Introduction

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Although cocaine is an indirect, nonselective monoaminergic agonist, it is thought that dopamine (DA) reuptake inhibition is primarily involved in the reinforcing effects of cocaine and other psychostimulants (Wise, 1998). The DA system has therefore been targeted for therapeutic development in the treatment of psychostimulant abuse, and drugs that bind to dopamine transporters (DATs) are being tested as possible agonist substitution therapies (Grabowski et al., 2001). Animal models offer a systematic means to evaluate potential compounds for their effects on psychostimulant self-administration, and studies in nonhuman primates have provided encouraging data concerning pharmacotherapy development (Howell and Wilcox, 2001). Decreases in cocaine self-administration, for example, have been documented after pretreatments with long-acting DA reuptake inhibitors (Glowa et al., 1996; Nader et al., 1997; Howell et al., 2000).

With respect to the use of DAT blockers as candidate agonist therapies, the abuse liability of these types of compounds must be considered in addition to their ability to decrease psychostimulant intake. A positive relationship has been reported between the affinity of drugs for DAT and their potency to maintain responding (Ritz et al., 1987; Bergman et al., 1989; Wilcox et al., 1999). In drug self-administration studies, DAT inhibitors, such as 2-carbomethoxy-3-phenyltropane (-CIT), CFT, and 3-(4-chlorophenyl) tropane-2-carboxylic acid phenyl ester (RTI-113), function as reinforcers in nonhuman primates (Spealman et al., 1991; Weed et al., 1995; Howell et al., 2000). PTT is a high-affinity, DAT-selective inhibitor, with 10-fold higher affinity at DAT and 50-fold greater inhibition of DA reuptake compared with cocaine (Bennett et al., 1995). In all studies of self-administration conducted with PTT in monkeys, response rates were lower and more variable than those maintained by cocaine (Nader et al., 1997; Birmingham et al., 1998; Lile et al., 2000).

Although PTT has high affinity at DAT, it is possible that the low, variable rates of responding maintained by PTT were due to its kinetic profile. For example, a drug's duration of action appears to influence the rate that it is self-administered (Winger et al., 1975; Panlilio and Schindler, 2000). PTT has a long duration of action in neurochemical and behavioral assays (Hemby et al., 1995; Porrino et al., 1995; Nader et al., 1997), which likely resulted in long interinjection intervals, and thus low rates of self-administration. Additionally, a drug's onset of action has been shown to determine the rate at which it maintains responding (Balster and Schuster, 1973; Panlilio et al., 1998). Therefore, one intent of the present study was to measure PTT's rate of onset to occupy DAT to determine whether it was different from cocaine (experiment 1).

While rates of responding are indicative of a drug's potency to maintain self-administration, response rates are not a measure of the strength of a drug to function as a reinforcer (Woolverton and Johanson, 1984). One method frequently used to assess the efficacy of a reinforcing event is a progressive-ratio (PR) schedule (Hodos and Kalman, 1963). Under PR schedules, reinforcement is contingent upon the completion of successively increasing ratio sizes; the number of responses necessary for reinforcement is systematically increased until the animal stops responding (termed its "breaking point"; BP). This schedule has been used to compare the reinforcing efficacy of cocaine with other psychostimulants (Stafford et al., 1998). Using a PR schedule, Roberts et al. (1999) found a correlation between the ratio of serotonin transporter (5-HTT) to DAT binding affinity and the reinforcing efficacy of several cocaine analogs, including PTT. In fact, PTT was found to maintain higher BPs than cocaine, but lower overall response rates, consistent with PTT's long duration of action compared with cocaine.

Because PTT's prolonged duration of action clearly impacted response rates, another goal of this study was to compare the reinforcing efficacy of PTT to the short-acting psychostimulant cocaine in primates under conditions where interpretation of the data is less dependent on rate of responding (experiments 2 and 3) (Katz, 1990). In experiment 2, a PR schedule similar to the one used by Roberts et al. (1999) was used to assess the reinforcing efficacy of PTT and cocaine. In experiment 3, these findings were extended using another behavioral evaluation of reinforcing efficacy, a drug-drug choice paradigm. In this procedure, the reinforcing efficacy of two drugs or doses of a drug are directly compared by making one drug solution available as an alternative to another for self-administration (Johanson and Schuster, 1975; Woolverton and Johanson, 1984). Furthermore, the present study allowed for a comparison of BP measures with drug choice, in an effort to validate both measures of reinforcing efficacy.

	Materials and Methods

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Experiment 1: Ex Vivo DAT Binding

Subjects and Apparatus. Male Sprague-Dawley rats weighing between 250 and 300 g were used. They were initially housed in groups of three in plastic cages and with a 12:12-h light/dark cycle (lights on at 6:00 AM). Food and water were available ad libitum.

Procedure. DAT binding was studied ex vivo using methods similar to those published previously using mice (Scheffel et al., 1991; Stathis et al., 1995; Gatley et al., 1999). Drugs were given i.v. via a surgically implanted catheter. For surgery, rats were anesthetized with pentobarbital (50 mg/kg i.p.), and a femoral catheter was implanted using standard techniques. The exteriorized tip of the catheter was sealed by heating. After surgery, animals were housed individually for 48 h and then used experimentally.

ED₅₀ Determination. To establish relative potency for DAT occupancy, injections of various doses of PTT were given before [³H]CFT to permit maximum (or equilibrium) binding of PTT to DAT. Initially, catheterized rats were placed in a plastic restrainer and injected i.v. (0.4 ml/rat/10 s) with 3 µmol/kg PTT. This dose was selected based upon preliminary studies with PTT and the in vitro potency ratio between cocaine and PTT. At various time points after drug injection (0.5-30 min), rats were injected intravenously with [³H]CFT (86 Ci/mmol; PerkinElmer Life Sciences, Boston, MA) 10 µCi/rat over 10 s. Because preliminary studies showed that the striatal/cerebellar ratio (S/C) of [³H]CFT reached a maximum 45 min after injection of [³H]CFT, rats were decapitated at this time. After decapitation, brains were removed, and the striatum (high DAT density) and the cerebellum (no DAT, nonspecific binding) were dissected. Striatum and cerebellum were weighed and placed into separate 5-ml glass vials. Solvable (10 µl/mg tissue) was added, and the vial was allowed to sit for 24 h at room temperature. After 24 h, glacial acetic acid (1 µl/mg tissue) was added, and 200 µl of the tissue solution was immediately pipetted into each well of 24-well scintillation plates (3-6 wells/sample). Microscint-20 cocktail (1000 µl) was then added to each well, and the plate was sealed. This preparation was allowed to sit for 4 h to further solubilize tissue and reduce chemiluminescence of the Microscint-20 cocktail. Radioactivity was then counted. A complete dose-response function was then determined for PTT using the time point at which the decrease in [³H]CFT was maximal. The ED₅₀ value was calculated for reduction in [³H]CFT binding.

Time Course Determination. To establish the rate of onset of DAT binding, an injection of a selected dose of PTT was given at the time point at which [³H]CFT was asymptotic. The decrease in binding was measured at various time points after drug injection and compared with the same points after saline injection (Stathis et al., 1995). Specifically, saline or the ED₅₀ dose of PTT was given 45 min after injection of [³H]CFT. Animals were decapitated at various time points (30 s-120 min) after injection of test drug.

Experiments 2 and 3: Comparison of the Reinforcing Efficacy of PTT and Cocaine

Subjects. Six individually housed adult male rhesus monkeys (Macaca mulatta) served as subjects. Monkeys R-1242, R-1253, and R-1272 were experimentally naive at the beginning of training under the choice procedure. At the completion of the choice study, monkey R-1272 was trained under the progressive-ratio procedure. Monkey R-1248 had previously self-administered cocaine under both fixed interval and fixed-ratio (FR) schedules. Monkey R-1286 had previously self-administered PTT and two other long-acting cocaine analogs. Monkey R-1322 had a history of pretreatments with HD-23, another long-acting cocaine analog, on a baseline of self-administered food and cocaine under FR schedules. Monkeys R-1248 and R-1322 began training on the PR schedule immediately following the completion of prior experiments; R-1272 and R-1286 were drug abstinent for approximately 1 year before training under the PR schedule. Subjects weighed between 9 and 14 kg under free-feeding conditions. Their body weights were maintained at approximately 90 to 95% of free-feeding weights by supplemental feeding of Lab Diet high-protein monkey diet (100-150 g/day; PMI Nutrition International Inc., Brentwood, MO). Monkeys were weighed approximately once a month, and if necessary, their diet was adjusted to maintain stable weights. In addition, they were given fresh fruit or peanuts at least 3 days/week. Monkeys lived in a temperature- and humidity-controlled colony room; lighting was maintained on a 6:00 AM/8:00 PM on/off schedule. Environmental enrichment was provided as outlined in the Animal Care and Use Committee of Wake Forest University Nonhuman Primate Environmental Enrichment Plan.

Apparatus. Monkeys were individually housed in sound-attenuating cubicles (91 cm³; Plas Labs, Lansing, MI). The front wall of each cubicle was constructed of Plexiglas to allow the monkey visual access to the laboratory. For the choice study (experiment 3), the front wall was covered with a drape during the session. In an effort to increase environmental enrichment, the monkeys in the PR study (experiment 2) were trained to respond during sessions while uncovered to allow for uninterrupted visual access to other animals. Each cubicle was equipped with two response levers (BRS/LVE, Beltsville, MD) and one (experiment 2) or two (experiment 3) peristaltic infusion pumps (Cole-Parmer Instrument, Chicago, IL) for delivering drug infusions at a rate of approximately 1.5 ml/10 s. Above each lever were two sets of jeweled stimulus lights. For experiment 2, the four lights above each lever were covered with alternating red and white lens caps. For experiment 3, the four lights above the left lever (lever 1) were covered with white lens caps, whereas two lights above the right lever (lever 2) were covered with alternating red and green lens caps. Each animal was fitted with a stainless steel restraint harness and spring arm (Restorations Unlimited, Chicago, IL) that was attached to the rear of the cubicle. Experimental events were controlled and counted by a Macintosh II computer and associated interfaces.

Surgery. Each animal was anesthetized with a combination of ketamine (15 mg/kg i.m.; Fort Dodge Animal Health, Fort Dodge, IA) and butorphanol (0.05 mg/kg i.m.; Fort Dodge Animal Health), and a chronic indwelling venous catheter was surgically implanted under sterile conditions. The proximal end of the catheter was inserted into a major vein (internal jugular, external jugular, brachial, or femoral), terminating in the vena cava. The distal end of the catheter was threaded subcutaneously and exited through a small incision between the scapula on the back of the animal. The catheter was contained within the spring arm and attached to an infusion pump. For experiment 3, a double-lumen silicone catheter (Ronsil Rubber Products, Blackstone, VA) was used and each lumen was attached to a separate infusion pump. Antibiotics (25 mg/kg Kefzol; cefazolin sodium, Marsam Pharmaceuticals, Inc., Cherry Hill, NJ) were administered prophylactically for 7 days starting on the day of surgery.

Procedure

Experiment 2: PR Schedule. Before the beginning of each test session, the catheter was flushed for approximately 20 s with the concentration of drug available for self-administration. We have calculated that this infusion duration is sufficient to fill the catheter with the drug solution available for that session without administering a significant amount of drug to the animal. All experimental sessions were conducted 6 to 7 days/week. Because session length was determined by individual session performance (see below), monkeys were fed at approximately 10:00 AM each day and sessions began at 2:00 PM, allowing for a maximum of a 20-h session. The next morning, each monkey's catheter was flushed with heparinized saline (100 U/ml) to help prevent clotting and the animals were fed.

Experiment 3: Cocaine-PTT Choice. Before the beginning of the session, each lumen of the double-lumen catheter was flushed for approximately 20 s with the concentration of drug available for self-administration. As noted above, we have calculated that this infusion duration is sufficient to fill the catheter with the drug solution available for that session without administering a significant amount of drug to the animal. Sessions typically began at 9:00 AM and were conducted 5 to 7 days/week. Sessions lasted 7 h or until 30 trials were completed. The catheter lumens were flushed with heparinized saline (100 U/ml) after the session to help prevent clotting and the monkeys were fed at least 30 min later.

Data Analysis

Experiment 1: Ex Vivo Binding. The S/C was calculated. Data were normalized to S/C  1 so that complete inhibition of binding approached zero. Transporter occupancy was calculated using the equation % occupancy = (A  x/A  B) × 100 (Gatley et al., 1999). In this equation, A and x are S/C measured after injection of radioligand alone and drug plus radioligand, respectively. B is the S/C measured after a high dose of cold 1-(2-bis-(4-fluorophenyl)-methoxy)-ethyl)-4-(3-phenyl-propyl) piperazine (GBR-12900), a selective DAT ligand, which is assumed to reflect 100% occupancy of the transporter. The difference between B and 1.0 presumably reflects differences in nonspecific binding. ED₅₀ values (the dose of competing drug displacing half the specific binding) were calculated using iterative curve fitting (Prism 3.0; GraphPad Software, San Diego, CA). Time course data for inhibition of binding by PTT were converted to percentage of control with saline pretreatment data using the same time points as control. Data for PTT were compared with saline control groups using a two-way analysis of variance followed by adjusted Bonferroni t tests; p < 0.05 was considered statistically significant.

Experiment 2: PR Schedule. The primary dependent variables were number of drug injections, breaking points, and drug intake in milligrams per kilogram. A separate mixed model was fit for each dependent variable using monkey as a random effect to account for variations in responding between animals (SAS 8.0; SAS, Inc., Cary, NC). Post hoc multiple comparisons were performed using a Tukey-Kramer adjustment. Mean interinfusion intervals were compared for the doses of PTT and cocaine that maintained peak BPs by t test assuming unequal variances. Data are the mean values from the last three sessions that each drug was available for self-administration. For all analyses, p < 0.05 was considered statistically significant.

Experiment 3: Cocaine-PTT Choice. The primary dependent variables were percentage of cocaine choice, total number of trials completed, and cocaine intake. The data analyzed were from the last three sessions for each comparison. Because there were doses of cocaine that were not tested in every monkey, the data were subdivided into two nonmutually exclusive categories for analysis. The first of these subsets compared PTT at the 0.01, 0.03, and saline levels to cocaine at 0.1 and 0.3. The second subset compared PTT at 0.01 and 0.03 with cocaine at 0.03, 0.1, and 0.3. Mixed models were used to compare the two drugs at their various levels for each dependent variable, using monkey as a random effect to better model the variation between monkeys (SAS 8.0). Post hoc multiple comparisons were performed using a Tukey-Kramer adjustment to get a pairwise comparison of the drug-dose interaction. For all analyses, p < 0.05 was considered statistically significant.

Drugs. ()-Cocaine HCl, provided by the National Institute on Drug Abuse (Bethesda, MD), was dissolved in sterile saline. (±)-PTT fumarate was synthesized according to the procedure described by Davies et al. (1993) and dissolved in sterile saline. Drug concentrations were calculated according to the salt form.

	Results

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Experiment 1: Ex Vivo Binding. Data regarding the maximum DAT occupancy and rate of onset for cocaine under these conditions have been reported elsewhere (Woolverton et al., manuscript submitted for publication). The maximum decrease in [³H]CFT binding by PTT was seen when PTT was given 10 min before [³H]CFT (data not shown). When various doses of PTT were administered at this pretreatment time, PTT inhibited the binding of [³H]CFT in a dose-related manner with an ED₅₀ value for PTT of 1.82 µmol/kg (compared with 8.82 µmol/kg for cocaine). When the ED₅₀ dose of PTT was given at various times before sacrifice, DAT occupancy was significant beginning at the 30-min time point (compared with 2 min postinjection for cocaine; Fig. 1).

View larger version (11K): [in this window] [in a new window]   Fig. 1.   Time course of DAT occupancy by PTT in rats. PTT (1.82 µmol/kg) was given 45 min after [3H]CFT, and rats were sacrificed at the indicated time points. Each point represents the S/C of radioactivity after [3H]CFT + PTT, calculated as a percentage of the same ratio when [3H]CFT was followed by saline injections with the identical sacrifice times. Each point represents the mean data from three to five rats and vertical lines are the S.E.M. Asterisks (*) indicate a statistically significant difference above saline controls: *, p < 0.05; ***, p < 0.001.

Experiment 2: PR Schedule. The training dose of cocaine maintained self-administration in monkeys exposed to daily PR cocaine self-administration sessions, with at least 10 injections received per session. Initial substitution of saline for either 0.03 mg/kg/injection cocaine (R-1248, R-1286, and R-1322) or 0.1 mg/kg/injection cocaine (R-1272) resulted in low levels of responding in all monkeys (range of 0-6 saline injections; Fig. 2). On average, 6.5 sessions (range 5-11) of saline availability were required for the number of saline injections to stabilize below 20% of the mean number of injections maintained by cocaine.

View larger version (26K): [in this window] [in a new window]   Fig. 2.   Final ratio requirement completed (BP; left ordinate) and number of injections received (right ordinate) when cocaine (0.003-0.56 mg/kg/injection) (A) or PTT (0.003-0.03 mg/kg/injection) (B) was substituted for the training dose of cocaine (0.03 mg/kg/injection for R-1248, R-1322, and R-1286; 0.1 mg/kg/injection for R-1272). sal denotes BP and number of injections self-administered when saline was substituted for cocaine. Each point represents the mean (±S.D.) of the last three sessions under each condition; different symbols represent individual animal data. Asterisks (***) indicate a statistically significant difference (p < 0.001) in the group mean number of drug injections received compared with saline. The 0.01 and 0.56 mg/kg/injection doses of cocaine were not included in the analysis due to incomplete data for R-1272. Dagger symbols () indicate a statistically significant difference (p < 0.05) between the group mean maximum number of injections maintained by cocaine and PTT.

Experiment 3: Cocaine-PTT Choice. The maximum number of trials available per session was 30. For monkey R-1242, when the lowest cocaine dose (0.03 mg/kg/injection) was tested as an alternative to saline, less than three trials were completed during the experimental session. At all other cocaine doses tested as alternatives to saline, between 25 and 30 trials were completed per session in each monkey (Fig. 3A). When choice was between saline and PTT (0.01 or 0.03 mg/kg/injection), the maximum number of trials completed was not greater than 15. For sessions in which at least five trials were completed, the frequency of choice for PTT and saline was approximately equal (Fig. 3B).

View larger version (19K): [in this window] [in a new window]   Fig. 3.   Total number of trials completed when cocaine (A; closed bars; 0.01-0.3 mg/kg/injection) or PTT (B; closed bars; 0.01, 0.03 mg/kg/injection) when presented as an alternative to saline (open bars). Data are the mean of the last 3 days that a dose combination was available and represent individual animals. N.D., not determined.

View larger version (32K): [in this window] [in a new window]   Fig. 4.   Percentage of trials in which cocaine (0.01-0.3 mg/kg/injection) was chosen for self-administration when presented as an alternative to saline (squares), 0.01 mg/kg/injection PTT (triangles), or 0.03 mg/kg/injection PTT (circles) under a discrete-trials choice procedure. Panels labeled R-1242, R-1272, and R-1253 represent individual animal data and are the mean (±S.D.) of the last three sessions a drug combination was available. Data in the group panel are the estimated mean (±S.E.M.) of the last 3 days a drug combination was available for all monkeys according to the mixed model used for analysis. The dashed line denotes no preference (i.e., 50% choice). Asterisks (*) indicate that the group mean percentage of cocaine choice was significantly different from 50% (95% confidence interval of the least square means did not contain 0.5). Dagger symbols () indicate a statistically significant difference from when saline was an alternative: , p < 0.001. Double-dagger symbols () indicate a statistically significant difference in cocaine choice when cocaine was an alternative to 0.01 mg/kg/injection PTT compared with 0.03 mg/kg/injection PTT: , p < 0.001.

View larger version (31K): [in this window] [in a new window]   Fig. 5.   Total number of trials completed during an experimental session in which various doses of cocaine (0.01-0.03 mg/kg/injection) was made available for self-administration as an alternative to saline, 0.01 mg/kg/injection PTT, or 0.03 mg/kg/injection PTT. Panels labeled R-1242, R-1272, and R-1253 represent individual animal data and are the mean (±S.D.) of the last three sessions a drug combination was available. Data in the group panel are the estimated mean (±S.E.M.) of the last 3 days; a drug combination was available for all monkeys according to the mixed model used for analysis. Asterisks (*) indicate a statistically significant difference from when saline was an alternative: **, p < 0.01; ***, p < 0.001. Dagger symbols () indicate a statistically significant difference from when cocaine was an alternative to 0.01 mg/kg/injection PTT compared with 0.03 mg/kg/injection PTT. , p < 0.001.

View larger version (30K): [in this window] [in a new window]   Fig. 6.   Cocaine intake (milligrams per kilogram per session) as a function of cocaine dose when the alternative was saline, 0.01 mg/kg/injection PTT, or 0.03 mg/kg/injection PTT. All other details are as in Fig. 5.

	Discussion

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The present study was undertaken to further investigate the variables that influence the reinforcing efficacy of psychostimulants. In experiment 1, the onset of action of the high-affinity DAT blocker PTT was measured in rodents using an ex vivo binding assay and was found to differ considerably from cocaine. In experiments 2 and 3, the reinforcing strength of PTT and cocaine was compared with the two most frequently used procedures for determining relative reinforcing efficacy, PR schedules and choice studies (Katz, 1990). Monkeys responding under an exponentially increasing, within-session PR schedule received significantly more injections and had higher breaking points maintained by cocaine compared with PTT. When concurrently available under a mutually exclusive, drug-drug choice paradigm, PTT was not preferred over saline and cocaine was chosen over the dose of PTT that maintained peak BPs under the PR schedule. The findings from these two measures of reinforcing efficacy are in agreement, and suggest that PTT is less efficacious at maintaining responding relative to cocaine. When a higher PTT dose, representing the descending limb of the dose-response curve under the PR schedule, was available as a choice to cocaine, cocaine preference decreased to approximately 50%, suggesting that self-administered PTT attenuated the reinforcing effects of cocaine.

Pharmacokinetic variables have previously been shown to influence the rate of drug self-administration and the strength of a drug to maintain responding. In particular, the duration of action and the onset of action can significantly impact the reinforcing effects of a drug. For example, it appears that as a drug's duration of action increases, the interval separating injections increases as well (Winger et al., 1975; Panlilio and Schindler, 2000). Consistent with this relationship, PTT, which has a longer half-life than cocaine, maintains lower rates of responding in rats (Roberts et al., 1999) and monkeys (Nader et al., 1997; Birmingham et al., 1998; Lile et al., 2000; present study, experiment 2) across a range of conditions.

However, there are data suggesting that duration of action does not affect a drug's efficacy as a reinforcer under PR (Panlilio and Schindler, 2000; Ko et al., 2002) and choice (Johanson and Schuster, 1975) conditions. In agreement with these studies, Roberts et al. (1999) reported that PTT maintained higher BPs compared with cocaine when studied under PR schedules in rodents, indicating that the longer half-life of PTT did not influence its reinforcing efficacy. In contrast, we found that the final ratio requirements completed for PTT were significantly less than for cocaine in nonhuman primates, and PTT was not preferred over saline under choice conditions. There are several possible reasons for these discrepant findings. One possibility is that duration of action impacted the reinforcing effects of PTT in monkeys to a larger degree than in rodents. This is unlikely because in a subsequent study using the identical PR schedule, a series of cocaine and methylphenidate analogs differing in duration of action were compared and differences in reinforcing efficacy were not related to duration of action (Lile et al., 2002).

A second possibility for the differences in results may be due to the direct effects of PTT on response rates. It was apparent that under the PR conditions PTT had substantial rate-decreasing effects in the monkey that may have artificially reduced its BP. Consistent with this possibility, an experiment designed to attenuate some of the rate-decreasing effects of a high dose of PTT, resulted in higher BP values (experiment 2). Stafford et al. (1998), in their review on variables that impact measures of reinforcing efficacy, pointed out "satiety", disruption in operant responding, and unconditioned aversive effects as possible reasons for decreases in performance at higher drug doses. These direct effects of PTT on ongoing behavior, in particular satiety, may have also impacted the results from the drug-drug choice studies, in which the total trials completed when the high dose of PTT was studied were decreased by at least 50%. Taken together, these results further support the supposition that although not directly dependent on rate of responding, PR and choice performance are measures of reinforcing efficacy that are, to some degree, contaminated by the direct effects of drugs (described as "multiply determined" by Katz, 1990).

The onset of action, or the interval between a response and the delivery of a reinforcing stimulus, has also been shown to influence the rate at which that stimulus maintains responding. For example, as this interval is lengthened, rates of drug self-administration tend to decrease (Beardsley and Balster, 1993). Similarly, lower rates of responding resulted when the infusion duration for contingent cocaine injections was increased (Balster and Schuster, 1973; Panlilio et al., 1998), which could be considered analogous to delaying the onset of action of cocaine. In experiment 1, the onset of DAT occupancy for PTT was determined to be much longer than cocaine, which may also have contributed to its low rates of self-administration. Data from a recent study by Winger et al. (2002) in rhesus monkeys responding under a PR schedule suggested that delays in onset of action can affect the strength of drug reinforcers as well. Consistent with these findings, PTT was less efficacious as a reinforcer in the present study under both choice and PR conditions. Interestingly, in the Roberts et al. (1999) study, PTT maintained higher BPs than cocaine, despite its prolonged rate of onset to bind DAT. Although the reasons for this are not clear, it is possible that rodents are less sensitive to the delay to reinforcement as a determinant of the reinforcing efficacy of a drug stimulus compared with monkeys.

In addition to the kinetic variables described above, the pharmacodynamic properties of a drug also contribute to its reinforcing effects. Cocaine binds with approximately equal affinity to DAT, 5-HTT, and the norepinephrine transporters, preventing the reuptake of these three monoamines. As noted in the Introduction, blockade of DA reuptake is thought to be primarily responsible for mediating the behavioral effects of psychostimulants (Wise, 1998). It has also been proposed that increased serotonin receptor activation acts as a negative modulator of the reinforcing effects of psychostimulants (but see Walsh and Cunningham, 1997). Although there is less information available on the interactions of psychostimulants with the norepinephrine system, it appears that blockade of norepinephrine transporters is not a major determinant of cocaine's reinforcing effects (Woolverton, 1987; Mello et al., 1990). In the study by Roberts et al. (1999), a positive relationship was demonstrated between the reinforcing efficacy of a series of cocaine analogs and the ratio of their affinity for 5-HTT/DAT; however, no relationship was observed in the present study. Monoamine transporter binding values for the phenyltropanes were calculated using rodent tissue, so DAT and 5-HTT affinity of these drugs may be different in monkeys. However, binding data comparing the selectivity and affinity of the monoamine reuptake inhibitor 1-(2-bis-(4-fluorophenyl)-methoxy)-ethyl)-4-(3-phenyl-propyl) piperazine (GBR-12900) in rat and monkey striatum demonstrated similar affinity ratios between the species (Dutta et al., 2001). It is also possible that differences in the relative activation of DA versus serotonin systems in the rodent versus the primate brain may have affected the reinforcing effects of PTT (Loh and Roberts, 1990; Richardson and Roberts, 1991). Further research to investigate the role of monoamine transporter selectivity in mediating the reinforcing efficacy of psychostimulants, with emphasis on comparing across species, is clearly warranted.

Noncontingent PTT has previously been shown to decrease cocaine self-administration when administered as a pretreatment (Nader et al., 1997). The present findings extend these results by showing that self-administered PTT significantly decreased cocaine intake under choice conditions. It has been proposed that compounds having some cocaine-like behavioral and neurochemical properties, but with a different pharmacokinetic profile would be desirable as potential medications for cocaine abuse (Howell and Wilcox, 2001). To that extent, the experiments presented here were undertaken in an effort to understand the properties of a drug that contribute to its abuse liability and to further investigate the potential for indirect DA agonists as medications using nonhuman primate models of drug use. Studies with the phenyltropane PTT have demonstrated that a high-affinity DAT inhibitor with a protracted rate of onset and offset can maintain less self-administration than cocaine across a range of conditions, as well as decrease cocaine intake. These findings support the continued investigation of monoamine reuptake inhibitors in an effort to develop an efficacious pharmacotherapy for the treatment of cocaine abuse and dependence.