Competitive and glycine/NMDA receptor antagonists attenuate withdrawal-induced behaviours and increased hippocampal acetylcholine efflux in morphine-dependent rats

Effects of inhaled low-concentration xenon gas on naltrexone-precipitated withdrawal symptoms in morphine-dependent mice

BACKGROUND

Opioid withdrawal symptoms (OWS) are highly aversive and prompt unprescribed opioid use, which increases morbidity, mortality, and, among individuals being treated for opioid use disorder (OUD), recurrence. OWS are driven by sympathetic nervous system (SNS) hyperactivity that occurs when blood opioid levels wane. We tested whether brief inhalation of xenon gas, which inhibits SNS activity and is used clinically for anesthesia and diagnostic imaging, attenuates naltrexone-precipitated withdrawal-like signs in morphine-dependent mice.

METHODS

Adult CD-1 mice were implanted with morphine sulfate-loaded (60 mg/ml) minipumps and maintained for 6 days to establish morphine dependence. On day 7, mice were given subcutaneous naltrexone (0.3 mg/kg) and placed in a sealed exposure chamber containing either 21% oxygen/balance nitrogen (controls) or 21% oxygen/added xenon peaking at 30%/balance nitrogen. After 10 minutes, mice were transferred to observation chambers and videorecorded for 45 minutes. Videos were scored in a blind manner for morphine withdrawal behaviors. Data were analyzed using 2-way ANOVAs testing for treatment and sex effects.

RESULTS AND CONCLUSIONS

Xenon-exposed mice exhibited fewer jumps (P = 0.010) and jumping suppression was detectible within the first 10-minute video segment, but no sex differences were detected. Brief inhalation of low concentration xenon rapidly and substantially attenuated naltrexone-precipitated jumping in morphine-dependent mice, suggesting that it can inhibit OWS. If xenon effects translate to humans with OUD, xenon inhalation may be effective for reducing OWS, unprescribed opioid use, and for easing OUD treatment initiation, which could help lower excess morbidity and mortality associated with OUD.

Sex differences in the expression of morphine withdrawal symptoms and associated activity in the tail of the ventral tegmental area

Recent studies, in male rodents, have begun to elucidate a role for the GABAergic neurons in the tail of the ventral tegmental area (tVTA) in morphine withdrawal. To date, the mechanisms underlying morphine withdrawal have been studied almost exclusively in male animals. As a result, there is a considerable gap in our current understanding of the processes underlying sex differences in morphine withdrawal behaviors and its effects on cellular activity in the tVTA in females. The purpose of the present study was to investigate the influence of sex on the expression and duration of spontaneous somatic morphine withdrawal syndrome, and to characterize the relationship between spontaneous somatic withdrawal symptoms and cellular activation (measured as phosphorylated CREB; pCREB), in the GABAergic tVTA in male and female rats. Morphine-dependent adult male and female Long Evans rats underwent 72 h of spontaneous withdrawal, and somatic withdrawal symptoms were assessed every 12 h. Male morphine-dependent rats expressed more severe symptoms during the early phases of withdrawal compared to females. Although, females demonstrated lower overall symptom severity, their symptoms persisted for a longer period of time, thus demonstrating higher withdrawal-symptom severity than males during late withdrawal. pCREB activity in the tVTA was elevated in morphine-withdrawn rats and was positively correlated with the severity of withdrawal symptoms. These results demonstrate sex differences in the timing of the expression of somatic withdrawal. Our data add to the growing body of evidence demonstrating a role for the tVTA in morphine withdrawal and begin to establish a sex-dependent behavioral and molecular profile within this brain region.

Glial and neuroinflammatory targets for treating substance use disorders

BACKGROUND

The plenary session at the 2016 Behavior, Biology and Chemistry: Translational Research in Addiction Conference focused on glia as potential players in the development, persistence and treatment of substance use disorders. Glia partake in various functions that are important for healthy brain activity. Drugs of abuse alter glial cell activity producing several perturbations in brain function that are thought to contribute to behavioral changes associated with substance use disorders. Consequently, drug-induced changes in glia-driven processes in the brain represent potential targets for pharmacotherapeutics treating substance use disorders.

METHODS

Four speakers presented preclinical and clinical research illustrating the effects that glial modulators have on abuse-related behavioral effects of psychostimulants and opioids. This review highlights some of these findings and expands its focus to include other research focused on drug-induced glia abnormalities and glia-focused treatment approaches in substance use disorders.

RESULTS

Preclinical findings show that drugs of abuse induce neuroinflammatory signals and disrupt glutamate homeostasis through their interaction with microglia and astrocytes. Preclinical and clinical studies testing the effects of glial modulators show general effectiveness in reducing behaviors associated with substance use disorders.

CONCLUSIONS

The contribution of drug-induced glial activity continues to emerge as an intriguing target for substance use disorder treatments. Clinical investigations of glial modulators have yielded promising results on substance use measures and indicate that they are generally safe and well-tolerated. However, results have not been entirely positive and more questions remain for continued exploration in the development and testing of glial-directed treatments for substance use disorders.

Effects of prototypic calcium channel blockers in methadone-maintained humans responding under a naloxone discrimination procedure

Accumulating evidence suggests that L-type calcium channel blockers (CCBs) attenuate the expression of opioid withdrawal and the dihydropyridine L-type CCB isradipine has been shown to block the behavioral effects of naloxone in opioid-maintained humans. This study determined whether two prototypic L-type CCBs with differing chemical structures, the benzothiazepine diltiazem and the phenylalkamine verapamil, attenuate the behavioral effects of naloxone in methadone-maintained humans trained to distinguish between low-dose naloxone (0.15 mg/70 kg, i.m.) and placebo under an instructed novel-response drug discrimination procedure. Once discrimination was acquired, diltiazem (0, 30, 60, 120 mg) and verapamil (0, 30, 60, 120 mg), alone and combined with the training dose of naloxone, were tested. Diltiazem alone produced 33-50% naloxone- and novel-appropriate responding at 30 and 60 mg and essentially placebo-appropriate responding at 120 mg. Verapamil alone produced 20-40% naloxone- and 0% novel-appropriate responding. Diltiazem at 60 mg decreased several ratings associated with positive mood and increased VAS ratings of "Bad Drug Effects" relative to placebo, whereas verapamil increased ratings associated with euphoria. When administered with naloxone, diltiazem produced 94-100% naloxone-appropriate-responding with 6% novel-appropriate responding at 60 mg (n=3). When administered with naloxone, verapamil produced 60-80% naloxone- and 0% novel-appropriate responding (n=5). Diltiazem decreased diastolic blood pressure and heart rate whereas verapamil decreased ratings of arousal relative to placebo. These results suggest that CCBs with different chemical structures can be differentiated behaviorally, and that diltiazem and verapamil do not attenuate the discriminative stimulus effects of naloxone in humans at the doses tested.

Nucleus accumbens NMDA receptor subunit expression and function is enhanced in morphine-dependent rats

We have previously shown, using radioligand binding studies, that N-methyl-d-aspartate (NMDA) NR1 and NR2A receptor subunits density was decreased in the forebrain of morphine-dependent rats. We have now determined if morphine-dependent rats display regional differences in NMDA receptor expression and whether such changes are functionally relevant. In morphine-dependent rats, the expression of NR1 and NR2A subunits protein, as determined by Western blotting with NMDA receptor subunit antibodies, were decreased in frontal cortex and hippocampus but significantly increased in the nucleus accumbens. The expression of the NR2B subunit was unchanged in all regions examined. In separate groups of morphine-dependent rats, MK-801-induced hyperactivity (thought to be mediated via modulation of nucleus accumbens dopamine release) was significantly enhanced in morphine-dependent animals. Similarly, the MK-801-induced increase of dopamine metabolism was significantly increased in the nucleus accumbens of morphine-dependent animals as compared to sham controls. Results provide both biochemical and behavioural evidence to suggest that NMDA receptor function in the nucleus accumbens, at least with respect to an interaction with the limbic dopamine system, is markedly enhanced in morphine-dependent rats. This increase in function may be associated with an enhanced expression of NMDA receptors, particularly those in the nucleus accumbens containing the NR2A subunit. Taken together, these data support several studies in the literature indicating that NMDA receptors in the nucleus accumbens are involved in the process of opiate dependence.

Ethological analysis of scopolamine treatment or pretreatment in morphine dependent rats

Although scopolamine is currently used to treat morphine addiction in humans, its extensive actions on behaviors have not been systematically analyzed yet, and the underlying mechanisms of its effects still remain ambiguous. The present study was carried out to clarify the possible mechanisms by evaluating the effects of scopolamine pretreatment and treatment on naloxone-precipitated withdrawal signs and some of other general behaviors in morphine dependent rats. Our results showed that scopolamine pretreatment and treatment attenuated naloxone-precipitated withdrawal signs including jumping, writhing posture, weight loss, genital grooming, teeth-chattering, ptosis, diarrhea and irritability, except for wet dog shakes, while general behaviors such as water intake, urine volume and morphine excretion in urine were increased. Our findings suggest that scopolamine has significant actions in the treatment of opiate addiction, which might result from increasing morphine excretion from urine.

Ionotropic glutamatergic neurotransmission in the ventral tegmental area modulates ΔFosB expression in the nucleus accumbens and abstinence syndrome in morphine withdrawal rats

The present study sought to assess whether the blockade of ionotropic glutamate receptors in the ventral tegmental area could modulate morphine withdrawal in morphine-dependent rats and the expression of stable DeltaFosB isoforms in the nucleus accumbens during morphine withdrawal. Rats were injected (i.p.) with increasing doses of morphine for 1 week to develop physical dependence, and withdrawal was then precipitated by one injection of naloxone (2 mg/kg, i.p.). Abstinence signs such as jumping, wet-dog shake, writhing posture, weight loss, and Gellert-Holtzman scale score were recorded to evaluate naloxone-induced morphine withdrawal. Two ionotropic glutamate receptor antagonists, dizocilpine (MK-801) and 6, 7-dinitroquinnoxaline-2, 3-dione (DNQX), were microinjected unilaterally into the ventral tegmental area 30 min before naloxone precipitation. A second injection of naloxone (2 mg/kg i.p.) was given 1 h after the first naloxone injection to sustain a maximal level of withdrawal so that the expression of stable DeltaFosB isoforms in the nucleus accumbens could be measured. This would enable determination of the correlation between the MK-801 or DNQX-induced decrease in somatic withdrawal signs and the change in neuronal activity in the nucleus accumbens. The results showed that both MK-801 and DNQX significantly alleviated all symptoms of morphine withdrawal except for weight loss and reduced the expression of stable DeltaFosB isoforms within the nucleus accumbens. These data suggest that ionotropic glutamatergic neurotransmission in the ventral tegmental area regulates the levels of stable DeltaFosB isoforms in the nucleus accumbens, which play a very important role in modulating opiate withdrawal.

Opiate physical dependence and N-methyl-D-aspartate receptors

The present review focused the involvement of N-methyl-D-aspartate (NMDA) receptors in morphine physical dependence. The increased levels of extracellular glutamate, NMDA receptor zeta subunit (NR1) mRNA, NMDA receptor epsilon 1 subunit (NR2A) protein, phosphorylated Ca(2+)/calmodulin kinase II (p-CaMKII) protein, c-fos mRNA, c-Fos protein, are observed in the specific brain areas of mice and/or rats showing signs of naloxone-precipitated withdrawal. In preclinical and clinical studies, a variety of NMDA receptor antagonists and pretreatment with an antisense oligonucleotide of the NR1 have been reported to inhibit the development, expression and/or maintenance of opiate physical dependence. In contrast to data obtained in adult animals, NMDA receptor antagonists are neither effective in blocking the development of opiate dependence nor the expression of opiate withdrawal in neonatal rats. In the NMDA receptor-deficient mice, the NR2A knockout mice show the marked loss of typical withdrawal abstinence behaviors precipitated by naloxone. The rescue of NR2A protein by electroporation into the nucleus accumbens of NR2A knockout mice reverses the loss of abstinence behaviors. The activation of CaMKII and increased expression of c-Fos protein in the brain of animals with naloxone-precipitated withdrawal syndrome are prevented by NMDA receptor antagonists, whereas the increased levels of extracellular glutamate are not prevented by them. These findings indicate that glutamatergic neurotransmission at the NMDA receptor site contributes to the development, expression and maintenance of opiate dependence, and suggest that NMDA receptor antagonists may be a useful adjunct in the treatment of opiate dependence.

Blockade of ionotropic glutamatergic transmission in the ventral tegmental area attenuates the physical signs of morphine withdrawal in rats

The present study sought to assess whether the blockade of ionotropic glutamate receptors in the ventral tegmental area (VTA) could modulate the morphine withdrawal in male Sprague-Dawley rats. The effects of dizocilpine (MK-801) or 6,7-dinitroquinnoxaline-2,3-dione (DNQX), ionotropic glutamate receptor antagonists, microinjected unilaterally into the VTA 30 min before naloxone [2 mg/kg, intraperitoneally (i.p.)] administration on the morphine withdrawal were assessed. Morphine dependence was developed with increasing morphine injection (i.p.), and morphine withdrawal was induced by injection of naloxone (2 mg/kg, i.p.). Jumping, wet-dog shakes, writhing posture, wall clamber, weight loss and Gellert-Holtzman scale were used as the indices to evaluate the intensity of morphine withdrawal. The results showed that unilateral microinjection of MK-801 or DNQX into the VTA significantly increased the incidence of wall clamber, had no effect on weight loss, and reduced all other symptoms of morphine withdrawal. These data suggest that the ionotropic glutamate receptors in the VTA are involved in mediating naloxone-precipitated opiate withdrawal.

Facilitation of morphine withdrawal symptoms and morphine-induced conditioned place preference by a glutamate transporter inhibitor DL-threo-beta-benzyloxyaspartate in rats

There is a body of evidence implying the involvement of the central glutamatergic system in morphine dependence. In this study, we examined the effect of intracerebroventricular (i.c.v.) administration of a potent glutamate transporter inhibitor, DL-threo-beta-benzyloxyaspartate (DL-TBOA), on acute morphine-induced antinociception, expression of somatic and negative affective components of morphine withdrawal, and acquisition of morphine-induced conditioned place preference in rats. I.c.v administration of DL-TBOA (10 nmol) to naive rats did not affect the acute antinociceptive effect of morphine. I.c.v. administration of DL-TBOA (10 nmol) to morphine-dependent rats significantly facilitated the expression of naloxone-precipitated somatic signs and conditioned place aversion. DL-TBOA (3 and 10 nmol) significantly facilitated acquisition of morphine-induced conditioned place preference. DL-TBOA itself produced neither conditioned place aversion nor place preference in naive rats. These results suggest that central glutamate transporters play inhibitory roles in the expression of somatic and negative affective components of morphine withdrawal and the reinforcing effect of morphine.

Involvement of Glial Glutamate Transporters in Morphine Dependence

There are several lines of evidence implying the involvement of the central glutamatergic system in morphine dependence. Extracellular glutamate released from nerve terminals is counterbalanced by glutamate transporters in neurons (EAAC1 and EAAT4) and glial cells (GLT-1 and GLAST), thereby modulating the glutamatergic system and protecting neurons from an excitotoxic action of glutamate. Here we show that a glial glutamate transporter GLT-1 could be involved in physical and psychological morphine dependence. By Northern blot analysis, the expression of glial glutamate transporter GLT-1, but not GLAST, mRNA was decreased in the striatum/nucleus accumbens (NAc) and thalamus of morphine-dependent rats. Subcutaneous administration of a glutamate transporter activator suppressed the development of physical morphine dependence and morphine-induced conditioned place preference. Intracerebroventricular administration of a glutamate transporter inhibitor to morphine-dependent rats facilitated the expression of naloxone-precipitated morphine withdrawal-induced somatic signs and conditioned place aversion. Furthermore, gene transfer techniques using recombinant adenoviruses revealed that GLT-1 in the locus coeruleus and NAc shell plays inhibitory roles in physical and psychological morphine dependence, respectively. These findings may provide evidence that a glial glutamate transporter GLT-1 could be a new target for preventing physical and psychological morphine dependence.

Isradipine and dextromethorphan in methadone-maintained humans under a naloxone discrimination procedure

In seven methadone-maintained human participants trained to distinguish between a low dose of naloxone (0.15 mg/70 kg, i.m.; i.e., Drug A) and placebo (i.e., Drug B) under an instructed novel-response drug discrimination procedure, the calcium channel blocker isradipine (0-10 mg/70 kg, p.o.; N=7) and the N-methyl-D-aspartic acid (NMDA) receptor antagonist dextromethorphan (0-60 mg/70 kg, p.o.; N=6) were tested each alone and in combination with the training dose of naloxone. Isradipine alone produced some naloxone- and novel-appropriate responding, minimal changes in self-reports and decreases in blood pressure. Dextromethorphan alone produced some novel-appropriate responding and minimal changes in self-reports and vital signs. When combined with naloxone, isradipine significantly attenuated naloxone-occasioned responding, without increasing novel-appropriate responding, and attenuated naloxone-induced increases in opioid receptor antagonist ratings and ratings measuring sedation. Dextromethorphan significantly attenuated naloxone-appropriate responding, increased novel-appropriate responding, and enhanced naloxone's effects on ratings of dysphoric effects. These results suggest that isradipine attenuates and dextromethorphan enhances some of the behavioral effects of naloxone in opioid-dependent humans.

D-cycloserine-Naloxone interactions in opioid-dependent humans under a novel-response naloxone discrimination procedure

Six participants currently in opioid maintenance treatment were trained to distinguish between naloxone (0.15 mg/70 kg, intramuscularly [i.m.]) and placebo under an instructed novel-response drug-discrimination procedure. Doses of the partial glycine agonist D-cycloserine (0-500 mg/70 kg, per os [P.O.]) alone and combined with naloxone (0.15 mg/70 kg) were then tested. D-cycloserine alone produced minimal drug-appropriate responding, no significant changes in self-reported effects, and increases only in systolic blood pressure. When combined with the naloxone, D-cycloserine partially attenuated naloxone-appropriate responding. D-cycloserine attenuated naloxone-induced increases in visual analog scale ratings of "like naloxone" and scores on the Lysergic Acid Diethyl Amide subscale of the Addiction Research Center Inventory (D. R. Jasinski, 1977; W. R. Martin, J. W. Sloan, J. D. Sapiro, & D. R. Jasinski, 1971). Naloxone attenuated D-cycloserine-induced increases in systolic blood pressure. These results suggest that D-cycloserine at doses up to 500 mg/70 kg may have some limited usefulness in relieving symptoms of opioid withdrawal.

Morphine tolerance and reward but not expression of morphine dependence are inhibited by the selective glutamate carboxypeptidase II (GCP II, NAALADase) inhibitor, 2-PMPA

Inhibition of glutamate carboxypeptidase II (GCP II; NAALADase) produces a variety of effects on glutamatergic neurotransmission. The aim of this study was to investigate effects of GCP II inhibition with the selective inhibitor, 2-PMPA, on: (a) development of tolerance to the antinociceptive effects, (b) withdrawal, and (c) conditioned reward produced by morphine in C57/Bl mice. The degree of tolerance was assessed using the tail-flick test before and after 6 days of twice daily (b.i.d.) administration of 2-PMPA and 10 mg/kg of morphine. Opioid withdrawal was measured 3 days after twice daily morphine (30 or 10 mg/kg) administration, followed by naloxone challenge. Conditioned morphine reward was investigated using conditioned place preference with a single morphine dose (10 mg/kg). High doses of 2-PMPA inhibited the development of morphine tolerance (resembling the effect of 7.5 mg/kg of the NMDA receptor antagonist, memantine) while not affecting the severity of withdrawal. A high dose of 2-PMPA (100 mg/kg) also significantly potentiated morphine withdrawal, but inhibited both acquisition and expression of morphine-induced conditioned place preference. Memantine inhibited the intensity of morphine withdrawal as well as acquisition and expression of morphine-induced conditioned place preference. In addition, 2-PMPA did not affect learning or memory retrieval in a simple two-trial test, nor did it produce withdrawal symptoms in morphine-dependent, placebo-challenged mice. Results suggest involvement of GCP II (NAALADase) in phenomena related to opioid addiction.

Substance P(1-7) affects the expression of dopamine D2 receptor mRNA in male rat brain during morphine withdrawal

Previous studies have confirmed an important role of the undecapeptide substance P (SP) in opioid reward and dependence. It is further shown that the SP N-terminal metabolite SP(1-7) may attenuate the intensity of opioid withdrawal in mice. In this study we have investigated the effect of the heptapeptide fragment on the expression of the brain dopamine D2 receptor mRNA and on the withdrawal reaction, as well, in morphine-dependent rats. Male Sprague-Dawley rats were randomly distributed into two groups. Guide cannula was implanted and aimed at the lateral ventricle and animals were subsequently made opioid dependent by two daily injections of morphine (10 mg/kg) for 7 days. Half an hour before naloxone challenge (2 mg/kg) one group of rats received an injection of SP(1-7) (28 nmol per rat) and the other, serving as control, was injected with saline through the cannula. Animals were decapitated 4 h following SP(1-7) or saline injections. The results indicated that the level of the dopamine D2 receptor transcript was significantly reduced by SP(1-7) in nucleus accumbens and frontal cortex but not altered in the striatum. In behavioral tests it was found that the heptapeptide attenuated several somatic withdrawal symptoms. The observed reduction in the receptor transcript in nucleus accumbens and frontal cortex is suggested to reflect an increased dopamine activity in these areas, which in turn may counteract the withdrawal reaction.

Therapeutic potential of NMDA receptor antagonists in the treatment of alcohol and substance use disorders

Despite the fact that the use of alcohol, nicotine and other drugs is the major external factor contributing to mortality in industrialised countries, there are few medications available to treat alcohol and substance use disorders. In recent years, major advances have been made in the understanding of the neurobiological basis for these disorders and these advances should lead to the development of new pharmacotherapeutics. A substantial amount of the research suggests that N-methyl-D-aspartate (NMDA) receptor neurotransmission contributes to mediating the behavioural effects of alcohol and other drugs of abuse. This research supports the therapeutic potential of NMDA receptor antagonists in alcohol and substance use disorders. In this paper the authors present their opinion on the goals and stages of pharmacological treatment of these complex psychiatric disorders. Available preclinical research using designs that model aspects of alcohol and substance use disorders is summarised, with an emphasis on research published in the last two years. In animal models, NMDA antagonists inhibit physical dependence and the reinforcing effects of a variety of abused substances. The ability of NMDA antagonists to inhibit tolerance to drug effects and contribute possible antidepressant and anxiolytic effects are also important from the perspective of drug development. This review summarises the relevant clinical laboratory and treatment data. Finally, it presents the status of the current development of NMDA receptor antagonists and discusses candidates with the greatest potential for clinical development.

Intracerebroventricular injection of the N-terminal substance P fragment SP(1-7) regulates the expression of the N-methyl-D-aspartate receptor NR1, NR2A and NR2B subunit mRNAs in the rat brain

Studies have implicated substance P (SP) in the regulation of affective behaviour, memory function, pain influx, stress and opioid reward. All these dimensions are known to involve glutamate transmission mediated through the N-methyl-D-aspartate (NMDA) receptor. The SP N-terminal fragment SP(1-7) is shown to share some but oppose other effects of the parent compound. We have examined the effect of intracerebroventricular injections of SP(1-7) on the expression of the NMDA receptor subunits NR1, NR2A and NR2B mRNAs in the spinal cord and in discrete areas of the male rat brain. The results indicated that the heptapeptide induced a dose-dependent upregulation of the NR2A transcript in hippocampus, periaqueductal grey and ventral tegmental area, already within a few hours. The level of the NR2B mRNA was increased in hippocampus and nucleus accumbens. The expression of the transcript of the NR1 was enhanced in hippocampus and nucleus accumbens but attenuated in spinal cord. The observed effects of the SP(1-7) fragment are in agreement with what could be expected from the known effects of the heptapeptide on various behaviours involving glutamate transmission.

The effect of the glycine/NMDA receptor antagonist, (+)-HA966, on morphine dependence in neuropathic rats

We have previously shown that rats with a painful peripheral neuropathy develop dependence without tolerance after repetitive doses [3mg/kg subcutaneously (s.c.)] of morphine. After injections of a higher dose (10mg/kg s.c.) the animals develop tolerance that can be prevented by the glycine/N-methyl-D-aspartate (NMDA) receptor antagonist, (+)-HA966. This study examined whether (1) dependence develops also after repetitive doses of 10mg/kg of morphine and, if so, (2) whether (+)-HA966 prevents the development of dependence after both the low and the higher morphine pretreatment doses. A 4day pretreatment regimen (post-operative days 12-16) with two daily s.c. injections of saline+saline, saline+morphine (3 or 10mg/kg), (+)-HA966 (2.5 or 5mg/kg)+morphine or (+)-HA966 (5mg/kg)+saline was used, and withdrawal was precipitated by an injection of naloxone [2mg/kg intravenously (i.v.)] at 17h after the last pretreatment injection. Three signs of withdrawal (exploring, writhing, ptosis) appeared after pretreatment with both doses of morphine alone, while other signs (teeth chattering, pilo-erection) developed only after injections at the 3mg/kg dose. One sign (penile grooming/erection) appeared only after the higher morphine dose. Pretreatment with the combination of (+)-HA966 and morphine at 3mg/kg prevented the development of all withdrawal signs. By contrast, except for exploring, (+)-HA966 did not modify the incidence of the withdrawal signs observed after pretreatment with doses of 10mg/kg of morphine. The results suggest that prevention of the development of morphine dependence by glycine/NMDA receptor antagonism depends on the degree of morphine dependence.

In search of a new pharmacological treatment for drug and alcohol addiction: N-methyl-D-aspartate (NMDA) antagonists

The most challenging aspect of treating alcohol and drug addiction is the relapsing course of these disorders. Although substitution therapies for nicotine and opioid dependence have proven to be relatively effective, there is a need for new pharmacotherapies designed to decrease the frequency and severity of relapse. The aim of this paper is to provide an overview of the potential utility of N-methyl-D-aspartate (NMDA) receptor antagonists as treatments for substance abuse as shown in preclinical models and preliminary clinical trials. It is hypothesized that NMDA receptors mediate the common adaptive processes that are involved the development, maintenance, and expression of drug and alcohol addiction. Modulation of glutamatergic neurotransmission with NMDA receptor antagonists offers a novel treatment approach. It is proposed that NMDA antagonists may have multiple functions in treating addictions, including an attenuation of withdrawal effects, normalization of the affective changes following initiation of abstinence which arise from neurochemical changes resulting from chronic addiction, and an attenuation of conditioned responses arising from drug-related stimuli.

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.

Prevention of morphine-induced muscarinic (M2) receptor adaptation suppresses the expression of withdrawal symptoms

Treatment of opiate addiction is generally directed at the suppression of withdrawal symptoms through maintenance of the 'addicted' state with methadone. Yet relatively little is known regarding the neural substrates that contribute to, and maintain the prolonged state of withdrawal experienced by addicts. Opiates can profoundly alter the dynamics of brain and peripheral cholinergic systems, and central administration of anticholinergic drugs in dependent rats has been shown to decrease the expression of precipitated withdrawal symptoms. The purpose of this study was to determine whether the adaptive changes to M2 muscarinic receptors in autonomic centers are linked to the expression of withdrawal phenomena. During the peak period of withdrawal, there was a significant increase in both the expression of M2 muscarinic receptors and its corresponding mRNA within the rostral ventrolateral medulla, a primary vasomotor region. That most of these changes in receptor expression were adaptive in nature was suggested by the fact that when the acetylcholinesterase inhibitor DFP was co-administered with morphine, both the increased mRNA expression and the appearance of withdrawal symptoms were inhibited. Thus, interference with morphine-induced M2 muscarinic receptor adaptation in critical brain regions was correlated with a reduction in the development of physical dependence.