Inhibition of morphine tolerance by NMDA receptor antagonists in the formalin test

Diverse roles of neurotensin agonists in the central nervous system

Neurotensin (NT) is a tridecapeptide that is found in the central nervous system (CNS) and the gastrointestinal tract. NT behaves as a neurotransmitter in the brain and as a hormone in the gut. Additionally, NT acts as a neuromodulator to several neurotransmitter systems including dopaminergic, sertonergic, GABAergic, glutamatergic, and cholinergic systems. Due to its association with such a wide variety of neurotransmitters, NT has been implicated in the pathophysiology of several CNS disorders such as schizophrenia, drug abuse, Parkinson's disease (PD), pain, central control of blood pressure, eating disorders, as well as, cancer and inflammation. The present review will focus on the role that NT and its analogs play in schizophrenia, endocrine function, pain, psychostimulant abuse, and PD.

Interactions of the potent D-amino acid oxidase inhibitor CBIO with morphine in pain and tolerance to analgesia

A series of experiments using technologies of gene mutation and silencing as well as chemical biology have demonstrated that spinal D-amino acid oxidase (DAAO) contributes to the development of central sensitization-mediated chronic pain and might be a potential molecular target for the treatment of chronic pain. DAAO inhibitors are now under clinical investigations for the management of chronic neuropathic pain. This study examined the interactions between morphine and the DAAO inhibitor CBIO (5-chloro-benzo[d]isoxazol-3-ol) in pain and analgesia tolerance mainly in the formalin test. Given subcutaneously CBIO acutely interacted with morphine in analgesia in an additive manner both in the acute nociception settings (the formalin acute phase nociception, hot-plate test and tail immersion test) and in formalin-induced tonic pain. Bi-daily exposure of CBIO given subcutaneously for 7 days did not produce self-tolerance to analgesia or cross-tolerance to morphine whereas 7-day subcutaneous morphine induced self-tolerance to analgesia but not cross-tolerance to CBIO. More importantly, subcutaneous co-administrations or even single dose of CBIO completely prevented or reversed morphine tolerance to analgesia (exhibited by a single dose or a dose-response curve of morphine) in both formalin-induced acute phase nociception and tonic phase pain. These results, for the first time, identified DAAO as an efficacious molecule mediating morphine tolerance, in addition to clarifying the complex interactions between morphine and DAAO inhibitors probed by CBIO, and provided a pharmacological basis for DAAO inhibitors in combination with morphine to clinically manage pain.

Botulinum neurotoxin A enhances the analgesic effects on inflammatory pain and antagonizes tolerance induced by morphine in mice

Over the recent years compelling evidence has accumulated indicating that botulinum neurotoxin serotype A (BoNT/A) results in analgesic effects on neuropathic as well as inflammatory pain, both in humans and in animal models. In the present study, the pharmacological interaction of BoNT/A with morphine in fighting inflammatory pain was investigated in mice using the formalin test. Moreover, the effects of BoNT/A on the tolerance-induced by chronic administration of morphine were tested and the behavioral effects were correlated with immunofluorescence staining of glial fibrillary acidic protein, the specific marker of astrocytes, at the spinal cord level. An ineffective dose of BoNT/A (2 pg/paw) combined with an ineffective dose of morphine (1 mg/kg) exerted a significant analgesic action both during the early and the late phases of formalin test. A single intraplantar injection of BoNT/A (15 pg/paw; i.pl.), administered the day before the beginning of chronic morphine treatment (7 days of s.c. injections of 20 mg/kg), was able to counteract the occurrence of tolerance to morphine. Moreover, BoNT/A reduces the enhancement of the expression of astrocytes induced by inflammatory formalin pain. Side effects of opiates, including the development of tolerance during repeated use, may limit their therapeutic use, the possibility of using BoNT/A for lowering the effective dose of morphine and preventing the development of opioid tolerance would have relevant implications in terms of potential therapeutic perspectives.

Enhancement of morphine antinociception with the peptide N-methyl-d-aspartate receptor antagonist [Ser1]-histogranin in the rat formalin test

Opiates may be used to attenuate chronic pain, but long-term use is complicated by the possible increase in pain over time, escalating dose requirements, and untoward side effects. Adjuncts such as ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, may be added to opiates to provide more consistent analgesia. However, the unwanted motor side effects of NMDA receptor antagonists prevent their widespread clinical usage. In the current study, an analogue of the naturally-derived peptide histogranin, [Ser(1)]histogranin (SHG), an NMDA receptor modulator without adverse side effects like those in current clinical use, was evaluated for its potential to enhance the antinociceptive effect of intrathecal morphine in the rat formalin test. Intrathecal injection of a combination of SHG and morphine resulted in significantly reduced hind paw flinching compared with morphine alone in the first and second phases. The effective dose of SHG used in the combination had no efficacy when tested alone. These results were similar to the increased efficacy that was obtained with a combination of ketamine and morphine. Thus, enhancement of opiate efficacy is possible using a novel peptide NMDA receptor modulator with a potentially improved safety profile.

ACEA 1021: flip or flop?

Inasmuch as glutamate is the main excitatory neurotransmitter in the central nervous system, strategies aimed at counteracting glutamate excitotoxicity, which is at least partially involved in many acute neurologic, chronic neurodegenerative and psychiatric diseases, are challenging. Blockade of the NMDA receptor was identified as one way of achieving selective antagonism and overcoming glutamate neurotoxicity, yet not without liabilities. Glycine site antagonism of the NMDA receptor in 1987 offered a significant advance in blocking this receptor because such drugs were shown to lack most of the side effects, such as memory impairment, ataxia, lack of motor coordination and psychotomimetic effects, which accompanied competitive and non-competitive NMDA receptor antagonists. To date, much has been done to improve the structure-activity relationship (SAR) of compounds resulting in the synthesis of ACEA 1021. It is unclear, however, whether further chemical substitutions will lead to an improved compound. Many studies have been performed with ACEA 1021 and although there are much in vitro and in vivo data to support its neuroprotective effects and improved safety profile, there is very little published information regarding its clinical pharmacology. In order to properly evaluate the true potential for ACEA 1021 in acute and chronic CNS disorders additional longer term safety and efficacy data in humans are needed.

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.

Opioid receptor desensitization contributes to thermal hyperalgesia in infant rats

Central nociceptive processing includes spinal and supraspinal neurons, but the supraspinal mechanisms mediating changes in pain threshold remain unclear. We investigated the role of forebrain neurons in capsaicin-induced hyperalgesia. Long-Evans rat pups at 21 days were randomized to undisturbed control group, or to receive tactile stimulation, saline injection (0.9% w/v) or capsaicin injection (0.01% w/v) applied to each paw at hourly intervals. Thermal paw withdrawal latency was measured 1 h later, forebrains were removed and purified forebrain neuronal membranes were assayed for adenylyl cyclase activity and opioid receptor function. Capsaicin-injected rats had decreased thermal latency (P < 0.0001) compared to the other groups. Neuronal membranes showed increased basal (P = 0.0003) and forskolin-stimulated (P=0.0002) adenylyl cyclase activity in the capsaicin group compared to other groups. The selective mu-opioid receptor agonist, [D-Ala2, N-Me-Phe4, Gly5-ol]enkephalin (DAMGO) was less effective in inhibiting adenylyl cyclase activity in the capsaicin group (P < 0.001) compared to other groups. These effects were naloxone-reversible and pertussis toxin-sensitive (P < 0.01) in the control, tactile stimulation and saline injection groups but not in the capsaicin group. Binding capacity and affinity for micro-opioid receptors were similar in all four groups, suggesting that receptor downregulation was not involved. Exposure to DAMGO increased [35S]GTPgammaS binding to neuronal membranes from the control, tactile and saline groups (P<0.001) in a naloxone-reversible and pertussis toxin-sensitive manner (P < 0.01) but not in the capsaicin group, suggesting mu-opioid receptor desensitization. Dose responses to systemic morphine were also reduced in the capsaicin group compared to the tactile group (P < 0.05). Capsaicin-induced hyperalgesia in 21-day-old rats was associated with an uncoupling of micro-opioid receptors in the forebrain. Opioid receptor desensitization in the forebrain may reduce opioidergic inputs to the descending inhibitory controls, associated with behavioral hyperalgesia and reduced responsiveness to morphine analgesia in capsaicin-injected young rats.

Antinociceptive and nociceptive actions of opioids

Although the opioids are the principal treatment options for moderate to severe pain, their use is also associated with the development of tolerance, defined as the progressive need for higher doses to achieve a constant analgesic effect. The mechanisms which underlie this phenomenon remain unclear. Recent studies revealed that cholecystokinin (CCK) is upregulated in the rostral ventromedial medulla (RVM) during persistent opioid exposure. CCK is both antiopioid and pronociceptive, and activates descending pain facilitation mechanisms from the RVM enhancing nociceptive transmission at the spinal cord and promoting hyperalgesia. The neuroplastic changes elicited by opioid exposure reflect adaptive changes to promote increased pain transmission and consequent diminished antinociception (i.e., tolerance).

Induction of pain facilitation by sustained opioid exposure: relationship to opioid antinociceptive tolerance

Opioid analgesics are frequently used for the long-term management of chronic pain states, including cancer pain. The prolonged use of opioids is associated with a requirement for increasing doses to manage pain at a consistent level, reflecting the phenomenon of analgesic tolerance. It is now becoming clearer that patients receiving long-term opioid therapy can develop unexpected abnormal pain. Such paradoxical opioid-induced pain, as well as tolerance to the antinociceptive actions of opioids, has been reliably measured in animals during the period of continuous opioid delivery. Several recent studies have demonstrated that such pain may be secondary to neuroplastic changes that result, in part, from an activation of descending pain facilitation mechanisms arising from the rostral ventromedial medulla (RVM). One mechanism which may mediate such pain facilitation is through the increased activity of CCK in the RVM. Secondary consequences from descending facilitation may be produced. For example, opioid-induced upregulation of spinal dynorphin levels seem to depend on intact descending pathways from the RVM reflecting spinal neuroplasticity secondary to changes at supraspinal levels. Increased expression of spinal dynorphin reflects a trophic action of sustained opioid exposure which promotes an increased pain state. Spinal dynorphin may promote pain, in part, by enhancing the evoked release of excitatory transmitters from primary afferents. In this regard, opioids also produce trophic actions by increasing CGRP expression in the dorsal root ganglia. Increased pain elicited by opioids is a critical factor in the behavioral manifestation of opioid tolerance as manipulations which block abnormal pain also block antinociceptive tolerance. Manipulations that have blocked enhanced pain and antinociceptive tolerance include reversible and permanent ablation of descending facilitation from the RVM. Thus, opioids elicit systems-level adaptations resulting in pain due to descending facilitation, upregulation of spinal dynorphin and enhanced release of excitatory transmitters from primary afferents. Adaptive changes produced by sustained opioid exposure including trophic effects to enhance pain transmitters suggest the need for careful evaluation of the consequences of long-term opioid administration to patients.

GV196771A, an NMDA receptor/glycine site antagonist, attenuates mechanical allodynia in neuropathic rats and reduces tolerance induced by morphine in mice

The effects of the N-methyl-D-aspartate (NMDA) receptor/glycine site antagonist, GV196771A (E-4,6-dichloro-3-(2-oxo-1-phenyl-pyrrolidin-3-ylidenemethyl)-1H-indole-2-carboxylic acid sodium salt), on mechanical allodynia and on tolerance to the antinociceptive effects induced by morphine were evaluated. Its antiallodynic properties were studied in a model of chronic constriction injury applied to rat sciatic nerve. GV196771A (0.3-10 mg/kg, p.o.) dose-dependently inhibited established mechanical allodynia when tested 14 or 21 days after nerve ligation. In the formalin test in mice, GV196771A (10 or 20 mg/kg, p.o.), administered for 8 days together with morphine 10 mg/kg, i.p. inhibited morphine tolerance development in both early and late phases of the test. This finding reinforces the key role of the NMDA receptors in the plastic event, such as allodynia, which develops in some conditions of painful neuropathy. Moreover, the capability to strongly reduce morphine-induced tolerance suggests that GV196771A could be an alternative agent for the treatment of difficult pain states not only when given alone, but also in combination, in order to prolong the analgesic effects of the opiates.

NMDA receptors as targets for drug action in neuropathic pain

Hyperalgesia and allodynia following peripheral tissue or nerve injury are not only due to an increase in the sensitivity of primary afferent nociceptors at the site of injury but also depend on NMDA receptor-mediated central changes in synaptic excitability. Functional inhibition of NMDA receptors can be achieved through actions at different recognition sites such as the primary transmitter site (competitive), strychnine-insensitive glycine site (glycine(B)), polyamine site (NR2B selective) and phencyclidine site located inside the cationic channel. Unfortunately, most agents which completely block NMDA receptors cause numerous side effects such as memory impairment, psychotomimetic effects, ataxia and motor incoordination. There is now, however, considerable evidence that moderate affinity channel blockers, glycine(B) and NR2B selective antagonists show a much better profile in animal models than high affinity channel blockers and competitive NMDA receptor antagonists. These "therapeutically" safe NMDA receptor antagonists are also able to slow or prevent the development of opioid tolerance, indicating the utility of their combination with opioids in the treatment of chronic pain. The antinociceptive effects of NMDA receptor antagonists and opioids could be predicted to be synergistic and the presence of an NMDA receptor antagonist should block both the development of chronic pain states and inhibit the development of tolerance to the analgesic effects of morphine. Peripheral NMDA receptors offer a very attractive target for NMDA receptor antagonists that do not cross the blood brain barrier in inflammatory and visceral pain. Such agents might be predicted to be devoid of CNS side effects at doses producing powerful antinociception at peripheral NMDA receptors.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

Differential potentiative effects of glutamate receptor antagonists in the production of antinociception induced by opioids administered intrathecally in the mouse

The effect of (+/-)-5-methyl-10,11-dihydro-5H-dibenzo(a,d) cyclohepten-5, 10-imine maleate (MK-801) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) injected intrathecally (i.t.) on the inhibition of the tail-flick response induced by morphine, D-Ala(2)-NmePhe(4)-Gly-ol-enkephalin (DAMGO), beta-endorphin, D-Pen(2,5)-enkephalin (DPDPE), or ¿(trans-3, 4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl] benzeocetamide)¿ (U50, 488H) administered i.t. was studied in ICR mice. The i.t. injection of MK-801 (2 microg) or CNQX (1 microg) alone did not affect the basal tail-flick response. Morphine (0.2 microg), DAMGO (0.8 ng), beta-endorphin (0.1 microg), DPDPE (0.5 microg) or U50, 488H (6 microg) caused only slight inhibition of the tail-flick response. CNQX injected i.t., but not MK-801, enhanced the inhibition of the tail-flick response induced by i.t. administered morphine, DAMGO, DPDPE or U50, 488H. However, CNQX or MK-801 injected i.t. was not effective in enhancing the inhibition of the tail-flick response induced by beta-endorphin administered i.t. The potentiating effect of CNQX on tail-flick inhibition induced by morphine, DAMGO, DPDPE or U50, 488H was blocked by naloxone (from 1 to 20 microg), yohimbine (from 1 to 20 microg) or methysergide (from 1 to 20 microg) injected i.t. in a dose-dependent manner. Our results suggest that the blockade of AMPA/kainate receptors located in the spinal cord appears to be involved in enhancing the inhibition of the tail-flick response induced by stimulation of spinal mu-, delta-, and kappa-opioid receptors. Furthermore, this potentiating action may be mediated by spinal noradrenergic and serotonergic receptors. However, N-methyl-D-aspartate receptors may not be involved in modulating the inhibition of the tail-flick response induced by various opioids administered spinally.

Prevention of cocaine-induced convulsions and lethality in mice: effectiveness of targeting different sites on the NMDA receptor complex

N-methyl-D-aspartate (NMDA) receptors appear to be involved in the behavioral toxic effects of cocaine. Therefore, different classes of NMDA receptor antagonists were compared for their ability to attenuate cocaine-induced convulsions and lethality in male, Swiss Webster mice. The mice were pre-treated (i.p.) with vehicle or an antagonist from one of the following classes: NMDA/glycine site antagonist (7-chlorokynurenic acid, ACEA-1021, ACEA-1031, ACEA-1328, DCQX, R(+)-HA-966), competitive antagonist (CPP, D-AP7), channel blocker (MK-801, memantine), or allosteric modulator (ifenprodil, CP-101,606, Co 101022, haloperidol). After a 15 min pre-treatment period, the mice were administered a convulsive (60 mg/kg, i.p.) or lethal (125 mg/kg, i.p.) dose of cocaine, equivalent to the calculated ED/LD97 values. Pre-treatment with competitive or NMDA/glycine site antagonists dose-dependently attenuated cocaine-induced convulsions and lethality (P<0.05). Pre-treatment with channel blockers or allosteric modulators of the NMDA receptor protected against cocaine-induced convulsions (P<0.05), but were ineffective or less effective than the competitive and glycine site antagonists in preventing death. The glutamate release inhibitor riluzole failed to prevent both the convulsions and lethality induced by cocaine. Significantly, post-treatment with NMDA/glycine site antagonists (ACEA-1021, ACEA-1031, ACEA-1328) after a cocaine overdose prevented death in a significant number of animals. The data suggest that NMDA receptors are involved in the pathophysiology of a cocaine overdose.

Effects of ACEA-1328, a NMDA receptor/glycine site antagonist, on U50,488H-induced antinociception and tolerance

Previously, we have shown that inhibition of the glycine site associated with the N-methyl-D-aspartate (NMDA) receptor is another viable approach to blocking morphine tolerance. In the present study, we sought to investigate the involvement of the NMDA receptor/glycine site in kappa-opioid receptor-mediated antinociception and tolerance in CD-1 mice. In antinociception studies, mice were injected with 5-nitro-6,7-dimethyl-1,4-dihydro-2, 3-quinoxalinedione (ACEA-1328), a systemically bioavailable NMDA receptor/glycine site antagonist, or the vehicle (Bis-Tris, 0.2 M) and then immediately with trans-(+/-)-3, 4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamid e methanesulfonate (U50,488H), a kappa-opioid receptor agonist. Thirty minutes later, mice were tested for changes in nociceptive responses in the tail flick assay. ACEA-1328, per se, prolonged tail flick latencies with an ED(50) of approximately 50 mg/kg. Concurrent administration of ACEA-1328, at doses that did not produce antinociception, with U50,488H increased the potency of U50,488H in a dose-dependent manner. In tolerance studies, mice were treated, either once a day for 9 days or twice daily for 4 days, with the vehicle or ACEA-1328. Immediately after the initial injection, mice then received an injection of saline or U50,488H. On the test day, mice were injected with U50,488H alone and tested for antinociception 30 min later. Chronic treatment with U50,488H by either method produced tolerance. Unlike the acute effect of the drug, chronic treatment with ACEA-1328 decreased the antinociceptive potency of U50,488H. Taken together, the data suggest that acute and chronic administration of ACEA-1328 differentially affected the antinociceptive effect of U50,488H. Furthermore, the decreased in the potency of U50,488H induced by chronic treatment with ACEA-1328 also confounded the interpretation of the tolerance data.

NMDA and opioid receptors: their interactions in antinociception, tolerance and neuroplasticity

Over the last several years, significant progress has been made in our understanding of interactions between the N-methyl-D-aspartate (NMDA) and opioid receptors. Such interactions have been demonstrated at two distinct sites: (1) modulation of NMDA receptor-mediated electrophysiological events by opioids; and (2) intracellular events involving interactions between NMDA and opioid receptors. Furthermore, a considerable number of studies have shown the involvement of such interactions in neural mechanisms of nociceptive transmission, antinociception in acute and chronic pain states, opioid tolerance/dependence, and neuroplasticity. Importantly, emerging evidence indicates that activation of NMDA receptors may differentially modulate functions mediated by distinct opioid receptor subtypes, namely mu, delta, and kappa receptors. These studies have greatly enriched our knowledge regarding both NMDA and opioid receptor systems and have shed light on neurobiology of both acute and chronic pain. The advancement of such knowledge also promotes new strategies for better clinical management of pain patients.

ACEA-1328, a NMDA receptor/glycine site antagonist, acutely potentiates antinociception and chronically attenuates tolerance induced by morphine

The effect of ACEA-1328, a competitive and systemically bioavailable NMDA receptor/glycine site antagonist, was studied on morphine-induced antinociception and tolerance in CD-1 mice using the tail flick test. To study the effect of acute administration of ACEA-1328 on morphine-induced antinociception, mice were injected with either ACEA-1328 (1, 5, and 10 mg kg(-1)) or Bis-Tris (0.2 m) immediately followed by an injection of morphine and tested for antinociception 30 min later. ACEA-1328 significantly increased the antinociceptive potency of morphine. To study the effect of chronic administration of ACEA-1328 on morphine-induced antinociception and tolerance, mice were treated, either once per day for 9 days or twice daily for 4 days, with ACEA-1328 or with the vehicle. Mice were then, within 1 min, injected daily with either morphine or saline. On the day of the test, mice were injected with only morphine and tested for antinociception 30 min later. In comparison to the acute effect of ACEA-1328, chronic treatment with the NMDA receptor/glycine site antagonist did not affect the antinociceptive potency of morphine. Chronic treatment with morphine, by both methods, produced a significant degree of tolerance. Concurrent administration of ACEA-1328 with the opioid analgesic completely blocked morphine tolerance. Our results demonstrate that acute, but not chronic, treatment with ACEA-1328 increased the antinociceptive potency of morphine. Furthermore, co-administration of the NMDA receptor antagonist with morphine abolished the development of tolerance. Overall, the data support a growing body of evidence showing that activation of the NMDA receptor plays a functional role in opioid-induced antinociception and tolerance.

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.

Novel NMDA/glycine site antagonists attenuate cocaine-induced behavioral toxicity

N-Methyl-D-aspartate (NMDA)/glycine site antagonists were tested for their ability to prevent cocaine-induced convulsions and lethality in Swiss Webster mice. Pre-treatment of mice with the novel NMDA/glycine site antagonists ACEA-1021 (5-nitro-6,7-dichloro-1,4-dihydro-2,3-quinoxalinedione) or ACEA-1328 (5-nitro-6,7-dimethyl-1,4-dihydro-2,3-quinoxalinedione) attenuated cocaine-induced convulsions; these effects were pharmacologically antagonized with D-cycloserine. The structurally-related NMDA/glycine site antagonist DCQX (6,7-dichloroquinoxaline-2,3-dione) and the structurally-unrelated NMDA/glycine site partial agonist HA-966 (3-amino-1-hydroxy-2-pyrrolidinone) also attenuated cocaine-induced convulsions, with the R(+)-isomer of HA-966 being more effective than the S(-)-isomer. In contrast, the selective alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) receptor antagonist, NBQX (1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide) , failed to provide statistically significant protection although it shares the 2,3-quinoxalinedione structure of DCQX and the ACEA compounds. Pre-treatment with ACEA-1021, ACEA-1328, DCQX, or R(+)-HA-966 also attenuated cocaine-induced lethality in mice. Significantly, post-treatment with ACEA-1021, immediately prior to or after the onset of seizures, prevented death in up to 86% of mice receiving a lethal dose of cocaine; post-treatment with vehicle resulted in death of all mice. The results suggest the utility of targeting excitatory mechanisms for the treatment of cocaine overdose and offer a novel base structure from which effective pharmacotherapies can be developed.