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

Endocannabinoid system and phytocannabinoids in the main species of veterinary interest: a comparative review

Since the discovery of the endocannabinoid system and due to the empirical evidence of the therapeutic effects on several illnesses both in humans and animals that follow the administration of exogenous cannabinoids (i.e., phytocannabinoids), numerous studies have been conducted. These investigations aimed to identify the expression and distribution of cannabinoid receptors in healthy and pathologic organs and tissues of different animal species and to define the interactions of phytocannabinoids with these receptors. In the last decade, pharmacokinetics, efficacy and tolerability of many Cannabis derivatives formulations, mainly containing cannabidiol, in the main species of veterinary interest, have been also investigated. This manuscript summarizes the findings reported by the scientific studies published so far on the molecular mode of action of the main phytocannabinoids, the localization of cannabinoid receptors in organs and tissues, as well as the pharmacokinetics, efficacy and tolerability of Cannabis derivatives in dogs, cats, horses and other species of veterinary interest. A deep knowledge of these issues is crucial for the use of phytocannabinoids for therapeutic purposes in animal species.

Improved quality of life and pain relief in mature horses with osteoarthritis after oral transmucosal cannabidiol oil administration as part of an analgesic regimen

The aim of this study was to evaluate the effect of oral cannabidiol (CBD) administration in addition to a conventional analgesic protocol on the clinical signs of 20 horses with mild joint osteoarthritis. The horses were randomly assigned to either the control group (C group) or the cannabidiol group (CBD group). Both groups were treated with phenylbutazone for 5 days. The CBD group received 0.03 mg/kg cannabidiol in hemp oil orally once daily for 14 days in addition to phenylbutazone treatment. All subjects were monitored for clinical parameters, oxidative status and blood counts. Pain and quality of life were also assessed using the Horse Chronic Pain Scale (HCPS). The CBD group showed a significant reduction in heart rate, respiratory rate, white blood cell count and oxidative stress (malondialdehyde lipid peroxidation). A significant reduction in HCPS scores was seen in both groups. Lower scores were recorded in the CBD group (3 med; range: 2/4) than in the C group (7 med; range: 4/10). The addition of a cannabidiol-based product to an analgesic protocol was well tolerated and showed positive effects on the treated subjects, improving their quality of life and pain relief.

Expression of Cannabinoid Receptors in the Trigeminal Ganglion of the Horse

Cannabinoid receptors are expressed in human and animal trigeminal sensory neurons; however, the expression in the equine trigeminal ganglion is unknown. Ten trigeminal ganglia from five horses were collected post-mortem from an abattoir. The expression of cannabinoid receptors type 1 (CB1R) and type 2 (CB2R), and the cannabinoid-related receptors like transient receptor potential vanilloid type 1 (TRPV1), peroxisome proliferator-activated receptor gamma (PPARɣ), and G protein-related receptor 55 (GPR55) in the trigeminal ganglia (TG) of the horse were studied, using immunofluorescence on cryosections and formalin-fixed paraffin-embedded (FFPE) sections. Neurons and glial cells were identified using fluorescent Nissl staining NeuroTrace® and an antibody directed against the glial marker glial fibrillary acidic protein (GFAP), respectively. Macrophages were identified by means of an antibody directed against the macrophages/microglia marker ionized calcium-binding adapter molecule 1 (IBA1). The protein expression of CB1R, CB2R, TRPV1, and PPARɣ was found in the majority of TG neurons in both cryosections and FFPE sections. The expression of GPR55 immunoreactivity was mainly detectable in FFPE sections, with expression in the majority of sensory neurons. Some receptors were also observed in glial cells (CB2R, TRPV1, PPARγ, and GPR55) and inflammatory cells (PPARγ and GPR55). These results support further investigation of such receptors in disorders of equine trigeminal neuronal excitability.

The use of cannabidiol as a novel treatment for oral stereotypic behaviour (crib-biting) in a horse

Behaviour is the response of living things to their environment and external stimulation, and is one of the parameters to be observed when assessing animal welfare. Any alteration from the conditions found in nature can lead to the occurrence of some specific behaviours, called stereotypies which are characterised as repetitive, consistent patterns of behaviour usually defined as having no apparent ultimate or proximal functions. It has been reported that once stabled or subjected to stressful activities, horses have more susceptibility of developing behavioural disturbances; therefore, behavioural disorders in horses are a strong indicator of poor welfare. Cannabis spp.-derived molecules have been studied under different medical conditions; the therapeutic potentials of phytocannabinoids are related to the effects of delta-9-tetrahydrocannabinol, cannabidiol (CBD), and other compounds. Cannabidiol has many activities within the central nervous system, such as anxiolytic, antidepressant, antipsychotic, anticonvulsant, and anti-inflammatory activities. Some studies have recently shown the potential and successful therapeutic use of phytocannabinoids in veterinary medicine. This clinical case report described a 22-year-old mare suffering from chronic crib-biting and wind-sucking, and the successful outcome of four weeks-therapy with CBD. This is the first report of the successful therapeutic use of phytocannabinoids in equine behavioural disorders.

Expression of cannabinoid (CB1 and CB2) and cannabinoid-related receptors (TRPV1, GPR55, and PPARα) in the synovial membrane of the horse metacarpophalangeal joint

Background

The metacarpophalangeal joint undergoes enormous loading during locomotion and can therefore often become inflamed, potentially resulting in osteoarthritis (OA). There are studies indicating that the endocannabinoid system (ECS) modulates synovium homeostasis, and could be a promising target for OA therapy. Some cannabinoid receptors, which modulate proliferative and secretory responses in joint inflammation, have been functionally identified in human and animal synovial cells.

Objective

To characterize the cellular distribution of the cannabinoid receptors 1 (CB1R) and 2 (CB2R), and the cannabinoid-related receptors transient receptor potential vanilloid type 1 (TRPV1), G protein-related receptor 55 (GPR55) and peroxisome proliferator-activated receptor alpha (PPARα) in the synovial membrane of the metacarpophalangeal joint of the horse.

Animals

The dorsal synovial membranes of 14 equine metacarpophalangeal joints were collected post-mortem from an abattoir.

Materials and methods

The dorsal synovial membranes of 14 equine metacarpophalangeal joints were collected post-mortem from an abattoir. The expression of the CB1R, CB2R, TRPV1, GPR55, and PPARα in synovial tissues was studied using qualitative and quantitative immunofluorescence, and quantitative real-time reverse transcriptase PCR (qRT-PCR). Macrophage-like (MLS) and fibroblast-like (FLS) synoviocytes were identified by means of antibodies directed against IBA1 and vimentin, respectively.

Results

Both the mRNA and protein expression of the CB2R, TRPV1, GPR55, and PPARα were found in the synoviocytes and blood vessels of the metacarpophalangeal joints. The synoviocytes expressed the mRNA and protein of the CB1R in some of the horses investigated, but not in all.

Conclusions and clinical importance

Given the expression of the CB1R, CB2R, TRPV1, GPR55, and PPARα in the synovial elements of the metacarpophalangeal joint, these findings encouraged the development of new studies supporting the use of molecules acting on these receptors to reduce the inflammation during joint inflammation in the horse.

Cannabinoid receptors are expressed in equine synovium and upregulated with synovitis

BACKGROUND

Osteoarthritis (OA) is a major cause of equine lameness. Cannabinoid receptors (CB) are now considered to be promising therapeutic targets in human rheumatology for pain and inflammation, however little is known about the equine endocannabinoid system.

OBJECTIVES

The primary goal was to assess the presence and expression pattern of CB1 and CB2 in the synovium of healthy joints. A secondary goal was to explore the relationship between the CB expression, degree of synovitis and OA pathology.

STUDY DESIGN

Ex vivo experimental study.

METHODS

Metacarpophalangeal joints (n=25) from a tissue bank were studied. The joints were dissected, and the articular cartilage lesions were scored. Synovial membrane specimens (n=45) were harvested, fixed and the degree of synovitis was graded on histological sections. Co-localised synovial sections were also immunostained with antibodies to CB1 and CB2. Five regions of interest (ROIs) were randomly selected from digital images of manually segmented synovial intima and scored blindly for positive cellular immunoreactive staining by 2 independent observers. Interobserver agreement was calculated with an intraclass correlation coefficient (ICC). Relationships between CB1 and CB2 immunoreactive scores and synovitis or joint OA grade were explored with mixed linear models.

RESULTS

CB1 was expressed in synovial intimal cells in all specimens studied whereas CB2 expression was identified in 94%. Both receptors were also expressed in the subintimal blood vessel walls. ICCs were 84.6% (CB1) and 92.9% (CB2) for the immunoreactivity scores. Both CB1 and CB2 expression were significantly upregulated (p=0.04 and p=0.03 respectively) with increasing degree of synovitis. Conversely, CB1 expression significantly decreased (p=0.03) with increasing severity of OA.

MAIN LIMITATIONS

The type of synovial cell expressing CB1 or CB2 was not investigated.

CONCLUSIONS

Equine synovial intimal cells constitutively express both CB1 and CB2 receptors that are upregulated with synovitis and may have a role in joint pain. They are potential targets for therapy with cannabinoid molecules or their derivatives. This article is protected by copyright. All rights reserved.

The cannabinoid receptors system in horses: Tissue distribution and cellular identification in skin

BACKGROUND

The endocannabinoid system (ECS) is composed of cannabinoid receptors type 1 (CBR1) and type 2 (CBR2), cannabinoid-based ligands (endogenous chemically synthesized phytocannabinoids), and endogenous enzymes controlling their concentrations. Cannabinoid receptors (CBRs) have been identified in invertebrates and in almost all vertebrate species in the central and peripheral nervous system as well as in immune cells, where they control neuroimmune homeostasis. In humans, rodents, dogs, and cats, CBRs expression has been confirmed in the skin, and their expression and tissue distribution become disordered in pathological conditions. Cannabinoid receptors may be a possible therapeutic target in skin diseases.

OBJECTIVES

To characterize the distribution and cellular expression of CBRs in the skin of horses under normal conditions.

ANIMALS

Fifteen healthy horses.

METHODS

Using full-thickness skin punch biopsy samples, skin-derived primary epidermal keratinocytes and dermal-derived cells, we performed analysis of Cnr1 and Cnr2 genes using real-time PCR and CBR1 and CBR2 protein expression by confocal microscopy and Western blotting.

RESULTS

Normal equine skin, including equine epidermal keratinocytes and dermal fibroblast-like cells, all exhibited constant gene and protein expression of CBRs.

CONCLUSIONS AND CLINICAL IMPORTANCE

Our results represent a starting point for developing and translating new veterinary medicine-based pharmacotherapies using ECS as a possible target.

Cannabidiol and the possibilities of its use in veterinary medicine of dogs and horses: A brief review

In connection with the use of cannabinoids for therapeutic purposes in human medicine, there is increased attention for their use in veterinary medicine, particularly by the owners of companion animals and horses. Therefore, veterinarians are expected to face this interest and have the corresponding knowledge on these substances. Presently, it is not possible to use medical marijuana (in terms of the dried cannabis flowers) for veterinary purposes in many countries, but there is increasing evidence that isolated cannabinoids also have beneficial effects (namely cannabidiol - CBD). Thus, this review summarises the possible therapeutic implications of CBD within the scope of evidence-based medicine, particularly in dogs and horses in association with the treatment of pain, epilepsy and anxiety in order to provide veterinarians with a concise overview of scientific findings in this field.

Role of primary sensory neurone cannabinoid type-1 receptors in pain and the analgesic effects of the peripherally acting agonist CB-13 in mice

BACKGROUND

Cannabinoid type-1 receptors (CB1Rs) are expressed in primary sensory neurones, but their role in pain modulation remains unclear.

METHODS

We produced Pirt-CB1R conditional knockout (cKO) mice to delete CB1Rs in primary sensory neurones selectively, and used behavioural, pharmacological, and electrophysiological approaches to examine the influence of peripheral CB1R signalling on nociceptive and inflammatory pain.

RESULTS

Conditional knockout of Pirt-CB1R did not alter mechanical or heat nociceptive thresholds, complete Freund adjuvant-induced inflammation, or heat hyperalgesia in vivo. The intrinsic membrane properties of small-diameter dorsal root ganglion neurones were also comparable between cKO and wild-type mice. Systemic administration of CB-13, a peripherally restricted CB1/CB2R dual agonist (5 mg kg-1), inhibited nociceptive pain and complete Freund adjuvant-induced inflammatory pain. These effects of CB-13 were diminished in Pirt-CB1R cKO mice. In small-diameter neurones from wild-type mice, CB-13 concentration-dependently inhibited high-voltage activated calcium current (HVA-ICa) and induced a rightward shift of the channel open probability curve. The effects of CB-13 were significantly attenuated by AM6545 (a CB1R antagonist) and Pirt-CB1R cKO.

CONCLUSION

CB1R signalling in primary sensory neurones did not inhibit nociceptive or inflammatory pain, or the intrinsic excitability of nociceptive neurones. However, peripheral CB1Rs are important for the analgesic effects of systemically administered CB-13. In addition, HVA-ICa inhibition appears to be a key ionic mechanism for CB-13-induced pain inhibition. Thus, peripherally restricted CB1R agonists could have utility for pain treatment.

Peripherally administered cannabinoid receptor 2 (CB2R) agonists lose anti-allodynic effects in TRPV1 knockout mice, while intrathecal administration leads to anti-allodynia and reduced GFAP, CCL2 and TRPV1 expression in the dorsal spinal cord and DRG

The transient receptor potential (TRP) superfamily of cation channels, of which the TRP vanilloid type 1 (TRPV1) receptor plays a critical role in inflammatory and neuropathic pain, is expressed on nociceptors and spinal cord dorsal horn neurons. TRPV1 is also expressed on spinal astrocytes and dorsal root ganglia (DRG) satellite cells. Agonists of the cannabinoid type 2 receptor (CB2R) suppress allodynia, with some that can bind TRPV1. The neuroimmune C-C class chemokine-2 (CCL2) expressed on injured DRG nociceptor cell bodies, Schwann cells and spinal astrocytes, stimulates immune cell accumulation in DRG and spinal cord, a known critical element in chronic allodynia. The current report examined whether two CB2R agonists, AM1710 and AM1241, previously shown to reverse light touch mechanical allodynia in rodent models of sciatic neuropathy, require TRPV1 activation that leads to receptor insensitivity resulting in reversal of allodynia. Global TRPV1 knockout (KO) mice with sciatic neuropathy given intrathecal or intraperitoneal AM1710 were examined for anti-allodynia followed by immunofluorescent microscopy analysis of lumbar spinal cord and DRG of astrocyte and CCL2 markers. Additionally, immunofluorescent analysis following intrathecal AM1710 and AM1241 in rat was performed. Data reveal that intrathecal AM1710 resulted in mouse anti-allodynia, reduced spinal astrocyte activation and CCL2 expression independent of TRPV1 gene deletion. Conversely, peripheral AM1710 in TRPV1-KO mice failed to reverse allodynia. In rat, intrathecal AM1710 and AM1241 reduced spinal and DRG TRPV1 expression, with CCL2-astrocyte and -microglial co-expression. These data support that CB2R agonists can impact spinal and DRG TRPV1 expression critical for anti-allodynia.

Cellular distribution of cannabinoid-related receptors TRPV1, PPAR-gamma, GPR55 and GPR3 in the equine cervical dorsal root ganglia

BACKGROUND

The activation of cannabinoid and cannabinoid-related receptors by endogenous, plant-derived or synthetic cannabinoids may exert beneficial effects on pain perception. Of the cannabinoids contained in Cannabis sativa, cannabidiol (CBD) does not produce psychotropic effects and seems to represent a molecule having great therapeutic potential. Cannabidiol acts on a great number of cannabinoid and cannabinoid-related G-protein-coupled receptors and ionotropic receptors which have, to date, been understudied in veterinary medicine particularly in equine medicine.

OBJECTIVES

To localise the cellular distribution of four putative cannabinoid-related receptors in the equine cervical dorsal root ganglia (DRG).

STUDY DESIGN

A qualitative and quantitative immunohistochemical study.

METHODS

The cervical (C6-C8) DRG of six slaughtered horses were obtained from a local slaughterhouse. The tissues were fixed and processed for immunohistochemistry, and the resulting cryosections were used to investigate immunoreactivity for the following putative CBD receptors: Transient receptor potential vanilloid type 1 (TRPV1), nuclear peroxisome proliferator-activated receptor gamma (PPARγ), and G protein-coupled receptors 55 (GPR55) and 3 (GPR3).

RESULTS

Large percentages of neuronal cell bodies showed immunoreactivity for TRPV1 (80 ± 20%), PPARγ (100%), GPR55 (64 ± 15%) and GPR3 (63 ± 11%). The satellite glial cells (SGCs) were immunoreactive for TRPV1, PPARγ and GPR55. In addition, GPR55 immunoreactivity was expressed by DRG interneuronal macrophages. In addition, microglia cells were observed surrounding the neuron-SGC complex.

MAIN LIMITATIONS

The limited number of horses included in the study.

CONCLUSIONS

Cannabinoid-related receptors were distributed in the sensory neurons (TRPV1, PPARγ, GPR55 and GPR3), SGCs (TRPV1, PPARγ and GPR55), macrophages (GPR55) and other interneuronal cells (PPARγ and GPR55) of the equine DRG. Given the key role of DRG cellular elements and cannabinoid receptors in the pathophysiology of pain, the present findings provided an anatomical basis for additional studies aimed at exploring the therapeutic uses of non-psychotropic cannabinoid agonists for the management of pain in horses.