Excitatory amino acid antagonists (kynurenic acid and MK-801) attenuate the development of morphine tolerance in the rat

Interactive role of acid sensing ion channels and glutamatergic system in opioid dependence

Dysregulation in glutamatergic receptors and transporters has been found to mediate drugs of abuse, including morphine. Among glutamate receptors, ionotropic glutamate receptors (iGluRs) are altered with exposure to drugs of abuse. Acid-sensing ion channels (ASICs) are ligand (H+)-gated channels, which are expressed at the excitatory synaptic clefts and play a role in drug dependence. Overexpression of a specific ASIC subtype, ASIC1a, attenuated reinstatement of cocaine. ASICs are revealed to be involved in cocaine and morphine seeking behaviors, and these effects are mediated through modulation of glutamatergic receptors. In this review, we discussed the interactive role of ASICs and glutamate receptors, mainly iGluRs, in opioid dependence. ASICs are also expressed in astrocytes and are suggested to be involved on regulating glutamate uptake. However, little is known about the coupling between ASICs and the astroglial glutamate transporters. In addition, this review discussed the role of nitric oxide in the modulation of ASIC function and potentially opioid dependence. We also discussed the role of ASICs in the modulation of the function of both glutamatergic receptors in post-synaptic neurons and glutamatergic transporters in astrocytes in animals exposed to drugs of abuse.

Synthesis of the Mechanisms of Opioid Tolerance: Do We Still Say NO?

The use of morphine as a first-line agent for moderate-to-severe pain is limited by the development of analgesic tolerance. Initially opioid receptor desensitization in response to repeated stimulation, thought to underpin the establishment of tolerance, was linked to a compensatory increase in adenylate cyclase responsiveness. The subsequent demonstration of cross-talk between N-methyl-D-aspartate (NMDA) glutamate receptors and opioid receptors led to the recognition of a role for nitric oxide (NO), wherein blockade of NO synthesis could prevent tolerance developing. Investigations of the link between NO levels and opioid receptor desensitization implicated a number of events including kinase recruitment and peroxynitrite-mediated protein regulation. Recent experimental advances and the identification of new cellular constituents have expanded the potential signaling candidates to include unexpected, intermediary compounds not previously linked to this process such as zinc, histidine triad nucleotide-binding protein 1 (HINT1), micro-ribonucleic acid (mi-RNA) and regulator of G protein signaling Z (RGSZ). A further complication is a lack of consistency in the protocols used to create tolerance, with some using acute methods measured in minutes to hours and others using days. There is also an emphasis on the cellular changes that are extant only after tolerance has been established. Although a review of the literature demonstrates a lack of spatio-temporal detail, there still appears to be a pivotal role for nitric oxide, as well as both intracellular and intercellular cross-talk. The use of more consistent approaches to verify these underlying mechanism(s) could provide an avenue for targeted drug development to rescue opioid efficacy.

Repetitive hypoglycemia reduces activation of glucose-responsive neurons in C1 and C3 medullary brain regions to subsequent hypoglycemia

The impaired ability of the autonomic nervous system to respond to hypoglycemia is termed "hypoglycemia-associated autonomic failure" (HAAF). This life-threatening phenomenon results from at least two recent episodes of hypoglycemia, but the pathology underpinning HAAF remains largely unknown. Although naloxone appears to improve hypoglycemia counterregulation under controlled conditions, hypoglycemia prevention remains the current mainstay therapy for HAAF. Epinephrine-synthesizing neurons in the rostroventrolateral (C1) and dorsomedial (C3) medulla project to the subset of sympathetic preganglionic neurons that regulate peripheral epinephrine release. Here we determined whether or not C1 and C3 neuronal activation is impaired in HAAF and whether or not 1 wk of hypoglycemia prevention or treatment with naloxone could restore C1 and C3 neuronal activation and improve HAAF. Twenty male Sprague-Dawley rats (250-300 g) were used. Plasma epinephrine levels were significantly increased after a single episode of hypoglycemia (n = 4; 5,438 ± 783 pg/ml vs. control 193 ± 27 pg/ml, P < 0.05). Repeated hypoglycemia significantly reduced the plasma epinephrine response to subsequent hypoglycemia (n = 4; 2,179 ± 220 pg/ml vs. 5,438 ± 783 pg/ml, P < 0.05). Activation of medullary C1 (n = 4; 50 ± 5% vs. control 3 ± 1%, P < 0.05) and C3 (n = 4; 45 ± 5% vs. control 4 ± 1%, P < 0.05) neurons was significantly increased after a single episode of hypoglycemia. Activation of C1 (n = 4; 12 ± 3%, P < 0.05) and C3 (n = 4; 19 ± 5%, P < 0.05) neurons was significantly reduced in the HAAF groups. Hypoglycemia prevention or treatment with naloxone did not restore the plasma epinephrine response or C1 and C3 neuronal activation. Thus repeated hypoglycemia reduced the activation of C1 and C3 neurons mediating adrenal medullary responses to subsequent bouts of hypoglycemia.

Inflammatory mediators of opioid tolerance: Implications for dependency and addiction

Each year, over 50 million Americans suffer from persistent pain, including debilitating headaches, joint pain, and severe back pain. Although morphine is amongst the most effective analgesics available for the management of severe pain, prolonged morphine treatment results in decreased analgesic efficacy (i.e., tolerance). Despite significant headway in the field, the mechanisms underlying the development of morphine tolerance are not well understood. The midbrain ventrolateral periaqueductal gray (vlPAG) is a primary neural substrate for the analgesic effects of morphine, as well as for the development of morphine tolerance. A growing body of literature indicates that activated glia (i.e., microglia and astrocytes) facilitate pain transmission and oppose morphine analgesia, making these cells important potential targets in the treatment of chronic pain. Morphine affects glia by binding to the innate immune receptor toll-like receptor 4 (TLR4), leading to the release of proinflammatory cytokines and opposition of morphine analgesia. Despite the established role of the vlPAG as an integral locus for the development of morphine tolerance, most studies have examined the role of glia activation within the spinal cord. Additionally, the role of TLR4 in the development of tolerance has not been elucidated. This review attempts to summarize what is known regarding the role of vlPAG glia and TLR4 in the development of morphine tolerance. These data, together, provide information about the mechanism by which central nervous system glia regulate morphine tolerance, and identify a potential therapeutic target for the enhancement of analgesic efficacy in the clinical treatment of chronic pain.

The influence of AMN082, metabotropic glutamate receptor 7 (mGlu7) allosteric agonist on the acute and chronic antinociceptive effects of morphine in the tail-immersion test in mice: Comparison with mGlu5 and mGlu2/3 ligands

Preclinical data indicated that the metabotropic glutamate receptors 5 (mGlu5) and glutamate receptors 2/3 (mGlu2/3) are involved in modulating morphine antinociception. However, little is known about the role of metabotropic glutamate receptors 7 (mGlu7) in this phenomenon. We compared the effects of AMN082 (0.1, 1 or 5mg/kg, ip), a selective mGlu7 allosteric agonist, LY354740 (0.1, 1 or 5mg/kg, ip), an mGlu2/3 agonist and MTEP (0.1, 1 or 5mg/kg, ip), a selective mGlu5 antagonist, on the acute antinociceptive effect of morphine (5mg/kg, sc) and also on the development and expression of tolerance to morphine analgesia in the tail-immersion test in mice. To determine the role of mGlu7 in morphine tolerance, and the association of the mGlu7 effect with the N-methyl-d-aspartate (NMDA) receptors regulation, we used MMPIP (10mg/kg, ip), a selective mGlu7 antagonist and MK-801, a NMDA antagonist. Herein, the acute administration of AMN082, MTEP or LY354740 alone failed to evoked antinociception, and did not affect morphine (5mg/kg, sc) antinociception. However, these ligands inhibited the development of morphine tolerance, and we indicated that MMPIP reversed the inhibitory effect of AMN082. When given together, the non-effective doses of AMN082 and MK-801 did not alter the tolerance to morphine. Thus, mGlu7, similarly to mGlu2/3 and mGlu5, are involved in the development of tolerance to the antinociceptive effects of morphine, but not in the acute morphine antinociception. Furthermore, while mGlu7 are engaged in the development of morphine tolerance, no interaction exists between mGlu7 and NMDA receptors in this phenomenon.

Interactions between the Kynurenine and the Endocannabinoid System with Special Emphasis on Migraine

Both the kynurenine and the endocannabinoid systems are involved in several neurological disorders, such as migraine and there are increasing number of reports demonstrating that there are interactions of two systems. Although their cooperation has not yet been implicated in migraine, there are reports suggesting this possibility. Additionally, the individual role of the endocannabinoid and kynurenine system in migraine is reviewed here first, focusing on endocannabinoids, kynurenine metabolites, in particular kynurenic acid. Finally, the function of NMDA and cannabinoid receptors in the trigeminal system-which has a crucial role in the pathomechanisms of migraine-will also be discussed. The interaction of the endocannabinoid and kynurenine system has been demonstrated to be therapeutically relevant in a number of pathological conditions, such as cannabis addiction, psychosis, schizophrenia and epilepsy. Accordingly, the cross-talk of these two systems may imply potential mechanisms related to migraine, and may offer new approaches to manage the treatment of this neurological disorder.

Kynurenic acid and its analogue can alter the opioid receptor G-protein signaling after acute treatment via NMDA receptor in rat cortex and striatum

Previously, we have shown that the N-methyl d-aspartate (NMDA)-receptor antagonist kynurenic acid (KYNA) and its analogue KYNA1 do not bind directly to mu, kappa and delta opioid receptors in vitro. On the other hand, chronic administration of KYNA and KYNA1 resulted in region (cortex vs striatum) and opioid receptor-type specific alterations in G-protein activation of mouse brain homogenates. Here we describe for the first time the acute effect of KYNA and KYNA1 on opioid receptor function with the possible involvement of the NMDA receptor. The acute 30minute in vivo KYNA1 and KYNA treatments altered opioid receptor G-protein signaling or ligand potency depending on the opioid receptor type and brain region (rat cortex vs striatum) using [³⁵S]GTPγS binding assays. Pretreatment with the NMDA receptor antagonist MK-801 impaired or reversed the effects of KYNA1 and KYNA. These results suggest an NMDA receptor mediated effect. After acute 30minute treatment HPLC measurements revealed a similar KYNA1 and a higher KYNA plasma concentration compared to cerebrospinal fluid concentrations. Finally, KYNA, KYNA1 and MK-801 showed comparable results in opioid receptor G-protein activity and ligand potency with acute in vivo treatments when they were administered in vitro for 30min on isolated cortex and striatum slices. We previously demonstrated that KYNA1 and KYNA acutely altered opioid receptor function in vivo and in vitro through the NMDA receptor depending on the opioid receptor type and brain region. This study may lead to a new, indirect approach to influence opioid receptor signaling.

Atorvastatin attenuates the antinociceptive tolerance of morphine via nitric oxide dependent pathway in male mice

The development of morphine-induced antinociceptive tolerance limits its therapeutic efficacy in pain management. Atorvastatin, or competitive inhibitor of 3-hydroxy-methyl-glutaryl coenzyme A (HMG-CoA) reductase, is mainstay agent in hypercholesterolemia treatment. Beyond the cholesterol-lowering activity, exploration of neuroprotective properties of this statin indicates its potential benefit in central nervous disorders. The aim of the present study was to assess the effects of atorvastatin in development and expression of morphine-induced analgesic tolerance in male mice and probable involvement of nitric oxide. Chronic and acute treatment with atorvastatin 10 and 20mg/kg, respectively, could alleviate morphine tolerance in development and expression phases. Chronic co-administration of nitric oxide synthase (NOS) inhibitors including L-NAME (non selective NOS inhibitor; 2mg/kg), aminoguanidine (selective inducible NOS inhibitor; 50mg/kg) and 7-NI (selective neuronal NOS inhibitor; 15mg/kg) with atorvastatin blocked the protective effect of atorvastatin in tolerance reversal. Moreover, reversing the atorvastatin effect was also observed in acute simultaneous treatment of L-NAME (5mg/kg) and aminoguanidine (100mg/kg) with atorvastatin. Co-treatment of guanylyl cyclase inhibitor, ODQ (chronic dose: 10mg/kg and acute dose: 20mg/kg) was associated with prevention of atorvastatin anti-tolerance properties. Our results revealed that the atorvastatin modulating role in morphine antinociceptive tolerance is mediated at least in part via nitric oxide in animal pain models of hot plate and tail flick.

Viral vectors encoding endomorphins and serine histogranin attenuate neuropathic pain symptoms after spinal cord injury in rats

Background

The treatment of spinal cord injury (SCI)-induced neuropathic pain presents a challenging healthcare problem. The lack of available robust pharmacological treatments underscores the need for novel therapeutic methods and approaches. Due to the complex character of neuropathic pain following SCI, therapies targeting multiple mechanisms may be a better choice for obtaining sufficient long-term pain relief. Previous studies in our lab showed analgesic effects using combinations of an NMDA antagonist peptide [Ser¹]histogranin (SHG), and the mu-opioid peptides endomorphins (EMs), in several pain models. As an alternative to drug therapy, this study evaluated the analgesic potential of these peptides when delivered via gene therapy.

Results

Lentiviruses encoding SHG and EM-1 and EM-2 were intraspinally injected, either singly or in combination, into rats with clip compression SCI 2 weeks following injury. Treated animals showed significant reduction in mechanical and thermal hypersensitivity, compared to control groups injected with GFP vector only. The antinociceptive effects of individually injected components were modest, but the combination of EMs and SHG produced robust and sustained antinociception. The onset of the analgesic effects was observed between 1–5 weeks post-injection and sustained without decrement for at least 7 weeks. No adverse effects on locomotor function were observed. The involvement of SHG and EMs in the observed antinociception was confirmed by pharmacologic inhibition using intrathecal injection of either the opioid antagonist naloxone or an anti-SHG antibody. Immunohistochemical analysis showed the presence of SHG and EMs in the spinal cord of treated animals, and immunodot-blot analysis of CSF confirmed the presence of these peptides in injected animals. In a separate group of rats, delayed injection of viral vectors was performed in order to mimic a more likely clinical scenario. Comparable and sustained antinociceptive effects were observed in these animals using the SHG-EMs combination vectors compared to the group with early intervention.

Conclusions

Findings from this study support the potential for direct gene therapy to provide a robust and sustained alleviation of chronic neuropathic pain following SCI. The combination strategy utilizing potent mu-opioid peptides with a naturally-derived NMDA antagonist may produce additive or synergistic analgesic effects without the tolerance development for long-term management of persistent pain.

Does modulation of spinal N-methyl-D-aspartic acid receptor by pre-activation accelerate the development of morphine tolerance?

Repeated morphine administration usually leads to a number of neuroadaptive processes, including tolerance and sensitization. However, research has shown that the induction and maintenance of central sensitization is dependent on N-methyl-d-aspartic acid receptor (NMDAR) activation. Chronic morphine exposure has been shown to result in spinal sensitization and activation of spinal NMDARs. Chronic morphine treatment and the activation of spinal NMDARs may be synergistic and form a closed loop that may worsen the development of morphine analgesic tolerance and spinal sensitization. Inhibition of NMDARs with an antagonist could effectively alleviate the development of morphine analgesic tolerance. So, what is the effect of modulating spinal NMDAR activation with exogenous agonists or neuropathic input on the development of morphine-induced analgesic tolerance? Our hypothesis is that chronic morphine treatment may worsen the already activation of spinal NMDARs and spinal sensitization after agonist application or neuropathic input to shorten the process of morphine-induced analgesic tolerance.

Low-dose ketamine as a potential adjuvant therapy for painful vaso-occlusive crises in sickle cell disease

The hallmark of sickle cell disease (SCD) is the acute painful vaso-occlusive crisis (VOC). Among SCD patients, vaso-occlusive pain episodes vary in frequency and severity. Some patients rarely have painful crises, while others are admitted to the hospital multiple times in a year for parenteral analgesics. Opioids are the mainstay of therapy for SCD-related pain. However, a subset of patients report continued pain despite escalating doses of opioids. Tolerance and opioid-induced hyperalgesia (OIH) have been considered as possible explanations for this phenomenon. The activation of the N-methyl-d-aspartate (NMDA) receptor has been implicated in both tolerance and OIH. As a NMDA receptor agonist, ketamine has been shown to modulate opioid tolerance and OIH in animal models and clinical settings. Low-dose ketamine, by virtue of its NMDA receptor agonist activity, could be a useful adjuvant to opioid therapy in patients with refractory SCD-related pain. Based on limited studies of adjuvant ketamine use for pain management, low-dose ketamine continuous infusion appears safe. Further clinical investigations are warranted to fully support the use of low-dose ketamine infusion in patients with SCD-related pain.

Randomized, placebo-controlled pilot trial of gabapentin during an outpatient, buprenorphine-assisted detoxification procedure

This pilot study examined the efficacy of the N-type calcium channel blocker gabapentin to improve outcomes during a brief detoxification protocol with buprenorphine. Treatment-seeking opioid-dependent individuals were enrolled in a 5-week, double-blind, placebo-controlled trial examining the effects of gabapentin during a 10-day outpatient detoxification from buprenorphine. Participants were inducted onto buprenorphine sublingual tablets during Week 1, were randomized and inducted onto gabapentin or placebo during Week 2, underwent a 10-day buprenorphine taper during Weeks 3 and 4, and then were tapered off gabapentin/placebo during Week 5. Assessments included thrice-weekly opioid withdrawal scales, vitals, and urine drug screens. Twenty-four individuals (13 male; 17 Caucasian, 3 African American, 4 Latino; mean age 29.7 years) participated in the detoxification portion of the study (gabapentin, n = 11; placebo, n = 13). Baseline characteristics did not differ significantly between groups. Self-reported and observer-rated opioid withdrawal ratings were relatively low and did not differ between groups during the buprenorphine taper. Urine results showed a Drug × Time interaction, such that the probability of opioid-positive urines significantly decreased over time in the gabapentin versus placebo groups during Weeks 3 and 4 (OR = 0.73, p = .004). These results suggest that gabapentin reduces opioid use during a 10-day buprenorphine detoxification procedure.

Morphine and hydromorphone-induced hyperalgesia in a hospice patient

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

Metabotropic glutamate receptors as targets for analgesia: antagonism, activation, and allosteric modulation

The metabotropic glutamate receptors (mGluRs) are expressed pre- and post-synaptically throughout the nervous system where they serve as modulators of synaptic transmission and neuronal excitability. Activation of mGluRs can be pro- or anti-nociceptive, depending on their anatomic location and the signaling cascades to which they couple. Antagonists of Group I mGluRs and agonists of Group II and III mGluRs have shown therapeutic promise in animal pain models. This article reviews the potential therapeutic utility of several agents that act predominantly via mGluRs, specifically focusing on their analgesic efficacy and discussing possible off-target effects. Glutamate, the primary excitatory neurotransmitter in the vertebrate nervous system, mediates its effects via activation of two main classes of receptors: ligand-gated ion channels known as ionotropic receptors and G-protein coupled metabotropic receptors. Antagonists of ionotropic glutamate receptors, such as ketamine, have robust analgesic properties; however, their analgesic utility is limited to monitored clinical settings due to the potential for psychomimetic effects.

The role of cyclo-oxygenase inhibitors in attenuating opioid-induced tolerance, hyperalgesia, and dependence

There is no denying that opioids are the most important analgesic drugs which are widely used in clinical situations. Still, prolonged administration of these drugs can cause to reduce their analgesic efficacy due to the development of tolerance. These drugs can also cause induction of hyperalgesia. In addition, long-term administration of opioids through reinforcing- and rewarding pathways of limbic system can result in expression of opioid dependence with the unintended consequences of opioid abuse/misuse and finally opioid addiction. As studies show, over-activity in cyclo-oxygenase pathways and production of prostaglandins due to long-term exposures of opioid have a critical role in the development of tolerance to antinociceptive effect of opioid, hyperalgesia, and opioid dependence. The present study aims at suggesting the hypothesis that through blending a non-steroid anti-inflammatory drug with opioid actively causes reduction in unwanted effects of opioid i.e. by inhibition of opioid-induced cyclo-oxygenase overactivity whereas it is well-known that the combination therapy via reducing opioid dosage reduces the unwanted effects.

Intra-accumbal NMDA but not AMPA/kainate receptor antagonist attenuates WIN55,212-2 cannabinoid receptor agonist-induced antinociception in the basolateral amygdala in a rat model of acute pain

Previous studies showed the role of basolateral amygdala (BLA) in cannabinoid-induced antinociception. Several lines of evidence indicated that the nucleus accumbens (NAc) receives excitatory glutamatergic inputs primarily from limbic-related structures, including the hippocampus, BLA, and various thalamic nuclei. Additionally, it has been shown that the NAc plays an important role in mediating the suppression of animal models of pain. In the present study, we examined the role of NMDA and AMPA/kainate receptors within the NAc in antinociception induced by intra-BLA cannabinoid receptor agonist WIN55,212-2 in rats. 126 adult male albino Wistar rats weighing 230-280 g were unilaterally implanted by two separate cannulae into the BLA and NAc. Dose-response antinociceptive effects of different doses of intra-BLA WIN55,212-2 (5, 10 and 15 μg/0.3 μl/rat) were evaluated in this study. Moreover, animals received intra-accumbal infusions of either NMDA receptor antagonist, AP5 (0.5, 2.5 and 5 μg/0.5 μl saline) or AMPA/kainate receptor antagonist, CNQX (0.1, 0.5 and 2.5 μg/0.5 μl DMSO), just 2 min before microinjection of WIN55,212-2 into the BLA. Antinociceptive responses of drugs were obtained by tail-flick analgesiometer and represented as maximal possible effect (%MPE) at 5, 15, 30, 45 and 60 min after their administrations. Results showed that intra-accumbal AP5 dose-dependently prevented antinociception induced by intra-BLA administration of WIN55,212-2 (15 μg/rat) in time set intervals. Nonetheless, administration of AMPA/kainate receptor antagonist, CNQX, could not affect WIN-induced analgesia. Additionally, solely administration of intra-accumbal injection of CNQX (2.5 μg/0.5 μl DMSO), but not AP5 (5 μg/0.5 μl saline), could significantly change the baseline tail-flick latencies in the rats. It seems that NMDA receptors located in the NAc, in part, mediate the antinociceptive responses of cannabinoid within the BLA in acute model of pain.

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.

Ascorbic Acid inhibits development of tolerance and dependence to opiates in mice: possible glutamatergic or dopaminergic modulation

In a recent study, it has been demonstrated that ascorbic acid possessed antidopaminergic activity and modulate the glutamatergic neurotransmission in mice. With this background, the present study was undertaken to study the effect of ascorbic acid on the development of tolerance and dependence to opiate and its mechanism of action. Male Swiss mice weighing 20-25 g were used in the present study. Mice were made physically dependent on opioid by the chronic administration of morphine (10 mg/kg, twice a day, for 9 days) intraperitoneally. Ascorbic acid, haloperidol (dopamine antagonist) or MK 801 (NMDA receptor antagonist) was administered daily for 9 d before challenging the animals with morphine. The development of tolerance was assessed by noting the tail-flick latency on day 1, 3, 9 and 10. On the 10(th) day after the measurement of tail-flick latency, animals were challenged with naloxone (2 mg/kg., i.p.) and incidence of escape jumps were recorded by placing the animals in 45 cm high plexiglass container. Ascorbic acid (400-1600 mg/kg) dose dependently inhibited development of tolerance and dependence to morphine as noted from tail-flick latency. When given along with MK 801 (0.01 mg/kg., i.p) or haloperidol (0.1 mg/kg i.p.), ascorbic acid (800 mg/kg., i.p.) potentiated the response of MK 801 or haloperidol. In conclusion, it is hypothesized that inhibition of development of tolerance and dependence to morphine by ascorbic acid appears to have two components, namely dopaminergic and glutamatergic.

Effects of opioids, cannabinoids, and vanilloids on body temperature

Cannabinoid and opioid drugs produce marked changes in body temperature. Recent findings have extended our knowledge about the thermoregulatory effects of cannabinoids and opioids, particularly as related to delta opioid receptors, endogenous systems, and transient receptor potential (TRP) channels. Although delta opioid receptors were originally thought to play only a minor role in thermoregulation compared to mu and kappa opioid receptors, their activation has been shown to produce hypothermia in multiple species. Endogenous opioids and cannabinoids also regulate body temperature. Mu and kappa opioid receptors are thought to be in tonic balance, with mu and kappa receptor activation producing hyperthermia and hypothermia, respectively. A particularly intense research focus is TRP channels, where TRPV1 channel activation produces hypothermia whereas TRPA1 and TRPM8 channel activation causes hyperthermia. The marked hyperthermia produced by TRPV1 channel antagonists suggests these warm channels tonically control body temperature. A better understanding of the roles of cannabinoid, opioid, and TRP systems in thermoregulation may have broad clinical implications and provide insights into interactions among neurotransmitter systems involved in thermoregulation.

Effects of opioids, cannabinoids, and vanilloids on body temperature

Cannabinoid and opioid drugs produce marked changes in body temperature. Recent findings have extended our knowledge about the thermoregulatory effects of cannabinoids and opioids, particularly as related to delta opioid receptors, endogenous systems, and transient receptor potential (TRP) channels. Although delta opioid receptors were originally thought to play only a minor role in thermoregulation compared to mu and kappa opioid receptors, their activation has been shown to produce hypothermia in multiple species. Endogenous opioids and cannabinoids also regulate body temperature. Mu and kappa opioid receptors are thought to be in tonic balance, with mu and kappa receptor activation producing hyperthermia and hypothermia, respectively. A particularly intense research focus is TRP channels, where TRPV1 channel activation produces hypothermia whereas TRPA1 and TRPM8 channel activation causes hyperthermia. The marked hyperthermia produced by TRPV1 channel antagonists suggests these warm channels tonically control body temperature. A better understanding of the roles of cannabinoid, opioid, and TRP systems in thermoregulation may have broad clinical implications and provide insights into interactions among neurotransmitter systems involved in thermoregulation.

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.

Opioids in chronic non-cancer pain

INTRODUCTION

The use of chronic opioid therapy for chronic non-cancer pain is growing and is now accepted as an effective treatment modality.

AREAS COVERED

Although there are guidelines and reviews for chronic opioid therapy for chronic non-cancer pain patients, physicians may still have concerns and be reluctant to prescribe strong opioids for chronic non-cancer pain. Common issues and concerns when prescribing opioid for chronic pain management are reviewed and discussed. The literature search was done using Medline with key words 'chronic non-cancer pain', 'chronic opioid therapy', 'effectiveness', 'opioid tolerance', 'opioid-induced hyperalgesia', 'adverse effect', 'opioid dependency', 'addiction', 'monitoring', 'opioid contract' and various combinations with these key words. Studies from 1990 - 2010 have been included. This article helps readers to update, clarify and understand the common concerns when using opioid for chronic non-cancer pain. Clinical effectiveness and adverse effects with chronic opioid therapy, opioid tolerance and opioid-induced hyperalgesia, opioid dependency and addiction, monitoring during chronic opioid use, and opioid contact are discussed in detailed.

EXPERT OPINION

Not much strongly positive data supports the long-term use of opioids for pain relief, and the evidence for an improvement in functional activity is inconclusive. With careful selection of patients, meticulous prescription and monitoring protocol, chronic non-cancer pain patients who are likely to benefit from potent opioids should not be prevented from obtaining this treatment.

Long-term treatment with morphine increases the D-serine content in the rat brain by regulating the mRNA and protein expressions of serine racemase and D-amino acid oxidase

Recent studies indicate that an endogenous co-agonist for an N-methyl-D-aspartate (NMDA) receptor-related glycine site, D-serine, is synthesized by serine racemase and is metabolized by D-amino acid oxidase (DAO) and that acute treatment with morphine augments the gene expression of serine racemase and DAO in rat brain. To further elucidate the mechanism underlying the activation of NMDA receptors following chronic opioid administration, we have evaluated the effects of the chronic administration of morphine on the mRNA and protein expressions of serine racemase and DAO and on the contents of D-serine in several areas of the rat brain. Repeated administration of morphine for 30 days produced a significant augmentation of both the mRNA and protein expressions of serine racemase in all the brain regions, whereas no significant change in the protein expression of DAO was observed in all the brain regions. Furthermore, the chronic administration caused a slight but significant elevation in the concentration of D-serine in the cortex, striatum, and hippocampus. These results indicate the elevated D-serine level following the chronic morphine treatment could at least in part be involved in the activation of NMDA receptors via the glycine site.

Beta-lactam antibiotic prevents tolerance to the hypothermic effect of a kappa opioid receptor agonist

Beta-lactam antibiotics are the only clinically approved drugs which directly increase glutamate uptake. They activate the glutamate transporter subtype 1 (GLT-1), the protein responsible for 90% of glutamate uptake in the mammalian brain. The capacity of GLT-1 to clear extracellular glutamate suggests that glutamate transporter activators be explored for therapeutic approaches to clinical conditions caused by increased glutamatergic transmission. One of the most common drug effects mediated by increased glutamatergic signaling is opioid tolerance. Therefore, we tested the hypothesis that a beta-lactam antibiotic (ceftriaxone), by increasing glutamate uptake, prevents tolerance to hypothermia induced by a kappa opioid receptor agonist (U-50,488H). A single injection of U-50,488H (20mg/kg, s.c.) caused significant hypothermia in rats. Tolerance to the hypothermic effect of U50,488H was induced by injecting U50,488H (20mg/kg) twice daily for 7days. Pretreatment with ceftriaxone (200mg/kg, i.p.) for 7days did not alter the acute hypothermic response to U50,488H (20mg/kg) but did prevent tolerance to U50,488H-induced hypothermia. Central administration of dl-threo-beta-benzyloxyaspartic acid (TBOA) (0.2micromol, i.c.v.), a glutamate transporter inhibitor, abolished the effect of ceftriaxone. These results identify a functional interaction between ceftriaxone and U50,488H in vivo and provide pharmacological evidence that a beta-lactam antibiotic abolishes tolerance to hypothermia induced by a kappa opioid receptor agonist.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

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.

Memantine produces modest reductions in heroin-induced subjective responses in human research volunteers

RATIONALE

Previous studies have demonstrated an interaction between opioids and noncompetitive antagonists at N-methyl-D: -aspartate (NMDA) receptors, but few studies have examined the utility of these medications for treating opioid dependence.

OBJECTIVE

In this 8-week inpatient study, participants were maintained on the low-affinity, noncompetitive NMDA receptor antagonist memantine (0, 30, and 60 mg per day, PO) and under each maintenance dose condition, the effects of intranasal heroin (0, 12.5, and 50 mg, IN) were examined.

METHODS

During the first week after admission to the hospital, participants were detoxified from heroin. All of the volunteers received all of the memantine and heroin dose combinations. Participants (N = 8) first sampled a dose of heroin and $20. During a subsequent choice session, participants could self-administer heroin and/or money. Responses, which consisted of finger presses on a computer mouse, were made under a modified progressive ratio schedule (PR 50, 100, 200, 400, 800, 1,200, 1,600, 2,000, 2,400, and 2,800) during a ten-trial self-administration task. Subjective, performance, and physiological effects were measured repeatedly during laboratory sessions.

RESULTS

Memantine produced modest reductions in subjective ratings of drug quality, liking, willingness to pay for the drug, and craving for heroin. However, memantine produced few changes in the reinforcing effects of heroin.

CONCLUSIONS

These data demonstrate that memantine was well tolerated and modestly effective in reducing the subjective but not the reinforcing effects of heroin. Although it is unlikely that memantine will be useful as a stand-alone maintenance medication for opioid dependence, it may have some utility as an adjunct treatment medication.

Activation of supraspinal NMDA receptors by both D-serine alone or in combination with morphine leads to the potentiation of antinociception in tail-flick test of rats

Although there is a variety of information concerning the effects of the N-methyl-D-aspartate (NMDA) receptor on opioid-induced antinociception at the spinal level, little is known about the effects at the supraspinal level. To clarify the role of the NMDA receptor on the morphine-induced antinociception at the supraspinal level, we investigated the effects of the intracerebroventricular (i.c.v.) administration of D-serine, a selective agonist for the glycine site of the NMDA receptors, alone or in combination with morphine using the tail-flick test. The i.c.v. administration of D-serine, but not L-serine, alone produced a dose-dependent antinociception in the tail-flick response. D-Serine also dose-dependently potentiated the antinociceptive effect induced by the i.c.v. administration of morphine and the simultaneous administration produced an additive effect. The potentiation of the antinociception produced by both D-serine alone or in combination with morphine was dose-dependently attenuated by the i.c.v. administration of L-701,324, a selective antagonist for the glycine site of the NMDA receptors. In addition, the potentiation of the D-serine-induced antinociception was antagonized by the i.c.v. administration of naloxone, a nonselective opioid receptor antagonist. These observations, together with the fact that D-serine is an endogenous and selective co-agonist for the glycine site of the NMDA receptors, strongly suggested that the activation of the supraspinal NMDA receptors by D-serine leads to the potentiation of the antinociception in the tail-flick test and that endogenous D-serine could modulate the mu-opioid receptor mediated antinociception via the glycine site of the NMDA receptors at the supraspinal level.

Additive Effect of Dextromethorphan on the Inhibitory Effect of Anti-NT4 on Morphine Tolerance

It has been proposed that opioid tolerance is a model of neuronal plasticity similar to learning and memory. Recent evidence suggests that neurotrophins may be involved in synaptic development and plasticity. Observations indicate that neurotrophin 4 (NT4) is required for the synaptic plasticity mediating both tolerance and memory. Also there are lines of evidence to indicate that NMDA receptors are involved in the neural plasticity underlying the development of opiate tolerance. Neurotrophins affect central transmission postsynaptically by enhancing NMDA receptor responsiveness. So we used the clinically available NMDA receptor antagonist, dextromethorphan, and the neurotrophin 4 antibody, anti-NT4, concomitantly and alone to investigate their effects on morphine tolerance. Tolerance was induced by injecting morphine (7 and 10 mg/kg i.p.) once per day for 4 days. Anti-NT4 (1 microg/rat i.c.v.) was administered 15 min before morphine. Results showed that chronic concomitant treatment of anti-NT4 with morphine in both doses inhibited the development of morphine tolerance. Also acute treatment of anti-NT4 significantly reversed the tolerance that was induced by morphine 7 mg/kg but failed to reverse the tolerance of morphine 10 mg/kg. Dextromethorphan in both doses (10 or 30 mg/kg) has an additive effect on the inhibitory effect of anti-NT4 on the reversal of morphine tolerance (7 mg/kg). These findings provide additional support for the hypothesis that NMDA receptor and NT4 may be involved in neural plasticity underlying opiate tolerance.

CNS arousal mechanisms bearing on sex and other biologically regulated behaviors

It now seems possible to move beyond analyzing only the mechanisms for specific sexual behaviors to the analysis of 'generalized arousal' that underlies all motivated behaviors. Our science has advanced sufficiently to attack mechanisms linking specific motivations to these general arousal mechanisms that intrinsically activate all biologically-regulated behaviors including ingestive behaviors. Learning from the well-developed reproductive behavior paradigm, we know that sex hormone effects on hypothalamic neurons have been studied to a point where receptor mechanisms are relatively well understood, a neural circuit for a sex steroid-dependent behavior has been worked out, and several functional genomic regulations have been discovered. Here we focus for the first time on three chemical systems that signal 'generalized arousal' and which impact hormone-dependent hypothalamic neurons of importance to sexual arousal: histamine, norepinephrine and enkephalin. Progress in linking generalized arousal to specific motivational mechanisms is reviewed.

Functional genomics of sex hormone-dependent neuroendocrine systems: specific and generalized actions in the CNS

Sex hormone effects on hypothalamic neurons have been worked out to a point where receptor mechanisms are relatively well understood, a neural circuit for a sex steroid-dependent behavior has been determined, and several functional genomic regulations have been discovered and conceptualized. With that knowledge in hand, we approach deeper problems of explaining sexual arousal and generalized CNS arousal. After a brief summary of arousal mechanisms, we focus on three chemical systems which signal generalized arousal and impact hormone-dependent hypothalamic neurons of behavioral importance: histamine, norepinephrine and enkephalin.

Effects of the NMDA receptor antagonist MK-801 on short-interval timing in rats

Effects of MK-801, an N-methyl-D-aspartate antagonist, on short-interval timing were examined using the peak-interval (PI) and PI-gap procedures. Fisher 344 rats were given daily injections of 0.025 mg/kg, 0.05 mg/kg, and 0.2 mg/kg MK-801. The main results were (a) 0.2 mg/kg MK-801 produced an immediate overestimation of the criterion time; (b) MK-801 increased peak rate of responding; (c) 0.2 mg/kg MK-801 produced an increase in variability; (d) during the PI-gap procedure, a reset pattern was observed for all rats (MK-801 and saline). Results suggest that MK-801 has at least 2 effects. First, MK-801 interferes with short-interval timing by producing an overestimation of time and a nonscalar increase in variability. Second, MK-801 increases response rate, suggesting a decrease in response inhibition.

Morphine tolerance does not develop in mice treated with endothelin-A receptor antagonists

Long-term use of morphine leads to development of antinociceptive tolerance. We provide evidence that central endothelin (ET) mechanisms are involved in development of morphine tolerance. In the present study, we investigated the effect of ET(A) receptor antagonists, BQ123 and BMS182874, on morphine antinociception and tolerance in mice. Mechanism of interaction of ET(A) receptor antagonists with morphine was investigated. BQ123 (3 microg, i.c.v.) and BMS182874 (50 microg, i.c.v.) significantly enhanced antinociceptive effect of morphine (P < 0.05), through an opioid-mediated effect. Treatment with a single dose of BQ123 (3 microg, i.c.v.) reversed tolerance to morphine antinociception in morphine-tolerant mice. BQ123 or BMS182874 did not affect naloxone binding in the brain. Therefore, ET(A) receptor antagonists did not bind directly to opioid receptors. [35S]GTPgammaS binding was stimulated by morphine and ET-1 in non-tolerant mice. Morphine- and ET-1-induced GTP stimulation was significantly lower (P < 0.05) in morphine-tolerant group (33% and 42%, respectively) compared to control group. BQ123 and BMS182874 did not activate binding in non-tolerant mice. BQ123 and BMS182874 significantly increased G protein activation in morphine-tolerant mice (96% and 86%, respectively; P < 0.05). These results provide evidence that uncoupling of G protein occurs in morphine-tolerant mice, and ET(A) antagonists promote coupling of G protein to its receptors, thereby restoring antinociceptive effect. These findings indicate that ET(A) receptor antagonists potentiate morphine antinociception and reverse antinociceptive tolerance in mice, through their ability to couple G proteins to opioid receptors.

An opiate cocktail that reduces morphine tolerance and dependence

Morphine is an exceptionally effective analgesic whose utility is compromised by the development of tolerance and dependence to the drug. Morphine analgesia and dependence are mediated by its activity at the mu opioid peptide (MOP) receptor [1]. The MOP receptor is activated not only by morphine, but also by other opiate drugs such as methadone and endogenous opioids such as endorphins. Morphine, however, is a unique opioid agonist ligand because it fails to induce endocytic trafficking of the MOP receptor [2], whereas the endogenous ligands and methadone do facilitate endocytosis [3]. Using the unique pharmacology of the MOP receptor and its proposed existence as an oligomeric structure [4], we designed a pharmacological cocktail that facilitates endocytosis of the MOP receptor in response to morphine. This cocktail consists of morphine and a small dose of methadone. Importantly, this cocktail, while retaining full analgesic potency, does not promote morphine dependence. We further demonstrate that dependence is reduced, at least in part, because endocytosis of the MOP receptor in response to morphine prevents the upregulation of N-methyl-D-aspartate (NMDA) receptors.

Pharmacodynamics of morphine-induced neuronal nitric oxide production and antinociceptive tolerance development

Elevated nitric oxide (NO) production has been implicated in the development of morphine antinociceptive tolerance. This study was conducted to establish the temporal relationship between morphine-induced increases in neuronal NO and loss of pharmacologic activity. Five groups of rats equipped with microdialysis probes in the jugular vein and hippocampus received an intravenous infusion of saline or morphine (0.3, 1, 2, or 3 mg/kg/h) for 8 h. Morphine concentrations in the blood and hippocampal microdialysate were determined by LC/MS-MS; NO production was quantified with an amperometric sensor implanted in the contralateral hippocampus. Antinociceptive effect was monitored at selected time points during and following infusion by electrical stimulation vocalization. The data were fit with a pharmacokinetic/pharmacodynamic model to obtain parameters governing morphine disposition, stimulation of NO production, antinociception, and antinociceptive tolerance development. An additional three groups of rats were pretreated with l-arginine, the NO precursor (100, 300, or 500 mg/kg/h for 8 h), to elevate NO concentrations prior to morphine infusion. Morphine administration resulted in a dose-dependent increase in NO production; the time course of altered NO production coincided with the development of antinociceptive tolerance. l-arginine pretreatment initially enhanced morphine-induced analgesia early in the morphine infusion. However, this NO-associated increase in opioid response dissipated rapidly due to a dominant NO-induced loss of antinociception. Pharmacodynamic modeling suggested that this latter effect was consistent with a hyperalgesic response. These data define a strong, time-dependent relationship between morphine-induced stimulation of NO production and tolerance development, identify specific NO-induced alterations in nociceptive processing after morphine administration, and indicate that NO is a key mediator of antinociceptive tolerance development.

Neuronal nitric oxide modulates morphine antinociceptive tolerance by enhancing constitutive activity of the mu-opioid receptor

NO is a key mediator of morphine antinociceptive tolerance. This work was conducted to evaluate the specific effects of NO on mu-opioid receptor activity. To investigate the effects of morphine- and L-arginine (the NO precursor)-induced increases in NO, five groups of rats were treated with saline, l-arginine (100-, 300-, or 500-mg/kg/h), or morphine 3-mg/kg/h for 8h on Day 1; brain tissue was collected on Day 2. To evaluate the effects of additional increases in NO on morphine-induced alterations of the mu-opioid receptor, six groups of rats were treated with 8-h intravenous infusions for two consecutive days as per the following scheme (Day 1:Day 2): saline:saline (control); saline:morphine 3-mg/kg/h (tolerant); L-arginine 500-mg/kg/h:saline (NO control); L-arginine 100-mg/kg/h:morphine 3-mg/kg/h; L-arginine 300-mg/kg/h:morphine 3-mg/kg/h; and L-arginine 500-mg/kg/h:morphine 3-mg/kg/h (supertolerant). Brain tissue was collected at the end of Day 2. The time course of effects on morphine-induced receptor alterations due to increased NO also was evaluated. Brain tissue was analyzed for changes in radioligand (agonist and antagonist) binding and [(35)S]GTPgammaS binding (agonist and antagonist). In the absence of agonist exposure, NO produced an alteration in the mu-opioid receptor that increased receptor activity. In the presence of agonist, NO increased constitutive activation of the mu-opioid receptor and reduced the ability of a selective mu-opioid agonist to activate the mu-opioid G-protein-coupled receptor; these molecular effects occurred in a time course consistent with the development of antinociceptive tolerance. This work establishes important NO-induced alterations in mu-opioid receptor functionality, which directly lead to the development of opioid antinociceptive tolerance.

Opioid receptors

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

Neurochemical changes of the extracellular concentrations of glutamate and aspartate in the nucleus accumbens of rats after chronic administration of morphine

The effects of discontinuing a chronic morphine treatment on the concentrations of glutamate and aspartate were analyzed in the nucleus accumbens of unrestrained unanesthetized rats. The administration of naloxone or the cessation of morphine administration resulted in increased concentrations of glutamate and aspartate in this central nervous system area. These increased amino acid concentrations were observed a few minutes after naloxone administration and persisted in the controls 48 h after the last dose of the opiate. Morphine withdrawal was also studied in rats not injected with naloxone. In these latter animals, increased concentrations of glutamate and aspartate persisted in controls 96 h after the last dose of the opiate. Single doses of morphine, acamprosate or riluzole administered to rats previously withdrawn from chronic morphine treatment restored the amino acid concentrations to normal levels. These results suggest that the maintenance of increased levels of amino acids could be the expression of new adjustments in central nervous system neurotransmission after discontinuation of the chronic morphine treatment.