Control of meal size by central noradrenergic action.

Previous investigations of central noradrenergic effects on food intake have concentrated on the use of high doses of noradrenaline (two to 200 times brain noradrenaline content). In this work we examined the effect of low doses of noradrenaline (not exceeding brain noradrenaline content) on the parameters of spontaneous ingestive behavior. By arranging for rats to trigger remote infusions of noradrenaline into their own anterior forebrains at the beginning of spontaneously initiated meals, meal size was very reliably increased more than 200% by doses of 0.015--0.37 nmol (2.5--62 ng of noradrenaline base) (n = 12). The effect was alpha-adrenergic. It was blocked by phentolamine but not by propranolol. Infusions of noradrenaline at the above doses, which nearly triple meal size, did not elicit eating when made during an intermeal interval, nor did they influence the length of the intermeal interval when made 60 min after the termination of an uninfused meal. These results show that noradrenaline increased food intake at doses less than 1% of the brain's endogenous noradrenaline content. Meal size is more susceptible to alteration by noradrenaline manipulations than is meal frequency. The brain's own noradrenergic system may function to sustain food intake once feeding is initiated. This function of brain noradrenaline may control spontaneous meal size.