Crude protein and supplemental dietary tryptophan effects on growth and tissue neurotransmitter levels in the broiler chicken.

Indian River male broiler chickens growing from 7 to 28 d of age were fed on diets containing 120, 210 and 300 g crude protein/kg diet and 0, 1.67 or 16.7 g added tryptophan (TRP)/kg diet. The hypothesis tested was that crude protein levels and TRP would affect both growth and neurotransmitter metabolism. Heart, brain and pancreatic neurotransmitter (noradrenaline (NA), dopamine (DA), serotonin (5-HT) and 5-hydroxy-indole-3-acetic acid (5-HIAA)) concentrations were determined by HPLC separation and electrochemical detection. Malate dehydrogenase (2-oxoglutarate decarboxylating) (NADP+) (MDH(NADP+); EC 1.1.1.40), isocitrate dehydrogenase (NADP+) (ICD(NADP+); EC 1.1.1.42) and aspartate aminotransferase (AAT; EC 2.6.1.1) activities were also measured. Supplemental TRP decreased growth and feed intake. Increasing dietary crude protein decreased MDH(NADP+), but increased (ICD(NADP+) and AAT activities. Additional dietary TRP decreased MDH(NADP+) activity, but had no effect on other enzyme activities. Cardiac NA concentrations were directly related to dietary crude protein levels while pancreatic levels were inversely related. An increase in dietary crude protein decreased both brain NA and DA. Supplemental dietary TRP increased both 5-HIAA and 5-HT. Changes in feed intake caused by different levels of both dietary crude protein and TRP are accompanied by altered levels of neurotransmitters. The present study indicates that much larger amounts of TRP are required to make simultaneous changes in feed intake and neurotransmitters.