Regulation of Retinal Endogenous Excitatory Neurotransmitters by Isoprostanes: Role of EP-Receptors

Abstract

Purpose: : We investigated the effect of the isoprostane, 8-isoprostaglandin E₂ (8-isoPGE₂) on endogenous levels of excitatory amino acid neurotransmitter glutamate and its metabolite, glutamine in the bovine retina, ex vivo

Methods: : Freshly isolated bovine eyeballs were injected intravitreally with either vehicle (for control) or 8-isoPGE₂ (0.01µM -100µM) and retina and vitreous humors were isolated 30 min after treatment for measurement of amino acids using High Performance Liquid Chromatography

Results: : 8-isoPGE₂ caused a concentration-dependent reduction of retinal glutamate and glutamine levels. For instance, 8-isoPGE₂ (1 µM and 100 µM) significantly (p<0.001) attenuated retinal glutamate levels by 34% and 48%, respectively. Likewise, 8-isoPGE₂ (100 µM) inhibited retinal glutamine levels by 38% (p<0.01). On the other hand, 8-isoPGE₂ exhibited no effect on glutamine levels in the vitreous humor. Glutamate was not detected either before or after 8-isoPGE₂ treatment in vitreous humor. Pretreated bovine eyeballs with the enzyme inhibitors flurbiprofen (nonselective COX inhibitor), NS-398 (selective COX-2 inhibitor) or furegrelate (thromboxane synthase inhibitor) did not interfere with the inhibitory effects of 8-isoPGE₂ on retinal glutamate and glutamine concentrations. To determine the role of prostanoid receptors in the inhibitory effects of 8-isoPGE₂, eyeballs were pretreated with the prostanoid receptor antagonists SC-19220 (EP₁), AH-6809 (EP_1/2/3), AH-23848 (EP₄) or SQ-29548 (TP). SC-19220, AH-6809 and AH-23848 blocked the inhibitory effects of 8-isoPGE₂ on retinal glutamate and glutamine levels in a concentration-dependent manner. Interestingly, the TP antagonist, SQ-29548 did not prevent the inhibitory effects of 8-isoPGE_2. Used as a positive control, the EP receptor agonist, PGE₂ (10 µM) reduced retinal glutamate and glutamine levels by 48% (p<0.001) and 45% (p<0.001), respectively and its effects were reversed by both SC-19220 and AH-6809

Conclusions: : The isoprostane, 8-isoPGE₂ can decrease the levels of endogenous excitatory neurotransmitter, glutamate and its metabolite glutamine in bovine retina, ex vivo presumably via an effect on EP-receptors. This finding suggests a potential neuroprotective effect of 8-isoPGE₂ in mammalian retina.

Keywords: retina • oxidation/oxidative or free radical damage • neuroprotection