Research reportThe expression of a high level of morphine antinociceptive tolerance in mice involves both PKC and PKA
Introduction
The cellular mechanisms responsible for antinociceptive tolerance to opioids have been the subject of numerous research studies for several decades. Low intrinsic efficacy agonists such as morphine appear to produce tolerance in the absence of affecting mu-opioid receptor density and mRNA levels [5], [7], [48]. Instead, morphine may mediate tolerance through downstream Ca2+-dependent/independent-PKC and cAMP-PKA pathways, as opposed to high intrinsic efficacy agonists like etorphine that induce mu-opioid receptor desensitization and downregulation through G-protein-coupled receptor kinases and β-arrestin [22], [52]. In support of this, Bernstein and Welch [2] reported the PKA inhibitor KT-5720, but not the PKG inhibitor KT-5823, reversed antinociceptive tolerance in morphine pellet-implanted mice. Recently, it was reported that i.c.v. injections of the anti-sense oligodeoxynucleuotide to PKA mRNA blocked the tolerance induced to morphine [33]. Phospholipid signal transduction systems have also been implicated in opioid tolerance. We reported that inhibitors of phosphatidylcholine- and phosphatidylinositol-specific phospholipase C reversed morphine tolerance when injected 30-min before testing [36]. Furthermore, the inositol tris-phosphate (IP3) receptor antagonist low molecular-weight heparin also reversed tolerance in the same study. Studies have also focused on the role of PKC in tolerance. For example, PKC inhibitors such as chelerythrine chloride, H7 and calphostin C were able to prevent or reverse acute antinociceptive tolerance to mu- and delta-opioid agonists [3], [13], [29], [30]. In other studies, tolerance following chronic opioid administration was prevented by concomitantly infusing PKC inhibitor i.c.v. or spinally at the time opioids were administered [15], [28]. These PKC inhibitors were also effective in reversing opioid antinociceptive tolerance [15], [35], [36]. Recent studies demonstrated that spinal infusion of anti-sense oligodeoxynucleuotide to PKCalpha mRNA blocked the tolerance to spinally infused morphine [19].
All of these studies utilized models that induced a moderate 5- to 8-fold level of opioid tolerance. For this study, we developed a model of 45-fold tolerance to determine the effectiveness of PKC and PKA inhibitors to reverse a high level of morphine tolerance. We hypothesized that it would be necessary to co-administer both PKC and PKA inhibitors to completely reverse high levels of tolerance, since both PKC and PKA contribute to tolerance expression. Our results demonstrate that the PKC or PKA inhibitors administered alone only partly reversed the high level of morphine tolerance, but that co-administration of both classes of inhibitors completely reversed the tolerance.
Section snippets
Methods of handling mice
Male Swiss Webster mice (Harlan Laboratories, Indianapolis, IN) weighing 25–30 g were housed six to a cage in animal care quarters maintained at 22±2 °C on a 12-h light–dark cycle. Food and water were available ad libitum. The mice were brought to a test room (22±2 °C, 12-h light–dark cycle), marked for identification and allowed 24-h to recover from transport and handling. The Institutional Animal Care and Use Committee (IACUC) at the Virginia Commonwealth University School of Medicine approved
Influence of PKC and PKA inhibitors alone on high morphine tolerance
The antinociceptive tolerance observed following the implantation of a 75-mg morphine pellet and daily supplemental morphine administration resulted in a 45-fold reduction in the potency of morphine. In previous studies, implantation of the 75-mg morphine pellet alone yielded approximately a 5–8-fold level of tolerance [2], [35], [36]. Seventy-two hours after pellet implantation, the mice were tested with vehicle injections i.c.v. or the PKC inhibitor bisindolylmaleimide I. The 11.1 nmol dose
Reversal of moderate versus high morphine tolerance
The efficacy of PKC and PKA inhibitors to reverse morphine tolerance was determined utilizing animal models that express different degrees of tolerance. Each of these inhibitors were completely effective in reversing the 5–8-fold rightward shifts in the morphine dose–response curves [2], [35], [36]. Yet, when the degree of tolerance was increased to 45-fold, a threshold was crossed whereby tolerance reversal by either the PKC or PKA inhibitors alone was impossible. During high level opioid
Acknowledgements
This research was funded by the National Institute on Drug Abuse grants DA-01647-26 and K05-DA-00480; R.J. was supported by T32-DA-07027.
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2021, Neuroscience LettersCitation Excerpt :Moreover, pharmacological investigations have suggested the implication of several signaling proteins in morphine tolerance such as nitric oxide synthase (NOS) [69], protein kinase A (PKA), protein kinase C (PKC) [70], and calmodulin kinase ІІ (CaMKІІ) [71]. In line with these findings, some evidence has reported that tolerance to the anti-nociceptive effects of morphine can be abolished by administration of PKC inhibitors [72,73]. It seems that cross regulation of μ-opioid and NMDA receptors causes desensitization of μ-opioid receptors [74].
Agonist-dependent μ-opioid receptor signaling can lead to heterologous desensitization
2010, Cellular SignallingEvidence for an important role of protein phosphatases in the mechanism of morphine tolerance
2007, Brain ResearchCitation Excerpt :Shen et al. (2000) demonstrated that i.c.v. administration of the anti-sense oligodeoxynucleuotide to PKA mRNA blocked the antinociceptive tolerance to morphine in mice (Shen et al., 2000). We previously reported that i.c.v. administration of the inhibitors of PKA, PKI-(14-22)-amide and 4-cyano-3-methylisoquinoline and KT-5720 completely reversed 5- to 8-fold and only partly 45-fold morphine antinociceptive tolerance (Smith et al., 2003; Javed et al., 2004; Dalton et al., 2005). We also demonstrated that PKA inhibition reinstated antinociception, hypothermia and Straub tail in mice implanted with morphine pellets 72-h earlier (Smith et al., 2006).