This competing renewal application will examine biochemical/metabolic mechanisms underlying the effects of food on the synthesis of two brain neurotransmitters, serotonin (5HT) and dopamine (DA). 5HT and DA synthesis rates reflect brain levels of their respective substrates, tryptophan (TRP) and tyrosine (TYR), because the rate-limiting enzymes TRP and TYR hydroxylase are unsaturated with substrate. Brain TRP and TYR levels, in turn, are influenced by their uptake from blood, mediated by a competitive transport system shared by all large neutral amino acids [LNAA], including TRP and TYR. Brain TRP uptake may also be influenced by TRP binding to serum albumin. Our proposed studies for TRP->5HT will examine mechanisms by which food ingestion modifies brain TRP uptake (and 5HT synthesis), studying in particular the importance of the meal's protein content, acting via changes in serum LNAA levels, and its fat content, acting via changes in albumin binding of TRP (post-meal changes in serum non-esterified fatty acid [NEFA] levels alter TRP binding to albumin). We will also examine the effects of exercise, and its interaction with meals on serum TRP binding, brain TRP levels and 5HT synthesis: Exercise elevates serum NEFA levels, reduces TRP binding, and may thus raise brain TRP and 5HT. For TYR->DA, we will examine PHE as a substrate for DA synthesis, and the relative roles of PHE and TYR in overall DA [catecholamine (CAM)] synthesis, in vitro and in vivo. We will also study whether PHE and TYR uptake into brain and adrenal medulla, a model for peripheral CAN neurons, are similar, and whether food ingestion produces similar effects on PHE and TYR levels, (and CAM synthesis) at each site. For studies in vivo, we will employ rats; for studies in vitro, we will employ PC12 cells and bovine adrenal medullary cells. TRP, TYR and PHE levels, and 5HT and DA synthesis will be measured by HPLC in brain and adrenal medulla following acute or chronic food ingestion, or amino acid injection, or in cell cultures incubated with TYR, PHE or both together. Results should provide a more definitive understanding of one mechanism by which diet appears to inform the brain regarding what has recently been consumed, alone and possibly within the context of other metabolic events (here, those stimulated by exercise). Our data will also add important new information on the relative roles of TYR and PHE as substrates in CAM synthesis, and the impact of diet on CAM synthesis, acting via changes it produces in the neuronal pools of each substrate.
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