Effects of food and light on norepinephrine turnover

To study diurnal differences in norepinephrine turnover, groups of rats were housed in rooms with alternating 12 h light and 12 h dark where half the animals were in a normal light cycle and half were in a reversed light cycle. Norepinephrine turnover was measured in all groups beginning at 1000 by inhibition of tyrosine hydroxylase. When rats are fed ad libitum, the turnover of norepinephrine in interscapular brown adipose tissue and heart was significantly higher in the normal cycle than in the reversed cycle. In addition, there was an interaction between the turnover of norepinephrine and the feeding and lighting schedules. When animals were adapted to eating from 0700 to 1900, the turnover of norepinephrine was somewhat faster when the animals ate in the reversed cycle than when they ate in the normal cycle. Conversely, when the feeding schedule was reversed and animals ate between 1900 and 0700, the turnover of norepinephrine in the absence of food was slightly slower in the animals eating in the reversed cycle than in those eating in the light cycle. This interaction between lighting and food intake observed in both male and female rats was abolished by ventromedial hypothalamic lesions. These data suggest that food intake and lighting conditions interact as controllers of the sympathetic nervous system and that these interactions are modulated by the ventromedial hypothalamus.