Background and Purpose: Corneal injury can result in dysfunction of corneal nociceptive signaling and corneal sensitization. Activation of the endocannabinoid system has been reported to be analgesic and anti-inflammatory. The purpose of this research was to investigate the antinociceptive and anti-inflammatory effects of cannabinoids with reported actions at cannabinoid 1 (CB1R) and cannabinoid 2 (CB2R) receptors and/or noncannabinoid receptors in an experimental model of corneal hyperalgesia.Methods: Corneal hyperalgesia (increased pain response) was generated using chemical cauterization of the corneal epithelium in wild-type (WT) and CB2R knockout (CB2R−/−) mice. Cauterized eyes were treated topically with the phytocannabinoids Δ8-tetrahydrocannabinol (Δ8THC) or cannabidiol (CBD), or the CBD derivative HU-308, in the presence or absence of the CB1R antagonist AM251 (2.0 mg/kg i.p.), or the 5-HT1A receptor antagonist WAY100635 (1 mg/kg i.p.). Behavioral pain responses to a topical capsaicin challenge at 6 h postinjury were quantified from video recordings. Mice were euthanized at 6 and 12 h postcorneal injury for immunohistochemical analysis to quantify corneal neutrophil infiltration.Results: Corneal cauterization resulted in hyperalgesia to capsaicin at 6 h postinjury compared to sham control eyes. Neutrophil infiltration, indicative of inflammation, was apparent at 6 and 12 h postinjury in WT mice. Application of Δ8THC, CBD, and HU-308 reduced the pain score and neutrophil infiltration in WT mice. The antinociceptive and anti-inflammatory actions of Δ8THC, but not CBD, were blocked by the CB1R antagonist AM251, but were still apparent, for both cannabinoids, in CB2R−/− mice. However, the antinociceptive and anti-inflammatory actions of HU-308 were absent in the CB2R−/− mice. The antinociceptive and anti-inflammatory effects of CBD were blocked by the 5-HT1A antagonist WAY100635.Conclusion: Topical cannabinoids reduce corneal hyperalgesia and inflammation. The antinociceptive and anti-inflammatory effects of Δ8THC are mediated primarily via CB1R, whereas that of the cannabinoids CBD and HU-308, involve activation of 5-HT1A receptors and CB2Rs, respectively. Cannabinoids could be a novel clinical therapy for corneal pain and inflammation resulting from ocular surface injury.

Cannabinoid receptor 1 (CB1) activation has been reported to reduce transient receptor potential cation channel subfamily V member 1 (TRPV1)-induced inflammatory responses and is anti-nociceptive and anti-inflammatory in corneal injury. We examined whether allosteric ligands, can modulate CB1 signaling to reduce pain and inflammation in corneal hyperalgesia. Corneal hyperalgesia was generated by chemical cauterization of cornea in wildtype and CB2 knockout (CB2−/−) mice. The novel racemic CB1 allosteric ligand GAT211 and its enantiomers GAT228 and GAT229 were examined alone or in combination with the orthosteric CB1 agonist Δ8-tetrahydrocannabinol (Δ8-THC). Pain responses were assessed following capsaicin (1 µM) stimulation of injured corneas at 6 h post-cauterization. Corneal neutrophil infiltration was also analyzed. GAT228, but not GAT229 or GAT211, reduced pain scores in response to capsaicin stimulation. Combination treatments of 0.5% GAT229 or 1% GAT211 with subthreshold Δ8-THC (0.4%) significantly reduced pain scores following capsaicin stimulation. The anti-nociceptive effects of both GAT229 and GAT228 were blocked with CB1 antagonist AM251, but remained unaffected in CB2−/− mice. Two percent GAT228, or the combination of 0.2% Δ8-THC with 0.5% GAT229 also significantly reduced corneal inflammation. CB1 allosteric ligands could offer a novel approach for treating corneal pain and inflammation.

Background and PurposeDamage to corneal tissue results in intense ocular pain, dysfunction in nociceptive signaling and release of inflammatory mediators. Current treatments for corneal pain and inflammation are frequently ineffective. The endocannabinoid system (ECS) is an emerging therapeutic target in the modulation of pain and inflammation. The ECS consists of endogenous cannabinoid ligands that mediate their actions via G‐protein coupled receptors, cannabinoid 1 receptor (CB1R) and cannabinoid 2 receptor (CB2R). Cannabinoids that bind to CB1R, like tetrahydrocannabinol (THC), have shown utility in treating pain, however, their therapeutic applications are limited by CB1R‐mediated behavioral side‐effects. The non‐psychoactive phytocannabinoid, cannabidiol (CBD), and CBD derivatives, CBD‐DMH and HU‐308, have reported anti‐inflammatory effects, which may be mediated independent of CB1R, and could offer an alternative to CB1R ligands in the treatment of ocular pain and inflammation. Therefore, the purpose of this research is to investigate the antinociceptive and anti‐inflammatory properties of CBD, CBD‐DMH and HU308 in a mouse model of corneal injury.MethodsExperimental corneal hyperalgesia and inflammation were generated using chemical cauterization of the cornea in wildtype (WT) and CB2R knockout (CB2R−/−) mice. Cauterized eyes were treated with topical cannabinoids (0.2–5% w/v) in the presence or absence of the CB1R antagonist, AM281 (2.5mg/kg ip). The ocular blink response, indicative of corneal hyperalgesia following chemical stimulation by capsaicin, was recorded 6 hours post‐injury. Mice were euthanized and eyes were enucleated at 12 hours and neutrophil infiltration into the cornea, a marker for inflammation, was analyzed using immunohistochemistry in the corneal sections.ResultsCorneal cauterization resulted in an increased blink response to capsaicin 6 hours post‐injury compared to sham control eyes (p < 0.0001). Application of 5% CBD, 5% CBD‐DMH and 1.5% HU308 reduced blinks in WT mice compared to vehicle‐treated eyes (p < 0.01). The antinociceptive effects of CBD and HU308 (p < 0.01 & p < 0.05, respectively), but not CBD‐DMH, were reduced in CB2R−/− mice. The antinociceptive effects of CBD‐DMH, but not CBD and HU308 (p < 0.0001), were blocked by the CB1R antagonist, AM281. Neutrophil infiltration into the cornea was increased 12 hours post injury, compared to non‐cauterized eyes in WT mice (p < 0.0001). Neutrophil infiltration was exacerbated in CB2R−/− mice compared to WT mice (p < 0.001). 5% CBD and 1.5% HU308 reduced neutrophil infiltration (p <0.001 & p <0.0001, respectively); these effects were reduced in CB2R−/− mice (p < 0.01 & p < 0.05, respectively).ConclusionCBD‐DMH had antinociceptive actions through CB1R, whereas, the antinociceptive and anti‐inflammatory actions of CBD and HU308 were independent of CB1R and mediated via CB2R activation. Therefore, the cannabinoids, CBD and HU308, could offer a novel therapy for ocular pain and inflammation with reduced CB1R mediated side‐effects.Support or Funding InformationCanadian Institutes of Health Research (CIHR).