Role of Prostanoid Receptors in the Excitatory Effect of Neuroprostanes on Potassium Induced [3H]D-Aspartate Release in Isolated Bovine Retina

Abstract

Purpose: : There is evidence that neuroprostanes (nPs), a series of isoprostane (IsoP)-like compounds that are spontaneously generated via free-radical catalyzed peroxidation of long chain polyunsaturated fatty acids are elevated in neurodegenerative conditions (Musiek et al., Brain Pathol. 15:149,2005). However, their effect on excitatory neurotransmitter release in neuronal tissues has not been fully elucidated. Purpose: In this study, we investigated the regulatory effect of the eicosapentanoic aid (EPA)-derived epimer pair, 5(S)-F_3t-IsoP (CO5-667) and 5-epi-5-F_3t-IsoP(CO5-668) and the docosahexaenoic acid (DHA)-derived nP, 4(S)-F4t-nP (CO5-738) on K⁺-induced glutamate release (using [³H]D-aspartate as a marker) in isolated bovine retina. We also examined the mechanism by which CO5-738 regulates this excitatory neurotransmitter release.

Methods: : Freshly isolated bovine retinae were incubated in oxygenated Krebs solution (pH 7.45; 37 ºC) containing 200nM of [³H] D-aspartate for 60 mins and then prepared for studies of neurotransmitter release using the well established superfusion method. Release of [³H]D-aspartate was evoked by iso-osmotic concentration of K⁺ (50mM)-stimuli applied at 80-88 mins (S₁) and 116-124 mins (S₂) after the onset of superfusion. When used, the antagonist was present before and during S₁ and S₂.

Results: : In the concentration range, 1 nM to 1 µM, the nPs enhanced K⁺-induced [³H]D-aspartate release from bovine retina without affecting basal [³H]D-aspartate efflux. Of the EPA-derived epimer-pair examined, the cis-conformer, C05-668 exhibited a higher potency, achieving a maximal excitatory response of 80% (p<0.01, n=4) at the 10 nM concentration of the nP. At an equimolar concentration of 10 nM, the rank order of activity of the nPs was as follows: CO5-668> CO5-738> CO5-667. Interestingly, the excitatory effect elicited by the DHA-derived nP, CO5-738 (0.1 µM) was completely reversed by the prostanoid TP/DP₂-receptor inhibitor, ramatroban (BAY-U3405) (10µM).

Conclusions: : In conclusion, the EPA- and DHA-metabolites enhance K⁺-induced [³H]D-aspartate release in bovine retina. Moreover, prostanoid TP/DP₂-receptors mediate the excitatory action exhibited by the DHA-metabolite, CO5-738 on the neurotransmitter release.

Keywords: excitatory neurotransmitters • retina • neurotransmitters/neurotransmitter systems