Effects of rearing temperature on sympathoadrenal activity in young adult rats

Animals reared at 18°C exhibit enhanced innervation of brown adipose tissue (BAT) and greater cold tolerance as adults, yet gain more weight when fed an enriched diet compared with rats reared at 30°C. To explore this paradox, sympathoadrenal activity was examined using techniques of [³H]norepinephrine ([³H]NE) turnover and urinary catecholamine excretion in male and female rats reared until 2 mo of age at 18 or 30°C. Gene expression in BAT was also analyzed for several sympathetically related proteins. Although [³H]NE turnover in heart did not differ between groups, [³H]NE turnover in BAT was consistently elevated in the 18°C-reared animals, even 2 mo after removal from the cool environment. Gene expression for uncoupling proteins 1 and 3, GLUT-4, leptin, and the α_1A-adrenergic receptor was more abundant in BAT and the increase in epinephrine excretion with fasting suppressed in 18°C-reared animals. These studies demonstrate that obesity consequent to exposure to 18°C in early life occurs despite tonic elevation of sympathetic input to BAT. Diminished adrenal epinephrine responsiveness to fasting may play a contributory role.