Mu Opioid Splice Variant MOR-1K Contributes to the Development of Opioid-Induced Hyperalgesia

Abstract

Background: A subset of the population receiving opioids for the treatment of acute and chronic clinical pain develops a paradoxical increase in pain sensitivity known as opioid-induced hyperalgesia. Given that opioid analgesics are one of few treatments available against clinical pain, it is critical to determine the key molecular mechanisms that drive opioid-induced hyperalgesia in order to reduce its prevalence. Recent evidence implicates a splice variant of the mu opioid receptor known as MOR-1K in the emergence of opioid-induced hyperalgesia. Results from human genetic association and cell signaling studies demonstrate that MOR-1K contributes to decreased opioid analgesic responses and produces increased cellular activity via Gs signaling. Here, we conducted the first study to directly test the role of MOR-1K in opioid-induced hyperalgesia.

Methods and results: In order to examine the role of MOR-1K in opioid-induced hyperalgesia, we first assessed pain responses to mechanical and thermal stimuli prior to, during, and following chronic morphine administration. Results show that genetically diverse mouse strains (C57BL/6J, 129S6, and CXB7/ByJ) exhibited different morphine response profiles with corresponding changes in MOR-1K gene expression patterns. The 129S6 mice exhibited an analgesic response correlating to a measured decrease in MOR-1K gene expression levels, while CXB7/ByJ mice exhibited a hyperalgesic response correlating to a measured increase in MOR-1K gene expression levels. Furthermore, knockdown of MOR-1K in CXB7/ByJ mice via chronic intrathecal siRNA administration not only prevented the development of opioid-induced hyperalgesia, but also unmasked morphine analgesia.

Conclusions: These findings suggest that MOR-1K is likely a necessary contributor to the development of opioid-induced hyperalgesia. With further research, MOR-1K could be exploited as a target for antagonists that reduce or prevent opioid-induced hyperalgesia.

Fig 1. Strains exhibit divergent behavioral responses to mechanical stimuli at baseline.

Compared to C57BL/6J mice, 129S6, and CXB7/ByJ mice exhibit differences in (A) paw withdrawal threshold, (B) the number of responses to repeated presentation of a noxious mechanical stimulus sensitivity, and (C) the number of responses to continuous thermal heat. N = 15-16/group. Data expressed as mean ± SEM. ***p<0.001, **p<0.01, *p<0.05 different from C57BL/6J. ^### p<0.001 different from 129S6.

Fig 2. Strains exhibit divergent morphine-dependent analgesic and allodynic/hyperalgesic profiles with corresponding changes in MOR-1K gene expression.

(A) A timeline of chronic morphine administration and the assessments of pain behavior and gene expression are shown. (B-E) Compared to C57BL/6J, 129S6 mice exhibit increased paw withdrawal thresholds and fail to develop allodynia or hyperalgesia. In contrast, CXB7/ByJ exhibit decreased paw withdrawal thresholds and increased allodynia and hyperalgesia. (F) 129S6 mice exhibit decreased MOR-1K gene expression levels on days 1 and 4, corresponding to their analgesic behavioral profile. In contrast, CXB7/ByJ mice exhibit increased MOR-1K gene expression levels on day 4, corresponding to their hyperalgesic behavioral profile. For behavioral graphs, blue background indicates a decrease in pain sensitivity (analgesia), and red background indicates an increase in pain sensitivity (allodynia/hyperalgesia). Panels B-E: N = 15-16/group. Data expressed as mean ± SEM. ***p<0.001, **p<0.01, *p<0.05 different from C57BL/6J. ^### p<0.001, ^## p<0.01, ^# p<0.05 different from 129S6. Panel F: N = 7/group. Data expressed as means ± SEM. *p<0.05 different from baseline.

Fig 3. CXB7/ByJ mice treated with exon 13 antisense siRNA fail to develop OIH.

(A) A timeline of sustained i.t. siRNA delivery, chronic morphine administration, and assessments of pain behavior and gene expression is shown. CXB7/ByJ mice receiving antisense siRNA exhibit analgesia following morphine administration and cessation, and fail to exhibit (B) decreased paw withdrawal thresholds, or increased responses to (C) repeated presentation of an innocuous or (D) noxious mechanical stimulus. In contrast, mice receiving sense siRNA or sham exhibited analgesia following morphine administration, which was then followed by allodynia/hyperalgesia on days 5–7. (E) CXB7/ByJ mice receiving antisense siRNA exhibit decreased MOR-1K gene expression levels in the spinal cord as compared to those receiving sense siRNA. For all behavioral graphs, blue background indicates a decrease in pain sensitivity (analgesia), and red background indicates an increase in pain sensitivity (allodynia/hyperalgesia). Panels B-D: N = 6-9/group. Data expressed as mean ± SEM.****p<0.0001, ***p<0.001, **p<0.01, *p<0.05 different from Sham. Panel E: N = 3-4/group. **p<0.01 different from Sense.