Hypoxia reduces the hypothalamic thermogenic threshold and thermosensitivity.

Area of Science:

• Physiology
• Neuroscience
• Thermoregulation

Background:

• Hypoxia is known to reduce mammalian body temperature (T(b)).
• The neural mechanisms underlying this regulated hypothermia remain unclear.
• Previous studies suggest hypoxia lowers T(b) via reduced thermal neutral zone and conductance.

Purpose of the Study:

• To investigate if altered central thermosensitivity causes the metabolic response to hypoxia.
• To examine the role of the hypothalamus in mediating hypoxic hypothermia.
• To determine changes in hypothalamic temperature thresholds and thermosensitivity under hypoxic conditions.

Main Methods:

• Ground squirrels were exposed to normoxia and hypoxia (7%, 10%, 12% O(2)).
• The preoptic area of the hypothalamus was heated and cooled using indwelling thermodes.
• Metabolic rate and hypothalamic temperature were continuously monitored.

Main Results:

• The threshold hypothalamic temperature for metabolic response to cooling decreased significantly under hypoxia.
• Metabolic thermosensitivity (alpha) was reduced 5-9 fold during hypoxic conditions.
• Hypoxia proportionally reduced the hypothalamic temperature threshold for thermogenesis.

Conclusions:

• The fall in body temperature during hypoxia results from reduced activation of thermogenic mechanisms.
• Decreased central thermosensitivity is a critical factor in altering mammalian T(b) during hypoxia.
• Altered central thermosensitivity may explain reduced T(b) and circadian fluctuations under hypoxic stress.