Distribution of Cannabinoid Receptors in Keratinocytes of Healthy Dogs and Dogs With Atopic Dermatitis

Abstract

It is commonly accepted that some form of skin barrier dysfunction is present in canine atopic dermatitis (AD), one of the most common cutaneous pruritic inflammatory diseases of dogs. The impaired skin barrier function facilitates the penetration of allergens and subsequently stronger sensitization responses. The role of the endocannabinoid system (ECS) in the physiology and pathology of the skin is becoming increasingly established. It has been demonstrated that cannabinoid receptors are expressed in healthy and diseased skin and, based on current knowledge, it could be stated that cannabinoids are important mediators in the skin. The present study has been designed to immunohistochemically investigate the expression of the cannabinoid receptors type 1 (CB1R) and 2 (CB2R) and the cannabinoid-related receptors G protein-coupled receptor 55 (GPR55), transient receptor potential vanilloid 1 (TRPV1) and ankyrin 1 (TRPA1), peroxisome proliferator-activated receptors alpha (PPARα), and serotoninergic receptor 1a (5-HT1aR) in keratinocytes of healthy dogs and of dogs with AD. Samples of skin tissues were collected from 7 healthy controls (CTRL-dogs) and from 8 dogs with AD (AD-dogs). The tissue samples were processed using an immunofluorescence assay with commercially available antibodies, and the immunolabelling of the receptors studied was quantitatively evaluated. The keratinocytes of the CTRL- and the AD-dogs showed immunoreactivity for all the receptors investigated with a significant upregulation of CB2R, TRPA1, and 5-HT1aR in the epidermis of the AD-dogs. The presence of cannabinoid and cannabinoid-related receptors in healthy keratinocytes suggested the possible role of the ECS in canine epidermal homeostasis while their overexpression in the inflamed tissues of the AD-dogs suggested the involvement of the ECS in the pathogenesis of this disease, having a possible role in the related skin inflammation and itching. Based on the present findings, the ECS could be considered a potential therapeutic target for dogs with AD.

Cannabinoid receptors in the inflammatory cells of canine atopic dermatitis

“…Sensory neurons expressing TRPA1-IR usually co-express TRPV1-IR ( 86 ). However, TRPA1 is also functionally expressed in skin cells, such as keratinocytes, MCs, macrophages, DCs, melanocytes, and endothelial cells ( 45 , 87 – 89 ). The observation of TRPA1-IR in the MCs of the AD-dogs was consistent with the findings of Oh et al ( 87 ) who showed TRPA1 in the MCs of AD lesions of humans and mice.…”

“…Eight client-owned dogs diagnosed with spontaneous AD, based on predefined diagnostic criteria ( 45 ) and on the exclusion of other causes of pruritus (flea bite, allergic dermatitis and adverse food reaction), were enrolled ( Table 1 ). Cutaneous samples were collected from the AD-dogs on which no treatment had been made in the previous 6 months.…”

“…Written client consent was obtained prior to the enrollment of all cases. The skin samples utilized in the current study derived from the same dogs included in a previous study ( 45 ).…”

“…The homology between the specific amino acid sequences of the dog and the rat immunogens was 87.51%. The specificity of the rabbit anti-TRPV1 antibody, which has recently been tested on the canine nervous system ( 49 ), was tested using a preadsorption test on the canine skin ( 45 ).…”

“…The role of the ECS in the keratinocytes of healthy dogs was recently analyzed, and the upregulation of the cannabinoid receptors (CB1R and CB2R) and cannabinoid-related receptors (GPR55, TRPV1, TRPA1; PPARα, serotonin 1A [5-HT1aR]) was evaluated in dogs with AD ( 45 ). In that study, CB2R, GPR55, TRPV1 and TRPA1 immunoreactivity was also observed on different cellular elements of the dermis.…”