Mediation of ocular tetrahydrocannabinol effects by adrenergic nervous system

Indirect sympatholytic actions at β-adrenoceptors account for the ocular hypotensive actions of cannabinoid receptor agonists

Intraocular pressure (IOP) is the primary risk factor for glaucoma, a blinding eye disease. Cannabinoid agonists have long been known to decrease IOP, suggesting they may be useful in glaucoma treatment. However, the specific mechanism by which cannabinoids generate this ocular hypotensive effect remains unknown. The current evidence suggests the cannabinoids reduce IOP through actions at cannabinoid 1 (CB(1)) receptors within the eye, and adrenergic receptors (ARs) may also contribute to this action of cannabinoids. Considering this, the present study aimed to elucidate the mechanism behind the ocular hypotensive properties of cannabinoids through the use of mice genetically lacking either cannabinoid receptors or βARs. Cannabinoid agonists, βAR antagonists, and βAR agonists decreased IOP in wild-type mice and CB(2)(-/-) mice. In contrast, none of these compounds were found to reduce IOP in βAR(-/-) or CB(1)(-/-) mice. Desensitization of the βARs and depletion of catecholamines in wild-type mice also eliminated the ability of the cannabinoid agonist (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate (WIN 55,212-2) to reduce IOP, strongly implicating a role for both βARs and catecholamines in the ocular hypotensive properties of cannabinoids. Finally, CB(1) receptors were shown to colocalize with tyrosine hydroxylase, a marker for adrenergic neurons. Taken together, these findings suggest that βARs are required for the ocular hypotensive properties of cannabinoids, and cannabinoids reduce IOP by acting as indirect sympatholytics and inhibiting norepinephrine release within the eye.

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.

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

The purpose of this study was to characterize the intraocular pressure (IOP) lowering activity and possible mechanism of action of the synthetic, non-psychotropic cannabinoid dexanabinol (HU-211) [(+)(3S,4S), 7-hydroxy-delta-6-tetrahydrocannabinol 1,1 dimethylheptyl], following intravenous (i.v.) administration in the rabbit. IOP (pneumatonometry), aqueous humor inflow rate (fluorophotometry), blood pressure, and heart rate (computerized physiograph system connected to central ear artery cannula) were measured in unanesthetized albino rabbits. Intravenous administration of HU-211 resulted in a dose-related reduction in IOP; a maximal IOP reduction of 5.0 +/- 0.2 mmHg was observed 4 hr after a 0.5 mg/kg dose. No significant changes in blood pressure or heart rate were observed during the first hr following this dose of HU-21 1. Pupil diameter did not change significantly during the 5 hr following the 0.5 mg/kg i.v. dose. No significant change in the rate of aqueous humor inflow occurred during the 6 hr after a 0.5 mg/kg dose of HU-211, thereby implicating outflow changes as the major source of IOP reduction. IOP reduction by HU-211 following pre-treatment with the alpha2 adrenergic antagonist, yohimbine (1 mg/kg, i.v.), was only 30% of that of HU-211 alone. IOP reduction following pretreatment with the alpha2 agonist, clonidine (0.5 mg/kg i.v.), was twice as large as that of HU-211 alone. Pretreatment with the beta-adrenergic antagonist, propranolol (0.5 mg/kg i.v.), resulted in a 50% reduction in the IOP-lowering effect of HU-211. In summary, HU-211, administered i.v., is an effective IOP-lowering agent, devoid of any significant side effects (blood pressure, heart rate or pupil diameter, all of which have been reported previously for cannabinoids). Involvement of the adrenergic system is indicated in mediating the IOP-lowering effects of HU-211 that appear to reflect a change in fluid outflow from the eye.

A synopsis of recent developments in antiglaucoma drugs

Open-angle glaucoma is treated primarily with drugs, some of which have been used clinically for years. These drugs include: 1) cholinergic agonists that increase aqueous humor outflow, 2) adrenergic agonists and antagonist that affect both aqueous humor formation and outflow, and 3) carbonic anhydrase inhibitors that decrease aqueous humor formation. Several new classes of drugs are being tested for efficacy and mechanism of action. They include: 1) the D-isomer of timolol that reduces aqueous humor formation without producing adrenergic blockade, 2) dopaminergic agonists and antagonists, including bromocriptine and butyrophenones that reduce intraocular pressure, and 3) cannabinoids that reduce aqueous humor formation and increase outflow. In addition, several other types of drugs, such as prostaglandins, diuretics, Na+,K+-ATPase inhibitors, and adenyl cyclase stimulators are just now beginning to be studied.

Effect of delta 9-tetrahydrocannabinol on intraocular pressure after removal of autonomic input

Sympathetic input to the anterior segment of the eyes of cats was unilaterally removed by either superior cervical ganglionectomy or treatment with 6-hydroxydopamine. Parasympathetic input was unilaterally removed by extirpation of the ciliary ganglion. delta 9-tetrahydrocannabinol (delta 9-THC; 20 micrograms/hr) was delivered unilaterally to the denervated eyes and to eyes of surgically intact control cats via osmotic minipumps and connecting extraocular cannulas over a total period of nine days. The results indicated that the degree of reduction of intraocular pressure by delta 9-THC was not affected by removal of input from either branch of the autonomic nervous system. Outflow facility during chronic administration of THC showed a two-to-three fold increase. Ciliary ganglionectomy alone produced a moderate decrease in intraocular pressure that endured for one week. These findings indicate that neither adrenergic nor cholinergic input to the cat eye is apparently required for the mediation of the tension lowering effect of THC. They additionally suggest that cholinergic input may normally play a role in the regulation of steady-state intraocular pressure levels, presumably by modulating aqueous humor formation.

Topical delta 9-tetrahydrocannabinol and aqueous dynamics in glaucoma

Systemic delta 9-tetrahydrocannabinol (THC), administered either by smoking marihuana or as synthetic THC in soft gelatin capsules, lowers ocular tension in various glaucomas, but at the expense of significant decreases in systolic blood pressure. Topical THC in light mineral oil vehicles, though effective in laboratory animals, was not shown effective in 0.05 and 0.1% topical solutions when administered to six subjects with primary open-angle glaucoma in a randomized, balanced, double-masked protocol. Light mineral oil, which has an affinity for corneal epithelium, is an optimum vehicle for administering drugs whose mechanisms of action are systemic rather than local within the eye. Further glaucoma research should therefore proceed with marihuanas containing insignificant levels of THC (less than 0.4%) and with various local delivery systems of the ocular-active cannabinoid found in Cannabis sativa.

Cannabinoids in glaucoma: a primary screening procedure

A procedure was developed for screening of cannabinoids for their ability to reduce intraocular pressure (IOP) using normal rabbits. Eight animals per group were used for statistical significance of data. A negative control group was used for every screen as well as a positive control with 1.5 mg/kg delta 9-THC given intravenously (I.V.). All compounds were tested by I.V. injection and IOP measurements were taken periodically for 5 hours. Data were analyzed by a computer program which takes into account the change in IOP of the control group. Following this procedure we found that delta 8-THC, delta 9-THC, cannabinol, and nabilone were active while cannabidiol was inactive.

Lack of effect of cannabinoids on carbonic anhydrase

Cannabinoids have significant anticonvulsant effects, the mechanism of which is obscure. In addition, these agents reduce intraocular pressure. Inhibitors of carbonic anhydrase also share these two pharmacological actions. In order to investigate whether these actions of cannabinoids are mediated through inhibition of carbonic anhydrase (CA), commercial solutions of the enzyme were exposed to delta 9-tetrahydrocannabinol and cannabidiol. No inhibition was observed at concentrations of 10(-7)--10(-5) M.

The influence of delta 9-tetrahydrocannabinol on intraocular pressure in the anaesthetized cat

delta 9-Tetrahydrocannabinol (delta 9-THC) was injected both intravenously and into the brain stem via the left vertebral artery. Contrary to results obtained with clonidine, neither the fall in intraocular pressure (IOP), nor the arterial hypotension induced by delta 9-THC, were enhanced after the 'central' administration of the drug. For clonidine, a central mechanism underlying the ocular hypotensive effect has recently been proposed. This suggestion is based upon the enhanced fall in IOP after 'central' administration of clonidine. The pontomedullary area is considered to be the main initial target of this drug. Obviously, the IOP-lavering mechanism of delta 9-THC is different from that of clonidine.

Effect of marihuana on intraocular and blood pressure in glaucoma

Marihuana inhalation was accompanied by increased heart rate and decreased intraocular and blood pressure in 18 subjects with heterogenous glaucomas. The hypotensive effects appeared in 60 to 90 minutes as the decrease in intraocular pressure (IOP) appeared to follow the decrease in blood pressure. In addition to any local effect, the mechanism of lowered to any local effect, the mechanism of lowered IOP may also involve the decreased pressure perfusing the ciliary body vasculature as a result of the peripheral vasodilatory properties of marihuana. Postural hypotension, tachycardia, palpitations, and alterations in mental status occurred with such frequency as to mitigate against the routine used in the general glaucoma population. Our data indicate that further research should be directed to local means of delivering the ocular hypotensive cannabinoid to the glaucomatous eye.

The influence of acute and chronic cervical sympathectomy on the ocular hypotensive effect of clonidine

The influence of clonidine on intraocular pressure was studied in chloralose-anesthetized cats. The cats were subjected to unilateral cervical sympathectomy, either acute or chronic. The drug was injected either intravenously or into the left vertebral artery. With the aim of investigating the role of the sympathetic nervous system in the central IOP-lowering effect of clonidine, the fall in IOP induced by the drug in these preparations was compared to the decrease in ocular pressure caused by clonidine in intact cats. An enhanced depressor effect in both eyes of unilaterally sympathectomized cats became evident after the intravenous injection of clonidine. In contrast, the effect of central administration was similar or even smaller than in intact cats. We conclude that the central IOP-lowering effect of clonidine is diminished after cervical sympathectomy, indicating that the peripheral sympathetic nervous system is the efferent pathway of this centrally initiated effect.