Anorexia elicited by different catecholamines in rats

Adrenaline (A) produces a strong anorexic effect, possibly by acting on hepatic receptors (nerve endings on hepatocytes). To study whether this is mediated by alpha- or beta-adrenergic mechanisms, or both, the anorexigenic effects of intraperitoneal injections of A, noradrenaline (NA) and isoproterenol (I) were studied under four different experimental conditions: (I) at the beginning of the dark period in rats fed ad libitum, or (II) on a 24 h-feeding/24 h-fasting schedule; (III) during the light period, under the same feeding schedule; (IV) after an acute 24 h fast. In condition I, the three catecholamines produced a marked decrease in feeding, slightly larger for A. In condition II (dark), they elicited a decrease in food intake about double that in condition III (light), their relative potencies also differed: A greater than I greater than NA in II and A greater than I = NA in III. In IV, the same relative potencies were obtained as in III. A mixture of half-doses of NA and I had the same effect in III and IV as either NA or I alone, suggesting that the alpha and beta effects are additive. However, even a mixture of the full doses of NA and I was not as effective as A in condition IV. This suggests that A is more potent than NA or I at stimulating hepatic adrenergic receptors that cannot be classified as either alpha or beta.