Damage to the cornea can result in the pain signals from the cornea becoming dysfunctional, increasing sensitivity and producing random and intense pain. As damaged corneas are very common, due to infections, neurological diseases, traumas, or surgery, effective treatment of the neuropathic pain that results is important. “persistent hyperalgesia, debilitating pain, photoallodynia, burning, stinging, dryness, and inflammation”, as well as inflammation, are common symptoms, according to the authors of this study.
Many of the existing pain-relieving drugs are intolerable, ineffective, unsafe, or addictive in the short and long-term, making them unsuitable for many patients. The anti-inflammatory and analgesic effects of cannabinoids are especially suited to the neuropathic pain that results from damaged corneas. The authors decided to study whether they could be used. Delta-8 THC, CBD and the synthetic cannabinoid HU-308 were administered.
The results were positive, with topical administration reducing corneal hyperalgesia and inflammation by targeting CB1R, CB2R, and 5-HT1A receptors, which are some of the main regulators of pain and inflammation in the body. This proof of concept needs more study.
CB1R, CB2R and 5-HT1A are all important endocannabinoid receptors, involved in inflammatory response, pain signaling, and immune responses. When the scientists who carried out this study looked at the body’s reaction to damaged corneas in rats, they found that the endocannabinoid system, of which these receptors are a large part, was very much involved in the pain and inflammatory response to damage.
Each of the cannabinoid receptors studied in this study was involved in some part of the neuropathic and inflammatory pain that results from corneal damage. Administering delta-8 THC and CBD caused activation of these receptors and a resulting lowering of inflammation and hyperalgesia. Each receptor is distinct but each produced analgesic effects when they were activated. Equally, each cannabinoid was demonstrably analgesic.
This is believed to be through the altering of nociceptive and inflammatory pathways. By changing the threshold a pain signal can pass through a neuron, the number of pain signals that reach the brain are reduced, altering the perception of pain. Inflammation causes a lowered pain threshold in sensory neurons; by reducing inflammation, the threshold is raised and fewer pain signals are passed. This appears to be mainly mediated by CB2R.
When the damaged eyes of the test subject animals were administered combinations of delta-8 THC and CBD, inflammation and hyperalgesia were lowered. This effect was also seen when either CBD or delta-8 THC was administered individually.
Through the different endocannabinoid pathways, they were each able to provide significant pain relief to the animals by reducing nociceptive and inflammatory signaling. Unfortunately, this is pre-clinical data, and more research will reveal the safest and most effective cannabinoids to be used, as well as appropriate doses.
The pain-relieving and anti-inflammatory effects of cannabinoids have been demonstrated for many parts of the body and different conditions. This study proves that the inflammation and pain caused by damage to the cornea can be effectively treated by targeting the endocannabinoid system.