Limited bedding and nesting material changes indices of cellular metabolism and behavioral thermal regulation in Long-Evans rats during the first two weeks of life

Abstract

A quality nest can buffer pups from the cold climate in typical laboratory housing by insulating them from cool ambient air and facilitating pup huddling during early life when many thermal regulation strategies are not yet developed. The limited nesting material (LBN) manipulation prevents the dam from constructing a quality nest. In the present study, we investigate whether LBN presents a thermal challenge to pups across the first two postnatal weeks. Behavioral thermal regulation (huddling), physiological thermogenesis (activation of brown adipose tissue; BAT), changes in cellular metabolism (mitochondrial biogenesis) in peripheral and central tissues, and maternal behavior were measured in environments with LBN or abundant bedding on postnatal day (P) 2, P6, P10, and P14. The huddle of LBN litters had greater area, greater perimeter, and reduced circularity, and more pups visible. Control male pups were visible longer than control female pups during home cage recordings and LBN pups were visible longer than control pups. Heart weight relative to body weight was higher in LBN pups after P2. LBN affected BAT, but not white adipose tissue (WAT), activation in a sex and age-specific manner, with reductions in UCP1 expression after P2. Pups appear especially affected by LBN on P6, when mitochondrial DNA copy number was reduced in WAT. Mitochondrial DNA copy number was reduced in LBN pups in the hippocampus but not the brainstem. Our data provide evidence of changes in indices of thermal regulation and cellular metabolism in LBN pups which may be indicative of changes in energy allocation during development.