PROJECT SUMMARY/ABSTRACT
Chronic systemic opioid use leads to misuse, addiction and overdoses. Topical and peripheral application
of low doses of opioids are safer alternatives, as they provide efficient local analgesia with limited penetration in
the central nervous system (CNS). Unfortunately, tolerance decreases peripheral opioid analgesia and the
mechanisms of this phenomenon remain unknown. We propose the innovative hypothesis that peripheral
tolerance could develop because of opioid action on skin keratinocytes and activation of peripheral
PDGFRß signaling. CNS-tolerance involves activation of the platelet-derived growth factor receptor beta
(PDGFRß) by the platelet-derived growth factor B (PDGF-B) ligand, which is released in response to activation
of the µ-opioid receptor (MOR) by opioids. Accordingly, intrathecal injections of imatinib, a PDGFRß inhibitor,
prevented opioid tolerance. Previous reports have hypothesized that central and peripheral tolerance occur by
different mechanisms. However, we found that imatinib can also prevent tolerance to peripherally administered
morphine, suggesting a shared mechanism that involves PDGFRß signaling. In separate studies, using mice
that express channelrhodopsin (ChR2) in keratinocytes (Krt14-ChR2), we found that optogenetic stimulation of
keratinocytes activates a subpopulation of peripheral sensory neurons (PSNs), likely through release of
neuroactivators from activated keratinocytes. Since PDGF-B is expressed in keratinocytes, these findings led to
the idea that keratinocytes are a component of peripheral tolerance. In exciting preliminary studies, we
discovered that repeated optogenetic stimulation of keratinocytes generated peripheral tolerance that could be
blocked by imatinib, providing evidence that peripheral tolerance involves keratinocyte and PDGFRß signaling.
We now hypothesize that peripheral tolerance is mediated by the release of PDGF-B from MOR-activated
keratinocytes with opioids. We will test this hypothesis by first, evaluating the role of PDGF-B signaling from
keratinocytes in the development of peripheral tolerance, using behavioral pharmacology, optogenetics,
proteomics and mRNA/protein imaging methods. Second, we will determine if MORs on keratinocytes mediate
peripheral tolerance, by generating mice in which MOR will be specifically deleted either from keratinocytes or
PSNs. We will also determine if keratinocytes are essential for peripheral tolerance, by evaluating if their selective
optogenetic inhibition alters the development of tolerance.
This project explores new cellular and molecular substrates for peripheral tolerance that involve epithelial-
neural communication. Outcomes from this study could lead to new therapeutic strategies to treat chronic pain
by repurposing PDGFRß inhibitors to prevent peripheral opioid tolerance. This approach has the potential to
significantly increase opioid safety in chronic pain treatment by minimizing undesirable central side effects.