7-Nitroindazole blocks conditioned place preference but not hyperactivity induced by morphine

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.

Involvement of the insular nitric oxide signaling pathway in the expression of morphine-induced conditioned place preference in rats

Nitric oxide (NO) has been recently reported to play an important role in the rewarding effects of addictive drugs. The regional NO signaling in the brain, however, is not completely clear. Here, we studied the effects of insular NO signaling on the expression of morphine-induced conditioned place preference (CPP). Insular microinjection of the NO inhibitors N-nitro L-arginine methyl ester (L-NAME) and 7-nitroindazole reduced the expression of morphine-induced CPP. The NO donor molsidomine, in contrast, reversed L-NAME-induced reduction of CPP expression. These results suggest that insular NO signaling is involved in the expression of morphine-CPP.

The role of acetaldehyde in ethanol reinforcement assessed by Pavlovian conditioning in newborn rats

RATIONALE

Animal studies indicate that central acetaldehyde, dependent on catalase metabolism of ethanol (EtOH), modulates ethanol reinforcement. Brain catalase activity and acetaldehyde (ACD) production are significantly higher in rat pups compare d with adults. Interestingly, infant rats show high EtOH affinity for alcohol consumption and are particularly sensitive to the drug's reinforcing effects.

OBJECTIVES

We tested whether central ACD is necessary and sufficient to induce appetitive conditioning in newborn rats through the artificial nipple technique.

METHODS

Vehicle, EtOH (100 mg%), and acetaldehyde (0.35 μmol) were administered into the cisterna magna (1 μl). Half of the animals also received a central administration of 75 μg (experiment 1) or 40 μg of D-penicillamine (experiment 2). Afterwards, pups were exposed to an olfactory cue (conditioned stimulus). One hour later, neonates were tested with an artificial nipple in the presence of the conditioned cue. Nipple attachment duration, mean grasp duration, and number of nipple disengagements served as dependent variables.

RESULTS

Positive responses to the scented nipple occurred in neonates conditioned with EtOH or ACD (experiments 1 and 2). In experiment 1, there were indications that D-penicillamine weakened the reinforcing effects of EtOH and ACD. In experiment 2, D-penicillamine (40 μg) significantly inhibited appetitive conditioned responses dependent upon EtOH or ACD.

CONCLUSIONS

Appetitive conditioning was observed when employing either central EtOH or ACD as unconditioned stimuli. Central abduction of ACD inhibited conditioned appetitive responsiveness to the surrogate nipple. Central ACD is involved in the determination or modulation of EtOH's motivational properties during early stages in development.

Lithium chloride disrupts consolidation of morphine-induced conditioned place preference in male mice: the nitric oxide/cyclic GMP signaling pathway

Lithium effects on brain functions such as cognition, attention, learning and memory are well-established for ages; however, the way it affects these functions and its precise mechanism of action remains unknown. The purpose of this study was to determine the effects of lithium on the consolidation of morphine-associated conditioned place preference and the possible involvement of the NO/cGMP pathway. Using an unbiased conditioned place preference (CPP) model, the effects of lithium (1-100 mg/kg, i.p.), nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) (5-100 mg/kg, i.p.), nitric oxide precursor L-arginine (50-150 mg/kg, i.p.) and phosphodiesterase inhibitor sildenafil (5-40 mg/kg, i.p.) on the consolidation of morphine-induced CPP were assessed. In addition, the possible interaction between lithium, L-arginine and sildenafil or subeffective doses of lithium and L-NAME on the consolidation of morphine-induced contextual memory was evaluated. NMRI mice were used in all studies. Lithium (5-30 mg/kg, i.p.), immediately after conditioning trials, significantly reduced the time spent by mice in the reward-paired compartment. Although post-training administration of L-arginine, sildenafil or L-NAME had no significant effect on the consolidation of CPP, concomitant administration of L-arginine (50-150 mg/kg) and sildenafil (5-10 mg/kg) with lithium (30 mg/kg) prevented the impairing effect of lithium. Also, co-administration of sub-effective doses of lithium (1 mg/kg) and L-NAME (5 mg/kg) disrupted consolidation of CPP. However, delayed administration of effective doses of lithium, which shows specific effect on memory consolidation, did not affect morphine-induced CPP. Lithium seems to inhibit consolidation of morphine-induced CPP and this impairing effect might be via nitric oxide/cyclic GMP pathway.

An acetaldehyde-sequestering agent inhibits appetitive reinforcement and behavioral stimulation induced by ethanol in preweanling rats

Ethanol's motivational consequences have been related to the actions of acetaldehyde, a metabolic product of ethanol oxidation. The present study assessed the role of acetaldehyde in the motivational effects of ethanol on preweanling rats. In Experiment 1 pups (postnatal days 13-14, PD 13-14) were given systemic administration of D-penicillamine (DP, a drug that sequesters acetaldehyde: 0, 25, 50 or 75 mg/kg) before pairings of 1.0 g/kg ethanol and a rough surface (sandpaper, conditioned stimulus, CS). At test, pups given sandpaper-ethanol pairings exhibited greater preference for the CS than unpaired controls, but this preference was not expressed by pups given DP. Pre-training administration of 25 or 50 mg/kg DP completely blocked the expression of ethanol-mediated appetitive conditioning. D-penicillamine did not alter blood ethanol levels. Subsequent experiments revealed that ethanol-induced activation was blocked by central (intra-cisterna magna injections, volume: 1 μl, dose: 0 or 75 μg) but not systemic treatment with DP (0, 25, 50 or 75 mg/kg; ip). These results indicate that: (a) preweanling rats are sensitive to the reinforcing effect of ethanol, and (b) that this effect is associated with the motor activating effect of the drug. These effects seem to be mediated by the first metabolite of ethanol, acetaldehyde.

Intracerebroventricular administration of nitric oxide-sensitive guanylyl cyclase inhibitors induces catalepsy in mice

RATIONALE

Catalepsy is a preclinical test that predicts extrapyramidal symptoms in humans. It models symptoms of acute extrapyramidal side effects induced at the beginning of antipsychotic treatment. Nitric oxide (NO) plays a role in a series of neurobiological functions underlying behavior. For example, inhibition of NO synthesis disrupts rodent exploratory behavior and induces catalepsy. Although several effects mediated by NO involve the activation of soluble guanylyl cyclase (sGC), the transduction mechanism of the catalepsy-inducing effect of NO has not yet been investigated.

OBJECTIVES

The study was designed to test if intracerebroventricular (i.c.v.) microinjection of NO-sensitive inhibitors of sGC (NO-sGC) induces catalepsy in mice similar to that induced by NO synthase (NOS) inhibitors. Exploratory behavior was tested in the open field. In addition, the effects of a NOS inhibitor on oxidative metabolites of NO were measured in the striatum.

MATERIALS AND METHODS

Drug effects were examined in the hanging-bar test after the following i.c.v. treatments: oxadiazolo-quinoxalin (ODQ, 30-300 nmol) or methylene blue (MB, 3-100 nmol), selective and nonselective sGC inhibitors, respectively, or 7-nitroindazole (7-NI, 3-90 nmol) and G-nitro-L: -arginine methyl ester (L: -NAME, 3-90 nmol), selective and nonselective neuronal NOS inhibitors. To test if the effects were related to interference with the NO system, additional groups received 7-NI (30 nmol), ODQ (100 nmol), or L-NAME (90 nmol) preceded by L: -arginine (L: -arg, 30-100 nmol, i.c.v. 30 min before). A possible interference of ODQ and 7-NI on exploratory behavior was tested in an open field. The concentration of nitrites and nitrates (NO( x )) in striatum homogenates was measured by the Griess reaction.

RESULTS

Both NO-sGC and NOS inhibitors induced catalepsy in mice that lasted for at least 2 h. The range of effective doses of these drugs, however, was limited, and the dose-effect curves had an inverted U shape. The cataleptic effect induced by L: -NAME was inversely correlated with NO( x ) products in the striatum. The cataleptic effect of 7-NI and ODQ was prevented by pretreatment with L: -arginine. No drug changed exploratory behavior in the open field.

CONCLUSION

This study showed that pharmacological disruption of the endogenous NO-sGC signaling in the central nervous system induces long-lasting catalepsy in mice. Moreover, the cataleptic effect of NOS inhibition correlates with the decrease in NO( x ) products formation in the striatum. The results give further support to the hypothesis that NO plays a role in motor behavior control mediated, at least in part, by cyclic guanosine monophosphate production in the striatum.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Morphine-induced antinociception in the formalin test: sensitization and interactions with D1 and D2 dopamine receptors and nitric oxide agents

In this study, the effects of dopamine receptor antagonists and nitric oxide agents on morphine-induced sensitization in the formalin test in mice have been investigated. Repeated daily intraperitoneal administration of morphine (30 mg/kg for 3 days) followed by a 11-day wash out period increased morphine-induced antinociception in the formalin test, which may be due to sensitization. The antinociceptive response to higher doses of morphine (6 and 9 mg/kg) but not 3 mg/kg was significantly increased in sensitized animals compared with control groups. Pretreatment of animals with an opioid receptor antagonist, naloxone (4 mg/kg), during repeated administration of morphine, attenuated the morphine-induced sensitization. In the second part of the study, the animals received SCH23390 (D1 receptor antagonist), sulpiride (D2 receptor antagonist), L-Arg (nitric oxide precursor) and NG-nitro-L-Arg methylester (nitric oxide synthase inhibitor) during repeated morphine administration, to evaluate the role of dopamine receptor antagonists and nitric oxide agents in this phenomenon. Pretreatment of animals with NG-nitro-L-Arg methylester (20 mg/kg) and sulpiride (100 mg/kg) during morphine sensitization decreased the antinociceptive response to higher doses of morphine in the formalin test. It is concluded that D2 dopamine receptor and nitric oxide mechanisms may be involved at least partly in morphine-induced sensitization in the formalin test.

Involvement of Nitric Oxide in Spatial Memory Deficits in Status Epilepticus Rats

Status epilepticus (SE) is associated with a significant risk of cognitive impairment, and the increase of nitric oxide (NO) releasing has been reported during SE. We investigated the effects of neuronal nitric oxide synthase (nNOS) inhibitor, 7-nitroindazole (7-NI) and inducible nitric oxide synthase (iNOS) inhibitor, aminoguanidine (AG), on spatial performance of rats in the Morris water maze. Treatment with 7-NI, but not with AG, improved the performance of rats after SE not only in acquisition of the task but also in probe test. Furthermore, the level of SE-induced malondialdehyde (MDA), end product of lipid peroxidation, was significantly decreased only in animals receiving 7-NI injection. Taken together, the results of the present study provided evidence that the NO pathway contributed to oxidative stress after SE, and nNOS/NO pathway may underlie one of the potential mechanisms contributing to SE-induced spatial memory deficits.

Tissue-specific Autophagy Alterations and Increased Tumorigenesis in Mice Deficient in Atg4C/Autophagin-3

Atg4C/autophagin-3 is a member of a family of cysteine proteinases proposed to be involved in the processing and delipidation of the mammalian orthologues of yeast Atg8, an essential component of an ubiquitin-like modification system required for execution of autophagy. To date, the in vivo role of the different members of this family of proteinases remains unclear. To gain further insights into the functional relevance of Atg4 orthologues, we have generated mutant mice deficient in Atg4C/autophagin-3. These mice are viable and fertile and do not display any obvious abnormalities, indicating that they are able to develop the autophagic response required during the early neonatal period. However, Atg4C-/--starved mice show a decreased autophagic activity in the diaphragm as assessed by immunoblotting studies and by fluorescence microscopic analysis of samples from Atg4C-/- GFP-LC3 transgenic mice. In addition, animals deficient in Atg4C show an increased susceptibility to develop fibrosarcomas induced by chemical carcinogens. Based on these results, we propose that Atg4C is not essential for autophagy development under normal conditions but is required for a proper autophagic response under stressful conditions such as prolonged starvation. We also propose that this enzyme could play an in vivo role in events associated with tumor progression.

[Effects of new nitric oxide synthase inhibitors on spontaneous locomotor activity]

INTRODUCTION

New nitric oxide synthase (NOS) inhibitors: 3-bromo-7-nitroindazole (3-Br-7-NI), 1-(2-trifluoromethylphenyl) imidazole (TRIM), S-methyl-L-thiocitrulline (S-Me-TC) and 7-nitroindazole (7-NI) reduce spontaneous locomotor activity in mice.

MATERIAL AND METHODS

In order to elucidate central effects of NOS inhibitors on locomotor activity, the influence of 7-NI on electroencephalographic (EEG) power spectrum in rats was investigated.

RESULTS

7-NI reduced the EEG power density in all frequency bands in rats, suggesting a depression of the central neuronal activity. The electrophysiologic power was most reduced in the range of 7-9 Hz of the rhythmic slow activity (theta rhythm), which is in accordance with decreased locomotor activity observed following administration of NOS inhibitors.

CONCLUSION

The present results indicate that nitric oxide exerts an excitatory effect on central neuronal structures involved in regulation of locomotion.

Involvement of nitric oxide system in enhancement of morphine-induced conditioned place preference by agmatine in male mice

Agmatine recently has been suggested as a neurotransmitter, is able to interact with various effects of morphine like analgesia and dependence. In this study, the effects of agmatine on rewarding properties of morphine, and the possible involvement of nitric oxide (NO) system has been evaluated in an unbiased conditioned place preference (CPP) paradigm. Agmatine (1, 5 and 10mg/kg, i.p.) alone induced neither CPP nor conditioned place aversion (CPA). Morphine (0.01, 0.05, 0.1 and 0.5mg/kg, s.c.), while unable to show CPP or CPA, induced CPP in mice pretreated with agmatine. L-arginine (200mg/kg, i.p.), a NO precursor, significantly enhanced the effect of agmatine (5mg/kg) on morphine (0.5mg/kg)-induced place preference. NG-nitro-L-arginine methyl ester (L-NAME; 2.5mg/kg, i.p.), a non specific nitric oxide synthase (NOS) inhibitor, and aminoguanidine (50 and 100mg/kg, i.p.), a specific inducible NOS inhibitor, significantly reduced the effect of agmatine (5mg/kg) on morphine (0.5mg/kg)-induced place preference. These results suggest the possible involvement of inducible nitric oxide system in potentiating effects of agmatine on morphine-induced place preference.

Rewarding properties of sildenafil citrate in mice: role of the nitric oxide-cyclic GMP pathway

RATIONALE

Sildenafil citrate is widely prescribed for erectile dysfunction and acts by inhibiting phosphodiesterase type-5, resulting in accumulation of cyclic-guanosine monophosphate (cGMP) via activation of nitric oxide synthase (NOS). The nitric oxide (NO) system is relevant to the rewarding effects of various drugs of abuse. Several epidemiologic studies indicate that sildenafil is abused in a recreational fashion.

OBJECTIVES

In the present study, the rewarding properties of sildenafil and probable involvement of the NO-cGMP pathway were investigated in adult male NMRI mice.

METHODS

The ability of sildenafil citrate (1-40 mg/kg) to produce conditioned place preference (CPP) was studied in an unbiased CPP paradigm. The effects of NO precursor L-arginine, nonselective NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), and the inhibitor of guanylyl cyclase methylene blue (MB) on sildenafil-induced CPP were assessed.

RESULTS

Mice that received sildenafil (20 and 40 mg/kg) in one environment during conditioning phase displayed a preference for this environment. Both L-NAME (5 mg/kg) and MB (1 mg/kg) in combination with sildenafil (20 mg/kg) suppressed the acquisition of sildenafil-induced place preference. Lower and per se noneffective dose of sildenafil (10 mg/kg) and L-arginine (60 mg/kg), when coadministered, exerted a significant place conditioning.

CONCLUSIONS

Sildenafil shows rewarding properties that may involve the NO-cGMP pathway.

Ethanol-induced conditioned place preference, but not aversion, is blocked by treatment with D -penicillamine, an inactivation agent for acetaldehyde

RATIONALE

There is evidence to suggest that acetaldehyde is involved in the control of ethanol-seeking behavior and reward. D -penicillamine, a thiol amino acid, is a highly selective agent for the inactivation of acetaldehyde. Previous studies from our laboratory have demonstrated that D -penicillamine prevents both behavioral stimulation induced by ethanol and acetaldehyde-produced locomotor depression in mice.

OBJECTIVES

The contribution of ethanol-derived acetaldehyde to the affective effects of ethanol (preference and aversion) was assessed using an unbiased place conditioning design.

METHODS

Male mice received four pairings of a distinctive floor stimulus (CS+: GRID+ or HOLE+) with injections of saline and ethanol (2 g/kg) given before (preference) or after (aversion) the 5-min exposure to the place conditioning apparatus. A different floor stimulus (CS-: GRID- or HOLE-), associated with saline-saline injections on alternate days, was presented. For a different group of animals, the pairings with the CS+ were associated with saline and ethanol injections, but on alternate days, they received D -penicillamine (50 or 75 mg/kg) and ethanol injections paired with the CS-floor stimulus. A 60-min preference test was carried out 24 h after the last conditioning trial. A similar procedure was followed to test the effect of D -penicillamine on morphine (16 mg/kg) and cocaine-induced (20 mg/kg) conditioned place preference (CPP).

RESULTS

CPP and conditioned place aversion (CPA) were observed for ethanol, but D -penicillamine only blocked CPP. D -penicillamine, by itself, did not produce either rewarding or aversive effects. CPP observed for morphine and cocaine was unaffected by D -penicillamine pretreatment.

CONCLUSIONS

The results of the present study suggest that the selective inactivation of acetaldehyde blocked the rewarding, but not aversive, effects of ethanol and support the role of this ethanol metabolite in the affective properties of ethanol.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Long-lasting rewarding effects of morphine induced by drug primings

To evaluate the persistence of the rewarding effects of morphine, the acquisition, maintenance, extinction and reinstatement of a conditioned place preference (CPP) was assessed in OF1 mice. In Experiment 1, the persistence of morphine-induced CPP was evaluated weekly. Mice showed CPP after four sessions of conditioning with 5, 10, 20 and 40 mg/kg of morphine, which lasted 0, 1, 2 and 4 weeks, respectively. In Experiment 2, after four sessions of conditioning with 40 mg/kg of morphine, the effects of four schedules of extinction differing in the time interval (2, 4, 6 or 8 weeks) between sessions were evaluated. CPP was no longer evident after 8 weeks for the groups examined each 2 or 8 weeks and after 12 weeks for the groups examined each 4 and 6 weeks. After extinction, the reinstating effects of a priming dose of 20 mg/kg of morphine were demonstrated. This procedure of extinction/reinstatement was repeated with a decreasing priming dose of morphine (10, 5, 2.5 and 1.25 mg/kg) until a noneffective dose was found. These results show that morphine-induced CPP is very persistent over time, suggesting that drug exposure induces long-lasting changes in the brain, which supports the idea that drug addiction must be considered as a chronic, lifelong disorder.

Reduced nNOS expression induced by repeated nicotine treatment in μ-opioid receptor knockout mice

To determine whether neuronal nitric oxide synthase (nNOS) is involved in nicotine-induced behavioral sensitization in mu-opioid receptor knockout mice we adopted an immunohistochemical approach. Our results confirm that repeated nicotine administration increased locomotor activity in wild-type mice, but failed to increase locomotor activity in mu-opioid receptor knockout mice, thus suggesting that the mu-opioid receptor is involved in behavioral sensitization. Higher numbers of nNOS-positive cells were observed in the striatum of wild-type mice repeatedly treated with nicotine than in saline-treated wild-type mice. However, mu-opioid receptor knockout mice showed significantly lower nicotine-induced nNOS expression in the striatum versus wild-type mice. No differences were found in the hilus of the dentate gyrus between wild-type and mu-opioid receptor knockout mice. These findings demonstrate that the absence of mu-opioid receptors can cause a significant reduction in the expression of nNOS in the striatum, as induced by repeated nicotine treatment.

NMDA glutamate but not dopamine antagonists blocks drug-induced reinstatement of morphine place preference

The effects of dopaminergic and glutamatergic antagonists on the drug-induced reinstatement of a previously extinguished morphine conditioned place preference (CPP) in mice were evaluated. Following extinction of a place preference induced by morphine (40 mg/kg), a non-contingent injection of the dopaminergic antagonists SCH 23390 (0.125, 0.5 mg/kg), raclopride (0.3, 1.2 mg/kg), haloperidol (0.1, 0.2 mg/kg) and the dopamine (DA) release inhibitor CGS 10746B (1, 10 mg/kg) or glutamatergic NMDA antagonists memantine (10, 20, 40 mg/kg) and MK-801 (0.1, 0.2, 0.3 mg/kg) alone or with 10 mg/kg morphine was given. Neither the dopaminergic nor the glutamatergic antagonists alone reinstated the place preference. Dopamine antagonists failed to block the morphine-induced reinstatement of place preference while memantine and MK-801 blocked it with intermediate and high doses. These results suggest that drug-induced reinstatement of place preference may be largely independent of dopamine and more closely related to glutamatergic neurotransmission.