Growth performance and brain neurotransmitters in pigs as affected by tryptophan, protein, and sex

The effects of dietary levels of tryptophan (TRP) and protein on voluntary feed intake, growth performance, muscle pH, and brain indoleamine and catecholamine concentrations were studied in an experiment involving 48 crossbred Piétrain x Large White pigs (38 kg initial weight). Equal numbers of females, intact males, and barrows were used and they were given ad libitum access to feed during the 28-d experiment. Four dietary treatments were compared in a 2 x 2 factorial arrangement, with the following levels of analyzed TRP and CP: 1) .12 and .16% TRP, suboptimum and optimum for growth, respectively, 2) 12.7% CP of a diet based on corn-soybean meal and adequately balanced for limiting essential amino acids other than TRP; and 16.7% CP diet with additional protein from corn gluten meal. Dietary CP interacted with TRP on feed intake and growth, with a greater depressive effect at .12% TRP than at .16% TRP in the three genders. Norepinephrine concentration, which was the highest in the preoptic area, was higher in females than in intact or castrated males. Serotonin concentration was increased by TRP supplementation in the different brain regions. Additional CP depressed serotonin concentration more at .12% TRP than at .16% TRP. The greater sensitivity of feed intake and growth of pigs, especially females, to TRP deficiency in the presence of additional CP may have been related to a more critical serotonergic activity, when hypothalamic serotonin concentration fell below a threshold level. Supplemental TRP increased muscle pH, both at 45 min and 24 h after death, in ham (adductor femoris and semimembranosus) and loin (longissimus), suggesting a sedative effect of TRP for reducing stress response.