Intraocular Pressure In Rabbits Research Articles

The Ocular Blood Flow Tonograph: A New Instrument for the Measurement of Intraocular Pressure in Rabbits

The Ocular Blood Flow Tonograph (OBFT) and Digilab Pneumatonograph (PTG) were calibrated for use on New Zealand White rabbits using a pressure manometer and performing tonometry at varying known pressures. The calibration experiments showed that a good correlation exists between OBFT measurements and manometric readings of IOP. Moreover, no difference between open and closed stopcock measurements were observed. Corrective equations for conversion of OBFT readings to mm Hg are documented. The OBFT was compared with the PTG and the results showed there was no significant difference between the IOP measured by either instrument in a group of rabbits displaying a range of different IOPs. Regression analysis of the data gave a slope of 0.88 and a correlation co-efficient of 0.78. The OBFT proved easier to use than the PTG under restricted lighting conditions. The OBFT was also used to measure the circadian variation in IOP and to measure the IOP after the topical application of prostaglandin E 2and various serotonergic agonists. The combined data demonstrated that the OBFT is an accurate and reliable tonometer for the measurement of IOP in rabbits.

Ocular hypotensive mechanism of topical isopropyl unoprostone, a novel prostaglandin metabolite-related drug, in rabbits.

This study was performed to clarify the ocular hypotensive mechanism of topical isopropyl unoprostone (unoprostone), a novel prostaglandin (PG) metabolite-related drug, in the rabbit eye. The intraperitoneal administration of indomethacin (50 mg/kg) 1 hour before administration of topical 0.12% unoprostone partially diminished the intraocular pressure (IOP) reduction, and completely blocked the increase in aqueous PGE2 concentration caused by unoprostone. Aqueous humor dynamics measurements in the unoprostone- and the vehicle-treated contralateral eyes with indomethacin pretreatment revealed that aqueous flow determined by fluorophotometry was not significantly different, 2.3 +/- 0.3 and 2.4 +/- 0.2 microliters/min, respectively; the total outflow facility measurements determined by the two-level constant pressure perfusion method were 0.20 +/- 0.01 and 0.14 +/- 0.01 microliters/min/mmHg, respectively (p < 0.05); the uveoscleral outflow measurements determined by the fluorescein isothiocyanate-dextran perfusion method were 0.49 +/- 0.02 and 0.46 +/- 0.02 microliters/min, respectively (p < 0.05). The magnitude of the IOP reduction induced by unoprostone was estimated to be 5.2 mmHg, which agrees well with the actual IOP reduction. In conclusion, unoprostone lowers the IOP by affecting aqueous outflow pathways, primarily the pressure-dependent conventional pathway and the secondary uveoscleral outflow pathway in rabbits. Endogenous PGs induced by topical unoprostone are also involved in lowering the IOP in rabbits.

Comparative Efficacy of the SLT versus LASAG Nd

The efficacy of two types of contact transscleral Nd:YAG laser delivery systems, a synthetic sapphire-tipped laser probe connected to a Surgical Laser Technologies (SLT) laser and a quartz fiberoptic tip connected to a LASAG Microrupter III, was compared. A total of nine pigmented rabbits underwent transscleral LASAG cycloablation in one eye (3.2 W, 0.5 s); an additional eight rabbits underwent Surgical Laser Technologies cycloablation (3.0 W, 0.5 s). The maximum difference in intraocular pressure between the experimental versus fellow eyes was - 15.09 mm Hg (+/-0.25) with the LASAG and -13.16 mm Hg (+/-3.12) with the SLT laser; in both delivery systems, this difference was achieved 4 days postoperatively. The differences between the two groups in duration of pressure reduction was not statistically significant (p = 0.83). These data suggest that these delivery systems are equally effective in lowering intraocular pressure in rabbits. Histological changes of the ciliary processes and adjacent ocular tissue are presented.

The ocular hypotensive effect of the ETB receptor selective agonist, sarafotoxin S6c, in rabbits.

Endothelin-1 (ET-1) is known to affect intraocular pressure (IOP) in rabbits, and the IOP response is likely to be mediated by the receptors ETA, ETB, or both. Sarafotoxin S6c (STX-S6c) is a selective agonist to ETB receptors. The authors attempted to clarify the role of ETB receptors in changes in IOP induced by ET-1 in rabbits using STX-S6c and to determine the relationship between the IOP response and various doses of STX-S6c. Each concentration (10(-4) to 10(-7) M) of STX-S6c was injected intravitreally (20 microliters/eye) into one eye. The contralateral eye of each was used as a control. The IOP was measured periodically using a calibrated pneumatonometer. Indomethacin (50 mg kg-1) or vehicle (10 ml kg-1; 0.05 M phosphate buffer) was administered intraperitoneally twice, before and after intravitreal injection of STX-S6c (10(-5) M), and IOP was measured in the same protocol for 24 hours. In the STX-S6c (10(-4) and 10(-5) M) group, the IOP reduction was significant compared with the baseline (P < 0.05 to P < 0.01), starting from 6 and 4 hours and continuing until 192 and 72 hours after injection, respectively. A solution of 10(-6) M STX-S6c also resulted in significant reduction of IOP observed from 24 to 72 hours after injection (P < 0.05). The 10(-7) M solution of STX-S6c failed to affect IOP. The area under the curve of IOP response exhibited a significant correlation with the doses of STX-S6c (r = -0.856; P = 0.0001) in the treated eyes. Treatment with indomethacin failed to affect the IOP reduction caused by STX-S6c (10(-5) M). Ciliary injection and some dilatation of the iridial vessels were observed in eyes that received higher doses of STX-S6c. STX-S6c reduces the IOP in rabbits in a dose-dependent fashion, presumably mediated through the ETB receptors.

Selective prostaglandin D 2 receptor stimulation elicits ocular hypotensive effects in rabbits and cats

The effects of the selective prostaglandin D 2 (DP) receptor agonists, 572C85 ((±)-5-(3-carboxypropylthio)-1-(2-cyclohexyl-2-hydroxyethyl-amino)hexahydrocyclopenta( d)imidazol-2(1 H)-one) and 192C86 ((±)-5-(3-carboxypropylthio)-1-(2-cyclohexyl-2-hydroxyethylidene-amino)-3-ethylhexahydrocyclopenta( d)imidazol-2(1 H)-one), were determined on intraocular pressure regulation in rabbits and cats. 572C85 (50 μg) in rabbits maximally lowered intraocular pressure by 4.3 mm Hg, and significantly for 4 h compared to control. In cats 572C85 had a similar effect. 192C86 (50 μg) reduced intraocular pressure by 2.8 mm Hg for 2 h in rabbits. Following exposure to the specific DP receptor antagonist, BW A868C ((±)-3-benzyl-5-(6-carboxyhexyl)-1-(2-cyclohexyl-2-hydroxyethylamino)-hydantoin; 50 μg), which had no effect on intraocular pressure by itself, 572C85 (50 μg) did not reduce intraocular pressure in rabbits and cats. The intraocular pressure lowering effect of the mixed DP and EP receptor agonist, BW245C (5-(6-carboxyhexyl)-1-(3-cyclohexyl-3-hydroxypropyl)-hydantoin; 50 μg), in cats was suppressed by only 64% by BW A868C (50 μg). These results clearly show that the DP receptors in rabbit and cat eyes are involved in intraocular pressure regulation. However, under baseline conditions DP receptor activity does not contribute to this regulation.

Suprachiasmatic nucleus in the neural circuitry for the circadian elevation of intraocular pressure in rabbits.

Intraocular pressure (IOP) in light-dark entrained rabbits changes in a circadian pattern. Previous studies indicate that signals for the circadian IOP elevation in the early dark phase come from the central nervous system (CNS) via the ocular sympathetic nerves. Its neural circuitry in the CNS was totally unclear. In the present study, the role of the suprachiasmatic nucleus in the circuitry was investigated. Adult New Zealand albino rabbits were entrained in a daily 12 hr/12 hr light-dark cycle, and their circadian IOP elevations around the onset of dark were determined. Bilateral lesions of the suprachiasmatic nuclei were made by heat probe and verified by histology. Change in the circadian IOP elevation was monitored for up to 3 months. After the lesion, there was a significant reduction of the circadian IOP elevation for 2 weeks. Elevations in the plasma concentration of melatonin and the aqueous humor concentration of norepinephrine (NE) in the early dark phase were significantly reduced. When exposed to constant light, the residual circadian IOP elevation in the postoperative rabbits was further reduced to the level seen in intact rabbits exposed to constant light. In postoperative weeks 6-10, the circadian IOP elevation returned to the initial value. A full, inhibitory effect of constant light on the circadian IOP elevation was observed. In postoperative months 2-3, concentrations of plasma melatonin and aqueous humor NE regained the circadian pattern. These results indicate that the suprachiasmatic nucleus is in the neural circuitry which propagates the circadian elevation of IOP in rabbits.

Short-wavelength light reduces circadian elevation of intraocular pressure in rabbits

In light-dark entrained rabbits, the intraocular pressure (IOP) increases around the onset of dark due to the increased activities of ocular sympathetic nerves. This circadian elevation of IOP can be eliminated by exposing the rabbits to constant white light in the early subjective dark phase. The spectral effectiveness of light causing the reduction in IOP was studied using lights selectively filtered through various optic filters. Light which passed through a long-pass filter, with a cut-on wavelength at 570 nm or 530 nm, caused no change of the circadian IOP elevation. A significant reduction of the IOP elevation was observed with filtered light via the 495 nm long-pass filter and, to a larger extent, with light via the 475 nm long-pass filter. Short-wavelength light (380–480 nm, peak at 432 nm) passed through a band-pass filter completely eliminated the circadian IOP elevation. The aqueous humor concentration of norepinephrine (NE) under the short-wavelength light was significantly lower than the NE concentration under the filtered light via the 530 nm long-pass filter. Sensation of the short-wavelength light plays an important role in synchronizing the circadian elevation of IOP in rabbits.

Focal Scleral Permeability Enhanced by Collagenase Digestion: Experimental Model of Enzymatic Sclerostomy

The purpose of this study is to demonstrate that restricted application of clostridial collagenase to the animal sclerolimbal area produces increased tissue permeability in vitro and results in a lasting significant decrease in the intraocular pressure (IOP) in vivo. A special miniature device was developed to apply the carefully characterized enzyme to a small (1-mm diameter) and defined sclerolimbal area isolated from the surrounding tissue. The device was applied to the rabbit limbus for 4 hours and then removed. The enzyme was inactivated and washed out with an inhibitory solution. The IOP was measured by pneumotonometry, and the treated tissue was investigated by light and electron microscopy. Application and removal of the drug-delivery microapplicator was performed under topical anesthesia. Enzymatic degradation propagated without lateral spreading and reached at least two thirds of tissue width. Results of histologic examination of the treated area showed a thin tissue, characterized by degraded and disorganized collagen fibers, which were sharply separated from the intact fibers in the untreated area. This was sufficient to cause a tenfold increase in the basic transscleral flow in vitro, and a significant, relatively long-lasting (> 30 days) lowering of the mean IOP in 15 albino rabbits. In the first 10-day period after treatment, the IOP was reduced by 40%, and during the rest of the follow-up the IOP remained lowered by more than 20% (P > 0.0001) without any adjuvant treatment. Despite the small size of the collagenase-affected tissue, the evacuation rate of the aqueous humor was sufficient to cause a significant and relatively long-lasting lowering of IOP in rabbits. This mild treatment, performed under topical anesthesia, might present the basis for the development of an ambulatory treatment of human glaucoma.

Selective effects of retinal dopamine depletion on partial ischemia-induced electroretinographic hyperresponses in rabbits.

The interaction of retinal dopamine depletion and partial ischemia on the a- and b-wave amplitudes and implicit times of the electroretinogram was examined in adult pigmented rabbits. Seven days after 6-hydroxydopamine treatment, which resulted in a depletion of the amine, partial retinal ischemia was induced by raising the intraocular pressure. As expected, moderate elevation of intraocular pressure produced increases in both a- and b-wave amplitudes. Amplitude hyperresponses were significantly reduced in dopamine-depleted retinas. These reductions were more prominent with relatively lower intensities. However, response delays were not shortened but lengthened by 6-hydroxydopamine pretreatment. Together, these results point to a selective role of dopamine in partial retinal ischemia induced by moderate elevation of intraocular pressure in rabbits.

Neuromodulatory effect of sulprostone on the circadian elevation of intraocular pressure in rabbits.

Topical application of sulprostone, a preferential prostaglandin EP3 receptor agonist, caused a dose-dependent reduction of the circadian elevation of intraocular pressure (IOP) in New Zealand albino rabbits which were entrained to 12-hr/12-hr light-dark environment. Corresponding to the effect on IOP, 0.2 micrograms, 2 micrograms, and 20 micrograms sulprostone decreased the norepinephrine (NE) concentration in the aqueous humor in the dark phase. There was no breakdown of the blood-aqueous barrier. Bilateral applications of 2 micrograms and 20 micrograms sulprostone to entrained rabbits that had undergone unilateral, preganglionic transection of the cervical sympathetic trunk reduced the circadian IOP elevation in the intact eye, but caused little IOP change in the decentralized eye. These results indicate that the IOP-lowering effect of topical sulprostone in rabbits is dependent on sympathetic neural activity and that prejunctional inhibition of NE release may be an important mechanism of action.

Further studies on ocular responses to DP receptor stimulation

Prostaglandin D 2 (PGD 2) and the selective DP receptor agonist BW 245C have been previously shown to lower intraocular pressure in rabbits, while PGD 2, but not BW 245C, caused plasma extravasation, eosinophil infiltration, and goblet cell depletion. In these present studies definition of the ocular pharmacology of prostaglandin D 2 (PGD 2) has been extended by using a further selective DP receptor agonist SQ 27986 and a potent and selective DP receptor antagonist BW A868C. In cats and rabbits SQ 27986 caused ocular hypotension. The ocular hypotensive effect of PGD 2 in rabbits was blocked by pretreatment with the DP receptor antagonist BW A868C, whereas the activities of PGE 2 and PGF 2α remained unaltered. THe singular involvement of the DP receptor in changes in rabbit intraocular pressure evoked by PGD 2 was thereby verified by using the antagonist BW A868C. In terms of effects on the ocular surface, SQ 27986 caused no increase in conjunctival microvascular permeability, no eosinophil infiltration, and no depletion of the goblet cell population. These findings reinforce the concept that selective DP receptor agonists may be useful for lowering intraocular pressure without causing ocular surface pathology. PGD 2 induced increases in conjunctival microvascular permeability were inhibited by BW A868C, despite the fact that DP receptor agonists failed to evoke a plasma exudation response. This finding was unexpected and suggests a possible subdivision of the DP receptor designation.

Acetazolamide-like carbonic anhydrase inhibitors with topical ocular hypotensive activity

New carbonic anhydrase (EC 4.2.1.1) inhibitors were synthesized as potential drugs for the topical treatment of glaucoma. They were obtained by substituting the acetyl group of acetazolamide and methazolamide with bicarboxylic acids of different chain length (C4-C6). The terminal carboxyl was either kept free or esterified with alcohols of different size (C1-C12). A gamma-aminovaleric derivative was also prepared. All compounds proved active as carbonic anhydrase inhibitors in vitro, with an average IC50 of about 0.5 microM. Some proved also to be topically active in vivo in lowering the artificially elevated intraocular pressure in rabbits. The most active compound, carrying a succinic acid side chain, is the most soluble in aqueous buffers. Its duration of action is about 8 h and it is under evaluation as a topical antiglaucoma drug. It is hypothesized that the duration of action could be longer in compounds having both the same high water solubility and partition coefficient.

Effects of topical ethacrynic acid adducts on intraocular pressure in rabbits and monkeys.

We evaluated the effect of topical ethacrynic acid on rabbit and monkey intraocular pressure. In a preliminary experiment, 100-mmol/L ethacrynic acid applied topically to Dutch-Belted rabbit eyes was associated with an 8-mm Hg lowering of intraocular pressure. However, corneal edema was severe, and the corneal epithelium sloughed off. To try to maintain the pressure-lowering effect but reduce the corneal side effects, we attempted to create an adduct of ethacrynic acid by utilizing ethacrynic acid's sulfhydryl reactivity. Ethacrynic acid was mixed with equimolar cysteine to bind the sulfhydryl-reactive sites on ethacrynic acid. The goal was to expose the cornea to adducted ethacrynic acid, which might then dissociate in the anterior chamber via a retro-Michael reaction. Intraocular pressure decreased 8.9 mm Hg (n = 40) with this treatment, and corneal edema was lessened (32 of 40 eyes had mild to no edema). However, we observed that when the eye was treated before ethacrynic acid-cysteine administration with topical acetylcysteine, the corneal side effects were reduced further and the intraocular pressure effect remained. In living cynomolgus monkeys receiving a single pretreatment drop of 75-mmol/L acetylcysteine followed by two drops of 130-mmol/L ethacrynic acid and 130-mmol/L cysteine, an intraocular pressure lowering of 9.9 mm Hg was observed (n = 7). However, in three of seven eyes corneal edema developed. Pretreatment with two drops of acetylcysteine eliminated the pressure-lowering effect but did not confer any added corneal protection. Our results indicate that topical ethacrynic acid-cysteine is effective in lowering the intraocular pressure of rabbits and cynomolgus monkeys and, when combined with acetylcysteine pretreatment, may offer the potential for a new topical therapeutic regimen for use in glaucoma.

A comparative study between cod liver oil and liquid lard intake on intraocular pressure on rabbits

Cod liver oil administered intramuscularly (0.2 ml/day) lowered intraocular pressure (IOP) of the rabbit from 21 mmHg to 18 ± 0.4 mmHg (p < 0.05 n = 8). A higher dose of cod liver oil (1 ml) further lowered intraocular pressure to 14.5 ± 0.3 mmHg which remained at this level for up to 80 days when the rabbits were sacrificed. Intramuscular injections of liquid lard did not alter IOP in rabbits. When treatment with cod liver oil was stopped, IOP rose to baseline levels. Topical treatment with cyclooxygenase inhibitors had no effect on the IOP lowering effect of cod liver oil. Topical treatment of rabbit eyes with 1% aspirin solution twice daily (up to 28 days) and flurbiprofen t.i.d. (up to 15 days) caused no alteration in the decrease in IOP seen with intramuscular cod liver oil treatment for up to 28 and 15 days respectively.

A submicron emulsion as ocular vehicle for delta-8-tetrahydrocannabinol: effect on intraocular pressure in rabbits.

delta 8-Tetrahydrocannabinol (delta 8-THC), a known antiglaucoma lipophilic drug, was incorporated in a submicron emulsion for ocular administration. The mean droplet size of the emulsion was 130 +/- 41 nm, and no droplet was larger than 400 nm. No change in pH, particle size distribution or zeta potential was noted after sterilization by steam autoclaving or long-term storage over 9 months. An intense and long-lasting intraocular pressure (IOP)-depressant effect was observed after ocular application (50 microliters) of the THC emulsion, 0.4% (w/w), to rabbits with ocular hypertension (chymotrypsin model). Lesser effects were observed in normotensive rabbits. No irritation effect of either the emulsion vehicle or THC emulsion on the rabbit eyes was detected. These results underline the promising properties of submicron emulsions as vehicles for lipophilic ophthalmic drugs. The mechanism by which the emulsion induced the marked delta 8-THC antiglaucoma effect remains unclear. However, the possible involvement of delta 8-THC systemic absorption in the hypotensive effect induced by the emulsion cannot be excluded and will be the subject of further investigation.

Effect of the application of acetazolamide soaked contact lenses on intraocular pressure of rabbits

Topical application of acetazolamide has no known effect on intraocular pressure (IOP). We tried to detect the hypotensive effect on IOP of acetazolamide soaked onto soft contact lenses (CL). We applied CLs soaked in either 1%, 3%, or 5% acetazolamide solution onto one eye of 29 rabbits while the contralateral eye served as a control. There was an average 32% reduction of IOP amongst all acetazolamide applied eyes, and an average 19% reduction of IOP amongst all control eyes. Amongst the 1% acetazolamide-CL applied eyes there was a mean 37% reduction of IOP, amongst the 3% acetazolamide-CL applied eyes a mean 36% reduction, amongst the 5% acetazolamide-CL applied a mean 30% reduction, and a mean 19% reduction in control eyes. The longest period of IOP reduction followed the application of 1% acetazolamide-CLs, probably owing to improved drug corneal penetration at this concentration. Our results reveal that the application of acetazolamide soaked soft CLs has a significant hypotensive effect on IOP in both the applied and contralateral control eyes of rabbits.

Effects of new clonidine derivatives on rabbit intraocular pressure

AbstractEffects of 25 topically applied clonidine derivatives on rabbit IOP recovery model using intravenous hypertonic saline were studied. Among them, 12 compounds were dissolved in aqueous medium, 11 compounds in polyethylene glycol (PEG 200) and 2 compounds in dimethyl sulfoxide (DMSO). Four compounds showed marked delay in IOP recovery and were more active than S‐timolol and clonidine. Five compounds were about equiactive to S‐timolol and more active than clonidine in suppression of IOP recovery and prolongation of the duration of drug action. There were three compounds that were less active than S‐timolol yet they were still as active or were active than clonidine. Thirteen out of 25 compounds were inactive in suppressing IOP recovery curves. Most of these active compounds (9 out of 12) suppressed IOP recovery in the same magnitude for both treated and non‐treated contralateral eyes, indicating that these effects were manifested though central actions. Three out of 12 compounds showed more effect with treated eyes than with nontreated contralateral eyes, indicating that the local effects were predomiant.

Effects of the R(+)- and S(−)-isomers of β-adrenoceptor blockers with intrinsic sympathomimetic activity, befunolol and carteolol, on rabbit intraocular pressure

Effects of the R(+)- and S(-)-isomers of befunolol and carteolol, beta-adrenoceptors with intrinsic sympathomimetic activity, on the rabbit intraocular pressure were tested. The intraocular pressure was decreased by instillation of the R(+)- and S(-)-isomers of befunolol (0.1 and 0.3%) and of carteolol (1.0%) to the eye and attained the minimum level at 60 min. However, 0.3% of the R(+)- and S(-)-isomers of carteolol did not influence the pressure. The corresponding time courses for the intraocular pressure for the R(+)- and S(-)-isomers did not differ, suggesting that in the treatment of glaucoma, the therapeutic advantage of the R(+)-isomers of befunolol and carteolol may be similar to the S(-)-isomers.

Aqueous humor cyclic AMP and circadian elevation of intraocular pressure in rabbits.

Rabbits entrained in a daily light-dark environment show a circadian elevation of intraocular pressure (IOP) around the onset of dark. It was reported that concentration of cyclic AMP (cAMP) in aqueous humor increases significantly during this time period. Whether or not the increase of ocular cAMP-mediated activities is related to the circadian elevation of IOP is unclear. Light-dark entrained rabbits maintain the circadian IOP elevation in a constant dark environment. Corresponding to this IOP elevation in the constant dark, no change of aqueous humor cAMP concentration was found. Thus, the circadian IOP elevation in the constant dark is probably unrelated to the cAMP-mediated activities. This finding also suggests that a significant portion of the circadian IOP elevation in the light-dark environment is unrelated to the cAMP-mediated activities.

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.