Morphine-3-glucuronide's neuro-excitatory effects are mediated via indirect activation of N-methyl-D-aspartic acid receptors: mechanistic studies in embryonic cultured hippocampal neurones

CYX-5, a G-protein biassed MOP receptor agonist, DOP receptor antagonist and KOP receptor agonist, evokes constipation but not respiratory depression relative to morphine in rats

BACKGROUND

Strong opioid analgesics such as morphine alleviate moderate to severe acute nociceptive pain (e.g. post-surgical or post-trauma pain) as well as chronic cancer pain. However, they evoke many adverse effects and so there is an unmet need for opioid analgesics with improved tolerability. Recently, a prominent hypothesis has been that opioid-related adverse effects are mediated by β-arrestin2 recruitment at the µ-opioid (MOP) receptor and this stimulated research on discovery of G-protein biassed opioid analgesics. In other efforts, opioids with MOP agonist and δ-opioid (DOP) receptor antagonist profiles are promising for reducing side effects c.f. morphine. Herein, we report on the in vivo pharmacology of a novel opioid peptide (CYX-5) that is a G-protein biassed MOP receptor agonist, DOP receptor antagonist and kappa opioid (KOP) receptor agonist.

METHODS

Male Sprague-Dawley received intracerebroventricular bolus doses of CYX-5 (3, 10, 20 nmol), morphine (100 nmol) or vehicle, and antinociception (tail flick) was assessed relative to constipation (charcoal meal and castor oil-induced diarrhoea tests) and respiratory depression (whole body plethysmography).

RESULTS

CYX-5 evoked naloxone-sensitive, moderate antinociception, at the highest dose tested. Although CYX-5 did not inhibit gastrointestinal motility, it reduced stool output markedly in the castor oil-induced diarrhoea test. In contrast to morphine that evoked respiratory depression, CYX-5 increased tidal volume, thereby stimulating respiration.

CONCLUSION

Despite its lack of recruitment of β-arrestin2 at MOP, DOP and KOP receptors, CYX-5 evoked constipation, implicating a mechanism other than β-arrestin2 recruitment at MOP, DOP and KOP receptors, mediating constipation evoked by CYX-5 and potentially other opioid ligands.

Morphine ameliorates pentylenetetrazole-induced locomotor pattern in zebrafish embryos; mechanism involving regulation of opioid receptors, suppression of oxidative stress, and inflammation in epileptogenesis

Zebrafish (Danio rerio) is becoming an increasingly important model in epilepsy research. Pentylenetetrazole (PTZ) is a convulsant agent that induces epileptic seizure-like state in zebrafish and zebrafish embryos and is most commonly used in antiepileptic drug discovery research to evaluate seizure mechanisms. Classical antiepileptic drugs, such as valproic acid (VPA) reduce PTZ-induced epileptiform activities. Opioid system has been suggested to play a role in epileptogenesis. The aim of our study is to determine the effects of morphine in PTZ-induced epilepsy model in zebrafish embryos by evaluating locomotor activity and parameters related to oxidant-antioxidant status, inflammation, and cholinergic system as well as markers of neuronal activity c-fos, bdnf, and opioid receptors. Zebrafish embryos at 72 hpf were exposed to PTZ (20 mM), VPA (1 mM), and Morphine (MOR) (100 µM). MOR and VPA pretreated groups were treated with either MOR (MOR + PTZ) or VPA (VPA + PTZ) for 20 min before PTZ expoure. Locomotor activity was quantified as total distance moved (mm), average speed (mm/sec) and exploration rate (%) and analyzed using ToxTrac tracking programme. Oxidant-antioxidant system parameters, acetylcholinesterase activity, and sialic acid leves were evaluated using spectrophotometric methods. The expression of c-fos, bdnf, oprm1, and oprd1 were evaluated by RT-PCR. MOR pretreatment ameliorated PTZ-induced locomotor pattern as evidenced by improved average speed, exploration rate and distance traveled. We report the restoration of inflammatory and oxidant-antioxidant system parameters, c-fos, bdnf, and opioid receptor oprm1 as the possible mechanisms involved in the ameliorative effect of MOR against PTZ-induced epileptogenic process in zebrafish embryos.

Effect of morphine administration after status epilepticus on epileptogenesis in rats

INTRODUCTION

Morphine is widely used in patients and has been reported to alter seizure threshold, but its role in the development of epilepsy is unknown. In this study, role of morphine administration in the development of epilepsy using the status epilepticus (SE) model was determined in rats.

METHODS

Rats experiencing SE with lithium-pilocarpine (LiP) were randomized into four groups- saline, morphine low dose (5 mg/kg, s.c.), morphine high dose (5-20 mg/kg, s.c.), and naloxone (1 mg/kg, s.c.). Treatments were started 90 min after termination of SE and repeated twice daily for next three days. Rats were video monitored daily for 21 days to determine onset and frequency of spontaneous convulsive seizures (SS).

RESULTS

Morphine in low doses increased frequency of SS (1.51 ± 0.15 vs LiP 0.60 ± 0.12 seizures/rat/day, p-value = 0.0026) and seizures occurred during handling (SDH) (0.08 ± 0.02 vs LiP control 0.01 ± 0.01) (p-value = 0.0018). In high doses, no significant change in SS and SDH was found as compared to LiP. No effect of morphine on the onset of SS and percentage of rats experienced SS was found. No effect of naloxone per se was found on SS.

CONCLUSION

Morphine administration after SE does not affect epileptogenesis as no change in the onset of SS and percentage of rats experiencing SS was found. However, it might alter the susceptibility and frequency of SS. As no other study is available with a similar finding, it needs further evaluation.

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.

Drug-Induced Seizures: Considerations for Underlying Molecular Mechanisms

A broad spectrum of chemical entities have been associated with drug-induced seizure (DIS), emphasizing the importance of this potential liability across various drug classes (e.g., antidepressants, antipsychotics, antibiotics, and analgesics among others). Despite its importance within drug safety testing, an understanding of the molecular mechanisms associated with DIS is often lacking. The etiology of DIS is understood to be a result of either a deficit in inhibitory (e.g., gamma aminobutyric acid) or an elevated excitatory (e.g., glutamate) signaling, leading to synchronous neuronal depolarization affecting various brain regions and impairing normal neurological functions. How this altered neuronal signaling occurs and how these changes interact with other non-brain receptor driven DIS-associated changes such as metabolic disturbances, electrolyte imbalances, altered drug metabolism, and withdrawal effects are poorly understood. Herein, we discuss important molecular mechanisms identified in DIS for several drugs and/or drug classes. With a better understanding of the molecular mechanisms associated with DIS, in vivo or in vitro models may be applied to characterize and mitigate DIS risk during drug development. Susceptibility stratification for DIS presents species differences in the following order beagle dogs > rodents and cynomolgus monkeys > Göttingen minipigs with a more than 2-fold difference between canines and minipigs, which is important to consider during non-clinical species selection. While clinical signs such as myoclonus, severe muscle jerks, or convulsions are often associated with abnormal epileptiform EEG activity, tremors are most of the time physiological and rarely observed with concurrent epileptiform EEG activity which need to be considered during DIS risk evaluation.

Stimulatory and inhibitory effects of morphine on pentylenetetrazol-induced epileptic activity in rat

AIMS

The present study attempts to evaluate the effects of different doses of morphine on experimental epileptiform activity caused by pentylenetetrazol (PTZ) in rats.

METHODS

Thirty adult male rats were assigned to saline (n = 5), morphine (2, 5, and 10 mg/kg, n = 15), naloxone (1 mg/kg, n = 5), and pre-treated with naloxone+morphine (1 + 10 mg/kg, n = 5) groups. The animals were anesthetized with ketamine + xylazine (80 + 8 mg/kg), and then a bipolar electrode was implanted into the CA1 (AP: -2.76 mm, ML: -1.4 mm and DV: 3 mm). To evaluate the effects of drugs on spike count and their amplitudes by elab amplifier, after drug administration for 25 min, PTZ (80 mg/kg, i.p.) was injected to induce epileptiform activity. Finally, diazepam (10 mg/kg) was used to suppress epileptic activity.

RESULTS

The results revealed that morphine at a dose of 2 mg/kg decreased, and at doses of 5 and 10 mg/kg had an increasing effect on seizure-like events (SLEs). Nevertheless, morphine at a dose of 10 mg/kg enhanced SLEs significantly (p < 0.01). Naloxone at a dose of 1 mg/kg had no significant effect on the spike count but increased amplitude of them (p < 0.001). Moreover, being pretreatment with naloxone at a dose of 1 mg/kg, the morphine group showed significantly increased in the spike count (p < 0.05).

CONCLUSIONS

Morphine has biphasic effects on PTZ-induced epileptiform activities that way at a low dose has an inhibitory effect, but if the dose is increased, it will intensify the desired event and that the stimulatory effects of morphine appear not to be via opioid receptors.

κ-Opioid receptor antagonism reverses heroin withdrawal-induced hyperalgesia in male and female rats

Although opioids are potent analgesics, a consequence of chronic opioid use is hyperalgesia during withdrawal, which may contribute to opioid misuse. Dynorphin, the endogenous ligand of κ-opioid receptors (KORs), is upregulated in opioid-dependent rats and in animal models of chronic pain. However, the role of KORs in opioid withdrawal-induced hyperalgesia remains to be determined. We hypothesized that KOR antagonism would reverse opioid withdrawal-induced hyperalgesia in opioid-dependent rats. Male and female Wistar rats received daily injections of heroin (2-6 mg/kg, SC) and were tested for mechanical sensitivity in the electronic von Frey test 4-6 h into withdrawal. Female rats required significantly more heroin than male rats to reach comparable levels of both heroin-induced analgesia and hyperalgesia (6 mg/kg vs. 2 mg/kg). Once hyperalgesia was established, we tested the effects of the KOR antagonists nor-binaltorphimine (norBNI; 30 mg/kg, SC) and 5'-guanidinonaltrindole (5'GNTI; 30 mg/kg, SC). When the animals continued to receive their daily heroin treatment (or saline treatment in the repeated saline group) five times per week throughout the experiment, both KOR antagonists reversed heroin withdrawal-induced hyperalgesia. The anti-hyperalgesia effect of norBNI was more prolonged in males than in females (14 days vs. 7 days), whereas 5'GNTI had more prolonged effects in females than in males (14 days vs. 4 days). The behavioral effects of 5'GNTI coincided with higher 5'GNTI levels in the brain than in plasma when measured at 24 h, whereas 5'GNTI did not reverse hyperalgesia at 30 min posttreatment when 5'GNTI levels were higher in plasma than in the brain. Finally, we tested the effects of 5'GNTI on naloxone-induced and spontaneous signs of opioid withdrawal and found no effect in either male or female rats. These findings indicate a functional role for KORs in heroin withdrawal-induced hyperalgesia that is observed in rats of both sexes.

Morphine plasmatic concentration in a pregnant mare and its foal after long term epidural administration

Background

Epidural administration of morphine has been shown to be an effective analgesic strategy in horses; however, the possible occurrence of side effects limits its usage. In order to decrease their frequency, it is important to target the minimal effective plasma concentration and avoid overdosing. As to date species-specific pharmacokinetics data are not available for epidural morphine, the dosing regimen is usually established on the basis of clinical reports and personal experience. In certain physiological conditions, like gestation, the outcome of an empirical dosing scheme can be unpredictable. The aim of this case report is to describe the pharmacological profile of morphine and its metabolites after prolonged epidural administration in a pregnant mare and her foal.

Case presentation

A 20 years old pregnant mare was presented to our hospital because of severe lameness, 2 months before delivery. Following an ineffective systemic pain treatment, an epidural catheter was inserted and morphine administered (initial dose 0.1 mg/kg every 8 h). Due to its efficacy in controlling pain, it was continued until end of gestation. Plasmatic concentration of morphine and its metabolites were assessed in the mare 6 weeks after starting the treatment, and in both the mare and foal during the first days after delivery. Plasmatic values similar to those previously reported in the literature following morphine short term administration through various routes and not accompanied by side effects were found in the mare, except during an excitatory period. Moreover, no evidence of dangerous drug accumulation or significant milk passage was noticed in the foal. Mild reduction of feces production with no signs of colic and two self-limiting episodes of excitement occurred during treatment in the mare. No side effects occurred during gestation and first phases of life in the foal.

Conclusion

Prolonged epidural administration of morphine in a pregnant mare allowed good pain control in absence of clinically relevant side effects, in both the mare and her foal. Sudden increase in morphine plasmatic concentration can occur and side effects appear; careful treatment to the lowest effective dose and continuous monitoring of the clinical condition of the treated horse should be performed.

Lipopolysaccharide and Morphine-3-Glucuronide-Induced Immune Signalling Increases the Expression of Polysialic Acid in PC12 Cells

Polysialic acid (polySia), a long homopolymer of 2,8-linked sialic acids, is abundant in the embryonic brain and is restricted largely in adult brain to regions that exhibit neurogenesis and structural plasticity. In the central nervous system (CNS), polySia is highly important for cell-cell interactions, differentiation, migration and cytokine responses, which are critical neuronal functions regulating intercellular interactions that underlie immune signalling in the CNS. In recent reports, a metabolite of morphine, morphine-3-glucuronide (M3G), has been shown to cause immune signalling in the CNS. In this study, we compared the effects of neurite growth factor (NGF), lipopolysaccharide (LPS) and M3G exposure on the expression of polySia in PC12 cells using immunocytochemistry and Western blot analysis. PolySia was also extracted from stimulated cell proteins by endo-neuraminidase digestion and quantitated using fluorescent labelling followed by HPLC analysis. PolySia expression was significantly increased following NGF, M3G or LPS stimulation when compared with unstimulated cells or cells exposed to the TLR4 antagonist LPS-RS. Additionally, we analyzed the effects of test agent exposure on cell migration and the oxidative stress response of these cells in the presence and absence of polySia expression on their cell surface. We observed an increase in oxidative stress in cells without polySia as well as following M3G or LPS stimulation. Our study provides evidence that polySia expression in neuronal-like PC12 cells is influenced by M3G and LPS exposure alike, suggestive of a role of TLR4 in triggering these events.

Evaluation of morphine-like effects of the mixed mu/delta agonist morphine-6-O-sulfate in rats: Drug discrimination and physical dependence

Morphine-6-O-sulfate (M6S) is as a mixed-action mu/delta (μ/δ) opioid receptor agonist with high potency and analgesic efficacy. These studies used assays of drug discrimination and schedule-controlled responding to assess abuse-liability, tolerance, and physical dependence as compared to morphine in rats. Attempts to train 0.3 mg/kg (IP) M6S from saline failed, but all rats rapidly acquired the discrimination when the training dose was changed to 3.0 mg/kg morphine, and substitution tests showed that morphine and fentanyl both fully substituted for the training dose, M6S and M3A6S (3-O-acetyl ester of M6S) only partially substituted, and salvinorin A did not elicit morphine-like effects. Tolerance to response rate-decreasing effects was studied in rats administered either 1.0 or 3.0 mg/kg morphine or M6S before food-reinforced operant sessions. At both unit doses, tolerance to M6S-elicited rate suppression developed more slowly than tolerance to morphine-induced reductions in response rates. To assess dependence, rats were maintained on 1.0 mg/kg morphine or 1.0 mg/kg M6S until food-reinforced response rates were stable for at least 5 days. Rats were then administered saline or increasing doses of the opioid antagonist naltrexone (NTX) (0.3, 1.0, 3.0, or 10.0 mg/kg) in order to determine antagonist-precipitated withdrawal. NTX precipitated withdrawal was similar in both morphine-maintained and M6S-maintained rats. In conclusion, the mixed μ/δ agonist activity of M6S failed to completely protect against the development of physical dependence, but delayed tolerance development to behavioral effects and resulted in decreased morphine-like subjective effects, perhaps implying a decreased abuse liability over μ agonists.

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

Opioid-induced hyperalgesia (OIH) is a paradoxical increase in pain perception that may manifest during opioid treatment. For morphine, the metabolite morphine-3-glucuronide (M3G) is commonly believed to underlie this phenomenon. Here, in three separate studies, we empirically assess the role of M3G in morphine-induced hyperalgesia. In the first study, CD-1 mice injected with morphine (15 mg/kg subcutaneously) after pretreatment with the opioid receptor antagonist naltrexone (NTX) (15 mg/kg) showed tail withdrawal latency reductions indicative of hyperalgesia (2.5 ± 0.1 s at t = 30 min, P < 0.001 versus baseline). In these mice, the morphine/M3G concentration ratios versus effect showed a negative correlation (r(p) = -0.65, P < 0.001), indicating that higher morphine relative to M3G concentrations are associated with increased OIH. In the second study, similar hyperalgesic responses were observed in mice lacking the multidrug resistance protein 3 (MRP3) transporter protein (Mrp3(-/-) mice) in the liver and their wild-type controls (FVB mice; latency reductions: 3.1 ± 0.2 s at t = 30 min, P < 0.001 versus within-strain baseline). In the final study, the pharmacokinetics of morphine and M3G were measured in Mrp3(-/-) and FVB mice. Mrp3(-/-) mice displayed a significantly reduced capacity to export M3G into the systemic circulation, with plasma M3G concentrations just 7% of those observed in FVB controls. The data confirm previous literature that morphine causes hyperalgesia in the absence of opioid receptor activation but also indicate that this hyperalgesia may occur without a significant contribution of hepatic M3G. The relevance of these data to humans has yet to be demonstrated.

Opioid therapies and cytochrome p450 interactions

Adverse drug reactions are common and associated with substantial economic and human costs. Particularly among older adult populations, preventable adverse drug reactions are often caused by drug-drug interactions. All analgesics have side effect profiles and many have known drug-drug interactions. Opioids are recognized as a necessary option for managing moderate-to-severe pain, yet many opioid side effects can be enhanced by metabolic interactions within the liver, involving other drugs, diseases, or genetics.

Morphine and hydromorphone-induced hyperalgesia in a hospice patient

Opioids including morphine and hydromorphone are widely used for control of moderate to severe pain and dyspnea in hospice and palliative care patients. Accumulation of the active morphine-3-glucuronide (M3G) and hydromorphone-3-glucuronide (H3G) metabolites is one proposed mechanism for the development of neuroexcitatory effects including allodynia and opioid-induced hyperalgesia (OIH). We report the case of a 43-year-old female hospice patient with metastatic non-small cell lung cancer who initially developed allodynia following morphine administration and again following administration of hydromorphone. The allodynia resolved both times following the discontinuation of the opioid and rotation to a different opioid regimen. Potential opioid-induced neuroexcitatory treatment options include opioid rotation to an agent with inactive metabolites, use of adjuvant pain medications for opioid-sparing effects, management of undesired symptoms (e.g., myoclonus), or increasing opioid clearance with intravenous (IV) fluids. Although the incidence is not well defined in the literature, hospice and palliative care clinicians should suspect OIH in patients with allodynia and/or hyperalgesia, especially when repeated dose escalations do not improve analgesia or pain escalates following opioid dose titration.

Neuroexcitatory effects of morphine-3-glucuronide are dependent on Toll-like receptor 4 signaling

BACKGROUND

Multiple adverse events are associated with the use of morphine for the treatment of chronic non-cancer pain, including opioid-induced hyperalgesia (OIH). Mechanisms of OIH are independent of opioid tolerance and may involve the morphine metabolite morphine-3-glucuronide (M3G). M3G exhibits limited affinity for opioid receptors and no analgesic effect. Previous reports suggest that M3G can act via the Toll-like receptor 4 (TLR4)/myeloid differentiation protein-2 (MD-2) heterodimer in the central nervous system to elicit pain.

METHODS

Immunoblot and immunocytochemistry methods were used to characterize the protein expression of TLR4 present in lumbar dorsal root ganglion (DRG). Using in vitro intracellular calcium and current clamp techniques, we determined whether TLR4 activation as elicited by the prototypical agonists of TLR4, lipopolysaccharide (LPS) and M3G, contributed to changes in intracellular calcium and increased excitation. Rodents were also injected with M3G to determine the degree to which M3G-induced tactile hyperalgesia could be diminished using either a small molecule inhibitor of the MD-2/TLR4 complex in rats or TLR4 knockout mice. Whole cell voltage-clamp recordings were made from small- and medium-diameter DRG neurons (25 μm < DRG diameter <45 μm) for both control and M3G-treated neurons to determine the potential influence on voltage-gated sodium channels (NaVs).

RESULTS

We observed that TLR4 immunoreactivity was present in peptidergic and non-peptidergic sensory neurons in the DRG. Non-neuronal cells in the DRG lacked evidence of TLR4 expression. Approximately 15% of assayed small- and medium-diameter sensory neurons exhibited a change in intracellular calcium following LPS administration. Both nociceptive and non-nociceptive neurons were observed to respond, and approximately 40% of these cells were capsaicin-insensitive. Increased excitability observed in sensory neurons following LPS or M3G could be eliminated using Compound 15, a small molecule inhibitor of the TLR4/MD-2 complex. Likewise, systemic injection of M3G induced rapid tactile, but not thermal, nociceptive behavioral changes in the rat, which were prevented by pre-treating animals with Compound 15. Unlike TLR4 wild-type mice, TLR4 knockout mice did not exhibit M3G-induced hyperalgesia. As abnormal pain sensitivity is often associated with NaVs, we predicted that M3G acting via the MD-2/TLR4 complex may affect the density and gating of NaVs in sensory neurons. We show that M3G increases tetrodotoxin-sensitive and tetrodotoxin-resistant (NaV1.9) current densities.

CONCLUSIONS

These outcomes provide evidence that M3G may play a role in OIH via the TLR4/MD-2 heterodimer complex and biophysical properties of tetrodotoxin-sensitive and tetrodotoxin-resistant NaV currents.

Clinical pharmacology of analgesic medicines in older people: impact of frailty and cognitive impairment

Pain is highly prevalent in frail older people who often have multiple co-morbidities and multiple medicines. Rational prescribing of analgesics in frail older people is complex due to heterogeneity in drug disposition, comorbid medical conditions, polypharmacy and variability in analgesic response in this population. A critical issue in managing older people with pain is the need for judicious choice of analgesics based on a comprehensive medical and medication history. Care is needed in the selection of analgesic medicine to avoid drug-drug or drug-disease interactions. People living with dementia and cognitive impairment have suboptimal pain relief which in part may be related to altered pharmacodynamics of analgesics and challenges in the systematic assessment of pain intensity in this patient group. In the absence of rigorously controlled trials in frail older people and those with cognitive impairment a pharmacologically-guided approach can be used to optimize pain management which requires a systematic understanding of the pharmacokinetics and pharmacodynamics of analgesics in frail older people with or without changes in cognition.

[Do opioids induce hyperalgesia?]

Opioids are the most potent drugs for treatment of acute and chronic pain. However, accumulating evidence suggests that opioids may paradoxically also enhance pain, often referred to as opioid-induced hyperalgesia. Opioid-induced hyperalgesia is defined as an increased sensitivity to pain or a decreased pain threshold in response to opioid therapy. Several mechanisms have been proposed to support opioid-induced hyperalgesia. However, it remains unclear whether opioid-induced hyperalgesia develops during continuous chronic application of opioids or on their withdrawal. This review provides a comprehensive summary of clinical research concerning opioid-induced hyperalgesia and the molecular mechanisms of opioid withdrawal and opioid tolerance and other potential mechanisms which might induce hyperalgesia during opioid therapy will be discussed. The status quo of our knowledge will be summarized and the clinical relevance of opioid-induced hyperalgesia will be discussed.

Evidence that intrathecal morphine-3-glucuronide may cause pain enhancement via toll-like receptor 4/MD-2 and interleukin-1beta

Morphine-3-glucoronide (M3G) is a major morphine metabolite detected in cerebrospinal fluid of humans receiving systemic morphine. M3G has little-to-no affinity for opioid receptors and induces pain by unknown mechanisms. The pain-enhancing effects of M3G have been proposed to significantly and progressively oppose morphine analgesia as metabolism ensues. We have recently documented that morphine activates toll-like receptor 4 (TLR4), beyond its classical actions on mu-opioid receptors. This suggests that M3G may similarly activate TLR4. This activation could provide a novel mechanism for M3G-mediated pain enhancement, as (a) TLR4 is predominantly expressed by microglia in spinal cord and (b) TLR4 activation releases pain-enhancing substances, including interleukin-1 (IL-1). We present in vitro evidence that M3G activates TLR4, an effect blocked by TLR4 inhibitors, and that M3G activates microglia to produce IL-1. In vivo, intrathecal M3G (0.75 microg) induced potent allodynia and hyperalgesia, blocked or reversed by interleukin-1 receptor antagonist, minocycline (microglial inhibitor), and (+)-and (-)-naloxone. This latter study extends our prior demonstrations that TLR4 signaling is inhibited by naloxone nonstereoselectively. These results with (+)-and (-)-naloxone also demonstrate that the effects cannot be accounted for by actions at classical, stereoselective opioid receptors. Hyperalgesia (allodynia was not tested) and in vitro M3G-induced TLR4 signaling were both blocked by 17-DMAG, an inhibitor of heat shock protein 90 (HSP90) that can contribute to TLR4 signaling. Providing further evidence of proinflammatory activation, M3G upregulated TLR4 and CD11b (microglial/macrophage activation marker) mRNAs in dorsal spinal cord as well as IL-1 protein in the lumbosacral cerebrospinal fluid. Finally, in silico and in vivo data support that the glucuronic acid moiety is capable of inducing TLR4/MD-2 activation and enhanced pain. These data provide the first evidence for a TLR4 and IL-1 mediated component to M3G-induced effects, likely of at least microglial origin.

Comparative studies of the neuro-excitatory behavioural effects of morphine-3-glucuronide and dynorphin A(2-17) following spinal and supraspinal routes of administration

Morphine-3-glucuronide (M3G) administered centrally produces dose-dependent neuro-excitatory behaviours in rodents via a predominantly non-opioid mechanism. The endogenous opioid peptide, dynorphin A (Dyn A) (1-17), is rapidly cleaved in vivo to the relatively more stable fragment Dyn A(2-17) which also produces excitatory behaviours in rodents via a non-opioid mechanism. This study investigated the possible contribution of Dyn A(2-17) to the neuro-excitatory behaviours evoked by supraspinally and spinally administered M3G in male Sprague-Dawley (SD) rats. Marked qualitative differences in behaviours were apparent following administration of M3G and Dyn A(2-17). Administration of 11 nmol i.c.v. doses of M3G produced intermittent myoclonic jerks, tonic-clonic convulsions, and ataxia, as well as postural changes, whereas i.c.v. Dyn A(2-17) at 15 nmol produced effects on body posture alone. Administration of 11 nmol i.t. doses of M3G produced intermittent explosive motor activity, and touch-evoked agitation, as well as postural changes, whereas i.t. Dyn A(2-17) at 15 nmol produced postural changes, touch-evoked agitation, and paralysis. Pre-treatment with Dyn A antiserum (200 microg) markedly attenuated total behavioural excitation following i.c.v. and i.t. administration of Dyn A(2-17) by approximately 94% and 78%, respectively. However, total behavioural excitation following i.c.v. and i.t. administration of M3G was less markedly attenuated (both approximately 27%) by pre-treatment with Dyn A antiserum, with reductions in tonic-clonic convulsions ( approximately 43%), explosive motor behaviour ( approximately 28%), and touch-evoked agitation ( approximately 22%). The present findings discount a major role for Dyn A in mediating the neuro-excitatory effects of M3G, although it may contribute to maintaining some individual neuro-excitatory behaviours.

Antinociception versus serum concentration relationships following acute administration of intravenous morphine in male and female Sprague-Dawley rats: differences between the tail flick and hot plate nociceptive tests

1. Antinociception versus serum morphine concentration relationships were defined in male and female Sprague-Dawley rats administered single intravenous (i.v.) bolus doses of morphine, using the hot plate (2.1-14 mg/kg) and tail flick tests (1-8 mg/kg). 2. Serum concentrations of morphine and morphine-3-glucuronide (M3G), its major metabolite in the rat, were assayed using high-performance liquid chromatography (HPLC) with electrochemical detection. 3. Significantly higher (P < 0.05) values of peak antinociception (approximately 1.7-fold), as well as the extent and duration of antinociception (approximately fourfold), were observed in male compared with female rats administered 10 mg/kg morphine in the hot plate test. Although there were no significant sex-related differences in the area under the serum morphine concentration versus time curve (AUC) at this dose, systemic exposure to M3G (M3G AUC) was significantly higher (approximately twofold; P < 0.05) in female than male rats. 4. In contrast with most previous studies investigating sex differences in morphine antinociception in rats, the antinociceptive effects of single i.v. doses of morphine (1-8 mg/kg) in the tail flick test did not differ significantly between male and female rats. 5. Morphine ED(50) and EC(50) values (95% confidence intervals) for antinociception in the hot plate test were significantly lower (P < 0.05) in male rats (ED(50) 8.4 mg/kg (7.6-9.2); EC(50) 1.8 nmol/L (1.5-2.1)) compared with female rats (ED(50) 10.6 mg/kg (9.1-12.0); EC(50) 3.7 nmol/L (3.4-4.1)). However, in the tail flick test, there was no significant difference between male and female rats in ED(50) (1.8 (0.4-3.3) and 1.4 mg/kg (0.4-2.5), respectively) or EC(50) (0.5 (0.3-0.6) and 0.4 nmol/L (0.2-0.5), respectively) values. 6. Supraspinal attenuation of morphine antinociception by M3G may account for these differences.

Current concepts in pain management: pharmacologic options for the pediatric, geriatric, hepatic and renal failure patient

This article provides a review for current practice. Strict guidelines are not available on some topics, and they may never be drafted because pain is such a unique individual experience. It is recommended to coordinate care with other medical specialties when patients present with organ dysfunctions or are at the extremes of age. More data are required in the field of pain management, particularly with regard to renal and hepatic dysfunction. In turn, these data serve as a foundation for physicians making practice decisions based on current evidence. Until this is achieved, clinicians must rely on anecdotal evidence and the experiences of others to treat a complex issue: pain.

Withdrawal from acute morphine dependence is accompanied by increased anxiety-like behavior in the elevated plus maze

Pretreatment with a single moderate dose of morphine (e.g. 5.6-10 mg/kg) 4-24 hr prior to challenge with an opioid antagonist such as naloxone results in reliable expression of behaviors that resemble aversive or emotional consequences of withdrawal from chronic opioid exposure, including suppression of operant responding, elevations in brain reward thresholds, and conditioned place aversion. Repeated daily or weekly treatment with these same morphine doses results in a progressive increase in naloxone potency to elicit these withdrawal signs. The current study sought to determine whether increased anxiety-like behavior during withdrawal from chronic opioid dependence is also seen after acute morphine exposure, and progresses with repeated intermittent treatment. Male Wistar rats were handled and injected with either vehicle or morphine for 4 consecutive days. Three injection regimens were employed: Morphine Naive (4 vehicle injections), Acute Morphine (3 vehicle injections, 4th injection 5.6 or 10 mg/kg morphine), or Repeat Morphine (all 4 injections with 5.6 or 10 mg/kg morphine). Acute pretreatment with 5.6 mg/kg or 10 mg/kg morphine resulted in time-dependent increases in exploration of the open arms of the plus maze in naloxone-naive rats when tested at 2, 4 or 8 hr after the final pretreatment injection, with the effects at the higher dose appearing later (4 hr) than after the lower dose (2 hr). This pattern of results, in combination with a separate study which confirmed a significant anxiolytic-like effect of a low dose of morphine (0.56 mg/kg) administered 15 min prior to test, suggested that low residual morphine levels remaining in plasma at 2-4 hr after 5.6 and 10 mg/kg morphine may be sufficient to elicit anxiolytic-like effects. Repeat treatment with either dose of morphine resulted in a further increase in the magnitude and duration of this anxiolytic-like effect. These effects had dissipated by 8 hr post-morphine, and therefore precipitation of withdrawal by one of several doses of naloxone (0.10-3.3 mg/kg) was assessed in separate cohorts of rats 8 hr after the final pretreatment under Morphine Naïve, Acute Morphine, or Repeat Morphine conditions. Naloxone resulted in a significant dose-dependent expression of anxiety-like behavior with no effects on general activity after Acute Morphine pretreatment at either 5.6 or 10 mg/kg morphine. A further significant shift in naloxone potency was observed after Repeat Morphine pretreatment at the 10 mg/kg but not the 5.6 mg/kg dose. Thus, anxiety-like behavior is a prominent feature of the negative emotional consequences of naloxone-precipitated withdrawal from acute opioid dependence.

Cholestasis and endogenous opioids: liver disease and exogenous opioid pharmacokinetics

A class of endogenous opioids is upregulated in liver disease particular to cholestasis, which contributes to symptoms in liver disease such as pruritus, hypotension and encephalopathy. Symptoms associated with cholestasis are reversed or at least ameliorated by mu opioid receptor antagonists. Palliation of symptoms related to cholestatic liver disease also involves bile acid binding agents. Opioid receptor antagonists, unlike bile acid binding agents, have been reported to relieve multiple symptoms, except for pruritus, and improve liver function as demonstrated in experimental cholestasis. Exogenous opioid pharmacology is altered by liver disease. Dose reduction or prolongation of dose intervals is necessary depending on the severity of liver disease.

Nociception increases during opioid infusion in opioid receptor triple knock-out mice

Opioids are extensively used analgesics yet can paradoxically increase pain sensitivity in humans and rodents. This hyperalgesia is extensively conceptualized to be a consequence of opioid receptor activity, perhaps providing an adaptive response to analgesia, and to utilize N-methyl-D-aspartate (NMDA) receptors. These assumptions were tested here in opioid receptor triple knock-out (KO) mice lacking all three genes encoding opioid receptors (mu, delta, and kappa) by comparing their thermal nociceptive responses to the opioids morphine and oxymorphone with those of B6129F(1) controls. Injecting acute opioid bolus doses in controls caused maximal analgesia that was completely abolished in KO mice, confirming the functional consequence of the KO mouse opioid receptor deficiency. Continuous opioid infusion by osmotic pump in control mice also initially caused several consecutive days of analgesia that was shortly thereafter followed by several consecutive days of hyperalgesia. In contrast, continuously infusing KO mice with opioids caused no detectable analgesic response, but only immediate and steady declines in nociceptive thresholds culminating in several days of unremitting hyperalgesia. Finally, injecting the non-competitive NMDA receptor antagonist MK-801 during opioid infusion markedly reversed hyperalgesia in control but not KO mice. These data demonstrate that sustained morphine and oxymorphone delivery causes hyperalgesia independently of prior or concurrent opioid or NMDA receptor activity or opioid analgesia, indicating the contribution of mechanisms outside of current conceptions, and are inconsistent with proposals of hyperalgesia as a causative factor of opioid analgesic tolerance.

Role of morphine's metabolites in analgesia: concepts and controversies

The metabolites of morphine, morphine-6-glucuronide (M6G) and morphine-3-glucuronide (M3G), have been extensively studied for their contribution to clinical effects following administration of morphine. Those contributions to both the desired effect (ie, analgesia) and the undesired effects (eg, nausea, respiratory depression) are the subject of clinical controversy. Much attention and effort have been directed at investigating the properties of M6G because of interest in this substance as a possible substitute for morphine. It exhibits increased potency and the possibility of a better side effect profile compared with morphine, although the reported relative benefits vary widely. M3G is not analgesic, but its role in producing side effects, including the development of clinical tolerance, has been proposed. This review is focused on M6G and the factors that contribute to its clinical utility. The formation and distribution of M6G are presented, as are the analgesic effect and the onset of this effect. The impact of genetics, age, and gender on M6G and its effects is also reviewed.

Opioid receptor-mediated hyperalgesia and antinociceptive tolerance induced by sustained opiate delivery

Opiates are commonly used to treat moderate to severe pain and can be used over prolonged periods in states of chronic pain such as those associated with cancer. In addition, to analgesic actions, studies show that opiate administration can paradoxically induce hyperalgesia. At the pre-clinical level, such hyperalgesia is associated with numerous pronociceptive neuroplastic changes within the primary afferent fibers and the spinal cord. In rodents, sustained opiate administration also induces antinociceptive tolerance. The mechanisms by which prolonged opiate exposure induces hyperalgesia and the relationship of this state to antinociceptive tolerance remain unclear. The present study was aimed at determining whether sustained opiate-induced hyperalgesia, associated neuroplasticity and antinociceptive tolerance are the result of specific opiate interaction at opiate receptors. Enantiomers of oxymorphone, a mu opioid receptor agonist, were administered to rats by spinal infusion across 7 days. Sustained spinal administration of (-)-oxymorphone, but not its inactive enantiomer (+)-oxymorphone or vehicle, upregulated spinal dynorphin content, produced thermal and tactile hypersensitivity, and produced antinociceptive tolerance. These results indicate that these pronociceptive actions of sustained opiate administration require specific interaction with opiate receptors and are unlikely to be the result of accumulation of potentially excitatory metabolic products. While the precise mechanisms, which may account for these pronociceptive changes remain to be unraveled, the present data point to plasticity initiated by opiate receptor interaction.

Subanesthetic ketamine infusion therapy: a retrospective analysis of a novel therapeutic approach to complex regional pain syndrome

Complex Regional Pain Syndrome (CRPS) is a disorder that can be accompanied by severe pain that is often both chronic and resistant to conventional therapy. Harbut and Correll previously reported the successful treatment of a 9-year case of intractable Type I CRPS with an intravenous inpatient infusion of ketamine in an adult female patient.

OBJECTIVE

The purpose of this study was to ascertain if indeed the use of subanesthetic inpatient infusions of ketamine provide meaningful improvements in pain scores, and thus, quality of life, in patients suffering from CRPS. To achieve this objective we focused our analysis on the relief of pain obtained by patients undergoing this novel treatment option developed at Mackay Base Hospital, Queensland, Australia.

METHODS

Case notes of 33 patients whose CRPS pain was treated by the inpatient administration of a continuous subanesthetic intravenous infusion of ketamine were reviewed. The dose and duration of ketamine therapy and the degree and duration of relief obtained were recorded. Notable side effects were also recorded. The degree of relief obtained (immediately after the infusion) was assessed using pre- and posttreatment numeric pain scores. The duration of relief obtained (throughout the follow-up period) was analyzed using a Kaplan-Meier cumulative survival curve analysis.

RESULTS

A total of 33 patients with diagnoses of CRPS who had undergone ketamine treatment at least once were identified. Due to relapse, 12 of 33 patients received a second course of therapy, and two of 33 patients received a third. The degree of relief obtained following the initial course of therapy was impressive (N=33); there was complete pain relief in 25 (76%), partial relief in six (18%), and no relief in two (6%) patients. The degree of relief obtained following repeat therapy (N=12) appeared even better, as all 12 patients who received second courses of treatment experienced complete relief of their CRPS pain. The duration of relief was also impressive, as was the difference between the duration of relief obtained after the first and after the second courses of therapy. In this respect, following the first course of therapy, 54% of 33 individuals remained pain free for >/=3 months and 31% remained pain free for >/=6 months. After the second infusion, 58% of 12 patients experienced relief for >/=1 year, while almost 33% remained pain free for >3 years. The most frequent side effect observed in patients receiving this treatment was a feeling of inebriation. Hallucinations occurred in six patients. Less frequent side effects also included complaints of lightheadedness, dizziness, and nausea. In four patients, an alteration in hepatic enzyme profile was noted; the infusion was terminated and the abnormality resolved thereafter.

CONCLUSION

This retrospective review suggests that limited subanesthetic inpatient infusions of ketamine may offer a promising therapeutic option in the treatment of appropriately selected patients with intractable CRPS. More study is needed to further establish the safety and efficacy of this novel approach.

Opioid receptors

Opioid receptors belong to the large superfamily of seven transmembrane-spanning (7TM) G protein-coupled receptors (GPCRs). As a class, GPCRs are of fundamental physiological importance mediating the actions of the majority of known neurotransmitters and hormones. Opioid receptors are particularly intriguing members of this receptor family. They are activated both by endogenously produced opioid peptides and by exogenously administered opiate compounds, some of which are not only among the most effective analgesics known but also highly addictive drugs of abuse. A fundamental question in addiction biology is why exogenous opioid drugs, such as morphine and heroin, have a high liability for inducing tolerance, dependence, and addiction. This review focuses on many aspects of opioid receptors with the aim of gaining a greater insight into mechanisms of opioid tolerance and dependence.

Measurement of intracellular Ca2+ in cultured rat embryonic hippocampal neurons using a fluorescence microplate reader: potential application to biomolecular screening

INTRODUCTION

Fluorescence microplate readers for the measurement of cytosolic free Ca(2+) ([Ca(2+)](i)) are used as a drug screening tool, particularly for immortal cell lines. However, wider application of this methodological approach to more differentiated cells such as neurons would also be useful for the screening of compounds that modulate synaptic transmission. Such an approach has the potential to identify lead compounds for the development of novel drugs for the treatment of epilepsy, pathological pain states, Parkinson's disease, or other neurological disorders.

METHODS

In this paper, we describe the development of a microplate reader assay for the assessment of [Ca(2+)](i) in a primary culture of rat hippocampal neurons maintained in Neurobasal medium using the fluorescent calcium indicator, fluo-3.

RESULTS

The assay was appropriate for the screening of glutamate receptor agonists and antagonists. Furthermore, lowering the extracellular Mg(2+) concentration ([Mg(2+)](O)) produced consistent oscillations in neuronal [Ca(2+)](i) detected using the fluorescence microplate reader. These oscillations were inhibited by the GABA(B) agonist, baclofen, and the NMDA receptor antagonist, LY274614.

DISCUSSION

Our results indicate that assessment of the inhibitory effects of agents on spontaneous [Ca(2+)](i) oscillations in neurons may be useful for the identification of agents that act on targets for which specific screening methods are not currently available, or those which act via a previously unknown pathway to inhibit synaptic transmission. This technique also has the potential to increase the productivity of experiments designed to characterize changes in [Ca(2+)](i) (including calcium oscillations) in cultured neurons.

[Opioid-induced hyperalgesia. Pathophysiology and clinical relevance]

Opioids are the drugs of choice for the treatment of moderate to severe acute and chronic pain. However, clinical evidence suggests that opioids can elicit increased sensitivity to noxious stimuli suggesting that administration of opioids can activate both pain inhibitory and pain facilitatory systems. Acute receptor desensitization via uncoupling of the receptor from G-proteins, up-regulation of the cAMP pathway, activation of the N-methyl-D-aspartate (NMDA) receptor system, as well as descending facilitation, have been proposed as potential mechanisms underlying opioid-induced hyperalgesia. Numerous reports exist demonstrating that opioid-induced hyperalgesia is observed both in animal and human experimental models. Brief exposures to micro-receptor agonists induce long-lasting hyperalgesic effects for days, which might by reflected by clinical observations that large doses of intraoperative micro-receptor agonists increased postoperative pain and morphine consumption. Furthermore, the prolonged use of opioids in patients often requires increasing doses and may be accompanied by the development of abnormal pain. Successful strategies that may decrease or prevent opioid-induced hyperalgesia include the concomitant administration of drugs like NMDA-antagonists, alpha(2)-agonists, or non-steroidal anti-inflammatory drugs (NSAIDs), opioid rotation or combinations of opioids with different receptor selectivity.