Keith Green

Neuroprotective Effect of(−)Δ9-Tetrahydrocannabinol and Cannabidiol in N-Methyl-d-Aspartate-Induced Retinal Neurotoxicity : Involvement of Peroxynitrite

In glaucoma, the increased release of glutamate is the major cause of retinal ganglion cell death. Cannabinoids have been demonstrated to protect neuron cultures from glutamate-induced death. In this study, we test the hypothesis that glutamate causes apoptosis of retinal neurons via the excessive formation of peroxynitrite, and that the neuroprotective effect of the psychotropic Delta9-tetrahydroxycannabinol (THC) or nonpsychotropic cannabidiol (CBD) is via the attenuation of this formation. Excitotoxicity of the retina was induced by intravitreal injection of N-methyl-D-aspartate (NMDA) in rats, which also received 4-hydroxy-2,2,6,6-tetramethylpiperidine-n-oxyl (TEMPOL,a superoxide dismutase-mimetic), N-omega-nitro-L-arginine methyl ester (L-NAME, a nitric oxide synthase inhibitor), THC, or CBD. Retinal neuron loss was determined by TDT-mediated dUTP nick-end labeling assay, inner retinal thickness, and quantification of the mRNAs of ganglion cell markers. NMDA induced a dose- and time-dependent accumulation of nitrite/nitrate, lipid peroxidation, and nitrotyrosine (foot print of peroxynitrite), and a dose-dependent apoptosis and loss of inner retinal neurons. Treatment with L-NAME or TEMPOL protected retinal neurons and confirmed the involvement of peroxynitrite in retinal neurotoxicity. The neuroprotection by THC and CBD was because of attenuation of peroxynitrite. The effect of THC was in part mediated by the cannabinoid receptor CB1. These results suggest the potential use of CBD as a novel topical therapy for the treatment of glaucoma.

Free radicals and aging of anterior segment tissues of the eye: a hypothesis.

A hypothesis is proposed that aging processes in the eye occur as a consequence of degradation of enzymes that normally metabolize and detoxify hydrogen peroxide and other free radicals. The loss of enzyme activity allows hydrogen peroxide, which normally occurs within eye fluids, and free radicals to induce irreversible deleterious effects on different eye tissues. These processes may lead to cataract formation in the lens, loss of corneal endothelial cells, modification of the glycosaminoglycan secretory patterns of the cells of the trabecular meshwork, and other changes associated with ocular aging. These processes may be exacerbated during inflammation when oxidation products increase. Considerable circumstantial evidence points towards hydrogen peroxide as one of the major chemicals involved in the induction of these changes. Much remains to be determined to definitively identify this chemical or free radicals as the primary inducers of tissue alterations that occur in aging eyes.

Computer-simulated eye surgery : a novel teaching method for residents and practitioners

PURPOSE To describe an eye surgery simulator that uses a computerized graphic display to allow ophthalmic surgeons of all experience levels to enhance their surgical skills. METHODS The eye surgery simulation environment consists of a high-speed computer graphics workstation, a stereo operating system, a wrist rest, and a position tracking stylus connected to force feedback motors. The surgeon views computer-generated images of the eye and surgical instruments through the stereo operating system and controls the position and orientation of the chosen surgical instrument by moving the stylus. During the simulated instrument-tissue interactions, three feedback motors generate component force feedback along three orthogonal axes connected by thin rigid bars to the tip of the stylus. RESULTS The current proof-of-concept system provides a method for rapid learning experiences in a living eye simulation. Procedures can be recorded for playback and analysis, as well as for examination of techniques from different viewpoints (e.g., from inside the eye). Four simulated surgical instruments are available for use (scalpel, forceps, scissors, and phacoemulsifier). CONCLUSION Eye surgery simulation offers both beginning and experienced ophthalmic surgeons an opportunity to learn new techniques and skills and achieve a satisfactory level of proficiency before use of that procedure in the operating room. When fully developed, this system should shorten the learning curve for new surgeons (i.e., residents) and offer an opportunity for practice before doing a difficult case or development of new techniques by experienced surgeons. The goal of replacement of current standard training methods for surgeons awaits further refinement and adjustment of the model.

The fate of anterior chamber fluorescein in the monkey eye 1. The anterior chamber outflow pathways

Abstract As defined by freeze-dry tissue techniques, there are two distinct routes by which fluorescein leaves the anterior chamber of the living rhesus monkey eye. I. The conventional pathway: Within five minutes of the initiation of fluorescein perfusion through the anterior chamber, the conventional pathway filled. Tracer passed through the trabecular meshwork into Schlemms canal, and flowed back within scleral and episcleral venous derivatives to enter large veins in the extraocular muscles. II. The uveo-vortex pathway: Fluorescein rapidly moved into the iris stroma and into the anterior part of the ciliary body. The anterior chamber border of the ciliary body is little more than a sparse mesh from which fluorescein entered directly into the substance of the anterior ciliary muscle. Fluorescein penetrated blood vessels in the iris stroma and in the anterior ciliary body from where it could be traced posteriorly to the equatorial choroid and vortex veins. The specific sequential tissue distribution of fluorescein strongly supports the existence of a uveo-vortex pathway for aqueous outflow.

Effects of adrenergic agonists upon regional ocular blood flow in normal and ganglionectomized rabbits.

Abstract Radioactive microspheres were used to measure changes in regional ocular blood flow 30–45 min after the topical application of epinephrine, norepinephrine, phenylephrine, and isoproterenol, to both normal and ganglionectomized rabbits. In normal animals, epinephrine (4%), phenylephrine (8%), and norepinephrine (4%), in order of decreasing potency, caused significant reductions in blood flow to the iris and ciliary processes. Isoproterenol (4%) had no significant effect in these tissues. Ganglionectomized eyes received much lower drug concentrations (0·1% in all cases), and displayed a different order of responsiveness: phenylephrine caused a larger reduction in flow than epinephrine, and norepinephrine failed to provoke statistically significant change. None of the drugs affected choroidal blood flow in either normal or ganglionectomized animals, but retinal blood flow in ganglionectomized eyes was significantly reduced by phenylephrine or isoproterenol administration. Additional experiments, addressing epinephrines effects at 3 hr after administration, demonstrated a restoration of flow to the ciliary processes and an increased iridial blood flow in both normal and ganglionectomized animals. This was accompanied by an increased flow to the choroid, and a reduced retinal blood flow in the ganglionectomized state.

THE EFFECT OF BENZALKONIUM CHLORIDE ON THE ELECTROPOTENTIAL OF THE RABBIT CORNEA

The effect of benzalkonium chloride on the electropotential of the cornea has been examined. The anterior surface of the in vivo or in vitro cornea was exposed to various concentrations of the surfactant, from 0.005% to 0.02%, for either 1 or 2 min. The initial effect is a hyperpolarization lasting up to 30 sec, followed by a rapid fall in potential difference with a subsequent recovery. The degree of potential difference decrease and the recovery rate was dependent upon both the concentration of the detergent, and the exposure time. There is excellent correlation between the previous anatomical and physiological studies on tracer penetration across the in vivo and in vitro cornea and our present work. The data indicate that benzalkonium chloride acts by breaking down the physiological and anatomical diffusion barrier to solute and solvent which is located in the outer layer of the epithelium.

Effect of Calcium Channel Blockers on Intraocular Pressure

We determined the effects of either topical or systemic calcium channel antagonists on rabbit intraocular pressure (IOP). Topical nifedipine, verapamil or diltiazem had no significant effect on IOP. Intravenous verapamil and nifedipine caused statistically significant reductions in IOP between 2 and 6 h after administration; the nifedipine response followed an increase in IOP at 30 min. Diltiazem, given 3 times daily for 3 days, caused no pressure change. In the rabbit, therefore, calcium channel antagonists have no effect when given topically, but do reduce IOP when given systemically.

Aqueous Humor Flow Rate and Intraocular Pressure during and after Pregnancy

Intraocular pressure and aqueous humor formation rate have been determined from the first trimester of pregnancy through term, with further determinations 3 months postpartum, in 7 patients. The intraocular pressure showed a consistent, statistically significant fall during pregnancy, returning to values seen in early pregnancy after delivery. Aqueous humor formation rate showed no change during pregnancy. The data indicate that the sustained elevated hormonal levels during pregnancy, either directly or indirectly, cause an increase in fluid outflow conductance from the eye without altering the rate of fluid entry.

Effect of topical epinephrine and timolol on ocular and optic nerve blood flow in phakic and aphakic rabbit eyes

The effects of 4% epinephrine bitartrate and 0.5% timolol on ocular and optic nerve blood flow were studied in phakic and aphakic rabbit eyes using a radioactive microsphere (85Sr) technique. Either epinephrine or timolol was given topically as eight drops in the hour immediately preceding blood flow measurements (one drop every 7.5 minutes). Epinephrine decreased anterior uveal blood flow in both phakic and aphakic eyes, whereas timolol exhibited no effect. Retinal and choroidal blood flow were not affected by either timolol or epinephrine in phakic eyes. In aphakic eyes, epinephrine increased choroidal blood flow without altering retinal blood flow, whereas timolol increased both retinal and choroidal blood flow. Optic nerve blood flow was increased in epinephrine treated phakic and aphakic eyes and also in timolol treated aphakic eyes.

A comparison of topical cannabinoids on intraocular pressure

A comparison has been made of the relative efficacy of a series of naturally occurring cannabinoids, related to Δ9-tetrahydrocannabinol, on the reduction of the rabbit intraocular pressure. Mineral oil was compared to sesame oil as a vehicle for some compounds. Some compounds which in sesame oil did not evoke a fall in intraocular pressure were found to do so when mineral oil was the vehicle, illustrating more efficacious drug delivery to the eye. Of the drugs tested Δ8-THC was the most potent in reducing intraocular pressure. It was found that some drugs which have been shown to have greatly reduced psychoactive effects in man, relative to Δ9-THC, reduced rabbit intraocular pressure. Such drugs appear to merit testing under clinical conditions.