MK-801 inhibits the development of morphine tolerance at spinal sites

Glycine Transporter 1 Inhibitors: Predictions on Their Possible Mechanisms in the Development of Opioid Analgesic Tolerance

The development of opioid tolerance in patients on long-term opioid analgesic treatment is an unsolved matter in clinical practice thus far. Dose escalation is required to restore analgesic efficacy, but at the price of side effects. Intensive research is ongoing to elucidate the underlying mechanisms of opioid analgesic tolerance in the hope of maintaining opioid analgesic efficacy. N-Methyl-D-aspartate receptor (NMDAR) antagonists have shown promising effects regarding opioid analgesic tolerance; however, their use is limited by side effects (memory dysfunction). Nevertheless, the GluN2B receptor remains a future target for the discovery of drugs to restore opioid efficacy. Mechanistically, the long-term activation of µ-opioid receptors (MORs) initiates receptor phosphorylation, which triggers β-arrestin-MAPKs and NOS-GC-PKG pathway activation, which ultimately ends with GluN2B receptor overactivation and glutamate release. The presence of glutamate and glycine as co-agonists is a prerequisite for GluN2B receptor activation. The extrasynaptic localization of the GluN2B receptor means it is influenced by the glycine level, which is regulated by astrocytic glycine transporter 1 (GlyT1). Enhanced astrocytic glycine release by reverse transporter mechanisms as a consequence of high glutamate levels or unconventional MOR activation on astrocytes could further activate the GluN2B receptor. GlyT1 inhibitors might inhibit this condition, thereby reducing opioid tolerance.

Spinal Serum- and Glucocorticoid-Regulated Kinase 1 (SGK1) Signaling Contributes to Morphine-Induced Analgesic Tolerance in Rats

Accumulating evidence indicates that phosphorylated serum- and glucocorticoid-regulated kinase 1 (SGK1) is associated with spinal nociceptive sensitization by modulating glutamatergic N-methyl-D-aspartate receptors (NMDARs). In this study, we determined whether spinal SGK1 signaling contributes to the development of morphine analgesic tolerance. Chronic morphine administration markedly induced phosphorylation of SGK1 in the spinal dorsal horn neurons. Intrathecal injection of SGK1 inhibitor GSK-650394 reduced the development of morphine tolerance with a significant leftward shift in morphine dose-effect curve. Furthermore, spinal inhibition of SGK1 suppressed morphine-induced phosphorylation of nuclear factor kappa B (NF-κB) p65 and upregulation of NMDAR NR1 and NR2B expression in the spinal dorsal horn. In contrast, intrathecal administration of NMDAR antagonist MK-801 had no effect on the phosphorylation of SGK1 in morphine-treated rats. In addition, morphine-induced upregulation of NR2B, but not NR1, was significantly abolished by intrathecal pretreatment with PDTC, a specific NF-κB activation inhibitor. Finally, spinal delivery of SGK1 small interfering RNA exhibited similar inhibitory effects on morphine-induced tolerance, phosphorylation of NF-κB p65, as well as upregulation of NR1 and NR2B expression. Our findings demonstrate that spinal SGK1 contributes to the development of morphine tolerance by enhancing NF-κB p65/NMDAR signaling. Interfering spinal SGK1 signaling pathway could be a potential strategy for prevention of morphine tolerance in chronic pain management.

Ethanol Reversal of Oxycodone Tolerance in Dorsal Root Ganglia Neurons

Oxycodone is a semisynthetic opioid compound that is widely prescribed, used, and abused today, and has a well-established role in shaping the current opioid epidemic. Previously, we have shown that tolerance develops to the antinociceptive and respiratory depressive effects of oxycodone in mice, and that a moderate dose of acute ethanol or a protein kinase C (PKC) inhibitor reversed that tolerance. To investigate further if tolerance was occurring through neuronal mechanisms, our aims for this study were to assess the effects of acute and prolonged oxycodone in isolated dorsal root ganglia (DRG) neurons and to determine if this tolerance was reversed by either ethanol or a PKC inhibitor. We found that an acute exposure to 3 μM oxycodone reduced neuronal excitability, as measured by increased threshold potentials and reduced action potential amplitude, without eliciting measurable changes in resting membrane potential. Exposure to 10 μM oxycodone for 18-24 hours prevented oxycodone's effect on neuronal excitability, indicative of tolerance development. The development of opioid tolerance was mitigated in DRG neurons from β-arrestin 2 knockout mice. Oxycodone tolerance was reversed in isolated DRG neurons by the acute application of either ethanol (20 mM) or the PKC inhibitor, bisindolylmaleimide XI hydrochloride (Bis XI), when a challenge of 3 µM oxycodone significantly reduced neuronal excitability following prolonged exposure. Through these studies, we concluded that oxycodone acutely reduced neuronal excitability, tolerance developed to this effect, and reversal of that tolerance occurred at the level of a single neuron, suggesting that reversal of oxycodone tolerance by either ethanol or Bis XI involves cellular mechanisms.

Opioids: keeping the good, eliminating the bad

Two new studies show that mechanisms mediating the opioid side effects of tolerance, hyperalgesia and physical dependence are mediated spinally and can be dissociated from analgesia. These side effects can be selectively targeted by clinically available drugs without affecting their pain-relieving effects.

Curcumin attenuates opioid tolerance and dependence by inhibiting Ca2+/calmodulin-dependent protein kinase II α activity

Chronic use of opioid analgesics has been hindered by the development of opioid addiction and tolerance. We have reported that curcumin, a natural flavonoid from the rhizome of Curcuma longa, attenuated opioid tolerance, although the underlying mechanism remains unclear. In this study, we tested the hypothesis that curcumin may inhibit Ca(2+)/calmodulin-dependent protein kinase II α (CaMKIIα), a protein kinase that has been previously proposed to be critical for opioid tolerance and dependence. In this study, we used state-of-the-art polymeric formulation technology to produce poly(lactic-co-glycolic acid) (PLGA)-curcumin nanoparticles (nanocurcumin) to overcome the drug's poor solubility and bioavailability, which has made it extremely difficult for studying in vivo pharmacological actions of curcumin. We found that PLGA-curcumin nanoparticles reduced the dose requirement by 11- to 33-fold. Pretreatment with PLGA-curcumin (by mouth) prevented the development of opioid tolerance and dependence in a dose-dependent manner, with ED50 values of 3.9 and 3.2 mg/kg, respectively. PLGA-curcumin dose-dependently attenuated already-established opioid tolerance (ED50 = 12.6 mg/kg p.o.) and dependence (ED50 = 3.1 mg/kg p.o.). Curcumin or PLGA-curcumin did not produce antinociception by itself or affect morphine (1-10 mg/kg) antinociception. Moreover, we found that the behavioral effects of curcumin on opioid tolerance and dependence correlated with its inhibition of morphine-induced CaMKIIα activation in the brain. These results suggest that curcumin may attenuate opioid tolerance and dependence by suppressing CaMKIIα activity.

Phosphoproteomics and bioinformatics analyses of spinal cord proteins in rats with morphine tolerance

Introduction

Morphine is the most effective pain-relieving drug, but it can cause unwanted side effects. Direct neuraxial administration of morphine to spinal cord not only can provide effective, reliable pain relief but also can prevent the development of supraspinal side effects. However, repeated neuraxial administration of morphine may still lead to morphine tolerance.

Methods

To better understand the mechanism that causes morphine tolerance, we induced tolerance in rats at the spinal cord level by giving them twice-daily injections of morphine (20 µg/10 µL) for 4 days. We confirmed tolerance by measuring paw withdrawal latencies and maximal possible analgesic effect of morphine on day 5. We then carried out phosphoproteomic analysis to investigate the global phosphorylation of spinal proteins associated with morphine tolerance. Finally, pull-down assays were used to identify phosphorylated types and sites of 14-3-3 proteins, and bioinformatics was applied to predict biological networks impacted by the morphine-regulated proteins.

Results

Our proteomics data showed that repeated morphine treatment altered phosphorylation of 10 proteins in the spinal cord. Pull-down assays identified 2 serine/threonine phosphorylated sites in 14-3-3 proteins. Bioinformatics further revealed that morphine impacted on cytoskeletal reorganization, neuroplasticity, protein folding and modulation, signal transduction and biomolecular metabolism.

Conclusions

Repeated morphine administration may affect multiple biological networks by altering protein phosphorylation. These data may provide insight into the mechanism that underlies the development of morphine tolerance.

Mu opioids and their receptors: evolution of a concept

Opiates are among the oldest medications available to manage a number of medical problems. Although pain is the current focus, early use initially focused upon the treatment of dysentery. Opium contains high concentrations of both morphine and codeine, along with thebaine, which is used in the synthesis of a number of semisynthetic opioid analgesics. Thus, it is not surprising that new agents were initially based upon the morphine scaffold. The concept of multiple opioid receptors was first suggested almost 50 years ago (Martin, 1967), opening the possibility of new classes of drugs, but the morphine-like agents have remained the mainstay in the medical management of pain. Termed mu, our understanding of these morphine-like agents and their receptors has undergone an evolution in thinking over the past 35 years. Early pharmacological studies identified three major classes of receptors, helped by the discovery of endogenous opioid peptides and receptor subtypes-primarily through the synthesis of novel agents. These chemical biologic approaches were then eclipsed by the molecular biology revolution, which now reveals a complexity of the morphine-like agents and their receptors that had not been previously appreciated.

Roles of reactive oxygen and nitrogen species in pain

Peroxynitrite (PN; ONOO⁻) and its reactive oxygen precursor superoxide (SO; O₂•⁻) are critically important in the development of pain of several etiologies including pain associated with chronic use of opiates such as morphine (also known as opiate-induced hyperalgesia and antinociceptive tolerance). This is now an emerging field in which considerable progress has been made in terms of understanding the relative contributions of SO, PN, and nitroxidative stress in pain signaling at the molecular and biochemical levels. Aggressive research in this area is poised to provide the pharmacological basis for development of novel nonnarcotic analgesics that are based upon the unique ability to selectively eliminate SO and/or PN. As we have a better understanding of the roles of SO and PN in pathophysiological settings, targeting PN may be a better therapeutic strategy than targeting SO. This is because, unlike PN, which has no currently known beneficial role, SO may play a significant role in learning and memory. Thus, the best approach may be to spare SO while directly targeting its downstream product, PN. Over the past 15 years, our team has spearheaded research concerning the roles of SO and PN in pain and these results are currently leading to the development of solid therapeutic strategies in this important area.

Pathophysiology of medication overuse headache: insights and hypotheses from preclinical studies

INTRODUCTION

Medication overuse headache (MOH) is a clinical concern in the management of migraine headache. MOH arises from the frequent use of medications used for the treatment of a primary headache. Medications that can cause MOH include opioid analgesics as well as formulations designed for the treatment of migraine, such as triptans, ergot alkaloids, or drug combinations that include caffeine and barbiturates.

LITERATURE REVIEW

Gathering evidence indicates that migraine patients are more susceptible to development of MOH, and that prolonged use of these medications increases the prognosis for development of chronic migraine, leading to the suggestion that similar underlying mechanisms may drive both migraine headache and MOH. In this review, we examine the link between several mechanisms that have been linked to migraine headache and a potential role in MOH. For example, cortical spreading depression (CSD), associated with migraine development, is increased in frequency with prolonged use of topiramate or paracetamol.

CONCLUSIONS

Increased CGRP levels in the blood have been linked to migraine and elevated CGRP can be casued by prolonged sumatriptan exposure. Possible mechanisms that may be common to both migraine and MOH include increased endogenous facilitation of pain and/or diminished diminished endogenous pain inhibition. Neuroanatomical pathways mediating these effects are examined.

N-Methyl-D-aspartate receptor antagonist MK-801 attenuates morphine tolerance and associated glial fibrillary acid protein up-regulation: a proteomic approach

BACKGROUND

It is well known that long-term morphine administration results in tolerance, which limits the clinical use of this drug in pain management.

METHODS

Male Wistar rats were randomly assigned to receive one of four different infusions: morphine [15 microg/h, intrathecal (i.t.)], saline, MK-801 (5 microg/h, i.t.) plus morphine (15 microg/h, i.t.), or MK-801 (5 microg/h, i.t.) alone.

RESULTS

Morphine infusion induced a maximal antinociceptive effect on day 1 and tolerance on day 3, and the maximal anti-receptive tolerance was observed on day 5. Co-infusing MK-801 with morphine attenuated morphine's anti-receptive tolerance. Two-dimensional gel electrophoretic analysis of spinal proteins revealed that eight protein spots were up-regulated in morphine-tolerant rats, and that they were significantly inhibited by MK-801 co-infusion. Among the up-regulated proteins, glial fibrillary acid protein (GFAP), a glial-specific maker, was identified by mass spectrometry. This finding was also confirmed by Western blot analysis.

CONCLUSION

Using proteomic analysis, we identified eight GFAP protein spots that were up-regulated in the dorsal horn of morphine-tolerant rat spinal cords. This up-regulation was partly inhibited by N-methyl-D-aspartate receptor antagonist MK-801 co-infusion, which suggests that GFAP protein can be considered to be a pathogenesis marker of morphine tolerance.

NMDA receptor antagonists inhibit opiate antinociceptive tolerance and locomotor sensitization in rats

RATIONALE

N-Methyl-D: -aspartate (NMDA) receptors have an important role in different forms of behavioral and neural plasticity. Evidence suggests that these receptors may also be involved in plasticity arising from long-term treatment with different drugs of abuse, including tolerance, sensitization, and physical dependence. There is abundant evidence demonstrating that NMDA receptors are involved in tolerance to opiate-induced antinociception; however, the role of these receptors in sensitization to the locomotor effects of opiates is more controversial.

OBJECTIVE

The ability of NMDA receptor antagonists to modify the development of sensitization to the locomotor stimulant effect of three different opiates was examined. In selected studies, the ability of the antagonists to modify tolerance to the antinociceptive effects of the opiates was also examined.

MATERIALS AND METHODS

Adult male Sprague-Dawley rats were used to assess the effects of NMDA receptor antagonists (MK-801, memantine or LY235959) on tolerance and sensitization to three opiates: morphine, methadone, or buprenorphine. It was predicted that low, selective doses of the antagonists would inhibit the development of opiate tolerance and sensitization.

RESULTS

Consistent with our predictions, the noncompetitive NMDA receptor antagonists MK-801 and memantine and the competitive NMDA receptor antagonist LY235959 inhibited the development of sensitization to the locomotor stimulant effect of morphine. Additionally, MK-801 inhibited the development of tolerance and sensitization to methadone and buprenorphine in a similar manner.

CONCLUSIONS

The results, together with previous research, suggest that NMDA receptors are broadly involved in opiate-induced plasticity, including the development of opiate tolerance and sensitization.

Blockade of NMDA receptors prevents analgesic tolerance to repeated transcutaneous electrical nerve stimulation (TENS) in rats

Repeated daily application of transcutaneous electrical nerve stimulation (TENS) results in tolerance, at spinal opioid receptors, to the antihyperalgesia produced by TENS. Since N-methyl-D-aspartate (NMDA) receptor antagonists prevent analgesic tolerance to opioid agonists, we hypothesized that blockade of NMDA receptors will prevent tolerance to TENS. In rats with knee joint inflammation, TENS was applied for 20 minutes daily at high-frequency (100 Hz), low-frequency (4 Hz), or sham TENS. Rats were treated with the NMDA antagonist MK-801 (0.01 mg/kg to 0.1 mg/kg) or vehicle daily before TENS. Paw withdrawal thresholds were tested before and after inflammation and before and after TENS treatment for 4 days. On day 1, TENS reversed the decreased mechanical withdrawal threshold induced by joint inflammation. On day 4, TENS had no effect on the decreased withdrawal threshold in the group treated with vehicle, demonstrating development of tolerance. However, in the group treated with 0.1 mg/kg MK-801, TENS significantly reversed the mechanical withdrawal thresholds on day 4, demonstrating that tolerance did not develop. Vehicle-treated animals developed cross-tolerance at spinal opioid receptors. Treatment with MK-801 reversed this cross-tolerance at spinal opioid receptors. In summary, blockade of NMDA receptors prevents analgesic tolerance to daily TENS by preventing tolerance at spinal opioid receptors.

PERSPECTIVE

Observed tolerance to the clinical treatment of TENS could be prevented by administration of pharmaceutical agents with NMDA receptors activity such as ketamine or dextromethorphan.

The glycine site-specific NMDA antagonist (+)-HA966 enhances the effect of morphine and reverses morphine tolerance via a spinal mechanism

Using the C-fibre reflex as a nociceptive response elicited by a wide range of stimulus intensities in the rat, we recently reported that a single treatment with (+)-HA966, a glycine site-specific NMDA receptor antagonist: (1) potentiates morphine antinociception; and (2) reverses an established morphine tolerance. We presently aimed at determining whether our observation was likely to result from a direct effect on the spinal cord or an indirect effect of supraspinal origin. In a 2x2x2 experimental design, we compared the effects of 5 mg/kg morphine in: (1) sham-operated rats or animals whose brainstems had been transected at the level of the obex; (2) rats that were implanted with pellets, either 150 mg morphine or placebo; and (3) animals injected either with saline or 10 mg/kg (+)-HA966. The control C-fibre reflexes were similar in all groups of animals. As compared to "non-tolerant" rats, the depressive effect of morphine was weaker in "morphine-tolerant" animals where the threshold did not change following morphine but the gain of the stimulus-response curve decreased, albeit to a significantly lesser extent than in the "non-tolerant" group. Whether in "non-tolerant" or "tolerant" groups, the effects of morphine were stronger in "obex-transected" than in "sham-operated" animals. In all groups, the effects of morphine were potentiated by the preliminary administration of (+)-HA966. However, in the "morphine-tolerant" group, the preliminary administration of (+)-HA966 was more potent in the "sham-operated" than in the "obex-transected" groups. Since overall effects were very similar in "sham-operated" and "obex-transected" animals, we concluded for our model that the critical site for the expression of the neuronal plastic changes associated with morphine tolerance lies in the spinal cord.

Role of spinal metabotropic glutamate receptor subtype 5 in the development of tolerance to morphine-induced antinociception in rat

Prolonged intrathecal (i.t.) administration of morphine results in tolerance to morphine-induced antinociception. We found that co-administration of selective metabotropic glutamate receptor subtype 5 antagonist MPEP with morphine could suppress the loss of morphine-induced antinociception and inhibit the development of tolerance to morphine-induced antinociceptive effect. Whereas, the specific metabotropic glutamate receptor subtype 5 agonist CHPG does the opposite. As the activation of NMDA receptor after chronic morphine administration has been verified, we suppose there is an enhanced activation of mGluR5 during the development of tolerance to morphine-induced antinociception. Activation of mGluR5 may mobilize the release of intracellular Ca(2+) and activate PKC, leading to morphine-induced antinociception suppression. We conclude that mGluR5 contributes to the development of tolerance to morphine-induced antinociception after chronic morphine exposure.

Resistance to morphine analgesic tolerance in rats with deleted transient receptor potential vanilloid type 1-expressing sensory neurons

Deletion of transient receptor potential vanilloid type 1 (TRPV1)-expressing afferent neurons reduces presynaptic mu opioid receptors but paradoxically potentiates the analgesic efficacy of mu opioid agonists. In this study, we determined if removal of TRPV1-expressing afferent neurons by resiniferatoxin (RTX), an ultrapotent capsaicin analog, influences the development of opioid analgesic tolerance. Morphine tolerance was induced by daily intrathecal injections of 10 microg of morphine for 14 consecutive days or by daily i.p. injections of 10 mg/kg of morphine for 10 days. In vehicle-treated rats, the effect of intrathecal or systemic morphine on the mechanical withdrawal threshold was gradually diminished within 7 days. However, the analgesic effect of intrathecal and systemic morphine was sustained in RTX-treated rats at the time the morphine effect was lost in the vehicle group. Furthermore, the mu opioid receptor-G protein coupling in the spinal cord was significantly decreased ( approximately 22%) in vehicle-treated morphine tolerant rats, but was not significantly altered in RTX-treated rats receiving the same treatment with morphine. Additionally, there was a large reduction in protein kinase Cgamma-immunoreactive afferent terminals in the spinal dorsal horn of RTX-treated rats. These findings suggest that loss of TRPV1-expressing sensory neurons attenuates the development of morphine analgesic tolerance possibly by reducing mu opioid receptor desensitization through protein kinase Cgamma in the spinal cord. These data also suggest that the function of presynaptic mu opioid receptors on TRPV1-expressing sensory neurons is particularly sensitive to down-regulation by mu opioid agonists during opioid tolerance development.

Primary afferent NMDA receptors increase dorsal horn excitation and mediate opiate tolerance in neonatal rats

Repeated exposure to opiates produces analgesic tolerance, which limits their clinical usefulness. Whole-cell voltage-clamped lamina I cells in spinal slices from opiate-tolerant neonatal rats show an increase in miniature, spontaneous, and primary afferent-evoked EPSCs when compared with lamina I cells from opiate-naive rat spinal slices. This increased excitation can be blocked by the NMDA receptor (NMDAR) antagonist APV, apparently acting at NMDARs on primary afferents. Consistent with these results, electron microscopy demonstrates an increased incidence of NMDARs in substance P-containing spinal dorsal horn primary afferent terminals in opiate-tolerant rats. Moreover, superfusion of spinal slices from opiate-tolerant rats with NMDA produces a reversible increase in miniature EPSC (mEPSC) frequency in contrast to a decrease in mEPSC frequency produced by NMDA in opiate-naive slices. Finally, NMDAR antagonists inhibit the expression of opiate tolerance both in inhibiting EPSCs in spinal slices and in inhibiting behavioral nociceptive responses to heat. NMDAR antagonists have been reported in many studies to inhibit morphine analgesic tolerance. Our studies suggest that an increase in primary afferent NMDAR expression and activity mediates a hypersensitivity to noxious stimuli and causes the inhibition of opiate efficacy, which defines tolerance.

Tolerance to morphine analgesia: Evidence for stimulus intensity as a key factor and complete reversal by a glycine site-specific NMDA antagonist

N-methyl-D-aspartate (NMDA) receptors are widely involved in opioid tolerance. However, it is less clear whether NMDA receptor antagonists reverse already-established tolerance and whether the intensity of the nociceptive stimulus influences morphine tolerance. Three days after implantation of morphine or control pellets the effects of i.v. morphine and pre-administration of saline or (+)-HA966 (a glycine site-specific NMDA receptor antagonist), were studied on the C-fibre reflex elicited by a wide range of stimulus intensities. Morphine both increased the threshold and decreased the slope of the recruitment curve in the "non-tolerant" group of animals. In the "morphine-tolerant" group, the threshold did not change but the gain of the stimulus-response curve decreased. The expression of tolerance to morphine depended on the intensity of the stimulus, being maximal when threshold stimulus intensities were used but considerably less with supra-threshold stimulation. As expected, a single treatment with (+)-HA966, potentiated morphine antinociception in "non-tolerant" rats. However, in "morphine-tolerant" rats (+)-HA966 reversed established morphine tolerance and increased the antinociceptive effects of morphine. These results suggest that (+)-HA966 interfered with expression of morphine tolerance, and offered an encouraging therapeutic approach for pain management in opioid abusers.

Reversal of Morphine Antinociceptive Tolerance and Dependence by the Acute Supraspinal Inhibition of Ca2+/Calmodulin-Dependent Protein Kinase II

Previous studies have suggested that Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) can modulate opioid tolerance and dependence via its action on learning and memory. In this study, we examined whether CaMKII could directly regulate opioid tolerance and dependence. CaMKII activity was increased after the treatment with morphine (100 mg/kg s.c. or 75 mg s.c. of morphine/pellet/mouse); the effect exhibited a temporal correction with the development of opioid tolerance and dependence. In mice treated with morphine (100 mg/kg s.c.), morphine tolerance and dependence developed in 2 to 6 h. An acute supraspinal administration of KN93 [2-[N-(2-hydroxyethyl)]-N-(4-methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine)], a CaMKII inhibitor, was able to dose-dependently reverse the already-established antinociceptive tolerance to morphine (p < 0.001 for 15-30 nmol; not significant for 5 nmol). KN92 [2-[N-(4-methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine] (30 nmol i.c.v.), a kinase-inactive analog of KN93, did not affect opioid tolerance. Neither KN92 nor KN93 affected basal nociception or acute morphine antinociception (1-10 nmol i.c.v.). Likewise, dependence on morphine was abolished by the acute administration of KN93, but not KN92, in a dose-dependent manner. Pretreatment of mice with KN93 also prevented the development of morphine tolerance and dependence. The effect of acute CaMKII inhibition was not limited to the particular experimental model, because KN93 also acutely reversed the established opioid tolerance and dependence in mice treated with morphine (75 mg/pellet/mouse s.c.) for 6 days. Taken together, these data strongly support the hypothesis that CaMKII can act as a key and direct factor in promoting opioid tolerance and dependence. Identifying such a direct mechanism may be useful for designing pharmacological treatments for these conditions.

Dextromethorphan differentially affects opioid antinociception in rats

Opioid drugs such as morphine and meperidine are widely used in clinical pain management, although they can cause some adverse effects. A number of studies indicate that N-methyl-D-aspartate (NMDA) receptors may play a role in the mechanism of morphine analgesia, tolerance and dependence. Being an antitussive with NMDA antagonist properties, dextromethorphan (DM) may have some therapeutic benefits when coadministered with morphine. In the present study, we investigated the effects of DM on the antinociceptive effects of different opioids. We also investigated the possible pharmacokinetic mechanisms involved. The antinociceptive effects of the mu-opioid receptor agonists morphine (5 mg kg(-1), s.c.), meperidine (25 mg kg(-1), s.c.) and codeine (25 mg kg(-1), s.c.), and the kappa-opioid agonists nalbuphine (8 mg kg(-1), s.c.) and U-50,488H (20 mg kg(-1), s.c.) were studied using the tail-flick test in male Sprague-Dawley rats. Coadministration of DM (20 mg kg(-1), i.p.) with these opioids was also performed and investigated. The pharmacokinetic effects of DM on morphine and codeine were examined, and the free concentration of morphine or codeine in serum was determined by HPLC.It was found that DM potentiated the antinociceptive effects of some mu-opioid agonists but not codeine or kappa-opioid agonists in rats. DM potentiated morphine's antinociceptive effect, and acutely increased the serum concentration of morphine. In contrast, DM attenuated the antinociceptive effect of codeine and decreased the serum concentration of its active metabolite (morphine). The pharmacokinetic interactions between DM and opioids may partially explain the differential effects of DM on the antinociception caused by opioids.

Analgesic pharmacology: I. Neurophysiology

The transmission of a pain signal from the periphery to the central nervous system is complex and only partially understood. Tissue damage results in peripheral release of endogenous chemicals that can directly activate nociceptive afferent fibers, sensitize nociceptors, or cause increased local extravasation and vasodilatation. These algesiogenic substances may be found in local tissues, plasma, and nerve terminals. Release of these substances may be caused by mechanical injury, radiation, or heat, or release may be stimulated by the by-products of tissue injury (ie, catecholamines or collagen). Peripheral nociceptors may be further sensitized by repeated noxious stimuli. Nociceptive afferents have their neurons in the dorsal root ganglion and contact second-order neurons in the dorsal horn or, less often, in the medulla. Modulation of the pain signal in the dorsal horn involves local inhibitory and facilitatory interneurons as well as diverse excitatory and inhibitory neurotransmitters. The neuronal circuitry in the dorsal horn can change and modulate with time so that pain signals sometimes long outlast the original peripheral tissue injury. This central sensitization is thought to be mediated largely through the NMDA receptor complex.

Effects of NMDA receptor antagonists on acute mu-opioid analgesia in the rat

Mixed research findings have led to a debate regarding the effect of N-methyl-D-aspartate (NMDA) receptor antagonists on opiate analgesia. NMDA antagonists have been found in various studies to enhance, to inhibit, or to have no effect on opiate analgesia. The present research used a single protocol to explore the effects of six NMDA receptor antagonists on acute morphine (3.0 mg/kg s.c.) and fentanyl (0.05 mg/kg s.c.) analgesia in adult male Sprague-Dawley rats. NMDA receptor antagonists were selected based on their abilities to block various sites on the NMDA receptor complex, including the noncompetitive antagonists MK-801 (0.1 and 0.3 mg/kg i.p.), dextromethorphan (10.0 and 30.0 mg/kg i.p.), and memantine (3.0 and 10.0 mg/kg i.p.), a glycine site antagonist, (+)-HA-966 (10.0 and 30.0 mg/kg i.p.), a competitive antagonist, LY235959 (1.0 and 3.0 mg/kg i.p.), and a polyamine site antagonist, ifenprodil (1.0 and 3.0 mg/kg i.p.). Analgesia was assessed using the tail-flick test. A single dose of each opiate was used. The low doses of the antagonists, which are known to produce significant neural and behavioral actions at NMDA receptors, had no effect on morphine or fentanyl analgesia. At the higher doses, morphine analgesia was significantly enhanced by LY235959 (3.0 mg/kg), and fentanyl analgesia was significantly enhanced by LY235959 (3.0 mg/kg), dextromethorphan (30.0 mg/kg), and (+)-HA-966 (30.0 mg/kg). Enhancement of analgesia occurred without any apparent adverse side effects. None of the NMDA antagonists affected tail-flick responses on their own, except the higher dose of LY235959 (3.0 mg/kg), which produced a mild analgesic effect. Because no consistent effects were observed, the data suggest that NMDA receptors are not involved in acute mu-opioid analgesia. The mechanisms underlying the enhancement of opiate analgesia by selected NMDA antagonists, such as LY235959, dextromethorphan, and (+)-HA-966, remain to be determined.

Increasing of intrathecal CSF excitatory amino acids concentration following morphine challenge in morphine-tolerant rats

Excitatory amino acids (EAAs) are involved in the development of opioid tolerance. The present study reveals that an increasing of CSF EAAs concentration might be responsible for the losing of morphine's antinociceptive effect in morphine tolerant rats. Male Wistar rats were implanted with two intrathecal (i.t.) catheters and one microdialysis probe, then continuously infused i.t. for 5 days with saline (1 microl/h; control group), morphine (15 micrograms/h), the NMDA antagonist, MK-801 (5 micrograms/h), or morphine (15 micrograms/h) plus MK-801 (5 micrograms/h). Each day, tail-flick responses were measured; in addition, CSF dialysates were collected and CSF amino acids measured by high performance liquid chromatography using a fluorescence detector. Morphine started to lose its analgesic effect on day 2 and this effect was overcome by MK-801. The AD(50) (AD: analgesic dose) was 1.33 micrograms in control animals, 83.83 micrograms in morphine-tolerant rats (a 63-fold shift), and 11.2 micrograms (a 8.4-fold shift) in rats that had received MK-801 plus morphine. No significant differences were observed in CSF amino acid release between the groups from day 1 to day 5. On day 5, after basal dialysate collection, a 10-micrograms challenge of morphine was administered i.t., and CSF samples collected over the next 3 h. After morphine challenge, morphine-tolerant rats showed a significant increase in the release of glutamate and aspartate (131+/-9.5% and 156+/-12% of basal levels, respectively), and no antinociceptive effect in the tail-flick latency test, while MK-801/morphine co-infused rats showed no increase in morphine-induced EAA release and a partial antinociceptive effect (MPE=40%). The present study provides direct evidence for a relationship between EAA release and a lack of an antinociceptive response to morphine, and shows that the NMDA antagonist, MK-801, attenuates both of these effects.

Spinal amino acid release and repeated withdrawal in spinal morphine tolerant rats

1. We used spinal microdialysis in awake rats to investigate whether the repeated withdrawal with naloxone during continuous spinal infusion of morphine would lead to a progressively greater spinal glutamate release and a more pronounced intrathecal tolerance. 2. Rats received lumbar intrathecal (IT) infusion of morphine (IT-M: 20 nmol microl(-1) h(-1)) or saline (IT-S: 1 microl h(-1)) continuously for 3 days. Both groups were further subdivided to receive intraperitoneal (i.p.) injection of naloxone (IP-N: 0.6 mg kg(-1)) or saline (IP-S: 3 ml kg(-1)) every 24 h after the beginning of IT infusion. Daily thermal escape latencies, withdrawal signs, the resting basal release of spinal amino acids before IP injection and the release immediately after the injection (evoked) were measured. 3. Rats receiving IT morphine showed a maximum increase in thermal escape latency on day 1, after which this value declined, with the fastest decline observed in IT morphine + IP naloxone group. On day 1, no significant difference was observed among groups in the resting basal release of amino acids. Rats in IT morphine + i.p. naloxone group displayed a progressive increase in this value. The release was not significantly altered in other groups. 4. For the IT-M + IP-N group, basal resting dialysate concentrations of Glu, Asp and Tau rose steadily over the 3-day infusion interval. No change in basal resting release was noted for any other treatment. 5. Evoked release (after i.p. naloxone) in IT-M animals displayed a progressive increase over the three repeated exposures. Evoked release did not change significantly in other treatment groups. 6. The degree of precipitated withdrawal significantly correlated with the increase in glutamate acutely evoked by i.p. injection. 7. The present results show that periodic transient withdrawal of spinal opiate agonist activity leads to a progressive increase in glutamate outflow and withdrawal signs, in a manner consistent with an enhanced development of spinal tolerance.

Nitroglycerin inhibits the development of morphine tolerance and dependence in rats

The development of tolerance to and physical dependence on opioids remains a significant barrier to their clinical use. N-Methyl-D-aspartate (NMDA) receptor antagonists inhibit tolerance and dependence. However, many NMDA antagonists have undesirable side effects. It has been shown that nitroglycerin (NTG) can antagonize NMDA receptor activity. This study was designed to determine whether NTG could inhibit the development of morphine tolerance and dependence. Rats were anesthetized and implanted with either morphine or placebo pellets, and pumps infusing vehicle or NTG (doses from 0.1 microg/kg/day to 10 mg/kg/day). Tolerance development was assessed by tail-flick latency (TFL). After 6 days, withdrawal was precipitated by subcutaneous injection of 2 mg/kg naloxone. Withdrawal signs were observed for 15 min. Placebo-pelleted rats showed no changes in TFL over the course of the study and no withdrawal signs. Morphine-pelleted rats developed tolerance. The 0.1 mg/kg/day NTG dose significantly attenuated tolerance development, while the other doses had no significant effect. The 0.1 mg/kg/day dose also attenuated some withdrawal signs. Higher or lower doses were not effective, possibly because of competing biochemical effects.

Differential mechanisms of morphine antinociceptive tolerance revealed in (beta)arrestin-2 knock-out mice

Morphine induces antinociception by activating mu opioid receptors (muORs) in spinal and supraspinal regions of the CNS. (Beta)arrestin-2 (beta)arr2), a G-protein-coupled receptor-regulating protein, regulates the muOR in vivo. We have shown previously that mice lacking (beta)arr2 experience enhanced morphine-induced analgesia and do not become tolerant to morphine as determined in the hot-plate test, a paradigm that primarily assesses supraspinal pain responsiveness. To determine the general applicability of the (beta)arr2-muOR interaction in other neuronal systems, we have, in the present study, tested (beta)arr2 knock-out ((beta)arr2-KO) mice using the warm water tail-immersion paradigm, which primarily assesses spinal reflexes to painful thermal stimuli. In this test, the (beta)arr2-KO mice have greater basal nociceptive thresholds and markedly enhanced sensitivity to morphine. Interestingly, however, after a delayed onset, they do ultimately develop morphine tolerance, although to a lesser degree than the wild-type (WT) controls. In the (beta)arr2-KO but not WT mice, morphine tolerance can be completely reversed with a low dose of the classical protein kinase C (PKC) inhibitor chelerythrine. These findings provide in vivo evidence that the muOR is differentially regulated in diverse regions of the CNS. Furthermore, although (beta)arr2 appears to be the most prominent and proximal determinant of muOR desensitization and morphine tolerance, in the absence of this mechanism, the contributions of a PKC-dependent regulatory system become readily apparent.

Loss of intrathecal morphine analgesia in terminal cancer patients is associated with high levels of excitatory amino acids in the CSF

PURPOSE

To examine excitatory amino acid (EAA) levels in the cerebrospinal fluid (CSF) of patients on long-term morphine treatment for terminal cancer pain relief and to correlate these with morphine's analgesic effect.

METHODS

Fourteen terminal cancer patients suffering severe pain and requiring long-term opioid treatment for pain relief were included. An intrathecal (IT) catheter was implanted at the L(3-4)/L(4-5) level and advanced 10 cm in a cephalad direction. IT morphine injection was started at 100 microgram q 12 hr with a daily incremental dose of 50 microgram until the effective dose was reached. The CSF was sampled (2 mL) as follows: 1) before the first IT morphine injection, 2) when the effective dose of morphine was reached, 3) when loss of morphine's analgesic effect at the effective dose (pain visual analogue scale > 5), and 4) after consecutive increases of the morphine dose (50 microgram, IT, daily) for satisfactory pain relief and up to double the effective dose. The concentrations of glutamate and aspartate in the CSF were determined.

RESULTS

CSF levels of glutamate and aspartate at the effective dose of morphine were lower than the baseline levels and increased when pain intensity increased and when morphine's analgesic effect was lost.

CONCLUSION

Long-term IT morphine administration was accompanied by an increase of EAA level in the CSF that was associated with a loss of morphine's analgesic effect.

Tolerance to the antinociceptive effect of Crotalus durissus terrificus snake venom in mice is mediated by pharmacodynamic mechanisms

Crotalus durissus terrificus venom exerts central and peripheral antinociceptive effect mediated by opioid receptors. The present work investigated the tolerance to the antinociceptive effect of the venom and characterised the mechanisms involved in this phenomenon. The hot plate test, applied in mice, was used for pain threshold determination. The venom (200 microg/kg) was administered by oral route, daily, for 14 days, and the nociceptive test was applied before and on days 1, 7 and 14 of the treatment. Prolonged treatment with venom lead to the development of tolerance to the antinociceptive effect. Tolerant animals exhibited increased sodium pentobarbital-induced sleeping time, although total hepatic microsomal cytochrome P450 was not altered. The antinociceptive effect of a single dose of venom (200 microg/kg) is mediated by kappa opioid receptors. Mice long-term-treated with venom showed cross-tolerance to U-TRANS, an agonist of kappa-opioid receptor, but not to morphine or DAMGO, two mu-opioid receptor agonists. Prolonged administration of venom did not cause symptoms of abstinence syndrome. These data indicate that prolonged treatment with C. durissus terrificus venom induces tolerance to the antinociceptive effect and that pharmacodynamic mechanisms are involved in the genesis of this phenomenon.

Effects of sufentanil and NMDA antagonists on a C-fibre reflex in the rat

The effects of intravenous sufentanil and pre-administration of N-methyl-D-aspartate (NMDA) receptor antagonists were tested on a reflex triggered by C-fibre activation. The reflex was elicited by electrical stimulation of the sural nerve and recorded from the ipsilateral biceps femoris muscle in halothane anaesthetized rats either (1) with an intact neuraxis or (2) in which the brain had previously been transected at the level of the obex. All four doses of sufentanil (0.33, 0.6, 1 and 2 microg kg(-1)) elicited a depression of the reflex in a dose-dependent manner. However, following the expected depression, all doses of sufentanil elicited both facilitation of the reflex and tonic inter-stimulus discharges. The C-fibre reflex was not modified following intravenous ketamine (1 mg kg(-1)) or (+)-HA966 (5 or 10 mg kg(-1)) but, when administered 5 min before sufentanil, these drugs enhanced both the extent and the duration of the depression and strongly reduced the facilitations. In the obex-transected rats, the depressive effect of 1 microg kg(-1) sufentanil increased, while the facilitation of the C-fibre reflex and the tonic inter-stimulus discharges disappeared. Pre-administration of 10 mg kg(-1) (+)-HA966 reinforced and prolonged the depressive effect of sufentanil. These results extend previous studies suggesting the involvement of NMDA receptors in the spinal transmission of nociceptive signals. They illustrate the potential of spinal NMDA receptor blockade to both enhance the analgesic, and prevent the pro-nociceptive, effects of sufentanil.

Activation of brainstem N-methyl-D-aspartate receptors is required for the analgesic actions of morphine given systemically

The analgesic actions of opioids are in large part mediated by activation of brainstem pain modulating neurons that depress nociceptive transmission at the level of the dorsal horn. The present study was designed to characterize the contribution of N-methyl-D-aspartate (NMDA)- and non-NMDA-mediated excitatory transmission within the rostral ventromedial medulla (RVM) to the activation of brainstem inhibitory output neurons and analgesia produced by systemic morphine administration. The NMDA receptor antagonist D-2-amino-5-phosophonopentanoic acid (AP5), the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX) or saline was infused into the RVM of lightly anesthetized rats while recording the activity of identified pain modulating neurons: 'off-cells', thought to inhibit nociceptive transmission, and 'on-cells', thought to facilitate nociception. Nociceptive responsiveness (tail flick latency) was not affected by either antagonist. AP5, but not CNQX, attenuated or blocked activation and disinhibition of off-cells and the antinociception produced by systemically administered morphine. Reflex-related discharge of on-cells was unaffected by AP5, but significantly attenuated by CNQX. The present results highlight two important aspects of RVM pain modulatory circuits. First, morphine given systemically produces its analgesic effect at least in part by recruiting an NMDA-mediated excitatory process to activate off-cells within the RVM. This excitatory process may play a role in the analgesic synergy produced by simultaneous mu-opioid activation at different levels of the neuraxis. Second, reflex-related activation of on-cells is mediated by a non-NMDA receptor, and this activation does not appear to play a significant role in regulating reflex responses to acute noxious stimuli. Excitatory amino acid-mediated excitation thus has at least two distinct roles within the RVM, activating off-cells and on-cells under different conditions.

Spinal cord neuroplasticity following repeated opioid exposure and its relation to pathological pain

Convincing evidence has accumulated that indicates neuroplastic changes within the spinal cord in response to repeated exposure to opioids. Such neuroplastic changes occur at both cellular and intracellular levels. It has been generally acknowledged that the activation of N-methyl-D-aspartate (NMDA) receptors plays a pivotal role in the development of neuroplastic changes following repeated opioid exposure. Intracellular cascades can also be activated subsequent to NMDA receptor activation. In particular, protein kinase C has been shown to be a key intracellular element that contributes to the behavioral manifestation of neuroplastic changes. Moreover, interactions between NMDA and opioid receptors can lead to potentially irreversible degenerative neuronal changes in the spinal cord in association with the development of opioid tolerance. Interestingly, similar cellular and intracellular changes occur in the spinal cord following peripheral nerve injury. These findings indicate that interactions exist in the spinal cord neural structures between two seemingly unrelated conditions-chronic opioid exposure and a pathological pain state. These observations may help understand mechanisms of chemical intolerance and multiple chemical sensitivity as well as have significant clinical implications in pain management with opioid analgesics.

Intrathecal Zn2+ attenuates morphine antinociception and the development of acute tolerance

Vesicular Zn2+, released in the brain and from small dorsal root ganglion neurons, interacts with opioid as well as N-methyl-D-aspartate (NMDA) receptors. We investigated the effect of Zn2+ on morphine antinociception in mice (tail flick assay), as well as acute tolerance and dependence, phenomena associated with NMDA activity. Administered intrathecally (i.t.), Zn2+ inhibited morphine antinociception in a dose-related fashion. Zn2+ also inhibited acute tolerance to morphine antinociception (5 h after 100 mg/kg of morphine). Injection i.t. of di-sodium calcium ethylenediamine tetra acetic acid (Na+Ca2+ EDTA), a chelator of divalent cations, had no effect on analgesia, acute tolerance or acute dependence. However, withdrawal jumps produced by naloxone (1 mg/kg s.c.) in morphine-pellet implanted mice (3 days) were potentiated by injections twice daily of 10 nmol of Na+Ca2+ EDTA, suggesting that endogenous Zn2+ tends to inhibit long-term development of withdrawal. These data suggest that the availability of Zn2+ is an important factor in opioid activity.

Morphine tolerance increases [3H]MK-801 binding affinity and constitutive neuronal nitric oxide synthase expression in rat spinal cord

N-Methyl-D-aspartate (NMDA) receptor antagonists and nitric oxide synthase (NOS) inhibitors inhibit morphine tolerance. In the present study, a lumbar subarachnoid polyethylene (PE10) catheter was implanted for drug administration to study alterations in NMDA receptor activity and NOS protein expression in a morphine-tolerant rat spinal model. Antinociceptive tolerance was induced by intrathecal (i.t.) morphine infusion (10 micrograms h-1) for 5 days. Co-administered (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) (10 micrograms h-1 i.t.) with morphine was used to inhibit the development of morphine tolerance. Lumbar spinal cord segments were removed and prepared for [3H]MK-801 binding assays and NOS western blotting. The binding affinity of [3H]MK-801 was higher in spinal cords of morphine-tolerant rats (mean (SEM) KD = 0.41 (0.09) nM) than in control rats (1.50 (0.13) nM). There was no difference in Bmax. Western blot analysis showed that constitutive expression of neuronal NOS (nNOS) protein in the morphine-tolerant group was twice that in the control group. This up-regulation was partially prevented by MK-801. The results suggest that morphine tolerance affects NMDA receptor binding activity and increases nNOS expression in the rat spinal cord.

Interactions of intrathecally administered ziconotide, a selective blocker of neuronal N-type voltage-sensitive calcium channels, with morphine on nociception in rats

Ziconotide is a selective, potent and reversible blocker of neuronal N-type voltage-sensitive calcium channels (VSCCs). Morphine is an agonist of mu-opioid receptors and inhibits N-type VSCC channels via a G-protein coupling mechanism. Both agents are antinociceptive when they are administered intrathecally (spinally). The present study investigated the acute and chronic (7-day) interactions of intrathecally administered ziconotide and morphine on nociception in several animal models of pain. In the acute study, intrathecal bolus injections of morphine and ziconotide alone produced dose-dependent inhibition of formalin-induced tonic flinch responses and withdrawal responses to paw pressure. The combination of ziconotide and morphine produced an additive inhibition of formalin-induced tonic flinch responses and a significant leftward shift of the morphine dose-response curve in the paw pressure test. After chronic (7-day) intrathecal infusion, ziconotide enhanced morphine analgesia in the formalin test. In contrast, chronic intrathecal morphine infusion produced tolerance to analgesia, but did not affect ziconotide antinociception. Antinociception produced by ziconotide alone was the same as that observed when the compound was co-administered with morphine to morphine-tolerant rats. In the hot-plate and tail immersion tests, chronic intrathecal infusion of morphine lead to rapid tolerance whereas ziconotide produced sustained analgesia with no loss of potency throughout the infusion period. Although ziconotide in combination with morphine produced an apparent synergistic analgesic effects during the initial phase of continuous infusion, it did not prevent morphine tolerance to analgesia. These results demonstrate that (1) acute intrathecal administrations of ziconotide and morphine produce additive or synergistic analgesic effects; (2) chronic intrathecal morphine infusion results in tolerance to analgesia but does not produce cross-tolerance to ziconotide; (3) chronic intrathecal ziconotide administration produces neither tolerance nor cross-tolerance to morphine analgesia; (4) intrathecal ziconotide does not prevent or reverse morphine tolerance.

Spinal coadministration of ketamine reduces the development of tolerance to visceral as well as somatic antinociception during spinal morphine infusion

This study was designed to investigate the effects of ketamine, an N-methyl-D-aspartate receptor antagonist, on the development of tolerance to morphine and morphine antinociception during intrathecal infusion. Two intrathecal catheters were implanted in the subarachnoid space in male rats under pentobarbital anesthesia. One catheter was used for the intrathecal infusion with the following solutions: morphine 1 microg x kg(-1) x hr(-1)(M1) and 5 microg x kg(-1) x hr(-1) (M5);ketamine 250 microg x kg(-1) x hr(-1) (K250); morphine plus ketamine, 1 microg x kg(-1) x hr(-1) plus 250 microg x kg(-1) x hr(-1) (M1 + K250) and 5 microg x kg(-1) x hr(-1) + 250 microg x kg(-1) x hr(-1) (M5 + K250); or saline. The other catheter was used for morphine challenge tests. The responses to noxious somatic and visceral stimuli were measured by tail flick (TF) and colorectal distension (CD) tests, respectively. Measurements were performed once a day for 7 days. Challenge tests with intrathecal morphine were performed to assess the magnitude of tolerance on Day 5 and Day 7. The antinociceptive effect was evaluated by using the percent of maximal possible effect (%MPE). Morphine infusion produced significant increases in %MPEs in TF and CD tests, while the saline and K250 infusions did not show any changes. The M1 + K250 infusion significantly increased the %MPEs in TF and CD tests, although the M1 and K250 infusions alone showed no changes. M5 + K250 enhanced the increases of %MPEs in TF and CD tests compared with the M5 infusion alone. In the challenge tests, the M1 + K250 infusion showed no significant decrease in %MPEs and TF and CD tests. The M5 + K250 infusion significantly inhibited those decreases in %MPEs, although the M5 infusion showed significant decreases in TF and CD tests. We concluded that ketamine attenuated the development of morphine tolerance to antinociceptive effects and increased the somatic and visceral antinociception of morphine.

IMPLICATIONS

Intrathecally coinfused ketamine attenuated morphine tolerance to somatic and visceral antinociception and increased morphine antinociception at the spinal level. These results suggest that a combination of morphine with ketamine may have an advantage in long-term use of opioids for controlling visceral as well as somatic pain.

Pain facilitatory circuits in the mammalian central nervous system: their behavioral significance and role in morphine analgesic tolerance

Sensitivity to noxious stimulation is not invariant; rather, it is modulated by discrete pain inhibitory and facilitatory circuits. This paper reviews the neural circuits for pain facilitation, describes the conditions governing their environmental activation, and examines their role in an animal's behavioral repertoire. Mechanisms for pain facilitation are contrasted at both the neural and behavioral level with mechanisms for pain inhibition. In addition, the involvement of mechanisms for pain facilitation in morphine analgesic tolerance is discussed, and the implications of this involvement for accounts of the role of associative processes in analgesic tolerance are highlighted.

Effects of NMDA receptor antagonists on morphine tolerance: a c-Fos study in the lumbar spinal cord of the rat

This study investigated the contribution of NMDA receptors to the development of tolerance to the antinociceptive properties of morphine at the level of the spinal cord dorsal horn. The expression of c-Fos protein following intraplantar (i.pl.) injection of carrageenin (6 mg/150 microl of saline) was used. In naive rats, acute intravenous (i.v.) administration of morphine (3 mg/kg) decreased the total number per section of Fos-Like-Immunoreactive (Fos-LI) neurons by 51%, observed at 2 h after injection of carrageenin. In tolerant rats, acute morphine did not significantly modify the total number of Fos-like immunoreactive neurons/section. In rats receiving chronic morphine and chronic injections of the non-competitive ((+)-MK 801 maleate: (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,1 0-imine) or the competitive (LY 235959: [3S-(3alpha,4a alpha,6beta,8a alpha)]-Decahydro-6-(phosphonomethyl)-3-isoquinolinecarboxylic+ ++ acid) NMDA receptor antagonists, only partial tolerance to the acute effects of morphine were observed (decrease of 42% and 38%, respectively). Administration of an antagonist at the strychnine-insensitive glycine site of the NMDA receptor ((+)-HA-966: R(+)-3-Amino-1-hydroxypyrrolidin-2-one) did not affect the development of morphine tolerance. These findings suggest that compounds attenuating the actions of the NMDA receptor via blockade of the glycine modulatory site may be substantially different from those acting at the ion channel of the NMDA receptor complex. This in vivo experiment in freely moving animals demonstrates for the first time an attenuation of tolerance at the cellular level.

Opiate tolerance to daily heroin administration: an apparent phenomenon associated with enhanced pain sensitivity

From a classical viewpoint, tolerance to analgesic effects of opiates refers to the decreased effectiveness of a given opiate following its repeated use. Despite much research, it has not been conclusively demonstrated in vivo that functional changes observed at the opioid receptor level in the responsiveness to opiates account for development of tolerance. An alternative hypothesis is that opioid receptors remain operative following repeated opiate administration but that opioid receptor activation rapidly induces a prolonged increase in pain sensitivity which opposes the predominant opiate analgesic effect following repeated opiate administration. We recently showed that a single heroin administration induces an enhanced pain sensitivity for several days, a phenomenon which is prevented by the non-competitive N-methyl-D aspartate receptor antagonist MK-801. Herein we report that repeated once-daily heroin injections induced a gradual lowering of the nociceptive threshold which progressively masked a sustained heroin analgesic functional effect. MK-801 prevented such opiate-induced allodynia and thereby prevented development of an apparent decrease in the effectiveness of heroin. These results indicate that intermittent heroin administration induced a persistent increase in the basal pain sensitivity which, if not taken into account gives the impression of less analgesia, i.e. apparent tolerance.

Alteration in the brain content of substance P (1-7) during withdrawal in morphine-dependent rats

Previous studies have shown that substance P (SP) may modulate the abstinence reaction to opioid withdrawal. Its N-terminal fragment SP1-7 may inhibit the intensity of the withdrawal reactions in morphine dependent mice. This study was designed to determine whether the endogenous concentrations of the SP1-7 fragment in the brain are affected during naloxone-precipitated withdrawal in the male rat. The amounts of the peptide was assessed by a specific radioimmunoassay in extracts of discrete brain regions (including the cerebral cortex, hippocampus, hypothalamus, nucleus accumbens, striatum, substantia nigra, ventral tegmental area and the spinal cord) during morphine tolerance and withdrawal. The results indicated that the concentrations of SP1-7 were significantly elevated in the ventral tegmental area both in morphine tolerant rats and during naloxone-precipitated withdrawal. During morphine withdrawal significant increases in the peptide concentration were also observed in the hypothalamus and the spinal cord. It was concluded that the enhanced content of SP1-7 may also indicate the involvement of the SP system during opioid withdrawal in the rat. The enhanced production of SP1-7 may reflect an increased release and/or metabolism of SP, which, in turn, counteracts the withdrawal.

Up-regulation of [3H]DTG but not [3H](+)-pentazocine labeled sigma sites in mouse spinal cord by chronic morphine treatment

To monitor the possible effect of morphine on sigma sites, binding characteristics of [3H](+)-pentazocine and [3H]1,3-di-(2-tolyl)guanidine (DTG) to brain and spinal cord membranes of morphine-treated and control mice were compared. For morphine treatment, a single injection (100 mg/kg, s.c.) of morphine was followed 4 h later by pellet implantation (75 mg morphine free base). Animals were sacrificed 24, 72 h or 7 days later. The equilibrium dissociation value (Kd) and the density (Bmax) of [3H](+)-pentazocine binding remained unaffected by morphine treatment. Also, no change was found in Kd and Bmax values of [3H]DTG labeled sigma2 subtypes after any morphine treatment schedule when measured in the presence of 100 nM (+)-pentazocine. However, the Bmax of [3H]DTG binding in the spinal cord in the absence of 100 nM (+)-pentazocine, was significantly elevated 72 h after implantation of the morphine pellet and recovered by 7 days, a time when the antinociceptive effect produced by the morphine pellet had dissipated. These data suggest that one population of sigma sites, that has a high affinity for [3H]DTG, but is not equivalent with the [3H](+)-pentozocine labeled sigma1 subtype or the [3H]DTG labeled sigma2 subtype, is upregulated by morphine and, therefore, may play a role in the development of tolerance to or dependence on the effects of morphine.

N-Methyl-D-aspartate attenuates opioid receptor-mediated G protein activation and this process involves protein kinase C

The effects of N-methyl-D-aspartate (NMDA) on opioid receptor-mediated G protein activation were explored in neuroblastoma X glioma hybrid (NG108-15) cells. Treatment of the cells with NMDA resulted in a remarkable attenuation of [35S]guanosine-5'-O-(3-thio)triphosphate binding stimulated by [D-Pen2,D-Pen5]-enkephalin (DPDPE), a delta-opioid receptor agonist. The effects of NMDA were dose and time dependent with an IC50 value of 5 nM and could be blocked by NMDA receptor antagonists. After NMDA treatment, the DPDPE dose-response curve shifted to the right (EC50 value increased approximately 7-fold, from 6 to 40 nM), and the maximal response induced by DPDPE was reduced by approximately 60%. The effects of NMDA were reversible, and the DPDPE response could recover within 60 min. The functional responses of delta-, mu-, and kappa-opioid receptors in primarily cultured neurons also were attenuated significantly by NMDA treatment. The inhibitory effects of NMDA on opioid receptor-mediated G protein activation could be blocked by coadministration of the protein kinase C (PKC) inhibitors or by elimination of the extracellular Ca2+. Correspondingly, NMDA treatment of NG108 cells significantly elevated cellular PKC activity and stimulated Gialpha2 phosphorylation. Transient transfection into NG108-15 cells of the wild-type Gialpha2 and a mutated Gialpha2 (Ser144Ala) resulted in a 2-fold increase in DPDPE-stimulated G protein activation. The DPDPE responses were greatly inhibited by NMDA treatment in the wild-type Gialpha2-transfected cells but much less affected in the mutant Gialpha2-transfected cells. In summary, NMDA attenuates opioid receptor/G protein coupling, and this process requires activation of PKC.

Acute effects of morphine on the expression of mRNAs for NMDA receptor subunits in the rat hippocampus, hypothalamus and spinal cord

The acute effects of subcutaneously injected morphine on transcripts of the NMDA receptor subunits NR1, NR2A and NR2B in certain areas of the central nervous system of male rats were examined by Northern blot analysis. The result clearly indicated that a single dose (10 mg/kg) of the opioid alters the expression of the mRNA for receptor subunits in the hippocampus and hypothalamus 4 h after drug injection. No change in the mRNA levels was observed 30 min following injection, and after 24 h most of the levels were restored to control values. The observation suggests that morphine affects this type of glutamate receptor already in the acute phase of its administration.

Effect of NMDA antagonists on development of rapid tolerance to various barbiturates

We recently reported that the noncompetitive antagonists, (+)-MK-801 and ketamine, block the development of rapid tolerance to ethanol. We now show that pretreatment with these NMDA antagonists also blocks rapid tolerance to the various barbiturates (pentobarbital, barbital, and phenobarbital) examined. Tolerance to pentobarbital occurred under three difference conditions: (a) in groups of rats that were tested at repeated times on day 1 (intoxicated practice or testing group), (b) in groups of rats that were not tested on the apparatus but handled at the same times on day 1 (dummy testing or associative learning group), and (c) in groups of rats that were not subjected to testing at all on day 1 (nontesting). However, NMDA antagonists blocked intoxicated practice and associative tolerance, but not tolerance produced in the nontesting group. In the last experiment NMDA antagonist failed to block tolerance (unlearned) when animals were treated in the animal quarters and tested in a different room (i.e., in the laboratory). These findings suggest that NMDA antagonists affect barbiturate tolerance in a manner similar to their effect on ethanol tolerance.

The effects of LY293558, an AMPA receptor antagonist, on acute and chronic morphine dependence

In rodents, noncompetitive and competitive NMDA receptor antagonists have been shown to attenuate and, in some cases, reverse tolerance to the analgesic effects of morphine. However, the ability of these same excitatory amino acid (EAA) receptor antagonists to modulate morphine dependence is controversial, and very little is known about the role of AMPA receptors in morphine dependence. LY293558, a novel, systemically active, competitive AMPA receptor antagonist and the NMDA receptor antagonists, MK-801 and/or LY235959, were evaluated in tolerant or dependent CD-1 mice. In mice rendered tolerant by morphine injection or pellet implantation, continuous s.c. infusion of LY293558 (60 mg/kg per 24 h) or MK-801 (1 mg/kg per 24 h) attenuated the development of tolerance. Neither LY293558 nor MK-801 produced analgesia or altered the ED50 value of morphine. Continuous s.c. infusion of LY293558 (60 mg/kg per 24 h), MK-801 (1 mg/kg per 24 h) or LY235959 (12 mg/kg per 24 h) attenuated the development of acute (3 h) morphine dependence (i.e., decreased naloxone-precipitated withdrawal jumping). In contrast, continuous s.c. infusion of LY293558 (60 mg/kg per 24 h) or LY235959 (12 mg/kg per 24 h) did not significantly attenuate the development of chronic dependence produced by morphine pellet implantation. These data indicate that the development of morphine tolerance is more sensitive to modulation by EAA receptor antagonists than is the development of morphine dependence as assessed by naloxone-precipitated withdrawal jumping.

Decrease of tolerance to, and physical dependence on morphine by, glutamate receptor antagonists

The effects of the non-competitive antagonists of the glutamate complex receptor, dizocilpine (MK 801) and ketamine and of the competitive antagonist CGP 39551 were examined on the induction of tolerance to morphine, the development of physical dependence and the expression of the abstinence syndrome to the opiate in mice. Morphine was administered in a single dose (300 mg/kg) of a slow release preparation. Dizocilpine (0.005 or 0.01 mg/kg given at 3, 12 and 24 h after the priming dose of morphine), ketamine (2, 4 or 8 mg/kg, 30 min before and 3, 6, 9 and 24 h after the priming dose) and DL-(E)-2-amino-4-methyl-5-phosphonopentanoate carboxy-ethylester (CGP 39551) (1.5 or 3 mg/kg, but not 6 or 12 mg/kg 30 min before and 12 and 24 h after the priming dose) reduced the intensity of tolerance to, and physical dependence on morphine. The drugs also reduced the intensity of the abstinence behaviour when given in a single dose, 30 min before (s.c.) naloxone (4 mg/kg)-precipitated withdrawal syndrome in mice chronically treated with morphine. Thus, the results of this study indicate that competitive and non-competitive NMDA receptor antagonists prevent morphine tolerance and decrease the development of physical dependence on the opiate in mice.

Functionally differentiating two neuronal nitric oxide synthase isoforms through antisense mapping: evidence for opposing NO actions on morphine analgesia and tolerance

Several isoforms of neuronal nitric oxide synthase (nNOS) have been identified. Antisense approaches have been developed which can selectively down-regulate nNOS-1, which corresponds to the full-length nNOS originally cloned from the brain, and nNOS-2, a truncated form lacking two exons which is generated by alternative splicing, as demonstrated by decreases in mRNA levels. Antisense treatment also lowers nNOS enzymatic activity. Down-regulation of nNOS-1 prevents the development of morphine tolerance. Whereas morphine analgesia is lost in control and mismatch-treated mice given daily morphine injections for 5 days, mice treated with antisense probes targeting nNOS-1 show no decrease in their morphine sensitivity over the same time period. Conversely, an antisense probe selectively targeting nNOS-2 blocks morphine analgesia, shifting the morphine dose-response curve over 2-fold to the right. Both systems are active at the spinal and the supraspinal levels. An antisense targeting inducible NOS is inactive. Studies with NG-nitro-L-arginine, which does not distinguish among NOS isoforms, indicate that the facilitating nNOS-2 system predominates at the spinal level while the inhibitory nNOS-1 system is the major supraspinal nNOS system. Thus, antisense mapping distinguishes at the functional level two isoforms of nNOS with opposing actions on morphine actions. The ability to selectively down-regulate splice variants opens many areas in the study of nNOS and other proteins.