Mechanisms of Nociception Evoked by Intrathecal High-dose Morphine

Interaction of Opioids with TLR4-Mechanisms and Ramifications

Simple Summary

Recent evidence indicates that opioids can be active at a receptor that is abundantly expressed on innate immune cells as well as cancer cells: the receptor is termed toll-like receptor 4 (TLR4). TLR4 is increasingly recognised as playing key roles in tumour biology and anticancer defences. However, the issue of whether TLR4 mediates some of the effects of opioids on tumour growth and metastasis is entirely unknown. We review existing evidence, mechanisms, and functional consequences of the action of opioids at TLR4. This opens new avenues of research on the role of opioids in cancer.

Abstract

The innate immune receptor toll-like receptor 4 (TLR4) is known as a sensor for the gram-negative bacterial cell wall component lipopolysaccharide (LPS). TLR4 activation leads to a strong pro-inflammatory response in macrophages; however, it is also recognised to play a key role in cancer. Recent studies of the opioid receptor (OR)-independent actions of opioids have identified that TLR4 can respond to opioids. Opioids are reported to weakly activate TLR4, but to significantly inhibit LPS-induced TLR4 activation. The action of opioids at TLR4 is suggested to be non-stereoselective, this is because OR-inactive (+)-isomers of opioids have been shown to activate or to inhibit TLR4 signalling, although there is some controversy in the literature. While some opioids can bind to the lipopolysaccharide (LPS)-binding cleft of the Myeloid Differentiation factor 2 (MD-2) co-receptor, pharmacological characterisation of the inhibition of opioids on LPS activation of TLR4 indicates a noncompetitive mechanism. In addition to a direct interaction at the receptor, opioids affect NF-κB activation downstream of both TLR4 and opioid receptors and modulate TLR4 expression, leading to a range of in vivo outcomes. Here, we review the literature reporting the activity of opioids at TLR4, its proposed mechanism(s), and the complex functional consequences of this interaction.

Novel opioid-neurotensin-based hybrid peptide with spinal long-lasting antinociceptive activity and a propensity to delay tolerance development

The behavioral responses exerted by spinal administration of the opioid-neurotensin hybrid peptide, PK23, were studied in adult male rats. The antinociceptive effect upon exposure to a thermal stimulus, as well as tolerance development, was assessed in an acute pain model. The PK23 chimera at a dose of 10 nmol/rat produced a potent pain-relieving effect, especially after its intrathecal administration. Compared with intrathecal morphine, this novel compound was found to possess a favourable side effect profile characterized by a reduced scratch reflex, delayed development of analgesic tolerance or an absence of motor impairments when given in the same manner, though some animals died following barrel rotation as a result of its i.c.v. administration (in particular at doses higher than 10 nmol/rat). Nonetheless, these results suggest the potential use of hybrid compounds encompassing both opioid and neurotensin structural fragments in pain management. This highlights the enormous potential of synthetic neurotensin analogues as promising future analgesics.

New βN-octadecanoyl-5-hydroxytryptamide: antinociceptive effect and possible mechanism of action in mice

The present study examined the potential antinociceptive activity of C18 5-HT (^βN-octadecanoyl-5-hydroxytryptamide) using chemical and thermal nociception models in mice. Orally administered C18 5-HT (0.1, 1 and 10 mg/kg) produced significant dose-dependent antinociceptive effects in formalin-, capsaicin- and glutamate-induced licking models. This compound also induced a significant increase in the response to thermal stimuli in the hot plate test, and its antinociceptive effect was not related to muscle relaxant or sedative actions. In a thermal hyperalgesia model, C18 5-HT presented an anti-hyperalgesic profile as evidenced by the increase in the response time of the animals. Furthermore, intraperitoneal (i.p) pretreatment with naloxone (a non-selective opioid receptor antagonist, 1 mg/kg), ondansetron (serotoninergic receptor antagonist (5-HT3 subtype), 0.5 mg/kg) or AM241 (CB1 cannabinoid receptor antagonist, 1 mg/kg) reversed the antinociceptive effects of C18 5-HT in the hot plate model. In the formalin-induced licking model, pretreatment with naloxone reversed the antinociceptive effects of C18 5-HT, as demonstrated by an increase in the paw licking response when compared with the C18 5-HT-treated group. These findings suggest that C18 5-HT has peripheral and central antinociceptive effects and that its mechanism of action involves, ate least in part, opioid, serotoninergic and cannabinoid pathways.

[Study of Supplementary Analgesics Capable of Reducing the Dosage of Morphine]

Morphine with its potent analgesic property has been widely used for the treatment of various types of pain. However, the intrathecal (i.t.) administration of morphine at doses far higher than those required for antinociception exhibited nociceptive-related behaviors consisting of scratching, biting and licking, hyperalgesia, and allodynia in mice. Morphine-3-glucuronide (M3G), one of the major metabolites of morphine, has been found to evoke nociceptive behaviors similar to those after high-dose i.t. morphine. It is plausible that M3G may be responsible for nociception seen after high-dose i.t. morphine treatment. This article reviews the potential mechanism of spinally mediated nociceptive behaviors evoked by i.t. M3G in mice. We discuss the possible presynaptic release of nociceptive neurotransmitters/neuromodulators such as substance P, glutamate, dynorphin, and Leu-enkephalin in the primary afferent fibers following i.t. M3G administration. It is possible to speculate that i.t. M3G could indirectly activate NK1, NMDA, and δ2-opioid receptors that lead to the release of nitric oxide (NO) in the dorsal spinal cord. The major function of NO is the production of cGMP and the activation of protein kinase G (PKG). The NO-cGMP-PKG pathway plays an important role in M3G-induced nociceptive behavior. The phosphorylation of extracellular signal-related kinase (ERK) in the dorsal spinal cord was also evoked via the NO-cGMP-PKG pathway through the activation of δ2-opioid, NK1, and NMDA receptors, contributing to M3G-induced nociceptive behaviors. The demonstration of a neural mechanism underlying M3G-induced nociception provides a pharmacological basis for improved pain management with morphine at high doses.

The effect of propofol on intrathecal morphine-induced pruritus and its mechanism

BACKGROUND

Previous studies have shown that a low dose of propofol IV bolus had a beneficial effect on intrathecal morphine-induced pruritus in humans. However, its exact mechanism has not been fully understood. In this study, we hypothesized that propofol relieved intrathecal morphine-induced pruritus in rats by upregulating the expression of cannabinoid-1 (CB[1]) receptors in anterior cingulate cortex (ACC).

METHODS

Twenty-four Sprague-Dawley rats were divided into a control group and 20, 40, 80 μg/kg morphine groups to create an intrathecal morphine-induced scratching model. The effects of propofol on intrathecal 40 μg/kg morphine-induced scratching responses were then evaluated. Sixty rats were randomly assigned to control, normal saline, intralipid, and propofol groups, with pruritus behavior observation or killed 8 minutes after venous injection of normal saline, intralipid, or propofol, and brain tissues were then collected for assay. Immunohistochemistry was then performed to identify the expression of CB (1) receptor in ACC, and the concentration of CB(1) receptor in ACC was determined by Western blot analysis.

RESULTS

Compared with the control group, rats in the 20, 40, 80 μg/kg morphine groups had higher mean scratching response rates after intrathecal morphine injection (P =0.020, 0.005, and 0.002, respectively). There was a statistical difference between 20 and 40 μg/kg morphine groups at 10 to 15 and 15 to 20 timepoints after intrathecal morphine injection (P = 0.049 and 0.017, respectively). Propofol almost abolished the scratching response that was induced by 40 μg/kg intrathecal morphine injection (F[2, 15] = 46.87, P < 0.001; F[22, 165] = 2.37, P = 0.001). Compared with the intralipid and normal saline groups, the scratching behavior was significantly attenuated in the propofol group (P < 0.001). Compared with control, normal saline, and intralipid groups, the protein expression of CB(1) receptor in ACC (Western blot) in the propofol group increased (0.86 ± 0.21, 0.94 ± 0.18, 0.86 ± 0.13, and 1.34 ± 0.32, respectively, P < 0.001). There was no significant difference among control, normal saline, and intralipid groups. Compared with the control, normal saline, and intralipid groups, the average number of neurons of CB(1) receptor in the ACC area were higher in the propofol group (21.0 ± 1.4, 19.3 ± 1.8, 24.8 ± 7.7, and 37.2 ± 3.3, respectively, P < 0.001).

CONCLUSIONS

Morphine elicits dose-independent scratching responses after intrathecal injection in rats. Morphine 40 μg/kg intrathecal injection-induced scratching responses can be prevented by propofol. Increased protein expression of CB(1) receptors in ACC may contribute to the reversal of intrathecal morphine-induced scratching.

Compact, power-efficient architectures using microvalves and microsensors, for intrathecal, insulin, and other drug delivery systems

This paper describes a valve-regulated architecture, for intrathecal, insulin and other drug delivery systems, that offers high performance and volume efficiency through the use of micromachined components. Multi-drug protocols can be accommodated by using a valve manifold to modulate and mix drug flows from individual reservoirs. A piezoelectrically-actuated silicon microvalve with embedded pressure sensors is used to regulate dosing by throttling flow from a mechanically-pressurized reservoir. A preliminary prototype system is demonstrated with two reservoirs, pressure sensors, and a control circuit board within a 130cm(3) metal casing. Different control modes of the programmable system have been evaluated to mimic clinical applications. Bolus and continuous flow deliveries have been demonstrated. A wide range of delivery rates can be achieved by adjusting the parameters of the manifold valves or reservoir springs. The capability to compensate for changes in delivery pressure has been experimentally verified. The pressure profiles can also be used to detect catheter occlusions and disconnects. The benefits of this architecture compared with alternative options are reviewed.

PolyMorphine: an innovative biodegradable polymer drug for extended pain relief

Morphine, a potent narcotic analgesic used for the treatment of acute and chronic pain, was chemically incorporated into a poly(anhydride-ester) backbone. The polymer termed "PolyMorphine", was designed to degrade hydrolytically releasing morphine in a controlled manner to ultimately provide analgesia for an extended time period. PolyMorphine was synthesized via melt-condensation polymerization and its structure was characterized using proton and carbon nuclear magnetic resonance spectroscopies, and infrared spectroscopy. The weight-average molecular weight and the thermal properties were determined. The hydrolytic degradation pathway of the polymer was determined by in vitro studies, showing that free morphine is released. In vitro cytocompatibility studies demonstrated that PolyMorphine is non-cytotoxic towards fibroblasts. In vivo studies using mice showed that PolyMorphine provides analgesia for 3 days, 20 times the analgesic window of free morphine. The animals retained full responsiveness to morphine after being subjected to an acute morphine challenge.

Antinociceptive activity of the monoterpene α-phellandrene in rodents: possible mechanisms of action

OBJECTIVES

The aim of this work was to investigate the antinociceptive property of α-phellandrene (α-PHE) in experimental nociception models and possible mechanisms involved.

METHODS

Mass spectrometry was used to evaluate the purity and molecular mass of α-PHE. Macrophages from mice peritoneal cavity were used in an MTT test. Rodents were used in tests of chemical and mechanical nociception. In the study of the mechanisms, the animals were treated with pharmacological tools and then submitted to the glutamate test.

KEY FINDINGS

α-PHE purity was 98.2% and molecular mass 136.1 Da. α-PHE did not show cytotoxicity. In the writhing and capsaicin tests, α-PHE promoted the antinociceptive effect in all evaluated doses (minimum dose 3.125 mg/kg). In the formalin test, α-PHE (50 mg/kg) was effective in inhibiting both phases. In the glutamate test, the monoterpene (12.5 mg/kg) decreased the nociceptive response. In carrageenan-induced hyperalgesia, α-PHE (50 mg/kg) decreased the hypernociception index. In the study of the mechanisms involved, pretreatment with naloxone reversed the α-PHE antinociceptive effect, the same occurred with glibenclamide, l-arginine, atropine and yohimbine. α-PHE did not show muscle relaxant activity or central depressant effects in open field and rota rod tests.

CONCLUSIONS

α-PHE has an antinociceptive effect and it possibly involves the glutamatergic, opioid, nitrergic, cholinergic and adrenergic systems.

Involvement of NMDA receptor in nociceptive effects elicited by intrathecal [Tyr6] gamma2-MSH(6-12), and the interaction with nociceptin/orphanin FQ in pain modulation in mice

The mas-related genes (Mrgs, also known as sensory neuron-specific receptors, SNSRs) are specifically expressed in small diameter sensory neurons in the trigeminal and dorsal root ganglia, suggesting an important role of the receptors in pain transmission. The present study aimed to investigate the underlying mechanism of the nociceptive effects after activation of MrgC, and the interaction between MrgC and N/OFQ-NOP receptor system in modulation of nociception in mice. Intrathecal (i.t.) administration of [Tyr(6)] gamma2-MSH(6-12), the most potent agonist for MrgC receptor, produced a significant hyperalgesic response as assayed by tail withdrawal test and a series of characteristic nociceptive responses, including biting, licking and scratching, in a dose-dependent manner (0.01-10 pmol and 0.01-10 nmol, respectively) in mice. These pronociceptive effects induced by [Tyr(6)] gamma2-MSH(6-12) were inhibited dose-dependently by co-injection of competitive NMDA receptor antagonist D-APV, non-competitive NMDA receptor antagonist MK-801, and nitric oxide (NO) synthase inhibitor L-NAME. However, the tachykinin NK(1) receptor antagonist L-703,606, and tachykinin NK(2) receptor antagonist MEN-10,376, had no influence on pronociceptive effects elicited by [Tyr(6)] gamma2-MSH(6-12). In other groups, [Tyr(6)] gamma2-MSH(6-12)-induced nociceptive responses were bidirectionally regulated by the co-injection of N/OFQ. N/OFQ inhibited nociceptive responses at high doses (0.01-1 nmol), but potentiated the behaviors at low doses (1 fmol-3 pmol). Furthermore, both hyperalgesia and nociceptive responses were enhanced after the co-administration with NOP receptor antagonist [Nphe(1)]N/OFQ(1-13)-NH(2). These results suggest that intrathecal [Tyr(6)] gamma2-MSH(6-12)-induced pronociceptive effects may be mediated through NMDA receptor-NO system in the spinal cord, and demonstrate the interaction between MrgC and N/OFQ-NOP receptor system in pain transmission.

Morphine: A Protective or Destructive Role in Neurons?

Morphine has received intensive research interest for a long time. However, until recently, the protective versus destructive roles of morphine in the neuronal system have not been studied. There is evidence suggesting that morphine induces apoptotic cell death in neuronal and glial cells, whereas controversial studies support a neuroprotective role for morphine. The exact mechanisms for both protective and destructive pathways are not clear and are still under investigation. Improved understanding of morphine neuroprotection and neurotoxicity will be helpful to control morphine side effects in medical applications and to identify new targets for potential therapies and prevention strategies to opioid addiction. NEUROSCIENTIST 14(6):561-570, 2008. DOI:

Abnormal heat and pain perception in remitted heroin dependence months after detoxification from methadone-maintenance

Patients receiving methadone maintenance therapy (MMT) for opiate dependence have altered nociception, complicating analgesic treatment. Increasing numbers of patients are choosing opiate-free treatment programs, yet data on the course of this abnormality months after detoxification from methadone is contradictory and based exclusively on cold pressor experiments. Heat and pain thresholds were measured by quantitative sensory testing (QST) in 23 subjects with heroin dependence in full, sustained remission months after detoxification from methadone and 27 healthy non-drug using controls. Self reports of pain intensity and unpleasantness were also collected. Test scores were compared across groups and correlated with measures of drug use history. There were significant differences between remitted opiate-dependent subjects and controls on the measures of heat threshold (38.83 vs. 35.96; Mann-Whitney U=177.5, p=0.006), and the measure of pain threshold (48.73 vs. 47.62; Mann-Whitney U=217.5, p=0.043). There was no correlation of any measure of drug use history with the heat or pain experience. Abstinent, formerly opioid-dependent patients continue to demonstrate abnormal noxious perception months after detoxification from methadone.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 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, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Bis selenide alkene derivatives: A class of potential antioxidant and antinociceptive agents

Bis and tris-selenide alkene derivatives, a class of organoselenium compounds, were screened for antinociceptive and antioxidant activities. In vitro, bis-selenide alkene 1c (R=2,4,6-Me(3)C(6)H(2)), 1d (R=4-ClC(6)H(4)) and 1e (R=4-MeOC(6)H(4)) protected against lipid peroxidation about 50%, whereas 1b (R=C(6)H(5)) and 1a (R=C(4)H(9)) protected only 23%. Compound 1d presented lesser IC(50) against lipid peroxidation than other bis-selenide alkene compounds (1d>1e> or =1c>1a=1b). The maximal inhibitory effect of tris-selenide alkenes on lipid peroxidation was in the following order 2c>2a=2b. Compound 1e increased the rate of GSH, but not DTT, oxidation. Tris-selenide alkene 2c (R=4-MeOC(6)H(4)) demonstrated the higher rate of thiol oxidation, while 2a (R=C(6)H(5)) did not change DTT oxidation but oxidized GSH. Conversely, compound 2b (R=4-ClC(6)H(4)) did not change the rate of GSH oxidation, but oxidized DDT. Bis-selenide alkene derivatives 1c, 1d and 1e were the most promising compounds tested in vitro. In vivo, compounds 1c and 1d (5-50mg/kg, subcutaneously) produced significant inhibition of acetic acid- and capsaicin-induced pain. Compounds 1c and 1d increased the tail-flick response latency time. The antinociception effect of 1c and 1d was not abolished by naloxone (an antagonist of opioid receptor, 1mg/kg, subcutaneously), suggesting that the antinociceptive effect is not influenced by the opioidergic mechanism.