Possible abuse potential of the NMDA antagonist MK-801

Effects of the noncompetitive N-methyl-d-aspartate receptor antagonists ketamine and MK-801 on pain-stimulated and pain-depressed behaviour in rats

BACKGROUND

Pain is a significant public health concern, and current pharmacological treatments have problematic side effects and limited effectiveness. N-methyl-d-aspartate (NMDA) glutamate receptor antagonists have emerged as one class of candidate treatments for pain because of the significant contribution of glutamate signalling in nociceptive processing.

METHODS

This study compared effects of the NMDA receptor antagonists ketamine and MK-801 in assays of pain-stimulated and pain-depressed behaviour in rats. The nonsteroidal anti-inflammatory drug ketoprofen was examined for comparison as a positive control. Intraperitoneal injection of dilute acid served as an acute visceral noxious stimulus to stimulate a stretching response or depress intracranial self-stimulation (ICSS) in male Sprague-Dawley rats.

RESULTS

Ketamine (1.0-10.0 mg/kg) blocked acid-stimulated stretching but failed to block acid-induced depression of ICSS, whereas MK-801 (0.01-0.1 mg/kg) blocked both acid-stimulated stretching and acid-induced depression of ICSS. These doses of ketamine and MK-801 did not alter control ICSS in the absence of the noxious stimulus; however, higher doses of ketamine (10 mg/kg) and MK-801 (0.32 mg/kg) depressed all behaviour. Ketoprofen (1.0 mg/kg) blocked both acid-induced stimulation of stretching and depression of ICSS without altering control ICSS.

CONCLUSION

These results support further consideration of NMDA receptor antagonists as analgesics; however, some NMDA receptor antagonists are more efficacious at attenuating pain-depressed behaviours.

WHAT DOES THIS STUDY ADD

NMDA receptor antagonists produce dissociable effects on pain-depressed behaviour. Provides evidence that pain-depressed behaviours should be considered and evaluated when determining the antinociceptive effects of NMDA receptor antagonists.

Intracranial self-stimulation to evaluate abuse potential of drugs

Intracranial self-stimulation (ICSS) is a behavioral procedure in which operant responding is maintained by pulses of electrical brain stimulation. In research to study abuse-related drug effects, ICSS relies on electrode placements that target the medial forebrain bundle at the level of the lateral hypothalamus, and experimental sessions manipulate frequency or amplitude of stimulation to engender a wide range of baseline response rates or response probabilities. Under these conditions, drug-induced increases in low rates/probabilities of responding maintained by low frequencies/amplitudes of stimulation are interpreted as an abuse-related effect. Conversely, drug-induced decreases in high rates/probabilities of responding maintained by high frequencies/amplitudes of stimulation can be interpreted as an abuse-limiting effect. Overall abuse potential can be inferred from the relative expression of abuse-related and abuse-limiting effects. The sensitivity and selectivity of ICSS to detect abuse potential of many classes of abused drugs is similar to the sensitivity and selectivity of drug self-administration procedures. Moreover, similar to progressive-ratio drug self-administration procedures, ICSS data can be used to rank the relative abuse potential of different drugs. Strengths of ICSS in comparison with drug self-administration include 1) potential for simultaneous evaluation of both abuse-related and abuse-limiting effects, 2) flexibility for use with various routes of drug administration or drug vehicles, 3) utility for studies in drug-naive subjects as well as in subjects with controlled levels of prior drug exposure, and 4) utility for studies of drug time course. Taken together, these considerations suggest that ICSS can make significant contributions to the practice of abuse potential testing.

Dissociable effects of the noncompetitive NMDA receptor antagonists ketamine and MK-801 on intracranial self-stimulation in rats

RATIONALE

The noncompetitive NMDA antagonist ketamine produces rapid antidepressant effects in treatment-resistant patients suffering from major depressive and bipolar disorders. However, abuse liability is a concern.

OBJECTIVES

This study examined abuse-related effects of ketamine using intracranial self-stimulation (ICSS) in rats. The higher-affinity NMDA antagonist MK-801 and the monoamine reuptake inhibitor cocaine were examined for comparison.

METHODS

Male Sprague Dawley rats were implanted with electrodes targeting the medial forebrain bundle and trained to respond to brain stimulation under a frequency-rate ICSS procedure. The first experiment compared the potency and time course of ketamine (3.2-10.0 mg/kg) and MK-801 (0.032-0.32 mg/kg). The second experiment examined effects of repeated dosing with ketamine (3.2-20.0 mg/kg/day) and acute cocaine (10.0 mg/kg).

RESULTS

Following acute administration, ketamine (3.2-10 mg/kg) produced only dose- and time-dependent depressions of ICSS and failed to produce an abuse-related facilitation of ICSS at any dose or pretreatment time. In contrast, MK-801 (0.032-0.32 mg/kg) produced a mixed profile of rate-increasing and rate-decreasing effects; ICSS facilitation was especially prominent at an intermediate dose of 0.18 mg/kg. Repeated dosing with ketamine produced dose-dependent tolerance to the rate-decreasing effects of ketamine (10.0 and 18.0 mg/kg) but failed to unmask expression of ICSS facilitation. Termination of ketamine treatment failed to produce withdrawal-associated decreases in ICSS. As reported previously, 10.0 mg/kg cocaine facilitated ICSS.

CONCLUSIONS

The dissociable effects of ketamine and MK-801 suggest differences in the pharmacology of these nominally similar NMDA antagonists. Failure of ketamine to facilitate ICSS contrasts with other evidence for the abuse liability of ketamine.

Ketamine - reves et realites

Ketamine is an anaesthetic used in human medicine and veterinary practice, synthesised on 1962 and marketed on 1970 in France. Recreational uses were described during 1992 in the medical communauty and in 1996 in the dance settings. The chemical name of ketamine is 2 - (2chlorophenyl) 2-(methylamine)-cyclohexanone, an aryl cyclohexylamine, structurally related to phencyclidine. Ketamine is known under the following street names : Keta K, Kate, Special K, Vitamine K, la Golden, la Vétérinaire. Ketamine is used intranasally, orally and intramusculary in recreational use. Ketamine is manufactured by the chemical industry. Due to the complicated synthesis, it is sold illicitly for recreational use. Ketamine is a dissociative drug, and the user enters in a psychedelic dream with hallucinations, floating sensation, feeling of dissociation of the mind from the body. The dream is forgotten, the user fulls in reality with loss of self control, risk of acute intoxication. In long term exposure, tolerance, dependence, withdrawal signs and flash back are described. Ketamine trademarks are subject to control in France through medicine legislation Ketamine and its salts are subject to control under the national legislation on narcotics and psychotropics substance. From September 2001, the theft of medical and veterinary trademarks have to be declared to police, care health authority Pharmacy control authority and French Health Products Safety Agency.

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.

Blockade of astrocytic glutamate uptake in rats induces signs of anhedonia and impaired spatial memory

Mood disorders are associated with regional brain abnormalities, including reductions in glial cell and neuron number, glutamatergic irregularities, and differential patterns of brain activation. Because astrocytes are modulators of neuronal activity and are important in trafficking the excitatory neurotransmitter glutamate, it is possible that these pathologies are interrelated and contribute to some of the behavioral signs that characterize depression and related disorders. We tested this hypothesis by determining whether depressive-like signs were induced by blocking central astrocytic glutamate uptake with the astrocytic glutamate transporter (GLT-1) inhibitor, dihydrokainic acid (DHK), in behavioral tests that quantify aspects of mood, including reward and euthymia/dysthymia: intracranial self-stimulation (ICSS) and place conditioning. We found that DHK elevated ICSS thresholds, a depressive-like effect that could reflect reduced sensitivity to reward (anhedonia) or increased aversion (dysphoria). However, DHK treatment did not establish conditioned place aversions, suggesting that this treatment does not induce dysphoria. To identify the brain regions mediating the behavioral effects of DHK, we examined c-Fos expression in areas implicated in motivation and emotion. DHK increased c-Fos expression in many of these regions. The dentate gyrus of the hippocampus was robustly activated, which led us to explore whether DHK alters hippocampal learning. DHK impaired spatial memory in the MWM. These findings identify disruption of astrocyte glutamate uptake as one component of the complex circuits that mediate anhedonia and cognitive impairment, both of which are common symptoms of depression. These finding may have implications for the etiology of depression and other disorders that share the features of anhedonia and cognitive impairment.

Differential effects of acute and subchronic clozapine and haloperidol on phencyclidine-induced decreases in voluntary sucrose consumption in rats

Prior exposure to the psychotomimetic drug phencyclidine (PCP) decreases voluntary sucrose consumption in rats. This may be indicative of reduced reward function, a phenomenon associated with negative schizophrenic symptomatology. Given that atypical antipsychotics have been shown to ameliorate negative symptoms of schizophrenia more effectively than typical neuroleptics, this effect should be reversed by clozapine but not haloperidol. PCP (15 mg/kg) or saline was administered 20 h prior to testing for voluntary sucrose consumption in non-deprived rats. In the acute experiments, rats were treated with clozapine (5 mg/kg), haloperidol (0.2 mg/kg), or vehicle 45 min prior to testing. In the subchronic experiments, rats were treated with clozapine (3 mg/kg, bid), haloperidol (0.5 mg/kg, bid), or vehicle for 10 days prior to PCP administration. Acute clozapine exacerbated the PCP-induced decrease in sucrose consumption without altering water consumption. Acute haloperidol produced an overall decrease in sucrose consumption in both PCP-pretreated and control groups. Subchronic treatment with clozapine, but not haloperidol, reversed PCP-induced decreases in sucrose consumption. The synergistic effect of acute clozapine and PCP may reflect a PCP-induced increase in the reward-reducing properties of CLZ, normally seen only at higher doses. The observation that subchronic clozapine, but not haloperidol, reversed PCP-induced decreases in sucrose consumption supports the hypothesis that this effect of PCP represents a plausible animal model for negative schizophrenic symptomatology.

Lowered brain stimulation reward thresholds in rats treated with a combination of caffeine and N-methyl-D-aspartate but not alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate or metabotropic glutamate receptor-5 receptor antagonists

Previous studies suggested that adenosine A1 and A2A receptor agonists counteract behavioral effects of N-methyl-D-aspartate (NMDA) receptor antagonists while adenosine receptor antagonists may produce opposite effects enhancing the actions of NMDA receptor antagonists. To further evaluate the effects of combined administration of adenosine receptor antagonist caffeine and various NMDA and non-NMDA glutamate receptor antagonists on brain stimulation reward (discrete-trial threshold current intensity titration procedure), rats with electrodes implanted into the ventral tegmental area were tested after pretreatment with NMDA receptor channel blocker MK-801 (0.01-0.3 mg/kg), competitive antagonist D-CPPene (0.3-5.6 mg/kg), glycine site antagonist L-701,324 (1.25-5 mg/kg), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor antagonist GYKI-53655 (1-10 mg/kg), metabotropic glutamate receptor 5 (mGluR5) antagonist MPEP (1-10 mg/kg) alone and in combination with caffeine (1-30 mg/kg). MK-801 (0.056 and 0.1 mg/kg) was the only tested glutamate antagonist that lowered self-stimulation thresholds, while D-CPPene (5.6 mg/kg) and MPEP (5.6 and 10 mg/kg) had the opposite effects. Threshold-increasing effects of D-CPPene, but not of MPEP, however, were associated with marked impairment of operant performance, reflected by longer latencies to respond and higher rates of responding during the inter-trial intervals. Operant performance was also disrupted by the highest dose of MK-801 (0.3 mg/kg). For subsequent experiments, caffeine (1-30 mg/kg) was combined with the highest doses of NMDA receptor antagonists that did not lower the brain stimulation reward thresholds and did not impair operant performance. Caffeine had no appreciable effects on self-stimulation behavior when given alone. A low dose of caffeine (3 mg/kg) significantly lowered self-stimulation thresholds only when given together with MK-801 (0.03 mg/kg) or D-CPPene (3 mg/kg). Combined with the same antagonist drugs, higher doses of caffeine (10 and 30 mg/kg) facilitated time-out responding. These results indicate that, within a limited dose range, caffeine in combination with an NMDA receptor channel blocker and a competitive antagonist significantly lowers brain stimulation reward thresholds in rats.

Effects of group I metabotropic glutamate receptor antagonists on the behavioral sensitization to motor effects of cocaine in rats

RATIONALE

Metabotropic glutamate receptors (mGluRs) were reported to regulate various behavioral effects of addictive drugs.

OBJECTIVE

The present study evaluated the role of group I mGluRs in the progressive augmentation ("sensitization") of the behavioral effects observed after repeated, intermittent cocaine exposure.

MATERIALS AND METHODS

After habituation to handling and baseline activity measurement (days 1-2), rats received eight injections of cocaine (10 mg/kg) or saline on days 3-6, 8-11, and then, were tested twice with acute saline and cocaine given in a counterbalanced manner on days 13 and 15. Before the test sessions, subjects were pretreated with mGluR1 antagonist EMQMCM (JNJ16567083, (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate) and mGluR5 antagonist MTEP ([(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine).

RESULTS

Pretreatment with EMQMCM (2.5-10 mg/kg) but not MTEP (2.5-10 mg/kg) significantly reduced expression of the sensitized ambulatory motor activity of the cocaine-experienced animals acutely challenged with cocaine. Both EMQMCM and MTEP significantly reduced vertical motor activity across all cocaine/saline treatment conditions.

CONCLUSIONS

These findings indicate that the expression of behavioral sensitization to cocaine-induced stimulation of locomotor activity may be modulated by group I mGluR antagonists (mGluR1 rather than mGluR5), but these effects occur at the dose levels that attenuate vertical activity.

Facilitation of brain stimulation reward by MK-801 (dizocilpine) may be independent of D2-like dopamine receptor stimulation in rats

RATIONALE

Dopamine (DA) and glutamate (Glu) interactions in the mesocorticolimbic pathway may regulate motivation and reward and contribute to schizophrenia and drug abuse. We have recently demonstrated synergistic effects of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor blockade and D(2/3) DA receptor stimulation in brain stimulation reward (BSR).

OBJECTIVES

This study was conducted to explore interactions between DA and Glu systems in BSR using the NMDA receptor antagonist MK-801 and the DA receptor agonists 7-OH-DPAT and apomorphine.

METHODS

Systemic effects of these compounds were measured in male Sprague-Dawley rats using rate-frequency threshold analysis of ventral tegmental area (VTA) BSR (n=27). Effects of bilateral applications of MK-801 and 7-OH-DPAT into the nucleus accumbens (NAS) shell subregion were also investigated (n=10).

RESULTS

MK-801 (0.03 or 0.13 mg kg(-1) i.p. or 0.66 mug intra-NAS) reduced reward thresholds while 7-OH-DPAT (0.03 mg kg(-1) s.c. or 5.0 microg intra-NAS) or apomorphine (0.05 mg kg(-1), s.c.) increased this measure. MK-801 combined with apomorphine or with 7-OH-DPAT, systemically or in the NAS shell, induced additive effects.

CONCLUSIONS

Lack of interaction between DA agonists and MK-801 in this study contrasts with our previous work showing synergistic reward-decreased effects of AMPA/kainate receptor blockade and D(2/3) DA receptor stimulation in the NAS shell, and indicates possible independence of DA and N-methyl-D-aspartate (NMDA) receptor effects in VTA electrical self-stimulation.

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.

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.

Memantine presents different effects from MK-801 in motivational and physical signs of morphine withdrawal

Adaptive changes in neural systems due to chronic opiate exposure are related to the neural plasticity phenomenon, NMDA receptors being implicated in these processes, e.g. tolerance, dependence or withdrawal. In this work, we investigated the effect of two non-competitive NMDA antagonists, memantine and MK-801, in motivational (Conditioned Place Aversion paradigm, CPA) and physical aspects of morphine withdrawal. After the induction of morphine dependence, animals in which the CPA was studied, received memantine (5 and 10 mg/kg) or MK-801 (0.3-0.006 mg/kg) either during the acquisition (conditioning) or expression (test) phase of this procedure. Both drugs were capable of inhibiting conditioned aversion when administered in any phase. In a second experiment, the effects of these drugs were evaluated in the intensity of the physical signs of withdrawal, only memantine administration being efficient. In addition to these studies, the intensity of morphine dependence was investigated under the blockade of NMDA receptors, i.e. morphine was co-administered with memantine or MK-801. These animals did not develop CPA and present less intensity in the physical signs of morphine withdrawal. Our results support the idea that NMDA receptors are involved in the behavioural changes and therefore in the neural adaptations produced by repeated morphine administration.

Phencyclidine-induced potentiation of brain stimulation reward: acute effects are not altered by repeated administration

Phencyclidine (PCP; 2.5 or 5.0 mg/kg) potentiated the effects of rewarding hypothalamic brain stimulation, causing parallel leftward shifts of the functions that relate rate of responding to stimulation frequency. Thus, like a number of other drugs of abuse, PCP lowered the "dose" of stimulation required to maintain responding at a given criterion. No progressive changes in the reward-potentiating effects of PCP were evident when the rats were tested once per week for 8 weeks; there was neither tolerance nor sensitization to the initial rewarding properties of PCP. However, in subsequent locomotor tests rats appeared to be already sensitized to PCP; this raises the possibility that the electrical stimulation of the lateral hypothalamus itself maximally sensitized the animals to the stimulant effects of the drug.

Effects of a novel uncompetitive NMDA receptor antagonist, MRZ 2/579 on ethanol self-administration and ethanol withdrawal seizures in the rat

It has been repeatedly reported that NMDA receptors may contribute to ethanol-induced discriminative stimulus effects and withdrawal syndrome. However, the role of NMDA receptors in the reinforcing properties of ethanol remains unclear. The aim of the present study was to evaluate effects of the novel low-affinity, uncompetitive NMDA receptor antagonist, 1-amino-1,3,3,5,5-pentamethyl-cyclohexane hydrochloride (MRZ 2/579), on ethanol self-administration and ethanol withdrawal-associated seizures in rats. Both an operant (lever pressing for ethanol) and non-operant two-bottle choice setups were employed to initiate ethanol self-administration. In another procedure, forced treatment with high doses (9--15 g/kg/day) was used to induce physical dependence on ethanol. MRZ 2/579 delivered chronically by osmotic minipumps (9.6 mg/day, s.c.) did not alter either operant or non-operant ethanol drinking behaviour in a maintenance phase of ethanol self-administration. In contrast, repeated daily injections of the drug (5 mg/kg, i.p.) led to a progressive decrease in operant responding for ethanol. MRZ 2/579 (0.5--7.5 mg/kg, i.p.) and another low-affinity NMDA receptor antagonist, memantine (1--10 mg/kg, i.p.) dose-dependently suppressed ethanol withdrawal seizures with efficacies comparable with that of a standard benzodiazepine derivative, diazepam. The results of the present study indicate that: (i) intermittent administration of MRZ 2/579 may lead to a gradual decrease of operant responding for ethanol; and (ii) the group of low-affinity uncompetitive NMDA receptor antagonists may be an interesting alternative to benzodiazepines in the treatment of alcohol withdrawal.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Expression and acquisition of the conditioned place preference response to cocaine in rats is blocked by selective inhibitors of the enzyme N-acetylated-?-linked-acidic dipeptidase (NAALADASE)

In this study we examined the effect of 2-(phosphonomethyl)pentanedioic acid (2-PMPA) and GPI 5693, selective inhibitors of the enzyme N-Acetylated-alpha-Linked-Acidic Dipeptidase (NAALADase; glutamate carboxypeptidase II; EC no. 3.4.17.21), which cleaves glutamate from the dipeptide N-acetyl-aspartyl-glutamate (NAAG), on the conditioned place preference (CPP) response to cocaine in male rats. The i.p. administration of 15 mg/kg of cocaine produced a significant CPP response. The acquisition and expression of the CPP response to cocaine was blocked by the i.p. administration of 100 mg/kg of 2-PMPA and the p.o. administration of 30 mg/kg of GPI 5693. In contrast, neither 2-PMPA nor GPI 5693 produced a CPP or conditioned place aversion response when administered alone. Furthermore, neither 2-PMPA or GPI 5693 altered the expression of the CPP response to food. These results indicate that NAALADase inhibitors block the incentive motivational value of cocaine, suggesting that such agents may be of use in treating cue-induced craving in cocaine addicts.

Effect of chronic food restriction on Fos-like immunoreactivity (FLI) induced in rat brain regions by intraventricular MK-801

The noncompetitive NMDA antagonist, MK-801, produces stimulant and rewarding effects that are mediated by a combination of dopamine-dependent and -independent mechanisms. It was recently demonstrated that, similar to amphetamine, the rewarding and locomotor effects of intraventricular (i.c.v.) MK-801 are potentiated by chronic food restriction. Because food restriction also increases c-Fos expression induced by i.c.v. amphetamine in several subcortical dopamine (DA) terminal areas, Fos-like immunoreactivity (FLI) induced by i.c.v. MK-801 was evaluated in an effort to identify responses that are common to amphetamine and MK-801 and similarly augmented by food restriction. Unlike amphetamine, MK-801 did not increase FLI in caudate-putamen, bed nucleus of the stria terminalis, or ventral pallidum. Similar to amphetamine, MK-801 increased FLI in cingulate cortex, central nucleus of the amygdala and nucleus accumbens (NAC) core, but in none of these areas was the response augmented by food restriction. In medial prefrontal cortex, retrosplenial cortex, and NAC shell, however, MK-801 induced FLI that was augmented by food restriction. An effect that is common to amphetamine and MK-801 is the augmentation of FLI by food restriction in NAC shell. It is therefore suggested that increased releasability of DA, or upregulation of the D-1 receptor linked signal transduction pathway, in NAC shell may mediate the enhanced behavioral sensitivity of food-restricted subjects to drugs of abuse.

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 cocaine-conditioned motor activity in rats

NMDA receptor antagonists have been reported to affect learned behaviors conditioned with abused drugs, with the outcome dependent, in part, on the class of NMDA receptor antagonist used. The present study tested the ability of various site-selective NMDA receptor antagonists to modify cocaine-conditioned motor activity. Two procedures were used for independently assessing drug effects on spontaneous activity and expression of cocaine-conditioned behavior. In the conditioning experiments, rats were administered i.p. injections of cocaine (30 mg/kg) or saline paired with distinctive environments. Spontaneous horizontal activity was dose-dependently enhanced by dizocilpine (0.03-0.3 mg/kg) and memantine (1-30 mg/kg), but not by D-CPPene (3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid; SDZ EAA 494; 1-10 mg/kg), ACEA-1021 (5-nitro-6,7-dichloro-1,4-dihydro-2, 3-quinoxalinedione; 3-56 mg/kg), or eliprodil (3-30 mg/kg). Higher doses of memantine, D-CPPene (1-10 mg/kg), eliprodil (3-30 mg/kg), or ACEA-1021 reduced vertical activity. Following five cocaine-environment pairings, rats displayed significant increases in motor activity when exposed to the cocaine-paired environment. The following antagonists were administered prior to the conditioning test: dizocilpine (MK-801; 0.03-0.1 mg/kg), memantine (1-10 mg/kg), D-CPPene (0.3-3 mg/kg), ACEA-1021 (3-10 mg/kg), and eliprodil (1-10 mg/kg). Of these, memantine, ACEA-1021 and, to the lesser degree, eliprodil attenuated expression of cocaine-conditioned motor activity at doses that did not significantly affect spontaneous motor activity. These results show that cocaine-conditioned behaviors can be selectively modulated by some, but not all, NMDA receptor antagonists.

Effects of N-methyl-D-aspartate receptor antagonists on reinstatement of cocaine-seeking behavior by priming injections of cocaine or exposures to cocaine-associated cues in rats

The reinstatement of extinguished cocaine self-administration behavior was studied in rats pretreated with N-methyl-D-aspartate receptor antagonists. Rats were trained to self-administer intravenous cocaine (0.32 mg/kg/infusion) during five consecutive daily sessions that were followed by five consecutive daily extinction sessions, during which cocaine was unavailable and cocaine-associated cues (sound and light) were absent. Neither the competitive N-methyl-D-aspartate receptor antagonist D-CPPene (0.3-3 mg/kg) nor the low-affinity N-methyl-D-aspartate receptor channel blocker memantine (1-10 mg/kg) reinstated extinguished responding. Priming injections of intravenous cocaine (Experiment 1), and exposures to cocaine-associated stimuli (buzzer and light; Experiment 2) engendered responding on the reinforced lever in excess of that on the non-reinforced lever. In Experiment 1, administration of D-CPPene or memantine prior to the priming injection of cocaine eliminated the difference between reinforced-lever and non-reinforced-lever response rates. For both D-CPPene and memantine, however, this effect was largely due to increased responding upon the non-reinforced lever rather than to decreased reinforced-lever responding. In Experiment 2, D-CPPene, but not memantine, abolished in a dose-dependent manner the selective increase in reinforced-lever over non-reinforced-lever responding that was induced by exposures to cocaine-related stimuli. This effect of D-CPPene was not due to increased non-reinforced-lever responding. These data help define the boundaries within which N-methyl-D-aspartate receptor antagonists can prevent reinstatement of cocaine-seeking behavior (e.g. type of antagonist used and reinstatement procedure).

Memantine does not substantially affect brain stimulation reward: comparison with MK-801

The non-competitive N-methyl-D-aspartate (NMDA)-receptor antagonist MK-801 (dizocilpine) has been shown to potentiate brain stimulation reward (BSR). Memantine (1-amino-3,5-dimethyladamantane) also binds to the PCP binding site of the NMDA receptor but with markedly different kinetics and affinity than MK-801. Here, we examined the effects of memantine on BSR and compared its effects to those of MK-801. MK-801 (0.05 mg/kg-0.4 mg/kg) produced clear, dose-dependent decreases in threshold frequency, manifest in clear leftward shifts of the function relating stimulation frequency to response rate. Memantine (1 mg/kg-17.5 mg/kg) had only small effects on threshold frequencies and only at high doses, manifest in only small shifts in the frequency-response function. The highest dose of each drug also produced a decrease in maximum response rate. This study shows that memantine failed to substantially influence BSR at low to intermediate doses, suggesting that this substance is likely to be largely devoid of rewarding effects in a therapeutic dose range.

Effects of abused drugs on thresholds and breaking points of intracranial self-stimulation in rats

The present study aimed to compare the effects of various abused drugs on threshold current intensities and the breaking points of intracranial self-stimulation. Effects of morphine (1-10 mg/kg, s.c.), d-amphetamine (0.3-3.2 mg/kg, i.p.), nicotine (0.1-3.2 mg/kg, s.c.), ethanol (0.6-2.4 g/kg, p.o.), caffeine (1-20 mg/kg, i.p.) and phencyclidine (0.3-5.6 mg/kg, i.p.) were studied in male Wistar rats trained to lever-press for electrical stimulation of ventral tegmental area. Morphine and d-amphetamine were the only two drugs that both decreased threshold currents and increased the maximal ratio of reinforced and non-reinforced responses. Nicotine (1 mg/kg) and ethanol (1.2 g/kg) lowered threshold currents while both decreases and increases in threshold current were seen after administration of low (5 mg/kg) and high (20 mg/kg) doses of caffeine, respectively. Nicotine, ethanol and caffeine had no effects in the progressive ratio procedure. Effects of phencyclidine did not reach levels of statistical significance in either procedure although high doses of phencyclidine disrupted performance in the progressive ratio procedure.

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).

Conditioned increase in place preference by access to novel objects: antagonism by MK-801

In three separate place conditioning experiments with rats, repeated access to novel objects in one of two distinct environments conditioned an increase in preference for the novelty-paired environment. A conditioned increase in preference was found whether novel objects were paired with a randomly chosen environment or with the less preferred of two environments (conditioned against a preference). This enhanced preference did not depend on the control group employed. Control groups exposed only to the place conditioning apparatus or to both the apparatus and the novel objects showed no systematic shift in place preference. Intravenous infusions of cocaine also produced an increase in preference using the procedures employed with novel objects. Pretreatment with the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (0.03 mg kg(-1)) blocked acquisition of the enhanced place preference conditioned by access to novel objects without decreasing time spent with objects or inducing a place aversion in controls. Combined, these results show that access to novel objects can serve as an appetitive stimulus like drugs of abuse and that this novelty-induced appetitive process involves NMDA receptors. These place-conditioning procedures may provide a good model for determining the behavioral and neural process underlying the appetitive effects of novelty.

Modulation of the discriminative stimulus and rate-altering effects of cocaine by competitive and noncompetitive N-methyl-D-aspartate antagonists

The purpose of this study was to determine the extent to which N-methyl-D-aspartic acid (NMDA) antagonists modified the discriminative stimulus effects of cocaine in rats trained to discriminate 5 mg/kg cocaine from vehicle on a fixed-ratio schedule of food presentation as well as the rate-altering effects of cocaine in rats maintained on a fixed-interval schedule of food presentation. NMDA-associated ion channel blockers (dizocilpine, phencyclidine, and magnesium chloride) and competitive NMDA antagonists (NPC 17742 and CGP 37849) displayed similar behavioral effects when administered alone: each drug engendered intermediate levels of cocaine-appropriate responses and rate-dependent effects on food-reinforced operant responding. Selected doses of dizocilpine, magnesium chloride, and phencyclidine given in combination with 1 mg/kg cocaine produced more cocaine-appropriate responses than this dose of cocaine alone. In addition, dizocilpine and magnesium chloride each attenuated the discriminative stimulus effects of higher doses of cocaine. The competitive NMDA antagonists did not appreciably modify the discriminative stimulus effects of any dose of cocaine. Under the fixed-interval schedule, each NMDA antagonist attenuated the effects of 3 mg/kg cocaine, which normally produced maximal increases in response rate. Attenuation of the rate-decreasing effects of the highest dose of cocaine (30 mg/kg) also were observed after pretreatment with dizocilpine and magnesium chloride. These findings demonstrated differences in the way that NMDA-associated ion channel blockers and competitive NMDA antagonists interact with cocaine, and suggest that some NMDA-associated ion channel blockers may either enhance or antagonize the effects of cocaine, depending on the dose and type of behavioral procedure.

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.

Effects of amphetamine, morphine and dizocilpine (MK-801) on spontaneous alternation in the 8-arm radial maze

The induction of psychomotor activation, behavioural sensitization and of perseverative behaviours, resulting in reduced behavioural variability, have been proposed to be common properties of drugs of abuse. The present investigation tested whether these drug effects could be measured using spontaneous alternation in an 8-arm radial maze. Behavioural effects of repeated treatment with amphetamine (2 and 4 mg/kg, i.p.), morphine (1.25 and 10 mg/kg, i.p.) and the non-competitive NMDA receptor antagonist, MK-801 (0.1 and 0.2 mg/kg, i.p.), on spontaneous alternation were evaluated in this paradigm. All drugs induced psychomotor activation. Sensitized as well as reduced locomotor activity could be observed after repeated treatment depending on drug and dose. Analysis of the sequences of arm entries revealed that all drugs induced perseverative locomotor patterns, but the pattern induced by amphetamine and morphine differed qualitatively from the pattern induced by MK-801.

MK-801 increases locomotor activity without elevating extracellular dopamine levels in the nucleus accumbens

In vivo microdialysis was used in freely moving rats to determine whether the locomotor stimulant effects of dizocilpine maleate (MK-801) were related to increased dopamine (DA) release within the nucleus accumbens (N. Acc.). Each experiment began with a baseline period of at least 2 h (starting 15-20 h after insertion of concentric, removable dialysis probes), during with activity records and dialysate samples were collected every 20 min. Rats in the first experiment then were injected with MK-801 (0.125, 0.25, or 0.50 mg/kg, i.p.) or saline, and activity and extracellular levels of DA, dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) were measured for a further 160 min post-injection. In a second experiment, rats were given 1.5 mg/kg d-amphetamine sulphate 40 min after receiving the same doses of MK-801, and testing was continued for 120 min. Rats in a third experiment were given low, autoreceptor-preferring doses of apomorphine hydrochloride (25 or 50 micrograms/kg, s.c.) or its vehicle 40 min after injection of 0.25 mg/kg MK-801 and then monitored for 120 min. MK-801 produced strong and consistent increases in locomotor activity that were augmented by amphetamine and greatly reduced by the low doses of apomorphine. MK-801 did not increase extracellular DA levels within the N. Acc. when given alone, and it failed to influence the changes in extracellular DA produced by d-amphetamine and apomorphine. MK-801 did produce consistent, dose-related increases in DOPAC and HVA that were probably not related to transmitter release. These results indicate that the increases in locomotor activity seen following MK-801 do not arise from a drug-induced increase in DA levels within the N. Acc.

Rewarding actions of phencyclidine and related drugs in nucleus accumbens shell and frontal cortex

Rats learned to lever-press when such behavior was reinforced by microinjections of phencyclidine (PCP) directly into the ventromedial (shell) region of nucleus accumbens, indicating that the drug has direct rewarding actions in that region. Separate groups of rats learned to lever-press when reinforced with microinjections of dizoclipine (MK-801) or 3-((+/-)2-carboxypiperazin-4yl)propyl-1-phosphate (CPP), drugs known to block NMDA receptor function but not dopamine uptake, into the same region. Each drug was ineffective or markedly less effective when injected at a slightly more dorsal and lateral site in the core of nucleus accumbens. Self-administration of PCP, MK-801, or CPP directly into nucleus accumbens was not altered by co-infusion of a dose of the dopamine antagonist sulpiride that effectively blocked intracranial self-administration of the dopamine uptake inhibitor nomifensine, suggesting that the rewarding actions of the NMDA receptor antagonists are not dopamine-dependent. Rats also developed lever-pressing habits when PCP, MK-801, and CPP were each microinjected directly into frontal cortex, a region previously associated with the rewarding actions of cocaine but not nomifensine. Thus nucleus accumbens and frontal cortex are each potential substrates for the rewarding properties of PCP and related drugs, and the ability of these drugs to disrupt NMDA receptor function seems sufficient to account for their rewarding actions. When considered with independent evidence, the present results suggest a model of drug reward within which the critical event is inhibition of medium spiny neurons in nucleus accumbens.

Dopaminergic basis for the facilitation of brain stimulation reward by the NMDA receptor antagonist, MK-801

MK-801 (dizocilpine maleate), an antagonist of the NMDA receptor, was given alone or in combination with dopamine D1 and D2 receptor antagonists to rats self-stimulating in lateral hypothalamus to determine whether the dopamine neurons play a role in mediating the effects of MK-801. MK-801 given at a dose of 0.1 mg/kg i.p. to self-stimulators induced a prolonged facilitation of lever-pressing, but given to non-self-stimulators, the drug had no effects. Pretreatment of self-stimulators with the dopamine D1 receptor antagonist Schering 23390 (SCH 23390), 0.2 mg/kg given i.p. 15 min before MK-801, prevented the facilitation seen with MK-801, but did not suppress self-stimulation. SCH 23390 given alone suppressed self-stimulation. Pretreatment of self-stimulators with the dopamine D1/D2 receptor antagonist, haloperidol, 0.2 mg/kg given i.p. 15 min before MK-801, also prevented the facilitation of self-stimulation induced by MK-801 yet did not suppress self-stimulation. Haloperidol given alone suppressed self-stimulation. Pretreatment of self-stimulators with both SCH 23390 and haloperidol 15 min before MK-801 prevented the facilitation seen with MK-801 and suppressed self-stimulation. The combined treatment with SCH 23390 and haloperidol (without MK-801) suppressed self-stimulation, and the suppression lasted longer than the suppression seen when the two dopamine receptor antagonists were given as pretreatment, before MK-801. Pretreating self-stimulators with the combination of SCH 23390 and haloperidol 15 min before amphetamine (2 mg/kg) prevented the facilitatory response and suppressed responding for the brain reward. The suppression was of shorter duration than the suppression seen after the injection of SCH 23390 plus haloperidol. The treatment of self-stimulators with both MK-801 and amphetamine resulted in a greater and longer-lasting facilitation than the increase in responding produced by either drug alone. The similarity between the effects of MK-801 and those of amphetamine and between the effects of pretreatment with the dopamine receptor antagonists before MK-801 and before amphetamine suggests that dopaminergic activity played a significant role in the action underlying the effects of MK-801 on brain stimulation reward.

The effects of MK801 on place conditioning

The appetitive properties of MK801 were investigated using the conditioned place preference paradigm. Male Sprague-Dawley rats received conditioned place preference training for a four-day period. The conditioning box consisted of three chambers with distinctive visual and tactile cues, separated by removable doors. On alternating days rats received MK801 (0.05, 0.1, 0.25, 0.5 and 0.75 mg/kg) paired with one side of the chamber and saline paired with the other side. On the fifth day, rats were placed in the center chamber and the time spent in each chamber, as well as entries into each chamber were recorded by a trained observer. MK801 produced a conditioned place preference for side paired with drug for the four highest doses. Contrary to previous findings, these results indicate that MK801 is appetitive at doses higher than has previously been assumed.

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.

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

The noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 prevents the development of sensitization to the locomotor-activating effects of amphetamine. In the present study, the possibility that the NMDA receptor might also play a role in the rewarding effects of amphetamine (as measured in the conditioned place preference paradigm) was investigated. Male Sprague-Dawley rats received amphetamine (2.0 mg/kg IP) paired with one side of a two-compartment box and saline paired with the other side. During these pairings locomotor activity was measured. On the test day, the amount of time drug-free rats spent in each compartment was determined. Rats trained with amphetamine alone showed a significant increase in time spent on the drug-paired side from pre- to postconditioning, indicating a place preference. When rats were injected with MK-801 (0.03, 0.1, or 0.3 mg/kg SC) prior to amphetamine, no significant effects on amphetamine place conditioning were observed. Rats treated with MK-801 alone showed significant place conditioning, but only at the intermediate dose. On conditioning days, MK-801 produced a dose-dependent enhancement of amphetamine-induced locomotor activity; however, MK-801 alone caused a similar increase in activity. The preferential D2 dopamine receptor antagonist eticlopride (0.01, 0.05, or 0.1 mg/kg SC) significantly reduced amphetamine locomotor activity, and the highest dose blocked place conditioning. These data suggest that the NMDA receptor is not involved in either the rewarding or locomotor-activating effects of amphetamine.

MK-801 prevents the development of behavioral sensitization during repeated morphine administration

Acute administration of morphine (10 mg/kg) to rats elicited an increase in locomotion that became sensitized upon repeated treatment over 14 days. Administration of the noncompetitive N-methyl-D-aspartate receptor (NMDA) antagonist MK-801 (0.1 or 0.25 mg/kg) prior to each morphine injection prevented the development of behavioral sensitization to morphine, an effect that persisted even after a 7-day withdrawal from repeated treatment. Sensitization was also prevented by coadministration of the competitive NMDA receptor antagonist CGS 19755 (10 mg/kg). In contrast, acute pretreatment with MK-801 did not alter the response of sensitized rats to morphine challenge, indicating that MK-801 does not prevent the expression of sensitization. When administered alone, MK-801 produced stereotyped movements at moderate doses (0.25 mg/kg) and horizontal locomotion at higher doses (0.5 mg/kg). Repeated administration of 0.25 mg/kg MK-801 elicited sensitization to its own locomotor stimulatory effects, such that this dose became capable of eliciting horizontal locomotion. Sensitization was not seen during repeated administration of 0.1 mg/kg MK-801 or 10 mg/kg CGS 19755, although both of these pretreatments did produce a sensitized response to subsequent challenge with 0.25 mg/kg MK-801. This effect was enhanced by coadministration of morphine, even though repeated administration of morphine alone failed to sensitize rats to MK-801 challenge. These results suggest a complex interplay between NMDA and opioid receptors, such that NMDA antagonists prevent morphine sensitization while morphine enhances the ability of NMDA antagonists to elicit sensitization to their own locomotor stimulatory effects.

Morphine-induced potentiation of brain stimulation reward is enhanced by MK-801

Morphine (2.5 mg/kg, i.p.) and MK-801 (0.05 mg/kg, i.p.) each enhanced the rewarding impact of electrical stimulation of the medial forebrain bundle, causing small but reliable parallel leftward shifts of the functions relating response rate to stimulation frequency. Administration of MK-801 and morphine together caused a profound leftward shift in the functions. This effect was not due to sensitization to either drug, and suggests that disruption of glutamatergic function can potentiate the rewarding impact of opiates.

The NMDA receptor antagonist MK-801 produces ethanol-like discrimination in the rat

Two groups of rats, one derived from N/Nih stock and the second from the (putatively) serotonin-compromised Fawn-Hooded line, were trained to discriminate ethanol from its vehicle in a drug discrimination paradigm. Once each of the two groups attained discrimination criterion, dose-response relationships with lower doses of ethanol indicated that the Fawn-Hooded rats were less sensitive (ED50 value = 579.5 mg/kg) than the N/Nih rats (ED50 = 371.4 mg/kg). Testing of various doses of the NMDA (N-methyl-D-aspartate) receptor antagonist MK-801 produced complete generalization in each group of animals, with a similar difference in ED50 values between N/Nih and Fawn-Hooded lines. These results extend previous ethanol-to-MK-801 generalization reported in pigeon, and are discussed in light of a possible NMDA-mediated contribution to the ethanol-induced discriminative stimulus cues.

The NMDA receptor antagonist MK-801 elicits conditioned place preference in rats

(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate, (MK-801) a potent noncompetitive antagonist of central NMDA receptors, has been hypothesized to have rewarding properties indicative of abuse potential. To test this hypothesis, the effects of MK-801 on the acquisition of a conditioned place preference and on locomotor activity were assessed and compared with d-amphetamine. Both MK-801 (0.03 and 0.1 mg/kg, SC) and d-amphetamine (1.0 mg/kg, SC) administration resulted in the acquisition of a conditioned place preference. However, while both amphetamine and the higher dose of MK-801 produced a behavioral activation during the training period the lower dose of MK-801 did not. These results suggest that MK-801, at doses that produce behavioral activation and below, is rewarding and therefore may have abuse potential.

The NMDA antagonist dizocilpine (MK801) attenuates motivational as well as somatic aspects of naloxone precipitated opioid withdrawal

The non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine has recently been reported to antagonize certain overt withdrawal signs in morphine dependent rats. The purpose of the present study was to reassess this response and examine the effect of this drug in a model presumably reflective of the motivational impact of withdrawal using the place conditioning technique. Rats were made opiate dependent by the subcutaneous implantation of a 75 mg morphine pellet. Three-4 days later withdrawal was precipitated by naloxone 0.5 mg/kg. Dizocilpine (0.1-0.5 mg/kg) attenuated many of the subsequent behaviours elicited by naloxone, notably diarrhoea, mouth movements, paw shakes and ptosis. In a separate group of morphine dependent rats, naloxone (0.05 mg/kg) precipitated withdrawal produced a clear place aversion. This place aversion was blocked by dizocilpine (0.02-0.1 mg/kg) pre-treatment prior to conditioning. Therefore dizocilpine may modify both motivational and somatic aspects of opioid withdrawal.

Interactions between glutamatergic and monoaminergic systems within the basal ganglia--implications for schizophrenia and Parkinson's disease

Recent animal experiments suggest that dopamine plays a less crucial role than formerly supposed in the regulation of psychomotor functions. This is illustrated by the finding that even in the almost complete absence of brain dopamine, a pronounced behavioural activation is produced in mice following suppression of glutamatergic neurotransmission. This paper discusses the possibility that a deficient activity within the corticostriatal glutamatergic/aspartergic pathway may be an important pathophysiological component in schizophrenia, and that glutamatergic agonists may be beneficial in the treatment of this disease. In addition, it is suggested that glutamatergic antagonists may be valuable supplements in the treatment of Parkinson's disease.