Abstract
Whether tryptophan in the usual dose found in total parenteral nutrition (TPN) solutions caused a decrease in food intake was investigated in 16 adult male Fischer 344 rats. A jugular venous catheter was placed, and patency was maintained with a continuous infusion of normal saline at 3 ml/hr. After a 10-day recovery period, eight normal saline control rats were sacrificed and the remaining eight rats were given a parenteral nutrition solution (PN-50) at 3 ml/hr, providing 50% of the rat's daily energy requirement as 5% dextrose and 2% Intralipid. On day 14, study rats were further divided into two subgroups. Subgroup A received tryptophan (180 mg/day) added to the solution for 5 days. Subgroup B received the serotonin-receptor blocker, cyproheptadine 24 mg/kg/day, continuously for 5 days, and on day 16, tryptophan (0.25% = 180 mg/kg/day) was added to the solution for 3 days. Chow and water was continuously available and spontaneous food intake was measured daily. Study rats were sacrificed on day 19. Plasma amino acids and insulin and whole brain biogenic amines were compared with those in control rats using Student's t-test. With administration of PN-50, spontaneous food intake decreased to about 60% compared with the pre-PN level. The addition of tryptophan to the continuous infusion of PN-50 did not decrease spontaneous food intake further. Compared with pre-PN levels there were no changes in plasma immunoreactive insulin, plasma tryptophan concentration, or plasma Trp/Val+Leu+Ile+Phe+Tyr ratio, even though the concentration of tryptophan (0.25%) which was added to the PN solution was four times higher than that usually found in standard TPN solution. Furthermore, the administration of cyproheptadine did not increase spontaneous food intake. The data thus question the role that tryptophan, in its usual doses in TPN solution, is thought to have in the regulation of spontaneous food intake. (Journal of Parenteral and Enteral Nutrition
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