Hypoxia inhibits cold-induced huddling in rat pups

We asked to what extent hypoxia would modify the huddling behaviour of young rats during cold exposure. Sets of five animals (postnatal age 9+/-1 days) were placed at predetermined positions in a chamber maintained at approximately 33 degrees C (warm) or approximately 15 degrees C (cold), in normoxia or hypoxia (10% inspired O2), and their movements monitored for 30 min by a video camera. The surface areas (SA) of each individual pup (SAi) and of the whole set of pups (SAset) was measured every 5 min. In warm, the rats spread out, and both SAi and SAset were the greatest, whether in normoxia or hypoxia. In hypoxia, the total travelled distance (TTD) was much greater than in normoxia. In cold, during normoxia, SAi and SAset were decreased because of postural changes and huddling, and body temperature, measured at the end of the exposure, was also decreased. In hypoxic-cold, compared to normoxic-cold, fewer pups were in contact with one another, SAi and SAset did not decrease and the drop in body temperature was larger. Differently from hypoxia, hypercapnia (5% CO2) did not modify the responses observed during breathing air, whether in warm or cold conditions. We conclude that hypoxia, in addition to inhibiting shivering and non-shivering thermogenesis, can also limit behavioural thermogenesis, with the effect of further lowering body temperature.