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BEHAVIORAL PHARMACOLOGY

Effects of the Competitive N-Methyl-D-aspartate Receptor Antagonist, LY235959 [(-)-6-Phosphonomethyl-deca-hydroisoquinoline-3-carboxylic Acid], on Responding for Cocaine under Both Fixed and Progressive Ratio Schedules of Reinforcement

Department of Psychology, University of Colorado at Denver and Health Sciences Center, Denver, Colorado (R.M.A., T.L.S., C.V.E.); and Departments of Psychology (R.M.C., L.A.D.) and Pharmacology (L.A.D.), University of North Carolina, Chapel Hill, North Carolina

Received March 16, 2005; accepted July 11, 2005.

	Abstract

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It is difficult to determine the precise role of the N-methyl-D-aspartate (NMDA) receptor system in the reinforcing effects of cocaine since uncompetitive NMDA receptor antagonists alter cocaine self-administration in different ways, depending on the antagonist examined and the behavior being measured. To increase understanding of the role of the NMDA system in cocaine's reinforcing effects, this study measured the effects of the competitive NMDA receptor antagonist, LY235959 [(-)-6-phosphonomethyl-deca-hydroisoquinoline-3-carboxylic acid], in rats that self-administered cocaine under both fixed ratio (FR) 1 and progressive ratio (PR) schedules of reinforcement. Rats were trained to self-administer cocaine (0.33 mg/infusion) under an FR1 schedule of reinforcement. Thereafter, the effects of pretreatment with LY235959, or the uncompetitive antagonists dextromethorphan and dizocilpine, were examined. The number of infusions earned during the first 10 min of responding under the FR1 schedule was analyzed separately. When rats responded for 0.33 mg/infusion cocaine under an FR1 schedule of reinforcement, 3 mg/kg LY235959 decreased cocaine self-administration only during the first 10 min of the responding. This effect was dose and time dependent and blocked by the competitive NMDA receptor agonist, NMDA. LY235959 (3 mg/kg) decreased total responding for cocaine only when the self-administered dose of cocaine was small (0.02–0.04 mg/infusion) or when responding was reinforced under the PR schedule. In contrast, dizocilpine decreased responding under the FR1 schedule but increased responding under the PR schedule. These data suggest that LY235959 decreased the reinforcing effectiveness of cocaine, a finding reported with systemically administered NMDA receptor antagonists other than dizocilpine.

Behavioral data are consistent with the hypothesis that the normal acquisition of cocaine self-administration behavior requires activation of NMDA glutamate receptors. For example, rats treated with the noncompetitive NMDA receptor antagonist dizocilpine hydrogen maleate (MK-801) fail to develop lever discrimination for the active versus inactive levers during the acquisition of cocaine self-administration behavior (Schenk et al., 1993b). Dizocilpine also blocks the development of sensitization to repeated experimenter-administered amphetamine injections that otherwise facilitate the acquisition of cocaine self-administration behavior in rats (Schenk et al., 1993a).

In addition to the effects of NMDA receptor antagonists on the acquisition of operant responding reinforced with cocaine, NMDA receptor antagonists also alter the self-administration of cocaine in animals that have previously learned to self-administer the drug. For example, dizocilpine increases breakpoint under progressive ratio (PR) schedules of reinforcement (Rinaldi et al., 1996) and decreases the number of cocaine infusions self-administered by rats under fixed ratio (FR) schedules of reinforcement in a manner consistent with increasing the self-administered dose of cocaine (Pierce et al., 1997; Hyytiä et al., 1999). This suggests that the reinforcing effectiveness of cocaine is increased by dizocilpine and that NMDA receptors may mediate motivational effects of cocaine in well trained animals as well as acquisition of cocaine self-administration behavior.

However, the precise role of the NMDA receptor in the acute reinforcing effects of cocaine is difficult to determine because not all NMDA receptor antagonists tested to date alter cocaine self-administration in the same way. Indeed, dizocilpine appears unique in its ability to increase the reinforcing effectiveness of cocaine after systemic administration. Data from several laboratories show that some antagonists selective for the NMDA receptor reduce self-administration of cocaine under both FR and PR schedules of reinforcement when administered systemically, suggesting that NMDA receptor antagonists decrease the reinforcing effectiveness of cocaine. This effect has been demonstrated with the noncompetitive NMDA receptor antagonists dextromethorphan (Pulvirenti et al., 1997) and memantine (Hyytiä et al., 1999) and the strychnine-insensitive glycine/NMDA modulatory site partial agonist (+)-HA-966 administered systemically (Shoaib et al., 1995) and intracerebroventricularly (Cervo et al., 2004).

To date, only one published study has reported the effects of a systemically administered competitive NMDA receptor antagonist on cocaine self-administration behavior. In this study, the competitive NMDA receptor antagonist, CGP 39551, did not alter cocaine self-administration in rats responding for the drug under an FR5 schedule of reinforcement (Hyytiä et al., 1999). This finding is limited by the fact that CGP 39551 was only examined over a limited dose range and that the effect of CGP 39551 on responding for cocaine under a PR schedule of reinforcement was not examined.

The purpose of the experiments presented here was to characterize the effects of the competitive NMDA receptor antagonist, LY235959, on cocaine self-administration behavior under both FR1 and PR schedules of reinforcement. LY235959 is the active isomer of LY274614, a potent, selective, and systemically active competitive NMDA receptor antagonist (Schoepp et al., 1991). LY235959 has a mean plasma t_1/2 of 1.6 h after systemic administration to Sprague-Dawley rats (Dahlem and Eckstein, 1992) and has been shown to prevent NMDA-induced convulsions (Schoepp et al., 1991) and amphetamine-induced neurotoxicity (Fuller et al., 1992) in rats after systemic administration. However, the effect of LY235959 on cocaine self-administration has not been examined previously.

	Materials and Methods

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Subjects. Male Sprague-Dawley rats were purchased from Harlan (Indianapolis, IN) and weighed 275 to 324 g upon arrival at the animal colony. Rats were housed in an animal colony at the University of North Carolina (n = 7; Chapel Hill, NC) or the University of Colorado (n = 33; Denver, CO). A 12-h light/dark cycle was programmed, with lights on at 7:30 AM. Rats lived in the animal colony for 1 week before catheters were surgically implanted under anesthesia. During this time, food and water were both available ad libitum.

Indwelling Intravenous Catheter Implantation. The construction of catheters in the laboratory and surgical implantation of catheters under ketamine/xylazine anesthesia were conducted using established procedures (Caine et al., 1993). Rats were allowed to recover from surgery for at least 1 week before self-administration training began.

Cocaine Self-Administration Training Procedure. Cocaine self-administration sessions were conducted in Plexiglas experimental chambers (29 x 24 x 21 cm) that were housed within sound-attenuating cabinets (MED Associates, St. Albans, VT). Chambers contained two retractable levers mounted on the front wall of the chamber with a food or fluid delivery trough located between the levers. Stimulus lights were positioned 6 cm above each lever. A tone presentation speaker (Sonalert Tone Generator, 2900 Hz) and a speaker for white noise (90 dB) were both mounted 12 cm above the floor on the wall opposite to the levers. A house light (100 mA) was mounted 6 cm above the tone speaker. Cocaine infusions were controlled by an electronic circuit that operated a computer-controlled syringe pump. All behavioral events were monitored and controlled by a personal computer using MED-PC for Windows software (MED Associates).

During training (2-h sessions, 5 days/week) and the subsequent experiments, food was available ad libitum but daily water intake was controlled (30 ml/day/rat) to maintain stable body weights during the experimental procedures. The beginning of each self-administration session was signaled by the onset of a stimulus light located over a lever on the right side of the operant chamber and the extension of two retractable levers. Responses on the right side lever were followed by an intravenous infusion of cocaine (0.33 mg/infusion) according to an FR1 schedule of reinforcement. Each infusion was delivered over a 6-s period via a computer-controlled syringe pump. Initiation of drug delivery was signaled by the simultaneous onset of a tone (2900 Hz) and house light (100 mA) conditioned stimulus complex that remained on for 20 s after the drug-reinforced response. During the 20-s postreinforcement interval, responses on the right lever did not activate the pump. Responses on the left lever had no programmed consequences.

Stable self-administration behavior tended to emerge by session 10 and was characterized by two distinct phases: 1) an early phase (within the first 10 min of the responding for cocaine) in which rats pressed rapidly for cocaine and 2) a phase in which rats responded for cocaine with longer but regular interinfusion intervals. These two phases have been called the load-up and maintenance phases, respectively (Carelli and Deadwyler, 1996), and these operational definitions will be used here.

Testing Procedures. Five distinct experiments were performed in this study. In the first experiment, the effects of LY235959 and dextromethorphan on responding for cocaine under an FR1 schedule of reinforcement were measured. One rat in this group (n = 9) required multiple priming infusions each day to initiate self-administration behavior, and this rat was excluded from the final data analysis. Drugs were administered to rats subcutaneously 30 min prior to the start of self-administration sessions, typically on Tuesdays and Fridays and in the following order: LY235959 (3, 1, and 6 mg/kg) and dextromethorphan (30, 10, and 60 mg/kg). LY274614, the racemic mixture that contains LY235959, protects against NMDA-induced lethality in mice and NMDA-induced convulsions in neonatal rats when administered via intraperitoneal injection 30 min before systemic NMDA administration (Schoepp et al., 1991). Another group of rats (n = 8) was trained as described above, and then cocaine self-administration behavior was measured after vehicle or dizocilpine injections, given in the order of 0.1 and 0.03 mg/kg on different test days. Dizocilpine was also administered subcutaneously 30 min prior to the start of self-administration sessions.

The results from this first experiment suggested that LY235959 produced a selective effect on responding during the early phase of the session (i.e., the load-up phase). We chose to examine this effect further, because some research shows a distinct change in the phasic activity of nucleus accumbens cells during the transition from the load-up to the maintenance phase of a cocaine self-administration session (Carelli and Deadwyler, 1996). Animals self-administered 0.33 mg/infusion cocaine in all subsequent experiments (except when we determined the cocaine dose-response curve in experiment 4), and we measured the effect of 3 mg/kg LY235959 on behavior (30-min presession injection time) in all subsequent experiments (except when determining the time course of the effects of 3 mg/kg LY235959 in experiment 2) to further characterize the pharmacological specificity of this finding.

In a second experiment, the time course of the effect of LY235959 on responding for cocaine under an FR1 schedule of reinforcement was measured in another group of rats (n = 9). Three rats in this group had already completed the fourth experiment described below. Rats received subcutaneous injections with vehicle or 3 mg/kg LY235959 at each of the following pretreatment times arranged in the following order: 30, 0, 60, 120, 240, and 480 min. Rats typically received vehicle control injections on Mondays and Thursdays and injections with 3 mg/kg LY235959 on Tuesdays and Fridays. Two rats in this experiment had catheters fail during the experiment and were not included in the final data analysis.

In a third experiment, the effects of NMDA, alone (10 and 30 mg/kg), and in combination with 3 mg/kg LY235959 (10, 30, and 100 mg/kg) were measured in another group of rats (n = 8). LY235959 (3 mg/kg) was administered subcutaneously 30 min prior to the start of self-administration sessions, and NMDA was administered intraperitoneally 10 or 30 min before the start of self-administration sessions. The doses of NMDA used in this study were chosen after reviewing the literature on NMDA discriminative stimulus effects in Sprague-Dawley rats (Amrick and Bennett, 1987; Grech et al., 1995). The effects of 100 mg/kg NMDA alone were not measured to avoid convulsions that could emerge with the systemic administration of this dose of NMDA alone (Giménez-Llort et al., 1995).

In a fourth experiment, another group of trained rats (n = 8) was administered 3 mg/kg LY235959 in combination with a different dose of cocaine each week (0.33, 0.16, 0.08, 0.04, 0.66, and 0.02 mg/infusion during weeks 1, 2, 3, 4, 5, and 6, respectively). Each week, rats were injected subcutaneously with vehicle as a control on Mondays, Wednesdays, and Thursdays and with 3 mg/kg LY235959 on Tuesdays and Fridays. The effects of vehicle control injections and LY235959 injections each were averaged for every rat, and the averages were analyzed.

In a fifth experiment, the effects of LY235959 on responding for cocaine under a PR schedule of reinforcement were measured. Of the seven rats used in this experiment, six had previously completed experiment three described above. The PR schedule of reinforcement used in this experiment has been described previously (Richardson and Roberts, 1996). The delivery of each cocaine infusion resulted in an increase in the response requirement for a subsequent infusion, such that the response requirements for the first 25 infusions were 1, 2, 4, 6, 9, 12, 15, 20, 25, 32, 40, 50, 62, 77, 95, 118, 145, 178, 219, 268, 328, 402, 492, 603, and 737. Breakpoint was defined as the last response requirement completed before a 1-h period in which no cocaine infusions were earned. Each rat responded for cocaine under the PR schedule twice before the experiment began. Rats were injected with vehicle or 3 or 6 mg/kg LY235959 30 min before the start of self-administration sessions. The effects of a dose of LY235959 and vehicle control were measured during two consecutive daily PR sessions. The order of testing was counterbalanced so that some rats received vehicle on day 1 and LY235959 on day 2, and others received LY235959 on day 1 and vehicle on day 2. The effects of dizocilpine on breakpoint were measured in another group of rats (n = 7) using a similar procedure. Each rat responded for cocaine under the PR schedule twice before the experiment began. During the experiment, vehicle and drug injections were not counterbalanced but were given in the following order: vehicle, vehicle, 0.1 mg/kg dizocilpine, and 0.03 mg/kg dizocilpine. The effect of dizocilpine on cocaine responding under an FR1 schedule of reinforcement had been measured previously in these rats (experiment 1).

Data Analysis. In our experience, cumulative records from a session in which rats respond for cocaine under an FR1 schedule of reinforcement usually reveal a distinct break in responding that separates an early phase of rapid responding for cocaine from a subsequent phase in which rats self-administer cocaine infusions at slower but regular intervals. These two phases have been called the load-up and maintenance phases of responding, and the transition between these phases is associated with a shift in the phasic activity of nucleus accumbens cells (Carelli and Deadwyler, 1996). For the first experiment, these load-up phase responses were counted by hand. For these data and data from all other experiments, each self-administration session was subsequently divided into 10-min bins and the responses that occurred during the first 10 min of responding were analyzed separately from responses that occurred in the remainder of the session. Although this method occasionally extended beyond the obvious break in responding between the load-up and maintenance phases (and thus sometimes included the first maintenance phase infusion), it was more objective and the preferred choice over the manual counting method.

All data were analyzed using SPSS for Windows, version 11.0.1. (SPSS Inc., Chicago, IL). The data for each experiment were analyzed using repeated measures ANOVA, with drug dose or presession injection time entered as the independent variable and total infusions, infusions earned during the first 10 min of responding, infusions earned after the first 10 min of responding, interinfusion interval, or breakpoint as the dependent variable. Each analysis was entirely within subject design. When significant interactions or main effects were observed, post hoc analyses were conducted using the least significant difference (LSD) method.

Drugs. Cocaine hydrochloride was generously supplied by the National Institute on Drug Abuse. Cocaine was dissolved in a 0.9% sodium chloride solution and filtered through a disposable Nalgene filtration unit (0.2-µm surfactant-free cellulose acetate membrane). Heparin sodium was then added to the filtered solution to produce a final concentration of 1.67 U/ml. Dextromethorphan hydrobromide, dizocilpine hydrogen maleate, and NMDA were purchased from Sigma-Aldrich (St. Louis, MO). All drugs were dissolved in sterile water and injected subcutaneously (dextromethorphan and dizocilpine) or intraperitoneally (NMDA). LY235959 was generously provided by Eli Lilly & Co. (Indianapolis, IN). LY235959 powder was exposed to 1 N sodium hydroxide solution (1 µl of solution/1 mg of LY235959) and then dissolved in sterile water to yield a solution with pH 7.0. Dextromethorphan, dizocilpine, and LY235959 were all administered subcutaneously to rats in a volume of 1 ml/kg when possible. Dextromethorphan was insoluble in distilled water at concentrations greater than 20 mg/kg, and doses of 30 and 60 mg/kg were therefore given in higher injection volumes.

	Results

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Experiment 1: Self-Administration of Cocaine Alone and in Combination with LY235959, Dextromethorphan, or Dizocilpine (FR Schedule). The effect of a presession subcutaneous vehicle control injection on cocaine self-administration is presented in Fig. 1. Rats (n = 8) self-administered a mean of 26.3 ± 1.7 (mean ± S.E.M., throughout) 0.33-mg cocaine infusions during a 2-h session. The mean number of infusions earned during load-up was 4.0 ± 0.3 when counted by hand and 4.6 ± 0.3 when the number of infusions self-administered within the first 10 min of responding was measured.

View larger version (26K): [in this window] [in a new window]   Fig. 1. Effects of a vehicle control injection (V), LY235959, and dextromethorphan (left of vertical dashed line; n = 8) or vehicle (V) and dizocilpine (right of vertical dashed line; n = 8) on the number of 0.33-mg cocaine infusions earned during the first 10 min of responding in a 2-h self-administration session (top) and the remaining time in the session (bottom). Categories along the abscissa represent treatment condition and numerical values represent drug dose in milligrams per kilogram. Values along the ordinate represent the number of infusions earned. Asterisks represent the significance level revealed in post hoc tests comparing each treatment condition with vehicle control using the LSD method (*, P < 0.05; **, P < 0.01; ***, P < 0.001).

Figure 1 also shows the effect of systemic treatment with LY235959 (1, 3, and 6 mg/kg) on responding for cocaine. LY235959 dose-dependently reduced the number of cocaine infusions self-administered by rats both within the first 10 min of responding for cocaine [F(3,21) = 8.556, P = 0.001] and during the remainder of the session [F(3,21) = 6.742, P = 0.002]. Post hoc analyses of the effect of LY235959 on the latter phase of responding revealed a statistically significant difference between vehicle control and 6 mg/kg LY235959 (P = 0.007). A similar post hoc analysis of the effect of LY235959 on the number of cocaine infusions earned within the first 10 min of responding revealed statistically significant differences between vehicle and both 3 (P = 0.020) and 6 mg/kg LY235959 (P = 0.006).

The reduction in the number of infusions earned after the first 10 min of responding by rats injected with 6 mg/kg LY235959 was largely the result of single extended pauses in responding during the session in four rats and a complete suppression of responding in another rat. The interinfusion intervals from one rat after injection with vehicle and 6 mg/kg LY235959 are presented in Fig. 2a. These pauses in responding varied in length and time to onset (21.3, 69.3, 34.7, or 18.6 min in duration after infusion numbers 4, 8, 10, and 14, respectively). Responding during this phase appeared relatively normal both before and after the pauses. To note, when these single extended pauses were excluded from the analysis, the mean interinfusion interval during this phase of the sessions was 312.3 (± 19.4) and 316.9 s (± 25.3) after vehicle control and 6 mg/kg LY235959 injections, respectively.

View larger version (16K): [in this window] [in a new window]   Fig. 2. Effects of vehicle and 6 mg/kg LY235959 for one rat (a) and vehicle and 0.1 mg/kg dizocilpine for another rat (b) on interinfusion intervals when rats responded for 0.33 mg/infusion cocaine under an FR1 schedule of reinforcement. Session time in minutes is indicated on the abscissa. Vertical lines indicate the time in the session at which each cocaine infusion was self-administered. The numbers in parentheses above each axis indicate the number of load-up infusions calculated using the counting method.

Dextromethorphan (10, 30, and 60 mg/kg) did not decrease the total number of cocaine infusions self-administered by rats and had no statistically significant effect on the number of cocaine infusions self-administered during the first 10 min of responding (Fig. 1). However, the number of cocaine infusions self-administered during the first 10 min of responding decreased in five of eight rats injected with 60 mg/kg dextromethorphan. Doses of dextromethorphan higher than 60 mg/kg were not tested because of problems with skin lesions and drug solubility, and further testing with dextromethorphan was discontinued.

Data from another group of rats (n = 8) that self-administered cocaine after receiving subcutaneous injections with vehicle or dizocilpine (0.03 and 0.1 mg/kg) are also presented in Fig. 1. Dizocilpine reduced the number of cocaine infusions self-administered by rats in a statistically significant manner both during [F(2,14) = 19.427, P < 0.001] and after [F(2,14) = 4.256, P = 0.036] the first 10 min of responding for cocaine. Post hoc analyses showed that, compared with vehicle-treated rats, only the 0.1 mg/kg dose of dizocilpine reduced the number of cocaine infusions self-administered by rats, both during (P < 0.001) and after (P = 0.007) the first 10 min of responding for cocaine. Figure 2b shows interinfusion intervals from one rat that was injected with vehicle and 0.1 mg/kg dizocilpine. For the group of rats injected with 0.1 mg/kg dizocilpine, the reduction in the number of cocaine infusions self-administered after the first 10 min of responding was largely due to a lengthening of regularly spaced responses within the first hour of responding. Thereafter, interinfusion intervals gradually shortened. In the final hour of each session, responding for cocaine did not differ between rats treated with vehicle and 0.1 mg/kg dizocilpine (data not shown).

Experiment 2: Time Course of the Effect of LY235959 on Responding for Cocaine (FR Schedule). Figure 3 shows the effect of 3 mg/kg LY235959 on the number of self-administered cocaine infusions when LY235959 was administered 0, 30, 60, 120, 240, and 480 min before the start of self-administration sessions to another group of rats (n = 7). LY235959 produced a time-dependent reduction in the number of cocaine infusions self-administered during the first 10 min of responding. A repeated measures ANOVA using LY235959 dose and presession injection time as within-subject variables revealed a significant interaction between dose and time [F(5,30) = 2.718, P = 0.039]. Post hoc analyses revealed statistically significant differences between vehicle and 3 mg/kg LY235959 injections at 30 (P = 0.035) and 60 min (P = 0.020) but not at 0, 120, 240, or 480 min. In contrast to the time-dependent reduction in the number of cocaine infusions self-administered during the first 10 min of responding, LY235959 did not alter the number of cocaine infusions self-administered after the first 10 min of responding for cocaine.

View larger version (15K): [in this window] [in a new window]   Fig. 3. Time course of the effect of a vehicle control injection or injection of 3 mg/kg LY235959 on the number of 0.33-mg cocaine infusions earned during the first 10 min of responding in a 2-h self-administration session (top) and the remaining time in the session (bottom). Ordinate, number of infusions earned. Abscissa, pretreatment time in minutes. Asterisks represent the significance level revealed in post hoc tests (LSD method) comparing 3 mg/kg LY235959 with control at each time point (*, P < 0.05; **, P < 0.01; ***, P < 0.001).

View larger version (15K): [in this window] [in a new window]   Fig. 4. Effect of 3 mg/kg LY235959 alone and in combination with 100 mg/kg NMDA on the number of 0.33-mg cocaine infusions earned during the first 10 min of responding in a 2-h self-administration session (top) and the remaining time in the session (bottom). Asterisks represent the significance level revealed in post hoc analyses using the LSD method (*, P < 0.05; **, P < 0.01; ***, P < 0.001; n.s., not significant).

Experiment 4: Dose-Dependent Effects of Cocaine and LY235959. As shown in Fig. 5, the relationship between cocaine dose and the number of cocaine infusions self-administered by rats was biphasic; i.e., the dose-response curve had an inverted U shape. LY235959 produced several changes in responding for cocaine when administered in combination with a range of cocaine doses. LY235959 selectively reduced the number of 0.33-mg cocaine infusions self-administered during the first 10 min of responding for cocaine, and a statistical trend was observed for the effects of 3 mg/kg LY235959 on the number of 0.16-mg cocaine infusions self-administered during this phase (P = 0.060). In contrast, when LY235959 was combined with 0.02 and 0.04 mg/infusion cocaine, it reduced the number of cocaine infusions self-administered both during and after the first 10 min of responding for cocaine, reducing total consumption of the lowest doses of cocaine. LY235959 (3 mg/kg) did not alter the number of 0.66-mg cocaine infusions self-administered by rats during either phase of the self-administration session.

View larger version (17K): [in this window] [in a new window]   Fig. 5. Effect of 3 mg/kg LY235959 on the number of cocaine infusions earned during the first 10 min of responding in a 2-h self-administration session (top) and the remaining time in the session (bottom; n = 8). Ordinate, number of infusions earned. Abscissa, self-administered dose of cocaine. Asterisks represent significance level (*, P < 0.05; **, P < 0.01; ***, P < 0.001) when comparing the effects of 3 mg/kg LY235959 with control in post hoc analyses using the LSD method. Cocaine dose is plotted on a log scale.

Experiment 5: Effects of LY235959 and Dizocilpine on Responding for Cocaine under a PR Schedule. Figure 6 shows the effects of LY235959 and dizocilpine on cocaine self-administration under a PR schedule of reinforcement. Rats in the LY235959 and dizocilpine groups earned their last infusion of cocaine 107.9 ± 13.9 and 140.3 ± 6.7 min after the start of self-administration sessions under vehicle control conditions. LY235959 dose-dependently reduced the number of cocaine infusions self-administered by rats and the final ratio completed under the PR schedule in a statistically significant manner [infusions earned, F(2,12) = 12.000, P = 0.001; final ratio, F(2,12) = 10.656, P = 0.002]. Rats injected with 6 mg/kg LY235959 earned their last infusion of cocaine 53.5 ± 13.5 min after the start of the self-administration session, significantly sooner than when injected with vehicle before the start of the session (P < 0.05). In contrast, dizocilpine dose-dependently increased the number of infusions earned [F(2,12) = 5.213, P = 0.023] and the final ratio completed by rats responding for 0.33 mg/infusion cocaine [F(2,12) = 4.370, P = 0.038]. Rats injected with 0.1 mg/kg dizocilpine earned their last infusion 199.0 ± 20.8 min after the start of the self-administration session, significantly later than when injected with vehicle before the start of the session (P < 0.05).

View larger version (30K): [in this window] [in a new window]   Fig. 6. Effect of LY235959 (3 and 6 mg/kg; n = 7) and dizocilpine (0.03 and 0.1 mg/kg; n = 7) on responding for cocaine under a PR schedule of reinforcement. Both the total infusions earned (top) and breakpoint (bottom) are presented. Asterisks represent post hoc analyses performed using the LSD method comparing the effects of vehicle to each dose of the antagonist (*, P < 0.05; **, P < 0.01; ***, P < 0.001; n.s. = not significant).

	Discussion

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The present study used a rat model of intravenous drug self-administration to characterize the effects of the competitive NMDA receptor antagonist LY235959 on cocaine self-administration behavior. In this study, LY235959 decreased consumption of cocaine during the first 10 min of responding when rats self-administered cocaine (0.33 mg/infusion) under an FR1 schedule of reinforcement. This effect was blocked by NMDA. In addition, the highest dose of LY235959 produced either a midsession pause in responding or complete suppression of responding. LY235959 also decreased total responding for cocaine when the self-administered dose of cocaine was low or when the number of responses required for an infusion of 0.33 mg of cocaine increased under a PR schedule of reinforcement.

In general, when rats respond for cocaine under FR schedules of reinforcement, the total number of infusions self-administered in an experimental session is inversely related to the dose of cocaine (Pickens and Thompson, 1968). Carelli and Deadwyler (1996) have shown specifically that when rats self-administer cocaine under an FR1 schedule of reinforcement, the number of infusions earned during both the load-up and maintenance phases of each session is inversely related to doses that fall along the descending limb of the dose-consumption curve. The decrease in responding for higher doses of cocaine under these conditions is associated with an increase in the duration of regularly spaced interinfusion intervals (Carelli and Deadwyler, 1996). Furthermore, breakpoints and the number of cocaine infusions earned by rats responding under PR schedules of reinforcement also tend to increase with increases in the self-administered dose of cocaine (Roberts et al., 1989; Depoortere et al., 1993), suggesting an increase in the reinforcing effectiveness of cocaine with increasing dose (Richardson and Roberts, 1996).

Published data show that when rats are treated acutely with the noncompetitive NMDA receptor antagonist dizocilpine before the start of cocaine self-administration sessions, rats self-administer fewer cocaine infusions under an FR1 schedule of reinforcement and consume more cocaine and attain higher breakpoints under a PR schedule of reinforcement (Rinaldi et al., 1996; Pierce et al., 1997; Hyytiä et al., 1999). We have replicated those findings in the present experiment. These changes in responding for cocaine are similar to changes that occur by increasing the dose of cocaine that rats self-administer, and they suggest that dizocilpine increases the reinforcing effectiveness of cocaine.

Two findings in the present study suggest that treatment with the competitive NMDA receptor antagonist LY235959 decreases, rather than increases, the reinforcing effectiveness of cocaine. First, LY235959 (3 and 6 mg/kg) decreased breakpoint and reduced the number of 0.33-mg cocaine infusions self-administered by rats responding under a PR schedule of reinforcement. LY235959 (3 mg/kg) also decreased total consumption of cocaine for doses along the ascending limb of the dose-response curve (0.02–0.04 mg/infusion). Thus, treatment with LY235959 decreased responding for cocaine when the dose of cocaine infused was small and when the number of responses required for an infusion of cocaine was increasing, suggesting a decrease in the reinforcing effectiveness of cocaine.

Although these data with LY235959 are not consistent with the effects of dizocilpine on cocaine self-administration behavior, most other systemically administered NMDA receptor antagonists tested to date produce effects on cocaine self-administration behavior similar to those reported here. For example, the noncompetitive NMDA receptor antagonists dextromethorphan (Pulvirenti et al., 1997) and memantine (Hyytiä et al., 1999) and the strychnine-insensitive glycine/NMDA modulatory site partial agonist (+)-HA-966 (Shoaib et al., 1995; Cervo et al., 2004) decrease breaking point and the number of cocaine infusions earned by rats that self-administer cocaine under a PR schedule of reinforcement. To our knowledge, no other systemically administered competitive NMDA receptor antagonist has been administered to rats responding for cocaine under a PR schedule of reinforcement. The data from the present study extend our knowledge and, together with other published data, suggest that, across a range of specific pharmacological mechanisms of action, systemic administration of NMDA receptor antagonists tends to decrease the reinforcing effectiveness of cocaine.

It is not known why dizocilpine produces a different profile of effects on cocaine self-administration behavior when compared with other NMDA receptor antagonists. Recently, several high-affinity channel blockers, such as 1-(1-phenylcyclohexyl)piperidine (phencyclidine), ketamine, and dizocilpine, have been shown to bind with high affinity to the high-affinity state of the dopamine D2 receptor (D2^High; Seeman et al., 2005). Dizocilpine was 6- to 10-fold more selective for D2^High receptors over NMDA receptors and also showed efficacy at cloned human D2_Long receptors expressed in Chinese hamster ovary cells (Seeman et al., 2005). If differences exist between dizocilpine and other NMDA receptor antagonists in their selectivity for D2 receptors, then this might explain the differences observed between dizocilpine and other NMDA antagonists on cocaine self-administration behavior. D2 receptor agonists produce leftward and upward shifts in cocaine self-administration dose-response functions (Caine et al., 1999; Barrett et al., 2004), suggesting that D2 receptor agonists can enhance the reinforcing effectiveness of cocaine. Additional data from receptor binding assays together with data from studies that investigate the interactions between NMDA antagonists and D2 receptor agonists/antagonists on cocaine self-administration behavior are needed to support D2 receptor activation as a mechanism for this difference.

In this study, the highest dose of LY235959 tested reduced responding for cocaine under the FR1 schedule of reinforcement, but the pattern of responding for cocaine in the presence of LY235959 was qualitatively different from the pattern of responding observed after dizocilpine treatment. LY235959 did not produce an increase in the length of regularly spaced interinfusion intervals during the cocaine self-administration session, as was observed during the first hour of responding after dizocilpine treatment. The reduction in responding produced by 6 mg/kg LY235959 was accounted for by a reduction in the number of infusions earned during the first 10 min of the session, single extended midsession pauses in responding for some rats, and a complete suppression of behavior in another during the maintenance phase of the session. It is also important to note that a lower dose of LY235959 (3 mg/kg) decreased the number of cocaine infusions self-administered by rats during the first 10 min of responding but not during the remainder of the session. Thus, responding during these two phases of the cocaine self-administration session was differentially altered by NMDA receptor antagonism and tended to be selective to the earlier load-up phase of the session.

Although a selective effect on load-up responding by NMDA receptor antagonists has not been specifically described by other laboratories, it has been shown that NMDA receptor antagonists can produce temporally bound changes in cocaine responding. For example, Hyytiä et al. (1999) report that the noncompetitive NMDA receptor antagonist memantine only produced statistically significant reductions in cocaine consumption during the first 30 min of 2-h cocaine self-administration sessions. Similar effects were reported with systemic administration of dextromethorphan (Pulvirenti et al., 1997) and intracerebroventricular administration of (+)-HA-966 (Cervo et al., 2004). Whether these effects were restricted specifically to the load-up phase of self-administration is not known.

Neurobiological data show that the transition from the load-up to maintenance phases of a self-administration session is accompanied by a transition in nucleus accumbens cell firing from activity unrelated to the reinforced response to one of four types of neuronal discharge patterns (Carelli et al., 1993). This behavior and its associated neuronal correlates can be selectively altered by dopamine receptor antagonists (Carelli et al., 1999). Our data suggest that glutamatergic activity via NMDA receptors may be another mechanism controlling the behavioral transition between the load-up and maintenance phases. It is also possible that the emergence of nucleus accumbens cell firing that processes information about cocaine-directed behaviors (Carelli, 2004) may also be dependent upon glutamatergic activity via NMDA receptors. Extensive anatomic studies show that the nucleus accumbens receives afferent projections from areas including the prefrontal cortex (Brog et al., 1993), the basolateral amygdala (Brog et al., 1993; Wright et al., 1996), and the subiculum of the hippocampus (Groenewegen et al., 1991; Brog et al., 1993). Given this anatomic arrangement, it has been proposed that the nucleus accumbens functions as a site for the integration of limbic information related to memory, drive and motivation, and the generation of goal-directed movement (Mogenson, 1987; Pennartz et al., 1994). Future studies are needed to determine whether the emergence of nucleus accumbens patterned discharges that encode cocaine-related information after load-up is dependent upon activation of NMDA receptors in this structure.

The primary purpose of this study was to compare the effects of the competitive NMDA receptor antagonist LY235959 on cocaine self-administration behavior with what is known about the effects of other systemically administered NMDA receptor antagonists on cocaine self-administration behavior. The data presented in this study suggest that LY235959 decreases the reinforcing effectiveness of cocaine when administered systemically to rats, a finding reported with systemically administered NMDA receptor antagonists other than dizocilpine. In addition, this study characterized a selective effect of LY235959 on load-up responding for cocaine that was dose-dependent, time-dependent, and blocked by the NMDA receptor agonist, NMDA. Additional electrophysiological studies may help elucidate neurobiological changes associated with this effect.

	Footnotes

 
This work was supported by U.S. Public Health Service Grants DA14389 (to R.M.A.), DA 14339 (to R.M.C.), and DA 02749 (to L.A.D.) from the National Institute on Drug Abuse.

A preliminary report of these data was presented at the 2000 meeting of the Society for Neuroscience (San Diego, CA) and the 2004 meeting of the College on Problems of Drug Dependence (San Juan, Puerto Rico).

Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.

doi:10.1124/jpet.105.086355.

ABBREVIATIONS: NMDA, N-methyl-D-aspartate; PR, progressive ratio; FR, fixed ratio; (+)-HA-966, (+)-hydroxy-3-aminopyrrolidine-2-one; CGP 39551, DL-(E)-2-amino-4-methyl-5-phosphono-3-pentanoic acid carboxyethylester; LY235959, (-)-6-phosphonomethyl-deca-hydroisoquinoline-3-carboxylic acid; LSD, least significant difference; MK-801, (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (dizocilpine hydrogen maleate); ANOVA, analysis of variance.

Address correspondence to: Dr. Richard M. Allen, Department of Psychology, University of Colorado at Denver and Health Sciences Center, CB No. 173, P.O. Box 173364, Denver, CO 80217. E-mail: richard.allen{at}cudenver.edu

	References

Top Abstract Materials and Methods Results Discussion References

 

Amrick CL and Bennett DA (1987) N-Methyl-D-aspartate produces discriminative stimuli in rats. Life Sci 40: 585-591.[Medline]

Baker DA, Cornish JL, and Kalivas PW (2002) Glutamate and dopamine interactions in the motive circuit, in Contemporary Clinical Neuroscience: Glutamate and Addiction (Herman BH ed) pp 143-156, Humana Press, Totowa, NJ.

Barrett AC, Miller JR, Dohrmann JM, and Caine SB (2004) Effects of dopamine indirect agonists and selective D1-like and D2-like agonists and antagonists on cocaine self-administration and food maintained responding in rats. Neuropharmacology 47: 256-273.

Bonci A and Malenka RC (1999) Properties and plasticity of excitatory synapses on dopaminergic and GABAergic cells in the ventral tegmental area. J Neurosci 19: 3723-3730.[Abstract/Free Full Text]

Brog JS, Salyapongse A, Deutch AY, and Zahm DS (1993) The patterns of afferent innervation of the core and shell in the "accumbens" part of the ventral striatum: immunohistochemical detection of retrogradely transported fluoro-gold. J Comp Neurol 338: 255-278.[CrossRef][Medline]

Borgland SL, Malenka RC, and Bonci A (2004) Acute and chronic cocaine-induced potentiation of synaptic strength in the ventral tegmental area: electrophysiological and behavioral correlates in individual rats. J Neurosci 24: 7482-7490.[Abstract/Free Full Text]

Caine SB and Koob GF (1994) Effects of mesolimbic dopamine depletion on responding maintained by cocaine and food. J Exp Anal Behav 61: 213-221.[CrossRef][Medline]

Caine SB, Lintz R, and Koob GF (1993) Intravenous drug self-administration techniques in animals, in Behavioral Neuroscience: a Practical Approach (Sahgal A ed) pp 117-143, Oxford UP, Oxford, UK.

Caine SB, Negus SS, Mello NK, and Bergman J (1999) Effects of dopamine D_1-like and D_2-like agonists in rats that self-administer cocaine. J Pharmacol Exp Ther 291: 353-360.[Abstract/Free Full Text]

Carelli RM (2004) Nucleus accumbens cell firing and rapid dopamine signaling during goal-directed behaviors in rats. Neuropharmacology 47: 180-189.

Carelli RM and Deadwyler SA (1996) Dose-dependent transitions in nucleus accumbens cell firing and behavioral responding during cocaine self-administration sessions in rats. J Pharmacol Exp Ther 277: 385-393.[Abstract/Free Full Text]

Carelli RM, Ijames S, Konstantopoulos J, and Deadwyler SA (1999) Examination of factors mediating the transition to behaviorally correlated nucleus accumbens cell firing during cocaine self-administration sessions in rats. Behav Brain Res 104: 127-139.[CrossRef][Medline]

Carelli RM, King VC, Hampson RE, and Deadwyler SA (1993) Firing patterns of nucleus accumbens neurons during cocaine self-administration in rats. Brain Res 626: 14-22.[CrossRef][Medline]

Cervo L, Cocco A, and Carnovali F (2004) Effects on cocaine and food self-administration of (+)-HA-966, a partial agonist at the glycine/NMDA modulatory site, in rats. Psychopharmacology 173: 124-131.[Medline]

Dahlem AM and Eckstein JA (1992) Disposition of the novel NMDA antagonist, LY235959, in rats, dogs and monkeys. Pharm Res (NY) 9: S278.

Depoortere RY, Li DH, Lane JD, and Emmett-Oglesby MW (1993) Parameters of self-administration of cocaine in rats under a progressive ratio schedule. Pharmacol Biochem Behav 45: 539-548.[CrossRef][Medline]

Fuller RW, Hemrick-Luecke SK, and Ornstein PL (1992) Protection against amphetamine-induced neurotoxicity toward striatal dopamine neurons in rodents by LY274614 and excitatory amino acid antagonist. Neuropharmacology 31: 1027-1032.[Medline]

Giménez-Llort L, Ferré S, and Martínez E (1995) Effects of the systemic administration of kainic acid and NMDA on exploratory activity in rats. Pharmacol Biochem Behav 51: 205-210.[Medline]

Grech DM, Lunn WHW, and Balster RL (1995) Discriminative stimulus effects of excitatory amino acid agonists in rats. Neuropharmacology 34: 55-62.[Medline]

Groenewegen HJ, Berendse HW, Meredith GE, Haber SN, Voorn P, Walters JG, and Lohman AHM (1991) Functional anatomy of the ventral, limbic system-innervated striatum, in The Mesolimbic Dopamine System: From Motivation to Action (Willner P and Scheel-Kruger J eds) pp 19-59, John Wiley & Sons, New York.

Hyytiä P, Bäckström P, and Liljequist S (1999) Site-specific NMDA receptor antagonists produce differential effects on cocaine self-administration in rats. Eur J Pharmacol 378: 9-16.[CrossRef][Medline]

Kombian SB and Malenka RC (1994) Simultaneous LTP of non-NMDA and LTD of NMDA-receptor-mediated responses in the nucleus accumbens. Nature (Lond) 368: 242-246.[CrossRef][Medline]

Mogenson GF (1987) Limbic-motor integration. Prog Psychobiol Physiol Psychol 12: 117-169.

Pennartz CM, Groenewegen HJ, and Lopes da Silva FH (1994) The nucleus accumbens as a complex of functionally distinct neuronal ensembles: an integration of behavioral, electrophysiological and anatomical data. Prog Neurobiol 42: 719-761.[CrossRef][Medline]

Pettit HO, Ettenberg A, Bloom FE, and Koob GF (1984) Destruction of dopamine in the nucleus accumbens selectively attenuates cocaine but not heroin self-administration in the rat. Psychopharmacology 84: 167-173.[CrossRef][Medline]

Pickens R and Thompson T (1968) Cocaine-reinforced behavior in rats: effects of reinforcement magnitude and fixed-ratio size. J Pharmacol Exp Ther 161: 122-129.[Abstract/Free Full Text]

Pierce RC, Meil WM, and Kalivas PW (1997) The NMDA antagonist, dizocilpine, enhances cocaine reinforcement without influencing mesoaccumbens dopamine transmission. Psychopharmacology 133: 188-195.[CrossRef][Medline]

Pulvirenti L, Balducci C, and Koob GF (1997) Dextromethorphan reduces intravenous cocaine self-administration in the rat. Eur J Pharmacol 321: 279-283.[CrossRef][Medline]

Richardson NR and Roberts DCS (1996) Progressive ratio schedules in drug self-administration studies in rats: a method to evaluate reinforcing efficacy. J Neurosci Methods 66: 1-11.[CrossRef][Medline]

Rinaldi R, French E, and Roberts DCS (1996) Systemic pretreatment with MK-801 (dizocilpine) increases breaking points for self-administration of cocaine on a progressive ratio schedule in rats. Psychopharmacology 128: 83-88.[CrossRef][Medline]

Roberts DC, Loh EA, and Vickers G (1989) Self-administration of cocaine on a progressive ratio schedule in rats: dose-response relationship and effect of haloperidol pretreatment. Psychopharmacology 97: 535-538.[CrossRef][Medline]

Roberts DCS and Koob GF (1980) Extinction and recovery of cocaine self-administration following 6-hydroxydopamine lesions of the nucleus accumbens. Pharmacol Biochem Behav 12: 781-787.[CrossRef][Medline]

Schenk S, Valdez A, McNamara C, House DT, Higley D, Bankson MG, Gibbs S, and Horger BA (1993a) Development and expression of sensitization to cocaine's reinforcing properties: role of NMDA receptors. Psychopharmacology 111: 332-338.[CrossRef][Medline]

Schenk S, Valdez A, Worley CM, and McNamara C (1993b) Blockade of the acquisition of cocaine self-administration by the NMDA antagonist MK-801 (dizocilpine). Behav Pharmacol 4: 652-659.[Medline]

Schoepp DD, Ornstein PL, Salhoff CR, and Leander JD (1991) Neuroprotectant effects of LY274614, a structurally novel systemically active competitive NMDA receptor antagonist. J Neural Trans 85: 131-143.

Seeman P, Ko F, and Tallerico T (2005) Dopamine receptor contribution to the action of PCP, LSD and ketamine psychotomimetics. Mol Psychiatry, in press.

Shoaib M, Shippenberg TS, Goldberg SR, and Schindler CW (1995) Behavioral studies with the glycine partial agonist (+)-HA966 on cocaine-induced locomotor activity and reinforcement. Behav Pharmacol 6: 568-576.[Medline]

Thomas MJ, Malenka RC, and Bonci A (2000) Modulation of long-term depression by dopamine neurons in the mesolimbic system. J Neurosci 20: 5581-5586.[Abstract/Free Full Text]

Ungless MA, Whistler JL, Malenka RC, and Bonci A (2001) Single cocaine exposure in vivo induces long-term potentiation in dopamine neurons. Nature (Lond) 411: 583-587.[CrossRef][Medline]

Wright CI, Beijer AV, and Groenewegen HJ (1996) Basal amygdaloid complex afferents to the rat nucleus accumbens are compartmentally organized. J Neurosci 16: 1877-1893.[Abstract/Free Full Text]

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