A comparative study of topical natural ergot alkaloids on the intraocular pressure and aqueous humor dynamics in oclular normotensive and α-chymotrypsin-induced ocular hypertensive rabbits

Background Although it has been suggested that ergot derivatives may play a role in antiglaucoma therapy, little attention has been paid to the ocular hypotensive action of these drugs. Having previously reported that topical natural ergot alkaloids ergocristine α-ergocryptine and ergocornine dose-dependently reduce intraocular pressure in ocular normotensive and α-chymotrypsin-induced ocular hypertensive rabbits, the aim of the present work was to compare the effect of ergocristine, α-ergocryptine and ergocornine on the intraocular pressure and aqueous humor dynamics in ocular normotensive and α-chymotrypsin-induced ocular hypertensive rabbits, in order to further explore the ocular actions of these compounds. Methods Experiments were conducted in albino ocular normotensive and hypertensive rabbits by intracameral injection of α-chymotrypsin. Intraocular pressure responses to drug vehicle and seven different doses of topical natural ergot alkaloids were examined, in order to obtain dose–response relationships for comparing the intraocular pressure-lowering effect and potency of these drugs. Tonographies were also performed to ascertain the actions of natural ergot alkaloids on aqueous humor dynamics. Results All natural ergot alkaloids tested reduced intraocular pressure in a dose-related fashion. The ocular hypotensive effect was greater in α-chymotrypsin-induced ocular hypertensive rabbits for the three compounds tested. All natural ergot alkaloids tested decreased both tonographic outflow facility and, to a greater extent, aqueous humor inflow in ocular normotensive and in α-chymotrypsin-induced ocular hypertensive rabbits. Conclusion Taken together, our data suggest that these compounds decrease both tonographic outflow facility and, to a greater extent, aqueous humor inflow, which explains their final effect in ocular normotensive and in α-chymotrypsin-induced ocular hypertensive rabbits. Reductions in aqueous humor inflow observed after topical application of natural ergot alkaloids in α-chymotrypsin-induced ocular hypertensive rabbits can only be explained by a marked inhibition of active secretion of aqueous humor, since processes involved in aqueous humor formation may probably be altered after α-chymotrypsin injection.     

The effect of topical natural ergot alkaloids on the intraocular pressure and aqueous humor dynamics in rabbits with α-chymotrypsin-induced ocular hypertension

BACKGROUND: We previously reported that topical natural ergot alkaloids ergocristine, alpha-ergocryptine and ergocornine dose-dependently reduce intraocular pressure in ocular normotensive rabbits, most likely by decreasing aqueous humor inflow. In the present study, the effects of these compounds on intraocular pressure and aqueous humor dynamics in a rabbit model for ocular hypertension were assessed. METHODS: Experiments were conducted in albino rabbits made ocular hypertensive by intracameral injection of alpha-chymotrypsin. Intraocular pressure responses to drug vehicle and seven different doses of topical natural ergot alkaloids were examined in order to obtain dose-response relationships for comparing the intraocular pressure-lowering effect and potency of these drugs. Tonographies were also performed to ascertain the actions of natural ergot alkaloids on aqueous humor dynamics in alpha-chymotrypsin-induced ocular hypertensive rabbits. RESULTS: Topical application of the natural ergot alkaloids ergocristine, alpha-ergocryptine and ergocornine lowered intraocular pressure in alpha-chymotrypsin-induced ocular hypertensive rabbits in a dose-related fashion, with ergocristine displaying the greatest intraocular pressure-lowering effect. Tonographic studies revealed a decrease in the tonographic outflow facility following topical application of natural ergot alkaloids, although only the effects of both ergocristine and alpha-ergocryptine reached statistical significance. All natural ergot alkaloids tested significantly reduced the calculated aqueous humor inflow. CONCLUSION: This study suggests that the natural ergot alkaloids ergocristine, alpha-ergocryptine and ergocornine effectively decrease intraocular pressure in the alpha-chymotrypsin-induced model of ocular hypertension. Since these compounds reduce the tonographic aqueous humor outflow facility, their final ocular antihypertensive effect appears to result from a remarkable reduction of the aqueous humor inflow.     

Aqueous humor dynamics in alpha-chymotrypsin-induced ocular hypertensive rabbits.

Aqueous humor dynamics were studied in alpha-chymotrypsin-induced ocular hypertensive rabbits either by tonographic or two-level constant pressure perfusion techniques. A significant correlation was obtained between the values of outflow facility in alpha-chymotrypsin-induced ocular hypertensive rabbits as determined by tonography and constant pressure perfusion. The mean value of tonographic outflow facility in ocular hypertensive rabbits was not statistically different from that found in ocular normotensive rabbits. On the contrary, the estimated rate of aqueous inflow in ocular hypertensive rabbits was about 1.5-fold higher than that of ocular normotensive ones. While topical timolol lowered intraocular pressure and aqueous humor inflow in ocular hypertensive rabbits, pilocarpine did not produce any significant effect. Aqueous humor protein was significantly increased in ocular hypertensive eyes. The results of this study show that accurate measurements of outflow facility can be obtained in alpha-chymotrypsin-induced ocular hypertensive rabbits by tonographic technique. Our data suggest that the long-term ocular hypertension induced by alpha-chymotrypsin in albino rabbits may be secondary to an increase in the rate of aqueous humor inflow, likely produced by a breakdown of the blood-aqueous barrier. This finding strongly conflicts with the hypothesis of trabecular blockage as the cause of alpha-chymotrypsin-induced ocular hypertension in this species.     

Topical verapamil lowers outflow facility in the rabbit eye.

Results of studies examining the mechanism of the ocular hypotensive effect of topical calcium channel blockers are controversial. Whereas evidence obtained in perfused human eyes indicates that these drugs lower intraocular pressure by increasing the aqueous humor outflow, tonographic studies in rabbits have revealed that they reduce both the aqueous humor outflow and inflow. In order to clarify such a discrepancy, the aim of this study was to assess whether the effect of topical verapamil on the facility of aqueous humor outflow in the rabbit eye was dose-related. Total outflow facility was determined by two-level constant pressure perfusion in anesthetized rabbits. The effect of 5 different concentrations on aqueous humor outflow at 60 minutes postdrug was studied in groups of 10 rabbits each. Baseline outflow facility was also determined in a group of 15 rabbits. In order to check the reliability of the method for detecting drug-induced changes in aqueous outflow, the effect of pilocarpine was also tested. Topical verapamil was shown to lower outflow facility in the rabbit eye in a dose-related fashion. On the contrary, topical pilocarpine was found to significantly increase outflow facility. Our data indicate that topical verapamil reduces outflow facility in the rabbit eye.     

The effect of topical CS-088, an angiotensin AT1 receptor antagonist, on intraocular pressure and aqueous humor dynamics in rabbits

PURPOSE: To evaluate the ocular hypotensive effect of topical CS-088, an angiotensin AT1 receptor antagonist, and the effect of CS-088 on aqueous humor dynamics. METHODS: The effects of CS-088 on intraocular pressure (IOP) were studied in 2 models of rabbit ocular hypertension. Experimental ocular hypertension was induced in albino rabbits by injecting alpha-chymotrypsin into the anterior chamber (alpha-chymotrypsin rabbit). The effects of the single application of CS-088 were examined. Additionally, CS-088 was repeatedly administered over a period of 3 weeks to hereditary ocular hypertensive rabbits (buphthalmic rabbits, JWHR bu/bu) and the IOPs were monitored throughout the experiment. The effects of CS-088 on aqueous humor dynamics were also examined in normal rabbits. In this study, the methods of IOP recovery rate, two-level constant pressure perfusion and fluorescein-dextran perfusion were used respectively to determine the aqueous inflow, outflow facility and uveoscleral outflow (USF). RESULTS: CS-088 at 1% and 2% significantly lowered the IOP in the alpha-chymotrypsin rabbits with a maximum IOP reduction of 10.1 mmHg. The maximum effect obtained with 2% CS-088 was no greater than that with 1% CS-088. In the buphthalmic rabbits, 2% CS-088 also lowered IOP significantly. Timolol was effective in both models. In the study on aqueous humor dynamics, a slight increase in USF (17%) was seen after a topical application of CS-088 whereas changes in aqueous inflow or outflow facility were not observed. CONCLUSIONS: Topical CS-088 can decrease IOP in rabbits. Despite the USF change, the ocular hypotensive mechanism by CS-088 was not fully determined.     

The peripheral and central neural actions of clonidine in normal and glaucomatous eyes.

The peripheral and central neural actions of clonidine on normal and glaucomatous eyes have been investigated. Threshold doses of clonidine applied topically induced a monotonic decrease of intraocular pressure in the treated eye and had no effect on the contralateral eye. With increased clonidine dose, a decrease of intraocular pressure occurred in the untreated eye, and there was a concomitant decrease of systemic arterial blood pressure. Analysis of aqueous humor dynamics showed that the ocular response to the peripheral and the central neural actions of clonidine were without effect on the tonographic coefficient of outflow facility. The episcleral venous pressure decreased in both the treated and the untreated eyes, but the changes were too small to account for the observed decrease of intraocular pressure. The results are consistent with the concept that both the peripheral and central ocular hypotensive actions of clonidine are mediated by an inhibition of adrenergic neurogenic vasoconstriction in the eye.     

Effects of AGN 192024, a new ocular hypotensive agent, on aqueous dynamics

PURPOSE: To determine the mechanism of intraocular pressure lowering for the Ocular Hypotensive Lipid, AGN 192024 (Allergan, Inc, Irvine, California). METHODS: Twenty-five normal human volunteers between the ages of 21 and 48 took part in a randomized, double-masked, placebo-controlled, paired-comparison study in which intraocular pressure, aqueous humor flow, and tonographic resistance to outflow were studied. Measurements of aqueous flow were made during the day and at night while subjects slept. Intraocular pressure was measured with the Goldmann tonometer, and resistance to outflow was measured by electronic recording Schi?tz tonography. RESULTS: Intraocular pressure was decreased by 20% on day 3 in AGN 192024-treated eyes in comparison with placebo-treated eyes in normal subjects (P <.001). Aqueous humor flow was stimulated 13% during the day (P =.007) and 14% at night (P =.014) by the drug. Tonographic resistance to outflow was decreased 26% by AGN 192024 (P <.001), and apparent resistance to outflow (the ratio of intraocular pressure to aqueous flow) was decreased 31% (P <.001). Assuming that AGN 192024 does not cause prolonged lowering of episcleral venous pressure, the results show that pressure-insensitive outflow is enhanced by 50%, whereas tonographic facility of outflow (reciprocal of resistance) was enhanced 35%. CONCLUSIONS: AGN 192024 is an ocular hypotensive agent that works by enhancing both pressure-sensitive and pressure-insensitive aqueous humor outflow without diminishing aqueous humor formation.     

Topical vanadate lowers intraocular pressure in rabbits

In unanesthetized rabbits the topical application of vanadate lowered intraocular pressure. Tonographic outflow facility and episcleral venous pressure were unaltered by topical vanadate. As estimated from the tonographic data, aqueous humor flow was reduced by approximately 30%. Posterior chamber aqueous humor ascorbate increased in the eye receiving topical vanadate, and this was compatible with a decreased rate of aqueous humor flow. Topical vanadate did not alter anterior chamber aqueous humor protein or cyclic AMP. In five monkeys intraocular pressure was also significantly reduced by topical vanadate.     

Trabecular outflow facility determined by fluorophotometry in human subjects

We studied a new fluorophotometric method to determine trabecular outflow facility in man. Pilocarpine was selected to evaluate this non-invasive method, since its primary ocular hypotensive effect is to increase trabecular outflow facility. Levobunolol was used to decrease intraocular pressure and aqueous humor flow required for the calculation of trabecular outflow facility. In ten ocular hypertensive patients trabecular outflow facility in the pilocarpine-treated eyes was 0.42 +/- 0.16 (S.D.) microliters min-1 mmHg-1 compared with 0.17 +/- 0.14 microliters min-1 mmHg-1 in the vehicle-treated eyes (P less than 0.004). An ocular hypotensive additivity was also found in the eyes treated with both drugs. It was concluded that this non-invasive fluorophotometric method may be useful in evaluating the effect of a drug on trabecular outflow facility.     

The prostanoid EP2 receptor agonist butaprost increases uveoscleral outflow in the cynomolgus monkey

PURPOSE: To investigate the ocular hypotensive effect of the prostanoid EP2 receptor agonist butaprost and to establish its mechanism of action. METHODS: All experiments were performed in cynomolgus monkeys after topical application of butaprost (0.1%). The effects of butaprost on aqueous humor flow were determined by fluorophotometry. Total outflow facility was measured by the two-level, constant-pressure perfusion method, and uveoscleral outflow was determined by perfusion of FITC-labeled dextran through the anterior chamber. Effects on ocular morphology were studied after tissue fixation with transcardial perfusion by paraformaldehyde and immersion fixation of the globe, in animals subjected to long-term treatment with butaprost. Conscious ocular normotensive monkeys and monkeys with unilateral ocular hypertension were used for intraocular pressure (IOP) studies. RESULTS: Butaprost had no significant effect on aqueous humor flow or total outflow facility in ocular normotensive monkeys. Uveoscleral outflow was significantly higher in the butaprost treated eyes than in vehicle treated eyes, 1.03 +/- 0.20 vs. 0.53 +/- 0.18 microL.min(-1). After a 1-year treatment with butaprost, the morphology of the ciliary muscle was changed, showing increased spaces between ciliary muscle bundles and the apparent formation of new outflow channels. In many instances, changes were observed in the trabecular meshwork as well. Butaprost, in a single 0.1% dose, decreased IOP significantly in ocular normotensive monkeys and reduced IOP in laser-induced glaucomatous monkey eyes to the same level as that in the ocular normotensive contralateral eyes. CONCLUSIONS: The prostanoid EP2 receptor agonist butaprost appears to lower IOP by increasing uveoscleral outflow, according to both physiological and morphologic findings. Although the prostanoid EP2 receptor is structurally and functionally distinct from the FP receptor, the effects of EP2 and FP receptor stimulation on aqueous humor outflow are similar.     

A fluorophotometric study on the aqueous humor dynamics in primary open angle glaucoma

One hundred and ten eyes in 40 normal persons with no ocular pathology, 20 with ocular hypertension and 50 with primary open angle glaucoma were studied. The value of the aqueous humor flow was obtained and then compared with the different samples and with general (age, sex) and ocular factors (intraocular pressure, tonographic outflow facility, campimeter and disk/cup ratio, among others.     

Effects of topical epinephrine on aqueous humor dynamics in the cat

The purpose of this study was to investigate, in cats, the effects of topical epinephrine on aqueous humor dynamics as measured by the non-invasive method of fluorophotometry and by other methods. Measurements were carried out on 12 cats before and after one week of twice daily treatment with 2% epinephrine hydrochloride to one eye. Aqueous flow and outflow facility were determined using fluorophotometry. Uveoscleral outflow was calculated from these results and was evaluated with anterior chamber perfusion of FITC-dextran. Outflow facility also was measured by tonography. Epinephrine-treated eyes, compared with their baseline values, showed a 31% reduction in intraocular pressure (P<0.001), a 23% reduction in aqueous flow (P<0.05), a 60% increase in fluorophotometric outflow facility (P<0.05), and a 43% increase in tonographic outflow facility (P<0.05). Treated eyes, compared with contralateral control eyes, showed a 27% reduction in IOP (P<0.005), a 25% reduction in aqueous flow (P<0.005), a 38% increase in fluorophotometric outflow facility (P<0.05), and a 34% increase in tonographic outflow facility. When evaluated by both fluorophotometry and FITC-dextran tracer methods, epinephrine had no significant effect on uveoscleral outflow. It was concluded that, in cats treated with topical epinephrine twice daily for a week, a reduction in intraocular pressure is induced by an increase in outflow facility and decrease in aqueous flow.     

Iloprost, a stable prostacyclin analog, reduces intraocular pressure

Topical application of Iloprost caused a dose-dependent decrease in intraocular pressure (IOP) in rabbits and ocular hypertensive beagles. In rabbits, the IOP response was biphasic and miosis was observed. In beagles, there was no initial hypertensive phase, and the fall in IOP was more pronounced (up to 37%). In beagles, Iloprost did not influence pupillary diameter. A mild transient hyperemia was noted in both rabbit and beagle eyes. Iloprost led to an increase in the aqueous humor protein concentration in rabbits but not in beagles. The use of artificial tears as vehicle enhanced the effect on intraocular pressure but also aqueous protein in rabbits. The central corneal temperature was increased after application of Iloprost in both rabbits and beagles. In rabbits, tonography revealed an increase in outflow facility during both the hypertensive and the hypotensive phases. Iloprost caused a decrease in mean arterial pressure in beagles; the effect on pulse rate was inconsequential. It is suggested that similar low doses of an analog of Iloprost or carboprostacyclin that does not affect the hemodynamic equilibrium could be of value in the treatment of glaucoma.     

New perspectives in aqueous humor secretion and in glaucoma: the ciliary body as a multifunctional neuroendocrine gland

The discovery in the human ocular ciliary body of glaucoma-associated genes (i.e., MYOC, CYP1B1), neuroendocrine processing enzymes, neuroendocrine peptides, steroid-converting enzymes, glutamate transporters, glutamate-metabolizing enzymes, and anti-angiogenic factors requires a reevaluation of its function on aqueous humor secretion, intraocular pressure and its role in glaucoma. The ciliary body should be considered as a multifunctional and interactive tissue. The intrinsic hypotensive and/or hypertensive biological activities of many of the endocrine peptides released by the ciliary epithelium are best explained within the context of a neuroendocrine system, linking the inflow and the outflow of aqueous humor. This interpretation is consistent with physiological and genetic studies indicating that changes altering the inflow affects intraocular pressure. In the proposed endocrine system, regulatory peptides secreted by the ciliary epithelium may subserve multiple functions in the following: inflow and outflow pathways of aqueous humor, ciliary blood flow, the immune privilege status of the anterior segment and the diurnal circadian rhythms of aqueous humor secretion and intraocular pressure. These previously unsuspected and challenging functions of the ciliary epithelium should be considered when assessing the multifactorial events which lead to the pathophysiology of glaucoma affecting the outflow pathways of aqueous humor. This review highlights published, and ongoing studies on authors' labs supporting neuroendocrine, steroidogenic and glutamatergic features of the ciliary epithelium and the endocrine communication between the inflow and outflow pathways of aqueous humor. We also discuss how glaucoma-associated genes expressed in the ciliary body and their mutant proteins could influence intraocular pressure, contributing to the development of glaucoma.     

The effect of betablockers with and without ISA on tonographic outflow facility

Summary The response of two ophthalmic betablockers, Timolol (without ISA) and Pindolol (with marked ISA), on IOP and tonographic outflow facility was investigated in a single-blind clinical study on 20 patients with glaucoma or ocular hypertension. In two treatment groups, ten patients each, in a randomized order IOP and tonographic outflow facility measurements were performed before and 2 hrs after drug application. Timolol reduced IOP by 6,8 mm Hg and Pindolol eye drops by 4,5 mm Hg. Both topically applied betablockers did not influence tonographic facility of outflow. It is concluded that the intrinsic sympathomimetic activity of an ophthalmic betablocker has no effect on outflow facility of aqueous humor.     

Effect of topically applied forskolin on aqueous humor dynamics in cynomolgus monkey

Topical administration of a 1% forskolin suspension significantly reduced intraocular pressure in cynomolgus monkey eyes. The fall in intraocular pressure was associated with a significant (P less than 0.01) decrease in aqueous humor flow measured by a fluorophotometric technique. No significant change was found in tonographic outflow facility or pupillary diameter. A loss of effect on intraocular pressure to subsequent doses of 1% forskolin suspension occurred in cynomolgus monkeys by the third day of twice-a-day treatment.     

Correlation of aqueous humor ascorbate with intraocular pressure and outflow facility in hereditary buphthalmic rabbits

The hypotensive effect of ascorbate on intraocular pressure has been reported following topical application, oral administration, or anterior chamber infusion in animals. The present report describes the correlation of aqueous humor ascorbate concentration with intraocular pressure as well as outflow facility in vivo. Low aqueous ascorbate level was seen in buphthalmic eyes with high intraocular pressure and low outflow facility. The opposite correlation was observed in normal eyes. Ascorbate concentration in the anterior chamber of the rabbit eye is apparently related to the alteration of outflow facility and the movement of fluid in the anterior chamber.     

Effects of topical nipradilol, a beta blocking agent with alpha blocking and nitroglycerin-like activities, on intraocular pressure and aqueous dynamics in humans

AIMS: To study the effects of topical nipradilol, a non-selective beta blocker with alpha blocking and nitroglycerin-like activities, on intraocular pressure (IOP) and aqueous humour dynamics in normal humans and in patients with ocular hypertension. METHODS: Nipradilol (0.06%, 0.125%, 0.25%, 0.5%) was applied to normal volunteers (n = 12) to test for IOP lowering effects. In a second group of normal volunteers (n = 11), nipradilol (0.125% and 0.25%) and timolol (0. 5%) were compared for IOP lowering effects. After a single administration of 0.25% nipradilol, IOP, flare intensity in the anterior chamber, aqueous flow, uveoscleral outflow, tonographic outflow facility, and episcleral venous pressure were either directly measured or mathematically calculated. Topical nipradilol (0.25%) was administered to 24 patients with ocular hypertension twice daily for 8 weeks. RESULTS: Administration of 0.25% nipradilol decreased IOP with a maximum reduction of 4.2 mm Hg lasting 12 hours. A single instillation of both 0.25% nipradilol and 0.5% timolol reduced the IOP in normotensive human subjects to the same degree. A single instillation of 0.25% nipradilol decreased the aqueous flow rate in the treated eye by 20%. Nipradilol produced no significant effect in tonographic outflow facility or episcleral venous pressure, but uveoscleral outflow was increased. In patients with ocular hypertension, twice daily instillation of 0.25% nipradilol decreased IOP without tachyphylaxis for the 8 week test period. CONCLUSION: Topical nipradilol (0.25%) reduced IOP by decreasing the aqueous flow rate and probably also by increasing uveoscleral outflow. Nipradilol should be further investigated as a new antiglaucoma drug.     

The ocular hypotensive effects of demeclocycline, tetracycline and other tetracycline derivatives

Demeclocycline, tetracycline and other tetracycline derivatives lowered intraocular pressure (IOP) in rabbits following intravitreal injection, but the onset of this effect was not evident until 1 or more days after drug administration. Of the drugs tested, demeclocycline was the most active ocular hypotensive agent. Demeclocycline caused a dose-dependent decrease in IOP. The maximum IOP decrease of approximately 12 mm Hg occurred 5 days after intravitreal administration of 0.5 mg, with the effect persisting for over a week. Demeclocycline did not alter tonographically measured aqueous humor outflow facility or episcleral venous pressure. Based on calculated aqueous humor flow rates following 0.2 mg demeclocycline, a 62% decrease in aqueous humor formation occurred 7 days after intravitreal injection. The flow-to-diffusion ratio for ascorbate was reduced 54% 6 days after the intravitreal administration of demeclocycline, a change also consistent with suppression of aqueous humor formation. Anterior chamber aqueous humor protein concentration was increased 6 days after demeclocycline administration. No histologic changes were present in the treated eyes by light microscopy. Intravitreal demeclocycline similarly lowered IOP in cats, with the duration of effect lasting up to 20 days.     

Ocular hypotensive action of ergoline derivatives in rabbits: effects of sympathectomy and domperidone pretreatment

Ergot derivatives, such as bromocriptine, lergotrile and pergolide, are potent dopaminergic agonists in various biological systems. In topical doses ranging from 0.001 to 1% applied unilaterally, each agent produced dose-related ocular hypotension in normal rabbits. Utilizing an intraocular pressure (IOP) recovery rate method (aqueous formation index), each agent was observed to suppress the recovery rate of IOP in normal rabbits. Pretreatment with a prejunctional dopamine receptor antagonist (domperidone) inhibited the ocular hypotensive effect of bromocriptine and pergolide more than that of lergotrile. In rabbits with unilateral superior cervical ganglionectomies, IOP was lowered appreciably less by these compounds in the denervated eyes. These studies indicate that: a) lowering of IOP by these ergot derivatives is dependent, in part, on suppression of sympathetic neuronal activity; b) the most probable sites of action are DA2 receptors on sympathetic nerve endings or in sympathetic ganglia; c) ocular hypotension is produced, in part, by suppressing aqueous humor formation.