Characterization of G-protein coupled receptors in pain, depression and anxiety

Chronic pain and major depressive disorder are widespread conditions in the world. Interestingly, these conditions often occur comorbidly, with each individual disease amplifying the symptoms of the other. A significant amount of preclinical research in pain and depression focuses on G-protein coupled receptors (GPCRs), implying that GPCRs may be useful in treating this comorbidity. Our efforts have sought to characterize several poorly understood GPCRs, including the serotonin receptor subtypes 2C and 7 (5-HT2CR and 5-HT7R) and metabotropic glutamate receptor 5 (mGluR5), along with more well-known GPCRs such as the mu opioid receptor (MOR), and the role that they play in comorbid pain and depression. Our approach for targeting these receptors uses (1) 5-HTR targeting compounds isolated from marine cyanobacteria, (2) a repurposed mGluR5 existing drug and (3) a novel formulation of a MOR existing drug. (1) From our cyanobacterial collections, compounds that were screened and tested showed strong in vitro affinity for both the 5-HT2CR and 5-HT7Rs. Cyanobacterial extracts were screened for in vivo activity using a series of pain and depression behavioral assays. Extracts were delivered into male and female mice via intracerebroventricular (ICV) cannulas for initial characterization. Knockout animals and delivery into specific areas of the brain known to express serotonin receptors was also completed. Extracts were also tested in naïve mice or in mice subjected to a model of comorbid pain and depression, the Spared Nerve Injury (SNI). We found cyanobacterial compounds that induce antidepressant and anxiolytic-like effects in male mice. These antidepressant and anxiolytic-like effects for 5-HT7R were found to be specific to males, as no behavioral effects were found in female mice and were also found to induce no effects in knockout animals. (2) For targeting mGluR5, a drug originally made for treating anxiety, fenobam, was evaluated. Fenobam was used to determine the role of mGluR5 in the conditioned place preference assay, an assay that tests for the reduction of spontaneous chronic pain. Mice with the chronic pain model of SNI were tested for the development of preference for the drug compared to sham animals. Fenobam was found to block the development of preference in animals with SNI surgery without affecting sham animals. (3) Lastly, for targeting MOR, the novel polymer drug called PolyMorphine was utilized. PolyMorphine was characterized in several different pain models and paradigms, including experimenter-induced and spontaneous models of pain. PolyMorphine was found to be analgesic in both mechanical and cognitive pain assays, and last significantly longer than traditional morphine. Overall, our results suggest that cyanobacteria produce compounds with neuroactive properties and that these compounds, along with repurposed and novel drugs, may be useful in understanding as well as treating spontaneous pain and comorbid pain and depression