Cytochrome oxidase activity in brown fat varies with reproductive response and use of torpor in deer mice.

Reproductive responses and thermogenic properties of brown adipose tissue (BAT) were evaluated in individuals from an outbred population of deer mice (Peromyscus maniculatus nebrascensis) after 10 weeks exposure to short photoperiod (8:16 light:dark) and cold ambient temperature (2 degrees C). Deer mice populations are composed of phenotypes that differ in their reproductive response to environmental cues. These phenotypes also differ in body temperature regulation as indicated by their use of daily torpor. By comparing BAT responses among individuals of different phenotypes, we were able to assess the association between environmentally induced changes in reproduction and metabolism. Short/cold days caused increased proliferation of BAT and higher cytochrome oxidase activity. However, the magnitude of these changes varied with reproductive phenotype and use of daily torpor. BAT weight in short/cold day exposed males with normal sized testes more than doubled while total cytochrome oxidase activity increased by 30% as compared to controls. In contrast, short/cold day exposed deer mice with atrophic testes that employed daily torpor exhibited a 64% increase in BAT weight and 100% increase in total cytochrome oxidase activity, compared to control mice. Cytochrome oxidase activity in nontorpid deer mice with atrophic testes was intermediate to these two groups. Our results demonstrate a response of BAT to short/cold days that varies with individual reproductive response. This finding suggests that there exists a common integrative mechanism for temperature and photoperiod to regulate both seasonal reproductive and metabolic adjustments.