Ontogeny of alpha- and beta-adrenergic anorexia in rats.

The anorectic action of alpha- (phenylephrine) and beta- (isoproterenol) adrenergic agonists was studied in mildly deprived neonatal, weanling, prepubescent, and adult rats. Intraperitoneal phenylephrine produced a reduction of food intake at all ages but with reduced potency and with a maximum of 50% in neonates. Contrary to intramuscular epinephrine that has no effect on feeding at any age, intramuscular phenylephrine was as effective as intraperitoneal in neonates, probably because it is not as rapidly destroyed in tissues as epinephrine. However, in weanlings and adults intramuscular phenylephrine was much less anorectic than intraperitoneal, suggesting that this effect is exerted via the liver. Isoproterenol did not reduce milk intake at any age before adulthood. Lactate had no effect on milk intake before the age of 40 days. Thus catecholamine anorexia is a purely alpha-adrenergic effect in young rats and appears before the metabolic effect of lactate. beta-Adrenergic anorexia, on the other hand, can be obtained only after puberty, suggesting that the mechanism mediating it matures after the preparatory action of the sexual hormones.

Ontogeny of epinephrine-induced anorexia in rats.

Intraperitoneal or intraportal epinephrine elicits a strong inhibition of food intake in adult rats and dogs but has no effect when injected intramuscularly or intrajugularly, in spite of production of larger hyperglycemia and cardiovascular changes. These facts suggest that the effect of epinephrine on feeding is elicited via the liver. Ontogeny of this adrenergic control of food intake was studied in newborn and weanling rats. Anorexic effect of intraperitoneal epinephrine was clearly observed in dam-deprived 3-day-old neonatal rats (youngest in which it was tested), both when they were offered enriched milk through an anterior oral cannula while they were isolated from their dam and when they were allowed to suckle from her. However, anorectic effect was less in neonatal rats (day 3-13) than in adults. Weanling rats, 21-26 days old, were as sensitive to intraperitoneal epinephrine as adults. In 3- to 4-day-old rats it also reduced water intake, but this effect disappeared by day 12 and was not observed in mildly water-deprived adults. Peripheral adrenergic control of intake appears very early in ontogeny of rats. First, it affects food and water intake equally, but by day 12 it affects only food intake. Increase in sensitivity to epinephrine after weaning is probably due to an increase in number of hepatocytic adrenergic receptors and/or increase in enzymes necessary for hepatic effects of epinephrine.