Sustained-Release Delivery System of a Slow Hydrogen Sulfide Donor, GYY 4137, for Potential Application in Glaucoma.

Hydrogen sulfide (H2S) targets both underlying factors in glaucoma pathogenesis by reducing elevated intraocular pressure (IOP) and providing retinal neuroprotection, whereas the current clinical approaches targets only reducing IOP. Therefore, H2S could be a potential superior candidate for glaucoma pharmacotherapy. However, H2S could be toxic in a concentration greater than 200 µM and its donors are unstable in water. Therefore, this study investigated the preparation and characterization of a non-aqueous in situ gelling sustained-release delivery system for H2S donors. The delivery system was prepared by dissolving GYY 4137, a H2S donor, in poly lactide-co-glycolide polymer (PLGA) (Resomer® RG 502H) solution prepared by dissolving polymer in a mixture of benzyl alcohol and benzyl benzoate in a ratio of 7:3, respectively. The GYY 4137 formulation was characterized for syringeability/injectability, change in pH and tonicity, moisture content, GYY 4137 degradation, and toxicity using rheometer, pH and osmometer, Karl Fisher titrimeter, NMR spectrometer, and Y79 retinoblastoma cells, respectively. The formulation was easily syringeable and injectable as evidenced by rheological data (plastic flow pattern with 43.89 ± 3.21 cP viscosity and 1.12 ± 0.15 Pa yield value). The pH, tonicity, and moisture content values were within acceptable range. NMR spectroscopy indicated presence of 4-methoxyphenylphosphonic acid (GYY 4137 degradation product). The GYY 4137 formulation did not show any significant (p < 0.05) toxicity except the solvent mixture. A sustained release of H2S was observed up to 72 h. The in situ gel forming PLGA-based system can be manipulated to achieve sustained release of H2S from its donor GYY 4137.

Ocular pharmacology of bicyclic hexahydroaporphines.

This paper explores the ocular hypotensive actions of bicyclic analogs of hexahydroaporphine (HHA), specifically nor-HHA, in an attempt to shed light on the mechanism(s) by which they lower intraocular pressure (IOP). Studies involving the measurement of IOP and aqueous humor production were conducted in ocular normotensive albino rabbits, while those involving smooth muscle contractility utilized isolated bovine iris. The ability of nor-HHA to produce a sustained drop in IOP is linked to both a functioning adrenergic nervous system and the availability of the products of cyclooxygenase metabolism. Although aqueous flow is not impacted by the bicyclic structures, the significant enhancement of outflow facility points to a probable mechanism of IOP-lowering action. Nor-HHA had no direct contractile or relaxant action on bovine irides, but does cause a concentration-dependent inhibition of carbachol-evoked contractions. This inhibition was reversed by inhibitors of phospholipase A(2) and cyclooxygenase, but not by inhibitors of lipoxygenase, again indicating a role for prostaglandins in the ocular pharmacological action of bicyclic HHAs. Pretreatment with a nitric oxide (NO) scavenger also reversed the ability of nor-HHA to inhibit carbachol-induced contractions, implying a role for NO in the postjunctional actions of HHAs.

Regulation of uveal sympathetic neurotransmission by peroxides.

PURPOSE: To investigate the effect of naturally occurring and synthetic peroxides on norepinephrine release from isolated iris-ciliary bodies of several mammalian species. METHODS: Hemiirides (bovine) and iris-ciliary bodies (human, rabbit, and rat) were incubated in Krebs solution containing [3H]-norepinephrine ([3H]NE) for 60 minutes. After incubation, tissues were set up for studies of [3H]NE release using the superfusion method. Release of [3H]NE was elicited through electrical field stimulation. RESULTS: In bovine irides, hydrogen peroxide (H2O2), cumene hydroperoxide (cuOOH), and tert-butyl hydroperoxide (buOOH) caused a concentration-dependent potentiation of field-stimulated [3H]NE release with the following rank order of potency: cuOOH > H2O2 > buOOH. Furthermore, the free radical scavenger, melatonin (2 mM), prevented the enhancement of evoked [3H]NE overflow elicited by H2O2 and cuOOH. At equimolar concentrations, H2O2 (1 mM) increased stimulated [3H]NE release from rabbit, human (mean age, 29.7; range, 15 to 48 years), and Fischer 344 rat (4 months old) iris-ciliary bodies by 98%, 50%, and 40%, respectively. However, H2O2 (1 mM) caused a 9% increase in evoked [3H]NE release in tissue from aged Fischer 344 rats (30 months old) and a 5% decrease in neurotransmitter release in tissue from old human donors (mean age, 72.3 years; range, 69 to 74 years). CONCLUSIONS: Peroxides such as H2O2 can potentiate sympathetic neurotransmission in the anterior uvea of several mammalian species. In bovine irides, H2O2-induced enhancement of neurotransmitter release can be mimicked by synthetic peroxides and may involve the generation of reactive oxygen species.

Mechanism of peroxide-induced potentiation of sympathetic neurotransmission in bovine irides: role of extracellular calcium.

Hydrogen peroxide (H2O2) and enzymes that regulate its metabolism are present in tissues of the anterior segment of the eye. We have previously shown that in vitro, H2O2 can enhance sympathetic neurotransmission in irides from several mammalian species. In the present study, we investigated the role of extracellular calcium in H2O2-induced potentiation of sympathetic neurotransmission in the bovine isolated iris. Isolated bovine hemiirides were incubated in a bicarbonate-buffered, carbogen-gassed Krebs buffer solution containing [3H]-norepinephrine ([3H]NE) for 60 min. After incubation, tissues were prepared for studies of [3H]NE release using the superfusion method. Release of [3H]NE was elicited by consecutive trains of electrical field stimulation. Removal of calcium from the buffer solution attenuated field-stimulated [3H]NE overflow in isolated, superfused bovine irides without affecting basal tritium efflux. H2O2 (1 mM) enhanced evoked [3H]NE release to the same extent in tissues exposed to buffer solutions containing normal calcium (1.3 mM) as in those containing low calcium (0.13 mM) or zero calcium. However, in the presence of zero-calcium buffer solution containing the chelator, EDTA (1 mM), H2O2 (1 mM) caused a gradual and sustained increase in basal tritium efflux. In buffer solutions containing high calcium (1.95 mM), the magnitude of H2O2-induced increase in field-stimulated [3H]NE release was significantly (P < 0.05) attenuated. Although the neuronal calcium channel antagonist omega-conotoxin (20 nM) inhibited [3H]NE by 25%, it had no effect on H2O2 (1 mM)-induced potentiation of evoked [3H]NE overflow. We conclude that while trace amounts of extracellular calcium are necessary for H2O2-induced enhancement of sympathetic neurotransmission, increasing extracellular (buffer) calcium concentration impaired peroxide-induced enhancement of [3H]NE release. Furthermore, voltage-activated calcium channels may not be directly involved in peroxide-induced alteration of adrenergic neurosecretion in bovine isolated irides.

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.

Hypoxia-induced [(3)H]D-aspartate release from isolated bovine retina: modulation by calcium-channel blockers and glutamatergic agonists and antagonists.

PURPOSE: The aim of the present study was two-fold: (a) to examine the effect of hypoxia on [(3)H]D-aspartate release from isolated bovine and human retinae, and (b) to investigate the regulation of hypoxia-induced neurotransmitter release by glutamate receptor agonists and antagonists. METHODS: Isolated neural retinae were incubated in oxygenated Krebs buffer solution containing [(3)H]D-aspartate and then prepared for studies of neurotransmitter release using the superfusion method. Release of [(3)H]D-aspartate was evoked by K(+) (50 mM) applied at 90 minutes (S(1)) and hypoxia (induced by exposure of tissues to solutions pregassed with 95%N(2): 5% CO(2) for 60 minutes) at 108 minutes (S(2)) after onset of superfusion. RESULTS: Under hypoxic conditions, pO(2) in normal Krebs buffer solution was reduced from 14.53 +/- 0.26 ppm (n = 6) to 0.54 +/- 0.04 ppm (n = 9) after one hour of gassing with 95% N(2): 5% CO( 2). Exposure to hypoxia elicited an overflow of [(3)H]D-aspartate yielding S(2)/S(1) ratios of 0.62 +/- 0.06 (n = 12) and 0.54 +/- 0.03 (n = 8) in bovine and human tissues respectively. In isolated bovine retinae, L- and N-calcium-channel antagonists diltiazem, nitrendipine, verapamil and omega-conotoxin significantly (p < 0.01 or higher) attenuated hypoxia-induced [(3)H]D-aspartate release. L-glutamate (30 microM) significantly (p < 0.001) potentiated hypoxia-induced [(3)H]D-aspartate release whereas kainate (30 microM) inhibited this response. NMDA (in concentrations up to 1 mM) had no effect on hypoxia-induced [(3)H]D-aspartate release. Antagonists of glutamate receptors and the polyamine site on the NMDA receptor inhibited hypoxia-induced release of [(3)H]D-aspartate in bovine retina with the following rank order of activity: ifenprodil congruent with MCPG > L-AP3 > MK-801. At an equimolar concentration (10 microM), L-AP3 but not ifenprodil, MCPG, MK 801 or arcaine, caused a significant (p < 0.001) inhibition of hypoxia-induced [(3)H]D-aspartate release from human retinae. CONCLUSIONS: Hypoxia can induce the release of [( 3)H]D-aspartate from isolated bovine retinae by a calcium-dependent process. Hypoxia-induced [(3)H]D-aspartate release from isolated bovine retinae can be regulated by glutamate receptor agonists/antagonists and blockers of polyamine site on the NMDA receptor.

Role of cyclic AMP in hydrogen peroxide-induced potentiation of sympathetic neurotransmission in the bovine iris.

Hydrogen peroxide (H2O2) has been shown to enhance electrically-evoked norepinephrine (NE) release from isolated, superfused bovine irides. Since stimulation of presynaptic adenylyl cyclase can potentiate sympathetic neurotransmission in several tissues, the present study considered the possibility that cyclic AMP may mediate the effects of H2O2 in the iris. Isolated bovine irides were prepared for analysis of field stimulation-induced [3H]NE release using the superfusion method. Both the diterpene activator of adenylyl cyclase, forskolin and the cyclic AMP-specific phosphodiesterase inhibitor, RO-201724 enhanced evoked [3H]NE overflow by 32%. On the other hand, inhibition of cyclic AMP-dependent protein kinase I/II by Rp-cAMPS attenuated field-stimulated [3H]NE release by 20%. Interestingly, both RO-201724 and Rp-cAMPS did not alter the enhancement of electrically-evoked [3H]NE overflow caused by submaximal concentrations of H2O2. We conclude that cyclic AMP may be involved in the pathway leading to NE release from sympathetic nerves in the bovine isolated iris. However, cyclic AMP may not be a mediator of H2O2-induced potentiation of sympathetic neurotransmission in this tissue.

Role of cyclic AMP in prostaglandin mediated responses in the neural retina.

Exogenous prostaglandins (PGs) have been shown to inhibit dopamine (DA) release from the rabbit retina via an effect on presynaptic EP3-receptors. In the present study, we investigated the possible involvement of cyclic AMP in DA release and in the prostanoid receptor mediated regulation of DA release from the neural retina. Both forskolin and 8-bromo-cyclic AMP enhanced field stimulation-evoked [3H]DA release from isolated, superfused rabbit retinas without affecting basal tracer efflux suggesting that presynaptic cyclic AMP may be involved in the pathway leading to DA release. Forskolin attenuated inhibition of evoked [3H]DA release caused by low but not high concentrations of PGE2. Both PGE2 and sulprostone had no significant effect on basal cyclic AMP levels but inhibited forskolin-stimulated cyclic AMP formation. Furthermore, sulprostone was more potent than PGE2 in attenuating forskolin-activated cyclic AMP production. The inhibition of forskolin-elevated cyclic AMP levels caused by PGE2 was, however, unaffected by the EP1-receptor antagonist, AH6809. We conclude that the regulation of DA release by presynaptic prostanoid EP3-receptors may be mediated, at least in part, through an inhibitory effect on adenylyl cyclase.

Effect of prostaglandins on cyclic AMP production in cultured human ciliary muscle cells.

Prostaglandins (PGs) lower intraocular pressure by increasing uveoscleral outflow, presumably via a receptor-mediated mechanism coupled to a second messenger pathway in the ciliary muscle. In the present study, we examined the effect of prostanoids on cyclic AMP production in cultured human ciliary muscle cells. Cells were identified based on their expression of smooth muscle specific alpha-actin and monoclonal antibody against desmin. Cyclic AMP production in confluent cells incubated with buffer solution containing various concentrations of prostanoids was analyzed by radioimmunoassay. PGE2 caused a time-dependent increase in cyclic AMP concentrations which reached a maximum after 10 mins. With the exception of PGD2, all prostanoids produced a concentration-dependent increase in cyclic AMP levels with the following rank order of activity: PGE2 > 11-deoxy-PGE1 > 16,16-dimethyl PGE2 > sulprostone > PGF2alpha. PGE2-induced increase on cyclic AMP levels was unaffected by AH6809, an antagonist at both PGD2 (DP) and E2 (EP1) receptors. Flurbiprofen decreased basal cyclic AMP concentrations suggesting that intramurally-generated PGs stimulate the formation of the nucleotide in ciliary smooth muscle cells. PGE2-induced increases in cyclic AMP production was synergistic with those induced by the diterpene activator of adenylyl cyclase, forskolin. We conclude that prostanoids active at EP2-receptors can stimulate cyclic AMP production in cultured human ciliary muscle cells.

Assessment of the intraocular pressure-lowering activity of bicyclic derivatives of 1-substituted benzyloctahydroisoquinoline.

In our study of IOP-lowering agents, we have synthesized several bicyclic analogs of 1-benzyloctahydroisoquinoline. The target molecules were synthesized in an eleven-step process. Structures were proved through spectrometry, elemental analysis and, in selected cases, high resolution mass spectrometry. The final products were secondary or tertiary amines containing a 1-benzyl moiety substituted at the p-position with a methoxy, methyl or chloro group. All target molecules were analyzed in 1% solution in distilled water in normotensive rabbits. After topical administration, IOP was monitored in both eyes for up to seven hours. The 1-p-methoxybenzyl molecule 2 was the most active, and caused a maximal IOP drop of 8.8 +/- 1.9 (n = 7) mm Hg in the ipsilateral eye at 4 hours post-administration, with only partial recovery at seven hours. All other compounds tested either showed very weak activity (3-6) or were inactive (1). All compounds produced a contralateral effect, and 5 induced rebound ocular hypertension. We conclude that selected tertiary bicyclic 1-p-methoxybenzyl-octahydroisoquinolines, particularly N-methylated structures, exhibit a significant IOP-lowering effect in normotensive rabbits.

Role of catalase in pre- and postjunctional responses of mammalian irides to hydrogen peroxide.

In the present study, we examined the effect of inhibition of catalase with 3-aminotriazole (3-AT) on hydrogen peroxide (H2O2)-induced enhancement of sympathetic neurotransmission in bovine irides and on the inhibitory effect of this oxidant on norepinephrine (NE) release from human irides, in vitro. Furthermore, we investigated the effect of 3-AT on H2O2-induced attenuation of contractile responses to carbachol in the bovine isolated irides. Isolated mammalian irides were prepared for studies of [3H]NE release using the superfusion method and for contractile studies using isolated organ baths. At concentrations less than 100 microM, H2O2 had no significant effect on field-stimulated [3H]NE release from bovine or human irides. In bovine irides, 3-AT caused significant (P < 0.001) leftward shifts of concentration-response curves to H2O2 (10-300 microM). 3-AT also increased H2O2-induced attenuation of evoked [3H]NE release from human isolated irides. Low concentrations of H2O2 (< 100 microM) had no effect on carbachol contractions. However, 3-AT unmasked an inhibitory effect of low concentrations of H2O2 (3-100 microM) on carbachol-induced contractions. We conclude that inhibition of catalase causes both pre- and postjunctional responses of isolated mammalian irides to be more susceptible to oxidative stress induced by H2O2.

Bicyclic derivatives of hexahydroaporphine: novel agents that lower intraocular pressure.

We have synthesized a number of novel bicyclic hexahydroaporphines containing a phenethylamine moiety for preliminary study as intraocular pressure lowering agents. The target molecules were synthesized in a twelve step process. The final products were secondary or tertiary amines containing either an aromatic methoxy or phenolic substituent. These molecules, in hydrochloride salt form, were assayed in doses ranging from 0.1-1.5%. All products and vehicle were administered topically to one eye of normotensive rabbits and intraocular pressure was measured in both eyes for up to six hours. Four of the five compounds examined produced a significant and, in some cases, prolonged, ocular hypotensive response. Secondary and N-methylated tertiary amines were equally effective, as were compounds containing either the 10-methoxy group of free phenol. Studies are currently in progress to optimize potency and identify functional and molecular mechanisms of action.

Effect of hydrogen peroxide on amino acid concentrations in bovine retina and vitreous humor, ex vivo.

In a previous study, we showed evidence that oxidative stress induced by hydrogen peroxide (H2O2) can inhibit the release of [3H]-D-aspartate from the bovine isolated retina, in vitro. The aim of the present study was to investigate the effect of H2O2 on glutamate and glycine levels in the bovine retina and vitreous humor, ex vivo. Furthermore, we examined whether inhibition of catalase activity with 3-amino-triazole had any effect on the concentrations of these amino acids in the posterior segment of the bovine eye. Whole eye organ cultures were prepared by incubating tissues in oxygenated Krebs solution at 37 masculine C for 30 min. After incubation, H2O2 (1-100 microM) or sterile distilled water was injected intravitreally into each eye. Thirty minutes after injection, the retina and vitreous humor were removed for analysis of glutamate and glycine by high performance liquid chromatography (HPLC) with fluorescence detection. Exogenously applied H2O2 (1-100 microM) caused a concentration-related decrease in both glutamate and glycine levels in the bovine retina. Furthermore, while H2O2 (1-10 microM) caused a concentration-dependent decrease in glycine levels in the vitreous humor, it had no significant effect on glutamate levels. The catalase inhibitor, 3-amino-triazole (10 mM), caused a significant reduction in both glutamate and glycine levels in the bovine retina, ex vivo. Likewise, 3-AT caused an attenuation in both glutamate and glycine concentration in the vitreous humor. We conclude that oxidative stress induced by H2O2 can alter the release and/or availability of amino acids in the posterior segment of bovine eyes.

Role of intracellular calcium in peroxide-induced potentiation of norepinephrine release from bovine isolated irides.

1. BAPTA AM (10 microM), thapsigargin (10 microM), ruthenium red (30 microM) and oligomycin (30 microM) inhibited field-stimulated [3H]NE release from bovine isolated irides by 54%, 30%, 30% and 26%, respectively. 2. Both BAPTA AM and thapsigargin had no significant effect (P>0.05) on H2O2-induced potentiation of evoked [3H]NE release. 3. Ruthenium red prevented (but oligomycin enhanced) H2O2-induced enhancement of evoked [3H]NE release. 4. We conclude that, although intracellular calcium participates in field-stimulation evoked [3H]NE release from bovine isolated irides, only the mitochondrial pool of calcium may be involved in peroxide-induced enhancement of sympathetic neurotransmission.

Prejunctional alpha2-adrenoceptors and peroxide-induced potentiation of norepinephrine release from the bovine iris.

Peroxides can enhance field-stimulated [3H]norepinephrine ([3H]NE) release in isolated irides from several mammalian species. In the present study, we investigated the role of prejunctional alpha2-adrenoceptors in peroxide-induced potentiation of sympathetic neurotransmission in bovine isolated irides. Isolated hemi-irides were incubated in a Krebs buffered-solution containing [3H]NE and prepared for studies of neurotransmitter release using the superfusion method. Alpha2-adrenoceptor agonists, oxymetazoline, UK-14304 and clonidine inhibited field-stimulated [3H]NE overflow without affecting basal tritium efflux. Pretreatment of tissues with H2O2 (300 microM) had no effect on inhibition of evoked [3H]NE release caused by the alpha2-adrenergic agonists. However, H2O2 (300 microM) caused significant (P < 0.01) leftward shifts of excitatory concentration-response curves to yohimbine (10 nM-1 microM). In contrast, yohimbine (1 microM) did not prevent the enhancement of evoked [3H]NE overflow induced by H2O2 (300 microM). In conclusion, excitatory effects of peroxides on sympathetic neurotransmission in bovine irides are not mediated by prejunctional alpha2-adrenoceptors.

Effect of hydroxycitric acid on serotonin release from isolated rat brain cortex.

There is evidence that hydroxycitric acid (HCA), an extract of dried fruit rind of South Asian trees of the genus Garcinia cambogia, can reduce food intake in experimental animals. In the present study, we investigated the effect of HCA on basal and potassium-depolarization evoked increase in radiolabeled serotonin ([3H]-5-HT) release from rat brain cortex slices in vitro. HCA (10 microM-1 mM) altered the baseline of spontaneous tritium efflux but had no significant effect on potassium-evoked release of [3H]-5-HT. When applied on its own, HCA (10 microM-1 mM) elicited a concentration-dependent increase in efflux of [3H]-5-HT reaching a maximum at 300 microM. We conclude that HCA can increase the release of radiolabeled 5-HT from the isolated rat brain cortex.

Human, bovine, and rabbit retinal glutamate-induced [3H]D-aspartate release: role in excitotoxicity.

The pharmacological basis of glutamate-induced [3H]D-aspartate release was investigated in isolated human, bovine and rabbit retinas. Isolated mammalian retinas were preloaded with [3H]D-aspartate and then prepared for studies of neurotransmitter release using the superfusion method. Release of [3H]D-aspartate was elicited by K+ (50 mM) or by L-glutamate. In bovine retinas, L-glutamate, but not D-glutamate induced an overflow of [3H]D-aspartate that was partially inhibited by low external calcium, omega-conotoxin (10 nM) or nitrendipine (1 microM). Metabotropic glutamate receptor (GLUR) agonists also evoked [3H]D-aspartate release in both bovine and human retinas whereas polyamines only enhanced the excitatory effects of L-glutamate on [3H]D-aspartate release. Antagonists of GLURs and the polyamine site inhibited L-glutamate evoked [3H]D-aspartate overflow with the following rank order of potency: MCPG >ifenprodil > AP-5 > arcaine> MK-801. In conclusion, L-glutamate-induces a stereoselective, calcium-dependent release of [3H]D-aspartate from isolated mammalian retinas that can be mimicked by GLUR agonists (and blocked by both receptor and polyamine site antagonists).

[(3)H]-serotonin release from bovine iris-ciliary body: pharmacology of prejunctional serotonin (5-HT(7)) autoreceptors.

In the present study, we investigated the pharmacological characteristics of electrically stimulated [(3)H]-serotonin release from mammalian iris-ciliary bodies. Isolated bovine and human iris-ciliary bodies were loaded with [(3)H]-serotonin, superfused with Krebs buffer solution and then stimulated with trains of 300 direct current (d.c.) pulses to initiate the release of the transmitter. The modification of this [(3)H]-serotonin release process by various serotonergic agonists and antagonists was studied in order to define the pharmacology of serotonin receptor(s) present in the iris-ciliary body. In bovine iris-ciliary body, electrically-evoked [(3)H]-serotonin release was calcium-dependent, tetrodotoxin-sensitive and was enhanced by serotonin (EC(50) = 200 n M) and 5-carboxmidotryptamine (EC(50) = 4 n M). The rank order of potency of agonists in enhancing field-stimulated [(3)H]-serotonin release was: 5-carboamidotryptamine > m-chlorophenylbiguanide > 2-methyl-5-hydroxytryptamine = 5-methoxy-dimethyltryptamine > serotonin > 5-methoxy-tryptamine > L-694,247 = alpha-methyl-5-hydroxytryptamine > CGS 12066A = 8-hydroxy-2-(di- n -propylamino)tetraline. Serotonin and m-chlorophenylbiguanide also enhanced electrically-evoked [(3)H]-serotonin release from human iris-ciliary bodies with EC(50)s of 3 microM and 30 n M, respectively. The pharmacological profile displayed by serotonin receptor agonists was supported by the potent antagonism of the serotonin-induced enhancement of [(3)H]-serotonin release by 5HT(7)receptor antagonists SB-258718 (IC(50) = 18.6 +/- 1.2 nM; n = 4) and mesulergine (IC(50) = 0.26 +/- 0.05 nM; n = 4). However, antagonists at 5HT(6)and 5HT(3)receptors exhibited a relatively weak blockade of serotonin induced enhancement of field-stimulated [(3)H]-serotonin release. These studies have shown the presence of functionally active prejunctional 5HT(7)autoreceptors regulating the release of [(3)H]-serotonin from bovine iris-ciliary bodies. Excitatory prejunctional 5-HT autoreceptors also exist in human iris-ciliary bodies. It is possible that these serotonin autoreceptors may have relevance to the regulation of aqueous humor dynamics in the anterior uvea.

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

Isoprostanes (IsoP's) are prostaglandin-like compounds that are derived from free-radical catalyzed peroxidation of arachidonic acid independent of the cyclcooxygenase enzyme. In the present study, we investigated the effect of IsoP's on norepinephrine (NE) release from human isolated iris-ciliary bodies. Isolated human iris-ciliary bodies were prepared for studies of [3H]NE release using the superfusion method. Both 8-iso-prostaglandin F2alpha (F2-IsoP) and the thromboxane (Tx) receptor agonist, U46619 enhanced field-stimulated [3H]NE release from isolated, superfused human iris-ciliary bodies without affecting basal tritium efflux. On the other hand, an equimolar concentration (10 microM) of 8-iso-prostaglandin E2 (E2-IsoP) inhibited evoked [3H]NE overflow. The Tx-receptor antagonist, SQ 29548 blocked the enhancements of electrically-evoked [3H]NE release induced by F2-IsoP and U46619. However, the inhibitory responses elicited by E2-IsoP was not antagonized by SQ 29548. We conclude that IsoP's can produce both excitatory and inhibitory effects on sympathetic neurotransmission in human isolated iris-ciliary bodies. The stimulatory effects of IsoP's on NE release may be mediated by Tx-receptors.

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

In the present study, we investigated the effect of inhibition of cyclooxygenase (COX) with flurbiprofen (FBF) on peroxide-induced enhancement of field-stimulated [3H]norepinephrine ([3H]NE) release from bovine isolated irides. Furthermore, the effect of FBF was examined on peroxide-induced attenuation of contractions evoked by carbachol on this tissue. Irides were prepared for studies of neurotransmitter release and for measurement of contractile tension in vitro. Pretreatment of tissues with FBF (10 microM) caused significant (P < 0.001) rightward shifts of concentration-response curves to H2O2 and also decreased cumene hydroperoxide (cuOOH)-induced enhancement of evoked [3H]NE release. FBF (10 microM) partially prevented the attenuation of carbachol-induced contractions induced by H2O2 (300 microM) and cuOOH (300 microM). We conclude that inhibition of the biosynthesis of prostanoids reduced both the prejunctional stimulatory effects of H2O2 and cuOOH on sympathetic neurotransmission and inhibitory effects of peroxides on carbachol-induced contractions the in the bovine isolated iris.