Effects of Memantine, an NMDA Receptor Antagonist, on Place Preference Conditioned with Drug and Nondrug Reinforcers in Mice P

Original article 237

Effects of memantine, an NMDA receptor antagonist, on place preference conditioned with drug and nondrug reinforcers in mice P. Popika, M. Wrobela, R. Rygulaa, A. Bisagab and A.Y. Bespalovc

Antagonists of the N-methyl-D-aspartate (NMDA) receptor tioned with morphine and sexual encounter, but had no complex have been shown to inhibit the expression of effects in food-conditioned mice. These findings suggest place preferences conditioned with several drugs that are that the effects of NMDA receptor blockade may not be abused by humans, which suggests that this class of limited to drug-reinforced behaviors. Behavioural Phar- compounds may be beneficial in the treatment of sub- macology 14:237–244 c 2003 Lippincott Williams & stance dependence. Therefore it is important to assess the Wilkins. specificity of this effect, of whether inhibitory effects of NMDA receptor antagonists on conditioned drug stimuli Behavioural Pharmacology 2003, 14:237–244 generalize to behaviors produced by nondrug reinforcers. Keywords: conditioned place preference, NMDA receptor antagonist, The present study was designed to compare the effects of memantine, morphine, sexual interaction, food reward, mouse the NMDA receptor channel blocker, memantine, on the aInstitute of Pharmacology, Polish Academy of Sciences, Krako´ w, Poland, expression of place preferences conditioned with: bDepartment of Psychiatry, Columbia University College of Physicians and (1) consumption of regular laboratory food; (2) sexual Surgeons, New York, USA and cInstitute of Pharmacology, Pavlov Medical encounters with females; and (3) injection of morphine University, St. Petersburg, Russia. (10 mg/kg) in adult male Swiss mice. For all three Sponsorship: This study and preparation of the manuscript were supported by experiments reported here, unconditioned stimuli (food, Institute of Pharmacology Statutory Activity, State Committee for Scientific Research (KBN grant No P05A. 042-17 to P.P.), by grant DA-00429 from NIDA receptive female or morphine) were presented before the (A.B.), and the Fogarty International Research Collaboration Award exposures to the ‘to-be-conditioned’ environments. Sig- # R03TW00714 (A.Y.B.). nificant place preferences developed as a result of explicit Correspondence and requests for reprints to Piotr Popik, Institute of pairings of the environmental context and food consump- Pharmacology, Polish Academy of Sciences, 12 Sme˛tna Street, 31–343 Krako´ w, Poland. tion, sexual encounter and morphine administration. E-mail: [email protected] Memantine (7.5mg/kg, given prior to the post-conditioning test) inhibited the expression of place preferences condi- Received 22 January 2003 Accepted as revised 20 March 2003

Introduction sponses are expressed as increased time spent in the The behaviors that are involved in drug abuse are drug-associated environment and are thought to repre- known to be highly sensitive to environmental control. sent an indirect measure of a drug’s rewarding effects Environmental cues that were previously associated (Carr et al., 1989). It has been reported that memantine with drug intake can later elicit drug craving and trigger blocked the expression of morphine- (Popik and Danysz, relapse to drug taking in abstinent individuals (Childress 1997) and cocaine- (Kotlinska and Biala, 2000) induced et al., 1988; O’Brien et al., 1998). Therefore, agents conditioned place preferences in rats. that effectively reduce responding to such drug- associated cues may be considered beneficial for medica- The important feature of drug abuse pharmacotherapy is tion development. For example, preliminary clinical that it should selectively interfere with the drug-related evidence suggests that a clinically used N-methyl- responses while leaving other biologically significant D-aspartate (NMDA) receptor channel blocker, meman- appetitive behaviors intact. In experimental studies, tine (Parsons et al., 1999), may be capable of attenuating food-maintained behavior is often taken as a baseline craving and relapse rate in heroin addicts (Bespalov against which the drug-maintained behavior is assessed. et al., 2001). For instance, experimental medications are required to alter drug self-administration while having little or no Antagonists of the NMDA subtype of glutamate receptors effects on food-seeking and eating (e.g., Mello et al., have been shown repeatedly to attenuate both develop- 1993). Similarly, in place conditioning studies, effects of ment and expression of place preferences conditioned the potential medications are assessed in both drug- and with various drugs of abuse (reviewed by Bisaga and food-conditioned animals (Popik and Danysz, 1997; Papp Popik, 2000). In this paradigm, drug-conditioned re- et al., 2002).

0955-8810 c 2003 Lippincott Williams & Wilkins DOI: 10.1097/01.fbp.0000074774.49104.0e

There are at least two reasons to investigate the effects of Method NMDA receptor antagonists on behaviors maintained by Subjects other types of reinforcing stimuli than drugs of abuse. Male and female albino Swiss mice (Breeding Facility of First, one of the commonly held views on the mechanism the Institute of Pharmacology, Polish Academy of of compulsive drug seeking and taking expects both Sciences) weighing 25–30 g were housed separately in ‘drug’ (e.g. opiate-induced) and ‘natural’ (food, sex) groups of 3–9 in standard plastic cages (43 Â 27 Â 15 cm), rewards to be mediated by a common neuroanatomical in a colony room maintained at a temperature of 21 ± 11C pathway, most likely by the mesocorticolimbic systems and 50–70% humidity. Artificial light was provided daily (Di Chiara et al., 1998). If NMDA receptor antagonists from 07.00 hours to 19.00 hours, for all mice, except for have an effect on behaviors maintained by both drug and those used in the sexual conditioning experiment, which nondrug reinforcers, this could suggest a common were maintained under a reversed light/dark cycle (lights mechanism or pathway involved in the processing of off at 07.00 hours, on at 19.00 hours). Unless indicated these stimuli. Secondly, NMDA receptor antagonists otherwise, food and water were freely available. Experi- were shown to reduce the intensity of a variety of mental procedures were approved by the Institutional behaviors related to drug addiction and dependence Animal Care and Use Committee and were performed in (reviewed by Herman et al., 1995; Bisaga and Popik, accordance with the recommendations and policies of the 2000). As NMDA (and other glutamate) receptor US National Institute of Health Principles of Laboratory antagonists are probed as potential candidates Animal Care (1996). for developing an effective drug abuse medication, the specificity of their effects becomes of critical Apparatus significance. The conditioning apparatus consisted of three rectangular arms (30 Â15 Â20 cm) placed at 1201 to each other, all of The present study aimed to establish place preferences which were accessible from a triangular (central) platform conditioned with food, sexual encounter and morphine (Popik et al., 2003). The apparatus was made of Metaplex administration in male mice and to compare the effects and the three arms differed in distinctive visual, tactile of memantine on the expression of place pre- and olfactory cues. One arm was white, had a black floor ference responses. Mice have already been used in with small holes, and was marked with peppermint odor; place conditioning experiments. However, with some one arm was black, had a rough white floor and was exceptions (Hutcheson et al., 1998; Martin et al., 2000; marked with anise odor; the final arm was black with a Valjent and Maldonado, 2000; Manzanedo et al., plain black floor, and had no odor. These distinct cues 2001), most of the experiments were carried out served as conditioned stimuli. The use of visual, olfactory using the ‘biased’ version of this procedure. In the and tactile cues ensured direct contact with the con- ‘biased’ version, the rewarding treatment is paired with ditioned stimuli during the preference testing the initially nonpreferred environment, while its control (Mucha et al., 1982). The guillotine doors, colored condition is paired with the initially preferred according to the respective wall colors, were inserted environment. Because the use of the ‘biased’ procedure during the conditioning sessions and were removed has many limitations (Bozarth, 1987; Spyraki, 1988), during the pre- and post-conditioning tests. The ceiling the present study involved an ‘unbiased’ procedure. of the three arms was made of transparent Plexiglas. In a conventional place conditioning study design, During testing, the location of the mouse was monitored there is a potentially critical difference between drug through a closed-circuit TV camera positioned directly versus nondrug conditioning protocols. For drug above the apparatus. The testing room had dim indirect conditioning, animals are administered a drug before lighting, comprised of two 15 W bulbs positioned about being placed into the to-be-conditioned environ- 1 m above the apparatus. A loudspeaker positioned above ment. For nondrug conditioning, the unconditioned the apparatus delivered white noise. The apparatus was stimuli (e.g. food or receptive female) are typically repeatedly washed and dried. presented after the animals were placed into the conditioning apparatus. This difference may bias the General procedure interpretation of the effect of memantine on the The conditioned place preference experiments were expression of drug- versus nondrug-conditioned place conducted using procedure described previously (Popik preferences (see Discussion). Thus, to avoid this et al., 2003). Each experiment consisted of five separate potentially confounding difference in conditioning pro- test days. During adaptation (Day 1), male mice were tocols, in the present study mice, were exposed to the brought into the testing room, where the experimenter nondrug unconditioned stimuli before being placed into weighed and handled animals by moving them from one the conditioning apparatus. This allowed us to eliminate standard home cage to another, which was near the potential effects of memantine on the behavioral the apparatus. The aim of this phase was to reduce the performance and/or consummatory phase of behaviors novelty and stress associated with handling, in- that were to be conditioned. jections and exposure to the apparatus. During the

pre-conditioning test (Day 2), male mice were placed Conditioning with sexual encounter individually on to the central triangular platform of the Female mice were bilaterally ovariectomized under apparatus, with free access to all three arms for 20 min. pentobarbital anesthesia 3–4 weeks prior to testing. All Time spent in each arm, as well as the number of arm male mice used in this experiment were sexually naı¨ve entrances (a raw measure of locomotor activity), were before they were given four screening tests for masculine recorded. Based on the results of this pre-conditioning copulatory behavior. Screening tests were conducted at 7- test, two arms with the most similar preferences were day intervals during the dark phase of the light–dark cycle. identified individually for each mouse. One of these arms Each male mouse was placed into the testing chamber was later paired with the effects of ‘rewarding conditions’ (50 Â 50 Â 50 cm) with sawdust bedding, for 20 min of and the other arm was paired with the respective ‘control habituation, followed by the introduction of a sexually conditions’ (see below). During the first conditioning receptive female for 60 min. Females were rendered session (Day 3), animals were confined to the first of the sexually receptive by the administration of estradiol pre-assigned arms of the apparatus for 45 min. Before the benzoate (20 mg), 48 h before the test, and progesterone placement, one group of the male mice was fed with (500 mg), 6 h before the test. During the screening tests, standard laboratory food, another group was exposed to a sexual behavior of male mice was recorded by a trained receptive female, and the third group received an observer and only mice demonstrating at least three injection of morphine (active conditions). During the ejaculations in four screening tests were used in the second conditioning session (Day 4), animals were subsequent conditioning experiments (approximately 95% confined to the second of the pre-assigned arms for of mice tested). The conditioning procedure was essen- 45 min. Before the placement, the first group of animals tially the same as the one used in the food experiment, was not fed, the second group was exposed to a non- except that mice were transferred to the apparatus after receptive female, and the third group was given saline the interaction with the female mice that lasted until the injections (control conditions). Assignment to the treat- first ejaculation (receptive female conditioning) or for ment–arm pairings was counterbalanced, with half of the 15 min (nonreceptive female conditioning). Male mice animals receiving treatment in the reverse order. The that needed more than 15 min to achieve ejaculation post-conditioning test (Day 5) was carried out in the (B10%) were not used. Male mice did not interact with manner identical to the pre-conditioning test. Twenty the same female mouse more than once. minutes prior to the post-conditioning test, mice were injected with either saline or memantine (7.5 mg/kg). Conditioning with morphine The design of experiments with morphine was essentially Conditioning with food the same as for the food and sexual encounter condition- For the duration of this experiment, mice were deprived ing procedures, with the exception that there were no of food for 20 h/day. During the first 8 days, mice were pre-exposures to morphine. Saline and morphine (10 mg/ adapted to the testing room, ‘feeding cages’ and the kg) injections were given immediately before placement feeding procedure. Subjects were transferred into the into designated arms of the place conditioning apparatus. testing room, adjacent to the animal facility room, and Control groups received saline injections prior to both placed individually for 4 h (15.00–19.00 h) into the conditioning sessions. In order to reduce the number of ‘feeding cages’ (standard plastic laboratory cages measur- mice, this experiment did not include a control group ing 27.5 Â 21.5 Â 13 cm and equipped with standard lab that would receive a memantine injection prior to the chow, a bottle with tap water and sawdust bedding). The post-conditioning test in the nonmorphine condition. amount of laboratory food consumed was monitored by Such treatment conditions were already represented in weighing it before and after feeding. At the end of this 8- the food or sexual conditioning experiments, which day period, mice achieved B80% of their original body included control treatment groups that were not fed or weight and consumed an average of 1.9 ± 0.1 g of chow were exposed to nonreceptive females, respectively, prior per 4-hour period. There were no differences among to each conditioning session. Moreover, our previous data various treatment groups with regard to these measures. indicate that memantine does not affect place preference Preliminary data (not shown) indicated that mice of saline-conditioned mice (Popik et al., 2003). deprived of food for 20 h consumed the majority of food during the first hour of the 4-hour feeding period. Nine Drugs days after the beginning of the food-deprivation proce- Both morphine hydrochloride (Polfa, Krako´w, Poland) and dure, the pre-conditioning test took place, followed by memantine (Sigma-Aldrich, Poznan, Poland) were dis- the conditioning sessions as described above. Mice were solved in sterile physiological saline and administered placed in the ‘feeding cages’, with or without food, for intraperitoneally (i.p.). The dose of morphine was cal- 45 min. Then subjects were transferred into the desig- culated for the base form of the drug. Estradiol benzoate nated arm of the conditioning apparatus for 45 min. The and progesterone (Sigma-Aldrich, Poznan, Poland) were control groups were not fed prior to either of the two prepared in sunflower oil and administered subcuta- conditioning sessions. neously (s.c.). Injection volume was 10 ml/kg.

Data analysis technical reasons, such data were available only for food- The times spent in all three arms of the conditioning and morphine-conditioned place preference experi- apparatus before and after conditioning were subjected to ments). The post-hoc Newman–Keuls test was applied statistical analyses. Additionally, the post-conditioning to test between-group differences whenever indicated by data were summarized as the difference in time spent in ANOVA results. the designated arm relative to the pre-conditioning values (called ‘delta’). Data were analyzed using two- and three- Results way analyses of variance (ANOVA) with repeated During the pre-conditioning test, the mean time spent by measures whenever appropriate. Because of the unba- mice in the ‘to-be-conditioned’ arm ranged from 268 s to lanced design with unequal cell sizes, the General Linear 343 s and did not differ significantly among the experi- Model (GLM) procedure was applied (SAS-STAT soft- ments (Table 1). The sham conditioning procedure did ware, SAS Institute Inc., Cary, North Carolina). not affect the arm preference scores, as no change in time The impacts of the following main factors were analyzed: spent in the test arm was observed in mice that were (1) pre- versus post-conditioning test; (2) conditioning pretreated with either saline or memantine prior to the (food versus no food, receptive versus nonreceptive test (Fig. 1, open and black bars). Conditioning with food, female); (3) memantine treatment (memantine versus sexual encounter and morphine produced a significant saline). Because of the unbalanced design (see above), increase in time spent in the designated arms (hatched one- and two-way ANOVAs were used for the analysis of bars) and successful conditioning was confirmed by the morphine conditioning results (treatment groups: ANOVA for all three experiments [F(1,40) = 16.9, saline + saline, morphine + saline, morphine + meman- P < 0.001; F(1,28) = 5.2, P < 0.05; F(2,31) = 5.5, tine). One- and two-way ANOVAs were used to analyze P < 0.01, respectively]. Pretreatment with memantine the effect of treatments on the number of arm entrances (cross-hatched bars) blocked the expression of place recorded during the post-conditioning tests (due to preference conditioned with sexual encounter

Table 1 Time spent in each arm of the conditioning apparatus before and after conditioning with food, sexual encounter or morphine

Time spent per arm (s)

Unconditioned stimulus Saline/memantine N Test Conditioned Sham conditioned Neutral

Food Saline 19 Pre 284.4 ± 12.9 324.2 ± 12.1 384.1 ± 20.0 w Post 382.2 ± 18.7 306.7 ± 10.1 381.2 ± 19.1 Memantine 8 Pre 304.8 ± 17.9 338.6 ± 20.3 367.6± 31.9 w Post 371.8 ± 17.6 325.1 ± 17.7 335.9 ± 13.0

No food Saline 8 Pre 332.5 ± 13.4 364.8 ± 14.1 309.6 ± 29.6 Post 332.8 ± 18.2 321.4 ± 13.4 363.6 ± 19.7 Memantine 9 Pre 334.6 ± 14.5 358.0 ± 13.4 345.1 ± 22.8 Post 335.1 ± 21.0 342.6 ± 10.7 397.3± 16.0 Pre/post Â conditioning F(1,40) = 16.9* F(1,40) = 0.5 F(1,40) = 6.5* Pre/post Â memantine F(1,40) = 0.6 F(1,40) = 0.7 F(1,40) = 0.3 Pre/post Â conditioning Â memantine F(1,40) = 0.6 F(1,40) = 0.4 F(1,40) = 0.2

Receptive female Saline 8 Pre 291.9 ± 19.4 321.0 ± 17.3 361.4 ± 29.5 w Post 375.6 ± 28.5 327.8± 26.7 335.9 ± 21.2 Memantine 8 Pre 343.3 ± 16.8 366.0 ± 18.7 301.4 ± 28.7 Post 328.3 ± 16.3 323.9 ± 18.9 338.8 ± 20.2

Nonreceptive female Saline 8 Pre 307.4± 19.4 369.8 ± 31.5 330.6 ± 18.2 Post 317.5± 16.0 338.0 ± 30.8 343.0 ± 23.5 Memantine 8 Pre 343.5 ± 13.4 373.4 ± 17.9 296.5 ± 18.1 Post 329.6 ± 16.3 344.9 ± 11.9 343.8 ± 24.2 Pre/post Â conditioning F(1,28) = 5.2* F(1,28) = 0.3 F(1,28) = 0.8 Pre/post Â memantine F(1,28) = 14.9* F(1,28) = 0.7 F(1,28) = 3.3 Pre/post Â conditioning Â memantine F(1,28) = 5.5* F(1,28) = 0.9 F(1,28) = 0.3

Morphine Saline 10 Pre 268.2 ± 21.0 327.0± 21.9 389.2 ± 46.9 w Post 356.9 ± 17.0 330.4 ± 23.6 327.2± 23.5 Memantine 13 Pre 311.9 ± 15.2 350.0 ± 16.1 333.1 ± 36.9 Post 311.6 ± 22.1 335.5 ± 30.3 353.4 ± 26.3 Saline Saline 11 Pre 315.6 ± 14.9 332.7 ± 12.0 304.1 ± 23.3 Post 310.6 ± 18.2 337.0± 12.1 331.4 ± 19.3 Pre/post Â conditioning F(2,31) = 5.6* F(2,31) = 0.2 F(2,31) = 2.0

*Saline or memantine (7.5 mg/kg) was injected once prior to the post-conditioning session. Values are means ± SEM. w P < 0.05 versus pre-test; *P < 0.01.

Fig. 1 sexual encounter and morphine administration. Secondly, the NMDA receptor channel blocker, memantine, Food Sexual Morphine blocked the expression of place preference conditioned interaction with morphine and sexual encounter, without affecting 150 * * * place preference conditioned with food. Morphine- ** * 100 conditioned place preference is a well-established experimental phenomenon (Carr et al., 1989). The present 50 study confirmed that it can be observed readily in mice NT 0 after a single conditioning trial in an unbiased conditioning procedure. However, to our knowledge, this paradigm Place preference (s) −50 has never been applied to study food- and sexual Memantine −−++−−++−−++ interaction-induced place preferences in mice, the − US + + − + + − − + + − − species most commonly used in behavioral genetics research (e.g. Crawley et al., 1997). Moreover, the present design (three-arm apparatus) allowed us to demonstrate Effects of memantine on the expression of place preference conditioned with food, sexual encounter and morphine in male mice. that place preference responses observed in these studies See text for the conditioning and test details. Data are presented are cannot be explained by the ‘novelty’ account (Bevins and mean ( ± SEM) shift in time (in seconds) spent in the conditioned Bardo, 1999), because we found no increase of the time environment relative to the pre-conditioning values. Memantine (7.5mg/ kg) or its vehicle was administered 20 min prior to the post-conditioning spent in the ‘novel’ arms that could be related to the test. *P < 0.01 and **P < 0.001 (Newman–Keuls test), significant conditioning procedure and/or drug treatment. differences between groups are indicated by the brackets; NT, not tested; US, unconditioned stimulus. Each bar represents data of 8–19 mice/treatment. Behaviors maintained by nondrug reinforcers are often used as baseline controls for analyzing the drug effects in a variety of experimental procedures, such as intravenous drug self-administration (Mello et al., 1993) or drug- [F(1,28) = 14.9, P < 0.001] and morphine [F(2,31) = 5.5, conditioned place preference (Popik and Danysz, 1997; P < 0.01], but not with food [F(1,40) = 0.6, NS]. A Papp et al., 2002). For example, Papp et al. (2002) similar analysis conducted for the time spent in the sham- demonstrated recently that the partial agonist at the conditioned or novel arms did not reveal any significant glycine/NMDA site, ACPC (1-aminocyclopropane carb- increases in the time spent in these compartments as a oxylic acid), inhibited the acquisition and expression of result of conditioning or memantine treatment (Table 1). morphine-induced conditioned place preference in rats. However, as indicated in the Table 1, there was a ACPC (200 mg/kg) affected neither the acquisition nor significant interaction between pre-/post-conditioning expression of place preferences conditioned with natural test and food-conditioning factors for a neutral arm. This reinforcers (food, sucrose, social interaction and novelty). interaction was not due to the increased time spent in the Similarly, Popik and Danysz (1997) evaluated the effects neutral arm as a result of the conditioning procedure. of memantine (7.5 mg/kg) on the expression of morphine- Instead, for both food-conditioned groups (treated with and food-conditioned place preferences in rats, and found saline or memantine prior to the post-conditioning test), that morphine, but not the food-conditioned response, the time spent in the neutral arm was slightly decreased. was inhibited by memantine, indicating different me- The mean numbers ± SEM of entrances into the chanisms of food- and drug-induced conditioned place ‘rewarded’ arm recorded during the post-conditioning preference or, alternatively, a different sensitivity of these test for mice fed/treated with placebo, fed/treated with responses to NMDA receptor blockade. memantine, nonfed/treated with placebo and nonfed/ treated with memantine were: 41.9 ± 2.53, 35.5 ± 1.86, In contrast to previous studies with food place condition- 42.6 ± 5.08 and 35.3 ± 5.63, respectively. There were no ing (Popik and Danysz, 1997; Papp et al., 2002), in the significant differences among treatments [F(3,40) = 1.0, present study mice had no opportunity to consume food NS]; the effects of memantine were also not significant while in the place-conditioning apparatus. Instead, they [F(1,40) = 2.96, NS]. For mice treated with morphine/ were first allowed to consume food for a specified period placebo, morphine/memantine and placebo/placebo, of time (as in drug-conditioning sessions when morphine these values were 31.0 ± 1.96, 38.7 ± 3.76 and was given before mice were exposed to the conditioning 30.5 ± 1.91, respectively, and did not differ from each environment) and then were placed into the conditioning other [F(2,31) = 2.68, NS]. apparatus. Similarly, unlike the previous place-conditioning studies with sexual behavior (Miller and Baum, 1987; Discussion Hughes et al., 1990; Mehrara and Baum, 1990), in the There were two major findings in this study. First, present study conditioning sessions did not start until the significant preferences were observed in mice for places interaction with the females (receptive or nonreceptive) previously paired with food consumption, successful was complete. Thus, the design of our experiments was

similar to those used by Agmo and co-workers (Agmo and comparison condition (e.g. for evidence on the differ- Berenfeld, 1990; Agmo and Marroquin, 1997) and Lett et ential effects of naloxone on the intake of palatable and al. (2000), in that the unconditioned stimuli were regular types of food, see Segall and Margules, 1989; presented before rather than during the exposure to the Levine et al., 1995). Thus, future studies should probably conditioned environment. Although this approach may focus on sucrose as the potential candidate for analyzing resemble the backward conditioning paradigm, most conditioned place preference responding in nondeprived evidence suggests that backwardly conditioned cues subjects (e.g. as in Agmo and Marroquin, 1997; Papp et al., become inhibitory (Hall, 1984). In contrast, in the 2002). However, one should note that in a study by Papp present study significant conditioned place preferences and colleagues (2002), place preferences conditioned were observed in experiments with both food consump- with sucrose were not affected by ACPC, which is a tion and sexual encounters [see, however, Maes and glycine/NMDA receptor partial agonist (Popik et al., Vossen (1993), who reported that such a conditioning 1995), rather than a full antagonist of NMDA receptor procedure is less likely to produce the place preference complex. For example, it was observed in electrophysio- response compared with the conventional protocols, in logical studies that ACPC demonstrated intrinsic activity which the animals consume food while in the to-be- of 92% at the glycine/NMDA receptor (Karczkubicha et conditioned environment]. al., 1997) while acting as the low-affinity competitive antagonist at the glutamate site (Nahum-Levy et al., Memantine inhibited the expression of place preferences 1999). Thus, it is likely that the effects of ACPC are not conditioned with morphine and sexual encounter, while necessarily identical to those seen for NMDA receptor having little or no effect on the expression of food- antagonist with little or no intrinsic activity. Moreover, conditioned responses. These findings are somewhat Papp et al. (2002) reported that ACPC did not affect surprising, since most, if not all, previous studies drug-induced place aversions and had no effect on the reported the lack of effects of NMDA receptor antago- expression of amphetamine-conditioned place prefer- nists on place preferences conditioned with nondrug ence. This also contrasts with some previous studies reinforcers (e.g. Popik and Danysz, 1997; Papp et al., suggesting that NMDA receptor antagonists may be 2002). Inhibitory effects on morphine-induced place capable of inhibiting both conditioned place aversions preference confirm the previously reported inhibition of as well as amphetamine-conditioned place preference morphine- and cocaine-conditioned place preference by (Higgins et al., 1992; Bespalov, 1996; Popik and Danysz, memantine in rats (Popik and Danysz, 1997; Kotlinska 1997; Mead and Stephens, 1999). On the other hand, and Biala, 2000) and are also consistent with earlier it has been reported that while memantine blocked reports on the decreased expression of drug-conditioned the expression of cocaine-induced place preference place preference by other NMDA receptor antagonists (Kotlinska and Biala, 2000), neither the glycine/NMDA (Bespalov, 1996; Delpozo et al., 1996; Mead and Stephens, site antagonist L-701,324 (Kotlinska and Biala, 1999), 1999; Papp et al., 2002). Thus, the effects of memantine the partial agonist at the glycine/NMDA site, ACPC are most likely attributable to the NMDA receptor (Papp et al., 2002), nor the uncompetitive NMDA blockade. receptor antagonist, dizocilpine (Cervo and Samanin, 1995), were able to affect cocaine-induced place To explain these differential effects of memantine, one preferences. Other data indicate that while ACPC did should analyze the differences of drug, sexual interaction not block expression of amphetamine-induced place and food reinforcers. Satiety thresholds exist for various preference (Papp et al., 2002), the competitive types of drug and nondrug reinforcing stimuli (e.g. NMDA receptor antagonist ( ± )-CPP (conditioned place Hilliard and Domjan, 1995; Tsibulsky and Norman, preference) in rats (Bespalov, 1996) and the glycine/ 1999) and the deprivation state may facilitate responding NMDA antagonist 7-chloro-4-hydroxy-3-(2-phenoxy) to both drug- and nondrug-related stimuli, although it phenyl-2(1 H)-quinolone in mice, did block expression may not be a necessary condition. In contrast to the fact of amphetamine-induced place preferences (Mead that the pre-existing drug dependence is not a pre- and Stephens, 1999). These data indicate that various requisite for acquisition and maintenance of drug self- NMDA receptor antagonists do not affect the expression administration behavior (Katz, 1989), mice consume of psychostimulant-induced place preferences uniformly, regular types of food only when in the deprived state with the cocaine-induced response being most difficult and it is commonly assumed that food intake in food- to inhibit. Further data on the effects of memantine restricted subjects is ‘driven mainly by energy needs, with on expression of drug-induced place preferences taste or palatability being relatively less important’ are necessary. With regard to the natural rewards, (Zhang et al., 1998, p. 909). Since both humans and while the present study demonstrates an initial positive animals will consume highly palatable food even in the effect, it remains to be evaluated whether NMDA nondeprived (satiated) state, one could argue that the receptor antagonists inhibit the expression of place place preference established with palatable food in preferences conditioned with non-drug reinforcers in nondeprived subjects would be a more adequate the nondeprived state(s).

One of the behavioral models of motivation proposes that arm entrances recorded during the post-conditioning test two separate systems contribute to reward processes. The in food- and morphine-conditioned place preference first system is associated with the state of nondeprivation experiments did not suggest any potentially important (e.g. satiety) and the second system is associated with the effects of memantine on locomotor activity. Also, it state of deprivation (e.g. hunger in the case of food should be noted that, in previous studies, NMDA reward). This distinction is substantiated by the differ- receptor antagonism was found to inhibit the expression ences between behaviors acquired in the nondeprivation of drug-conditioned place preference irrespective of its and deprivation state. A number of studies have effect on locomotor activity (Popik and Kolasiewicz, suggested that these systems (states) are readily 1999). Finally, although it is possible that memantine discriminated by the neurobiological and pharmacological could make the retrieval of place preference state- interventions that selectively alter one of the systems/ dependent, the studies by Tzschentke and Schmidt states (e.g. Nader et al., 1997). We thus hypothesize that (1997) and Papp et al. (2002) provided arguments against the observed pattern of the effects of memantine is due a state-dependency explanation. to the selective interaction with motivated behaviors acquired in the nondeprived state. In addition, we further One of the limitations of the present study was that a suggest that in certain cases, food-reinforced behaviors single dose of memantine was used. However, this dose may not serve as appropriate controls for drug-maintained (7.5 mg/kg) is well within the dose range that is known to behaviors. result in brain concentrations (B1 mmol/l) sufficient to block NMDA receptors (Parsons et al., 1999). Tests with There are alternative hypotheses that may explain the higher doses of memantine are less desirable in behavioral differential effect of memantine on the expression of experiments, since they may result in the loss of conditioned place preferences induced by various stimuli. selectivity towards the NMDA receptors, as well as in a Experimental evidence suggests that separate neural substantial change in the behavioral profile. At doses circuits may be involved in the mediation of behaviors above 10 mg/kg, memantine is more likely to produce maintained by different reinforcers. For instance, non- behavioral effects typical for phencyclidine-like NMDA overlapping populations of neurons are known to encode receptor antagonists (Grant et al., 1996). Moreover, there information about abused drugs (cocaine) and food is no experimental evidence that would suggest signifi- reinforcement (Carelli et al., 2000). Moreover, it is cant sensorimotor, attention and discrimination deficits possible that cocaine activates populations of cells that produced by memantine in mice, at the dose level used in normally process information about the reinforcing the present study. In addition, at the same dose, properties of sexual behavior (Childress et al., 1998). memantine itself produces neither conditioned place Although cocaine-responsive neural circuits may not be preference nor aversion (Popik and Danysz, 1997) that the same as those for opiate drugs (Chang et al., 1998), it could explain its effects on drug- and nondrug-induced is still worth noting that the present results support the place preference. The results of this study demonstrate view on common mechanisms encoding behaviors main- that memantine blocked the expression of place prefer- tained by drug and sexual reinforcement. 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