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

Modulation of learning and memory by natural polyamines

Spermine and spermidine are natural polyamines that are produced mainly via decarboxylation of l-ornithine and the sequential transfer of aminopropyl groups from S-adenosylmethionine to putrescine by spermidine synthase and spermine synthase. Spermine and spermidine interact with intracellular and extracellular acidic residues of different nature, including nucleic acids, phospholipids, acidic proteins, carboxyl- and sulfate-containing polysaccharides. Therefore, multiple actions have been suggested for these polycations, including modulation of the activity of ionic channels, protein synthesis, protein kinases, and cell proliferation/death, within others. In this review we summarize these neurochemical/neurophysiological/morphological findings, particularly those that have been implicated in the improving and deleterious effects of spermine and spermidine on learning and memory of naïve animals in shock-motivated and nonshock-motivated tasks, from a historical perspective. The interaction with the opioid system, the facilitation and disruption of morphine-induced reward and the effect of polyamines and putative polyamine antagonists on animal models of cognitive diseases, such as Alzheimer's, Huntington, acute neuroinflammation and brain trauma are also reviewed and discussed. The increased production of polyamines in Alzheimer's disease and the biphasic nature of the effects of polyamines on memory and on the NMDA receptor are also considered. In light of the current literature on polyamines, which include the description of an inborn error of the metabolism characterized by mild-to moderate mental retardation and polyamine metabolism alterations in suicide completers, we can anticipate that polyamine targets may be important for the development of novel strategies and approaches for understanding the etiopathogenesis of important central disorders and their pharmacological treatment.

A Nonrewarding NMDA Receptor Antagonist Impairs the Acquisition, Consolidation, and Expression of Morphine Conditioned Place Preference in Mice

N-methyl-D-aspartate (NMDA) receptor antagonists block morphine-induced conditioned place preference (CPP). Although polyamines are endogenous modulators of the NMDA receptor, it is not known whether polyaminergic agents induce CPP or modulate morphine-induced CPP. Here, we examined whether polyamine ligands modify morphine CPP acquisition, consolidation, and expression. Adult male albino Swiss mice received saline (0.9 % NaCl, intraperitoneally (i.p.)) or morphine (5 mg/kg, i.p.) and were respectively confined to a black or a white compartment for 30 min for four consecutive days for CPP induction. The effect of arcaine (3 mg/kg, i.p.) or spermidine (30 mg/kg, i.p.), respectively, an antagonist and an agonist of the polyamine-binding site at the NMDA receptor, on the acquisition, consolidation, and expression of morphine CPP was studied. In those experiments designed to investigate whether spermidine prevented or reversed the effect of arcaine, spermidine (30 mg/kg, i.p.) was administered 15 min before or 15 min after arcaine, respectively. Arcaine and spermidine did not induce CPP or aversion per se. Arcaine (3 mg/kg, i.p.) impaired the acquisition, consolidation, and expression of morphine CPP. Spermidine prevented the impairing effect of arcaine on the acquisition of morphine CPP but not the impairing effect of arcaine on consolidation or expression of morphine CPP. These results suggest that arcaine may impair morphine CPP acquisition by modulating the polyamine-binding site at the NMDA receptor. However, the arcaine-induced impairment of consolidation and expression of morphine CPP seems to involve other mechanisms.

NMDA receptor glycine modulatory site in the ventral tegmental area regulates the acquisition, retrieval, and reconsolidation of cocaine reward memory

RATIONALE AND OBJECTIVES

Accumulating clinical and preclinical studies have shown that the memories of the rewarding effects of drugs and their paired cues may contribute to relapse and persistent cocaine use. Glutaminergic actions in the ventral tegmental area (VTA) have been shown to regulate the rewarding effect of drugs and conditioned responses to drug-associated cues, but the role of the VTA in the acquisition, retrieval, and reconsolidation of cocaine cues is not yet known.

METHODS

In the present study, we used 7-chlorothiokynurenic acid (7-CTKA), an N-methyl-D-aspartate (NMDA) receptor glycine modulatory site antagonist with no rewarding effects, to examine the role of the NMDA receptor glycine modulatory site in the acquisition, retrieval, and reconsolidation of cocaine-related reward memory using the conditioned place preference (CPP) paradigm.

RESULTS

Separate groups of Sprague-Dawley rats were trained to acquire cocaine-induced CPP. Vehicle or 7-CTKA was microinjected into the VTA or substantia nigra (SN) (5 μg/μl) at different time points: 10 min before each CPP training session (acquisition), 10 min before the reactivation of CPP (retrieval), and immediately after the reactivation of CPP (reconsolidation). Cocaine-induced CPP was retested 24 h and 1 and 2 weeks after 7-CTKA administration. 7-CTKA microinjected into the VTA, but not SN, significantly impaired the acquisition, retrieval, and reconsolidation of cocaine-induced CPP without affecting cocaine-induced locomotion.

CONCLUSIONS

Our findings suggest that the NMDA receptor glycine modulatory site in the VTA plays a major role in cocaine reward memory, and NMDA receptor glycine site antagonists may be potential pharmacotherapies for the management of relapse.

Comparison of the MK-801-induced appetitive extinction deficit with pressing for reward and associated pERK1/2 staining in prefrontal cortex and nucleus accumbens

Administration of the noncompetitive N-methyl-d-aspartate (NMDA)-receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) has been shown to produce extinction deficits on appetitive operant tasks. The present study sought to further explore this by comparing extinction pressing to pressing for the primary reward and examining associated neural correlates to determine if the MK-801 extinction profile resembled the behavioral and neural profile associated with pressing for primary reward. Immunohistochemical labeling of phosphorylated extracellular signal-regulated kinase-1 and -2(pERK1/2) in the prelimbic (PrL) and infralimbic (IL) cortices and nucleus accumbens shell (AcbSh) and core (AcbC) was examined after rewarded or extinction lever pressing conditions. A dose-response curve revealed a within-day extinction deficit following administration of 0.05 mg/kg MK-801. All doses of MK-801 were associated with reduced IL pERK1/2 staining but only the 0.05 mg/kg dose was associated with elevated AcbSh pERK1/2 labeling. Extinction pressing under the influence of MK-801 was elevated compared to that seen during rewarded pressing-whether on MK-801 or saline. Rewarded pressing following saline or MK-801 was associated with elevated pERK1/2 in the PrL with no similar patterns in the MK-801/extinction group. There was more pERK1/2 labeling in the AcbSh of the MK-801 extinction group than any other condition. These data suggest that the MK-801-induced extinction deficit may be due to the combination of an underactive cortical behavioral inhibition system and an overactive AcbSh reward system.

Effects of depressive-like behavior of rats on brain glutamate uptake

Learned helplessness paradigm is a widely accepted animal model of depressive-like behavior based on stress. Glutamatergic system is closely involved with the stress-neurotoxicity in the brain and recently it is pointed to have a relevant role in the pathophysiology of depression disorder. Glutamate uptake is the main mechanism to terminate the glutamatergic physiological activity and to neuroprotection against excitotoxicity. We investigated the profile of glutamate uptake in female rats submitted to the learned helplessness paradigm and to different classes of stress related to the paradigm, in slices of brain cortex, striatum and hippocampus. Glutamate uptake in slices of hippocampus differ between learned helplessness (LH) and non-learned helplessness (NLH) animals immediately persisting up to 21 days after the paradigm. In addition, there were a decrease of glutamate uptake in the three brain structures analyzed at 21 days after the paradigm for LH animals. These results may contribute to better understand the role of the glutamatergic system on the depressive-like behavior.

The role of NR2B containing NMDA receptor in place preference conditioned with morphine and natural reinforcers in rats

It has been reported that N-methyl-D-aspartate (NMDA) receptor is implicated in drug addiction and antagonists of the NMDA receptor complex can inhibit the development and expression of conditioned place preference (CPP) induced by several addictive drugs, implying that this class of compounds might be considered as candidate for the treatment of substance abuse. To explore this possibility, it is important to evaluate whether the inhibitory effect of NMDA receptor antagonists would be confined to behaviors produced by drugs of abuse only, but not by natural reinforcers. According to the quantitative changes of NMDA receptor subunits, including NR1, NR2A, and NR2B, induced by diverse types of reinforcers, we chose NR2B subunit as the target of research. Experimental results showed that (1) an augmented expression of NR2B subunit was revealed by Western blotting in the nucleus accumbens (NAc) and the hippocampus in rats with CPP induced by morphine, but not by natural rewards such as food, novel environment and social interaction. (2) Ifenprodil, an antagonist highly selective for NR2B subunit of the NMDA receptor, produced a dose-dependent reduction in CPP induced by morphine and novel environment, but not that by food consumption and social interaction. Taking together, these findings suggested that NR2B containing NMDA receptor may be more involved with morphine reward rather than natural rewards, and that antagonism of NR2B may have a potential for the treatment of morphine abuse.

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.

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.

Emerging experimental therapeutics for bipolar disorder: clues from the molecular pathophysiology

Bipolar affective disorder (manic-depressive illness) is a common, severe, chronic, and often life-threatening illness, associated with significant comorbidity. The recognition of the significant morbidity and mortality of patients with bipolar disorder, as well as the growing appreciation that a high percentage of patients respond poorly to existing treatments, has made the task of discovering new therapeutic agents, that are both efficacious and have few side effects increasingly more important. Most recent agents introduced into the pharmacopeia for the treatment of bipolar disorder have been anticonvulsants and atypical antipsychotics. We propose that novel treatments developed specifically for bipolar disorder will arise from (1) understanding more precisely the molecular mechanisms of treatments that are clearly efficacious or (2) developing medications based on the knowledge obtained of the underlying pathophysiology of bipolar disorder. Knowledge with regard to the underlying pathophysiology of bipolar disorder is increasing at a rapid pace, including alterations in intracellular signaling cascades as well as impairments of cellular plasticity and resilience in critical neuronal circuits. We propose that therapeutics designed to enhance cellular plasticity and resilience and that counter maladaptive stress-responsive systems may have considerable utility for the treatment of bipolar disorder. Therapeutic strategies designed to address cellular resilience and plasticity include the regulation of neurotrophic pathways, glucocorticoid signaling, phosphodiesterase activity, and glutamatergic throughput and mitochondrial function. While the task of developing novel medications for bipolar disorder is truly daunting, these and similar approaches will ultimately lead to better medications for the millions who suffer from this devastating illness.

The Role of NMDA Receptor Binding Sites in Ethanol Place Conditioning

Little is known about the specific role of glutamate, in particular its actions at N-methyl-D-aspartate (NMDA) receptors, in ethanol reward. Pretreatment with channel blockers MK-801 and ketamine, NMDA NR2B receptor subunit antagonists ifenprodil and CP-101,606, and the glycine(B) partial agonist (+)-HA-966 did not alter acquisition of ethanol-induced conditioned place preference (CPP) in mice. However, pretreatment with the competitive antagonist CGP-37849 attenuated acquisition of ethanol-induced CPP. Follow-up experiments indicated that CGP-37849 also blocked acquisition of ethanol-induced and lithium chloride-induced conditioned place aversion but did not produce rewarding or aversive effects on its own. These results suggest that the NMDA receptor glutamate binding site is important for ethanol place conditioning. Moreover, these results suggest CGP-37849 modulates ethanol place conditioning by impairing the ability to learn these tasks.

Acamprosate does not induce a conditioned place preference and reveals no state-dependent effects in this paradigm

To evaluate the putative rewarding properties of the anticraving substance acamprosate, male rats learned to associate injections of vehicle and acamprosate (200 mg/kg ip) with two visually contrasting compartments in a place conditioning paradigm. The degree of preference for the acamprosate-associated compartment was determined, in both a postconditioning test with undrugged animals and a consecutive test with drugged animals, to rule out the possibility that a putatively rewarding effect of acamprosate may have been masked by state-dependent effects. The animals did not show any preference for the substance-paired compartment, neither in the undrugged nor in the drugged state. In conclusion, acamprosate has no rewarding properties as shown in place preference. Therefore, it may prevent a relapse in detoxified alcoholics in a way other than by simply substituting the rewarding effects of ethanol. This adds to the therapeutic value of acamprosate in the treatment of drug craving.

The effect of (R)-HA966 or ACEA 1021 on dexfenfluramine or (S)-MDMA-induced changes in temperature, activity, and neurotoxicity

The glycine site-specific N-methyl-D-aspartate (NMDA) antagonist 5-nitro-6,7-dichloro-2,3-quinoxalinedione (ACEA 1021, 4x30 mg/kg, i.p.) given 30 min before dexfenfluramine (4x15 mg/kg, i.p., every 2 h) was unable to prevent dexfenfluramine-induced depletion of 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) content, and 5-HT transporter (5-HTT) density. Another glycine site-specific NMDA antagonist, R(+)-3-aminohydroxypyrrolidin-2-one [(R)-HA 966] (2x30 mg/kg, ip), given 30 min before dexfenfluramine (2x10 mg/kg, ip, 2 hourly) was also unable to prevent regional depletion of 5-HT, 5-HIAA, and 5-HTT density. However, ACEA 1021 (4x30 mg/kg, i.p.) given 30 min before (S)-3,4-methylenedioxymethamphetamine (MDMA, 4x10 mg/kg, 2 hourly, ip) attenuated the regional depletion of dopamine (DA), dihydroxyphenylacetic acid (DOPAC), 5-HT, 5-HIAA content, and 5-HTT density. ACEA 1021 combined with (S)-MDMA also prevented (S)-MDMA-induced hyperthermia without causing hypothermia or preventing an (S)-MDMA-induced increase in locomotor activity.

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.

Correlation between 5-HT content and uptake site density following (S)-MDMA and dexfenfluramine-induced depletion, and with neuroprotection by the glycine site-specific NMDA antagonist ACEA 1021

3,4-Methylenedioxymethamphetamine (MDMA) and fenfluramine are amphetamine analogues that both cause long-term depletion of serotonin (5-HT) and 5-HT uptake sites in brain tissue. Depletion caused by these amphetamines is commonly measured by labeling 5-HT uptake sites using 3H-paroxetine combined with autoradiography or, alternatively measuring the concentration of 5-HT in tissue using high-performance liquid chromatography (HPLC). A close correlation between the 5-HT concentration measured in micropunch samples and the density of 3H-paroxetine-labeled 5-HT uptake sites measured in corresponding 20 micron coronal slices was determined (R2 = 0.92). These methods combined demonstrated that the glycine-site specific NMDA antagonist ACEA 1021 (4 x 30 mg/kg, i.p., 2 hourly) given 30 minutes before (S)-MDMA (4 x 10 mg/kg, i.p., 2 hourly) was able to prevent the depletion of both 5-HT content and uptake site density but unable to prevent the depletion of 5-HT content and uptake site density caused dexfenfluramine (4 x 15 mg/kg, i.p., 2 hourly).

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.

The rewarding properties of NMDA and MK-801 (dizocilpine) as indexed by the conditioned place preference paradigm

N-Methyl-D-aspartate (NMDA) ([R]-2-[Methylamino]succinic acid) is a specific excitatory amino acid. Two experiments were conducted to determine the rewarding properties of this compound using the conditioned place preference paradigm. In the first experiment, 40 male Sprague-Dawley rats received place preference conditioning for a 4 day period. The conditioned place preference apparatus consisted of two chambers with distinct visual and tactile cues, separated by a removable door. On days 2 and 4, rats were systemically administered NMDA (1.0, 15.0, and 30.0 mg/kg) paired with one chamber. On days 3 and 5, rats were systemically administered saline paired with the other chamber. Day 6 was the test day, and the rat was allowed free run of the entire apparatus in a drug-free state. Time spent in each side of the apparatus was computer recorded. NMDA produced a significant increase in the amount of time spent on the side previously paired with drug for 15.0 and 30.0, but not 1.0 mg/kg NMDA. In the second experiment, systemic administration of NMDA (30.0 mg/kg) paired with the noncompetitive NMDA receptor antagonist, MK-801 (0.5 mg/kg), resulted in neither place preference nor place aversion.

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).

Involvement of the serotonergic neuronal system in phencyclidine-induced place aversion in rats

The possible involvement of the serotonergic neuronal system in aversive motivation produced by phencyclidine [1-(1-phenylcyclohexyl)piperidine; PCP] was investigated using a place-conditioning paradigm in rats. PCP (4 mg/kg, i.p.) produced place aversion in this task as reported previously (Kitaichi K, Noda Y, Hasegawa T, Furukawa H, Nabeshima T. Acute phencyclidine induces aversion, but repeated phencyclidine induces preference in the place conditioning test in rats. Eur J Pharmacol 1996;318:7-9). The blockade of serotonin2A (5-HT2A) receptors using the antagonist ritanserin (3 and 10 mg/kg, p.o.) significantly attenuated this aversive property of PCP whereas lesions of serotonergic neurons using 5,7-dihydroxytryptamine (5,7-DHT, 100 microg/animal, i.c.v.) failed to affect it. Repeated PCP treatment (10 mg/kg, i.p. for 14 days), which is enough to diminish the stereotyped 5-HT2A receptor-mediated head-twitch behavior, also decreased the place aversion. These results suggest that the serotonergic neuronal system, specifically the 5-HT2A receptor, may play a critical role in producing PCP-induced place aversion.

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.