Using radiolabeled free fatty acids, a mathematical model was developed to calculate transfer rate constants and rates of incorporation from plasma of plasma free fatty acid (FFA) into individual brain regions and brain phospholipids. Radiolabeled palmitic acid (PA, e.g. 3H-PA) is incorporated into the sn-1 of phosphatidylcholine; [3H]-arachidonic acid (3H-AA) is incorporated into the sn-2 position of phosphatidylinositol and phosphatidylcholine, while radiolabeled docosahexaenoic acid (DHA) is incorporated into the sn-2 position of phosphatidylethanolamine. [1- 14C]arachidonate was used in awake rats, with or without cholinergic stimulation to monitor the effect of lesioning of the nucleus basalis magnocellularis. We demonstrated an increased uptake of radiolabeled fatty acids in tumor cells relative to surrounding brain parenchyma. Administration of serotonergic agonists increased brain incorporation of 14C-AA but not 3H-PA. This again demonstrates the ability of our approach (e.g. 3H-AA) to study second messenger systems, in vivo. We have now synthesized C-11 radiolabeled PA and AA for use in imaging monkey brain phospholipids by positron emission tomography (PET) and PET studies using these compounds in monkey have been initiated with positive results. Biochemical analysis and modeling of phospholipid metabolism using radiolabeled fatty acids to monitor the transfer of fatty acid into brain phospholipid has also been further analyzed both in brain perfusion and blood infusion models while paying particular attention to the incorporation of fatty acid into phospholipid precursor pools i.e. brain free fatty acid and brain acyl-Coenzyme A. We are now in the last stage of determining parameters for our fatty acid mathematical model. A complete analysis of brain capillary phospholipids and its variation with age has also been completed. Work on endogenous phospholipase A2 and its pharmacological inhibition has been initiated in vitro and will be extended to the brain perfusion model (i.e. in vivo).
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