Functional and molecular approaches to study mu and delta opioid receptor hetero-oligomerization in a model of neuropathic pain
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Neuropathic (NP) pain is a debilitating form of chronic pain that can result from a variety of disease states that target the sensory nervous system. NP pain can manifest as burning or shooting pain, and is commonly refractory to traditional analgesics including opioids. Clinically used opioids target the mu opioid receptors (MORs); preclinical reports, however, suggest that the delta opioid receptor (DOR) may be a valid target. It is now accepted that MORs and DORs oligomerize in heterologous expression systems and exhibit novel pharmacology distinct from their monomers. This research aimed to utilize both functional and molecular evidence to identify if a heteromeric mu-delta opioid receptor oligomer (M/DOR) forms in vivo and whether it is upregulated in NP pain. In an animal model of NP pain, animals displayed characteristic behaviours including protecting of the ipsilateral hindpaw from environmental stimuli, and mechanical allodynia in this paw. Behavioural studies reported that acute injection of DOR- selective agonists that bind M/DOR produced enhanced thermal antinociception and reversed mechanical allodynia in NP rats. DOR agonists that have low binding affinity for M/DOR did not produce enhanced thermal antinociception but did reverse mechanical allodynia in NP rats. Molecular studies were employed to characterize the molecular species of ORs in the lumbar spinal cord. Isolated spinal cord membranes were subjected to coimmunoprecipitation with a M/DOR antibody. Co-immunoprecipitation was unable to conclusively identify changes in M/DOR levels in the dorsal horn but did confirm that such a species exists in vivo. Furthermore, antibody characterization was completed to determine if the commercial antibodies used were labeling the appropriate OR proteins. HEK293T cells transfected with MOR and/or DOR plasmids were used in Western blotting and immunocytochemistry protocols to test commercially available antibodies. These studies determined that MOR and DOR antibodies do label their respective OR type, but also recognize additional proteins (non-specifically) in Western blotting protocols. In conclusion, behavioural studies revealed a putative role for M/DOR agonists in the treatment of NP pain; however, more sensitive tools and protocols must be developed before molecular experiments are able to identify quantifiable changes with endogenous M/DORs.