The role of delta opioid receptors in trigeminovascular pain

Chronic use of commonly used migraine therapies can lead to medication overuse headache (MOH). This is a paradoxical increase in severity of migraine-associated symptoms and headaches which are refractory to other treatments. Currently, the first-line treatment for MOH is drug cessation. However, during this abstinence period, patients continue to suffer from severe migraine, and a majority of MOH patients return to these medications within the first year. Targeted therapies specifically for MOH would result in better headache management and increased patient quality of life. One of the accomplishments of the previous funding cycle of this grant was to test δ opioid receptor (δOR) agonists in multiple headache models, including models of MOH. We found that δOR activation completely reversed cephalic allodynia induced by chronic medication treatment, revealing δOR agonists as a novel therapeutic strategy for MOH. In the previous funding cycle, we also performed a large scale unbiased peptidomic screen to identify overlapping mechanisms between chronic migraine and MOH. We identified pituitary adenylate cyclase activating polypeptide (PACAP) binding through PAC1 receptor as a potential link between these two disorders; and that the PACAPergic system may be distinctly involved in pain facilitation by chronic medication exposure. Upon further analysis we also found that there is high co-expression between δOR and PACAP or PAC1 in pain-processing regions, including in the periaqueductal grey (PAG) and trigeminal complex. The overall goal of this renewal is to build upon these exciting findings and determine if δOR agonists relieve migraine and MOH through inhibition of the PACAPergic system. In Aim 1, we will test G protein biased δOR agonists in novel translationally significant models of cephalic MOH and determine if they cause tolerance in this model. These studies will strengthen the evidence for drug development of δOR for MOH. In Aim 2 we will map the co-expression of δOR with PACAP and PAC1 and use biochemical and electrophysiological assays to investigate how δOR modulates PACAPergic signaling. Finally, in Aim 3 we will generate conditional knockouts of δOR in PACAP and PAC1 expressing cells, which will reveal if the behavioral effects of δOR agonists are regulated through PACAPergic signaling. The experiments proposed in this application are highly innovative and use a multidisciplinary approach. They will provide important insight on the effectiveness of δOR agonist as a therapeutic target for MOH and headache disorders more broadly and will determine if δOR agonists work through inhibition of the PACAPergic system. Further, the modulation of the PACAPergic system by δOR may be fundamental to other δOR behavioral effects, including emotional modulation and peripheral analgesia.