IN VIVO IMAGING OF BRAIN SIGNAL TRANSDUCTION AND METABOLISM VIA ARACHIDONIC AND DOCOSAHEXAENOIC ACID IN ANIMALS AND HUMANS. The polyunsaturated fatty acids (PUFAs), arachidonic acid (AA, 20:4 n-6) and docosahexaenoic acid (DHA, 22:6 n-3), important second messengers in brain, are released from membrane phospholipid following receptor-mediated activation of specific PLA2 enzymes. We developed an in vivo method in rodents using quantitative autoradiography to image PUFA incorporation into brain from plasma, and showed that AA and DHA incorporation rates equal their rates of metabolic consumption by brain. We employ our imaging method in rodents to demonstrate signaling effects of mood stabilizers on brain AA/DHA incorporation during neurotransmission by muscarinic M(1,3,5), serotonergic 5-HT(2A/2C), dopaminergic D(2)-like (D(2), D(3), D(4)) or glutamatergic N-methyl-D-aspartic acid (NMDA) receptors, and effects of inhibition of acetylcholinesterase, of selective serotonin and dopamine reuptake transporter inhibitors, of neuroinflammation (HIV-1 and lipopolysaccharide) and excitotoxicity, and in genetically modified rodents. The method has been extended for the use with positron emission tomography (PET), and can be employed to determine how human brain AA/DHA signaling and consumption are influenced by diet, aging, disease and genetics. TRANSLATIONAL STUDIES ON REGULATION OF BRAIN DOCOSAHEXAENOIC ACID METABOLISM IN VIVO. One goal in the field of brain polyunsaturated fatty acid (PUFA) metabolism is to translate studies conducted in vitro and in animal models to the clinical setting. Doing so can elucidate the roles of PUFAs in the human brain, and effects of diet, drugs, disease and genetics on this role. In a review, we discussed new in vivo radiotracer kinetic and neuroimaging techniques that allow us to do this, with a focus on docosahexaenoic acid (DHA). We illustrated how brain PUFA metabolism is influenced by graded reductions in dietary n-3 PUFA content in unanesthetized rats, and how kinetic tracer techniques in rodents have helped to identify mechanisms of action of mood stabilizers used in bipolar disorder, how DHA participates in neurotransmission, and how brain DHA metabolism is regulated by calcium-independent iPLA(2)beta. In humans, regional rates of brain DHA metabolism can be quantitatively imaged with positron emission tomography following intravenous injection of 1-(11)CDHA. (2) D2-LIKE RECEPTOR ACTIVATION DOES NOT INITIATE A BRAIN DOCOSAHEXAENOIC ACID SIGNAL IN UNANESTHETIZED RATS. The polyunsaturated fatty acid, docosahexaenoic acid (DHA), participates in neurotransmission involving activation of calcium-independent iPLA2, which is coupled to muscarinic cholinergic and serotonergic neuroreceptors. Drug induced activation of iPLA2 can be measured in vivo with quantitative autoradiography using 14C-DHA as a probe. The present study used this approach to address whether a DHA signal is produced following D2-like receptor activation with the D2 agonist, quinpirole in rat brain. Unanesthetized rats were infused intravenously with 14C-DHA one minute after saline or quinpirole infusion, and serial blood samples were collected over a 20-minute period to obtain plasma. The animals were euthanized with sodium pentobarbital, their brains excised, coronally dissected and subjected to autoradiography. Plasma labeled and unlabeled unesterified DHA concentrations were measured. The brain incorporation coefficient, k*, for DHA did not differ significantly between quinpirole-treated and control rats in any of 81 identified brain regions. Conclusion: These findings demonstrate that D2-like receptor initiated signaling does not involve DHA as a second messenger, and likely does not involve iPLA2 activation. This paper is being prepared for submission IPLA2ΒKNOCKOUT MOUSE, A GENETIC MODEL FOR PROGRESSIVE HUMAN MOTOR DISORDERS, DEVELOPS AGE-RELATED NEUROPATHOLOGY. Calcium-independent phospholipase A2 group VIa (iPLA2β) preferentially releases docosahexaenoic acid (DHA) from the sn-2 position of phospholipids. Mutations of its gene, PLA2G6, are found in patients with several progressive motor disorders, including Parkinson disease. At 4 months, PLA2G6 knockout mice (iPLA2β(-/-)) show minimal neuropathology but altered brain DHA metabolism. By 1 year, they develop motor disturbances, cerebellar neuronal loss, and striatal α-synuclein accumulation. We hypothesized that older iPLA2β(-/-) mice also would exhibit inflammatory and other neuropathological changes. Real-time polymerase chain reaction and Western blotting were performed on whole brain homogenate from 15 to 20-month old male iPLA2β(-/-) or wild-type (WT) mice. These older iPLA2β(-/-) mice compared with WT showed molecular evidence of microglial (CD-11b, iNOS) and astrocytic (glial fibrillary acidic protein) activation, disturbed expression of enzymes involved in arachidonic acid metabolism, loss of neuroprotective brain derived neurotrophic factor, and accumulation of cytokine TNF-αmessenger ribonucleic acid, consistent with neuroinflammatory pathology. There was no evidence of synaptic loss, of reduced expression of dopamine active reuptake transporter, or of accumulation of the Parkinson disease markers Parkin or Pink1. iPLA2γexpression was unchanged. iPLA2βdeficient mice show evidence of neuroinflammation and associated neuropathology with motor dysfunction in later life. These pathological biomarkers could be used to assess efficacy of dietary intervention, antioxidants or other therapies on disease progression in this mouse model of progressive human motor diseases associated with a PLA2G6 mutation. (1)
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