“Marijuana extracts (cannabinoids) have been used for several millennia for pain treatment.

Regarding the site of action, cannabinoids are highly promiscuous molecules, but only two cannabinoid receptors (CB₁ and CB₂) have been deeply studied and classified.

Thus, therapeutic actions, side effects and pharmacological targets for cannabinoids have been explained based on the pharmacology of cannabinoid CB₁/CB₂ receptors. However, the accumulation of confusing and sometimes contradictory results suggests the existence of other cannabinoid receptors.

Different orphan proteins (e.g., GPR18, GPR55, GPR119, etc.) have been proposed as putative cannabinoid receptors.

According to their expression, GPR18 and GPR55 could be involved in sensory transmission and pain integration.

This work summarized novel data supporting that, besides cannabinoid CB₁ and CB₂receptors, GPR18 and GPR55 may be useful for pain treatment.

Conclusion: There is evidence to support an antinociceptive role for GPR18 and GPR55.”

https://www.ncbi.nlm.nih.gov/pubmed/30670965

https://www.frontiersin.org/articles/10.3389/fphar.2018.01496/full