Generalization of NMDA-Receptor Antagonists to the Discriminative Stimulus Effects of Κ-Opioid Receptor Agonists U-50,488H, but Not TRK-820 in Rats

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Generalization of NMDA-Receptor Antagonists to the Discriminative Stimulus Effects of κ-Opioid Receptor Agonists U-50,488H, but Not TRK-820 in Rats

Tomohisa Mori1,2, Mutsuko Nomura1, Kazumi Yoshizawa1, Hiroshi Nagase3, Toshiko Sawaguchi2, Minoru Narita1, and Tsutomu Suzuki1,* 1Department of Toxicology, Hoshi University School of Pharmacy and Pharmaceutical Sciences, Shinagawa-ku, Tokyo 142-8501, Japan 2Department of Legal Medicine, Tokyo Women’s Medical University, Shinkuku-ku, Tokyo 162-8666, Japan 3Basic Research Laboratories, Toray Industries, Inc., Kamakura, Kanagawa 248-8555, Japan

Received May 30, 2005; Accepted December 8, 2005

Abstract. Generalizations of NMDA-receptor antagonists to the discriminative stimulus effects of κ-opioid receptor agonists in rats were examined. Phencyclidine, MK-801, and ketamine, non- competitive NMDA-receptor antagonists, generalized to the discriminative stimulus effects of U-50,488H, but not those of TRK-820, whereas (±)-3-(2-carbaxypiperazine-4-yl) propyl-1-phosphonic acid (CPP), a competitive NMDA-receptor antagonist, and ifenprodil, an NR1/NR2B NMDA-receptor antagonist, did not, suggesting that non-competitive NMDA-receptor antagonists possess U-50,488H-like discriminative stimulus effects in rats. Since U-50,488H and phencyclidine both induce aversive effects, our findings indicate that the cue of the discriminative stimulus effects of U-50,488H and non-competitive NMDA-receptor antagonists may be associated with their aversive effects.

Keywords: drug discrimination, κ-opioid receptor agonist, NMDA-receptor antagonist

It is generally accepted that discriminative stimulus Previous reports showed that phencyclidine (PCP), a effects of drugs in animals serve as a model for the noncompetitive N-methyl-D-aspartate (NMDA)-receptor subjective effects in humans (1). Furthermore, drug antagonist, exerts the discriminative stimulus effects in discrimination procedures have been shown to be useful monkeys and rats. κ-Opioid receptor agonists or PCP for distinguishing drug effects mediated by κ-opioid induce place aversion using place conditioning para- receptors from those mediated by other receptors and digms (6). The goal of the present study was to compare for characterizing and quantifying such drug-receptor the possible similarities in the discriminative stimulus interactions (2 – 4). We recently demonstrated that the effects of two different types of κ-opioid receptor patterns of generalization of κ-opioid receptor agonists agonists, U-50,488H and TRK-820, and NMDA-recep- to the discriminative stimulus effects of other κ-opioid tor antagonists. Therefore, we examined the generaliza- receptor agonists, U-50,488H and TRK-820, were tion of NMDA-receptor antagonists such as PCP, somewhat different (5). Furthermore, the potency order MK-801, ketamine, ifenprodil, and (±)-3-(2-carbaxy- of κ-opioid receptor agonists to the discriminative piperazine-4-yl)propyl-1-phosphonic acid (CPP) to the stimulus effects of U-50,488H were parallel to the discriminative stimulus effects of κ-opioid-receptor potency of their aversive effects, suggesting that the agonists in rats. cue of the discriminative stimulus effects of U-50,488H Sixteen male Fischer 344 rats (Charles River Japan is related to aversive effects in rats (5). Inc., Atsugi) were maintained at 220 – 230 g (80% free- feeding weight). Water was available ad libitum for all *Corresponding author. [email protected] of the rats in their home cages. The rats were housed in Published online in J-STAGE: February 11, 2006 individual cages at a room temperature of 22 ± 1°C with DOI: 10.1254/jphs.SCJ05006X a 12-h light-dark cycle (light on 8:00 a.m. to 8:00 p.m.).

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These studies were conducted in accordance with the of drugs were used: 10 min for 0.5 – 2.0 mg/kg PCP and Guide for Care and Use of Laboratory Animals of Hoshi 1.0–10mg/kg ketamine and 30 min for 10 – 40 µg/kg University School of Pharmacy and Pharmaceutical TRK-820, 0.56 – 3.0 mg/kg U-50,488H, 10 – 80 µg/kg Sciences, which is accredited by the Ministry of Educa- MK-801, 5 – 15 mg/kg ifenprodil, and 0.3 – 5.6 mg/kg tion, Culture, Sports, Science and Technology. CPP. If the rats did not make 10 responses during each Experiments were conducted in operant-conditioning component, the response was judged to have been test boxes (Model GT 8810; O’hara & Co., Ltd., Tokyo) disrupted. equipped with 2 levers and a reinforcement cup mounted Drugs were obtained from the following sources: midway between the levers. White lamps were installed 17-cyclopropylmethyl-3,14b-[N-methyl-trans-3-(3-furyl) above each of the levers. Chambers were enclosed acrylamido] morphinan hydrochloride (TRK-820), within sound- and light-attenuating boxes and supplied trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclo- with white noise to mask extraneous sound. Reinforce- hexyl] benzenacetamide methanesufonate (U-50,488H; ment consisted of a 20-mg food pellet (O’hara & Co., Research Biochemicals International, Natick, MA, Ltd.). USA); phencyclidine hydrochloride (PCP; Taisho Before rats were trained to discriminate between Pharmaceutical Co., Ltd., Ohmiya); 5-methyl-10,11- cocaine and saline, all rats were trained to press lever. dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydro- Rats were trained to press either the right or left lever in gen maleate (MK-801, Research Biochemicals Inter- the daily sequence LRRLLRRLLR (R = right, L = left). national); ketamine hydrochloride (Sigma Chemical Training began under a fixed-ratio (FR) 1 reinforcement Co., St. Louis, MO, USA); (±)-3-(2-carbaxypiperazine- schedule in which the rat was presented with a food 4-yl)propyl-1-phosphonic acid (CPP, Research Bio- pellet each time it pressed a lever. When reinforcement chemicals International); and ifenprodil tartrate (Grelan was provided, the lamp above the lever was illuminated. Pharmaceutical Co., Ltd., Tokyo). TRK-820 was synthe- After the response rates had stabilized, the FR require- sized by us. Ifenprodil was dissolved in 9% Tween 80- ment was steadily increased to a reinforcement schedule saline solution at the final concentration of 5% dimethyl of FR 10. After that, the response rates became stabilized sulfoxide. All other drugs were dissolved in saline. under FR 10 and the rat received 40 reinforcements During the training sessions, accuracy was defined in during 4 consecutive sessions; both levers were pre- terms of percentage of correct responses out of the sented in the chamber. U-50,488H or TRK-820 and total number of responses before the first food pellet. saline was administered i.p. 15 min before each session During the test sessions, performance was expressed in in a daily sequence of SDDSSDDSSD (D = drug, S = terms of the number of drug-appropriate responses as a saline). Rats were required to respond on the stimulus- percentage of the total responses on completion of FR appropriate lever to obtain reinforcement; there were 10. Drugs were considered to have generalized to the no programmed consequences for responding on the discriminative stimulus effects of the training drug if incorrect lever. Training sessions were 15 min in dura- more than 80% of the responses were on the drug- tion and this phase of training continued until all rats appropriate lever. Response rate was calculated by performed up to the required criterion [during 5 consecu- dividing the total number of responses made on either tive training sessions, at least 83% of the responses that lever by the time (min) to complete the first ratio. The occurred before the first reinforcement (FRF) were made paired Student’s t-test was used to compare the response on the correct lever (i.e., FRF ≤12)]. rates during the generalization tests. After the animals attained the criterion, the dose- Rats required a mean of 23 sessions to acquire the U- response and generalization tests were initiated; test 50,488-saline discrimination and a mean of 25 sessions sessions were performed after the discrimination to acquire the TRK-820-saline discrimination. Once criterion described above had been satisfied for at least 3 rats attained the criterion, drug-saline discrimination consecutive sessions; if accuracy criteria were not stabilized and was maintained with a high degree of reached during the training session, rats were trained accuracy. until their accuracy was maintained for a minimum of 3 During the dose-response tests, U-50,488H (1.0 – 3.0 training sessions. During the test session, rats were left mg/kg) and TRK-820 (10 – 40 µg/kg) produced a dose- in the operant box until they had made ten responses on related increase in drug-appropriate responses in all of either lever or 5 min had elapsed. In the dose-response the rats (Figs. 1A and 2A), and a significant difference tests, rats were tested after the administration of various was observed for the response rates in the dose-response doses of U-50,488H or TRK-820. In generalization tests, tests of U-50,488H (Fig. 1A). Non-competitive NMDA various NMDA-receptor antagonists were tested instead antagonists such as PCP, MK-801, and ketamine of the training drugs. The pretreatment times and doses generalized to the discriminative stimulus effects of U-50,488H-like Effect of NMDA Antagonist 159

Fig. 1. Dose-response of U-50,488H (A) and generalization of PCP (B), MK-801 (C), ketamine (D), ifenprodil (E), and CPP (F) to the discriminative stimulus effects of U-50,488H in rats trained to discriminate between 3.0 mg/kg U-50,488H and saline. Each point represents the mean percentage of U-50,488H-appropriate responding and the mean response rates with S.E.M. of 8 animals.

Fig. 2. Dose-response of TRK-820 (A) and generalization of PCP (B) and MK- 801 (C) to the discriminative stimulus effects of TRK-820 in rats that had been trained to discriminate between 40 µg/kg TRK-820 and saline. Each point represents the mean percentage of TRK-820- appropriate responding and the mean response rates with S.E.M. of 8 animals.

U-50,488H in a dose-dependent manner, and they disrupted the responding in the 6 – 8 rats out of 8 rats decreased response rates when the doses of MK-801 (data not shown). On the other hand, PCP and MK-801 and ketamine were increased. CPP and ifenprodil at doses which generalized to the discriminative engendered intermediate levels of U-50,488H-appro- stimulus effects of U-50,488H, did not generalize to priate responding (Fig. 1), and a significant decrease the discriminative stimulus effects of TRK-820 (less was observed in the generalization test of CPP at 3.0 mg than 30% drug-appropriate responding) (Fig. 2). Saline /mg; however, noted behavioral changes was not did not engender drug-appropriate responses with mean observed in the case of 3.0 mg/kg of CPP. The highest response rate between 45 and 75 (responses/min) (Figs. doses of ifenprodil (20 mg/kg) and CPP (10 mg/kg) 1 and 2). 160 T Mori et al

Our previous study demonstrated that U-50,488H after a longer (30 min) pretreatment time (16) than that generalizes to the discriminative stimulus effects of used in the present study (10 min). Therefore, treatment TRK-820, whereas TRK-820 did not generalize to time may explain why PCP and other NMDA-related those of U-50,488H, indicating that the discriminative compounds generalized to the discriminative stimulus stimulus effects of TRK-820 and U-50,488H were effects of U-50,488H in the present study. somewhat different (5). In the generalization tests using In rats discriminating PCP from saline, MK-801 and κ-opioid receptor agonists, E-2078, but not R-84760, ketamine, but not CPP and ifenprodil, generalized to the generalized to the discriminative stimulus effects of both discriminative stimulus effects of PCP (17 – 19). The TRK-820 and U-50,488H, whereas KT-90, CI-977, and present study showed the lack of generalization of the ICI-199441 generalized to the discriminative stimulus competitive NMDA-receptor antagonist CPP and an effects of U-50,488H, but not to those of TRK-820 (5). antagonist of the NMDA-associated polyamine site Thus, the patterns of generalization with various κ- ifenprodil to the discriminative stimulus effects of U- opioid receptor agonists to the discriminative stimulus 50,488H. Furthermore, PCP induces place aversion effects of U-50,488H and TRK-820 were different. (6), whereas ifenprodil did not produce either place These results and the present data indicate that there preference or aversion using place conditioning para- are qualitative differences between the discriminative digms (20). These findings provide further evidence stimulus effects of TRK-820 and U-50,488H (5). In that there are subtle differences between non-competi- the present study, we demonstrated that non-competitive tive and competitive NMDA-receptor antagonists on NMDA-receptor antagonists including PCP, MK-801 the discriminative stimulus effects. Furthermore, to and ketamine generalized to the discriminative stimulus strengthen our hypothesis that there are differences effects of U-50,488H. On the other hand, these non- between the discriminative stimulus effects of TRK-820 competitive NMDA-receptor agonists did not generalize and U-50,488H, further examinations are needed, to the discriminative stimulus effects of TRK-820. especially generalization tests of other NMDA-receptor These results suggest that these non-competitive antagonists to the discriminative stimulus effects of NMDA-receptor antagonists possess the U-50,488H- TRK-820. like discriminative stimulus effects in rats. TRK-820 In summary, the results of the present study showed failed to produce a significant place aversion in rodents that qualitative differences exist between the discrimina- at doses that were sufficient to produce significant tive stimulus effects of U-50,488H and TRK-820, place aversion (7, 8) and induced place aversion only at and NMDA-receptor antagonists, such as PCP, MK-801, high doses (9). The ability of opioids to produce the and ketamine, possess U-50,488H-like discriminative discriminative stimulus effects is believed to be homo- stimulus effects in rats. The mechanism underlying the logous to the subjective effects that opioids produce in generalization of NMDA-receptor antagonists used in humans. It has been reported that κ-opioid receptor the present study for the discriminative stimulus effects agonists produce dysphoria and psychotomimetic effects of U-50,488H remains unclear. However, these findings in humans (10, 11) and aversive effects in rodents (12 – suggest the possibility that the cue of the discriminative 14). These results may support our hypothesis that the stimulus effects of U-50,488H may be, at least in part, cue of the discriminative stimulus effects of U-50,488H associated with its aversive effects. may be associated with aversive effects in rats, and the cue of the discriminative stimulus effects of U-50,488H Acknowledgment and some NMDA-receptor antagonists may be, at least in part, accompanied by their aversive effects. This work was supported by a Research Grant from It was reported that PCP failed to generalize to the the Ministry of Education, Culture, Sports, Science, and discriminative stimulus effects of a κ-opioid receptor Technology of Japan. agonist, U62,066, in rats (15) and that PCP partially generalized to the discriminative stimulus effects of U- References 50,488H (about 40% drug-lever responding) (16). 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