Date of Award
3-7-2025
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Opioids are commonly prescribed for the treatment of pain and headache disorders. However, chronic opioid use can result in a paradoxical increased sensitivity to pain as well as an extension of pain area known as opioid-induced hyperalgesia (OIH). One field OIH has been clearly identified is migraine, known as medication overuse headache (MOH). While opioids may provide acute relief, chronic use results in increased migraine pain severity and progression of migraine from an episodic to chronic state. The primary treatment is opioid cessation, which is difficult to implement as patients are reluctant to stop treatment they believe is helping them, and they may also experience opioid withdrawal. Opioid cessation has low retention rates, and one study reports a 50% relapse rate within the first year, which can ultimately feed into a pattern of opioid use disorder. There is thus a desperate need to find effective therapies for OIH, and this disorder would benefit greatly from a focused drug development strategy. In this thesis, I provide a greater understanding about the pathophysiology of OIH and MOH and characterize novel treatment options. Using an unbiased proteomic screen, our lab had previously identified the pituitary adenylate cyclase activating polypeptide (PACAP) as a possible target for both OIH and migraine. In Chapter 2 I showed that PACAP alone could induce allodynia at translationally significant doses. I further demonstrated efficacy of a delta opioid receptor (DOR) agonist, SNC80, in reducing PACAP-induced allodynia. I also elucidated the expression of DOR with PACAP and its receptor PAC1 using in situ hybridization and found high co-expression of DOR with the PAC1 receptor. Together, these results suggest that PACAP can cause allodynia, that this allodynia is blocked by DOR, and that DOR may act by inhibiting PAC1 signaling. Previous studies from our lab have demonstrated the effectiveness of a peptide PAC1 antagonist M65 in an OIH model. PACAPergic blockade was shown to decrease migraine clinically. Small molecule PAC1 receptor antagonists and antibodies have different properties to peptides and are worth investigating in our OIH/MOH models. In Chapter 3, I demonstrate that a small molecule PAC1 inhibitor could block established and prevent development of OIH. Additionally, a PACAP targeting antibody also blocked established OIH and prevented development of OIH and MOH. PACAP and the related peptide, vasoactive intestinal peptide (VIP), can both bind to VPAC1, VPAC2, and PAC1. PAC1 has a lower affinity for VIP than PACAP. PACAP may also activate the MAS-related GPR, member B2 (Mrgprb2) receptor. I determined the receptor and endogenous ligand expression of the PACAPergic, and mu and delta opioid receptors, with cell type resolution using in situ hybridization in the trigeminal nucleus caudalis (TNC). The results indicate that of all PACAP receptors, PAC1 had the highest total expression (~90% cells), followed by VPAC2 (~52%), VPAC1 (~45%), and MRGPRB2 (10%). Considering this high expression, approximately 97% of all mu opioid receptor positive cells co-expressed PAC1 in this region. These results suggest that the PAC1 receptor plays an important role in head pain processing regions. Previously our lab had shown upregulation of PACAP in the periaqueductal gray (PAG) in models of OIH and chronic migraine. In Chapter 4, I focused on the role of the PAG in OIH specifically. I determined that among PACAP receptors, PAC1 exhibited the highest overall expression at 66% of all cells, followed by VPAC2 (~44%), VPAC1 (~27%), and MRGPRB2 (15%). Notably, approximately 86% of all mu opioid receptor positive cells were found to also express PAC1. I used excitatory chemogenetic and shRNA knockdown approaches to characterize whether PAC1 in the ventrolateral periaqueductal gray (vlPAG) is necessary and sufficient to produce mechanical hypersensitivity, similar to OIH. Acute excitation of PAC1-expressing vlPAG neurons resulted in freezing behavior and analgesia, but chronic excitation produced sustained mechanical allodynia. We also used a shRNA approach to knockdown PAC1 receptors in the vlPAG. Compared to the control-OIH group, chronic morphine in animals with knockdown of PAC1 in the vlPAG developed OIH to a lesser extent and recovered more quickly. Together, our data suggests that cells expressing PAC1 in the vlPAG are sufficient to induce hyperalgesia and that the chronicity of OIH is partially dependent on activation of vlPAG PAC1 receptors. The results presented in this thesis provide a deeper understanding of the pathophysiology of OIH and MOH and identify DOR agonists, PAC1 antagonists, and PACAP antibodies as potential therapies for this disorder.
Language
English (en)
Chair and Committee
Amynah Pradhan
Committee Members
Hadas Nahman-Averbuch; Paul Taghert; Ream Al-Hasani; Yu-Qing Cao
Recommended Citation
Mangutov, Elizaveta, "Pituitary Adenylate Cyclase Activating Polypeptide and Its Receptor as Novel Therapeutic Targets for Opioid Induced Hyperalgesia" (2025). Arts & Sciences Electronic Theses and Dissertations. 3454.
https://openscholarship.wustl.edu/art_sci_etds/3454