Effect of isoprostanes on sympathetic neurotransmission in the human isolated iris-ciliary body

Evaluation of antioxidative enzymes levels and lipid peroxidation products levels in diabetic and non diabetic senile cataract patients

Background

The current study aimed to investigate the Superoxide dismutase, Glutathione Peroxidase activity, Advanced oxidation Protein products levels, Malondialdehyde levels, Baseline Conjugated Diene concentration, and 8-Isoprostaglandin F2α (8-IPG-F2α) quantification in diabetic and non-diabetic senile cataract patients to find out the pathomechanism of early onset of cataract in diabetic patients.

Methods

This case-control study was performed on 184 subjects undergoing cataract surgery. For 8-IPG-F2α quantification, 35 diabetics and non-diabetic lenses and the entire study included 22 patients with diabetic senile cataracts and non-diabetic senile cataracts of aqueous humor age ≥ 40 years. Ninety-two patients with diabetic senile cataract and non-diabetic senile cataract lenses aged ≥ 40 years were incorporated for all other studies. The student's t-test was used for statistical analysis.

Results

From the study population, the mean age was 63.82 ± 0.6, and 75% of them were female. Higher female prevalence in both groups was observed. The results revealed that the superoxide dismutase and glutathione peroxidase activity were significantly reduced in diabetic patients compared to non-diabetic patients (p < 0.001) in both lenses and aqueous humor. Malondialdehyde, conjugated diene, and 8-IPG-F2α levels significantly increased in diabetic patients when compared to non-diabetic (p < 0.01), and advanced oxidation protein products levels also significantly increased in diabetic patients when compared to non-diabetics in both lenses and aqueous humor (p < 0.001).

Conclusion

The current study revealed that oxidative stress and Lipid per-oxidation have an imperative role in the diabetic-related complication, specifically in the lens, may have a responsibility in the pathomechanism of early onset of cataracts coupled with diabetes mellitus.

Human presynaptic receptors

Presynaptic receptors are sites at which transmitters, locally formed mediators or hormones inhibit or facilitate the release of a given transmitter from its axon terminals. The interest in the identification of presynaptic receptors has faded in recent years and it may therefore be justified to give an overview of their occurrence in the autonomic and central nervous system; this review will focus on presynaptic receptors in human tissues. Autoreceptors are presynaptic receptors at which a given transmitter restrains its further release, though in some instances may also increase its release. Inhibitory autoreceptors represent a typical example of a negative feedback; they are tonically activated by the respective endogenous transmitter and/or are constitutively active. Autoreceptors also play a role under pathophysiological conditions, e.g. by limiting the massive noradrenaline release occurring during congestive heart failure. They can be used for therapeutic purposes; e.g., the α₂-adrenoceptor antagonist mirtazapine is used as an antidepressant and the inverse histamine H₃ receptor agonist pitolisant has been marketed as a new drug for the treatment of narcolepsy in 2016. Heteroreceptors are presynaptic receptors at which transmitters from adjacent neurons, locally formed mediators (e.g. endocannabinoids) or hormones (e.g. adrenaline) can inhibit or facilitate transmitter release; they may be subject to an endogenous tone. The constipating effect of the sympathetic nervous system or of the antihypertensive drug clonidine is related to the activation of inhibitory α₂-adrenoceptors on postganglionic parasympathetic neurons. Part of the stimulating effect of adrenaline on the sympathetic nervous system during stress is related to its facilitatory effect on noradrenaline release via β₂-adrenoceptors.

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.

Regulation of [³H]d-aspartate release by the 5-F(2t)-isoprostane and its 5-epimer in isolated bovine retina

We have evidence that 15-F₂-isoprostanes (15-F₂-IsoPs) regulate excitatory neurotransmitter release in ocular tissues. Although 5-F₂-IsoPs are abundantly produced in mammals, their pharmacological actions on neurotransmitter release remain unknown. In the present study, we compared the effect of the 5-F₂-IsoP epimer pair, 5-F(2t)-IsoP (C5-OH in β-position) and 5-epi-5-F(2t)-IsoP (C5-OH in α-position), on K⁺-evoked [³H]D-aspartate release in isolated bovine retina. We further examined the role of prostanoid receptors on the inhibitory action of 5-epi-5-F(2t)-IsoP on [³H]D-aspartate overflow. Isolated bovine retina were prepared for studies of K⁺-evoked release of [³H]D-aspartate using the superfusion method. 5-epi-5-F(2t)-IsoP (0.01 nM to 1 μM), attenuated K⁺-evoked [³H]D-aspartate release in a concentration-dependent manner, with the inhibitory effect of 26.9% (P < 0.001; IC₂₅ = 0.2 μM) being achieved at 1 μM concentration. Its 5-(S)-OH-epimer, 5-F(2t)-IsoP (0.1 nM-1 μM), exhibited an inhibitory biphasic action, yielding a maximal response of 35.7% (P < 0.001) at 10 nM concentration of the drug (IC₂₅ value of 3 nM). Although the prostanoid-receptor antagonists, AH 6809 (10 μM; EP₁₋₃/DP) and BAY-u3405 (10 μM; DP/Tx) exhibited no effect on 5-epi-5-F(2t)-IsoP (10 nM-1 μM)-mediated inhibition, SC-19220 (1 μM; EP₁) completely reversed 5-epi-5-F(2t)-IsoP (0.1 μM and 1 μM)-induced attenuation of K⁺-evoked [³H]D-aspartate release. Similarly, both SC-51322 (10 μM; EP₁ and AH 23848 (1 μM; EP₄) reversed the inhibitory action elicited by 5-epi-5-F(2t)-IsoP (0.1 μM) on the neurotransmitter release. We conclude that the 5-F₂-IsoP epimer pair, 5-F(2t)-IsoP and 5-epi-5-F(2t)-IsoP, attenuate K⁺-induced [³H]D-aspartate release in isolated bovine retina presumably via prostanoid receptor dependent mechanisms. The trans-orientation of the allylic hydroxyl group at position C5 accounts for the apparent biphasic response exhibited by 5-F(2t)-IsoP on excitatory neurotransmitter release.

Role of prostanoid production and receptors in the regulation of retinal endogenous amino acid neurotransmitters by 8-isoprostaglandin E2, ex vivo

The role of enzymes and receptors of the prostanoid pathway in the inhibitory effect of 8-isoprostaglandin E2 (8-isoPGE2) on endogenous amino acid neurotransmitter levels was examined, ex vivo. Freshly isolated bovine eyeballs were injected intravitreally with IsoPs, incubated in Krebs buffer for 30 min and retina prepared for HPLC-ECD detection of amino acids. 8-isoPGE2 attenuated retinal glutamate and its metabolite, glutamine and glycine in a concentration-dependent manner. The nonselective cyclooxygenase (COX)-inhibitor, flurbiprofen, COX-2 selective inhibitor, NS-398 and thromboxane (Tx) synthase inhibitor, furegrelate had no effect on both basal amino acid levels and the inhibitory effects of 8-isoPGE2 (1-100 μM) on the retinal amino acids. Whereas the TP-receptor antagonist SQ-29548(10 μM) exhibited no effect, SC-19220(EP1; 30 μM), AH-6809(EP(1-3); 30 μM) and AH-23848(EP4; 30 μM) reversed the inhibitory effects of 8-isoPGE2 (0.01-100 μM) on glutamate, glutamine and glycine levels. We conclude that prostanoid EP-receptors regulate the inhibitory effect of 8-isoPGE2 on basal levels of endogenous amino acids in bovine retina, ex vivo.

The level of isoprostanes as a non-invasive marker for in vivo lipid peroxidation in secondary progressive multiple sclerosis

Oxidative stress leads to lipid peroxidation and may contribute to the pathogenesis of lesions in multiple sclerosis (MS), an autoimmune disease characterized by inflammatory as well as degenerative phenomena. Isoprostanes are prostaglandin-like compounds which are formed by free radical catalysed peroxidation of arachidonic acid esterified in membrane phospholipids. They are a new class of sensitive specific markers for in vivo lipid peroxidation. In this study 26 patients (15 females and 11 males; mean age 48.2 ± 15.2 year; mean disease duration 10.0 ± 6.5 year) with secondary progressive MS (SPMS) and 12 healthy controls were enrolled. In patients with multiple sclerosis the lipid peroxidation as the level of urine isoprostanes and the level of thiobarbituric acid reactive species (TBARS) in plasma were estimated. Moreover, we estimated the total antioxidative status (TAS) in plasma. It was found that the urine isoprostanes level was over 6-fold elevated in patients with SPMS than in control (P < 0.001). In SPMS patients TBARS level was also statistically higher than in controls (P < 0.01). However, we did not observed any difference of TAS level in serum between SPMS patients and controls (P > 0.05). In patients with SPMS the lipid peroxidation and oxidative stress measured as the increased level of isoprostanes was observed. Thus, we suggest that the level of isoprostanes may be used as non-invasive marker for a determination of oxidative stress what in turn, together with clinical symptoms, may determine an specific antioxidative therapy in SPMS patients.

Characterization of prostanoid receptors present on adrenergic neurons innervating the porcine uterine longitudinal muscle

The cyclooxygenase-prostanoid pathway regulates myometrial contractility through activation of prostanoid receptors on uterine smooth muscles. However, the possible expression of prostanoid receptors on autonomic nerves cannot be excluded completely. The aim of the present study was to clarify the presence of neural prostanoid receptors on adrenergic nerves in the porcine uterine longitudinal muscle. In [(3)H]-noradrenaline-loaded longitudinal muscle strips of porcine uterus, electrical field stimulation (EFS) evoked [(3)H]-noradrenaline release in a stimulation frequency-dependent manner. The EFS-evoked release was completely abolished in Ca(2+)-free (EGTA, 1mM) incubation medium and by tetrodotoxin or omega-conotoxin GVIA, suggesting that [(3)H]-noradrenaline was released from neural components. The EFS-evoked [(3)H]-noradrenaline release was significantly enhanced by treatment with indomethacin. In the presence of indomethacin, PGE(2) and PGF(2alpha), but not PGD(2), inhibited the EFS-evoked [(3)H]-noradrenaline release. Of synthetic prostanoid receptor agonists examined, both U46619 (TP) and sulprostone (EP(1)/EP(3)) decreased the EFS-evoked [(3)H]-noradrenaline release in a concentration-dependent manner, while fluprostenol (FP), BW245C (DP) and butaprost (EP(2)) were almost ineffective. SQ29548 (TP receptor antagonist) blocked the effect of U46619, but SC19220 (EP(1) receptor antagonist) did not change the inhibition by sulprostone or PGE(2). Double immunofluorescence staining using protein gene product 9.5, tyrosine hydroxylase, EP(3) receptor and TP receptor antibodies suggested the localization of EP(3) or TP receptors on adrenergic nerves in the porcine uterus. These results indicated that neural EP(3) and TP receptors are present on adrenergic nerves of the porcine uterine longitudinal muscle. Endogenous prostanoid produced by cyclooxygenase can regulate noradrenaline release in an inhibitory manner through activation of these neural prostanoid receptors.

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.

E-ring 8-isoprostanes inhibit ACh release from parasympathetic nerves innervating guinea-pig trachea through agonism of prostanoid receptors of the EP3-subtype

1. In the present study, we examined the effect of E-ring 8-isoprostanes on cholinergic neurotransmission in guinea-pig trachea and identified the receptor(s) involved. As isoprostanes are isomeric with prostaglandins, PGE(2) and sulprostone (a selective EP(3)-receptor agonist) were examined in parallel. 2. 8-Iso-PGE(1), 8-iso-PGE(2) (0.1 nm-1 microM), sulprostone (1 nm-1 microM) and PGE(2) (1 microM) suppressed EFS-evoked [(3)H]ACh release from guinea-pig trachea in a concentration-dependent manner, producing 39.5, 53.9, 61.2 and 59.9% inhibition, respectively, at 1 microM. It should be noted that an established maximum effective concentration was not determined. 3. Neither SQ 29,548 (1 microm; a TP-receptor antagonist) nor AH 6809 (10 microM; an EP(1)-/EP(2)-/DP-receptor antagonist) reversed the inhibitory effect of these compounds. 4. L-798,106, a novel and highly selective EP(3)-receptor antagonist, produced a parallel shift to the right of the concentration-response curves that described the inhibitory action of sulprostone on EFS-evoked contractile responses in guinea-pig vas deferens (an established EP(3)-receptor-expressing tissue), from which a mean pA(2) of 7.48 was derived. On guinea-pig trachea, L-798,106 also antagonised sulprostone-induced inhibition of EFS-induced twitch responses, with similar potency (mean pA(2)=7.82). 5. The inhibitory effects of 8-iso-PGE(1), 8-iso-PGE(2), sulprostone and PGE(2) on EFS-induced [(3)H]ACh release was blocked by L-798,106 at a concentration (10 microM) that binds only weakly to human recombinant EP(1)-, EP(2)- and EP(4)-receptor subtypes expressed in HEK 293 cells. 6. These data suggest that E-ring 8-isoprostanes, PGE(2) and sulprostone inhibit EFS-evoked [(3)H]ACh release from cholinergic nerves innervating guinea-pig trachea, by interacting with prejunctional prostanoid receptors of the EP(3)-subtype.

Effects of 8-iso-prostaglandin E2 and 8-iso-prostaglandin F2 alpha on the release of noradrenaline from the isolated rat stomach

In the present experiment, we examined the effect of 8-iso-prostaglandin E(2) and 8-iso-prostaglandin F(2 alpha) on the release of noradrenaline from the isolated rat stomach. The postganglionic sympathetic nerves were electrically stimulated twice at 1 Hz for 1 min and test reagents were added during the second stimulation. 8-Iso-prostaglandin E(2) (10(-8)-10(-6) M) and 8-iso-prostaglandin F(2 alpha) (10(-7)-10(-5) M) dose-dependently reduced the evoked noradrenaline release, and these inhibitory potencies were as follows: 8-iso-prostaglandin E(2)>8-iso-prostaglandin F(2 alpha). The inhibitory effect of 8-iso-prostaglandin F(2 alpha), but not 8-iso-prostaglandin E(2), was abolished by 10(-6) M SQ-29548 ([1S-[1 alpha,2 alpha(Z),3 alpha,4 alpha]]-7-[3-[[2-[(phenylamino)carbonyl]hydrazino] methyl]-7-oxabicyclo[2,2,1]hept-2-yl]-5-heptenoic acid) (a prostanoid TP receptor antagonist). On the other hand, the inhibitory effect of 8-iso-prostaglandin E(2) was abolished by 10(-5) M AH-6809 (6-isopropoxy-9-oxoxanthene-2-carboxylic acid) (a prostanoid EP receptor antagonist), which also attenuated the inhibitory effects of ONO-AE-248 (16S-9-deoxy-9 beta-chloro-15-deoxy-16-hydroxy-17,17-trimethylene 19, 20-didehydro prostaglandin F(2)) (a selective EP(3) receptor agonist) on the evoked release of noradrenaline. The inhibitory effect of 8-iso-prostaglandin F(2 alpha), but not 8-iso-prostaglandin E(2), was abolished by pertussis toxin. These results suggest that 8-iso-prostaglandin F(2 alpha) inhibits noradrenaline release through TP receptors, whereas 8-iso-prostaglandin E(2) seems to inhibit noradrenaline release through EP(3) receptors, located on the gastric sympathetic nerve terminals in rats.

Potentiation of sympathetic neurotransmission in bovine isolated irides by isoprostanes

Isoprostanes (IsoP) are formed by free radical catalyzed peroxidation of arachidonic acid independent of the cyclooxygenase enzyme. In the present study, we examined the effect of IsoP on norepinephrine (NE) release from the bovine isolated iris. Furthermore, we studied the role of IsoP's in hydrogen peroxide (H2O2)-induced enhancement of NE release from this tissue. Isolated bovine irides were prepared for studies of [3H]NE release using the superfusion method. Release of [3H]NE was induced via electrical field stimulation. Both 8-iso-prostaglandin E2 (E2-IsoP) and 8-iso-prostaglandin F2 alpha (F2-IsoP) produced a concentration-related enhancement of field-stimulated [3H]NE release from isolated bovine irides, an effect that was mimicked by the thromboxane (Tx) receptor agonist, U46619 and by H2O2. The Tx-receptor antagonist, SQ 29548 inhibited responses to E2-IsoP (10 microM) with an IC50 of 370 +/- 50 nM. SQ 29548 (10 microM) also blocked the enhancement of electrically-evoked [3H]NE release induced by U46619 (10 microM) but not that caused by H2O2 (300 microM). The Tx synthetase inhibitor, carboxyheptylimidazole (10 microM) prevented the stimulatory effect of E2-IsoP on evoked [3H]NE release without affecting responses induced by H2O2. We conclude that IsoP's can enhance sympathetic neurotransmission in the bovine isolated iris, an effect that can be blocked by a Tx-receptor antagonist. Furthermore, endogenously produced Tx's mediate the stimulatory effect of IsoP's on NE release. However, endogenously generated IsoP's or Tx's are not involved in H2O2-induced potentiation of sympathetic neurotransmission.