Sympathetic activity in brown adipose tissue in lactating mice.

Sympathetic activity has been assessed, by measurements of norepinephrine turnover, in interscapular brown adipose tissue of mice during lactation. Norepinephrine turnover was reduced in brown adipose tissue from early lactation until weaning. The reduction in turnover occurred in dams suckling either large-or small-sized litters. Norepinephrine turnover returned to the control level after natural weaning and increased rapidly after abrupt weaning at peak lactation. Acute exposure to cold resulted in a large increase in norepinephrine turnover in brown adipose tissue of lactating mice, as in control animals. These results indicate that sympathetic activity is suppressed in brown adipose tissue during lactation, but sympathetic responsiveness is retained. The reduction in sympathetic activity is likely to be responsible for the decrease in brown adipose tissue thermogenesis in lactation. Norepinephrine turnover in the heart tended to be reduced at peak lactation, suggesting that there may be a general decrease in sympathetic activity in the lactating animal. In contrast to the normal animal, the hyperphagia of lactation does not lead to an activation of the sympathetic nervous system.