Morphine induces hyperalgesia without involvement of μ-opioid receptor or morphine-3-glucuronide

Challenges in management of refractory pain and sedation in infants

The survival of preterm infants continues to improve, along with an increased in neonatal intensive care unit (NICU) management of chronic infants who are medically complex infants who have prolonged hospital stays, sometimes up until 2 years of age. Despite advances in neonatal and infant care, the management of pain and sedation in chronic NICU patients continues to be a challenge. Challenges such as development of appropriate pain, sedation, and withdrawal scales along with unfamiliarity of the NICU care team with pediatric disease states and pharmacotherapy complicate management of these patients. Opioid induced hyperalgesia (OIH) and delirium may play a large role in these refractory cases, yet are often not considered in the NICU population. Drug therapy interventions such as gabapentin, ketamine, risperidone, and others have limited data for safety and efficacy in this population. This article summarizes the available literature regarding the evidence for diagnosis and management of infants with refractory pain and sedation along with the challenges that clinicians face when managing these patients.

Central metabolism as a potential origin of sex differences in morphine antinociception but not induction of antinociceptive tolerance in mice

BACKGROUND AND PURPOSE

In rodents, morphine antinociception is influenced by sex. However, conflicting results have been reported regarding the interaction between sex and morphine antinociceptive tolerance. Morphine is metabolised in the liver and brain into morphine-3-glucuronide (M3G). Sex differences in morphine metabolism and differential metabolic adaptations during tolerance development might contribute to behavioural discrepancies. This article investigates the differences in peripheral and central morphine metabolism after acute and chronic morphine treatment in male and female mice.

EXPERIMENTAL APPROACH

Sex differences in morphine antinociception and tolerance were assessed using the tail-immersion test. After acute and chronic morphine treatment, morphine and M3G metabolic kinetics in the blood were evaluated using LC-MS/MS. They were also quantified in several CNS regions. Finally, the blood-brain barrier (BBB) permeability of M3G was assessed in male and female mice.

KEY RESULTS

This study demonstrated that female mice showed weaker morphine antinociception and faster induction of tolerance than males. Additionally, female mice showed higher levels of M3G in the blood and in several pain-related CNS regions than male mice, whereas lower levels of morphine were observed in these regions. M3G brain/blood ratios after injection of M3G indicated no sex differences in M3G BBB permeability, and these ratios were lower than those obtained after injection of morphine.

CONCLUSION

These differences are attributable mainly to morphine central metabolism, which differed between males and females in pain-related CNS regions, consistent with weaker morphine antinociceptive effects in females. However, the role of morphine metabolism in antinociceptive tolerance seemed limited.

LINKED ARTICLES

This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Differential Effects of a Novel Opioid Ligand UTA1003 on Antinociceptive Tolerance and Motor Behaviour

Analgesic tolerance is a major problem in the clinic for the maintenance of opioid-induced long-term pain relief. Opioids with mixed activity on multiple opioid receptors promise reduced antinociceptive tolerance in preclinical studies, but these compounds typically show poor bioavailability upon oral, subcutaneous, intraperitoneal, or intravenous administration. We designed UTA1003 as a novel opioid that acts as a mu (MOP) and kappa (KOP) opioid receptor agonist and a partial agonist for delta (DOP) opioid receptor. In the present study, its antinociceptive effects, as well as its effects on antinociceptive tolerance and motor behaviour, were investigated in male rats. Acute antinociception was measured before (basal) and at different time points after subcutaneous injection of UTA1003 or morphine using the tail flick and hot plate assays. Various motor behavioural activities, including horizontal locomotion, rearing, and turning, were automatically measured in an open-field arena. The antinociceptive and behavioural effects of repeated administration of UTA1003 and morphine were determined over eight days. UTA1003 induced mild antinociceptive effects after acute administration but induced no tolerance after repeated treatment. Importantly, UTA1003 co-treatment with morphine prevented antinociceptive tolerance compared to morphine alone. UTA1003 showed less motor suppression than morphine in both acute and sub-chronic treatment regimens, while it did not affect morphine-induced motor suppression or hyper-excitation. Based on these activities, we speculate that UTA1003 crosses the blood-brain barrier after subcutaneous administration and, therefore, could be developed as a lead molecule to avoid opioid-induced antinociceptive tolerance and motor suppression. Further structural modifications to improve its antinociceptive effects, toxicity profile, and ADME parameters are nevertheless required.

Morphine-3-Glucuronide, Physiology and Behavior

Morphine remains the gold standard painkiller available to date to relieve severe pain. Morphine metabolism leads to the production of two predominant metabolites, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G). This metabolism involves uridine 5'-diphospho-glucuronosyltransferases (UGTs), which catalyze the addition of a glucuronide moiety onto the C3 or C6 position of morphine. Interestingly, M3G and M6G have been shown to be biologically active. On the one hand, M6G produces potent analgesia in rodents and humans. On the other hand, M3G provokes a state of strong excitation in rodents, characterized by thermal hyperalgesia and tactile allodynia. Its coadministration with morphine or M6G also reduces the resulting analgesia. Although these behavioral effects show quite consistency in rodents, M3G effects are much more debated in humans and the identity of the receptor(s) on which M3G acts remains unclear. Indeed, M3G has little affinity for mu opioid receptor (MOR) (on which morphine binds) and its effects are retained in the presence of naloxone or naltrexone, two non-selective MOR antagonists. Paradoxically, MOR seems to be essential to M3G effects. In contrast, several studies proposed that TLR4 could mediate M3G effects since this receptor also appears to be essential to M3G-induced hyperalgesia. This review summarizes M3G's behavioral effects and potential targets in the central nervous system, as well as the mechanisms by which it might oppose analgesia.

Opioid Analgesia and Opioid-Induced Adverse Effects: A Review

Opioids are widely used as therapeutic agents against moderate to severe acute and chronic pain. Still, these classes of analgesic drugs have many potential limitations as they induce analgesic tolerance, addiction and numerous behavioural adverse effects that often result in patient non-compliance. As opium and opioids have been traditionally used as painkillers, the exact mechanisms of their adverse reactions over repeated use are multifactorial and not fully understood. Older adults suffer from cancer and non-cancer chronic pain more than younger adults, due to the physiological changes related to ageing and their reduced metabolic capabilities and thus show an increased number of adverse reactions to opioid drugs. All clinically used opioids are μ-opioid receptor agonists, and the major adverse effects are directly or potentially connected to this receptor. Multifunctional opioid ligands or peripherally restricted opioids may elicit fewer adverse effects, as shown in preclinical studies, but these results need reproducibility from further extensive clinical trials. The current review aims to overview various mechanisms involved in the adverse effects induced by opioids, to provide a better understanding of the underlying pathophysiology and, ultimately, to help develop an effective therapeutic strategy to better manage pain.

A theoretical framework and nomenclature to characterize the iatrogenic contribution of therapeutic opioid exposure to opioid induced hyperalgesia, physical dependence, and opioid use disorder

Opioid use, misuse, and risky use contribute to a critically important and complex crisis in current healthcare. Consequences of long-term opioid use, including opioid induced hyperalgesia, physical dependence, and opioid use disorder, can be considered iatrogenic, or partially iatrogenic, in cases where therapeutic opioid exposures were contributory. Research investigation and presumptive clinical action are needed to attenuate the iatrogenic component of the opioid crisis; treatment of individuals already suffering from opioid use disorder will not prevent incident cases. This work will be challenged by a remarkably high degree of complexity involving myriad and highly variable factors along the continuum from initial opioid exposure to long-term opioid use. An organized view of this complex problem should accelerate the pace of innovation and facilitate clinical implementation of research findings. Herein, we propose a theoretical framework and modern nomenclature for characterizing therapeutic opioid exposure and the degree to which it contributes iatrogenically to adverse outcomes. In doing so, we separate the role of exposure from other factors contributing to long-term opioid use, clarify the relationship between opioid exposure and outcomes, emphasize that exposure source is an important consideration for health services research and practice in the areas of pain treatment and opioid prevention, and recommend terminology necessary to quantify therapeutic opioid exposure separately from nonmedical exposure.

miR‑320‑3p is involved in morphine pre‑conditioning to protect rat cardiomyocytes from ischemia/reperfusion injury through targeting Akt3

Morphine pre-conditioning (MPC) can significantly reduce myocardial ischemic injury and inhibit cardiomyocyte apoptosis, but the underlying mechanism still remains unclear. The aim of the present study was to investigate the protective mechanism of MPC in myocardial hypoxia/reoxygenation (H/R) injury at the microRNA (miR) level. H9c2 cells were used as a model of H/R and subjected to morphine pre-treatment. The protective effects of MPC on H/R injury in cardiomyocytes were evaluated using MTT and colorimetric assay, as well as flow cytometry. In addition, reverse transcription-quantitative PCR, western blotting and dual-luciferase reporter assay experiments were performed to determine the relationship between MPC, miR-320-3p and Akt3, and their effects on H/R injury. The present study demonstrated that MPC enhanced cell activity, decreased LDH content, and reduced apoptosis in rat cardiomyocytes, suggesting that MPC could protect these cells from H/R injury. Moreover, MPC partially reversed the increase in miR-320-3p expression and the decrease in Akt3 levels caused by H/R injury. Inhibition of miR-320-3p expression also attenuated the effects of H/R on cardiomyocyte activity, LDH content and apoptosis. Furthermore, Akt3 was predicted to be a target gene of miR-320-3p, and overexpression of miR-320-3p inhibited the expression of Akt3, blocking the protective effects of MPC on the cells. The current findings revealed that MPC could protect cardiomyocytes from H/R damage through targeting miR-320-3p to regulate the PI3K/Akt3 signaling pathway.

Opioid Induced Hyperalgesia, a Research Phenomenon or a Clinical Reality? Results of a Canadian Survey

BACKGROUND

Very little is known regarding the prevalence of opioid induced hyperalgesia (OIH) in day to day medical practice. The aim of this study was to evaluate the physician's perception of the prevalence of OIH within their practice, and to assess the level of physician's knowledge with respect to the identification and treatment of this problem.

METHODS

An electronic questionnaire was distributed to physicians who work in anesthesiology, chronic pain, and/or palliative care in Canada.

RESULTS

Of the 462 responses received, most were from male (69%) anesthesiologists (89.6%), in the age range of 36 to 64 years old (79.8%). In this study, the suspected prevalence of OIH using the average number of patients treated per year with opioids was 0.002% per patient per physician practice year for acute pain, and 0.01% per patient per physician practice year for chronic pain. Most physicians (70.2%) did not use clinical tests to help make a diagnosis of OIH. The treatment modalities most frequently used were the addition of an NMDA antagonist, combined with lowering the opioid doses and using opioid rotation.

CONCLUSIONS

The perceived prevalence of OIH in clinical practice is a relatively rare phenomenon. Furthermore, more than half of physicians did not use a clinical test to confirm the diagnosis of OIH. The two main treatment modalities used were NMDA antagonists and opioid rotation. The criteria for the diagnosis of OIH still need to be accurately defined.

Opioid-induced hyperalgesia in clinical anesthesia practice: what has remained from theoretical concepts and experimental studies?

PURPOSE OF REVIEW

This article reviews the phenomenon of opioid-induced hyperalgesia (OIH) and its implications for clinical anesthesia. The goal of this review is to give an update on perioperative prevention and treatment strategies, based on findings in preclinical and clinical research.

RECENT FINDINGS

Several systems have been suggested to be involved in the pathophysiology of OIH with a focus on the glutaminergic system. Very recently preclinical data revealed that peripheral μ-opioid receptors (MORs) are key players in the development of OIH and acute opioid tolerance (AOT). Peripheral MOR antagonists could, thus, become a new prevention/treatment option of OIH in the perioperative setting. Although the impact of OIH on postoperative pain seems to be moderate, recent evidence suggests that increased hyperalgesia following opioid treatment correlates with the risk of developing persistent pain after surgery. In clinical practice, distinction among OIH, AOT and acute opioid withdrawal remains difficult, especially because a specific quantitative sensory test to diagnose OIH has not been validated yet.

SUMMARY

Since the immediate postoperative period is not ideal to initiate long-term treatment for OIH, the best strategy is to prevent its occurrence. A multimodal approach, including choice of opioid, dose limitations and addition of nonopioid analgesics, is recommended.

GluN2B N-methyl-D-aspartate receptor and excitatory amino acid transporter 3 are upregulated in primary sensory neurons after 7 days of morphine administration in rats: implication for opiate-induced hyperalgesia

The contribution of the peripheral nervous system to opiate-induced hyperalgesia (OIH) is not well understood. In this study, we determined the changes in excitability of primary sensory neurons after sustained morphine administration for 7 days. Changes in the expression of glutamate receptors and glutamate transporters after morphine administration were ascertained in dorsal root ganglions. Patch clamp recordings from intact dorsal root ganglions (ex vivo preparation) of morphine-treated rats showed increased excitability of small diameter (≤30 μm) neurons with respect to rheobase and membrane threshold, whereas the excitability of large diameter (>30 μm) neurons remained unchanged. Small diameter neurons also displayed increased responses to glutamate, which were mediated mainly by GluN2B containing N-methyl-D-aspartate (NMDA) receptors, and to a lesser degree by the neuronal excitatory amino acid transporter 3/excitatory amino acid carrier 1. Coadministration in vivo of the GluN2B selective antagonist Ro 25-6981 with morphine for 7 days prevented the appearance of OIH and increased morphine-induced analgesia. Administration of morphine for 7 days led to an increased expression of GluN2B and excitatory amino acid transporter 3/excitatory amino acid carrier 1, but not of the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate, kainate, or group I metabotropic glutamate receptors, or of the vesicular glutamate transporter 2. These results suggest that peripheral glutamatergic neurotransmission contributes to OIH and that GluN2B subunit of NMDA receptors in the periphery may be a target for therapy.

siRNA capsulated brain-targeted nanoparticles specifically knock down OATP2B1 in mice: a mechanism for acute morphine tolerance suppression

Regulating main brain-uptake transporter of morphine may restrict its tolerance generation, then modify its antinociception. In this study, more than 2 fold higher intracellular uptake concentrations for morphine and morphine-6-glucuronide (M6G) were observed in stable expression cells, HEK293-hOATP2B1 than HEK293-MOCK. Specifically, the K_m value of morphine to OATP2B1 (57.58 ± 8.90 μM) is 1.4-time more than that of M6G (80.31 ± 21.75 μM); Cyclosporine A (CsA), an inhibitor of OATP2B1, can inhibit their intracellular accumulations with IC₅₀ = 3.90 ± 0.50 μM for morphine and IC₅₀ = 6.04 ± 0.86 μM for M6G, respectively. To further investigate the role of OATP2B1 in morphine brain transport and tolerance, the novel nanoparticles of DGL-PEG/dermorphin capsulated siRNA (OATP2B1) were applied to deliver siRNA into mouse brain. Along with OATP2B1 depressed, a main reduction was found for each of morphine or M6G in cerebrums or epencephalons of acute morphine tolerance mice. Furthermore, calcium/calmodulin-dependent protein kinase IIα (CaMKIIα) in mouse prefrontal cortex (mPFC) underwent dephosphorylation at Thr286. In conclusion, OATP2B1 downregulation in mouse brain can suppress tolerance via blocking morphine and M6G brain transport. These findings might help to improve the pharmacological effects of morphine.

Opioid-induced hyperalgesia: Cellular and molecular mechanisms

Opioids produce strong analgesia but their use is limited by a paradoxical hypersensitivity named opioid-induced hyperalgesia (OIH) that may be associated to analgesic tolerance. In the last decades, a significant number of preclinical studies have investigated the factors that modulate OIH development as well as the cellular and molecular mechanisms underlying OIH. Several factors have been shown to influence OIH including the genetic background and sex differences of experimental animals as well as the opioid regimen. Mu opioid receptor (MOR) variants and interactions of MOR with different proteins were shown important. Furthermore, at the cellular level, both neurons and glia play a major role in OIH development. Several neuronal processes contribute to OIH, like activation of neuroexcitatory mechanisms, long-term potentiation (LTP) and descending pain facilitation. Increased nociception is also mediated by neuroinflammation induced by the activation of microglia and astrocytes. Neurons and glial cells exert synergistic effects, which contribute to OIH. The molecular actors identified include the Toll-like receptor 4 and the anti-opioid systems as well as some other excitatory molecules, receptors, channels, chemokines, pro-inflammatory cytokines or lipids. This review summarizes the intracellular and intercellular pathways involved in OIH and highlights some mechanisms that may be challenged to limit OIH in the future.

Morphine Glucuronidation and Elimination in Intensive Care Patients: A Comparison with Healthy Volunteers

BACKGROUND

Although morphine is used frequently to treat pain in the intensive care unit, its pharmacokinetics has not been adequately quantified in critically ill patients. We evaluated the glucuronidation and elimination clearance of morphine in intensive care patients compared with healthy volunteers based on the morphine and morphine-3-glucuronide (M3G) concentrations.

METHODS

A population pharmacokinetic model with covariate analysis was developed with the nonlinear mixed-effects modeling software (NONMEM 7.3). The analysis included 3012 morphine and M3G concentrations from 135 intensive care patients (117 cardiothoracic surgery patients and 18 critically ill patients), who received continuous morphine infusions adapted to individual pain levels, and 622 morphine and M3G concentrations from a previously published study of 20 healthy volunteers, who received an IV bolus of morphine followed by a 1-hour infusion.

RESULTS

For morphine, a 3-compartment model best described the data, whereas for M3G, a 1-compartment model fits best. In intensive care patients with a normal creatinine concentration, a decrease of 76% was estimated in M3G clearance compared with healthy subjects, conditional on the M3G volume of distribution being the same in intensive care patients and healthy volunteers. Furthermore, serum creatinine concentration was identified as a covariate for both elimination clearance of M3G in intensive care patients and unchanged morphine clearance in all patients and healthy volunteers.

CONCLUSIONS

Under the assumptions in the model, M3G elimination was significantly decreased in intensive care patients when compared with healthy volunteers, which resulted in substantially increased M3G concentrations. Increased M3G levels were even more pronounced in patients with increased serum creatinine levels. Model-based simulations show that, because of the reduction in morphine clearance in intensive care patients with renal failure, a 33% reduction in the maintenance dose would result in morphine serum concentrations equal to those in healthy volunteers and intensive care patients with normal renal function, although M3G concentrations remain increased. Future pharmacodynamic investigations are needed to identify target concentrations in this population, after which final dosing recommendations can be made.

Targeting Opioid-Induced Hyperalgesia in Clinical Treatment: Neurobiological Considerations

Opioid analgesics have become a cornerstone in the treatment of moderate to severe pain, resulting in a steady rise of opioid prescriptions. Subsequently, there has been a striking increase in the number of opioid-dependent individuals, opioid-related overdoses, and fatalities. Clinical use of opioids is further complicated by an increasingly deleterious profile of side effects beyond addiction, including tolerance and opioid-induced hyperalgesia (OIH), where OIH is defined as an increased sensitivity to already painful stimuli. This paradoxical state of increased nociception results from acute and long-term exposure to opioids, and appears to develop in a substantial subset of patients using opioids. Recently, there has been considerable interest in developing an efficacious treatment regimen for acute and chronic pain. However, there are currently no well-established treatments for OIH. Several substrates have emerged as potential modulators of OIH, including the N-methyl-D-aspartate and γ-aminobutyric acid receptors, and most notably, the innate neuroimmune system. This review summarizes the neurobiology of OIH in the context of clinical treatment; specifically, we review evidence for several pathways that show promise for the treatment of pain going forward, as prospective adjuvants to opioid analgesics. Overall, we suggest that this paradoxical state be considered an additional target of clinical treatment for chronic pain.

Chronic administration of modafinil induces hyperalgesia in mice: reversal by L-NG-nitro-arginine methyl ester and 7-nitroindazole

Modafinil [2-((diphenylmethyl) sulfinyl) acetamide] is a central nervous system stimulant. It has received considerable attention as a potential psychotropic agent in several psychiatric disorders. The current study was carried out to investigate the effect of modafinil after acute administration on animal models of pain in mice. Also, this study evaluated the effect of L-NG-nitroarginine methyl ester (L-NAME), 7-nitroindazole (7-NI) and naloxone following chronic administration of modafinil. Modafinil was administered in the doses of 50, 100 or 200 mg/kg once in acute study and it showed significantly increased tail-flick latency (tfl) and paw-licking latency. In formalin test modafinil (100 mg/kg) significantly reduced licking/biting time in both early and late phases in comparison to control. In chronic study, modafinil 100 mg/kg administered for 10 days, produced a progressive decrease in the reaction time (i.e., tfl/paw-licking latency) in comparison to day 1 values which started building up from day 4 and fully established at day 6, indicating hyperalgesic response. Prior administration of 7-NI (on day 7) and L-NAME (on day 10) prevented the hyperalgesic response while naloxone on day 10 did not have a significant effect on modafinil-induced hyperalgesia. These results demonstrate that modafinil has a potential role in pain as it exhibited antinociceptive effect after acute administration in a dose-dependent manner and on chronic administration it caused hyperalgesia. This hyperalgesia is reversed by nitric oxide synthase inhibitors, suggesting the possibility of involvement of nitric oxide pathway. Further studies are required to evaluate the role of modafinil in clinical pain.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).