Extracellular-derived calcium does not initiate in vivo neurotransmission involving docosahexaenoic acid

Epolia Ramadan,Angelo O Rosa,Lisa Chang,Mei Chen,Stanley I Rapoport,Mireille Basselin

doi:10.1194/jlr.m006262

Epolia Ramadan, Angelo O Rosa + Show 4 more

Open Access

https://doi.org/10.1194/jlr.m006262

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Abstract

In vitro studies show that docosahexaenoic acid (DHA) can be released from membrane phospholipid by Ca(2+)-independent phospholipase A(2) (iPLA(2)), Ca(2+)-independent plasmalogen PLA(2) or secretory PLA(2 (sPLA2)), but not by Ca(2+)-dependent cytosolic PLA(2) (cPLA2), which selectively releases arachidonic acid (AA). Since glutamatergic NMDA (N-methyl-D-aspartate) receptor activation allows extracellular Ca(2+) into cells, we hypothesized that brain DHA signaling would not be altered in rats given NMDA, to the extent that in vivo signaling was mediated by Ca(2+)-independent mechanisms. Isotonic saline, a subconvulsive dose of NMDA (25 mg/kg), MK-801, or MK-801 followed by NMDA was administered i.p. to unanesthetized rats. Radiolabeled DHA or AA was infused intravenously and their brain incorporation coefficients k*, measures of signaling, were imaged with quantitative autoradiography. NMDA or MK-801 compared with saline did not alter k* for DHA in any of 81 brain regions examined, whereas NMDA produced widespread and significant increments in k* for AA. In conclusion, in vivo brain DHA but not AA signaling via NMDA receptors is independent of extracellular Ca(2+) and of cPLA(2). DHA signaling may be mediated by iPLA(2), plasmalogen PLA(2), or other enzymes insensitive to low concentrations of Ca(2+). Greater AA than DHA release during glutamate-induced excitotoxicity could cause brain cell damage.

Highlights

N-methyl-D-aspartic acid (NMDA) (N-methyl-D-aspartate) receptor activation allows extracellular Ca2+ into cells, we hypothesized that brain docosahexaenoic acid (DHA) signaling would not be altered in rats given NMDA, to the extent that in vivo signaling was mediated by Ca2+-independent mechanisms
Using our imaging method involving the intravenous infusion of [1-14C]arachidonic acid (AA) [24, 25], we reported that administration of a subconvulsive dose of NMDA (25 mg/kg i.p.) to unanesthetized rats increased AA signaling, measured as increased AA incorporation coefficients k*, in wide areas of brain [26,27,28]
We confirmed the report that this dose of NMDA significantly increased k* for AA in wide areas of the brain [26,27,28]

Summary

MATERIALS AND METHODS

Threemonth-old male Fischer F344 rats (Taconic Farms, Rockville, MD) were acclimated for 1 week in an animal facility having regulated temperature, humidity, and a light-dark cycle. The rats had ad libitum access to water and food (Rodent NIH-31 auto 18-4 diet, Zeigler Bros, Gardners, PA). The diet contained (as percent of total fatty acids) 20.0% saturated, 22.5% monounsaturated, 47.9% linoleic, 5.1% ␣-linolenic, 0.2% AA, 2.0% eicosapentaenoic, and 2.3% DHA acids [29]. Thin-layer chromatography was performed and radioactivity of each band was measured by scintillation counter. 98% of radioactivity was detected in the fatty acid band. Gas chromatography was performed to confirm the identity of the fatty acid [31]

Surgical procedures and tracer infusion

RESULTS

DISCUSSION