Spatial and temporal activation of brain regions in hibernation: c‐fos expression during the hibernation bout in thirteen‐lined ground squirrel

Abstract

Hibernation results in dramatic changes in body temperature and metabolism; however, the central nervous system remains active during deep torpor. By cloning c-fos cDNA from the 13-lined ground squirrel (Spermophilus tridecemlineatus) and using squirrel c-fos mRNA probe for in situ hybridization histochemistry, we systematically analyzed and identified specific brain regions that were activated during six different phases of the hibernation bout. During entrance into torpor, we detected activation of the ventrolateral subdivision of the medial preoptic area ('thermoregulatory center'), and the reticular thalamic nucleus, which is known to inhibit the somatomotor cortex. During torpor, c-fos expression in the cortex was suppressed while the reticular thalamic nucleus remained uniformly active. Throughout torpor the suprachiasmatic nucleus ('biological clock') showed increasing activity, likely participating in phase-change regulation of the hibernation bout. Interestingly, during torpor very strong c-fos activation was seen in the epithelial cells of the choroid plexus and in tanycytes at the third ventricle, both peaking near the beginning of arousal. In arousal, activity of the suprachiasmatic and reticular thalamic nuclei and choroid epithelial cells diminished, while ependymal cells in the lateral and fourth ventricles showed stronger activity. Increasing body temperature during arousal was driven by the activation of neurons in the medial part of the preoptic area. In interbout awake animals, we demonstrated the activation of hypothalamic neurons located in the arcuate nucleus and the dorsolateral hypothalamus, areas involved in food intake. Our observations indicate that the hibernation bout is closely regulated and orchestrated by specific regions of the central nervous system. J. Comp. Neurol. 505:443-458, 2007. (c) 2007 Wiley-Liss, Inc.

Circannual Transitions in Gene Expression

“…Like the heart, one area of the brain, the hypothalamus, remains active throughout the hibernation season (Kilduff et al, 1982; Kilduff et al, 1990; Bratincsak et al, 2007). This region of the brain is involved in many aspects important for hibernation and hibernation preparation, including food intake, circadian rhythms, sleep, and thermoregulation.…”

“…The SCN of the hypothalamus, containing the circadian clock, is a likely candidate for the control of circannual hibernation timing. c-fos mRNA, used as a marker for neuronal activation, increases in the SCN during torpor and peaks during arousal (Bitting et al, 1994; Bratincsak et al, 2007), suggesting that it is involved in torpor bout timing. Broad hypothalamic lesions prevented successful hibernation (Satinoff, 1967) and focused lesions specific to the SCN altered hibernation timing in ground squirrels (Ruby et al, 1996), which suggests that the hypothalamus is important for both hibernation induction and maintenance.…”

Myocyte enhancer factor-2 and cardiac muscle gene expression during hibernation in thirteen-lined ground squirrels

“…In anticipation of the winter season ground squirrels enter a phase of hyperphagia during the autumn (weight gains of up to 40%), accumulate large lipid depots, and alter fuel use during hibernation to rely primarily on the β-oxidation of fatty acids (Buck and Barnes, 2000;Storey and Storey, 2010). Furthermore, hibernating ground squirrels withstand major changes in organ perfusion rates (b10% of normal), respiration rates (2.5% of normal), and only specific regions of the brain demonstrate active neuronal firing (Bratincsák et al, 2007;Frerichs and Hallenbeck, 1998;Storey and Storey, 2004). The present research focuses on the thirteen-lined ground squirrel, Ictidomys tridecemlineatus, which are native to the central prairies of North America and survive the winter by hibernating in underground burrows.…”

Myostatin levels in skeletal muscle of hibernating ground squirrels

“…Male squirrels were used for hibernation studies as previously described; temperature transponders implanted under the skin allowed body temperature (T b ) to be monitored to identify sampling times (Bratincsák et al, 2007). Studies took place within the natural hibernation season (January and February) in the Northern Hemisphere.…”

“…ASP 1223-05). Skeletal muscle (all hindlimb thigh muscles from each animal homogenized) of thirteen-lined ground squirrels was sampled at multiple time points over hibernation bouts, as defined in Bratincsák et al (Bratincsák et al, 2007): control euthermic (CON); entrance into hibernation (ENT), with T b decreasing ranging from 31 to 18°C (at least two successive temperature readings showed a decreasing T b ); early hibernation (EHib), animals showed a stable T b of 5-8°C for 24h; late hibernation (LHib), animals in torpor for at least 3days with a stable T b of 5-8°C; early arousal (EAr), with T b rising and increased respiration rate ≥60min -1 ; and arousal (AR), with a T b of at least 34-37°C for about 18h. Note that CON combined data from two types of control animals, those housed at room temperature and those that were in the cold room (4-5°C), which were capable of entering torpor but had not reentered hibernation; initial trials found no significant differences between these groups for any parameter measured.…”