Thermoneutral housing has limited effects on social isolation-induced bone loss in male C57BL/6J mice

Social isolation stress has numerous known negative health effects, including increased risk for cardiovascular disease, dementia, as well as overall mortality. The impacts of social isolation on skeletal health, however, have not been thoroughly investigated. We previously found that 4 wk of social isolation through single housing led to a significant reduction in trabecular and cortical bone in male, but not female, mice. One possible explanation for these changes in male mice is thermal stress due to sub-thermoneutral housing and sex differences in thermal physiology. Single housing at room temperature (~20 to 25 °C)-below the thermoneutral range of mice (~26 to 34 °C)-may lead to cold stress, which has known negative effects on bone. Therefore, the aim of this study was to test the hypothesis that housing mice near thermoneutrality, thereby ameliorating cold-stress, will prevent social isolation-induced bone loss in male C57BL/6J mice. 16-wk-old mice were randomized into social isolation (1 mouse/cage) or grouped housing (4 mice/cage) at either room temperature (~23 °C) or in a warm temperature incubator (~28 °C) for 4 wk (N = 8/group). As seen in our previous studies, isolated mice at room temperature had significantly reduced bone parameters, including femoral bone volume fraction (-35% BV/TV), bone mineral density (-27% BMD), and cortical thickness (-12%). Contrary to our hypothesis, these negative effects on bone were not fully ameliorated by thermoneutral housing. There was no significant effect of housing or temperature on serum turnover markers. Social isolation increased glucocorticoid-related gene expression in bone and Ucp1 and Pdk4 expression in BAT across temperatures, while thermoneutral housing increased percent lipid area and decreased Ucp1 and Pdk4 expression in BAT across housing conditions. Overall, our data suggest thermal stress from single housing cannot fully explain social isolation-induced bone loss and provide a key insight into the mechanism mediating the effects of isolation on skeletal health.