European Journal of Pharmacology 423 (2001) 41-46
Role of the NMDA receptor subunit in the expression of the discriminative stimulus effect induced by ketamine
Minoru Narita, Kazumi Yoshizawa, Mutsuko Nomura, Kazue Aoki, Tsutomu Suzuki *
Department of Toxicology, School of Pharmacy, Hoshi University, 2-4-41 Ebara, Shinagawa, Tokyo 142-8501, Japan
Received 30 November 2000; received in revised form 21 May 200]; accepted z') May 2001
Abstract
Ketamine, which is a non-competitive NMDA receptor antagonist, has been used as a dissociative anesthetic agent However, chronic use of ketamine produces psychotomimetic effects, such as nightmares, hallucination and delusion, Therefore, the present study was designed to ascertain the role of the NMDA receptor and (J" receptor in the discriminative stimulus effect induced by ketamine. Fischer 344 rats were trained to discriminate between ketamine (5 mgy'kg, i.p.) and saline under a fixed-ratio 10 food-reinforced procedure, Non-competitive antagonists for both NR2A- and NR2B-coptaining NMDA receptors, such as phencyclidine (0,1-1 mgykg, i.p.) and dizocilpine (3-30 fLg/kg, i.p.), and the NR2A-containing NMDA receptor-preferred antagonist dextromethorphan (3-56 mg zkg, i.p.) fully substituted for the ketamine cue in a dose-dependent manner. By contrast, the NR2B-cont(}ining NMDA receptor antagonist ifenprodil (5-20 mg/kg, i.p.) exhibited no generalization. Additionally, the competitive NMDA antagonist 3-[( ± )-2-c(}rboxypiperazine-
-l-yl] propyl-J-phosphonic acid «±)-CPP; 0.3-5.6 mgjkg, i.p.) and a (J" receptor ligand DTG (0.3-3 mg z'kg, s.c.) displayed no generalization to the ketamine cue. These results suggest that NRl/NR2A subunit containing NMDA antagonism may be critical for the production of the ketamine cue. I
Keywords' Ketamine; Drug discrimination; NMDA receptor; Ifenprodil
1. Introduction morphine alone was effectively controlled by the co-infu- sion of ketamine with morphine (Yang et at, 1996). The NMDA receptor is the most clearly defined and the However, the chronic use of ketamine has been somewhat best characterized excitatory amino acid receptor subtype limited by its tendency to produce psychotomimetic effects in the mammalian central nervous system (Monaghan et such as nightmares, hallucination and delusion. Ketamine aI., 1989), This receptor is a ligand-gated, ion channel-as- has also been shown to produce either positive symptoms sociated receptor complex with several pharmacologically of psychosis, such (is illusions, disturbances in thought distinct sites at which activity can produce alterations in organization and delusions, or negative symptoms similar NMDA neurotransmission (Foster and Fagg, 1987; Mon- to those associated with schizophrenia including blunted aghan et al., 1989). emotional responses, emotional detachment and psychomo- The behavioral, cognitive, and electrophysiological ef- tor retardation (Krystal et al., 1994). Thus, the very proper- fects of antagonists to the NMDA receptor subtype in ties that restrict its clinical use have made ketamine an healthy subjects resemble some of the signs and symptoms increasingly popular drug of abuse (Hancock and Stam- of schizophrenia and dissociative disorders (Luby et aI., ford, 1999). 1959; Domino et aI., 1965; 0ye et al., 1993; Krystal et aI., A major explanatory hypothesis for the pathophysiology 1994; Malhotra et al., 1996). Ketamine, which is a non- of schizophrenia is the dopamine hypothesis, which main- competitive NMDA receptor antagonist, has been widely tains that dysfunction of the dopam ine neurotransmitter used as an intravenous or intramuscular anesthetic system underlies the behavioral abnormalities associated (Miyasaka and Domino, 1968). Furthermore, pain experi- with schizophrenia. An alternative explanation for the enced by cancer patients that could not be relieved by etiology of schizophrenia is dependent on dysfunction of the glutarninergic system. This hypothesis originates from
* Corresponding author. Tel.yfax: +81-3-5498-5831. the observation that intoxication induced by the non-com- E-mail [email protected] (T. SUZUki). petitive NMDA antagonist phencyclidine (PCP) closely 42 M. Narita et al.rEuropean Journal of Pharmacology 423() 2001 41–46 mimics schizophreniaŽ. Luby et al., 1959 . Like PCP, ke- which the rat was presented with a food pellet each time it tamine and dizocilpine, another NMDA receptor antago- pressed a lever. When reinforcement was provided, the nist, also produce schizophrenic symptomsŽ Ellison, 1995; light above the lever was illuminated. The FR requirement Abi-Saab et al., 1998. and exacerbate symptoms in pa- for food reinforcement was gradually increased to a value tients with schizophreniaŽ. Javitt and Zukin, 1991 . These of 10. After the response rates had stabilized under FR 10, findings suggest that PCP- and dizocilpine-treated animals rats were trained to discriminate between 5 mgrkg of may be useful as models for schizophreniaŽ Carlsson and ketamine and saline under an FR 10 schedule. In the Carlsson, 1990; Corbett et al., 1993, 1995. . discrimination training, ketamineŽ. D or saline Ž. S was It has been reported that PCP possesses an affinity for administered i.p. 10 min before each session in a daily sigma Ž.s receptors as well as NMDA receptors Ž Gund- sequence of DDSS, according to a double alternation lach et al., 1985. . The potential involvement of s recep- schedule, and the assignment of left and right levers to tors in schizophrenia has been considered ever since their drug and saline states was counter-balanced. Rats were discoveryŽ. Debonnel and de Montigny, 1996 . Further- required to respond on the stimulus-appropriate lever to more, a major role for s receptors is thought to regulate obtain reinforcement; there were no programmed conse- the glutamatergic system via NMDA receptorsŽ Debonnel quences for responding on the incorrect lever. Generaliza- and de Montigny, 1996. . tion sessions were only performed after the discrimination Therefore, the present study was designed to ascertain criterion described below had been satisfied for at least the role of the NMDA receptor and s receptor in the five consecutive daily discrimination-training sessionsŽ ac- discriminative stimulus effect induced by ketamine. curacy of at least 83% and fewer than 12 responses to obtain the first food pellet. .
2. Materials and methods 2.4. Generalization testing
The present study was conducted in accordance with the Following the successful acquisition of discrimination Guiding Principles for the Care and Use of Laboratory between ketamine and saline, test sessions were usually Animals, Hoshi University, as adopted by the Committee conducted once or twice per week with training sessions on Animal Research of Hoshi University, which is accred- on intervening days. During the test session, rats were ited by the Ministry of Education, Science, Sports and placed in the operant box until they had made 10 responses Culture of Japan. on either lever or 5 min had elapsed. The pretreatment 2.1. Animals times and doses of drugs used were: 10 min for ketamine Ž.Ž1.25–5 mgrkg, i.p. ; 30 min for PCP 0.1–1 mgrkg, i.p..Ž , dizocilpine 3–30 mgrkg, i.p. . , dextromethorphan Eight male Fischer 344 ratsŽ Charles River Japan, At- Ž.Ž.3–56 mgrkg, i.p. , ifenprodil 5–20 mgrkg, i.p. and sugi, Japan..Ž weighing between 210 and 230 g 80% DTGŽ.Ž 0.3–3 mgrkg, s.c. ; 60 min for ".-CPPŽ 0.3–5.6 free-feeding weight. were used in this study. Water was mgrkg, i.p.. . Control tests with ketamine and saline indi- freely available for all of the rats in their individual home cate that the animals used in this study were under good cages. The rats were housed at a room temperature of stimulus control throughout the duration of all generaliza- 22"1 8C with a 12-h lightrdark cycleŽ lights on 8:00– tion testing. 20:00. .
2.2. Apparatus 2.5. Drugs
Experiments were conducted in operant chambers Ketamine hydrochlorideŽ. Sigma, St. Louis, MO, USA , Ž.model GT8810; O’Hara, Tokyo, Japan equipped with phencyclidine hydrochlorideŽ PCP; Taisho Pharmaceutical, two levers, with a food cup mounted midway between the Tokyo, Japan.Ž , q .-5-methyl-10, 11-dihydro-5-H-dibenzo levers. White lamps were installed above each of the Ž.a,d cyclohepten-5, 10-imine maleateŽ dizocilpine; levers. White noise was used to mask extraneous sound. MerckrBanyu Pharmaceutical, Tokyo, Japan. and 3-wŽ." - Reinforcement consisted of 20-mg food pelletsŽ. O’Hara . 2-carboxypiperazine-4-ylx propyl-1-phosphonic acid ŽŽ" .- CPP; Sigma. were dissolved in saline. D-3-Methoxy-N- 2.3. Training procedure methylmorphinanŽ. dextromethorphan; Sigma , ifenprodil tartrateŽ. Grelan Pharmaceutical, Tokyo, Japan and 1,3-di- Discrimination training was performed according to the o-tolyguanidineŽ DTG; Aldrich Chemical, Milwaukee, WI, method described by Suzuki et al.Ž. 1997 . Briefly, before USA.Ž were dissolved in 5% DMSO dimethyl sulfoxide; they were trained to discriminate between ketamine and Wako, Tokyo, Japan. with 9% Tween 80rsaline. All drugs saline, all rats were trained to press a lever. Training began were administered in a volume of 1.0 mlrkg, except for under a reinforcement schedule of fixed ratio 1Ž. FR 1 , in dextromethorphanŽ. 2.0 mlrkg for the 56 mgrkg dose . M. Narita et al.rEuropean Journal of Pharmacology 423() 2001 41–46 43
was considered to be disrupted and the data obtained were
not included in the calculations. ED50 values and their 95% confidence limitsŽ. 95% CL were determined by the method of Litchfield and WilcoxonŽ. 1949 .
3. Results
3.1. Acquisition and dose–response test
All rats acquired ketamine–saline discrimination, as determined by successful completion of five consecutive tests with ketamine and saline, within 45 training sessions Ž.data not shown . Once rats attained the criterion, drug– saline discrimination stabilized and was maintained with a high degree of accuracy. KetamineŽ. 1.25–5 mgrkg was dose-dependently generalized to the training dose without a reduction in the response rateŽ. Fig. 1A .
3.2. Generalization testing with competitiÕe NMDA recep- tor antagonist
The results of a generalization test with the competitive NMDA recepor antagonist Ž." -CPP are shown in Fig. 1B.
Fig. 1.Ž. A Dose–response curve for % ketamine-lever respondingŽ left panel.Ž. and response rates right panel in rats trained to discriminate between 5 mgrkg ketamine and saline. Each point represents the mean percentage of ketamine- or saline-appropriate responding with S.E.M. of eight animals.Ž. B Lack of generalization to Ž." -CPP to the discrimina- tive stimulus properties of ketamine in rats trained to discriminate be- tween 5 mgrkg ketamine and saline. Each point represents the mean percentage of ketamine-appropriate respondingŽ. left panel and the mean response ratesŽ. right panel with S.E.M. of eight animals.Ž. C Lack of generalization to DTG to the discriminative stimulus properties of ke- tamine in rats trained to discriminate between 5 mgrkg ketamine and saline. Each point represents the mean percentage of ketamine-ap- propriate respondingŽ. left panel and the mean response ratesŽ. right panel with S.E.M. of four animals.
2.6. Data analysis
During the training sessions, accuracy was defined as the number of correct responses as a percentage of the Fig. 2.Ž. A Generalization to phencyclidineŽ. closed square , dizocilpine Ž. Ž . total responses before the first food pellet. During the test open square and dextromethorphan closed circle to the discriminative stimulus properties of ketamine in rats trained to discriminate between 5 sessions, performance was expressed as the number of mgrkg ketamine and saline. Each point represents the mean percentage drug–lever responses as a percentage of the total responses of ketamine-appropriate respondingŽ. left panel and the mean response upon completion of FR 10. Drugs were considered to have ratesŽ. right panel with S.E.M. of four to eight animals.Ž. B Lack of substituted for the discriminative stimulus effect of ke- generalization to ifenprodil to the discriminative stimulus properties of ketamine in rats trained to discriminate between 5 mgrkg ketamine and tamine if more than 80% of the responses were on the saline. Each point represents the mean percentage of ketamine-ap- drug-appropriate lever. Further, in rats that made fewer propriate respondingŽ. left panel and the mean response ratesŽ. right panel than 10 responses before 5 min had elapsed, responding with S.E.M. of eight animals. 44 M. Narita et al.rEuropean Journal of Pharmacology 423() 2001 41–46
Ž." -CPP displayed no generalization to a ketamine cue. 3.4. Generalization testing with s receptor ligand Ž." -CPP produced a maximum mean ketamine-lever se- lection of only 35% at a doseŽ. 5.6 mgrkg that caused The results of a generalization test with the s receptor about a 72% reduction in the response rate compared to ligand DTG are shown in Fig. 1C. The vehicle alone had the control training session. no effect on the generalization to ketamine. DTG displayed no generalization to a ketamine cue. DTG produced a maximum mean ketamine-lever selection of only 33% at 1 3.3. Generalization testing with non-competitiÕeNMDA mgrkg, which did not affect the response rate. receptor antagonists
4. Discussion The results of generalization tests with the non-competi- tive NMDA receptor antagonists are shown in Fig. 2. The vehicle alone had no effect on the generalization to ke- Drug-discrimination studies that provide information tamineŽ. data not shown . In contrast to the competitive relevant to the subjective effects of drugs in humans are a NMDA receptor antagonist, PCP and dizocilpine fully sensitive and pharmacologically specific measure of be- Ž. substituted for a ketamine cue in a dose-dependent manner. havior Balster, 1990; Bobelis and Balster, 1993 . The Both PCP and dizocilpine produced dose-related general- present study demonstrated that the non-competitive ization to ketamine, up to 92%. Furthermore, dex- NMDA receptor antagonist ketamine produced a discrimi- tromethorphan dose-dependently substituted for ketamine native stimulus effect in rats. After discrimination acquisi- with ketamine-lever responding of greater than 87.5%, tion, rats maintained ketamine-saline discrimination with a with a reduction in the response rate to about 74% com- high degree of accuracy. Under these conditions, the com- Ž." pared to the control training session. By contrast, the petitive NMDA receptor antagonist -CPP failed to NR2B-containing NMDA receptor antagonist ifenprodil substitute for a ketamine cue. Similar findings have been failed to substitute for a ketamine cue. Ifenprodil produced previously observed regarding the lack of generalization a maximum mean ketamine-lever selection of only 14% at between non-competitive and competitive NMDA receptor Ž 10 mgrkg, which did not affect the response rate. A antagonists in rats Willetts et al., 1989; Bobelis and . higher dose of ifenprodilŽ. 20 mgrkg was associated with Balster, 1993; Grant et al., 1996 . Since the non-competi- a reduction in the response rate to about 58% compared to tive NMDA receptor antagonist blocks NMDA receptor- the control training session. The ED values and their mediated responses by binding to the cation channel of the 50 Ž. 95% CL for all compound tested in the present study are NMDA receptor complex Collingridge and Lester, 1989 , presented in Table 1. the present data clearly suggest that the discriminative stimulus effects of ketamine may be related to its ability to block the NMDA receptor-associated channel. Most NMDA receptors in the brain are thought to be pentameric or tetrameric complexes of the NR1 subunit Table 1 r and one or more of four NR2 subunitsŽ. NR2A-2D ED50 valuesŽ. expressed as mg kg of each compound with 95% CL for percentage of ketamine-appropriate responding when different NMDA Ž.Kutsuwada et al., 1992; Monyer et al., 1992 . Thus, the receptor antagonists were substituted for the 5 mgrkg training dose of differential expression of NR2 subunits in various regions ketamine of the brain may account for the diversity of NMDA r Compound ED50 values n N receptor subtypes in the central nervous systemŽ Watanabe Ž.95% CL mgrkg et al., 1993; Standaert et al., 1994; Mori and Mishina, Competitive antagonist None 1r8 1995. . The distribution of NMDA receptor channel subunit Ž." -CPP mRNAs in the adult rodent brain has been examined by in Ž. r Non-competitive 1.71 1.01–2.91 8 8 situ hybridization analysesŽ. Kutsuwada et al., 1992 . NR1 antagonist Ketamine Ž.NR1rNR2A6NR1 rNR2B subunit mRNA is distributed ubiquitously in the brain. In Phencyclidine 0.35Ž. 0.19–0.65 7r8 contrast, the four NR2 subunit mRNAs show characteristic Ž.NR1rNR2A6NR1 rNR2B distributions in the brain. The molecular diversity of the Dizocilpine 0.0071Ž. 0.003–0.016 6r7 NR2 subunit family probably underlies the pharmacologi- Ž.r 6 r NR1 NR2A R1 NR2B cal and electrophysiological heterogeneity of the NMDA Dextromethorphan 17.74Ž. 7.33–42.93 4r4 Ž.NR1rNR2A)NR1rNR2B receptor channel. Generally, the NMDA receptor is com- Ifenprodil None 0r8 posed of a common NR1 subunit and one of four NR2 Ž.NR1rNR2A
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