Date of Award
Doctor of Philosophy (PhD)
Chronic itch is a major unmet problem that cannot be treated with anti-histamines. Itch information is relayed from the skin to the brain via the spinal cord. Previous studies show that systemic activation of classically Gαi-coupled -opioid receptor (KOR) inhibits histaminergic and non-histaminergic itch. Nalfurafine, a KOR agonist, has been used in clinical trials to treat pruritus (itch), and has been approved to treat uremic itch in Japan. However, the underlying mechanisms are unknown. Gastrin-releasing peptide (GRP), a neurotransmitter released from sensory neurons and its receptor, gastrin-releasing peptide receptor (GRPR), an itch-specific receptor expressed in the dorsal horn of the spinal cord, are primarily required for mediating non-histaminergic itch. GRPR is a Gαq-coupled receptor that is also crucial for the development of chronic itch in mouse models. Here, we hypothesize that spinal KOR activation inhibits histamine-independent itch by blocking GRPR function. We found that spinal KOR activation by an exogenous KOR agonist U-50,488 attenuated histamine-independent itch and GRP-induced calcium responses of GRPR neurons via a Gαi-independent pathway and these effects can be mimicked by protein kinase C (PKC) activation in a Ca2+-independent manner. Using isoform specific siRNA knockdown approaches, we identified PKCδ as the isoform that mediates KOR activation induced itch inhibition. Conversely, blocking PKC activation abolishes KOR activation induced inhibition of itch signaling. KOR is co-expressed with GRPR in the dorsal horn of the spinal cord, and KOR heteromerizes with GRPR in human embryonic kidney (HEK293) cells. KOR activation induced a rapid and robust GRP-independent cross-phosphorylation of GRPR in HEK293 cells, which could be blocked by PKC inhibition. Interestingly, U-50,488 blocked GRP- but not AMPA-induced inward currents in GRPR neurons, suggesting that glutamatergic transmission is not involved in KOR-mediated inhibition of GRPR function. Mice lacking KOR or dynorphin, the endogenous agonist for KOR, display normal acute and chronic itch behavior, indicating that the endogenous dynorphin/KOR system is not involved in the inhibition of itch. Taken together, our studies suggest that spinal KOR inhibits itch by blocking the function of GRPR via Gαi- and Ca2+-independent PKC activation and phosphorylation.
Chair and Committee
Thomas Baranski, Michael Bruchas, N Gautam, Robert Gereau, Qin Liu
Munanairi, Admire, "Spinal k-Opioid Receptor Activation Attenuates Itch by Inhibiting Gastrin-releasing Peptide Receptor Function" (2015). Arts & Sciences Electronic Theses and Dissertations. 669.
Available for download on Thursday, August 15, 2115