The noncompetitive NMDA antagonist MK-801 fails to block amphetamine-induced place conditioning in rats

Segregation of caffeine reward and aversion in the rat nucleus accumbens shell versus core

Caffeine, the most commonly consumed psychoactive drug in the world, is readily available in dietary sources, including soft drinks, chocolate, tea and coffee. However, little is known about the neural substrates that underlie caffeine's rewarding and aversive properties and what ultimately leads us to seek or avoid caffeine consumption. Using male Wistar rats in a place conditioning procedure, we show that systemic caffeine at a low intraperitoneal dose of 2 mg/kg (or 100 µM injected directly into the rostral, but not caudal, portion of the ventral tegmental area) produced conditioned place preferences. By contrast, high doses of systemic caffeine at 10 and 30 mg/kg produced conditioned place aversions. These aversions were not recapitulated by a caffeine analog restricted to the periphery. Both caffeine reward and aversion were blocked by systemic D1-like receptor antagonism using SCH23390, while systemic D2-like receptor antagonism with eticlopride had smaller effects on caffeine motivation. Most important, we demonstrated that pharmacological blockade of dopamine receptors using α-flupenthixol injected into the nucleus accumbens shell, but not core, blocked caffeine-conditioned place preferences. Conversely, α-flupenthixol injected into the nucleus accumbens core, but not shell, blocked caffeine-conditioned place aversions. Thus, our findings reveal two dopamine-dependent and functionally dissociable mechanisms for processing caffeine motivation, which are segregated between nucleus accumbens subregions. These data provide novel evidence for the roles of the nucleus accumbens subregions in mediating approach and avoidance behaviours for caffeine.

Blockade of ventral midbrain NMDA receptors enhances brain stimulation reward: a preferential role for GluN2A subunits

Ventral midbrain (VM) neurons that project to limbic structures play a role in reward and incentive motivation. It has been suggested that a reward-related signal is transmitted when the firing rate of VM dopamine neurons shifts from a tonic to a phasic mode. Since glutamate is necessary for this transduction process, it is likely to play a role in reward signaling. This study was aimed at determining the effect of VM N-Methyl-D-Aspartate (NMDA) receptor blockade on reward induced by electrical brain stimulation. Experiments were performed on rats trained to self-administer an electrical stimulation in the medial posterior mesencephalon. Reward thresholds were measured with the curve-shift paradigm before and after bilateral VM injections of the following NMDA receptor antagonists: R-CPP, 3-(R-2-Carboxypiperazin-4-yl)-propyl-1 phosphonic acid, (0, 20.6, 41.2 and 82.5 pmol/0.5 μl/side), PPPA, (2R,4S)-4-(3-Phosphonopropyl)-2-piperidinecarboxylic acid, (0, 0.825 and 1.65 nmol/0.5 μl/side) orRo04-5595, 1-[2-(4-Chlorophenyl)ethyl]-1,2,3,4-tetrqahydro-6-methoxy-2-methyl-7-isoquinolinol hydrochloride (0, 0.825, 1.65 nmol/0.5 μl/side). R-CPP and PPPA produced a dose and time dependent decrease in reward threshold, an effect that was, at some doses and times after the injection, accompanied by an increase in maximum responses. These effects were not observed with Ro04-5595 over the range of doses tested. While previous studies suggest a role for glutamate in reward signaling, the present results show that VM glutamate exerts a tonic inhibition on the reward-relevant pathway. The selectivity of Ro04-5595 for NMDA receptors composed of GluN2B subunits and the higher affinity of R-CPP and PPPA for GluN2A suggest that the inhibition is mediated by receptors composed of GluN2A subunits.

The competitive NMDA receptor antagonist CPP disrupts cocaine-induced conditioned place preference, but spares behavioral sensitization

Recently, the notion that memory and addiction share similar neural substrates has become widely accepted. N-methyl-d-aspartate receptors (NMDAR) are the cornerstones of synaptic models of memory. The present study examined the effect of the competitive NMDAR antagonist CPP on the induction of behavioral sensitization and conditioned place preference to cocaine. Conditioned place preference is an associative memory model of drug seeking, while sensitization is a non-associative model of the transition from casual to compulsive use. There were three principal findings: (1) co-administration of CPP and cocaine altered the acute response to cocaine, suggesting a direct interaction between the two drugs; (2) NMDAR antagonism had no effect on behavioral sensitization; and (3) NMDAR antagonism abolished conditioned place preference. A review of prior evidence supporting a role for NMDARs in sensitization suggests that NMDAR antagonists directly interfere with cocaine's psychostimulant effects, and this interaction could be misinterpreted as a disruption of sensitization. Finally, we suggest that addiction recruits multiple kinds of plasticity, with sensitization recruiting NMDAR-independent mechanisms.

Role of serine racemase in behavioral sensitization in mice after repeated administration of methamphetamine

Background

The N-methyl-D-aspartate (NMDA) receptors play a role in behavioral abnormalities observed after administration of the psychostimulant, methamphetamine (METH). Serine racemase (SRR) is an enzyme which synthesizes D-serine, an endogenous co-agonist of NMDA receptors. Using Srr knock-out (KO) mice, we investigated the role of SRR on METH-induced behavioral abnormalities in mice.

Methodology/Principal Findings

Evaluations of behavior in acute hyperlocomotion, behavioral sensitization, and conditioned place preference (CPP) were performed. The role of SRR on the release of dopamine (DA) in the nucleus accumbens after administration of METH was examined using in vivo microdialysis technique. Additionally, phosphorylation levels of ERK1/2 proteins in the striatum, frontal cortex and hippocampus were examined using Western blot analysis. Acute hyperlocomotion after a single administration of METH (3 mg/kg) was comparable between wild-type (WT) and Srr-KO mice. However, repeated administration of METH (3 mg/kg/day, once daily for 5 days) resulted in behavioral sensitization in WT, but not Srr-KO mice. Pretreatment with D-serine (900 mg/kg, 30 min prior to each METH treatment) did not affect the development of behavioral sensitization after repeated METH administration. In the CPP paradigm, METH-induced rewarding effects were demonstrable in both WT and Srr-KO mice. In vivo microdialysis study showed that METH (1 mg/kg)-induced DA release in the nucleus accumbens of Srr-KO mice previously treated with METH was significantly lower than that of the WT mice previously treated with METH. Interestingly, a single administration of METH (3 mg/kg) significantly increased the phosphorylation status of ERK1/2 in the striatum of WT, but not Srr-KO mice.

Conclusions/Significance

These findings suggest first, that SRR plays a role in the development of behavioral sensitization in mice after repeated administration of METH, and second that phosphorylation of ERK1/2 by METH may contribute to the development of this sensitization as seen in WT but not Srr-KO mice.

Repeated treatment with the NMDA antagonist MK-801 disrupts reconsolidation of memory for amphetamine-conditioned place preference

Long-lasting drug-associated memories can contribute to relapse; therefore these memories must be inactivated to enable sustainable success in addiction therapy. As drug associations are usually acquired over several conditioning events, we assume that an effective treatment should be repeatedly applied to achieve persistent effects. In this study, we examine whether 10 repeated memory reactivation tests followed by systemic N-methyl-D-aspartate receptor antagonist MK-801 (0.1 mg/kg) administrations can disrupt memory reconsolidation in rats, leading to a reduction of well-established amphetamine-conditioned place preference (CPP). We found that immediate (but not 60-min delayed) administration of MK-801 after the tests reduced amphetamine-CPP expression after at least four treatments. These effects were specific to CPP expression as no MK-801-induced change in locomotion was observed during all tests. We discuss these results as being caused by MK-801 disrupting memory reconsolidation and we propose the applied repeated-treatment regimen as a new therapeutic research strategy to persistently disrupt drug-associated memories.

Gene transfer of GLT-1, a glutamate transporter, into the nucleus accumbens shell attenuates methamphetamine- and morphine-induced conditioned place preference in rats

Several lines of evidence have suggested that the glutamatergic system in the nucleus accumbens (NAc) plays an important role in the conditioned rewarding effect of drugs of abuse. In addition, it is recognized that extracellular glutamate is rapidly removed from the synaptic cleft by Na+-dependent glutamate transporters in neurons and glial cells, thereby maintaining physiological levels of glutamate. We previously reported that activation of glutamate uptake by a glutamate transporter activator attenuated the acquisition of conditioned place preference induced by methamphetamine and morphine in mice. In the present study, we examined the effects of gene transfer of a glial glutamate transporter, GLT-1, into the NAc shell by recombinant adenoviruses on methamphetamine- and morphine-induced conditioned place preference in rats. Bilateral infusion of the recombinant adenoviruses into the NAc shell efficiently increased GLT-1 expression surrounding the infusion site, at least during the period 2-8 days after the infusion. In the conditioned place preference paradigm, animals were conditioned with repeated subcutaneous injections of methamphetamine (2 mg/kg) or morphine (3 mg/kg). Intra-NAc shell overexpression of GLT-1 before the conditioning significantly attenuated the conditioned place preference induced by methamphetamine or morphine, when compared with control. However, it had no effect on the somatic signs of naloxone-precipitated morphine withdrawal. These results suggest that GLT-1 within the NAc shell plays an inhibitory role in the conditioned rewarding effects of methamphetamine and morphine but not the physical dependence on morphine.

Effect of glutamate receptor antagonists on place aversion induced by naloxone in single-dose morphine-treated rats

The neurobiological mechanism underlying the negative motivational component of withdrawal from acute opiate dependence is far from understood. Our objectives were to determine whether the glutamatergic system is involved in the motivational component of morphine withdrawal in acutely dependent rats and such an involvement is associated with dopaminergic neurotransmission. We examined the effects of various kinds of glutamate receptor antagonists on conditioned place aversion (CPA) induced by naloxone-precipitated withdrawal from a single morphine exposure 24 h before. Furthermore, the influence of pretreatment with the dopamine receptor antagonist haloperidol on those effects of glutamate receptor antagonists was also investigated. CPA was attenuated in a dose-dependent manner by all glutamate receptor antagonists examined including the NMDA receptor antagonists (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine maleate (MK-801) and phencyclidine hydrochloride (PCP), AMPA receptor antagonist 1-(4-aminophenyl)4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466), and metabotropic receptor antagonists (+/-)-2-amino-3-phosphonopropionic acid (AP-3) and (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG). The effects of MK-801, GYKI 52466 and MCPG were blocked by haloperidol. These results suggest that the glutamatergic system involving multiple classes of receptors plays a role in the motivational component of withdrawal from acute morphine dependence, and the function of the glutamatergic system would be closely associated with dopaminergic neurotransmission.

NMDA or AMPA/kainate receptor blockade prevents acquisition of conditioned place preference induced by D(2/3) dopamine receptor stimulation in rats

RATIONALE

Recent experiments from this laboratory demonstrated synergistic effects of AMPA/kainate receptor blockade and D(2/3) dopamine (DA) receptor stimulation on brain stimulation reward and locomotor activity.

OBJECTIVES

Using place conditioning, this study explored further the interaction between DA and glutamate (Glu) using the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, the AMPA/kainate receptor antagonist NBQX, and the D(2/3) DA receptor agonist 7-OH-DPAT.

METHODS

Effects of these compounds, alone and combined, were measured in male Sprague--Dawley rats using an unbiased two-compartment place conditioning procedure.

RESULTS

7-OH-DPAT (0.03--5.0 mg kg(-1), s.c.) administered immediately prior to conditioning was ineffective; when administered 15 min prior to conditioning, only the highest dose (5.0 mg kg(-1), s.c.) induced conditioned place preference (CPP). Acquisition of 7-OH-DPAT-induced CPP was blocked by MK-801 (0.06 or 0.13 mg kg(-1), i.p.) or NBQX (0.5 microg) microinjected into the nucleus accumbens (NAS) shell subregion. Intra-NAS shell administration of 7-OH-DPAT (5.0 microg) or NBQX (0.5 microg), alone or combined, failed to induce place conditioning, and this lack of effect was not due to state dependency. Administration of MK-801 or 7-OH-DPAT (5.0 mg kg(-1)) during the conditioning phase acutely increased horizontal activity, but neither compound, alone or combined, induced conditioned locomotor effects.

CONCLUSIONS

Acquisition of place conditioning induced by systemic administration of 7-OH-DPAT is blocked by systemic NMDA receptor antagonism by MK-801 or by the AMPA/kainate receptor antagonist NBQX microinjected into the NAS shell subregion.

Effect of MS-153, a glutamate transporter activator, on the conditioned rewarding effects of morphine, methamphetamine and cocaine in mice

There is a body of evidence implying the involvement of the glutamatergic system in the conditioned rewarding effects of drugs of abuse. It is recognized that the release of extracellular glutamate from nerve terminals is counterbalanced by the functions of neuronal and glial glutamate transporters. In the present study, we investigated the effects of (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline (MS-153), a glutamate transporter activator, on the induction of the conditioned place preference to morphine, methamphetamine and cocaine in mice. In the conditioned place preference paradigm, mice were conditioned with repeated subcutaneous injections of morphine (5 mg/kg), methamphetamine (2 mg/kg) or cocaine (8 mg/kg) in combination with or without MS-153 (3 and 10 mg/kg). Co-administration of MS-153 at a dose of 10 mg/kg, but not 3 mg/kg, significantly attenuated the induction of conditioned place preference to morphine, methamphetamine and cocaine. However, MS-153 itself produced neither conditioned place preference nor aversion. On the other hand, co-administration of MS-153 (10 mg/kg) did not alter the acute locomotor activation elicited by a single injection of morphine, methamphetamine and cocaine. These results suggest that MS-153, a glutamate transporter activator, has an inhibitory effect on the conditioned rewarding effects of morphine, methamphetamine and cocaine without affecting their acute locomotor responses.

Effects of NMDA receptor antagonists (MK-801 and memantine) on the acquisition of morphine-induced conditioned place preference in mice

Several studies have shown that the systemic administration of a variety of N-methyl-D-aspartate (NMDA) receptor antagonists can block the development or expression of conditioned place preference (CPP) induced by rewarding drugs such as morphine. In the present study, we examined the effects of different doses of two non-competitive NMDA receptor antagonists, MK-801 (0.1, 0.2 and 0.3 mg/kg) and memantine (2.5, 5, 10, 20 and 40 mg/kg), in CPP induced by 40 mg/kg of morphine in male mice. The CPP was carried out with an unbiased procedure in terms of initial spontaneous preference. Animals received the different doses of drugs in the conditioning sessions. MK-801 and memantine, at all doses used, produced neither place preference nor place aversion, but the higher doses of memantine (20 and 40 mg/kg) were able to completely block morphine-induced CPP. The present data show that the NMDA receptor antagonists MK-801 and memantine have no reinforcing properties but memantine is capable of preventing the acquisition of morphine-induced CPP. These results suggest that the development of morphine-induced CPP may be closely related to NMDA receptors and that the glutamatergic system can modulate opiate reward.

Behavioural adaptations to addictive drugs in mice lacking the NMDA receptor epsilon1 subunit

N-methyl-D-aspartate (NMDA) receptors, a subtype of glutamate receptors (GluRs) formed by assembly of the GluRzeta subunit (called NR1 in rats) with any one of four GluRepsilon subunits (GluRepsilon1-4; NR2A-D), play an important role in excitatory neurotransmission, synaptic plasticity and brain development. Recent pharmacological studies have also indicated a role for NMDA receptors in drug addiction. In the present study, we investigated the behavioural adaptations to addictive drugs such as phencyclidine (PCP), methamphetamine (MAP) and morphine (MOR) in mice lacking the GluRepsilon1 subunit of the NMDA receptor. GluRepsilon1 mutant mice exhibited a malfunction of NMDA receptors, as evidenced by the reduction of [3H]MK-801 binding in an autoradiographic receptor binding assay. GluRepsilon1 mutant mice showed an attenuation of acute PCP- and MAP-induced hyperlocomotion. The development of sensitization by repeated treatment with PCP and MAP at a low, but not high, dose was also suppressed. The development of MOR-induced analgesic tolerance and naloxone-precipitated MOR withdrawal symptoms were attenuated in GluRepsilon1 mutant mice. In the place conditioning test, PCP-induced place aversion in naive mice and place preference in PCP-pretreated mice, as well as MOR-induced place preference, were diminished whereas MAP-induced place preference was not affected in GluRepsilon1 mutant mice. These findings provide genetic evidence that GluRepsilon1 subunit-containing NMDA receptors are involved in certain aspects of drug addiction.

Influence of acute or repeated restraint stress on morphine-induced locomotion: involvement of dopamine, opioid and glutamate receptors

The development of restraint stress-induced sensitization to the locomotor stimulating effect of morphine (2 mg/kg i.p.) was investigated. In experiment 1, both a single restraint session (2 h) and a repeated restraint stress (2 h per day for 7 days), similarly enhanced the effects of morphine on motor activity. In experiment 2, we observed that this sensitization was prevented by administration of both D(1) and D(2) dopaminergic antagonist [SCH-23390 (0.5 mg/kg i.p.) and (+/-)-sulpiride (60 mg/kg i.p.)] 10 min prior to the stress session. In experiment 3, we showed that an opioid antagonist pretreatment [naltrexone (1 mg/kg i.p.) 10 min prior to stress session, suppressed the stress-induced sensitization after morphine administration. In experiment 4, pretreatment with a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) type of glutamate receptors [(+)-MK-801 (0.1 mg/kg i.p.)], 30 min prior to the acute restraint session, prevented the development of sensitization to morphine. All these results suggest that: (1) sensitization to morphine on stimulating locomotor effect does not depend on the length of exposure to stress (acute vs. repeated); (2) stimulation of both D(1) and D(2) dopaminergic receptors is necessary for the development of restraint stress-induced sensitization to morphine; (3) an opioid system is also involved in this sensitization process; and (4) the stimulation of glutamatergic NMDA receptors is involved in this acute restraint-induced effect.

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.

Lack of effects of glycineB receptor ligands on the psychostimulant-induced discriminative stimuli in rats

To examine the role of glycineB receptors in the stimulus effects induced by psychostimulants, separate groups of rats were trained to discriminate amphetamine (AMPH; 1 mg/kg) from saline (SAL), or cocaine (COC; 10 mg/kg) from SAL, using a two-lever operant procedure. Substitution studies showed that neither 1-aminocyclopropanecarboxylic acid (ACPC; 200 mg/kg) nor 7-chloro-4-hydroxy-3-(3-phenoxy)phenyl-(H)quinolone (L-701,324; 3 mg/kg), being a partial agonist or an antagonist at glycineB receptors, respectively, generalized for the training drugs. Combination tests of glycineB ligands demonstrated that injection of a fixed dose of ACPC (200 mg/kg) or L-701,324 (3 mg/kg) together with different doses of AMPH or COC practically did not modify dose-response curves of the psychostimulants, nor did it affect their ED50 values. Our results indicate that glycineB receptors do not play a role in the discriminative effects of AMPH and COC.

Functional heterogeneity of the rat medial prefrontal cortex: effects of discrete subarea-specific lesions on drug-induced conditioned place preference and behavioural sensitization

While the principal components of the brain reward system, the nucleus accumbens septi and the ventral tegmental area have received much attention, their efferent and afferent structures have not been investigated to the same degree. One major input to this system originates from the medial prefrontal cortex (mPFC) which is not a homogenous structure but can be divided into different subareas that can be distinguished on anatomical and possibly functional grounds. We examined the effects of discrete bilateral quinolinic acid lesions (45 nmol/0.5 micro(L)) of each of the mPFC subareas, the infralimbic (il), prelimbic (pl) and the anterior cingulate (cg) mPFC, on the conditioned place preference (CPP) and psychomotor activation induced by several drugs. Lesions of the il mPFC blocked CPP induced by morphine (10 mg/kg) and CGP37849 [DL-(E)-2-amino-4-methyl-5-phosphono-3-pentic acid, a competitive N-methyl-D-aspartate receptor antagonist; 10 mg/kg]. Lesions of the pl mPFC blocked CPP induced by cocaine (15 mg/kg) and CGP37849, and lesions of the cg mPFC only blocked CGP37849-induced CPP. Lesions of the whole mPFC blocked morphine-, cocaine- and CGP37849-induced CPP. None of the lesions affected DL-amphetamine (4 mg/kg)-induced CPP. During the conditioning period, none of the lesions affected amphetamine-induced psychomotor activation and sensitization, whereas both phenomena were attenuated by pl and whole mPFC lesions in the case of cocaine, and by il and whole mPFC lesions in the case of morphine. These results show that the different mPFC subregions have distinct functional roles in the generation of behavioural effects produced by different classes of drugs. This heterogeneity should be taken into account in future studies addressing the role of the mPFC in drug reward and sensitization.

Effects of calcium channel blockers on behaviors induced by the N-methyl-D-aspartate receptor antagonist, dizocilpine, in rats

The present study assessed the ability of voltage-sensitive calcium channel (VSCC) blockers to affect the behavioral effects of the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine, in male Wistar rats. Dizocilpine produced dose-dependent increases in locomotor activity. Nimodipine, verapamil, and flunarizine suppressed dizocilpine-facilitated vertical activity, while horizontal activity was attenuated by verapamil and nimodipine but not flunarizine. Repeated dizocilpine injections resulted in the development of sensitization to its locomotor stimulating properties. Development of sensitization was not context specific, and was observed following repeated exposures to 0.1 but not 0.056 or 0.3 mg/kg of dizocilpine. Nimodipine retarded the development of sensitization to dizocilpine's stimulating effects on horizontal activity, while verapamil suppressed sensitization to the vertical stimulating effects of dizocilpine. Flunarizine had no significant effects on sensitization to dizocilpine's locomotor stimulating properties. In rats trained to discriminate between injections of 0.056 mg/kg of dizocilpine and vehicle, none of the tested VSCC blockers was able to completely antagonize the discriminative stimulus properties of dizocilpine. Nimodipine, when administered in combination with the training dose of dizocilpine, modestly decreased the dizocilpine-lever selection. Dizocilpine dose dependently decreased the self-determined stimulation threshold implanted in rats with electrodes into the ventral tegmental area. Nimodipine exhibited some tendency to block the facilitating effects of dizocilpine, while verapamil and flunarizine had no effects. In summary, in the present experiments VSCC blockers exerted only modest interactions with the behavioral effects of dizocilpine, and it is unlikely that VSCC blockers have remarkable potential as adjunct treatment aimed at correcting the negative side effects of NMDA receptor antagonists (e.g., dizocilpine).

Low-affinity NMDA receptor channel blockers inhibit acquisition of intravenous morphine self-administration in naive mice

Experimental evidence suggests that NMDA receptor antagonists modulate behavioral effects of morphine in models assessing abuse potential of drugs. The present study sought to evaluate the ability of NMDA receptor channel blockers to affect the acquisition of morphine i.v. self-administration in drug- and experimentally naive mice. DBA/2 mice were allowed to self-administer morphine (0.125-4.0 mg/ml) or saline during the 30-min test. Each nose-poke of the active mouse resulted in a 1.6-microl infusion to both the active mouse and the passive (yoked control) mouse. In vehicle-treated mice, differences between operant activity of active and passive mice were most obvious when active mice were allowed to self-inject morphine at the concentration of 0.5 mg/ml (the optimum concentration). Pretreatment with MRZ 2/579 (1-amino-1,3,3,5,5-pentamethyl-cyclohexan hydrochloride; 1, 3.2 and 10 mg/kg) shifted the optimum concentration to 0.75 mg/ml. Memantine (1-amino-3,5-dimethyladamantane hydrochloride; 0.3, 1, 3.2 and 10 mg/kg) suppressed both the morphine intake and the difference in nose-poke activity of active vs. passive mice across all tested concentrations of morphine. Dizocilpine ((+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate; 0.1 mg/kg) was ineffective. Taken together with earlier reports, the present results suggest that low-affinity NMDA receptor channel blockers--in contrast to dizocilpine--attenuate the rewarding potential of morphine.

Place conditioning of mice with the NMDA receptor antagonists, eliprodil and dizocilpine

Effects of noncompetitive and competitive NMDA receptor antagonists have been repeatedly characterized using place conditioning models. The present study aimed to characterize the effects in mice of another NMDA receptor antagonist acting at polyamine binding site, eliprodil. Five-day conditioning with eliprodil (1-30 mg/kg, i.p.) resulted in a dose-dependent avoidance of an eliprodil-paired compartment during post-conditioning tests. These effects were: (i) observed both with eliprodil and without drug, and (ii) less pronounced in individually housed mice subjected to repeated social defeats and mild footshocks prior to and during the conditioning period (compared to group-housed and individually housed nonstressed mice). In a parallel set of experiments, the effects of dizocilpine (MK-801; 0.03-0.3 mg/kg, i.p.) were evaluated using the same study design as for eliprodil. Conditioned place preference was established with the dizocilpine dose of 0.3 mg/kg and this effect was not affected by housing/stressing or drug exposures during the test.

Measuring reward with the conditioned place preference paradigm: a comprehensive review of drug effects, recent progress and new issues

This review gives an overview of recent findings and developments in research on brain mechanisms of reward and reinforcement from studies using the place preference conditioning paradigm, with emphasis on those studies that have been published within the last decade. Methodological issues of the paradigm (such as design of the conditioning apparatus, biased vs unbiased conditioning, state dependency effects) are discussed. Results from studies using systemic and local (intracranial) drug administration, natural reinforcers, and non-drug treatments and from studies examining the effects of lesions are presented. Papers reporting on conditioned place aversion (CPA) experiments are also included. A special emphasis is put on the issue of tolerance and sensitization to the rewarding properties of drugs. Transmitter systems that have been investigated with respect to their involvement in brain reward mechanisms include dopamine, opioids, acetylcholine, GABA, serotonin, glutamate, substance P, and cholecystokinin, the motivational significance of which has been examined either directly, by using respective agonist or antagonist drugs, or indirectly, by studying the effects of these drugs on the reward induced by other drugs. For a number of these transmitters, detailed studies have been conducted to delineate the receptor subtype(s) responsible for the mediation of the observed drug effects, particularly in the case of dopamine, the opioids, serotonin and glutamate. Brain sites that have been implicated in the mediation of drug-induced place conditioning include the 'traditional' brain reward sites, ventral tegmental area and nucleus accumbens, but the medial prefrontal cortex, ventral pallidum, amygdala and the pedunculopontine tegmental nucleus have also been shown to play important roles in the mediation of place conditioning induced by drugs or natural reinforcers. Thus, although the paradigm has also been criticized because of some inherent methodological problems, it is clear that during the past decade place preference conditioning has become a valuable and firmly established and very widely used tool in behavioural pharmacology and addiction research.

Continued trends in the conditioned place preference literature from 1992 to 1996, inclusive, with a cross-indexed bibliography

In light of the overwhelming response to the previous publication in Neuroscience and Biobehavioral Reviews (1993, 17, 21-41) regarding trends in place conditioning (either preference or aversion), the present work constitutes a five-year follow-up to review the empirical research in this behavioral paradigm from 1992 to 1996, inclusively. The behavioral technique has grown as indicated by the number of publications over the last five years which equals those authored over the 35 years covered by our last survey. The previous work used descriptive statistics to explore topical issues, whereas the present work discusses trends since that time and hopes to provide an exhaustive bibliography of the CPP literature, including articles, published abstracts, book chapters and reviews, as well as providing a cross-index of identified key words/drugs tested.

The role of excitatory amino acids in behavioral sensitization to psychomotor stimulants

Behavioral sensitization refers to the progressive augmentation of behavioral responses to psychomotor stimulants that develops during their repeated administration and persists even after long periods of withdrawal. It provides an animal model for the intensification of drug craving believed to underlie addiction in humans. Mechanistic similarities between sensitization and other forms of neuronal plasticity were first suggested on the basis of the ability of N-methyl-D-aspartate (NMDA) receptor antagonists to prevent the development of sensitization [Karler, R., Calder, L. D., Chaudhry, I. A. and Turkanis, S. A. (1989) Blockade of "reverse tolerance" to cocaine and amphetamine by MK-801. Life Sci., 45, 599-606]. This article will review the large number of subsequent studies addressing: (1) the roles of NMDA, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and metabotropic glutamate receptors in the development and expression of behavioral sensitization, (2) excitatory amino acids (EAAs) and the role of conditioning in sensitization, (3) controversies regarding EAA involvement in behavioral sensitization based on studies with MK-801, (4) the effects of acute and repeated stimulant administration on EAA neurochemistry and EAA receptor expression, and (5) the neuroanatomy of EAA involvement in sensitization. To summarize, NMDA, AMPA metabotropic glutamate receptors all participate in the development of sensitization, while maintenance of the sensitized state involves alterations in neurochemical measures of EAA transmission as well as in the expression and sensitivity of AMPA and NMDA receptors. While behavioral sensitization likely involves complex neuronal circuits, with EAAs participating at several points within this circuitry, EAA projections originating in prefrontal cortex may play a particularly important role in the development of sensitization, perhaps via their regulatory effects on midbrain dopamine neurons. The review concludes by critically evaluating various hypotheses to account for EAA involvement in the development of behavioral sensitization, and considering the question of whether EAA receptors are involved in mediating the rewarding effects of psychomotor stimulants and sensitization of such rewarding effects.

Blockade of morphine- and amphetamine-induced conditioned place preference in the rat by riluzole

Previous studies have shown that antagonists at glutamatergic N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazolpropionic acid (AMPA) receptors can disrupt the development or expression, respectively, of conditioned place preference (CPP) induced by drugs of abuse. The present study examined the effects of inhibition of presynaptic glutamate release by riluzole on the development of morphine- and amphetamine-induced CPP. Morphine (10 mg/kg), D,L-amphetamine (4 mg/kg) and riluzole (4 mg/kg) itself each produced a significant CPP; however, when riluzole was co-administered with morphine or amphetamine during the conditioning sessions, no CPP developed. It is concluded that non-specific disruption of glutamatergic neurotransmission prevents the development of morphine- and amphetamine-induced CPP.

MK-801 inhibits methamphetamine-induced conditioned place preference and behavioral sensitization to apomorphine in mice

Intraperitoneal administration of MK-801 (0.1 mg/ kg), an N-methyl-D-aspartate (NMDA) receptor antagonist, before and during methamphetamine treatment inhibited methamphetamine-induced conditioned place preference (CPP) in mice. Behavioral sensitization to a dopamine (DA) receptor agonist apomorphine that developed in methamphetamine-induced CPP mice was also inhibited by MK-801. Furthermore, MK-801 inhibited apomorphine-induced postsynaptic dopaminergic action, cage-climbing behavior. Therefore, the present studies suggest that methamphetamine-induced behaviors, such as CPP and behavioral sensitization, may be closely related to the dopaminergic activation mediated via the NMDA receptor. The behavioral sensitization to apomorphine may be a possible underlying mechanism of methamphetamine-induced CPP, because behavioral sensitization developed in methamphetamine-induced CPP mice, as well as apomorphine-induced climbing behavior in mice, were inhibited by MK-801.

Synergistic effects of cocaine and dizocilpine (MK-801) on brain stimulation reward

The rewarding effects of lateral hypothalamic electrical stimulation were assessed in animals treated with the combination of cocaine and dizocilpine (MK-801), a noncompetitive N-methyl-D-aspartate antagonist. Eight male Long-Evans rats were trained to perform a lever-press operant to deliver trains of cathodal rectangular pulses directly into the lateral hypothalamus. Response rate was determined across the range of effective stimulation frequencies. For each rat the frequency threshold was defined as the lowest frequency that sustained minimal responding. After thresholds had stabilized each rat was tested under 4 treatment conditions; saline + saline, dizocilpine (0.05 mg/kg, i.p., 30 min before test) + saline, saline + cocaine (4 mg/kg, i.p., 5 min before test) and dizocilpine + cocaine. The saline + saline, dizocilpine + saline and saline + cocaine treatments each failed to cause significant changes in threshold or maximum response rates. The dizocilpine + cocaine treatment produced a large reduction in thresholds indicating a synergism between the two drugs and the rewarding stimulation. These synergistic effects of dizocilpine and cocaine stand in contrast to the putative antagonism by dizocilpine of cocaine's psychomotor-sensitizing action.

Interactions of MK-801 and GYKI 52466 with morphine and amphetamine in place preference conditioning and behavioural sensitization

In an earlier study we showed that co-administration of the NMDA-receptor antagonist MK-801 during conditioning sessions blocks morphine-induced conditioned place preference (CPP). From this result, the question arose of whether this blockade is due to state-dependency effects induced by MK-801. Therefore, in a first experiment, animals were tested in the drugged state under which they had been conditioned. These animals did not show a CPP, thus it can be concluded that MK-801 does not make state-dependent the morphine conditioning. In the same experiment those animals receiving only morphine during conditioning sessions showed a significant CPP when tested in an undrugged state but failed to show CPP when tested after injection of MK-801 (i.e., in the drugged state). These results indicate that MK-801 not only blocks the development of morphine-induced CPP but is also able to block the expression of a conditioned response that has been acquired before. In the same experiment repeated injection of neither morphine nor MK-801 produced sensitized locomotor activity. However, a strong sensitization was observed following repeated injection of morphine plus MK-801. There was also cross-sensitization between morphine plus MK-801 and MK-801 alone but not with morphine alone, and also between morphine and MK-801, but not vice versa. In a second experiment the effects of the AMPA-receptor antagonist GYKI 52466 were examined. It was found that GYKI 52466 did not produce CPP or behavioural sensitization. Finally, in a third experiment, CPP was induced by morphine and amphetamine (animals tested in the drug-free state), and behavioural sensitization was induced by amphetamine. When animals were tested after an injection of GYKI 52466, neither the morphine- nor the amphetamine-conditioned animals showed a CPP. Likewise, challenge of sensitized animals with amphetamine plus GYKI 52466 failed to produce a sensitized response. It can be concluded, that GYKI 52466, like MK-801, can block the expression of a conditioned response, and can also block the expression of sensitized behaviour.

Differential effects of agents acting at various sites of the NMDA receptor complex in a place preference conditioning model

A conditioned place preference paradigm was used to assess the potential rewarding properties of the uncompetitive NMDA receptor antagonist, MK-801 (dizolcipine), the two competitive NMDA receptor antagonists, CGP 37849 (DL-(E)-2-amino-4-methyl-5-phosphono-3-pentonoic acid) and its (R)-enantiomer CGP 40116, as well as the partial agonist at strychnine-insensitive glycine receptors, ACPC (1-aminocyclopropanecarboxylic acid). MK-801 (0.3 mg/kg), CGP 37849 (1.25-10 mg/kg) and CGP 40116 (1.25-10 mg/kg), administered in association with either the initially non-preferred or initially preferred side of the two-arm chamber, caused a significant increase in the time spent on that side in a post-conditioning test. In contrast, ACPC did not support the conditioned place preference. Thus, the time spent on the drug-associated side following conditioning with ACPC (50-400 mg/kg) did not significantly differ from that measured in the pre-conditioning test, irrespective of whether it was associated with the initially non-preferred black side or the initially preferred white side. These results are consistent with both clinical and pre-clinical data demonstrating differences in psychopharmacological properties among compounds acting at the multiple, allosteric regulatory sites on the NMDA receptor complex. Moreover, these results indicate that the abuse potential of ACPC, which acts as a functional NMDA receptor antagonist, may be lower than that of either uncompetitive or competitive NMDA receptor antagonists.

Amphetamine and dopamine-induced immediate early gene expression in striatal neurons depends on postsynaptic NMDA receptors and calcium

Amphetamine and cocaine induce the expression of both immediate early genes (IEGs) and neuropeptide genes in rat striatum. Despite the demonstrated dependence of these effects on D1 dopamine receptors, which activate the cyclic AMP pathway, there are several reports that amphetamine and cocaine-induced IEG expression can be inhibited in striatum in vivo by NMDA receptor antagonists. We find that in vivo, the NMDA receptor antagonist MK-801 inhibits amphetamine induction of c-fos acutely and also prevents downregulation of IEG expression with chronic amphetamine administration. Such observations raise the question of whether dopamine/glutamate interactions occur at the level of corticostriatal and mesostriatal circuitry or within striatal neurons. Therefore, we studied dissociated striatal cultures in which midbrain and cortical presynaptic inputs are removed. In these cultures, we find that dopamine- or forskolin-mediated IEG induction requires Ca2+ entry via NMDA receptors but not via L-type Ca2+ channels. Moreover, blockade of NMDA receptors diminishes the ability of dopamine to induce phosphorylation of the cyclic AMP responsive element binding protein CREB. Although these results do not rule out a role for circuit-level dopamine/glutamate interactions, they demonstrate a requirement at the cellular level for interactions between the cyclic AMP and NMDA receptor pathways in dopamine-regulated gene expression in striatal neurons.

Inhibition by MK-801 of morphine-induced conditioned place preference and postsynaptic dopamine receptor supersensitivity in mice

Intraperitoncel injection of morphine (5 mg/kg) in mice every other day for 8 days produced conditioned place preference (CPP). CPP effects were evaluated by assessing the difference in time spent in the drug-paired compartment and the saline-paired compartment of the place conditioning apparatus. The injection of a noncompetitive NMDA antagonist, MK-801 (0.05 and 0.1 mg/kg. IP), prior to and during morphine treatment in mice inhibited morphine-induced CPP. The development of postsynaptic dopamine (DA) receptor supersensitivity in mice displaying a morphine-induced CPP was evidenced by the enhanced response in ambulatory activity to the DA agonist, apomorphine (2 mg/kg). MK-801 inhibited that development of postsynaptic DA receptor supersensitivity. MK-801 also inhibited apomorphine-induced climbing behavior, suggesting that MK-801 inhibits dopaminergic activation mediated via the NMDA receptor. These results suggest that the development of morphine-induced CPP may be associated with the development of postsynaptic DA receptor supersensitivity. The development of morphine-induced CPP and DA receptor supersensitivity may be closely related to NMDA receptor-mediated dopaminergic activity, because morphine-induced changes in sensitivity to apomorphine, as well as apomorphine-induced climbing behavior in morphine treated mice, were both blocked by MK-801.

Effects of dopaminergic and glutamatergic receptor antagonists on the establishment and expression of conditioned locomotion to cocaine in rats

A series of experiments were conducted to investigate the role of dopaminergic D1 and D2 and glutamatergic NMDA and AMPA/kainate receptors on the establishment and expression of cocaine-induced conditioned locomotion in rats. In the first experiment conditioned locomotion was demonstrated by testing the animals in an environment previously associated with 15 mg/kg i.p. cocaine. The D2-receptor antagonist (-)-sulpiride (50 and 100 mg/kg i.p.) administered before cocaine during the conditioning phase did not modify the establishment of conditioned locomotion whereas when administered before testing only at the higher dose it partially reduced rats' locomotion in the absence of cocaine (expression). At the higher dose (0.1 mg/kg i.p.) the D1-receptor antagonist SCH 23390 attenuated the expression of cocaine-induced conditioned locomotion whereas the lower dose (0.03 mg/kg i.p.) had no effect. Both doses of the NMDA receptor antagonist MK-801 (0.125 and 0.25 mg/kg i.p.) blocked the development of cocaine-induced conditioned locomotion but neither dose, when administered before testing, modified locomotion in the absence of cocaine. Both doses of the AMPA/kainate receptor antagonist DNQX administered intracerebroventricularly (1 and 3 micrograms/rat) blocked cocaine-induced conditioned locomotion when given before cocaine during conditioning but when given before testing only the higher dose attenuated the conditioned activity. The results confirm the importance of the interaction between glutamatergic and dopaminergic systems for the conditional factors maintaining drug seeking behaviour. The findings may have implications for the treatment of cocaine craving and relapse.

The NMDA receptor antagonist dizocilpine (MK-801) stereoselectively inhibits morphine-induced place preference conditioning in mice

The effect of the non-competitive NMDA receptor antagonist dizocilpine (MK-801) on conditioned place preference induced by morphine was studied in mice. As expected, morphine (1-8 mg/kg, i.p.) elicited a significant preference for the drug-paired compartment. Pretreatment of mice with (+)-dizocilpine (0.1 and 0.2 mg/kg, i.p.), the more active dizocilpine enantiomer, dose-dependently reversed the conditioned place preference produced by morphine (4 mg/kg, i.p.), whereas (-)-dizocilpine (0.2 mg/kg, i.p.) did not modify morphine-induced effects. In contrast, both enantiomers of dizocilpine (at a dose of 0.2 mg/kg, i.p.) elicited a conditioned place preference. These data suggest that (1) NMDA receptors play a role in morphine-induced place preference, and (2) dizocilpine-reinforcing properties in the place preference paradigm do not seem to be dependent on NMDA receptor blockade.

The expression of both amphetamine-conditioned place preference and pentylenetetrazol-conditioned place aversion is attenuated by the NMDA receptor antagonist (+/-)-CPP

The present study addressed the ability of the NMDA receptor antagonist (+/-)-CPP ((+/-)-3-(2-carboxy-piperazine-4-yl)-propyl-1-phosphonic acid) to block the expression of amphetamine (AMPH)-conditioned place preference (CPP) or pentylenetetrazol (PTZ)-conditioned place aversion (CPA) in male Wistar rats. During a 4-day conditioning period, daily injections of AMPH (1.5 mg/kg, s.c.), PTZ (20 mg/kg, i.p.) and their vehicles were paired with two distinctive compartments of the shuttle box (balanced procedure). Rats pretreated with saline showed significant preference for the AMPH-paired or aversion of PTZ-paired compartments. Pretreatment with (+/-)-CPP (10-30 mg/kg, i.p.) dose-dependently blocked the expression of both CPP and CPA. These data suggest that NMDA receptors are implicated in the expression of conditioned behaviors.

Inhibition by MK-801 of cocaine-induced sensitization, conditioned place preference, and dopamine-receptor supersensitivity in mice

Repeated administration of cocaine led to increases in ambulation-accelerating activity (sensitization) and conditioned place preference (CPP). Dopamine (DA)-receptor supersensitivity was also developed in cocaine-induced sensitized and CPP mice. An N-methyl-D-aspartate (NMDA)-receptor antagonist, MK-801, blocked simultaneously developments of cocaine-induced behavioral sensitization, CPP, and DA-receptor supersensitivity. Furthermore, MK-801 inhibited a apomorphine-induced striatal dopaminergic action: climbing behavior. These results suggest that the cocaine-induced dopaminergic behaviors such as sensitization to ambulatory activity and CPP may be produced via activation of the NMDA receptor. The development of postsynaptic DA-receptor supersensitivity may be an underlying common mechanism that mediates cocaine-induced behavioral sensitization and CPP.

N-methyl-D-aspartic acid-receptor antagonists block morphine-induced conditioned place preference in rats

We addressed the question of whether (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) and DL-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP37849), a non-competitive and a competitive N-methyl-D-aspartate (NMDA) receptor antagonist, respectively, are able to block morphine-induced conditioned place preference (CPP). MK-801 alone (0.1 mg/kg) produced neither a place preference nor a place aversion, but was able to completely block morphine-induced CPP. CGP37849 alone (10 mg/kg) produced a small but significant CPP, and was able to significantly attenuate morphine-induced CPP. These results cannot be due to simple additive effects of drug actions, but suggest that NMDA receptors play a complex role in the development of morphine CPP.

Excitatory amino acids and drugs of abuse: a role for N-methyl-D-aspartate receptors in drug tolerance, sensitization and physical dependence

N-methyl-D-aspartate (NMDA) receptors have been implicated in several types of neural and behavioral plasticity ranging from development to learning. The present paper reviews evidence suggesting that these receptors might also be involved in the neural and behavioral changes resulting from chronic administration of drugs of abuse. NMDA receptor antagonists have been found to interfere with tolerance, sensitization, physical dependence and conditioning to a variety of self-administered drugs, including psychomotor stimulants, opiates, ethanol and nicotine. The results indicate a broad role for NMDA receptors in drug-induced neural and behavioral plasticity, including changes in the brain and behavior that may lead to compulsive drug use, and suggest that drugs acting at the NMDA receptor complex may be clinically useful.

Effects of dopaminergic and glutamatergic receptor antagonists on the acquisition and expression of cocaine conditioning place preference

A balanced conditioning place preference (CPP) paradigm was used to study the role of dopamine D1 and D2 and glutamatergic NMDA and AMPA/kainate receptors on the acquisition and expression of cocaine place conditioning. The D1 receptor antagonist SCH 23390 (0.1-0.2 mg/kg i.p.), administered before cocaine during the training phase, significantly blocked the establishment of place conditioning (acquisition) but had no effect when administered before testing for place preference in the absence of cocaine (expression). Similar results were obtained with the non-competitive NMDA receptor antagonist MK-801 (0.1-0.5 mg/kg i.p.). The D2 receptor antagonist (-)-sulpiride (50-100 mg/kg i.p.) had no effect on either acquisition or expression of cocaine CPP. The AMPA/kainate receptor antagonist DNQX, administered intracerebroventricularly (0.2-3 micrograms/10 microliters), blocked cocaine CPP when given before testing but not when given before cocaine during the training trials. The results suggest that dopaminergic D1 (but not D2) and glutamatergic NMDA receptors are involved in the primary rewarding properties of cocaine (as assessed by the establishment of CPP) whereas the AMPA/kainate receptors are important only for the behaviour elicited by the stimuli previously associated with the drug action (CPP expression). The implications for the treatment of cocaine craving and relapse are discussed.