Effects Of Mu Opioid Receptor Agonists Research Articles

Opioid Antagonists from the Orvinol Series as Potential Reversal Agents for Opioid Overdose.

The opioid crisis continues to claim many lives, with a particular issue being the ready availability and use (whether intentional or accidental) of fentanyl and fentanyl analogues. Fentanyl is both potent and longer-acting than naloxone, the standard of care for overdose reversal, making it especially deadly. Consequently, there is interest in opioid reversal agents that are better able to counter its effects. The orvinol series of ligands are known for their high-affinity binding to opioid receptors and often extended duration of action; generally, compounds on this scaffold show agonist activity at the kappa and the mu-opioid receptor. Diprenorphine is an unusual member of this series being an antagonist at mu and only a partial agonist at kappa-opioid receptors. In this study, an orvinol antagonist, 14, was designed and synthesized that shows no agonist activity in vitro and is at least as good as naloxone at reversing the effects of mu-opioid receptor agonists in vivo.

Lack of Antinociceptive Cross-Tolerance With Co-Administration of Morphine and Fentanyl Into the Periaqueductal Gray of Male Sprague-Dawley Rats

Tolerance to the antinociceptive effect of mu-opioid receptor agonists, such as morphine and fentanyl, greatly limits their effectiveness for long-term use to treat pain. Clinical studies have shown that combination therapy and opioid rotation can be used to enhance opioid-induced antinociception once tolerance has developed. The mechanism and brain regions involved in these processes are unknown. The purpose of this study was to evaluate the contribution of the ventrolateral periaqueductal gray (vlPAG) to antinociceptive tolerance and cross-tolerance between administration and co-administration of morphine and fentanyl. Tolerance was induced by pretreating rats with morphine or fentanyl or low-dose combination of morphine and fentanyl into the vlPAG followed by an assessment of the cross-tolerance to the other opioid. In addition, tolerance to the combined treatment was assessed. Cross-tolerance did not develop between repeated vlPAG microinjections of morphine and fentanyl. Likewise, there was no evidence of cross-tolerance from morphine or fentanyl to the co-administration of morphine and fentanyl. Co-administration did not cause cross-tolerance to fentanyl. Cross-tolerance was only evident to morphine or morphine and fentanyl combined in rats pretreated with co-administration of low doses of morphine and fentanyl. This finding is consistent with the functionally selective signaling that has been reported for antinociception and tolerance after morphine and fentanyl binding to the mu-opioid receptor. This research supports the notion that combination therapy and opioid rotation may be useful clinical practices to decrease opioid tolerance and other side effects. PerspectiveThis preclinical study shows that there is a decrease in cross-tolerance between morphine and fentanyl within the periaqueductal gray, which is a key brain region in opioid antinociception and tolerance.

Long-Lasting Effects of Methocinnamox on Opioid Self-Administration in Rhesus Monkeys.

Opioid abuse remains a serious public health challenge, despite the availability of medications that are effective in some patients (naltrexone, buprenorphine, and methadone). This study explored the potential of a pseudoirreversible mu-opioid receptor antagonist [methocinnamox (MCAM)] as a treatment for opioid abuse by examining its capacity to attenuate the reinforcing effects of mu-opioid receptor agonists in rhesus monkeys. In one experiment, monkeys responded for heroin (n = 5) or cocaine (n = 4) under a fixed-ratio schedule. Another group (n = 3) worked under a choice procedure with one alternative delivering food and the other alternative delivering the mu-opioid receptor agonist remifentanil. A third group (n = 4) responded for food and physiologic parameters were measured via telemetry. The effects of MCAM were determined in all experiments and, in some cases, were compared with those of naltrexone. When given immediately before sessions, naltrexone dose-dependently decreased responding for heroin and decreased choice of remifentanil while increasing choice of food, with responding returning to baseline levels 1 day after naltrexone injection. MCAM also decreased responding for heroin and decreased choice of remifentanil while increasing choice of food; however, opioid-maintained responding remained decreased for several days after treatment. Doses of MCAM that significantly decreased opioid-maintained responding did not decrease responding for cocaine or food. MCAM did not impact heart rate, blood pressure, body temperature, or activity at doses that decreased opioid self-administration. Because MCAM selectively attenuates opioid self-administration for prolonged periods, this novel drug could be a safe and effective alternative to currently available treatments for opioid abuse.

Sex differences in peripheral mu-opioid receptor mediated analgesia in rat orofacial persistent pain model.

Unilateral ligation of the tendon of anterior superficial part of rat masseter muscle (TASM) leads to long-lasting allodynia. Sex differences in peripheral mu-opioid receptor (MOR)-mediated analgesia under persistent myogenic pain are not well understood. In this study, we examined (1) whether locally applied MOR agonists attenuate persistent pain following TASM ligation in a sex dependent manner, (2) whether there are sex differences of MOR expression changes in rat trigeminal ganglia (TG). The effects of MOR agonist, D-Ala2, N–Me-Phe4, Gly5-ol]-Enkephalin acetate salt (DAMGO), were assessed 14 days after TASM ligation in male, female and orchidectomized (GDX) male rats. MOR mRNA and protein levels in TG 14 days following tendon ligation were also determined. The mechanical thresholds of the injured side were significantly decreased in both male and female rats, from 3 days to 28 days after TASM ligation. A10 μg DAMGO significantly attenuated allodynia in male rats. A 10-fold higher dose of DAMGO was required in female and GDX male rats to produce the level of anti- allodynia achieved in male rats. The level of MOR mRNA in TG from male rats was significantly greater 14 days after TASM ligation compared with the sham-operated male rats, but not from female and GDX male rats. After TASM ligation, males had significantly more MOR immunoreactivity in TG compared to sham-operated males. The MOR levels increased to 181.8% of the sham level in male rats receiving tendon injury. But there was no significant change in female rats receiving tendon injury compared to the sham female rats. Taken together, our data suggest that there were sex differences in the effects of peripheral MOR agonists between male and female rats under TASM ligation developing long-lasting pain condition, which is partly mediated by sex differences in the changes of MOR expressions and testosterone is an important factor in the regulation of MOR.

The Influence of Opioids on Urokinase Plasminogen Activator on Protein and mRNA Level in MCF‐7 Breast Cancer Cell Line

Urokinase plasminogen activator plays a key role in tumor-associated processes, increasing cancer cell invasion and metastasis, and is therefore used as a marker in cancer prognosis. In this study, we have determined the effect of mu-opioid receptor agonists and antagonists on the urokinase plasminogen activator secretion in MCF-7 cell line. It was shown that mu-opioid receptor agonists, such as morphine and endomorphins, greatly stimulate urokinase plasminogen activator secretion, while naloxone and MOR-selective antagonists elicit the opposite effect. The same tendency was observed also on the urokinase plasminogen activator mRNA level. However, neither agonists nor antagonists had any effect on proliferation of MCF-7 cells. The findings reported in this study may be useful in designing further experiments aimed at elucidating the role of the opioid system in cancer cells.

Sustained Inhibition of Neurotransmitter Release from Nontransient Receptor Potential Vanilloid Type 1-Expressing Primary Afferents by μ-Opioid Receptor Activation-Enkephalin in the Spinal Cord

Removing transient receptor potential vanilloid type 1 (TRPV1)-expressing primary afferent neurons reduces presynaptic mu-opioid receptors but potentiates opioid analgesia. However, the sites and underlying cellular mechanisms for this paradoxical effect remain uncertain. In this study, we determined the presynaptic and postsynaptic effects of the mu-opioid receptor agonist [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (DAMGO) using whole-cell patch-clamp recordings of lamina II neurons in rat spinal cord slices. Treatment with the ultrapotent TRPV1 agonist resiniferotoxin (RTX) eliminated TRPV1-expressing dorsal root ganglion neurons and their central terminals in the spinal dorsal horn and significantly reduced the basal amplitude of glutamatergic excitatory postsynaptic currents (EPSCs) evoked from primary afferents. Although RTX treatment did not significantly alter the concentration-response effect of DAMGO on evoked monosynaptic and polysynaptic EPSCs, it causes a profound long-lasting inhibitory effect of DAMGO on evoked EPSCs. Subsequent naloxone treatment did not reverse the prolonged inhibitory effect of DAMGO on evoked EPSCs. Furthermore, brief application of DAMGO produced a sustained inhibition of miniature EPSCs in RTX-treated rats. However, the concentration response and the duration of the effects of DAMGO on G protein-coupled inwardly rectifying K+ currents in lamina II neurons were not significantly different between vehicle- and RTX-treated groups. These data suggest that stimulation of mu-opioid receptors on non-TRPV1 afferent terminals causes extended inhibition of neurotransmitter release to spinal dorsal horn neurons. The differential effect of mu-opioid receptor agonists on different phenotypes of primary afferents provides a cellular basis to explain why the analgesic action of opioids on mechanonociception is prolonged when TRPV1-expressing primary afferents are removed.

Differential effects of mu-opioid receptor agonists in a hippocampal hypoxia/hypoglycemia model

In rat hippocampal slices, a short hypoxia/hypoglycemia causes immediate loss of evoked potentials (population spike amplitude) in the CA1 region and the extent of electrophysiological restoration during reperfusion can serve as a parameter for cell function. Previous experiments using this model revealed that exposure to morphine aggravates the neurotoxic effects of a subsequent hypoxia/hypoglycemia in a concentration-dependent manner. Therefore, the aim of the present study was to evaluate the effects of additional mu-opioid receptor (MOPr) agonists on the electrophysiological restoration after hypoxia/hypoglycemia. Rat hippocampal slices were exposed to either morphine (10 microM), pethidine (10 microM), fentanyl (100 nM/1 microM) or to the synthetic peptide [d-Ala2, N-Me-Phe4, Glycinol5]-enkephalin (DAMGO, 10 microM) for 60 min; thereafter, slices underwent a brief hypoxic/hypoglycemic episode followed by reperfusion (drug-free) for 2.5 h. Electrophysiological recording consisted of determination of population spike amplitude in CA1 in response to constant stimulation of Schäffer's collaterals. Exposure to morphine prior to hypoxia/hypoglycemia resulted in a significantly impaired electrophysiological recovery during reperfusion when compared to controls. Following exposure to pethidine, the electrophysiological recovery was slightly reduced, whereas fentanyl or DAMGO did not affect restoration of population spike amplitude during reperfusion. The results of the present study demonstrate that different MOPr agonists differentially influence the electrophysiological recovery of hippocampal slices following a brief hypoxia/hypoglycemia. It is speculated that known receptor-internalizing opioids such as fentanyl or DAMGO may have less neurotoxic effect in hypoxia/hypoglycemia than the non-internalizing drug morphine.

Understanding How Opioids Contribute to Reward and Analgesia

Opioids acting at the mu opioid (MOP) receptor produce powerful analgesia. They also produce an intensely rewarding effect that can lead to addiction. The analgesic effect of MOP receptor agonists derives from a direct inhibitory effect on pain transmission at the spinal-cord level and through activation of a descending pain-modulatory pathway. The rewarding effect of MOP agonists is the result of their actions in the mesostriatal dopamine pathway classically associated with both natural and drug rewards. Both the analgesic and rewarding effect of MOP agonists are best understood in the context of decision making under conditions of conflict. Pain is one of many competing motivational states, and endogenous opioids suppress responses to noxious stimuli in the presence of conflicting motivations, such as hunger or a threatening predator. When a food reward is available, MOP agonists microinjected into the mesostriatal circuit promote its consumption, while concomitantly suppressing responses to noxious stimulation. The mesostriatal "reward" circuit, thus, appears to perform a function critical to decision making and can either amplify or suppress responses to noxious stimuli.

Effector antagonism by the regulators of G protein signalling (RGS) proteins causes desensitization of mu-opioid receptors in the CNS

In cell culture systems, agonists can promote the phosphorylation and internalization of receptors coupled to G proteins (GPCR), leading to their desensitization. However, in the CNS opioid agonists promote a profound desensitization of their analgesic effects without diminishing the presence of their receptors in the neuronal membrane. Recent studies have indicated that CNS proteins of the RGS family, specific regulators of G protein signalling, may be involved in mu-opioid receptor desensitization in vivo. In this work we review the role played by RGS proteins in the intensity and duration of the effects of mu-opioid receptor agonists, and how they influence the delayed tolerance that develops in response to specific doses of opioids. RGS proteins are GTPase-activating proteins (GAP) that accelerate the hydrolysis of GalphaGTP to terminate signalling at effectors. The GAP activity of RGS-R4 and RGS-Rz proteins restricts the amplitude of opioid analgesia, and the efficient deactivation of GalphazGTP subunits by RGS-Rz proteins prevents mu receptor desensitization. However, RGS-R7 proteins antagonize effectors by binding to and sequestering mu receptor-activated Galphai/o/z subunits. Thus, they reduce the pool of receptor-regulated G proteins and hence, the effects of agonists. The delayed tolerance observed following morphine administration correlates with the transfer of Galpha subunits from mu receptors to RGS-R7 proteins and the subsequent stabilization of this association. In the CNS, the RGS proteins control the activity of mu opioid receptors through GAP-dependent (RGS-R4 and RGS-Rz) as well as by GAP-independent mechanisms (RGS-R7). As a result, they can both antagonize effectors and desensitize receptors under certain circumstances.

Modification of morphine dependence under chronic pain and its mechanism

Clinical studies have demonstrated that when opiates are used to control cancer pain, psychological dependence and analgesic tolerance are not a major concern. The present study was, therefore, designed to investigate the modulation of rewarding effects of opiates under inflammatory chronic pain in SD rats. Formalin (2.5%, 50 microliters) or carrageenan (1%, 100 microliters) was injected into the plantar surface of the rat paw. Formalin and carrageenan reduced the paw pressure threshold. The hyperalgesia lasted for 9 to 13 days. Rewarding effect of morphine was evaluated by conditioned place preference paradigm. Morphine produced a significant place preference. This effect was significantly attenuated in inflamed groups as compared with the respective non-inflamed groups. Furthermore, the morphine-induced place preference in the inflamed group gradually recovered to the respective control level as the inflammation healed. On the other hand, we found that kappa-opioid receptor agonists markedly inhibit rewarding effect of mu-opioid receptor agonists. Therefore, to elucidate the mechanism of this attenuation, the effects of pretreatment with kappa- and delta-opioid receptor antagonists, nor-binaltorphimine (nor-BNI) and naltrindole (NTI), on the development of the morphine-induced place preference under inflammation were examined. Nor-BNI, but not NTI, eliminated the suppression of the morphine-induced place preference in inflamed groups. The morphine-induced increase in dopamine turnover in the limbic forebrain was suppressed under inflammation, and the suppression was abolished by the pretreatment with nor-BNI. These results suggest that endogenous kappa-opioid systems may be activated by chronic inflammatory nociception, resulting in the suppression of the development of rewarding effects produced by morphine.

Modulation of NMDA-induced currents by mu-opioid receptor agonist DAGO in acutely isolated rat spinal dorsal horn neurons.

The whole-cell patch-clamp technique was used to examine effects of mu-opioid receptor agonist Tyr-D-Ala-Gly-Me-Phe-Gly-ol-enkephalin (DAGO) on N-methyl-D-aspartate (NMDA)-induced currents in freshly enzymatically and/or mechanically dissociated rat spinal dorsal horn neurons (laminae I-III). Here we report that the responses of dorsal horn neurons to NMDA were modulated by DAGO in a complex manner. When applied simultaneously with, or prior to NMDA, DAGO (10-100 nM) initially suppressed the response to NMDA in 52% of examined cells. Following removal of DAGO, the NMDA responses were potentiated in 71% of the cells. The enhancing effect of DAGO was rapid in onset and lasted up to 50 min after removal of the peptide. Both the initial depressant and the late enhancing effect were reversed by naloxone. These results are consistent with the possibility that DAGO might directly modulate the NMDA receptor-ion channel complex and that this action may contribute to the regulation of the strength of excitatory amino acid-mediated primary afferent neurotransmission, including nociception.