain for adenosine-induced torpor during the hibernation season. The off-season, commonly referred to as the “summer-active” season, is indicated by a white background. During the off-season, overflow of adenosine that occurs as part of normal purinergic signaling fails to induce torpor. Here we use homeostatic sleep drive as an example of normal purinergic signaling (Porkka-Heiskanen et al., 1997; Basheer et al., 2004). The present report shows that an increase in the gain in purinergic signaling occurs during the hibernation season. The hibernation season is indicated by a dark background and the shading from light to dark illustrates an increase in gain in purinergic signaling as the season progresses. This increased gain in purinergic signaling during the hibernation season primes the brain such that overflow of endogenous adenosine with subsequent activation of A₁AR now induces torpor. The effect of endogenous adenosine is demonstrated by the ability of an A₁AR antagonist (CPT) to reverse onset of spontaneous torpor.