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Cellular Distribution of Canonical and Putative Cannabinoid Receptors in Canine Cervical Dorsal Root Ganglia

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Cellular Distribution of Canonical and Putative Cannabinoid Receptors in Canine Cervical Dorsal Root Ganglia

Roberto Chiocchetti et al. Front Vet Sci.

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Abstract

Growing evidence indicates cannabinoid receptors as potential therapeutic targets for chronic pain. Consequently, there is an increasing interest in developing cannabinoid receptor agonists for treating human and veterinary pain. To better understand the actions of a drug, it is of paramount importance to know the cellular distribution of its specific receptor(s). The distribution of canonical and putative cannabinoid receptors in the peripheral and central nervous system of dogs is still in its infancy. In order to help fill this anatomical gap, the present ex vivo study has been designed to identify the cellular sites of cannabinoid and cannabinoid-related receptors in canine spinal ganglia. In particular, the cellular distribution of the cannabinoid receptors type 1 and 2 (CB1 and CB2) and putative cannabinoid receptors G protein-coupled receptor 55 (GPR55), nuclear peroxisome proliferator-activated receptor alpha (PPARα), and transient receptor potential vanilloid type 1 (TRPV1) have been immunohistochemically investigated in the C6-C8 cervical ganglia of dogs. About 50% of the neuronal population displayed weak to moderate CB1 receptor and TRPV1 immunoreactivity, while all of them were CB2-positive and nearly 40% also expressed GPR55 immunolabeling. Schwann cells, blood vessel smooth muscle cells, and pericyte-like cells all expressed CB2 receptor immunoreactivity, endothelial cell being also PPARα-positive. All the satellite glial cells (SGCs) displayed bright GPR55 receptor immunoreactivity. In half of the study dogs, SGCs were also PPARα-positive, and limited to older dogs displayed TRPV1 immunoreactivity. The present study may represent a morphological substrate to consider in order to develop therapeutic strategies against chronic pain.

Keywords: G protein-coupled receptor 55; cannabinoid receptor 1; cannabinoid receptor 2; endocannabinoids; nuclear peroxisome proliferator-activated receptor alpha; satellite glial cells; transient receptor potential vanilloid type 1.

Figures

Figure 1
Figure 1
Representative image of Western blots (WB) analysis showing the specificity of the primary antibody rabbit anti-G protein-coupled receptor 55 (GPR55). The antibody revealed a single band of expected molecular weight (~40 kDa). The images of the different immunoblots were slightly adjusted in brightness and contrast to match their backgrounds.
Figure 2
Figure 2
(a–d) Photomicrographs of cryosections of canine cervical (C8) dorsal root ganglion showing cannabinoid receptor 1 (CB1) immunoreactivity. Small stars indicate small neurons showing CB1 receptor weak to moderate immunoreactivity. Large stars indicate CB1 receptor negative. Arrows indicate satellite glial cells showing weak CB1 receptor immunoreactivity. Bar: a–d = 50 μm.
Figure 3
Figure 3
Photomicrographs of cryosections of canine cervical (C8) dorsal root ganglion showing cannabinoid receptor 2- (CB2), glial fibrillary acidic protein- (GFAP), and CD31-immunoreactivity. (a–c) Stars indicate NeuroTrace labeled (a) dorsal root ganglion sensory neurons which were CB2 receptor negative (b), as well as the satellite glial cells (white arrows). (d–f) Stars indicate sensory neurons encircled by satellite glial cells (white arrows) which were GFAP-immunoreactive (e) and CB2 receptor negative. CB2 receptor immunoreactivity was expressed by Schwann cells and neuronal nuclei (open arrow). (g–i) The empty arrow indicates one neuronal axon that bifurcates (T-junction) in its central and peripheral portions (large white arrows). The small arrows indicate the nuclei of Schwann cells. (j–l) Open arrows indicate smooth muscle cells (vessel on the left) and pericyte-like cells (elongated and thin blood vessel on the right) showing CB2 receptor immunoreactivity (j). White arrows indicate endothelial cells showing CD31 immunoreactivity (k). Bar: a–f, j–l = 50 μm; g–i = 100 μm.
Figure 4
Figure 4
Photomicrographs of cryosections of canine cervical (C8) dorsal root ganglion showing GPR55 (a–f) and PPARalpha (g–i) immunolabeling. (a–c) Arrows indicate the Neurotrace-labeled nuclei of satellite glial cells (a) which showed bright GPR55 immunolabelling (b). White stars indicate unlabeled sensory neurons; open stars indicate empty spaces in which sensory neurons were no more evident. (d–f) White arrows indicate satellite glial cells which co-expressed bright GPR55- (d) and glial fibrillary acidic protein (GFAP) immunoreactivity; open arrows indicate SGCs which were GPR55 immunoreactive and GFAP negative (e). Stars indicate sensory neurons of different dimension, which expressed faint –to-moderate GPR55 immunoreactivity. (g–i) White arrows indicate the Neurotrace labeled nuclei of SGCs which showed PPARalpha immunoreactivity (h). Open arrow indicate autofluorescent pigment. Bar: a–i = 50 μm.
Figure 5
Figure 5
(a–f) Photomicrographs of cryosections of the C8 cervical dorsal root ganglia belonging to two aged dogs showing transient receptor potential vanilloid type 1 (TRPV1) immunoreactivity. White stars indicate neurons showing bright TRPV1 immunoreactivity, while open stars indicate larger neurons showing weaker TRPV1 immunoreactivity. Arrows indicate the Neurotrace labeled nuclei of satellite glial cells showing bright TRPV1 immunolabeling (b,e). Bar: a–f = 50 μm.

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