Extracellular hypothalamic serotonin and plasma amino acids in response to sequential carbohydrate and protein meals

In previous studies, we showed that carbohydrate and protein ingestion, respectively, increased and decreased hypothalamic extracellular serotonin and the plasma ratio tryptophan over its competitor amino acids (Trp/LNAAs), reflecting serotonin synthesis. Serotonin levels returned towards baseline 2 h after either meal while the ratio remained altered. The question addressed is the ability of serotonin to respond expectedly to a second meal of the alternate nutrient. Rats were fed with sequential meals of either carbohydrates first and then casein 2 h later or in reverse order. Hypothalamic serotonin was measured using microdialysis. Permanent blood sampling allowed to track in parallel plasma amino acids. A carbohydrate meal increased hypothalamic serotonin, so did a subsequent casein meal. Conversely, following a casein meal that reduced serotonin, a carbohydrate meal also decreased it. The plasma ratio Trp/LNAAs was enhanced by a carbohydrate meal and remained high for 2h. A subsequent casein meal reversed this change but the ratio remained higher than basal values. A first casein meal reduced the ratio that was not increased again by a subsequent carbohydrate meal. It is obvious that ingestion of specific nutrients induce long-lasting metabolic and neurochemical variations that prevent subsequent changes to occur. The lack of expected changes to a second meal addresses again the hypothesis of alternate appetites for carbohydrates and proteins driven by serotonin changes.