Imaging Brain Signal Transduction via Arachidonic and Docosahexaenoic Acids

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EXTRACELLULAR-DERIVED CALCIUM DOES NOT INITIATE NEUROTRANSMISSION VIA DOCOSAHEXAENOIC ACID. Docosahexaenoic acid (DHA, 22:6n-3), a nutritionally essential polyunsaturated fatty, is found in high concentrations in synaptic membrane phospholipids, but its role in neurotransmission in vivo is poorly understood. In vitro studies show that DHA is released from postsynaptic membrane phospholipid by calcium-independent phospholipase A2, iPLA2, but not by calcium-dependent cytosolic cPLA2, which selectively releases arachidonic acid (AA). Since glutamatergic N-methyl-D-aspartate (NMDA) receptor activation allows extracellular calcium into cells, we hypothesized that brain AA but not DHA signaling would be increased in rats injected with NMDA if iPLA2 also selectively modulated DHA signaling in vivo. We confirmed this hypothesis using quantitative autoradiography. Greater AA than DHA release during excess glutamate stimulation of NMDA receptors could contribute to excitotoxic brain damage (Ramadan et al., 2010). IMAGING DECREASED BRAIN DOCOSAHEXAENOIC ACID METABOLISM AND SIGNALING IN iPLA-DEFICIENT MICE. Calcium-independent phospholipase A2beta (iPLA2beta) selectively hydrolyzes docosahexaenoic acid (DHA, 22:6n-3) from membrane phospholipid. Mutations in the PLA2G6 gene encoding this enzyme occur in patients with neurological disorders including idiopathic neurodegeneration plus brain iron accumulation and dystonia-parkinsonism without iron accumulation. Mice lacking PLA2G6 show neurological dysfunction and neuropathology after 13 months, and we hypothesized that they would have disturbed DHA function. We confirmed this by showing with our quantitative imaging technique that brain DHA metabolism and cholinergic receptor mediated DHA-signaling were reduced in 4-month-old iPLA2beta-deficient mice, before overt neuropathology had developed. Thus, iPLA2beta is critical for maintaining normal brain DHA metabolism and signaling in the intact organism. Positron emission tomography (PET) would be expected to show disturbed brain DHA metabolism in patients with PLA2G6 mutations, and treatment with elevated dietary DHA may be helpful in such patients (Basselin et al., 2010). DOCOSAHEXAENOIC ACID (DHA) INCORPORATION INTO BRAIN FROM PLASMA IS AN IN VIVO BIOMARKER OF BRAIN DHA METABOLISM AND NEUROTRANSMISSION. Docosahexaenoic acid (DHA) is critical for maintaining normal brain structure and function, and is considered neuroprotective. Its brain concentration depends on dietary DHA content and hepatic conversion from its diet derived n-3 precursor, alpha-linolenic acid (alpha-LNA). Our in vivo method using quantitative autoradiography and intravenously injected radiolabeled DHA can be used to image net rate of incorporation into the brain of unesterified plasma DHA, which equals the rate of brain metabolic DHA consumption at rest and during activation. We also extended the method for use in humans with positron emission tomography (PET). Imaging in unanesthetized rats given acute N-methyl-d-aspartate (NMDA), regional DHA signaling is independent of extracellular calcium, and mediated by calcium-independent phospholipase A2 (iPLA2). Studies in mice in which iPLA2-VIA (beta) was knocked out confirmed that this enzyme is critical for baseline and cholinergic signaling involving DHA. Thus, quantitative imaging of DHA incorporation from plasma into brain, in animals or humans, can be used as an in vivo biomarker of brain DHA metabolism and neurotransmission (Rapoport et al., 2011).