Human Hibernation

What are thyroid hormones' roles?

Next time you get hit with a prolonged polar vortex, consider hibernating until the warmth of the sun returns.

Archeologists found evidence that our Neanderthal ancestors resorted to this metabolic state during their ‘ice ages’. In fossil pits’ bones, they noted changes similar to the rings seen on tree trunks, indicating periods of arrested growth just like with hibernating animals.

many mammals including primates such as bushbabies and lemurs do this. “This suggests that the genetic basis and physiology for such a hypometabolism could be preserved in many mammalian species including humans,” state Arsuaga and Bartsiokas.

Large mammals though, do not actually fully hibernate, but instead enter a more intermediate state similar to deep sleep, called ‘torpor’.

Suppression in basal metabolic rate and body temp characterize the torpid state, which is periodically interrupted by interbout arousals initiated by non-shivering thermogenesis via activation of brown adipose tissue. And yes, human primates do have brown fat, too.

Human Brown Fat Depots https://www.umassmed.edu/guertinlab/

Seasonal modulation of thermoregulation conserves energy through attenuation of the sympathetic drive, but retains heat generating capacity through activation of the hypothalamus-pituitary-thyroid (HPT) axis.

Thyroid hormones can stimulate thermogenesis from brown fat, by uncoupling the mitochondria energy generation, allow it to dissipate into heat. This occurs through a signal transduction pathway which includes adrenergic receptors and cAMP signal cascade.

Thyroid Hormones enhance the capacity of the cells to respond to most actions of catecholamines and maintain a metabolic rate geared up to make available and mobilize substrates essential to ensure vigorous adrenergic responses.

Molecular signaling involved in the stimulation of thermogenesis in brown adipose tissue, including exposure to cold, local production of triiodothyronine, adrenergic stimulation of the b(3) kind and a number of the small molecules that mimic one or another physiological stimulus. Abbreviations: UCP1 (Uncoupling Protein); b3-AR and b1-AR (b-adrenergic receptors); AC (adenyl cyclase); PKA (protein kinase A); ATP (Adenosine triphosphate); TR (thyroid hormone nuclear T3 receptors); PPARg (peroxisomeproliferator-activated receptor gamma); RXR (retinoid X receptor); IL-4 (interleukin 4). A. Birerdinc et al./Progress in Lipid Research 52 (2013) 51–6157

Thermogenic capacity develops during autumn season as the HPT axis is activated to reach maximum capacity in winter, by increasing free thyroid hormones in response to cooling challenges. However, thermogenesis is inhibited during torpor state as sympathetic premotor neuronal activation is lower in winter, until full arousal happens.

Kinda gives a different slant on the phrase, ‘netflix and chill’ eh?

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REFERENCES

Early humans may have survived the harsh winters by hibernating. https://www.theguardian.com/science/2020/dec/20/early-humans-may-have-survived-the-harsh-winters-by-hibernating

How Hibernation Works. https://animals.howstuffworks.com/animal-facts/hibernation.htm

Thermogenic Mechanisms and Their Hormonal Regulation. https://journals.physiology.org/doi/full/10.1152/physrev.00009.2005

The Raphe Pallidus and the Hypothalamic-Pituitary-Thyroid Axis Gate Seasonal Changes in Thermoregulation in the Hibernating Arctic Ground Squirrel (Urocitellus parryii). https://www.frontiersin.org/articles/10.3389/fphys.2018.01747/full

Manipulating molecular switches in brown adipose. Available from: https://www.researchgate.net/publication/276173666_UMS-Manipulating_molecular_switches_in_brown

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