Neurochemical coding for feeding in sheep and steers.

Feeding responses of sheep and steers were compared following intraventricular injection of alpha-adrenergic agonists and antagonists in 2-hr. tests. In sheep, injection of the alpha-agonist 1-norepinephrine (1-NE) (140-1,120 nmoles) increased feed intakes 288% and 388% compared with intakes following synthetic cerebrospinal fluid. This response was blocked by the alpha-antagonist phenoxybenzamine, whereas the antagonist alone decreased feed intake. The beta-adrenergic antagonist propranolol did not modify the 1-NE response. A purer but weaker alpha-agonist, 1-phenylephrine, also resulted in increased feed intake that was blocked by phenoxybenzamine. In contrast to the feeding responses of sheep, 1-NE caused hypophagia in steers, reducing intakes as much as 58% in 2-hr. tests over doses ranging 42-27,800 nmoles. Phenoxybenzamine blocked the 1-NE-induced hypophagia in steers; when injected alone, it increased feed intakes 181% of control values. Changes in feeding following 1-NE injections are probably not attributable to changes in temperature, fat mobilization, or stupor. The data support the hypothesis of an alpha-adrenergic-coded system for feeding in sheep and for satiety in steers.