Role of the Non-enzymatic Metabolite of Eicosapentaenoic Acid, 5-epi-5-F_3t-Isoprostane in the Regulation of [³H]d-Aspartate Release in Isolated Bovine Retina

We have evidence that F₂-isoprostanes (F₂-IsoPs) regulate the release of excitatory neurotransmitters in isolated bovine retina. Although 5-F₃-IsoPs are generated in mammals, in vivo, their pharmacological actions on neurotransmitter release remain unknown. In this study, we investigated the effect of 5-epi-5-F_3t-IsoP on K⁺-evoked [³H]d-aspartate release in isolated bovine retina using the superfusion method. Furthermore, we examined the role of arachidonic acid metabolites in the regulation of the neurotransmitter release by this novel IsoP. In the concentration range, 0.01 nM–0.1 µM, 5-epi-5-F_3t-IsoP inhibited K⁺-evoked [³H]d-aspartate release in a concentration-dependent manner, achieving a maximum inhibition of 46.9 % at 0.1 µM (IC₃₀ = 1 nM). The prostanoid receptor antagonists, AH 6809 (EP_1–3/DP; 10 µM), SC 51322 (EP₁; 10 µM) and SC 19220 (EP₁; 1 µM) partially reversed 5-epi-5-F_3t-IsoP-mediated inhibition of K⁺-induced [³H]d-aspartate release. Pretreatment of retinal tissues with the cyclooxygenase (COX) inhibitor, flurbiprofen (3 μM) unmasked a biphasic action of 5-epi-5-F_3t-IsoP that was inhibitory at lower (0.1–10 pM) and stimulatory at higher concentrations (≥0.1 nM). The prostanoid pathway antagonists, BAY-u3405 (10 μM; TP/DP-receptors), SQ 29548 (10 μM; TP-receptor) and ozagrel (10 μM; Tx-synthase inhibitor) abolished the stimulatory action of the 5-epi-5-F_3t-IsoP (0.1 μM) on neurotransmitter release. In conclusion, 5-epi-5-F_3t-IsoP attenuates K⁺-induced [³H]d-aspartate release in a concentration-dependent manner by mechanisms that are partially dependent on activation of pre-junctional prostanoid EP₁-receptors. Moreover, blockade of the COX-pathway unmasks a biphasic action for 5-epi-5-F_3t-IsoP that is inhibitory at low concentrations and stimulatory at higher concentrations. Products of the thromboxane synthase pathway may partially account for the stimulatory action of this F₃-IsoP on isolated bovine retina.