Spinal cannabinoid receptor type 2 activation reduces hypersensitivity and spinal cord glial activation after paw incision.
BACKGROUND
Cannabinoids bind to cannabinoid receptors type 1 and 2 and produce analgesia in several pain models, but central side effects from cannabinoid 1 receptors limit their clinical use. Cannabinoid 2 receptors reduce inflammatory responses in the periphery by acting on immune cells, and they are present on glia in the central nervous system. This study tested whether spinal cannabinoid activation would induce analgesia, glial inhibition, and central side effects in a postoperative model or incisional pain.


METHODS
Rats underwent paw incision surgery, with intrathecal injections of cannabinoid agonists and antagonists and assessment of withdrawal thresholds and behavioral side effects. Spinal glial activation was determined by immunohistochemistry.


RESULTS
Intrathecal administration CP55940 reduced postoperative hypersensitivity (91 +/- 9% maximum possible effect; P < 0.05), and this was prevented by intrathecal administration of both cannabinoid 1 receptor (AM281) and cannabinoid 2 receptor (AM630) antagonists. CP55940 also caused several behavioral side effects, and these were prevented by the cannabinoid 1 receptor but not by the cannabinoid 2 receptor antagonist. Intrathecal injection of the cannabinoid 2 receptor agonist JWH015 reversed postoperative hypersensitivity (89 +/- 5% maximum possible effect; P < 0.05), and this was reversed by the cannabinoid 2 but not by the cannabinoid 1 receptor antagonist. JWH015, which did not induce behavioral side effects, reduced paw incision induced microglial and astrocytic activation in spinal cord (P < 0.05).


CONCLUSIONS
These data indicate that intrathecal administration of cannabinoid receptor agonists may provide postoperative analgesia while reducing spinal glial activation, and that selective cannabinoid 2 receptor agonists may do so without central side effects.
Romero-Sandoval
Alfonso
A
Department of Anesthesiology, Center for the Study of Pharmacologic Plasticity in the Presence of Pain, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157-1009, USA.
Eisenach
James C
JC
D000818:Animals; D003510:Cyclohexanes / Q000627:therapeutic use; D003511:Cyclohexanols; D007211:Indoles / Q000627:therapeutic use; D008297:Male; D009457:Neuroglia / Q000187:drug effects / Q000502:physiology; D010149:Postoperative Pain / Q000188:drug therapy*; D010636:Phenols / Q000627:therapeutic use; D051381:Rats; D017207:Rats, Sprague-Dawley; D043885:Receptor, Cannabinoid, CB2 / Q000819:agonists*; D012018:Reflex / Q000187:drug effects; D013116:Spinal Cord / Q000502:physiology*
C469526:Cnr2 protein, rat; D003510:Cyclohexanes; D003511:Cyclohexanols; D007211:Indoles; D010636:Phenols; D043885:Receptor, Cannabinoid, CB2; C054649:3-(2-hydroxy-4-(1,1-dimethylheptyl)phenyl)-4-(3-hydroxypropyl)cyclohexanol; C402944:JHW 015