Morphine Induces Upregulation of Neuronally Expressed CB2 Receptors in the Spinal Dorsal Horn of Rats.

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Background: Cannabinoid receptors play a key role in regulating numerous physiological processes, including immune function and reward signaling. Originally, endocannabinoid contributions to central nervous system processes were attributed to CB1 receptors, but technological advances have confirmed the expression of CB2 receptors in both neurons and glia throughout the brain. Mapping of these receptors is less extensive than for CB1 receptors, and it is still not clear how CB2 receptors contribute to processes that involve endocannabinoid signaling. Objectives: The goal of our study was to assess the effects of peripheral nerve injury and chronic morphine administration, two manipulations that alter endocannabinoid system function, on CB2 receptor expression in the spinal dorsal horn of rats. Methods: Twenty-four male Sprague Dawley rats were assigned to chronic constriction injury (CCI), sham surgery, or pain naïve groups, with half of each group receiving once daily injections of morphine (5 mg/kg) for 10 days. On day 11, spinal cords were isolated and prepared for fluorescent immunohistochemistry. Separate sections from the deep and superficial dorsal horn were stained for neuronal nuclei (NeuN), CD11b, or 4',6-diamidino-2-phenylindole (DAPI) to mark neurons, microglia, and cell nuclei, respectively. Double labeling was used to assess colocalization of CB2 receptors with NeuN or microglial markers. Quantification of mean pixel intensity for each antibody was assessed using a fluorescent microscope, and CB2 receptor expressing cells were also counted manually. Results: Surgery increased DAPI cell counts in the deep and superficial dorsal horn, with CCI rats displaying increased CD11b labeling ipsilateral to the nerve injury. Surgery also decreased NeuN labeling in both regions, an effect that was blocked by morphine administration. CB2 receptors were expressed, predominantly, on NeuN-labeled cells with significant increases in CB2 receptor labeling across all surgery groups in both deep and superficial areas following morphine administration. Conclusions: Our findings provide supporting evidence for the expression of CB2 receptors on neurons and reveal upregulation of receptor expression in the dorsal spinal cord following surgery and chronic morphine administration, with the latter producing a larger effect. Synergistic effects of morphine-cannabinoid treatments, therefore, may involve CB2-mu opioid receptor interactions, pointing to novel therapeutic treatments for a variety of medical conditions.