Route of administration of tryptophan and tyrosine affects short-term food intake and plasma and brain amino acid concentrations in rats.

The effects of route of administration of tryptophan and tyrosine on food intake and diet selection and on plasma and brain tryptophan or tyrosine concentrations were studied. Tryptophan and tyrosine given intraperitoneally at 100 mg/kg body wt suppressed food intake by 33-45% over a 2-h feeding period beginning 30 min after injections. No preferential effect was shown for either the high carbohydrate or high protein diet choice. When given intragastrically at this dose, neither tryptophan nor tyrosine affected food intake. Tryptophan, but not tyrosine, at 200, 400 and 600 mg/kg body wt given intragastrically reduced food intake and carbohydrate diet intake by an average of 20 and 25%, respectively, in the first 2 h of feeding. Plasma and brain tryptophan were higher for 30 min following intraperitoneal tryptophan injections than after tryptophan given intragastrically at 100 mg/kg body wt. However, intraperitoneal tyrosine (100 mg/kg body wt) resulted in higher plasma tyrosine levels at 5-10 min but lower levels at 30 min than when tyrosine was given intragastrically. Brain tyrosine was higher after intraperitoneal treatment only at 10 min and was similar to intragastric treatment at other times. When these amino acids were given intragastrically at 400 mg/kg body wt, higher plasma tryptophan, plasma tyrosine and brain tyrosine were found than following intraperitoneal injection of the behaviorally effective dose (100 mg/kg body wt) at 30 min. Thus the reduced effect on food intake of tryptophan or tyrosine given intragastrically compared with intraperitoneally is not readily explained by their lower concentrations in plasma and brain preceding and at the time that the rats had access to food.