Pharmacology of the intraocular pressure (IOP) lowering effect of systemic dexanabinol (HU-211), a non-psychotropic cannabinoid

Evidence that cannabinoid CB1 receptors regulate intraocular pressure via two opposing mechanisms

The cannabinoid signaling system regulates intraocular pressure (IOP) in the mouse via a complex system that includes three receptors: CB1, GPR18 and GPR119. In each case, activating the receptor lowers IOP, but CB1 receptors are found both at sites of aqueous humor inflow and outflow. As such, knockout mice for any of these receptors would be expected to have higher-than average, or at least unchanged, intraocular pressure. The current study investigates the unexpected observation that CB1 knockout mice have lower pressure than wild type counterparts by testing various regulators of cannabinoid signaling in murine models of IOP. We now report that a CB1 antagonist has differential effects on IOP: SR141716 raises IOP in standard light cycle (SLC) but lowers IOP in reverse light cycle (RLC). This is mimicked by ABD1085, a negative allosteric modulator of CB1. CB1 inhibitors lower IOP in both normotensive and hypertensive mouse eyes. The pressure-lowering effect is absent in CB1 knockout mice. IOP rebounds after the end of treatment but shows no sign of desensitization with daily treatment for a week. Unlike the positive cannabinoid effect, antagonist effects are not sex-dependent. We propose that there are two mechanisms of action for CB1, one that lowers IOP upon activation and a second with inverse sign that lowers IOP when CB1 is antagonized. The relatively lower pressure in CB1 knockout mouse eyes suggests that this second negative regulation of IOP is dominant.

The In Vivo Effects of the CB1-Positive Allosteric Modulator GAT229 on Intraocular Pressure in Ocular Normotensive and Hypertensive Mice

PURPOSE

Orthosteric cannabinoid receptor 1 (CB₁) activation leads to decreases in intraocular pressure (IOP). However, use of orthosteric CB₁ agonists chronically has several disadvantages, limiting their usefulness as clinically relevant drugs. Allosteric modulators interact with topographically distinct sites to orthosteric ligands and may be useful to circumvent some of these disadvantages. The purpose of this study was to investigate the effects of the novel CB₁-positive allosteric modulator (PAM) GAT229 on IOP.

METHODS

IOP was measured using rebound tonometry in anesthetized normotensive C57Bl/6 mice and in a genetic model of ocular hypertension [nose, eyes, ears (nee) mice] before drug administration, and at 1, 6, and 12 h thereafter.

RESULTS

In normotensive mice, topical administration of 5 μL GAT229 alone at either 0.2% or 2% did not reduce IOP. However, a subthreshold dose (0.25%) of the nonselective orthosteric CB₁ agonist WIN 55,212-2, when combined with 0.2% GAT229, significantly reduced IOP compared with vehicle at 6 and 12 h. Similarly, combination of subthreshold Δ⁹-tetrahydrocannabinol (a nonselective orthosteric CB₁ agonist; 1 mg/kg) with topical 0.2% GAT229 produced IOP lowering at 6 h. In nee mice, administration of topical 0.2% GAT229 or 10 mg/kg GAT229 alone was sufficient to lower IOP at 6 and 12 h, and 12 h, respectively.

CONCLUSIONS

The CB₁ PAM GAT229 reduces IOP in ocular hypertensive mice and enhanced CB₁-mediated IOP reduction when combined with subthreshold CB₁ orthosteric ligands in normotensive mice. Administration of CB₁ PAMs may provide a novel approach to reduce IOP with fewer of the disadvantages associated with orthosteric CB₁ activation.

Nonpsychotropic cannabinoids, abnormal cannabidiol and canabigerol-dimethyl heptyl, act at novel cannabinoid receptors to reduce intraocular pressure

The objective of our study was to examine the pharmacology of the intraocular pressure (IOP)-lowering actions of the behaviorally inactive cannabinoids, abnormal cannabidiol (abn-CBD), and a cannabigerol analog, cannabigerol-dimethyl heptyl (CBG-DMH), in comparison to that of the nonselective cannabinoid 1 receptor (CB(1)R) and CB(2)R agonist, WIN55,212-2, in Brown Norway rats. The IOP was measured noninvasively using a hand-held tonometer in nonanesthetized animals. The IOP measurements were taken every 15 min for a period of 2 h after drug administration. All drugs were administered via intraperitoneal (i.p.) injections, and abn-CBD and CBG-DMH were also given topically. Both abn-CBD and CBG-DMH reduced IOP when administrated i.p. at doses of ≥2.5 mg/kg or topically at concentrations of 1%-2%. The IOP-lowering effects of abn-CBD and CBG-DMH were reduced by i.p. administration of O-1918 (2.5 mg/kg), a selective antagonist of the abn-CBD-sensitive cannabinoid-related receptor (CBx), but were unaffected by the CB(1)R antagonist, AM251 (2.5 mg/kg), or the CB(2)R antagonist, AM630 (2.5 mg/kg). In contrast, the IOP-lowering action of WIN55,212-2 was completely blocked by the CB(1)R-selective antagonist, AM251, and was unaffected by the CBx receptor antagonist, O-1918. However, similar to the nonpsychotropic cannabinoids, the ocular hypotensive actions of WIN55,212-2 were also insensitive to block by the CB(2)R antagonist, AM630. Consistent with this, the selective CB(2)R agonist, HU-308 (2 mg/kg) failed to reduce IOP in Brown Norway rats. Concurrent application of a dose of WIN55,212-2 that was subthreshold to reduce IOP (0.25 mg/kg), together with a topical dose of either abn-CBD (0.5%) or CBG-DMH (0.25%), respectively, potentiated the ocular hypotensive effect of either compound applied alone. This study demonstrates that the atypical cannabinoid, abn-CBD, and the cannabigerol analog, CBG-DMH, decrease IOP in the normotensive Brown Norway rat eye independent of CB(1)R or CB(2)R activation, via activation of CBx receptors. The enhanced decrease in IOP seen after coapplication of the CB(1)R agonist, WIN55,212-2, together with either abn-CBD or CBG-DMH, respectively, further suggests that the ocular pharmacodynamics of abn-CBD and CBG-DMH are mediated by receptor targets distinct from CB(1)R. These results indicate that both CBG-DMH and abn-CBD have the potential for further investigation as novel ocular hypotensive cannabinoids devoid of CB(1)R-mediated side-effects.

[Cannabinoid applications in glaucoma]

INTRODUCTION

Glaucoma is a slowly progressive optic neuropathy that is one of the leading causes of legal blindness throughout the world. Currently there is a limited group of topical drugs for the medical treatment of glaucoma is currently limited, and research needs to be focused on new therapeutic horizons, such as the potential usefulness of the cannabinoid agonists for the treatment of glaucoma.

AIM

To review the current scientific literature related to the beneficial effects derived from the different ways of administration of cannabinoids indicated for the glaucomatous optic neuropathy.

DEVELOPMENT

Cannabinoid receptors have shown an intense expression in ocular tissues implicated in the regulation of the intraocular pressure, as well as inner layers of the retina. Through activation of CB1 and CB1 specific receptors and through other still unknown pathways, the cannabinoid agonists have shown both a clear hypotensive, as well as an experimentally proved neuroprotective effect on retinal ganglion cells.

CONCLUSIONS

Some cannabinoid agonists (WIN 55212-2, anandamide) have demonstrated, in experimental studies, to act as «ideal drugs» in the management of glaucoma, as they have been shown to have good tolerability after topical application, efficiently reduce intraocular pressure, and behave as neuroprotectors on retinal ganglion cells. Further studies as regards the safety and clinical assays must be carried out in order to examine the effectiveness of these drugs for the treatment of glaucoma in our daily clinical practice.

Agonist-dependent cannabinoid receptor signalling in human trabecular meshwork cells

BACKGROUND AND PURPOSE

Trabecular meshwork (TM) is an ocular tissue involved in the regulation of aqueous humour outflow and intraocular pressure (IOP). CB1 receptors (CB1) are present in TM and cannabinoid administration decreases IOP. CB1 signalling was investigated in a cell line derived from human TM (hTM).

EXPERIMENTAL APPROACH

CB1 signalling was investigated using ratiometric Ca2+ imaging, western blotting and infrared In-Cell Western analysis.

KEY RESULTS

WIN55212-2, a synthetic aminoalkylindole cannabinoid receptor agonist (10-100 microM) increased intracellular Ca2+ in hTM cells. WIN55,212-2-mediated Ca2+ increases were blocked by AM251, a CB1 antagonist, but were unaffected by the CB2 antagonist, AM630. The WIN55,212-2-mediated increase in [Ca2+]i was pertussis toxin (PTX)-insensitive, therefore, independent of Gi/o coupling, but was attenuated by a dominant negative Galpha(q/11) subunit, implicating a Gq/11 signalling pathway. The increase in [Ca2+]i was dependent upon PLC activation and mobilization of intracellular Ca2+ stores. A PTX-sensitive increase in extracellular signal-regulated kinase (ERK1/2) phosphorylation was also observed in response to WIN55,212-2, indicative of a Gi/o signalling pathway. CB1-Gq/11 coupling to activate PLC-dependent increases in Ca2+ appeared to be specific to WIN55,212-2 and were not observed with other CB1 agonists, including CP55,940 and methanandamide. CP55940 produced PTX-sensitive increases in [Ca2+]i at concentrations>or=15 microM, and PTX-sensitive increases in ERK1/2 phosphorylation.

CONCLUSIONS AND IMPLICATIONS

This study demonstrates that endogenous CB1 couples to both Gq/11 and Gi/o in hTM cells in an agonist-dependent manner. Cannabinoid activation of multiple CB1 signalling pathways in TM tissue could lead to differential changes in aqueous humour outflow and IOP.

Medicinal Chemistry Endeavors around the Phytocannabinoids

Over the past 50 years, a considerable research in medicinal chemistry has been carried out around the natural constituents of Cannabis sativa L. Following the identification of Delta9-tetrahydrocannabinol (Delta9-THC) in 1964, critical chemical modifications, e.g., variation of the side chain at C3 and the opening of the tricyclic scaffold, have led to the characterization of potent and cannabinoid receptor subtype-selective ligands. Those ligands that demonstrate high affinity for the cannabinoid receptors and good biological efficacy are still used as powerful pharmacological tools. This review summarizes past as well as recent developments in the structure-activity relationships of phytocannabinoids.

Neuroprotective and Intraocular Pressure-Lowering Effects of (–)Δ9-Tetrahydrocannabinol in a Rat Model of Glaucoma

In glaucoma, retinal ganglion cell (RGC) death is induced by many risk factors, including ocular hypertension. It has been proposed that glutamate-mediated oxidative stress may also contribute to this RGC death. Cannabinoids are known to possess therapeutic properties including ocular hypotension and antioxidation. In this study, we test the hypothesis that (-)Delta(9)-tetrahydrocannabinol (THC) lowers intraocular pressure (IOP) and prevents RGC death in a rat model of glaucoma. Arat model of experimental glaucoma with chronic, moderately elevated IOP was produced unilaterally by cauterization of episcleral vessels. Rats received weekly injections of THC at a level of 5 mg/kg or vehicle for 20 weeks. IOP of both eyes was measured weekly on anesthetized animals immediately before THC treatment. RGCs were labeled in a retrograde fashion and counted in whole-mounted retinas. IOP was elevated in all operated eyes 1 day after the operation and remained elevated in the vehicle-treated rats throughout 20 weeks. In THC-treated rats, IOP elevation in operated eyes was diminished 2 weeks after operation and remained reduced. IOP in the contralateral control eyes was not affected by THC. In the operated eyes of vehicle-treated animals, there was a loss of approximately 50 and 40% of the RGCs in the peripheral and central retina, respectively. The RGC loss in the operated eyes of the THC-treated animals was reduced to 10-20%. These results demonstrate that THC is a neuroprotectant that preserves RGCs in an experimental model of glaucoma, possibly through a reduction in IOP.

Evidence for the use of nutritional supplements and herbal medicines in common eye diseases

PURPOSE

To provide a perspective by reviewing the evidence for the role of nutritional supplements and herbal medicines in the common causes of visual impairment.

DESIGN

Retrospective literature review.

METHODS

Published studies and information found in PubMed, International Bibliographic Information of Dietary Supplements, and selected websites were reviewed for the role of nutritional and herbal medicines in the treatment of age-related macular degeneration, cataract, diabetic retinopathy, and glaucoma. The studies were evaluated systematically for their study design, study population, benefits, risks, biases, and criteria for the categorization of the level of evidence.

RESULTS

The available evidence does support the use of certain vitamins and minerals in patients with certain forms of age-related macular degeneration. For cataracts, the available evidence does not support these supplements to prevent or treat cataracts in healthy individuals. For diabetic retinopathy and glaucoma, the available evidence does not support the use of these supplements. In the category of herbal medicines, the available evidence does not support the use of herbal medicines for any of these ocular diseases.

CONCLUSION

Because of the widespread use of nutritional supplements and herbal medicines, ophthalmologists should be aware of their use so that they can inform patients properly when the supplements and herbal medicine are being used for eye disease.

Stimulation of cannabinoid (CB1) and prostanoid (EP2) receptors opens BKCa channels and relaxes ocular trabecular meshwork

Prostanoids and cannabinoids have ocular hypotensive and neuroprotective properties. The effect of the prostanoid AH13205 (EP2), the thromboxane-mimetic U46619, the cannabinoid (CB) agonists WIN55212-2 and CP 55,940, endothelin-1 (ET-1) and 8-bromo-cAMP on the membrane currents of trabecular meshwork (TM) cells were measured using the patch-clamp technique and compared to their effects on TM contractility. Previous studies show relaxation of TM to AH 13205 and other substances that elevate cAMP, while U46619 and endothelin-1 contract TM. This study shows that after contraction (100%) with carbachol (10(-6)m), the CB agonist CP 55,940 dose-dependently reduced contractility to 83+/-4% (n=9) (10(-6)m) and 61+/-10%, (n=7) (10(-5)m). In the presence of both the CB1 antagonist AM251 (10(-6)m) and CP 55,940 (10(-5)m), the contractile response to carbachol reached 84+/-3% (n=6) of the original level. In patch-clamp experiments, membrane permeable 8-bromo-cAMP (10(-4)m) had no effect on currents of TM cells. In contrast, AH 13205 and two cannabinoids reversibly enhanced outward current through high-conductance Ca(2+)-activated K(+) channels (BKCa, BK, maxi-K) to the following values (in % of the initial value at 100 mV): AH 13205 (10(-5)m): 200+/-28% (n=6), CP 55,940 (10(-6)m): 196+/-33% (n=7), CP 55,940 (10(-5)m): 484+/-113% (n=7), WIN55212-2 (10(-5)m): 205+/-41% (n=10). Iberiotoxin (10(-7)m) completely blocked these responses. The current response to CP 55,940 (10(-5)m) could be partially blocked by the CB1 antagonist AM251 (10(-6)m). Conversely, the contractile agents in this study either caused a transient reduction in outward current (ET-1(5x10(-8)m)) or had no effect (U46619 (10(-6)m)). We conclude that stimulation of EP2 and CB1 receptors in TM is coupled to the activation of BKCa channels via a non-diffusible second messenger cascade. This effect may contribute to the relaxant activity of EP2 and CB1 agonists in isolated TM strips, modulating ocular outflow.

Dexanabinol: dexanabinone, HU 211, PA 50211, PRS 211007, sinnabidol

Dexanabinol [HU 211, dexanabinone, sinnabidol, PA 50211, PRS 211007] is a synthetic, non-psychotropic tetrahydro-cannabinoid. Dexanabinol lacks cannabimimetic activity, and is a functional antagonist of the NMDA receptor with antioxidant and anti-tumour necrosis factor-alpha properties. Dexanabinol is in clinical trials for traumatic brain injury (head injuries), glaucoma and mild cognitive impairment, and is being investigated preclinically for its potential in the treatment of multiple sclerosis. Pharmos has a licensing agreement with the Hebrew University, Israel, and is seeking a partner for development and commercialisation of the dexanabinol family of compounds. The Financial Times (ft.com) reported in March 2001 that the market for brain trauma could be worth approximately 500 million US dollars, according to estimates by Pharmos. The company was said to have 26 million US dollars in capital at the time, most of which would be used taking dexanabinol through regulatory submission, according to the Financial Times.

Neuroprotective effect of (-)Delta9-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.

Endogenous cannabinoid system as a modulator of food intake

The ability of Cannabis sativa (marijuana) to increase hunger has been noticed for centuries, although intensive research on its molecular mode of action started only after the characterization of its main psychoactive component Delta(9)-tetrahydrocannabinol in the late 1960s. Despite the public concern related to the abuse of marijuana and its derivatives, scientific studies have pointed to the therapeutic potentials of cannabinoid compounds and have highlighted their ability to stimulate appetite, especially for sweet and palatable food. Later, the discovery of specific receptors and their endogenous ligands (endocannabinoids) suggested the existence of an endogenous cannabinoid system, providing a physiological basis for biological effects induced by marijuana and other cannabinoids. Epidemiological reports describing the appetite-stimulating properties of cannabinoids and the recent insights into the molecular mechanisms underlying cannabinoid action have proposed a central role of the cannabinoid system in obesity. The aim of this review is to provide an extensive overview on the role of this neuromodulatory system in feeding behavior by summarizing the most relevant data obtained from human and animal studies and by elucidating the interactions of the cannabinoid system with the most important neuronal networks and metabolic pathways involved in the control of food intake. Finally, a critical evaluation of future potential therapeutical applications of cannabinoid antagonists in the therapy of obesity and eating disorders will be discussed.

Cannabinoids in the treatment of glaucoma

The leading cause of irreversible blindness is glaucoma, a disease normally characterized by the development of ocular hypertension and consequent damage to the optic nerve at its point of retinal attachment. This results in a narrowing of the visual field, and eventually results in blindness. A number of drugs are available to lower intraocular pressure (IOP), but, occasionally, they are ineffective or have intolerable side-effects for some patients and can lose efficacy with chronic administration. The smoking of marijuana has decreased IOP in glaucoma patients. Cannabinoid drugs, therefore, are thought to have significant potential for pharmaceutical development. However, as the mechanism surrounding their effect on IOP initially was thought to involve the CNS, issues of psychoactivity hindered progress. The discovery of ocular cannabinoid receptors implied an explanation for the induction of hypotension by topical cannabinoid applications, and has stimulated a new phase of ophthalmic cannabinoid research. Featured within these investigations is the possibility that at least some cannabinoids may ameliorate optic neuronal damage through suppression of N-methyl-D-aspartate receptor hyperexcitability, stimulation of neural microcirculation, and the suppression of both apoptosis and damaging free radical reactions, among other mechanisms. Separation of therapeutic actions from side-effects now seems possible through a diverse array of novel chemical, pharmacological, and formulation strategies.

Cannabinoid CB(1) receptor expression, activation and detection of endogenous ligand in trabecular meshwork and ciliary process tissues

Elevated intraocular pressure is the primary risk factor for glaucoma. Cannabinoids interact with molecular targets in the eye and lower intraocular pressure by an unknown mechanism. The purpose of the present study was to examine eye tissues for functional cannabinoid receptors of the neuronal, CB(1) class, and an endogenous ligand, anandamide. The trabecular meshwork and ciliary processes are the primary structures of the eye that contribute to intraocular pressure and thus were our focus. Total RNA, frozen sections, cellular membranes and primary cultures of cells were prepared from both bovine and cadaveric human tissues. Using cannabinoid CB(1) receptor-specific oligodeoxynucleotide primers, cannabinoid CB(1) receptor antiserum, and cannabinoid-specific compounds (CP-55,940, WIN55,212-2 and SR-141716A), the presence of cannabinoid CB(1) receptors in ciliary processes and trabecular meshwork was determined. Using reverse transcription-polymerase chain reaction, we identified mRNA encoding cannabinoid CB(1) receptor protein in ciliary process and trabecular meshwork cells. Specific binding of anti-CB(1) immunoglobulin-G in tissue sections localized cannabinoid CB(1) receptor protein to the non-pigmented epithelial cells of the ciliary process and cells of the trabecular meshwork. While CP-55,940 and WIN55,212-2 failed to stimulate [(35)S]GTP gamma S binding in membrane preparations from trabecular meshwork and ciliary process, CP-55,940 significantly stimulated whole cell [(35)S]GTP gamma S binding by 51% over basal in ciliary process epithelial cells and 69% over basal in trabecular meshwork cells permeabilized with 5 microM digitonin (p<0.001). Specificity of agonist stimulation was verified by complete blockade with the specific cannabinoid CB(1) receptor antagonist, SR-141716A. Moreover, activation of cannabinoid CB(1) receptors by CP-55,940 resulted in a 2.3+/-0.3 and 1.7+/-0.3-fold stimulation of cAMP accumulation in trabecular meshwork and ciliary process cells, respectively (p<0.01). Lastly, anandamide was detected in human trabecular meshwork (3.08 pmol/g), ciliary process (49.42 pmol/g) and neurosensory retinal (4.48 pmol/g) tissues. These data, for the first time, demonstrate in a single study the presence of both CB(1) mRNA and protein in trabecular meshwork and ciliary processes from two different species. Activation of heterotrimeric G-proteins and stimulation of cAMP accumulation by cannabinoids in vitro suggest that their intraocular pressure-lowering effects in vivo result from activation of cannabinoid CB(1) receptors in the trabecular meshwork and increase aqueous outflow.