Cell-autonomous regulation of Mu-opioid receptor recycling by substance P.
Bottom Line: SP, through activation of the neurokinin 1 (NK1R) receptor, increases the post-endocytic recycling of the mu-opioid receptor (MOR) in trigeminal ganglion (TG) neurons in an agonist-selective manner.SP-mediated protein kinase C (PKC) activation is both required and sufficient for increasing recycling of exogenous and endogenous MOR in TG neurons.Our results define a physiological pathway that cross-regulates opioid receptor recycling via direct modification of MOR and suggest a mode of homeostatic interaction between the pain and analgesic systems.
How neurons coordinate and reprogram multiple neurotransmitter signals is an area of broad interest. Here, we show that substance P (SP), a neuropeptide associated with inflammatory pain, reprograms opioid receptor recycling and signaling. SP, through activation of the neurokinin 1 (NK1R) receptor, increases the post-endocytic recycling of the mu-opioid receptor (MOR) in trigeminal ganglion (TG) neurons in an agonist-selective manner. SP-mediated protein kinase C (PKC) activation is both required and sufficient for increasing recycling of exogenous and endogenous MOR in TG neurons. The target of this cross-regulation is MOR itself, given that mutation of either of two PKC phosphorylation sites on MOR abolishes the SP-induced increase in recycling and resensitization. Furthermore, SP enhances the resensitization of fentanyl-induced, but not morphine-induced, antinociception in mice. Our results define a physiological pathway that cross-regulates opioid receptor recycling via direct modification of MOR and suggest a mode of homeostatic interaction between the pain and analgesic systems.
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Mentions: To test if SP regulated the resensitization of MOR-mediated analgesia in mice, we measured the development of acute tolerance to the antinociceptive effects of fentanyl, a short-acting MOR agonist, using a warm-water tail-withdrawal assay (Melief et al., 2010; Pradhan et al., 2010; Figure S3A). After baseline measurements, animals were injected with fentanyl, and tail-withdrawal latencies were measured every 30 min. A significant but sub-maximal increase in tail-withdrawal latencies, persisting for approximately 120 min, was observed with fentanyl (Figure 7A). Either saline (vehicle control) or SP was injected intrathecally 120 min after the first fentanyl challenge. In control mice, a fentanyl rechallenge, given 30 min later, attenuated (~40% of initial) the antinociceptive response, indicating acute tolerance to fentanyl (Figure 7A). In contrast, SP-injected mice showed an antinociceptive response to the rechallenge that was comparable to the initial response (Figures 7A and S3B). Calculation of the areas under the curve showed that saline-injected mice showed a significantly reduced response to the fentanyl rechallenge compared to the initial response, while SP-injected mice showed comparable responses to both fentanyl injections (Figure 7B). Because morphine-activated MORs were not subject to SP-regulated recycling, we next tested if SP could sensitize morphine-induced analgesia in mice. Consistent with our cellular data, a morphine rechallenge following SP injection did not increase tail-withdrawal latency in contrast to fentanyl (Figures 7C and 7D and S3C). Taken together, our results indicate that SP signaling through PKC inhibits acute tolerance to fentanyl, but not morphine, by increasing MOR recycling in peripheral neurons.