Stimulation of endogenous cyclic AMP levels in ciliary body by SK&F 82526, a novel dopamine receptor agonist

A review of systemic medications that may modulate the risk of glaucoma

With increasing longevity, patients are developing more and more chronic diseases that require treatment with medications. Yet, it is not fully understood the extent by which these systemic medications affect ocular structures and whether they may increase or decrease the risk of sight-threatening ocular diseases. This review provides a summary of reported associations between different systemic medications and the risk of developing glaucoma or experiencing disease progression. Medication classes covered in this review that are known to or may modulate the risk of open-angle glaucoma include corticosteroids, beta blockers, calcium channel blockers, metformin, statins, selective serotonin reuptake inhibitors, bupropion, postmenopausal hormones, and cannabinoids. Medication classes addressed in this review that may increase the risk of angle closure glaucoma include anticholinergics, adrenergic agonists, certain classes of antidepressants, sulfonamides, and topiramate.

Oseltamivir (Tamiflu)-induced bilateral acute angle closure glaucoma and transient myopia

A 27-year-old woman developed bilateral acute angle closure glaucoma (AACG) and transient myopia after taking oseltamivir for four days. On the fourth day, she received systemic and topical intraocular pressure (IOP)-lowering agents, and IOP decreased in both eyes. However, her visual acuity was unchanged. A myopic shift of -5.25 D OD and -5.0 D OS was estimated to have occurred in the acute phase. A-scan ultrasonography and Pentacam showed markedly shallow anterior chambers and increased lens thickness. Ultrasound biomicroscopy revealed an annular ciliochoroidal effusion with forward displacement of the lens-iris diaphragm. Ciliochoroidal effusion and transient myopia were resolved after discontinuation of oseltamivir.

Fenoldopam, a selective dopamine-1 receptor agonist, raises intraocular pressure in males with normal intraocular pressure

Recent studies have suggested that intravenous infusion of fenoldopam, a selective dopamine-1 receptor agonist, elevates intraocular pressure (IOP) in man. This study evaluated the effect of intravenous fenoldopam on IOP, aqueous humor outflow facility and gonioscopy in 12 healthy human subjects. Three doses (0.2, 0.5 and 1.0 microg/kg/min) were infused for 120 minutes in a double masked, placebo controlled, four-way crossover design. IOP was measured every 20 minutes in the supine position and every 40 minutes while sitting during the drug and placebo infusions. Tonography and gonioscopy were performed at baseline and after 120 minutes of infusion. Compared to placebo, IOP increased by 3.5 mm Hg (32%) for the lowest dose, 5.8 mm Hg (46%) for the intermediate dose, and 6.9 mm Hg (55%) for the highest dose (p<0.05 for all three doses). IOP returned to baseline within 30 minutes of stopping the infusion. The outflow facility decreased from baseline by 26% after 120 minutes of infusion for all drug doses. In contrast, outflow facility increased from baseline by 11% during placebo infusion. Compared to placebo, the fenoldopam induced changes in outflow were statistically significant (p<0.05). There was no change in the gonioscopic appearance of the anterior chamber angle during the infusion. This study shows that systemic administration of a selective dopamine-1 receptor agonist causes a significant dose-dependent increase in IOP that can be explained in part by diminished outflow facility. These results support a role for the dopamine-1 receptor in the modulation of IOP in general and suggest modulation of aqueous humor outflow by dopaminergic receptors.

Ocular actions of an octahydrobenzo[f]quinoline: Ha117

An octahydrobenzo[f]quinoline compound, Ha117, was examined for activity on: (1) intraocular pressure and in normal, sympathectomized and water loaded rabbits and aqueous flow rate in normal and sympathectomized rabbits, respectively; (2) contractions of the cat nictitating membrane elicited by electrical stimulation of cervical sympathetic nerves; (3) cAMP accumulation in the isolated rabbit iris root-ciliary body preparation. Ha117 lowered intraocular pressure and aqueous flow rate in normal but not in sympathectomized rabbits. Elevated intraocular pressure produced by water loading was suppressed by Ha117 and pretreatment with metoclopramide antagonized the inhibitory effect of Ha117. Neuronally mediated contractions of the nictitating membrane were inhibited in a dose-related fashion by Ha117, and inhibitory effects were antagonized by domperidone. Ha117 and an analog, Ha118, did not suppress isoproterenol- and vasoactive intestinal peptide-induced accumulation of cAMP in the rabbit iris root/ciliary body. In vivo results in rabbit and cat models suggest that the ocular hypotensive effect of Ha117 is mediated by an action on prejunctional dopamine (DA2) receptors. In vitro data in the rabbit iris root/ciliary body suggest that Ha117 and Ha118-induced lowering of intraocular pressure does not involve postjunctional suppression of cAMP accumulation.

Biphasic dose-dependent effects of dopamine and involvement of dopamine autoreceptors on intra-ocular pressure in the rabbit

1. This work was conducted to provide new data concerning the possible dose-dependent activity of dopamine (DA) after ocular instillation. Experiments were done in rabbits with normal intraocular pressure (IOP), or after transitory induced ocular hypertension in water-loaded animals. 2. In ocular normotensive animals, a biphasic dose-dependent activity is observed, with no significant effect for 0.001% and 0.003% DA, a decrease in IOP after 0.005% and 0.01% DA instillation, then an important increase in IOP at concentrations from 0.05% onwards. 3. During transitory ocular hypertension, this phenomenon was confirmed, with a marked ocular hypotensive activity for 0.01% DA, no effect after 0.005% DA, then an important ocular hypertension with 0.05% and 0.5% DA as compared to the control group (0.9% NaCl). 4. An immediate and similar ocular hypertensive effect with DA could be reproduced by a subsequent instillation at high concentration (1%), while the hypotensive activity induced at low concentration (0.01%) is followed by a long-lasting refractory period (about 18 h). 5. Such a dose-dependent biphasic effect was also observed with N-methyl-dopamine (NMDA) after ocular instillation. The effects of instilled dopaminergic compounds were tested and ocular hypotensive activities of the S(-)enantiomer of the DA analogues 3-(3-hydroxyphenyl)-N-propylpiperidine (3-PPP), and of thiothixene (TIX) were also demonstrated. 6. The possible relationships to DA1 and DA2 receptors of the dual effect is discussed.

Stimulatory and inhibitory cyclic AMP responses in rabbit ciliary processes after cervical ganglionectomy

Cyclic AMP production in response to agonists which act at a variety of receptors to either stimulate or inhibit cyclic AMP production has been studied in intact, dissected ciliary processes from rabbit eyes after unilateral surgical removal of the cervical ganglion. Cyclic AMP responses to stimulatory ligands vasoactive intestinal peptide (VIP), isoproterenol, and forskolin and inhibitory agonists neuropeptide Y (NPY), the synthetic somatostatin analogue SMS 201-995, and alpha-adrenergic agents were investigated in tissues from normal eyes and compared to the same responses in tissues from sympathetically denervated eyes. Neither stimulated cyclic AMP production nor inhibition of stimulated cyclic AMP production was significantly different in tissues from denervated vs. normal eyes. Inhibition of VIP-stimulated cyclic AMP production by epinephrine and paraaminoclonidine in tissues from both normal and denervated eyes was blocked by the alpha 2-adrenergic antagonist yohimbine but not by the alpha 1-adrenergic antagonist prazosin. These data indicate that the VIP, NPY, somatostatin, and alpha 2- and beta 2-adrenergic receptors which regulate cyclic AMP production in rabbit ciliary processes are postjunctional and suggest that ligands known to modulate cyclic AMP levels in this tissue may exert effects on aqueous humor formation independently of adrenergic innervation.

Biochemical and functional evidence for the presence of dopamine D1 receptors in the bovine ciliary body

The present paper reports both functional and biochemical evidence for the presence of dopamine D1 receptors in the bovine ciliary body. Dopamine (DA) and dopamine D1 agonists (such as SKF 38,393) but not D2 agonists (such as LY 141,865) produced a concentration-related decrease in the tone induced by a maximally active concentration of carbachol (1 x 10(-4)-5 x 10(-4) M). The maximal relaxation obtained was 100% of the carbachol response using 10(-5) M dopamine or 5 x 10(-6) M SKF 38,393. SCH 23,390, a D1 antagonist, but not (-)-sulpiride, antagonized the effect of DA and SKF 38,393. In accordance with the functional data, radioreceptor binding experiments revealed the existence of a high affinity saturable [3H]SCH 23,390 binding to membranes prepared from ciliary body (Bmax: 344 fmol mg protein-1; Kd: 0.87 nM). The binding was specifically displaced by SCH 23,390, dopamine and dopamine D1 agonists, but not by norepinephrine, D2 agonists, or antagonists such as LY 141,865 and sulpiride. No specific binding was found when using dopamine D2 ligands, such as tritiated spiroperidol.

Review: alpha 2 and DA2 agonists as antiglaucoma agents: comparative pharmacology and clinical potential

Alpha-2 (alpha 2) and DA2 agonists lower intraocular pressure (IOP) in laboratory animals and man. Like beta-blockers, alpha 2 and DA2 agonists appear to lower IOP by reducing aqueous inflow. These agents share a common mode of action on sympathetic nerve terminals, where they modulate the release of neurotransmitters. However, one can demonstrate that peripheral prejunctional alpha 2 and DA2 receptors on sympathetic neurons are separate entities by utilizing selective agonists and antagonists. In addition to their prejunctional actions, alpha 2 agonists act postjunctionally in the iris root/ciliary body (IRCB). Moreover, utilizing selective postjunctional alpha 2 adrenoceptor antagonists, heterogeneity can be demonstrated between ocular pre- and postjunctional adrenoceptors. Stimulation of postjunctional alpha 2 adrenoceptors in the IRCB can inhibit the cellular responses to endogenous neurotransmitters and hormones that are coupled positively to adenylate cyclase. Based upon these observations, one can predict that alpha 2 agonists should have a broader spectrum of action in the eye than beta-receptor antagonists. Three bioassays were used in the activity analysis of alpha 2 and DA2 agonists. Prejunctional (neuronal) activity was determined in the cat nictitating membrane preparation in which frequency-related (2-8 Hz), neuronally induced contractions were inhibited by these compounds. Postjunctional activity was assayed on isolated rabbit IRCB tissue where cAMP levels were stimulated by either isoproterenol or VIP in the absence and presence of the test agonist (alpha 2 or DA2). In this system, it has been demonstrated that alpha 2 agonists have inhibitory properties, but DA2 agonists are inactive.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine receptors in rabbit and rat eye: characterization and localization of DA1 and DA2 binding sites

In vitro autoradiography is used to characterize and localize DA1 and DA2 receptors in rabbit and rat eyes. The bindings of 125I-SCH 23982 (DA1 antagonist) and of 125I-Iodosulpride (DA2 antagonist) to slide mounted tissue sections take place with characteristics expected of substances that recognize DA1 and DA2 receptors respectively. They are saturable, have high affinity and exhibit an appropriate pharmacology. The regional distribution of these receptors indicates that they are mainly present in retina. Precise cellular localizations are visualized in retinal structures by means of histoautoradiography: DA1 receptors are found in inner plexiform layer, outer plexiform layer and inner nuclear layer and DA2 binding sites are present in the two plexiform layers.

Effects of selective dopamine-1 receptor activation on intraocular pressure in man

The lack of specific agonists and antagonists has, until recently, precluded investigation of a role for dopamine receptors in the control of intraocular pressure. In the present study, we have examined the effects of fenoldopam, a novel selective dopamine1 (DA1) receptor agonist, on intraocular pressure, in eight healthy human volunteers. Fenoldopam, infused intravenously at 0.5 micrograms kg-1 min-1, increased intraocular pressure from 14.6 +/- 0.9 to 17.6 +/- 1.4 mmHg (P less than 0.05) while a control saline infusion had no effect. Pupil diameter and blood pressure did not change. In the same subjects, i.v. norepinephrine or angiotensin II both increased intraocular pressure--from 13.8 +/- 1.4- to 17.6 +/- 1.4 mmHg and from 13.4 +/- 1.3- to 17.5 +/- 1.7 mmHg respectively (P less than 0.05), and mean arterial pressure by about 20 mmHg. These data suggest that: (1) DA1 receptor activation can modulate intraocular pressure; (2) the intraocular pressure effects of the DA1 receptor agonist, fenoldopam, are independent of changes in systemic blood pressure, in contrast to those of norepinephrine or angiotensin II where intraocular and systemic blood pressures increase in parallel; (3) the ability of a DA1 receptor antagonist to lower intraocular pressure merits investigation.

Neurotransmitters and intraocular pressure

The role of the ocular autonomic nervous system in IOP regulation has been well established. Pharmacological and autohistoradiographic studies confirmed the high density of beta 2 and alpha 2 receptors on ciliary processes and iris epithelium. Their respective pharmacological activation or blockade is discussed. The role of other ocular neurotransmitters is also complex, as shown by the paradoxical similar action of dopamine agonists and antagonists on IOP. Concerning the cholinergic system, ocular muscarinic receptors are pharmacologically not well documented. Numerous other neurotransmitters may modulate IOP without necessarily leading to the development of new drugs. Drugs of the future will probably concentrate on dopaminergic agonists, cAMP-stimulators such as forskolin, prostaglandins, and cannabinoids.

Interaction between alpha 2- and beta 2-adrenergic receptors in rabbit ciliary processes

The interaction between the alpha 2- and beta 2-adrenergic receptors of ciliary processes has been studied by examining dose-response curves for adrenergic agonist stimulation of cyclic AMP production by intact, excised rabbit ciliary processes. Stimulation of cyclic AMP production by 1-isoproterenol is maximum from 0.1 to 1.0 microM; at higher concentrations stimulation decreases and approaches basal levels. Decreased cyclic AMP production at high concentrations of isoproterenol is blocked by the specific alpha 2-adrenergic antagonist, yohimbine, but not by the alpha 1-adrenergic antagonist, prazosin. Ciliary processes from animals after bilateral cervical ganglionectomy also show reduced cyclic AMP production at high concentrations of isoproterenol and this reduction is blocked by yohimbine, but not prazosin. This experiment suggests that the inhibition at high concentrations of isoproterenol is mediated by postsynaptic alpha 2-adrenergic receptors. Cyclic AMP production is relatively insensitive to epinephrine and norepinephrine, but their responses are potentiated by yohimbine. Catecholamines and clonidine, a specific alpha 2-adrenergic agonist, exhibit dose-dependent inhibition of forskolin-stimulated cyclic AMP production by ciliary processes. I50s from the dose-response curves are consistent with the characteristic binding affinities of these adrenergic agonists for alpha 2-adrenergic receptors: clonidine = epinephrine greater than norepinephrine greater than isoproterenol. Inhibition of forskolin-stimulated cyclic AMP production by clonidine is blocked by yohimbine but not by prazosin.(ABSTRACT TRUNCATED AT 250 WORDS)