A major pathway for the regulation of intraocular pressure

Beta2 adrenergic receptor silencing change intraocular pressure in New Zealand rabbits

PURPOSE/AIM

Glaucoma consists of a group of progressive optic neuropathies that are characterized by degeneration of the optic nerve and irreversible visual filed loss. Elevated intraocular pressure is the only proven treatable risk factor and commercial products used for glaucoma treatment are focused in lowering intraocular pressure. These drugs can have various undesirable side effects and this invites to look for new strategies. The purpose of this work is to study the use of a siRNA (small interfering RNA) to selectively silence beta2 adrenergic receptors and to see whether it reduces IOP (intraocular pressure).

MATERIAL AND METHODS

Topical instillation of beta2 adrenergic receptors small-interfering RNA (siRNA, 25-250μg) was applied and IOP was measured with a Tonopen XL up to 9 consecutive days. The effect of such siRNA was compared to commercial compounds such as Timoftlol, Trusopt and Xalatan, and it was also analyzed if some anatomical changes occurred by microscopy.

RESULTS

siRNA designed for beta2 adrenergic receptor induced a reduction of intraocular pressure (IOP) of 30±5%, compared to a control (scrambled siRNA). The results in terms of IOP decrease were similar to that found with commercial compounds but a long-lasting hypotensive action was shown by beta2 adrenergic receptor siRNA treatment as compared to commercial drugs. No apparent side effects were observed in the ocular structures.

CONCLUSION

The use of siRNA against the beta2 adrenergic receptors could provide an interesting therapeutic strategy for glaucoma treatment.

Role of cyclic AMP in the eye with glaucoma

Glaucoma is characterized by a slow and progressive degeneration of the optic nerve, including retinal ganglion cell (RGC) axons in the optic nerve head (ONH), leading to visual impairment. Despite its high prevalence, the biological basis of glaucoma pathogenesis still is not yet fully understood, and the factors contributing to its progression are currently not well characterized. Intraocular pressure (IOP) is the only modifiable risk factor, and reduction of IOP is the standard treatment for glaucoma. However, lowering IOP itself is not always effective for preserving visual function in patients with primary open-angle glaucoma. The second messenger cyclic adenosine 3′,5′-monophosphate (cAMP) regulates numerous biological processes in the central nervous system including the retina and the optic nerve. Although recent studies revealed that cAMP generated by adenylyl cyclases (ACs) is important in regulating aqueous humor dynamics in ocular tissues, such as the ciliary body and trabecular meshwork, as well as cell death and growth in the retina and optic nerve, the functional role and significance of cAMP in glaucoma remain to be elucidated. In this review, we will discuss the functional role of cAMP in aqueous humor dynamics and IOP regulation, and review the current medications, which are related to the cAMP signaling pathway, for glaucoma treatment. Also, we will further focus on cAMP signaling in RGC growth and regeneration by soluble AC as well as ONH astrocytes by transmembrane ACs to understand its potential role in the pathogenesis of glaucoma neurodegeneration

Silencing of P2Y(2) receptors reduces intraocular pressure in New Zealand rabbits

BACKGROUND AND PURPOSE

P2 receptors are involved in the regulation of ocular physiological processes like intraocular pressure (IOP). In the present study, the involvement of P2Y(2) receptors in the hypertensive effect of nucleotides was investigated by use of antagonists and of a siRNA designed for the P2Y(2) receptor.

EXPERIMENTAL APPROACH

Agonists of the P2Y(2) receptor a as well as P2 antagonists were applied to eyes of New Zealand rabbits, and the changes in IOP were followed for up to 6 h. Cloning of the P2Y(2) receptor cDNA was done using a combination of degenerate reverse transcription PCR (RT-PCR) and rapid amplification of cDNA ends (RACE). siRNA was synthesized and tested by immunohistochemistry.

KEY RESULTS

Single doses of 2-thioUTP, UTP-γ-S and UTP increased IOP. This behaviour was concentration-dependent and partially antagonized by reactive blue 2. Silencing the P2Y(2) receptor was observed in the ciliary body by immunohistochemistry labelling, where a reduction in the immunofluorescence was observed. This reduction in the expression of the P2Y(2) receptor was concomitant with a reduction in IOP, which was measurable 24 h after treatment with the siRNA, maximal after 2 days, followed by a slow increase towards control values for the following 5 days. Application of the P2Y(2) agonists after pretreatment of the animals with this siRNA did not produce any change in IOP.

CONCLUSIONS AND IMPLICATIONS

P2Y(2) receptors increase IOP in New Zealand rabbits. The application of a siRNA for this receptor significantly reduced IOP, suggesting that this technology might be used for the treatment of glaucoma.

Repeated topical administration of fenoterol in rabbit reverses its initial ocular hypotensive effect and decreases sensitivity of adenylyl cyclase in ciliary processes to stimulatory agents

PURPOSE

The effects of repeated topical administration of the selective beta 2-adrenergic agonist fenoterol on the intraocular pressure and on the adenylyl cyclase activity in ciliary processes in rabbit were examined in order to detect their possible causal relationship.

METHODS

Intraocular pressure was measured by pneumatonometry. Adenylyl cyclase activity in homogenates of ciliary processes was assayed ex vivo by measurement of conversion of 32P-alpha-ATP to 32P-cyclic AMP.

RESULTS

A single topical dose of 1% solution of fenoterol elicited a clear-cut decrease of the intraocular pressure lasting for several h. Repeated administration of fenoterol for 2-5 days led to a significant increase of intraocular pressure, observable from the second to the fifth day. The stimulation of adenylyl cyclase activity ex vivo by isoproterenol, vasoactive intestinal polypeptide or forskolin was significantly decreased on the fifth day (24 h after the administration of the last dose of fenoterol).

CONCLUSIONS

Our data showed that repeated topical administration of the selective beta 2-adrenergic agonist increased intraocular pressure and desensitized adenylyl cyclase in ciliary processes; if these two effects are related then they would support the idea of direct relationship of decreased cAMP production in ciliary processes to the increase of intraocular pressure, and vice versa. However, conclusive evidence of this suggestion and of its possible significance in another animal species or man would require further study.

The additive intraocular pressure-lowering effect of latanoprost in combined therapy with other ocular hypotensive agents

Latanoprost, a prostaglandin F2 alpha analogue prodrug, has been shown to be an effective ocular hypotensive agent when used alone on ocular hypertensive or open angle glaucoma patients. In various studies, the ocular hypotensive effects of latanoprost have also been evaluated when used in addition to, or in combination with, other ocular hypotensive agents. Latanoprost produces an additional reduction of intraocular pressure (IOP) when used in combination with timolol, pilocarpine, acetazolamide and dipivefrin. These represent four different classes of glaucoma drugs-beta-adrenergic antagonists, cholinergic agonists, carbonic anhydrase inhibitors, and adrenergic agonists-all of which reduce the IOP by different mechanisms (reduction of aqueous humor production, increased outflow facility, or by a mixed effect on aqueous humor dynamics). All the available evidence shows that latanoprost produces a clinically significant additive ocular hypotensive effect when used in combination with any currently available ocular hypotensive agent.

Effects of mild, moderate and severe exercise on intraocular pressure of sedentary subjects

Previous studies have shown a reduction in intraocular pressure (IOP) from many forms of exertion, ranging from walking to exhausting exercise. The variability in these results may be due to several factors, such as age, diurnal and seasonal variations, drinking water or coffee before the test, acute hyperglycaemia, and physical fitness. The purpose of the study was to investigate the effects of the common means of exertion on IOP in the same subject, after elimination of above-mentioned factors. The effects of sitting, walking, jogging, and running fast till exhaustion were noted on the IOP of 15 healthy sedentary male volunteers at an interval of 4 days. Intraocular pressures were measured with a Goldmann applanation tonometer at the beginning and at 5th, 20th, 40th, and 60th minutes of the first three tests, but in test 4, only at the beginning and end. Post-exercise measurements were taken after every 10 minutes until IOP returned to pre-exercise levels. The effects of all tests were similar on both eyes. During sitting, walking and jogging, the maximum decreases were (mean +/- SD) 1.20 +/- 0.66, 3.20 +/- 1.19, and 5.07 +/- 1.76 mmHg, and occurred at the 20th, 40th and 60th minutes of the tests respectively. During sitting, walking and jogging, 66.7%, 75% and 68.4% of the maximum decreases occurred after 5 min respectively. After running the mean decrease was 5.7 +/- 1.09 mmHg and the average time of running was 10.53 +/- 2.17 min. In the sitting test, recovery occurred at the end of the test, while in walking, jogging and running tests it occurred after 12.67 +/- 44.48, 30.67 +/- 7.99, and 56.00 +/- 11.21 min, respectively. It is concluded that all forms of exertion decrease IOP. It would seem reasonable, at present, not to discourage patients who have glaucoma from walking; perhaps, on the contrary, it should be encouraged.

A cyclic AMP phosphodiesterase inhibitor, 8'-pivaloyloxymethyl ester (POM-ester) of griseolic acid, lowers rabbit intraocular pressure

The effects of griseolic acid (GA), a cyclic-AMP phosphodiesterase (PDE) inhibitor, and its 8'-pivaloyloxymethyl (POM) ester on intraocular pressure (IOP) in rabbits were investigated. When 50 microliters of 1 and 2% GA POM ester solutions were topically applied to one eye in normal rabbits, significant IOP decreases were detected at 2 hrs and at 1 to 5 hrs, respectively. Other than ocular hypotension, no other ocular effects were detected locally even after administration of 2% GA POM ester. A more marked reduction in IOP occurred after the intravitreal injection of the GA POM ester. IOP was also reduced when GA was used in an intravitreal injection but not when it was topically applied. The difference in permeability between GA and GA POM ester across the corneal epithelium may explain why GA failed to reduced IOP following topical administration. GA and the GA POM ester inhibited cAMP PDE in rabbit ciliary body at low concentrations, the I50 being 0.075 microM and 2.4 microM, respectively, with 0.25 microM cAMP as substrate. GA and the GA POM ester markedly increased cAMP levels in vitro in iris-ciliary body specimens. Possibly, GA POM ester or its analogues may represent a new mechanistic class of ocular hypotensive agents.

Quantitative autoradiography of beta-adrenergic receptors in rabbit eyes

Beta-adrenergic receptors were localized and quantified with autoradiographic methods in frozen sections of eyes of albino rabbits using [125I]cyanopindolol and [3H]dihydroalprenolol. Quantification in albino rabbits revealed highest beta 2-adrenoceptor densities in the ciliary body, the corneal epithelium and the corneal endothelium. Moreover, the iris showed significant amounts of specific binding, whereas the retina and sclera are nearly free of these receptors.

Inhibitory effects of neuropeptide Y on adenylate cyclase of rabbit ciliary processes

The inhibitory effect of neuropeptide Y (NPY) was studied on the adenylate cyclase (AC) activity in homogenates of rabbit ciliary processes and compared with that of the alpha 2-adrenergic agonist clonidine (CLN). NPY inhibited basal AC activity as well as AC activity stimulated by isoproterenol (ISO), vasoactive intestinal polypeptide (VIP) or forskolin (FSK). The extent of this inhibition corresponded well to the inhibition elicited by CLN. The inhibitory effects of NPY and CLN appeared to be nonadditive. AC activity stimulated by ISO was considerably more sensitive to the effects of either NPY or CLN than basal, VIP- or FSK-stimulated AC activity. It was inferred that NPY inhibitory effects were mediated by the activation of NPY receptors coupled negatively to the catalytic unit of AC via the inhibitory Gi protein. Moreover, involvement of NPY in physiological modulation of AC activity in ciliary processes and in the regulation of aqueous humor formation and intraocular pressure is suggested.

Excitatory amino acids interfere with normal eye growth in posthatch chick

This study examines the effects of excitotoxic amino acids on eye growth and retinal morphology. Day old chicks received a single intraocular injection of either 200 nmoles kainic acid (KA), 200 nmoles quisqualic acid (QUIS) or 400 nmoles N-methyl-D,L-aspartate (NMDA). Following survival periods of 7, 14 and 21 days, eyeballs were removed and weighed. Measurements of axial length, equatorial length, anterior chamber depth and corneal diameter were taken. Treatment with KA increased eye weight and equatorial length. Treatment with QUIS increased the anterior chamber depth but decreased the equatorial length. Treatment with NMDA increased anterior chamber depth, but to a lesser extent than QUIS. The effects of QUIS and NMDA could be distinguished from those of KA since the former excitotoxins resulted in a marked increase in anterior chamber depth with no enlargement of vitreal chamber. Changes in eye size were evident by day 7 and were sustained throughout the duration of the experiment. Examination of retinae revealed that KA lesions amacrine cells, bipolar cells, some ganglion cells and photoreceptors. Exposure to QUIS lesions amacrine cells, horizontal cells and causes mild disruption of photoreceptor outer segments. In contrast, NMDA predominantly lesions amacrine cells. The results demonstrate that these neurotoxins have different effects on eye growth, which may be associated with differences in retinal pathology. It is proposed that photoreceptors are ideally suited to play a role in the control of eye growth.

Intravitreal human chorionic gonadotropin decreases intraocular pressure in rabbits: mechanism of action

Intravitreal injection of purified human chorionic gonadotropin (hCG) in rabbits decreased intraocular pressure (IOP). A dose-dependent decrease in IOP was observed with intravitreal hCG concentrations at 30 nM and 100 nM. The onset of this effect was later than 10 hr following the injection and it lasted for more than 24 hrs. The purified beta-subunit of hCG caused a similar decrease in IOP with a short duration. The threshold intravitreal concentration was 10 nM. Unlike the intact hCG, the hCG beta-subunit was inactive as a gonadotropic agent to activate the adenylate cyclase in the rat testis. Intravitreal injection of rabbit luteinizing hormone, which was active as a gonadotropic agent, had no effect on IOP in 4 intravitreal concentrations ranging from 1 nM to 30 nM. These observations indicate that the mechanism of IOP decrease by intravitreal hCG is not related to its gonadotropic action. The IOP decrease in rabbits due to intravitreal hCG or its beta-subunit is probably related to a contaminant or an immune reaction.

The effect of hCG on cyclic AMP in rabbit ciliary processes

Experiments designed to address the role of cyclic AMP as a mediator of the eye pressure and aqueous flow reducing effects of human chorionic gonadotropin (hCG) in the rabbit have been conducted. No significant stimulatory effect of hCG on particulate preparations of adenylate cyclase from rabbit ciliary processes could be found under several different experimental conditions. Forskolin, an activator of adenylate cyclase can, in some biological systems, reveal a significant hormonal cyclic AMP response which is undetectable in the presence of hormone alone. Forskolin did potentiate the effect of isoproterenol on cyclic AMP production by intact rabbit ciliary processes. Cyclic AMP production by intact ciliary processes was not affected by hCG, either in the presence or absence of forskolin. Significant specific binding of iodinated hCG to particulate preparations of rabbit ciliary processes could not be detected. These results dispute the role of cyclic AMP as a mediator of the flow and pressure reducing effects of hCG; however, theoretical and methodological complications relating to this interpretation are discussed.

Transformation of human ciliary epithelial cells by simian virus 40: induction of cell proliferation and retention of beta 2-adrenergic receptors

Ciliary epithelial cells derived from human eye were successfully propagated through many generations after transformation with simian virus 40. The cell clone 8-SVHCE was isolated and characterized by immunoprecipitation and pharmacological studies that demonstrated the presence of several functional properties observed in the parent cells of this tissue. Immunoprecipitation revealed the presence of large tumor (T) antigen, and Southern blot analysis showed the incorporation of viral DNA into high molecular weight ciliary epithelial cell DNA. The presence of beta-adrenergic receptors was demonstrated by direct binding of a radiolabeled antagonist, [125I]iodopindolol, to membrane preparations of 8-SVHCE cells (Kd = 41.8 pM and Bmax = 67.1 fmol/mg of protein). Competition experiments with [125I]iodopindolol and selective drugs suggested that the receptors are of the beta 2-adrenergic subtype. Studies of catecholamine-stimulated cellular cAMP production and of isoproterenol-dependent protein phosphorylation of vimentin in 8-SVHCE indicated the functional conservation of beta-adrenergic receptor-mediated processes that are thought to be important in the regulation of aqueous humor production by the ciliary epithelium in vivo.

Localization and characterization of specific receptors for atrial natriuretic factor in the ciliary processes of the eye

By light and electron microscope radioautography in vivo, competitive binding sites for 125I-Arg 101-Tyr 126 atrial natriuretic factor were localized mostly on the "pigmented" epithelium of the rat ciliary process. Further investigation using isolated ciliary processes from rabbits demonstrated the presence of specific receptors for 125I-atrial natriuretic factor. In addition, synthetic atrial natriuretic factor inhibited basal and stimulated adenylate cyclase activity. These results demonstrate for the first time the presence of specific receptors for atrial natriuretic factor in the ciliary processes which are negatively coupled to adenylate cyclase. The possible role of this peptide in the control of intraocular pressure is suggested.

Intravitreal injection of purified human chorionic gonadotropin lowers IOP in rabbits

Human chorionic gonadotropin (hCG) is a glycoprotein hormone composed of two dissimilar subunits alpha and beta held together by noncovalent forces. Each subunit contains about 30% carbohydrate and is extensively crosslinked by disulfide bonds. Previous work from our laboratory with commercial preparations of hCG indicated that intravitreal injection of hCG lowered intraocular pressure (IOP). Our work has been extended by using purified hCG obtained by reverse phase high performance liquid chromatography of a commercial preparation. With a wide pore octyl silica column and a step gradient composed of dilute aqueous trifluoroacetic acid and methanol, several peaks were obtained. The major peak was shown by sodium dodecylsulfate-polyacrylamide gel electrophoresis and amino acid analysis to contain both alpha and beta subunits. That this major peak contained intact hormone rather than a mixture of subunits was revealed by its ability to enhance the fluorescence of 8-anilino-1-naphthalene sulfonate and stimulate the release of cyclic AMP from isolated rat testes; subunits of hCG lack these properties. Physiological doses of hCG from this major peak injected intravitreally in rabbit eyes resulted in significant decreases in IOP without associated irritation when compared with contralateral control eyes.

Regulation of aqueous flow by the adenylate cyclase receptor complex in the ciliary epithelium

The answer to how the beta-adrenergic receptor mediates a fall in intraocular pressure has been elusive. Methods of measurement have not been refined sufficiently. The separate changes after adrenergic treatment frequently are small, and the tissue effects are multiple. On a molecular basis, stimulation of the beta-adrenergic receptor activates intracellular adenylate cyclase to produce increased cyclic adenosine monophosphate. Acting by different cell-receptor mechanisms, but nonetheless potent, nonadrenergic stimulators of adenylate cyclase in the ciliary epithelium, such as cholera toxin and organic fluorides, have been studied in experimental animals. They reduce intraocular pressure by reducing net aqueous flow. When forskolin, a diterpene and potent stimulator of adenylate cyclase, became available, it was used in noninvasive topical form in the human eye to clarify the question of whether increased cyclic adenosine monophosphate reduces intraocular pressure and aqueous flow. Noninvasive studies in human eyes have demonstrated a 35% reduction in outflow pressure after the administration of forskolin in a 1% topical suspension, matched by a corresponding reduction in aqueous flow. Tonographic outflow facility was unaltered. Thus, the entire reduction in intraocular pressure can be accounted for by a reduction in net aqueous flow.

Radioautographic localization of 125I-atrial natriuretic factor (ANF) in rat tissues

Rats were injected either with synthetic 125I-Arg 101-Tyr 126 atrial natriuretic factor (ANF) or with 125I-ANF together with an excess of cold Arg 101-Tyr 126 ANF. Binding sites in various tissues were accepted depending on two criteria: displacement of radioactivity by cold ANF and absence of localization of silver grains on putative target cells in the presence of cold ANF. Binding sites were localized on zona glomerulosa cells and on adrenergic and noradrenergic cells of adrenal medulla, on hepatocytes, on the base of mature epithelial cells of villi in the small intestine, on smooth muscle cells of the muscularis layer of the colon and on the base of epithelial cells of the ciliary bodies. In addition, binding sites were localized in the vasculature of kidney, adrenal cortex, lung and liver. Binding sites were particularly numerous on renal glomerular endothelial cells. These results indicate that ANF may have important hemodynamic effects in kidney, lung, liver and adrenal cortex, may regulate water and ion transport in small intestine and ciliary bodies and may have metabolic effects in the liver. The presence of binding sites on the zona glomerulosa is in agreement with the important inhibitory effect of the peptide on aldosterone secretion.

Cellular mechanisms in the actions of antiglaucoma drugs

There are several classes of drugs currently in use for the therapeutic management of the glaucomas. Although the ocular hypotensive effects of these agents have been well characterized and described, little is known of their site of action and cellular mechanism. This review attempts to describe those cellular mechanisms that may be linked to the actions of several classes of antiglaucoma drugs. Special emphasis was placed on drug actions and 1) the adenylate cyclase system; 2) receptor-coupled phosphoinositide turnover; 3) prostaglandins and 4) ion transport processes. Models are presented depicting proposed cellular sites of the interaction of the antiglaucoma drugs with these cellular processes.

Combined effect of forskolin and acetazolamide on intraocular pressure and aqueous flow in rabbit eyes

Forskolin, a diterpene, a potent and direct stimulator of the catalytic unit of adenylate cyclase, can reduce intraocular pressure when administered topically to rabbit eyes in a 1% suspension. Using an intravitreal plant of fluorescein-dextran, aqueous flow was measured by a modification of the method of Maurice. Forskolin reduces net aqueous flow by 51% compared to the controls. Acetazolamide (25 mg kg-1) administered intravenously reduces net aqueous flow by 45%. Forskolin and acetazolamide together reduce net aqueous flow by 72%, an important additive effect undoubtedly produced by different pathways.

Forskolin lowers intraocular pressure in rabbits, monkeys, and man

Topical ocular application of forskolin, a diterpene that increases intracellular cyclic adenosine monophosphate by stimulating adenylate cyclase directly without cell surface mediation, lowered intraocular pressure (IOP) in rabbits, monkeys, and volunteers who were free from eye disease. In man 50 microliters of a topical suspension of 1% forskolin significantly lowered IOP in 1 h, the effect reaching a peak at 2 h but remaining significant for at least 5 h. Outflow pressure fell by 70% on average. Forskolin and its analogues represent a new class of drugs active against glaucoma which differ in their molecular actions from any previously used drug.