Localisation of cannabinoid and cannabinoid-related receptors in the equine dorsal root ganglia

Background: Growing evidence recognises cannabinoid receptors as potential therapeutic targets for pain. Consequently, there is increasing interest in developing cannabinoid receptor agonists for treating pain. As a general rule, to better understand the actions of a drug, it would be of extreme importance to know the cellular distribution of its specific receptors. The localisation of cannabinoid receptors in the dorsal root ganglia of the horse has not yet been investigated. Objectives: To localise the cellular distribution of canonical and putative cannabinoid receptors in the equine cervical dorsal root ganglia. Study design: Qualitative and quantitative immunohistochemical study. Methods: Cervical (C6-C8) dorsal root ganglia were collected from six horses (1.5 years of age) at the slaughterhouse. The tissues were fixed and processed to obtain cryosections which were used to investigate the immunoreactivity of canonical cannabinoid receptors 1 (CB1R) and 2 (CB2R), and for three putative cannabinoid-related receptors: nuclear peroxisome proliferator-activated receptor alpha (PPARα), transient receptor potential ankyrin 1 (TRPA1) and serotonin 5-HT1a receptor (5-HT1aR). Results: The neurons showed immunoreactivity for CB1R (100%), CB2R (80% ± 13%), PPARα (100%), TRPA1 (74% ± 10%) and 5-HT1aR (84% ± 6%). The neuronal satellite glial cells showed immunoreactivity for CB2R, PPARα, TRPA1 and 5-HT1aR. Main limitations: The low number of horses included in the study. Conclusions: This study highlighted the expression of cannabinoid receptors in the sensory neurons and glial cells of the dorsal root ganglia. These findings could be of particular relevance for future functional studies assessing the effects of cannabinoids in horses to manage pain.