Effect of inhibition of cyclooxygenase on pre- and postjunctional actions of peroxides in the iris-ciliary body

Lipid peroxidation: pathophysiological and pharmacological implications in the eye

Oxygen-derived free radicals such as hydroxyl and hydroperoxyl species have been shown to oxidize phospholipids and other membrane lipid components leading to lipid peroxidation. In the eye, lipid peroxidation has been reported to play an important role in degenerative ocular diseases (age-related macular degeneration, cataract, glaucoma, diabetic retinopathy). Indeed, ocular tissues are prone to damage from reactive oxygen species due to stress from constant exposure of the eye to sunlight, atmospheric oxygen and environmental chemicals. Furthermore, free radical catalyzed peroxidation of long chain polyunsaturated acids (LCPUFAs) such as arachidonic acid and docosahexaenoic acid leads to generation of LCPUFA metabolites including isoprostanes and neuroprostanes that may further exert pharmacological/toxicological actions in ocular tissues. Evidence from literature supports the presence of endogenous defense mechanisms against reactive oxygen species in the eye, thereby presenting new avenues for the prevention and treatment of ocular degeneration. Hydrogen peroxide (H2O2) and synthetic peroxides can exert pharmacological and toxicological effects on tissues of the anterior uvea of several mammalian species. There is evidence suggesting that the retina, especially retinal ganglion cells can exhibit unique characteristics of antioxidant defense mechanisms. In the posterior segment of the eye, H2O2 and synthetic peroxides produce an inhibitory action on glutamate release (using [(3)H]-D-aspartate as a marker), in vitro and on the endogenous glutamate and glycine concentrations in vivo. In addition to peroxides, isoprostanes can elicit both excitatory and inhibitory effects on norepinephrine (NE) release from sympathetic nerves in isolated mammalian iris ciliary bodies. Whereas isoprostanes attenuate dopamine release from mammalian neural retina, in vitro, these novel arachidonic acid metabolites exhibit a biphasic regulatory effect on glutamate release from retina and can regulate amino acid neurotransmitter metabolism without inducing cell death in the retina. Furthermore, there appears to be an inhibitory role for neuroprostanes in the release of excitatory amino acid neurotransmitters in mammalian retina. The ability of peroxides and metabolites of LCPUFA to alter the integrity of neurotransmitter pools provides new potential target sites and pathways for the treatment of degenerative ocular diseases.

Prejunctional inhibitory effects of isoprostanes on dopaminergic neurotransmission in bovine retinae, in vitro

We investigated the effect of isoprostanes (IsoPs) on potassium (K+)-depolarization-evoked release of [3H]dopamine from isolated bovine retinae. Isolated retinae were preloaded with [3H]dopamine and then prepared for studies of [3H]dopamine release using the superfusion method. 8-iso(15R)PGF 2alpha, 8-isoPGE2, 8-isoPGE1 and 8-isoPGF 2alpha attenuated [3H]dopamine release from isolated bovine retinae. At a concentration of 1 microM, the rank order of activity displayed by IsoP agonists was: 8-iso(15R)PGF 2alpha > 8-isoPGE2 > 8-isoPGE1 > 8-isoPGF 2alpha. Inhibition of cyclooxygenase (COX) with flurbiprofen reversed the effects caused by 8-isoPGE2 (10 nM and 10 microM), 8-iso(15R)PGF 2alpha (1 microM) and 8-isoPGE1 (1 microM). Although the EP1/EP2 antagonist, AH 6809 (10 microM) had no significant effect on K+-induced [3H]dopamine release, it blocked the inhibitory effect of both 8-isoPGE1 (10 microM) and 8-isoPGE2 (10 microM). In conclusion, IsoPs attenuate K+-induced [3H]dopamine release in isolated bovine retinae, presumably via an indirect action on COX pathway leading to the production of prostanoids, which in turn, activates EP receptors.

Pharmacological consequences of oxidative stress in ocular tissues

The eye is a unique organ because of its constant exposure to radiation, atmospheric oxygen, environmental chemicals and physical abrasion. That oxidative stress mechanisms in ocular tissues have been hypothesized to play a role in diseases such as glaucoma, cataract, uveitis, retrolental fibroplasias, age-related macular degeneration and various forms of retinopathy provides an opportunity for new approaches to their prevention and treatment, In the anterior uvea, both H2O2 and synthetic peroxides exert pharmacological/toxicological actions tissues of the anterior uvea especially on the sympathetic nerves and smooth muscles of the iris-ciliary bodies of several mammalian species. Effects produced by peroxides require the presence of trace amounts of extracellular calcium and the functional integrity of mitochondrial calcium stores. Arachidonic acid metabolites appear to be involved in both the excitatory action of peroxides on sympathetic neurotransmission and their inhibitory effect on contractility of the iris smooth muscle to muscarinic receptor activation. In addition to the peroxides, isoprostanes (products of free radical catalyzed peroxidation of arachidonic acid independent of the cyclo-oxygenase enzyme) can also alter sympathetic neurotransmission in anterior uveal tissues. In the retina, both H2O2 and synthetic peroxides produced an inhibitory action on potassium depolarization induced release of [3H] D-aspartate, in vitro and on the endogenous glutamate and glycine concentrations in vivo. Effects caused by peroxides in the retina are mediated, at least in part, by second messengers such as nitric oxide, prostaglandins and isoprostanes. The ability of H2O2 to alter the integrity of neurotransmitter pools from sympathetic nerves in the anterior uvea and glutaminergic nerves in the retina could underlie its role in the etiology of glaucoma.

Dual Effect of Isoprostanes on the Release of [3H]D-Aspartate from Isolated Bovine Retinae: Role of Arachidonic Acid Metabolites

The effect of 8-isoprostanes on potassium (K+)-depolarization-evoked release of [3H]D-aspartate from bovine isolated retinae was investigated. Isolated bovine retinae were prepared for studies of K(+)-evoked release of [3H]D-aspartate using the Superfusion Method. Low concentrations of 8-isoPGF(2alpha) (1-100 nM) inhibited whereas higher concentrations of this 8-isoprostane (100 nM-30 microM) enhanced K(+)-induced [3H]D-aspartate overflow. The excitatory effect of 8-isoPGF(2alpha) was mimicked by thromboxane receptor agonist, U-46619 and blocked by thromboxane receptor antagonist, SQ 29,548 (10 microM). Pretreatment of tissues with the cyclooxygenase (COX) inhibitor, flurbiprofen unmasked an inhibitory effect of high concentrations of 8-isoPGF(2alpha) (1-30 microM) on [3H]D-aspartate release that was attenuated by AH 6809 (10 microM). In conclusion, 8-isoPGF(2alpha) exhibits a dual regulatory effect on K(+)-induced [3H]D-aspartate release in isolated bovine retinae. The inhibitory action caused by 8-isoPGF(2alpha) is due to the activation of EP1/EP2 receptors while the excitatory effects are due to the activation of thromboxane receptors.

Non-synaptic release of [3H]noradrenaline in response to oxidative stress combined with mitochondrial dysfunction in rat hippocampal slices

Brain ischemia is frequently associated with oxidative stress in the reperfusion period. It is known that noradrenaline (NA) is released in excess under energy deprivation by the sodium-dependent reversal of the monoamine carrier. However, it is not known how oxidative stress affects NA release in the brain alone or in combination with energy deprivation. As a model of oxidative stress, the effect of H(2)O(2) (0.1-1.5 mM) perfusion was investigated in superfused rat hippocampal slices. It elicited a dose-dependent elevation of the release of [(3)H]NA and its tritiated metabolites as well as a simultaneous drop in the tissue energy charge. Mitochondrial inhibitors, i.e. rotenone (10 microM), and oligomycin (10 microM) in combination, also decreased the energy charge, but they had only a mild effect on [(3)H]NA release. However, when H(2)O(2) was added together with oligomycin and rotenone their effect on [(3)H]NA release was greatly exacerbated. H(2)O(2) and mitochondrial inhibitors also induced an increase in [Na(+)](i) in isolated nerve terminals, and their effect was additive. The effect of H(2)O(2) on tritium release was temperature-dependent. It was also attenuated by the glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (30 microM) and (+/-)-2-amino-5-phosphonopentanoic acid (10 microM), by the nitric oxide synthase inhibitors, N omega-nitro-L-arginine methyl ester (100 microM), or 7-nitroindazole (50 microM) and by the vesicular uptake inhibitor tetrabenazine (1 microM). Our results suggest that oxidative stress releases glutamate followed by activation of postsynaptic ionotropic glutamate receptors that trigger nitric oxide production and results in a flood of NA from cytoplasmic stores. The massive elevation of extracellular NA under conditions of oxidative stress combined with mitochondrial dysfunction may provide an additional source of highly reactive free radicals thus initiating a self-amplifying cycle leading to neuronal degeneration.