Single-molecule characterization of opioid receptor heterodimers reveals soluble µ-δ dimer blocker peptide alleviates morphine tolerance.

An Opiate Cocktail that Reduces Morphine Tolerance and Dependence

Long-term morphine administration leads to tolerance and a gradual reduction in analgesic potency. Noncoding microRNAs (miRNAs) modulate gene expression in a posttranscriptional manner, and their dysregulation causes various diseases. Emerging evidence suggests that miRNAs play a regulatory role in the development of morphine tolerance. In the present study, we hypothesized that miR-873a-5p is a key functional small RNA that participates in the development and maintenance of morphine tolerance through the regulation of A20 (tumor necrosis factor α-induced protein 3, TNFAIP3) in mice. We measured the percentage of maximum possible effect (MPE %) to evaluate the analgesic effect of morphine. The expression of miR-873a-5p and its target gene A20 were determined after the morphine-tolerant model was successfully established. Intrathecal injection with lentivirus to intervene in the expression of A20 and the miR-873a-5p antagomir was used to explore the role of miR-873a-5p in the development of morphine tolerance. Chronic morphine administration significantly increased the expression of miR-873a-5p, which was inversely correlated with decreased A20 expression in the spinal cord of morphine-tolerant mice. Downregulation of miR-873a-5p in the spinal cord attenuated and partly reversed the development of morphine tolerance accompanied by overexpression of A20. Similarly, A20 was upregulated by a recombinant lentivirus vector, which attenuated and reversed the pathology of morphine tolerance by inhibiting the activation of nuclear factor (NF)-κB. Collectively, our results indicated that miR-873a-5p targets A20 in the spinal cord to facilitate the development of morphine tolerance in mice. Downregulating the expression of miR-873a-5p may be a potential strategy to ameliorate morphine tolerance.

Morphine and related opiates are commonly used in the clinical management of various types of pain. However, the antinociceptive properties of morphine are often overshadowed by the development of tolerance and dependence following its chronic use. The mechanisms underlying opiate tolerance are not fully understood, but appear to involve numerous and complex physiological adaptations. Recently, a role for the heterodimerization of mu and delta opioid receptors in the development of morphine tolerance has been proposed. This novel mechanism could help us to understand several observations, such as the critical role of delta opioid receptor regulation, the impact of delta opioid receptor binding site occupancy, and the participation of beta-arrestin2, in the development of morphine tolerance.

Morphine tolerance in mice is independent of polymorphisms in opioid receptor sequences

The spinal nitric oxide involved in the inhibitory effect of midazolam on morphine-induced analgesia tolerance

Vigabatrin attenuates the development and expression of tolerance to morphine-induced antinociception in mice

Morphine is a powerful analgesic but its effect is often diminished owing to the development of tolerance. It has been suggested that morphine activates microglia through its action on the toll-like receptor 4 (TLR4) in the spinal cord, leading to suppression of the morphine effect. However, it has not been examined whether the development of morphine tolerance is affected by the deletion and mutation of the TLR4 gene. Mice were treated with morphine (60 mg/kg) or vehicle once daily for five consecutive days to induce morphine tolerance, which was assessed by the tail-flick test before and after the treatment period. The effect of the microglial inhibitor minocycline, and the effect of TLR4 mutation (C3H/HeJ mouse) and deletion (TLR4-knockout mouse) on the development of morphine tolerance were tested. The expression of the microglial activation marker, CD11b, in the spinal cords of TLR4-knockout and wild-type mice after morphine treatment for 5 days was assessed by reverse-transcription polymerase chain reaction. Minocycline attenuated the development of morphine tolerance in mice. Mutation or deletion of the TLR4 gene did not significantly affect the development of morphine tolerance. CD11b mRNA expression was increased after morphine treatment both in TLR4-knockout and wild-type mice. Microglial activation caused by a mechanism independent of TLR4 is involved in the development of morphine tolerance. Further studies are necessary to clarify the cellular mechanisms of morphine-induced microglial activation.

Differential regulation of β-arrestin 1 and β-arrestin 2 gene expression in rat brain by morphine

Spinal interaction between the highly selective μ agonist DAMGO and several δ opioid receptor ligands in naive and morphine-tolerant mice

The tail-flick test was used to investigate the effects of chronic administration of the N-methyl-D-aspartate (NMDA) receptor antagonists, dextromethorphan, memantine and MRZ 2/579, on the development and reversal of morphine tolerance in mice in three separate experiments. Experiment 1 investigated the effects of NMDA receptor antagonists on the development of tolerance. Morphine (10 mg/kg for 6 days, twice daily) produced a 5.9-fold rightward shift of the cumulative dose-response curves. Co-administration of dextromethorphan, memantine or MRZ 2/579 between tests 1 and 2 dose-dependently (5-10 mg/kg) inhibited the development of morphine tolerance. In experiment 2, in which the effects on the reversal were investigated, morphine-tolerant mice were treated b.i.d. for an additional 6 days (between tests 2 and 3) with vehicle+vehicle, NMDA receptor antagonist+vehicle, vehicle+morphine or NMDA receptor antagonist+morphine. Morphine-tolerant mice treated with vehicle+vehicle remained morphine tolerant, whereas this residual morphine tolerance was inhibited by administration of all three NMDA antagonists (each 10 mg/kg). Morphine-tolerant mice receiving vehicle+morphine injections demonstrated an unchanged degree of antinociceptive tolerance. In these mice, the co-administration of memantine and MRZ 2/579, but not dextromethorphan, resulted in the reversal of morphine tolerance. In experiment 3, memantine and MRZ 2/579 (10 mg/kg) inhibited the acute antinociceptive effect of morphine, but dextromethorphan did not. These data indicate that low-affinity, clinically available and/or therapeutically promising NMDA receptor antagonists may be used to inhibit ongoing morphine tolerance.

Morphine tolerance involves numerous and complex mechanisms. The heterodimerization of mu and delta opioid receptors put forward recently is one of the important mechanisms which cause the development of opiate tolerance. Once transported to the membrane of neural cells, delta opioid receptors (DOR) combine with mu opioid receptors (MOR) to form MOR-DOR heterodimers,leading to the formation of new signaling pathway. This novel mechanism plays the critical role of DOR in the development of morphine tolerance. This review summarizes the regulation of DOR on MOR function during morphine tolerance in light of the recent findings about the regulation of DOR membrane transport and the new complex: MOR-DOR heterodimer. Key words: Morphine tolerance; MOR-DOR heterodimerization; MOR gene knockout; β-arrestin; Signaling regulation

Tolerance to the analgesic effects of an opioid occurs after its chronic administration, a pharmacological phenomenon that has been associated with the development of abnormal pain sensitivity such as hyperalgesia. In the present study, we examined the role of spinal glutamate transporters (GTs) in the development of both morphine tolerance and associated thermal hyperalgesia. Chronic morphine administered through either intrathecal boluses or continuous infusion induced a dose-dependent downregulation of GTs (EAAC1 and GLAST) in the rat's superficial spinal cord dorsal horn. This GT downregulation was mediated through opioid receptors because naloxone blocked such GT changes. Morphine-induced GT downregulation reduced the ability to maintain in vivo glutamate homeostasis at the spinal level, because the hyperalgesic response to exogenous glutamate was enhanced, including an increased magnitude and a prolonged time course, in morphine-treated rats with reduced spinal GTs. Moreover, the downregulation of spinal GTs exhibited a temporal correlation with the development of morphine tolerance and thermal hyperalgesia. Consistently, the GT inhibitor l-trans-pyrrolidine-2-4-dicarboxylate (PDC) potentiated, whereas the positive GT regulator riluzole reduced, the development of both morphine tolerance and thermal hyperalgesia. The effects from regulating spinal GT activity by PDC were at least in part mediated through activation of the NMDA receptor (NMDAR), because the noncompetitive NMDAR antagonist MK-801 blocked both morphine tolerance and thermal hyperalgesia that were potentiated by PDC. These results indicate that spinal GTs may contribute to the neural mechanisms of morphine tolerance and associated abnormal pain sensitivity by means of regulating regional glutamate homeostasis.

Morphine tolerance does not develop in mice treated with endothelin-A receptor antagonists

The ventrolateral periaqueductal gray (vlPAG) is an important midbrain region that regulates the descending pain modulatory system and is implicated in the development of antinociceptive tolerance to opioid analgesics. Protein Kinase-C (PKC) is a Ser/Thr kinase that phosphorylates the μ-opioid receptor (MOPr) following morphine activation. This phosphorylation is responsible for desensitization of the receptor, resulting in the termination of G-Protein signaling. It is speculated that PKC-mediated desensitization of the MOPr does not readily lead to the internalization of the receptor, thus contributing to tolerance. Previous studies have shown that intracerebroventricular administration of PKC inhibitors prevented the development of morphine tolerance in mice. Further, a non-specific PKC/GRK inhibitor was able to block the development of morphine tolerance in the vlPAG. However, the contribution of PKC to morphine tolerance in the vlPAG has not been directly explored. Using immunofluorescence staining, we observed increased phosphorylated PKC that is co-localized with the MOPr at the cell membrane of vlPAG neurons in morphine-tolerant animals. Further, we assessed the development of morphine antinociceptive tolerance following pre-treatment with the PKC inhibitor, Gö-7874, which targets conventional PKC isoforms. Repeated microinjections of the compound into the vlPAG of wild-type mice in combination with morphine led to a reduction in the development of morphine tolerance. Interestingly, repeated microinjections of Gö-7874 alone led to an enhancement of subsequent morphine antinociception. Lastly, morphine-induced changes in intracellular cAMP levels were assessed in-vitro, following pre-treatment with Gö-7874. This study contributes to the understanding of PKC-mediated MOPr desensitization processes leading to morphine tolerance.

Grand opening of structure-guided design for novel opioids