Background & Aims
Opioids are the most successful alternative to relieve severe pain, with applications in diverse conditions such as post-surgical pain, musculoskeletal, abdominal, and chest pain, management of palliative/end-of-life care, and cancer (1). Among the well-documented drawbacks of addiction and tolerance, repeated opioid exposure also leads to paradoxical pain (2). Opioid activation of µ-opioid receptor-1 triggers ?-arrestin2, which, in turn, causes MOR1 internalization and desensitization of the transient receptor potential vanilloid 1 channel (TRPV1) (3-5). Our preliminary results show that TRPV1 is directly involved in post-inflammatory thermal hyperalgesia through activation by RNA-dependent protein kinase (PKR), a sentinel molecule that reacts to pro-inflammatory stimuli (6).
This study aimed to investigate the role played by PKR in the analgesic effect of morphine in a mice model of incisional and burning pain.
Methods
Male and female C57BL/6 mice, aged 3-4 months old, were used. The inflammatory pain model consisted of a plantar incision made through the left hind paw’s skin, fascia, and muscle. Burning pain consisted of immersing the left hind paw in water at 60oC for 8 seconds. Morphine was administered at 1.0, 2.0, or 4.0mg/Kg (s.c.), whereas PKR inhibitor was given at 250?g/Kg (i.p.). Thermal hyperalgesia and protein expression were analyzed 1 and 72 hours after lesion by behavioral tests and western blot, respectively. Opioid tolerance was investigated in both models by injecting 4.0 mg/Kg of morphine twice a day for 5 consecutive days. Thermal hyperalgesia was then monitored 30 min and 24 hours after the injection. The effect of PKR inhibition was investigated on opioid-induced hyperalgesia caused by a single dose of morphine at 1.0, 2.0, or 4.0 mg/Kg. Data were analyzed by one-way or two-way ANOVA followed by Tukey HSD post-hoc analysis. Diferences were considered significant at p < 0.05.
Results
Both models caused marked thermal hyperalgesia at 1 and 72 hours after lesion. Moreover, morphine-induced analgesia in a dose-dependent manner. Inhibition of PKR activity 15 minutes before opioid administration reduced significantly, and in a dose-dependent manner, the antinociceptive effect of morphine in male and female mice (p < 0.001). Chronic treatment with morphine caused significant tolerance after incisional inflammatory lesion, whereas PKR inhibition before opioid administration aggravated the drug tolerance (p < 0.01). Opioid-induced hyperalgesia was also investigated. Low doses of morphine (1.0 and 2.0 mg/Kg) caused hyperalgesia in a dose-dependent manner, whereas the higher dose (4.0 mg/Kg) induced analgesia. However, PKR inhibition before a low dose of morphine administration significantly reversed the hyperalgesic effect in the first 60 minutes after opioid administration (p < 0.001).
Conclusions
Together, these data indicate that PKR activity influences the antinociceptive effect mediated by the µ-opioid receptors as well as the development of opioid tolerance and opioid-induced hyperalgesia. A potential interaction likely mediates these effects between PKR and TRPV1 receptors. The action of PKR on the µ-opioid receptor activity represents a new view of the opioid mechanism of action. Several possibilities have been pursued, such as (1) PKR interaction with TRPV1 or MOR, (2) ?-arrestin 2 interaction with PACT or PKR, and (3) PKR interaction with MOR, PACT and ?-arrestin-2 during pathological pain states.
References
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3. Rowan MP, Bierbower SM, Eskander MA, Szteyn K, Por ED, Gomez R, Veldhuis N, Bunnett NW, Jeske NA. Activation of mu-opioid receptors sensitizes transient receptor potential vanilloid type 1 (TRPV1) via ?-arrestin-2-mediated cross-talk. PLoS One, 9(4):e93688, 2014.
4. Scherer PC, Zaccor NW, Neumann NM, Vasavda C, Barrow R, Ewald AJ, Rao F, Sumner CJ, Snyder SH. TRPV1 is a physiological regulator of ?-opioid receptors. Proc Natl Acad Sci U S A., 114(51):13561-13566, 2017.
5. Melkes B, Markova V, Hejnova L, Novotny J. ?-Arrestin 2 and ERK1/2 Are Important Mediators Engaged in Close Cooperation between TRPV1 and µ-Opioid Receptors in the Plasma Membrane. Int J Mol Sci., 21(13):4626, 2020.
6. Gal-Ben-Ari S, Barrera I, Ehrlich M, Rosenblum K. PKR: A Kinase to Remember. Front Mol Neurosci., 11:480, 2019.
Presenting Author
Guilherme Lucas
Poster Authors
Topics
- Mechanisms: Biological-Molecular and Cell Biology