Cannabis and the Cornea

Abstract

Purpose:

While cannabis has potential to reduce corneal pain, cannabinoids might induce side effects. This review article examines the effects of cannabinoids on the cornea. As more states and countries consider bills that would legalize medical and recreational adult use of cannabis, health care providers will need to recognize ocular effects of cannabis consumption.

Methods:

Studies included in this review examined the connection between cannabis and the cornea, more specifically anti-nociceptive and anti-inflammatory actions of cannabinoids. NCBI Databases from 1781 up to July 2019 were consulted.

Results:

Five studies examined corneal dysfunctions caused cannabis consumption (opacification, decreased endothelial cell density). 12 studies observed reduction in corneal pain and inflammation (less lymphocytes, decreased corneal neovascularization, increased cell proliferation and migration).

Conclusion:

More than half of the studies examined the therapeutic effects of cannabinoids on the cornea. As the field is still young, more studies should be conducted to develop safe cannabinoid treatments for corneal diseases.

INTRODUCTION

Due to the therapeutic and psychotropic properties of cannabinoids, cannabis is the most consumed and illegally traded drug worldwide. In 2016, more than 192 million people have used cannabis, either topically, orally, sublingually or more commonly through inhalation. After recreational cannabis’ legalization in Uruguay (2013) and in Canada (2018), more states and countries consider bills that would decriminalize and legalize adult use of cannabis. In the context of widespread use of recreational cannabis and cannabinoid-based treatments, health-care providers, including eye care professionals (ophthalmologists, optometrists), general internists, and emergency physicians, will need to recognize ocular effects of cannabis consumption in patients.

Corneal visual impairment is the fourth cause of blindness worldwide. Surgery is the only available treatment. As it requires graft donors, this treatment can be inaccessible. In order to prevent corneal visual impairment, it is important to treat corneal pain and inflammation effectively and safely.

A paucity of articles correlates cannabinoid pathways with reduction of corneal inflammation and pain. The objectives of this study are to summarize the current literature about cannabinoids and corneal hyperalgesia and assess if certain cannabinoids could be isolated and used as a reliable treatment to reduce the nociceptive and inflammatory effects associated to corneal diseases.

METHODS

Study Eligibility Criteria

Studies that were eligible examined the protective and toxic effects of cannabinoids on the function of the human or animal cornea.

Studies indirectly addressing the connection between cannabinoids and the cornea were excluded. Studies on glaucoma and inflammation, cannabinoids and intraocular pressure, transcorneal permeability of cannabinoids, corneal kindling model experiments using cannabinoids, as well as experiments using cannabinoids to treat pterygium were not included.

Search Methods and Terms Used

The literature search identified relevant articles in the NCBI Literature Databases (PubMed/PubMed Central) from 1781 (publication date of the oldest article available on PubMed) up to December 2019.

Article searches contained the following MeSH terms and keywords: cannabinoid receptors, cannabinoids, cannabis, cornea, corneal endothelial cell loss, corneal injuries, dry eye syndromes, marijuana abuse, marijuana smoking, marijuana use, medical marijuana.

Additional studies were found by consulting the reference lists of selected articles.

Results

Seventeen articles were selected. These studies examined the following cannabinoid effects on the cornea: corneal cellular properties (density, migration via chemotaxis, quantity), corneal opacification, corneal neovascularisation, corneal inflammation, and corneal pain. Only three review articles were included. The majority of the articles (10 out of 17) were experimental studies performed on animal models. The main limitation of these studies is the low number of experiment repetitions performed (Table 1).

Table 1. Main Publications, Type of Studies, Findings.

This original table breaks down all the publications selected for this review article. It indicates their type (i.e. systematic review, random control trial, case report, case-control study, cohort study), their most important findings (i.e. corneal opacification, corneal neovascularization in humans, corneal endothelial cell density reduction) and their overall effects on corneal health (protective effects: +; toxic effects: -).

Title of Publications	Type of Study	Findings	Effects
The ocular manifestations of the cannabinols6	Review Article	• Increase size and number of corneal nerves	NA
Evidence for a GPR18 Role in Chemotaxis, Proliferation, and the Course of Wound Closure in the Cornea7	Experimental study in culture bovine corneal cells and murine corneal explants	• Induction of chemotaxis and proliferation after corneal GPR18 activation (wound healing process)	+
The Cannabinoids Delta(8)THC, CBD, and HU-308 Act via Distinct Receptors to Reduce Corneal Pain and Inflammation8	Experimental study on murine animals	• Reduction of corneal pain and inflammation	+
Topical cannabinoid agonist, WIN55,212-2, reduces cornea-evoked trigeminal brainstem activity in the rat10	Experimental study on murine animals	• Modulation of CB1R and corneal-responsive neurons	+
The cannabinoid WIN 55,212-2 inhibits transient receptor potential vanilloid 1 (TRPV1) and evokes peripheral antihyperalgesia via calcineurin11	Experimental study in murine animals	• Co-Activation of CB1R and TRPV1 by Delta-8-THC	+
Cannabinoid-induced chemotaxis in bovine corneal epithelial cells12	Experimental study on bovine corneal epithelial cells	• Induction of chemotaxis after CB1 activation in bovine cells	+
Epidermal growth factor receptor transactivation by the cannabinoid receptor (CB1) and transient receptor potential vanilloid 1 (TRPV1) induces differential responses in corneal epithelial cells13	Experimental study on human corneal epithelial cells	• Proliferation and migration of corneal cells after CB1 and TRPV1 activation	+
Cannabinoid receptor 1 suppresses transient receptor potential vanilloid 1-induced inflammatory responses to corneal injury14	Experimental study on human and murine epithelial cells	• Reduction of corneal opacification and inflammation	+
Cannabinoid CB2R receptors are upregulated with corneal injury and regulate the course of corneal wound healing15	Experimental study in an in vitro murine model	• Increased activity of CB2R after corneal injury • Chemorepulsion	+
Gene therapy for corneal dystrophies and disease, where are we?16	Review Article	• Reduction of corneal neovascularisation	+
Genetic and pharmacologic inactivation of cannabinoid CB1 receptor inhibits angiogenesis17	Experimental Study
Turning Down the Thermostat: Modulating the Endocannabinoid System in Ocular Inflammation and Pain5	Systematic Review	• Corneal cell proliferation and migration via chemotaxis	+/−
Ocular hypotension, ocular toxicity, and neurotoxicity in response to marihuana extract and cannabidiol20	Experimental study on cats	• Corneal opacification (in cats and dogs)	-
Intraocular pressure, ocular toxicity and neurotoxicity in response to 11-hydroxy-delta 9-tetrahydrocannabinol and 1-nantradol19	Experimental study on animal models (cats, rats, monkeys)
Intraocular pressure, ocular toxicity and neurotoxicity after administration of delta 9-tetrahydrocannabinol or cannabichromene18	Experimental study on cats
Comparative Toxicities of Tetrahydropyridobenzopyrans21	Experimental study on animal models (dogs, rhesus monkeys, rats)
Corneal endothelial changes in long-term cannabinoid users22	Case-control study	• Reduction in corneal cell density	-

DISCUSSION

Cannabis and Cannabinoids

Cannabis

Extracted from hybrid cannabis plants, cannabis is a medical and recreational drug used for its psychotropic and physical effects. These properties are inherent to cannabinoids, which are more than 100 chemical compounds that are found in the cannabis plant.

Cannabinoids

Cannabinoids adhere to cannabinoid and non-cannabinoid receptors in order to generate different reactions. There are many varieties of cannabinoids, including tetrahydrocannabinol (THC), cannabidiol (CBD), cannabichromene (CBC). tetrahydrocannabinolic acid (THCA) and cannabinol (CBN).

Each cannabinoid can be further divided into components. For example, delta-9-THC and delta-8-THC are different psychoactive components of cannabis. THC is the major cannabinoid type causing cannabis’ mind-altering properties. CBD is a main cannabis component, which causes the drug’s neurologic and bioactive activity. Unlike THC’s psychoactivity, CBD triggers reactions that can be beneficial in the treatment of psychiatric diseases, epilepsy, and neurodegenerations.

Phytocannabinoids, Synthetic Cannabinoids and Endocannabinoids

The most common legalized cannabinoids are natural cannabinoids, which are extracted from hybrid cannabis plants. These phytocannabinoids differ from synthetic cannabinoids (which are produced in the laboratory) and endocannabinoids (which are physiologically important). (Figure 1) In North America, synthetic cannabinoids pose a serious health threat, as they are new psychoactive substances that are unregulated.

Figure 1: Schematic Representations of Cannabinoid-Related Pathways.

Cannabinoids bind to cannabinoid receptors, which are predominantly found in corneal epithelial cells and the basal ganglia. Non-cannabinoid receptors modulate the cerebral cannabinoid-signaling pathway involved in the regulation of ocular pain and inflammation.

Endocannabinoids and Corneal Receptors

Cannabinoid Receptors and Location in the Cornea^1,2

Cannabinoid receptor 1 (CB1) and cannabinoid receptor 2 (CB2) are the most prominent cannabinoid receptors that are part of the endocannabinoid network. These G-protein receptors (GPCRs) constitute the largest membrane protein family and modulate important cell signaling molecules. Cannabinoid receptors are involved in various neural pathways associated with cannabis and can be found abundantly in the basal ganglia.

In the eye, CB1 receptors are predominantly located in the corneal epithelium and endothelium. While CB2 receptors are not present in the cornea, they are involved in the turnover of the aqueous humor. (Figure 1)

Corneal Endocannabinoids³

Endocannabinoids that have been detected in the human cornea include: palmitoyethanolamide (PEA), 2-arachidonoylglycerol (2-AG) and anandamide (N-arachidonoylethanolamine, AEA). (Figure 1) PEA promotes the activity of AEA. 2-AG is a physiologically important CB1R agonist. While AEA also activate cannabinoid receptors, AEA specifically mediates the transient receptor potential cation channel subfamily V member 1 (TRPV1).

Non-Cannabinoid Receptors⁴

TRPV1 works concurrently with cannabinoid receptors in the modulation of corneal hyperalgesia and inflammation. Other non-cannabinoid receptors that are cannabinoid molecular targets include: transient receptor potential cation channel subfamily M member 8 (TRPM8), transient receptor potential ankyrin 1 (TRPA1), peroxisome proliferator-activated receptors (PPARs), and the G-protein-coupled receptor serotonin 1A receptor (5-HT1A). (Figure 1)

Cannabinoids’ actions are linked to various systems, including the gabaergic, serotonergic, cholinergic and dopaminergic pathways.

By modulating these receptors and systems, cannabis can produce anti-inflammatory and antinociceptive effects. However, the exact mechanisms responsible for the modulation of cannabinoids in corneal pain are still unknown. Studies are currently examining the correlation between corneal cell proliferation and migration via chemotaxis in the modulation of endocannabinoids and corneal receptors.⁵

In Vivo Evidence-Based Effects of Cannabinoids on the Cornea

Corneal Neuropathic Pain and Short-Term Cannabis Usage¹

A study conducted from 1968 to 1973 examined a cohort of 350 cannabinol consumers and found that large and numerous corneal nerves were significant in a large number of patients. Short-term cannabis use might therefore be related to corneal neuropathic pain. ⁶

Corneal Endothelial Cells and Long-Term Cannabis Usage¹⁷

Similarly to the process of aging, intraocular surgeries, glaucoma, trauma, alcohol, and tobacco, cannabis might cause a decrease in the endothelial cell count.

One study has noted the long-term effects of cannabis use in human eyes. Corneal endothelial cell density (number of cells per square millimeter) was significantly reduced in the cannabinoid group. It has been suggested that this reduction in the corneal endothelial density is the result of endothelial cell death.

On the other hand, the coefficient of variation in cell size and the hexagonal cell ratio in cannabinoid users did not significantly change compared to the control group. It is likely that cannabinoid toxicity counters cellular growth and migration, which differs from the findings in previous studies using animal models and acute administration of cannabinoids. The mitotic rate of endothelial cells was insufficient to restore the endothelium due to cannabinoid toxicity.

It is plausible that the cornea is particularly vulnerable to cannabinoid toxicity due to the profusion of CB1 receptors in the anterior segment of the eye. The literature supports that the activation of CB1 receptors promotes oxidative stress, mitogen-activated protein kinase (MAPK) pathways, and apoptosis. However, these studies on the protective effects of inactive CB1 receptors in endothelial cells are not specific to the human cornea.

There were no clinically significant observations (i.e. absence of corneal edema). The reduction in CD was not sufficient to significantly reduce the central corneal thickness (CCT). This result might be explained by the selection criteria for the cannabinoid group: this study included 28 patients who have only been using cannabis during the past year at a frequency of three times per week or more (and who received a formal psychiatric diagnostic of cannabinoid use disorder). In order to clarify the relationship between CCT and cannabis, it would be important to examine research participants who have been consuming cannabis regularly for more than a year.

Cannabinoids’ Protective Effects on the Cornea

Cannabinoids and Corneal Inflammation and Pain

Recent studies on mice have further validated that cannabis has an influence on corneal pain. Cannabinoids would reduce corneal pain and inflammation by activating receptors involved in cannabinoid pathways. The nociceptive actions of capsaicin on cauterized murine eyes were significantly reduce by topical applications of cannabinoids, including the synthetic derivative HU-308 at a concentration of 1.5%, CBD at 5% and Delta-8-THC at 1%. Corneal inflammation was also reduced at those same cannabinoid concentrations, as a reduction of neutrophil infiltration was quantified through immunohistochemistry. (Table 2) It appears that delta-8-THC is a CB1R agonist because mice were treated with AM251 (a CB1R antagonist) did not benefit from delta-8-THC anti-sensitization properties.^7,8

Table 2. Observations on the Nociceptive and Anti-Inflammatory of Different Cannabis Regimens in Mice.

This original table illustrates the findings from the 2018 study by Thapa et al.⁸

Animal	Topical applications of cannabinoids	Dose prescribed (% w/v)	Significant Pain Score Reduction	Significant Neutrophil Number Reduction
Mice	Delta-8-THC (CB1R agonist)	0.2	No	Not examined
		0.4	No	Not examined
		0.5	Yes	Not examined
		1	Yes	Yes
	CBD (5-HT1A agonist)	3	No	Not examined
		5	Yes	Yes
	HU-308 (CB2R agonist)	1	No	Not examined
		1.5	Yes	Yes

Delta-8-THC is thought to be involved in the activation of CB1R and TPRV1 because the co-localization of these two has been well established. Studies have demonstrated that human corneal epithelial turnover was favored by the activation of CB1 and TRPV1.⁹ Both receptors induce differential responses through epidermal growth factor receptor-mediated MAPK pathways. While TRPV1 activation by capsaicin leads to more IL-6 and IL-8 release through a EGFR-independent pathway, active CB1 blocks the IL-8 release induced by TRPV1.^10,11 (Figure 2)

As CB1 and CB2 have opposite effects, their balance is important in the regulation of efficient cell migration via chemotaxis when needed. Without the actions of these cannabinoid receptors, corneal wound healing is considerably impaired.

Other important receptors responsible for cannabinoids anti-inflammatory effects include the 5-HT1A receptor activated by CBD and GPR18, which was conducive to wound healing and triggered cell proliferation in vitro.⁷

A study done in bovine corneal epithelial cells (bCEC) observed that the directed migration of bCEC is enhanced by 2-AG and halted by a CB1R antagonist (SR141716). From their findings, they conclude that CB1 activation leads to ERK1/2 dephosphorylation in a concentration-dependent way, which does not promote bCEC proliferation. Active CB1 would decrease cAMP levels, contributing to the inactivation of mitogen-activated protein-kinase (MAPK) and therefore acting as an antagonist in the EGF pathway responsible for cell proliferation.¹² (Figure 2A)

The claim that CB1 acts as an antagonist of EGF-induced cellular proliferation is not supported by studies conducted by Yang in 2010 and in 2013. The discrepancy in results might be explained by the use of different animal models (mice vs. cows) or a lower dosage of the synthetic CB1R agonist (1 and 10 nm vs. 10μM). ¹³

While CB1 regulates attractive chemotaxis of CECs, CB2 would modulate repulsive chemotaxis by activating MAPK and raising the cAMP levels. HU-308 is an example of CB2 agonist, as its antinociceptive effects were halted in CB2−/− mice. CB2 activation accelerates wound healing in injured murine corneas. By subjecting bovine corneal epithelial cells with a CB2R agonist (JWH133), Murataeva further discovered that active CB2R promotes MAPK activation, ERK phosphorylation, adenylyl cyclase activity and cAMP quantities, without affecting the proliferation of corneal epithelial cells.

This study also conflicts with the 2013 study by Yang et al., as it suggests that the presence of AEA is more important than 2-AG in the mechanism responsible for repairing corneal damage. While Yang et al. noted the presence of 2-AG and the absence of AEA in corneal wounds, Murateava found that corneal injury can be correlated with an increased amount of AEA, which is synthesized by the NAPE-PLD pathway.¹⁴ According to Murataeva, the differences in results are mainly due to the quantity of cannabinoids used.¹⁵

Reduction of Corneal Neovascularization^16,17

Two studies have noted a significant reduction in corneal neovascularization by inhibiting the action of CB1. Gene therapies have examined the effects of different CB1 antagonists in the angiogenesis of human, murine and rabbit corneas in vitro. Some of these CB1 inhibitors also modulate cannabinoid pathways, leading to corneal haze and fibrosis reduction, fast wound healing and corneal density augmentation.

Because of the paucity of research into CB1 antagonist’s actions, it is necessary to proceed to in vivo experiments before confirming such protective effects.

Cannabis’ Destructive Effects on the Cornea

Cannabis-Induced Corneal Opacification and Dehydration

In two feline studies, severe corneal opacities were developed at the application site after chronic topical administration of cannabinoids. Delta-9-THC delivery via osmotic minipumps had a more intense corneal opacification in cats than cannabis extracts. Both treatments lasted 9 days. On the third day, irregularities in the corneal epithelium were noticed and forecasted the corneal opacities (visible from the fourth to the sixth day). Contrastingly, cannabichromene, cannabidiol and acute application of delta-9-THC did not induce ocular toxicity in cats. Other animal experimental study has reported corneal opacification and ulceration after oral administration of synthetic cannabinoids in dogs, but not in rhesus monkeys, nor in rats.^18–21

It has been inferred that these corneal opacities were caused by a decreased corneal hydration. Active CB1 inhibits the pumping action of the corneal endothelial cells, which stops the removal of aqueous humor out of the cornea.^19,20

The activation of these receptors can therefore lead to reduced lacrimation and corneal opacification. Further experimental studies must be undertaken to validate if these ophthalmic changes also occur in humans.

CONCLUSION

Future Perspectives

While the exact mechanisms underlying corneal pain, inflammation, and toxicity are still unclear, more than half of the studies have noted cannabinoids’ therapeutic effects on the cornea. Only a few studies warn about their potential toxic properties on the eye (Table 1).

Hence, the direct link between corneal endothelial cells to cannabinoids toxicity requires further validation. Studies examining patients using cannabinoids for more than a year and experimental research observing corneal endothelial cell death in vivo are required. Clinical trials attesting that cannabis’ benefits outweigh health hazards would allow the use of legal and safe cannabinoid treatments for corneal diseases.