How Does DHA Reach the Brain to Provide Neuroprotection after Hypoxic‐Ischemic Injury?

Abstract

BackgroundPrevious papers have reported that intravenously injected radiolabeled docosahexaenoic acid (DHA) rapidly enters rat brain. Recent studies from our laboratory have shown that acute intraperitoneal (i.p) injection of triglyceride (TG) emulsions rich in n‐3 fatty acids, especially tri‐DHA provides neuroprotection after hypoxic‐ischemic (HI) injury in neonatal mice (PLoS One. 2016;11(8):e0160870).AIMTo determine how injected tri‐DHA released DHA and perhaps other DHA derivatives are delivered to brain to provide neuroprotection after HI.MethodologyLaboratory made tri‐DHA emulsions were labeled with non‐metabolizable [3H]‐cholesteryl hexadecyl ether. Neonatal naïve or HI mice (C57BL/6J) were injected with radiolabeled tri‐DHA emulsion intraperitoneally (0.375 mg TG/g bw). At 4 and 24 hrs after injection, blood, peritoneal fluid, lung, liver, heart, brain, kidney, muscle, bone, spleen and adipose tissue were harvested and levels of radiolabeled emulsion in each organ determined. Tissue uptake was expressed as percent of recovered injected dose. HI was produced by right carotid artery ligation followed by hypoxia (8% O2 at 37°C) for 15 mins.ResultsNo significant differences were observed between naïve or HI mice in uptake of tri‐DHA emulsion particles at 4 hrs after injection. Among naïve and HI mice, the percent of recovered dose of radioactivity in blood and intraperitoneal fluid at 4 hrs were found to be: blood‐ 7.7 ± 5.5% naïve vs 7.4 ± 0.81% HI; intraperitoneal fluid‐ 12.6 ± 8.8% naïve vs 17.8 ± 7.6% HI. Among all organs assayed, liver contained most of the recovered dose of radiolabeled emulsion (40.5 ± 8.9% naïve vs 42.7 ± 0.1% HI). Lowest emulsion particle uptake was found to be in the brain among naïve and HI mice (<0.1% of the recovered dose among both naïve and HI groups). Compared to 4 hrs, no significant changes were observed in the uptake of emulsion particles at 24 hrs after the injection.ConclusionsThe present data demonstrates that, neonatal mice efficiently clear radiolabeled emulsion from intraperitoneal fluid after i.p. injection. Uptake of emulsion particles was high in liver and very low in brain indicating that whole emulsion particle uptake by brain does not mediate neuroprotection after HI. This suggests the involvement of other pathways in DHA mediated neuroprotection in HI brain. We also hypothesize that local neuroprotection provided by DHA in HI brain is mediated, in part through DHA derivatives produced locally in brain and in other tissues.