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Comparison of the discriminative stimulus effects of 3,4-methylenedioxymethamphetamine (MDMA) and cocaine: asymmetric generalization
Drug and Alcohol Dependence 74 (2004) 281–287
Comparison of the discriminative stimulus effects of 3,4-methylenedioxymethamphetamine (MDMA) and cocaine: asymmetric generalization Nantaka Khorana, Manik R. Pullagurla, Richard Young, Richard A. Glennon∗ Department of Medicinal Chemistry, School of Pharmacy, Virginia Commonwealth University, P.O. Box 980540, Richmond, VA 23298-0540, USA Received 19 August 2003; received in revised form 9 December 2003; accepted 13 January 2004
Abstract Evidence suggests that (±)3,4-methylenedioxymethamphetamine (MDMA) and psychostimulants produce similar but non-identical stimulus effects in animals. To examine this hypothesis, groups of rats were trained to discriminate either MDMA (1.5 mg/kg) or cocaine (8 mg/kg) from saline vehicle using a two-lever operant procedure under a variable interval (VI) 15 s schedule of reinforcement. Once the animals were trained, tests of stimulus generalization were conducted with (±)MDMA, cocaine, S(+)MDMA, and R(−)MDMA. As previously demonstrated, both S(+)MDMA and R(−)MDMA (ED50 = 0.8 and 1.2 mg/kg, respectively) substituted for (±)MDMA. Stimulus generalization also occurred upon administration of cocaine (ED50 = 4.6 mg/kg) to the (±)MDMA-trained animals. In the cocaine-trained animals, however, stimulus generalization did not occur to (±)MDMA, S(+)MDMA nor R(−)MDMA. Receptor binding profiles for MDMA and cocaine were compared in an effort to identify any novel and common receptor-based mechanism(s) to explain stimulus generalization of MDMA-trained animals to the effects of cocaine, but only their actions on neurotransmitter transporters seem applicable. Taken together, the results indicate that stimulus substitution between MDMA and cocaine is asymmetric and suggest that although similarities exist between the stimulus actions of MDMA and cocaine, differences might be explained by their differential effects on increasing synaptic concentrations of serotonin (5-HT), dopamine (DA), and/or norepinephrine (NE). © 2004 Elsevier Ireland Ltd. All rights reserved. Keywords: MDMA; Ecstasy; Cocaine; Psychostimulants; (−)MDMA; (+)MDMA; Drug discrimination
1. Introduction Racemic 3,4-methylenedioxymethamphetamine ((±) MDMA) (“Ecstasy”) is currently a popular drug of abuse. The agent, a ring-substituted phenylisopropylamine related in structure to methamphetamine, is thought to produce its behavioral effects in animals primarily via release of serotonin (5-HT) and dopamine (DA) (Johnson et al., 1986; McKenna et al., 1991; Rudnick and Wall, 1992). Moreover, the specific involvement of dopamine D2 receptors and serotonin 5-HT2 receptors has been suggested as mediating some of their actions (Glennon et al., 1992; Lyon et al., 1986; Schechter, 1989). Studies with human volunteers have demonstrated that the overall psychological effects of MDMA are largely dependent on carrier-mediated release ∗ Corresponding author. Tel.: +1-804-828-8487; fax: +1-804-828-7404. E-mail address: [email protected] (R.A. Glennon).
of 5-HT whereas the stimulant-like euphoric actions are related, at least in part, to stimulation of dopamine receptors (Liechti and Vollenweider, 2001; Vollenweider et al., 1998). The mild perceptual effects induced by MDMA might involve stimulation of 5-HT2 serotonin receptors (Liechti and Vollenweider, 2001; Vollenweider et al., 1998). MDMA users also display noradrenergic hyperreactivity in the interval following drug use (Stuerenberg et al., 2002). This latter finding is consistent with animal studies showing that norepinephrine (NE) levels in rat nucleus accumbens were decreased, and those of dopamine were increased even at 4 weeks after exposure to drug (Mayerhofer et al., 2001). Polydrug use is common among MDMA users, with MDMA often being taken in combination with stimulants such as amphetamine (the combination being known as “speedies”), methamphetamine (known as “hugs and kisses” or “super X”), or cocaine (known as “bumping up” with powdered cocaine, and “cloud nine” with smoked free-base cocaine) (Riley et al., 2001; Williams et al.,
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1998; Winstock et al., 2001). MDMA and cocaine share interesting and related biochemical effects even though they might act by non-identical mechanisms. That is, administered acutely, both agents increase synaptic levels of serotonin, dopamine, and norepinephrine, but do so in a somewhat different manner. For example, MDMA is nearly equipotent at 5-HT and NE release (EC50 ≈ 65 and 90 nM, respectively) but is several-fold less potent at releasing DA (EC50 ≈ 375 nM) (Rothman et al., 2001; Setola et al., 2003). Cocaine does not release any of the three neurotransmitters (EC50 > 10,000 nM) but blocks their reuptake with similar potency (EC50 : 5-HT = 304, DA = 478 and NE = 779 nM) (Rothman et al., 2001). MDMA also acts as an inhibitor of reuptake but is >5-fold less potent for DA reuptake than for 5-HT or NE reuptake (EC50 = 5-HT = 238, DA = 1572 and NE = 466 nM) (Rothman et al., 2001). Thus, the two agents might be differentiated primarily with respect to the manner and extent to which they increase synaptic neurotransmitter levels. Nevertheless, there is a commonality of action in that all three neurotransmitters seem to be affected, although perhaps to differing degrees. Drug discrimination studies have been employed to compare the stimulus properties of MDMA with those of stimulants. However, results have not always been consistent across studies. For example, (+)amphetamine substitutes for MDMA (Oberlender and Nichols, 1988), and MDMA substitutes for (+)amphetamine (Evans and Johanson, 1986; Glennon and Young, 1984; Kamien et al., 1986). The S(+)isomer, but not the R(−)isomer of MDMA also substitutes for (+)amphetamine (Glennon et al., 1988). But, Nichols (1986), Oberlender and Nichols (1988) and Schechter (1989) have found that MDMA does not fully substitute for amphetamine; these inconsistencies might reflect procedural differences (e.g. different schedules of reinforcement, different training doses). Three-lever drug discrimination procedures provide additional information on the actions of MDMA and stimulants. Animals can be trained to discriminate racemic MDMA from (+)amphetamine from saline, demonstrating that their stimulus effects are not identical (Goodwin and Baker, 2000). In animals trained to discriminate (+)amphetamine from a hallucinogen (i.e. mescaline or LSD) from saline, MDMA isomers failed to substitute for either the stimulant or the hallucinogen stimulus (Baker and Taylor, 1997). Such studies must be interpreted with caution, however, in light of the report by Appel et al. (1999) that the specific drug-pairings in a three-lever choice procedure can influence lever selection, and that results can be different from those obtained in a corresponding two-lever procedure. Overall, then, there is evidence for stimulus similarity between MDMA and amphetamine, but there is also considerable evidence that their stimulus effects are not identical. Furthermore, although capable of exhibiting various physiological and behavioral effects in humans, MDMA produces subjective effects similar to those seen with amphetamine (e.g. Vollenweider et al., 1998; Tancer and Johanson, 2001).
Surprisingly little has been published with respect to the stimulus actions of MDMA relative to cocaine. Perhaps it has been assumed that the results of drug discrimination studies with MDMA and cocaine would be similar to those with MDMA and (+)amphetamine because cocaine and (+)amphetamine have been shown to substitute for one another regardless of which is used as training drug (reviewed: Woolverton, 1991). With regard to MDMA and cocaine, Emmet-Oglesby et al. (1990) examined the two optical isomers of MDMA in rats trained to discriminate 10 mg/kg of cocaine from vehicle; they found that 3.5 mg/kg of R(−)MDMA substitutes for cocaine, but that S(+)MDMA does not. The work was reported only in abstract form and no additional information is available. Broadbent et al. (1989) examined the effect of MDMA optical isomers using rats trained to a low (3.5 mg/kg), more common (10 mg/kg), or high (20 mg/kg) dose of cocaine. R(−)MDMA did not dose-dependently or consistently mimic the effects of cocaine, except that in the high-dose animals 2.75 mg/kg of R(−)MDMA produced 92% drug-appropriate responding (with three of five animals responding). S(+)MDMA elicited between 57 and 67% cocaine-appropriate responding depending upon the training group. However, when examined in rats trained to discriminate R(−)MDMA (1.25 mg/kg) or S(+)MDMA (3.5 mg/kg) from vehicle, cocaine produced a maximum of only 60 and 40% drug-appropriate responding, respectively (Baker et al., 1995). The only study to examine cocaine in racemic MDMA-trained animals was that by Schechter (1998), who found that cocaine produced <40% drug-appropriate responding in rats trained to discriminate (±)MDMA from saline. However, the animals used in the study were serotonergically-dysfunctional Fawn-Hooded rats. Only limited conclusions can be drawn from the above findings, but they suggest there might be some similarity between the MDMA and cocaine stimuli; although substitution was asymmetric, it seems that R(−)MDMA can substitute for cocaine. The only study to examine cocaine in MDMA-trained animals used Fawn-Hooded rats, and (±)MDMA has never been examined in cocaine-trained animals using a two-lever drug discrimination procedure. MDMA and cocaine have not been previously examined in MDMA- and cocaine-trained animals in the same investigation under similar conditions. Because there is no evidence that the isomers of MDMA are commonly used on the street, and due to mechanistic similarities between MDMA and cocaine, we trained a group of animals to discriminate (±)MDMA from saline vehicle to determine if cocaine would substitute for the (±)MDMA stimulus. Conversely, we trained a group of animals to discriminate cocaine from saline vehicle to determine if (±)MDMA would substitute for the cocaine stimulus. Doses of the individual optical isomers of MDMA were also examined in each group of animals to assist with comparisons. The training doses employed for MDMA (1.5 mg/kg) and cocaine (8 mg/kg) are fairly common doses,
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and are identical to those we have used in previous studies (Glennon et al., 1992; Young and Glennon, 1997).
2. Materials and methods 2.1. Drug discrimination studies Fifteen male Sprague–Dawley rats (Charles River Laboratories), weighing 250–300 g at the beginning of the study, were trained to discriminate (15 min presession injection interval) either 1.5 mg/kg of MDMA (n = 6) or 8 mg/kg of cocaine (n = 9) from saline vehicle (sterile 0.9% saline) under a variable interval 15 s schedule of reward (i.e. sweetened condensed milk) using standard two-lever Coulbourn Instruments operant equipment as previously described (Dukat et al., 2002; Young and Glennon, 1997). Animal studies were conducted under an approved Institutional Animal Care and Use Committee protocol. In brief, animals were food-restricted to maintain body weights of approximately 80% that of their free-feeding weight, but were allowed access to water ad lib in their individual home cages. Daily training sessions were conducted with the training dose of the training drugs or saline. For approximately half the animals, the right lever was designated as the drug-appropriate lever, whereas the situation was reversed for the remainder of the animals. Learning was assessed every fifth day during an initial 2.5 min non-reinforced (extinction) session followed by a 12.5 min training session. Data collected during the extinction session included response rate (i.e. responses per minute) and number of responses on the drug-appropriate lever (expressed as a percent of total responses). Animals were not used in the subsequent stimulus generalization studies until they consistently made ≥80% of their responses on the drug-appropriate lever after administration of training drug and less than 20% of their responses on the same drug-appropriate lever after administration of saline. During the stimulus generalization (i.e. substitution) phase of the study, maintenance of the training-drug/saline discrimination was insured by continuation of the training sessions on a daily basis (except on a generalization test day). On 1 of the 2 days before a generalization test, approximately half the animals would receive the training dose of training drug and the remainder would receive saline; after a 2.5 min extinction session, training was continued for 12.5 min. Animals not meeting the original training criteria during the extinction session were excluded from the subsequent generalization test session. During the investigations of stimulus generalization, test sessions were interposed among the training sessions. The animals were allowed 2.5 min to respond under non-reinforcement conditions. An odd number of training sessions (usually five) separated any two generalization test sessions. Doses of test drugs were administered in a random order, using a 15 min presession injection interval, to the groups of rats. Stimulus generalization was
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considered to have occurred when the animals, after a given dose of drug, made ≥80% of their responses (group mean) on the training drug-appropriate lever. Animals making fewer than five total responses during the 2.5 min extinction session were considered as being disrupted. Percent drug-appropriate responding and response rate data refer only to animals making ≥5 responses during the extinction session. If >50% of the animals were disrupted following administration of a given drug dose, data were not plotted. Where stimulus generalization occurred, an ED50 dose was calculated by the method of Finney (1952). The ED50 dose represents the drug dose where animals would be expected to make 50% of their responses on the drug-appropriate lever. Drugs (doses) examined in the MDMA-trained animals include: cocaine (2.0, 4.0, 6.0, 8.0, 10 and 11 mg/kg), S(+)MDMA (0.5, 0.75, 1.0 and 1.25 mg/kg) and R(−) MDMA (0.75, 1.5 and 3.0 mg/kg). The following drugs were administered to cocaine-trained animals (doses): cocaine (1.0, 2.0, 4.0 and 8 mg/kg), MDMA (0.75, 1.5, 1.6, 1.75, and 2.5 mg/kg), S(+)MDMA (0.25, 0.75, 1.25, 1.5, 1.75 and 2.25 mg/kg), and R(−)MDMA (1.0, 1.5, 2.0, 2.25 and 2.5 mg/kg). 2.2. Binding profile (±)MDMA and cocaine were examined in a number of different radioligand binding assays by the NIMH Psychoactive Drug Screening Program. The two agents were initially Table 1 Receptor binding profile for (±)MDMA and cocainea Receptor population ␣1A -Adrenergic ␣1B Adrenergic ␣2A -Adrenergic ␣2B -Adrenergic ␣2C -Adrenergic 1 -Adrenergic 2 -Adrenergic Serotonin r5-HT1B m3 Cholinergic m4 Cholinergic m5 Cholinergic H1 Histaminergic I1 Imidazoline
(±)MDMA, Ki (±S.E.M.) >10,000 >10,000 2,530 (180) 1,790 (100) 1,120 (30) 1,350 (30) >10,000 >10,000 1,850 (190) 8,250 (800) 6,350 (1,050) 7,780 (700) 220 (40)
Cocaine, Ki (±S.E.M.) >10,000 >10,000 20,900 7,490 14,700 3,000 >10,000 3,400 >10,000 >10,000 >10,000 2,250 2,800
(3,400) (820) (1,400) (160) (700)
(550) (440)
Receptor populations represent human receptors except where designated r (rat). Additionally, (±)MDMA and cocaine lacked appreciable affinity (Ki > 10,000 nM) for 5-HT1A , 5-HT1D , 5-HT1E , r5-HT2C , 5-HT3 , 5-HT5A , 5-HT6 and 5-HT7 serotonin receptors, - and -opioid receptors, D1 -, rD2 -, rD3 -, rD4 -, D5 -dopamine receptors, rGABAA receptors, rBZ receptors, rPCP receptors, m1 - and m2 -acetylcholine receptors, H2 histamine receptors, and nicotinic ␣22, ␣24, ␣32, ␣34, ␣42, ␣44 acetylcholine receptors. Cocaine was also re-examined for binding at three neurotransmitter transporters and displayed affinity for bovine DAT (Ki = 54 ± 12 nM), human SERT (Ki = 120 ± 20 nM) and human NET (Ki = 1000 ± 300 nM). a Binding data were obtained in quadruplicate from the NIMH Psychoactive Drug Screening Program (http://kidb.bioc.cwru.edu).
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screened in quadruplicate at a concentration of 10,000 nM; where an agent produced >50% inhibition, a Ki value was determined in quadruplicate. Further detail can be found in Table 1. 2.3. Drugs Racemic 3,4-methylenedioxymethamphetamine HCl (MDMA), R(-)MDMA HCl, and S(+)MDMA HCl were obtained as gifts from NIDA. Cocaine HCl was purchased from Sigma–Aldrich (St. Louis, MO). All doses of all drugs were administered via intraperitoneal injection; doses refer to the weight of the salts. Solutions in sterile 0.9% saline were freshly prepared daily.
3. Results 3.1. MDMA discrimination One group of animals (n = 6) was trained to discriminate 1.5 mg/kg of racemic MDMA from saline vehicle as previously reported such that the animals made 97% of their responses on the MDMA-appropriate lever following administration of the training dose of MDMA, and 5% of their responses on the same lever following administration of saline. The animals’ response rate (21.2 response per min) was not substantially different from that following administration of saline vehicle (19.9 response per min). Administration of the optical isomers of MDMA to the MDMA-trained animals resulted in substitution in a dose-dependent fashion (Fig. 1). The animals’ response rates were somewhat reduced at doses that produced >80% MDMA-appropriate responding. ED50 doses calculated for S(+)- and R(−)MDMA were 0.8 (95% CL = 0.6–1.1) mg/kg and 1.2 (95% CL = 0.6–2.2) mg/kg, respectively. Administration of cocaine to the MDMA-trained animals resulted in substitution in a dose-dependent fashion [ED50 = 4.6 (95% CL = 2.8–7.4) mg/kg]. A total of six doses of cocaine were examined; at 4 mg/kg, the animals’ response rate was higher than that following saline but rates steadily declined at higher doses (Fig. 1). At a dose of 11 mg/kg, only four of five animals responded (making 88% of their responses on the MDMA-appropriate lever) and the animals’ response rate was decreased to about 25% of that following saline vehicle. 3.2. Cocaine discrimination A second group of animals (n = 9) was trained to discriminate 8 mg/kg of cocaine from saline vehicle such that they made 96% of their responses on the drug-appropriate lever following administration of the training dose of cocaine, and 8% of their responses on the same lever following administration of saline. The animals’ response rate (23.2 response per min) was similar to that following
Fig. 1. Results (±S.E.M.) of stimulus generalization studies (upper panel), and animals’ response rates (lower panel), in rats trained to discriminate (±)MDMA from saline vehicle. Tests of stimulus generalization were conducted with S(+)MDMA, R(−)MDMA, and cocaine; results with the training dose of MDMA are represented as M and that for saline as S. Drug doses are plotted using a logarithmic scale.
administration of saline (20.9 response per min). Tests of stimulus generalization were conducted with (±)MDMA, S(+)MDMA, and R(−)MDMA using four to nine animals at each drug dose. Shortly after the study began, three of the cocaine-trained animals were lost; consequently, most of the substitution studies used four to six animals. Racemic MDMA produced a maximum of 36% cocaine-appropriate responding at 1.5 mg/kg (Fig. 2); at 1.6 mg/kg only two of four animals responded. At doses of 1.75 and 2.5 mg/kg, the animals’ lever-pressing behavior was disrupted and only one of six and none of five animals made ≥5 responses during the entire 2.5 min extinction session. Following administration of S(+)MDMA doses, the animals never made >24% (at 1.5 mg/kg) of their responses on the cocaine-appropriate lever (Fig. 2). Drug-appropriate responding declined to 11% at 1.75 mg/kg with four of six animals responding, and at 2.25 mg/kg, only one of five animals made ≥5 responses during the 2.5 min extinction session (i.e. 54% cocaine-appropriate responding; response
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4. Discussion
Fig. 2. Results (±S.E.M.) of stimulus generalization studies (upper panel), and animals’ response rates (lower panel), in rats trained to discriminate cocaine from saline vehicle. Tests of stimulus generalization were conducted with cocaine, (±)MDMA, S(+)MDMA, and R(−)MDMA; results for saline vehicle are represented by S. Drug doses are plotted using a logarithmic scale.
rate = 4.4 response per min), Five doses of R(−)MDMA were examined; 2.0 mg/kg of R(−)MDMA elicited 14% cocaine-appropriate responding with three of five animals responding. Administration of higher doses of R(−)MDMA resulted in disruption of behavior with no animals responding at 2.25 mg/kg and only two of six animals responding at 2.5 mg/kg (i.e. 0 and 100% drug-appropriate responding; response rate = 2.8 and 3.2 response per min, respectively). 3.3. Radioligand binding (±)MDMA and cocaine were examined in about 40 different radioligand binding assays to determine if they shared any binding similarities (Table 1). Both agents lacked appreciable affinity for most receptor populations examined. Whereas one of the two agents might have displayed low micromolar affinity for a particular receptor population, the other did not. In other words, there was no common receptor population where both agents displayed affinities (i.e. Ki values) of <2000 nM.
Previous drug discrimination studies have reported that animals trained to distinguish racemic MDMA from vehicle generalize to both MDMA optical isomers and that S(+)MDMA is about two to three times more potent than R(−)MDMA (Glennon et al., 1986; Oberlender and Nichols, 1988; Schechter, 1987). In the present study, using animals trained to discriminate (±)MDMA from saline vehicle, S(+)MDMA (ED50 = 0.8 mg/kg) was found to be 1.5 times more potent than R(−)MDMA (ED50 = 1.2 mg/kg). Only once before has cocaine been administered to rats trained to discriminate (±)MDMA from vehicle; in Fawn-Hooded rats, drug-appropriate responding never exceeded 40% (Schechter, 1998). In the present investigation, cocaine substituted for (±)MDMA in a fairly orderly fashion (Fig. 1). The potency of cocaine (ED50 = 4.6 mg/kg) was several-fold less than that of either MDMA isomer. The apparent inconsistency in the results obtained in the current study, as compared to that of the Schechter (1998) study, might be related to differences in methodology or, more likely, to the different strains of animals used in the two studies (see Section 1). Nonetheless, the present findings suggest that there is some similarity of stimulus effect produced by the two agents; this action might be related to the effect that the two agents have on increasing synaptic levels of 5-HT, DA, and/or NE. However, at cocaine doses of 10 and 11 mg/kg (where cocaine produced 75 and 88% MDMA-appropriate responding, respectively), the animals’ response rates were markedly reduced (Fig. 1). Thus, it is also tempting to speculate that the reduced response rates observed at the higher doses of cocaine are the result of some degree of differential action(s) of the two agents on one or more of the three neurotransmitter systems mentioned above. In addition, studies with the serotonergically-dysfunctional Fawn-Hooded rats suggest that serotonergic systems might play an important role in the stimulus actions of MDMA or in the ability of MDMA-trained animals to recognize cocaine. In an attempt to determine if (±)MDMA and cocaine produce their stimulus effects by a common but as yet unidentified receptor mechanism, the binding of both agents was examined at about 40 receptor populations. Table 1 shows that both MDMA and cocaine share some affinity for ␣2 - and 1 -adrenergic receptors; but, in general, their affinities are relatively low (Ki values are in the 2000–20,000 nM range). MDMA (Ki = 220 nM) and cocaine (Ki = 2, 800 nM) also bind at imidazoline I1 receptors (Table 1) with low affinity. Nevertheless, because both agents bind at these receptor populations, a contributory role for one or more of these populations cannot be excluded at this time. Racemic MDMA has not been previously examined in cocaine-trained animals. However, it has been reported that R(−)- but not S(+)MDMA substitutes in cocaine-trained animals (Broadbent et al., 1989). The current results show that (±)MDMA produced a maximum
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of 36% cocaine-appropriate responding (Fig. 2). At first, this result seems inconsistent with that of the earlier study. However, the possibility exists that the presence of the S(+)isomer in the racemic mixture of MDMA could account for the behaviorally disruptive actions of the racemate. Specifically, in the Broadbent et al. (1989) study, doses of S(+)MDMA ≥ 1.0 mg/kg reduced the animals’ lever-pressing behavior; similar dose levels of (±)MDMA impaired lever-pressing in the present animals. Thus, it could be argued that the S(+)MDMA component of racemic MDMA doses might have produced a disruptive effect and, consequently, precluded the possibility that the animals would have been able to respond to the reported cocaine-like effect of R(−)MDMA. But, the results presented here also show that neither R(−)- nor S(+)MDMA substituted in rats trained to discriminate a dose of 8 mg/kg of cocaine from vehicle. However, it should be noted that in the Broadbent et al. (1989) study, R(−)MDMA substituted only in animals trained to discriminate 20 mg/kg of cocaine, and produced incomplete generalization in animals trained to discriminate either 3.5 or 10 mg/kg of cocaine. This present findings with R(−)MDMA are, therefore, not inconsistent with the idea proposed by other investigators that results of a stimulus generalization test in cocaine-trained animals may be highly dependent on the training dose employed for cocaine (Broadbent et al., 1989; Kantak et al., 1999; Schechter, 1997; Spealman, 1995; Terry et al., 1994). Could the asymmetric generalization that occurred between (±)MDMA and cocaine be consistent with the findings that pre-exposure to (±)MDMA elevates locomotor activity, cocaine self-administration, and extracellular dopamine levels in the nucleus accumbens following a cocaine challenge in animals (Fletcher et al., 2001)? In the drug discrimination procedure, the (±)MDMA-trained animals obviously have been pre-exposed to MDMA as the training stimulus, and the administration of cocaine to these animals produced MDMA-like responding (Fig. 1). The cocaine-trained animals have been pre-exposed to cocaine as their training drug, but the administration of (±)MDMA did not produce cocaine-like responding. Finally, might the pre-exposure to MDMA followed by the administration of cocaine help explain the phenomenon referred to as “bumping” and “cloud nine” (see Section 1)? Users of MDMA usually ingest the agent orally, in tablet or capsule form. The psychoactive effects of MDMA are thought to be related, at least in part, to its action of releasing neuronal monoamine neurotransmitters, including dopamine, and increasing synaptic activity in areas of the brain such as the nucleus accumbens. The behavioral effects of MDMA last approximately 4–6 h. Inhalation of cocaine powder produces a euphoric effect lasting, typically, from 15 to 30 min. However, administration of cocaine following administration of MDMA might be able to intensify or “bump” a state of euphoria because it can function to block the reuptake process of dopamine which likely exists at an
increased level due to the neurochemical actions of MDMA. Similarly, smoking of cocaine in its free base or “crack” form allows a relatively high dose of cocaine to reach the brain very quickly and brings about a very immediate and intense bump that might last only 5–10 min, but may form the basis for a “cloud nine” euphoric effect. These effects might be the result of acute drug co-administration and, in the present study, the effects of acute cocaine and MDMA co-administration were not examined. However, MDMA is capable of producing long term neurotransmitter-related effects in animals lasting several weeks following a single administration of drug (Mayerhofer et al., 2001); hence, it is possible that pre-exposure to MDMA (e.g. as training drug) might influence the actions of subsequently administered cocaine doses. In summary, the results of the present study show for the first time that (±)MDMA-trained rats recognize the stimulus effects of cocaine (at the specific drug doses employed); stimulus similarity might be explained on the basis of mechanistic similarities reported for the two agents with respect to their ability to increase synaptic concentrations of serotonin, dopamine, and/or norepinephrine. Generalization is asymmetric, however, in that cocaine-trained animals failed to recognize (±)MDMA or either MDMA optical isomer. Asymmetric generalization might be related to differences in the extent to which each agent increases synaptic levels of serotonin, dopamine, and/or norepinephrine, and to which of these mechanisms predominates in the respective cuing properties of MDMA and cocaine. Studies by Schechter (1998) showing lack of substitution by cocaine in MDMA-trained Fawn-Hooded rats have particular significance with respect to this concept. Extended radioligand binding studies identified no other common receptor population where both MDMA and cocaine displayed high affinity. Pre-exposure to MDMA followed by administration of cocaine is another aspect of the asymmetric relationship that might aid our understanding of the reason individuals combine the effects of MDMA and cocaine, but this remains to be further investigated.
Acknowledgements The present study was supported in part by DA-01642. We also wish to acknowledge the NIMH Psychoactive Drug Screening Program for providing us with much of the radioligand binding data presented in Table 1. NK, a Royal Thai Fellow, was supported in part by a scholarship from the government of Thailand.
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Comparison of the discriminative stimulus effects of 3,4-methylenedioxymethamphetamine (MDMA) and cocaine: asymmetric generalization Nantaka Khorana, Manik R. Pullagurla, Richard Young, Richard A. Glennon∗ Department of Medicinal Chemistry, School of Pharmacy, Virginia Commonwealth University, P.O. Box 980540, Richmond, VA 23298-0540, USA Received 19 August 2003; received in revised form 9 December 2003; accepted 13 January 2004
Abstract Evidence suggests that (±)3,4-methylenedioxymethamphetamine (MDMA) and psychostimulants produce similar but non-identical stimulus effects in animals. To examine this hypothesis, groups of rats were trained to discriminate either MDMA (1.5 mg/kg) or cocaine (8 mg/kg) from saline vehicle using a two-lever operant procedure under a variable interval (VI) 15 s schedule of reinforcement. Once the animals were trained, tests of stimulus generalization were conducted with (±)MDMA, cocaine, S(+)MDMA, and R(−)MDMA. As previously demonstrated, both S(+)MDMA and R(−)MDMA (ED50 = 0.8 and 1.2 mg/kg, respectively) substituted for (±)MDMA. Stimulus generalization also occurred upon administration of cocaine (ED50 = 4.6 mg/kg) to the (±)MDMA-trained animals. In the cocaine-trained animals, however, stimulus generalization did not occur to (±)MDMA, S(+)MDMA nor R(−)MDMA. Receptor binding profiles for MDMA and cocaine were compared in an effort to identify any novel and common receptor-based mechanism(s) to explain stimulus generalization of MDMA-trained animals to the effects of cocaine, but only their actions on neurotransmitter transporters seem applicable. Taken together, the results indicate that stimulus substitution between MDMA and cocaine is asymmetric and suggest that although similarities exist between the stimulus actions of MDMA and cocaine, differences might be explained by their differential effects on increasing synaptic concentrations of serotonin (5-HT), dopamine (DA), and/or norepinephrine (NE). © 2004 Elsevier Ireland Ltd. All rights reserved. Keywords: MDMA; Ecstasy; Cocaine; Psychostimulants; (−)MDMA; (+)MDMA; Drug discrimination
1. Introduction Racemic 3,4-methylenedioxymethamphetamine ((±) MDMA) (“Ecstasy”) is currently a popular drug of abuse. The agent, a ring-substituted phenylisopropylamine related in structure to methamphetamine, is thought to produce its behavioral effects in animals primarily via release of serotonin (5-HT) and dopamine (DA) (Johnson et al., 1986; McKenna et al., 1991; Rudnick and Wall, 1992). Moreover, the specific involvement of dopamine D2 receptors and serotonin 5-HT2 receptors has been suggested as mediating some of their actions (Glennon et al., 1992; Lyon et al., 1986; Schechter, 1989). Studies with human volunteers have demonstrated that the overall psychological effects of MDMA are largely dependent on carrier-mediated release ∗ Corresponding author. Tel.: +1-804-828-8487; fax: +1-804-828-7404. E-mail address: [email protected] (R.A. Glennon).
of 5-HT whereas the stimulant-like euphoric actions are related, at least in part, to stimulation of dopamine receptors (Liechti and Vollenweider, 2001; Vollenweider et al., 1998). The mild perceptual effects induced by MDMA might involve stimulation of 5-HT2 serotonin receptors (Liechti and Vollenweider, 2001; Vollenweider et al., 1998). MDMA users also display noradrenergic hyperreactivity in the interval following drug use (Stuerenberg et al., 2002). This latter finding is consistent with animal studies showing that norepinephrine (NE) levels in rat nucleus accumbens were decreased, and those of dopamine were increased even at 4 weeks after exposure to drug (Mayerhofer et al., 2001). Polydrug use is common among MDMA users, with MDMA often being taken in combination with stimulants such as amphetamine (the combination being known as “speedies”), methamphetamine (known as “hugs and kisses” or “super X”), or cocaine (known as “bumping up” with powdered cocaine, and “cloud nine” with smoked free-base cocaine) (Riley et al., 2001; Williams et al.,
0376-8716/$ – see front matter © 2004 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.drugalcdep.2004.01.005
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1998; Winstock et al., 2001). MDMA and cocaine share interesting and related biochemical effects even though they might act by non-identical mechanisms. That is, administered acutely, both agents increase synaptic levels of serotonin, dopamine, and norepinephrine, but do so in a somewhat different manner. For example, MDMA is nearly equipotent at 5-HT and NE release (EC50 ≈ 65 and 90 nM, respectively) but is several-fold less potent at releasing DA (EC50 ≈ 375 nM) (Rothman et al., 2001; Setola et al., 2003). Cocaine does not release any of the three neurotransmitters (EC50 > 10,000 nM) but blocks their reuptake with similar potency (EC50 : 5-HT = 304, DA = 478 and NE = 779 nM) (Rothman et al., 2001). MDMA also acts as an inhibitor of reuptake but is >5-fold less potent for DA reuptake than for 5-HT or NE reuptake (EC50 = 5-HT = 238, DA = 1572 and NE = 466 nM) (Rothman et al., 2001). Thus, the two agents might be differentiated primarily with respect to the manner and extent to which they increase synaptic neurotransmitter levels. Nevertheless, there is a commonality of action in that all three neurotransmitters seem to be affected, although perhaps to differing degrees. Drug discrimination studies have been employed to compare the stimulus properties of MDMA with those of stimulants. However, results have not always been consistent across studies. For example, (+)amphetamine substitutes for MDMA (Oberlender and Nichols, 1988), and MDMA substitutes for (+)amphetamine (Evans and Johanson, 1986; Glennon and Young, 1984; Kamien et al., 1986). The S(+)isomer, but not the R(−)isomer of MDMA also substitutes for (+)amphetamine (Glennon et al., 1988). But, Nichols (1986), Oberlender and Nichols (1988) and Schechter (1989) have found that MDMA does not fully substitute for amphetamine; these inconsistencies might reflect procedural differences (e.g. different schedules of reinforcement, different training doses). Three-lever drug discrimination procedures provide additional information on the actions of MDMA and stimulants. Animals can be trained to discriminate racemic MDMA from (+)amphetamine from saline, demonstrating that their stimulus effects are not identical (Goodwin and Baker, 2000). In animals trained to discriminate (+)amphetamine from a hallucinogen (i.e. mescaline or LSD) from saline, MDMA isomers failed to substitute for either the stimulant or the hallucinogen stimulus (Baker and Taylor, 1997). Such studies must be interpreted with caution, however, in light of the report by Appel et al. (1999) that the specific drug-pairings in a three-lever choice procedure can influence lever selection, and that results can be different from those obtained in a corresponding two-lever procedure. Overall, then, there is evidence for stimulus similarity between MDMA and amphetamine, but there is also considerable evidence that their stimulus effects are not identical. Furthermore, although capable of exhibiting various physiological and behavioral effects in humans, MDMA produces subjective effects similar to those seen with amphetamine (e.g. Vollenweider et al., 1998; Tancer and Johanson, 2001).
Surprisingly little has been published with respect to the stimulus actions of MDMA relative to cocaine. Perhaps it has been assumed that the results of drug discrimination studies with MDMA and cocaine would be similar to those with MDMA and (+)amphetamine because cocaine and (+)amphetamine have been shown to substitute for one another regardless of which is used as training drug (reviewed: Woolverton, 1991). With regard to MDMA and cocaine, Emmet-Oglesby et al. (1990) examined the two optical isomers of MDMA in rats trained to discriminate 10 mg/kg of cocaine from vehicle; they found that 3.5 mg/kg of R(−)MDMA substitutes for cocaine, but that S(+)MDMA does not. The work was reported only in abstract form and no additional information is available. Broadbent et al. (1989) examined the effect of MDMA optical isomers using rats trained to a low (3.5 mg/kg), more common (10 mg/kg), or high (20 mg/kg) dose of cocaine. R(−)MDMA did not dose-dependently or consistently mimic the effects of cocaine, except that in the high-dose animals 2.75 mg/kg of R(−)MDMA produced 92% drug-appropriate responding (with three of five animals responding). S(+)MDMA elicited between 57 and 67% cocaine-appropriate responding depending upon the training group. However, when examined in rats trained to discriminate R(−)MDMA (1.25 mg/kg) or S(+)MDMA (3.5 mg/kg) from vehicle, cocaine produced a maximum of only 60 and 40% drug-appropriate responding, respectively (Baker et al., 1995). The only study to examine cocaine in racemic MDMA-trained animals was that by Schechter (1998), who found that cocaine produced <40% drug-appropriate responding in rats trained to discriminate (±)MDMA from saline. However, the animals used in the study were serotonergically-dysfunctional Fawn-Hooded rats. Only limited conclusions can be drawn from the above findings, but they suggest there might be some similarity between the MDMA and cocaine stimuli; although substitution was asymmetric, it seems that R(−)MDMA can substitute for cocaine. The only study to examine cocaine in MDMA-trained animals used Fawn-Hooded rats, and (±)MDMA has never been examined in cocaine-trained animals using a two-lever drug discrimination procedure. MDMA and cocaine have not been previously examined in MDMA- and cocaine-trained animals in the same investigation under similar conditions. Because there is no evidence that the isomers of MDMA are commonly used on the street, and due to mechanistic similarities between MDMA and cocaine, we trained a group of animals to discriminate (±)MDMA from saline vehicle to determine if cocaine would substitute for the (±)MDMA stimulus. Conversely, we trained a group of animals to discriminate cocaine from saline vehicle to determine if (±)MDMA would substitute for the cocaine stimulus. Doses of the individual optical isomers of MDMA were also examined in each group of animals to assist with comparisons. The training doses employed for MDMA (1.5 mg/kg) and cocaine (8 mg/kg) are fairly common doses,
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and are identical to those we have used in previous studies (Glennon et al., 1992; Young and Glennon, 1997).
2. Materials and methods 2.1. Drug discrimination studies Fifteen male Sprague–Dawley rats (Charles River Laboratories), weighing 250–300 g at the beginning of the study, were trained to discriminate (15 min presession injection interval) either 1.5 mg/kg of MDMA (n = 6) or 8 mg/kg of cocaine (n = 9) from saline vehicle (sterile 0.9% saline) under a variable interval 15 s schedule of reward (i.e. sweetened condensed milk) using standard two-lever Coulbourn Instruments operant equipment as previously described (Dukat et al., 2002; Young and Glennon, 1997). Animal studies were conducted under an approved Institutional Animal Care and Use Committee protocol. In brief, animals were food-restricted to maintain body weights of approximately 80% that of their free-feeding weight, but were allowed access to water ad lib in their individual home cages. Daily training sessions were conducted with the training dose of the training drugs or saline. For approximately half the animals, the right lever was designated as the drug-appropriate lever, whereas the situation was reversed for the remainder of the animals. Learning was assessed every fifth day during an initial 2.5 min non-reinforced (extinction) session followed by a 12.5 min training session. Data collected during the extinction session included response rate (i.e. responses per minute) and number of responses on the drug-appropriate lever (expressed as a percent of total responses). Animals were not used in the subsequent stimulus generalization studies until they consistently made ≥80% of their responses on the drug-appropriate lever after administration of training drug and less than 20% of their responses on the same drug-appropriate lever after administration of saline. During the stimulus generalization (i.e. substitution) phase of the study, maintenance of the training-drug/saline discrimination was insured by continuation of the training sessions on a daily basis (except on a generalization test day). On 1 of the 2 days before a generalization test, approximately half the animals would receive the training dose of training drug and the remainder would receive saline; after a 2.5 min extinction session, training was continued for 12.5 min. Animals not meeting the original training criteria during the extinction session were excluded from the subsequent generalization test session. During the investigations of stimulus generalization, test sessions were interposed among the training sessions. The animals were allowed 2.5 min to respond under non-reinforcement conditions. An odd number of training sessions (usually five) separated any two generalization test sessions. Doses of test drugs were administered in a random order, using a 15 min presession injection interval, to the groups of rats. Stimulus generalization was
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considered to have occurred when the animals, after a given dose of drug, made ≥80% of their responses (group mean) on the training drug-appropriate lever. Animals making fewer than five total responses during the 2.5 min extinction session were considered as being disrupted. Percent drug-appropriate responding and response rate data refer only to animals making ≥5 responses during the extinction session. If >50% of the animals were disrupted following administration of a given drug dose, data were not plotted. Where stimulus generalization occurred, an ED50 dose was calculated by the method of Finney (1952). The ED50 dose represents the drug dose where animals would be expected to make 50% of their responses on the drug-appropriate lever. Drugs (doses) examined in the MDMA-trained animals include: cocaine (2.0, 4.0, 6.0, 8.0, 10 and 11 mg/kg), S(+)MDMA (0.5, 0.75, 1.0 and 1.25 mg/kg) and R(−) MDMA (0.75, 1.5 and 3.0 mg/kg). The following drugs were administered to cocaine-trained animals (doses): cocaine (1.0, 2.0, 4.0 and 8 mg/kg), MDMA (0.75, 1.5, 1.6, 1.75, and 2.5 mg/kg), S(+)MDMA (0.25, 0.75, 1.25, 1.5, 1.75 and 2.25 mg/kg), and R(−)MDMA (1.0, 1.5, 2.0, 2.25 and 2.5 mg/kg). 2.2. Binding profile (±)MDMA and cocaine were examined in a number of different radioligand binding assays by the NIMH Psychoactive Drug Screening Program. The two agents were initially Table 1 Receptor binding profile for (±)MDMA and cocainea Receptor population ␣1A -Adrenergic ␣1B Adrenergic ␣2A -Adrenergic ␣2B -Adrenergic ␣2C -Adrenergic 1 -Adrenergic 2 -Adrenergic Serotonin r5-HT1B m3 Cholinergic m4 Cholinergic m5 Cholinergic H1 Histaminergic I1 Imidazoline
(±)MDMA, Ki (±S.E.M.) >10,000 >10,000 2,530 (180) 1,790 (100) 1,120 (30) 1,350 (30) >10,000 >10,000 1,850 (190) 8,250 (800) 6,350 (1,050) 7,780 (700) 220 (40)
Cocaine, Ki (±S.E.M.) >10,000 >10,000 20,900 7,490 14,700 3,000 >10,000 3,400 >10,000 >10,000 >10,000 2,250 2,800
(3,400) (820) (1,400) (160) (700)
(550) (440)
Receptor populations represent human receptors except where designated r (rat). Additionally, (±)MDMA and cocaine lacked appreciable affinity (Ki > 10,000 nM) for 5-HT1A , 5-HT1D , 5-HT1E , r5-HT2C , 5-HT3 , 5-HT5A , 5-HT6 and 5-HT7 serotonin receptors, - and -opioid receptors, D1 -, rD2 -, rD3 -, rD4 -, D5 -dopamine receptors, rGABAA receptors, rBZ receptors, rPCP receptors, m1 - and m2 -acetylcholine receptors, H2 histamine receptors, and nicotinic ␣22, ␣24, ␣32, ␣34, ␣42, ␣44 acetylcholine receptors. Cocaine was also re-examined for binding at three neurotransmitter transporters and displayed affinity for bovine DAT (Ki = 54 ± 12 nM), human SERT (Ki = 120 ± 20 nM) and human NET (Ki = 1000 ± 300 nM). a Binding data were obtained in quadruplicate from the NIMH Psychoactive Drug Screening Program (http://kidb.bioc.cwru.edu).
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screened in quadruplicate at a concentration of 10,000 nM; where an agent produced >50% inhibition, a Ki value was determined in quadruplicate. Further detail can be found in Table 1. 2.3. Drugs Racemic 3,4-methylenedioxymethamphetamine HCl (MDMA), R(-)MDMA HCl, and S(+)MDMA HCl were obtained as gifts from NIDA. Cocaine HCl was purchased from Sigma–Aldrich (St. Louis, MO). All doses of all drugs were administered via intraperitoneal injection; doses refer to the weight of the salts. Solutions in sterile 0.9% saline were freshly prepared daily.
3. Results 3.1. MDMA discrimination One group of animals (n = 6) was trained to discriminate 1.5 mg/kg of racemic MDMA from saline vehicle as previously reported such that the animals made 97% of their responses on the MDMA-appropriate lever following administration of the training dose of MDMA, and 5% of their responses on the same lever following administration of saline. The animals’ response rate (21.2 response per min) was not substantially different from that following administration of saline vehicle (19.9 response per min). Administration of the optical isomers of MDMA to the MDMA-trained animals resulted in substitution in a dose-dependent fashion (Fig. 1). The animals’ response rates were somewhat reduced at doses that produced >80% MDMA-appropriate responding. ED50 doses calculated for S(+)- and R(−)MDMA were 0.8 (95% CL = 0.6–1.1) mg/kg and 1.2 (95% CL = 0.6–2.2) mg/kg, respectively. Administration of cocaine to the MDMA-trained animals resulted in substitution in a dose-dependent fashion [ED50 = 4.6 (95% CL = 2.8–7.4) mg/kg]. A total of six doses of cocaine were examined; at 4 mg/kg, the animals’ response rate was higher than that following saline but rates steadily declined at higher doses (Fig. 1). At a dose of 11 mg/kg, only four of five animals responded (making 88% of their responses on the MDMA-appropriate lever) and the animals’ response rate was decreased to about 25% of that following saline vehicle. 3.2. Cocaine discrimination A second group of animals (n = 9) was trained to discriminate 8 mg/kg of cocaine from saline vehicle such that they made 96% of their responses on the drug-appropriate lever following administration of the training dose of cocaine, and 8% of their responses on the same lever following administration of saline. The animals’ response rate (23.2 response per min) was similar to that following
Fig. 1. Results (±S.E.M.) of stimulus generalization studies (upper panel), and animals’ response rates (lower panel), in rats trained to discriminate (±)MDMA from saline vehicle. Tests of stimulus generalization were conducted with S(+)MDMA, R(−)MDMA, and cocaine; results with the training dose of MDMA are represented as M and that for saline as S. Drug doses are plotted using a logarithmic scale.
administration of saline (20.9 response per min). Tests of stimulus generalization were conducted with (±)MDMA, S(+)MDMA, and R(−)MDMA using four to nine animals at each drug dose. Shortly after the study began, three of the cocaine-trained animals were lost; consequently, most of the substitution studies used four to six animals. Racemic MDMA produced a maximum of 36% cocaine-appropriate responding at 1.5 mg/kg (Fig. 2); at 1.6 mg/kg only two of four animals responded. At doses of 1.75 and 2.5 mg/kg, the animals’ lever-pressing behavior was disrupted and only one of six and none of five animals made ≥5 responses during the entire 2.5 min extinction session. Following administration of S(+)MDMA doses, the animals never made >24% (at 1.5 mg/kg) of their responses on the cocaine-appropriate lever (Fig. 2). Drug-appropriate responding declined to 11% at 1.75 mg/kg with four of six animals responding, and at 2.25 mg/kg, only one of five animals made ≥5 responses during the 2.5 min extinction session (i.e. 54% cocaine-appropriate responding; response
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4. Discussion
Fig. 2. Results (±S.E.M.) of stimulus generalization studies (upper panel), and animals’ response rates (lower panel), in rats trained to discriminate cocaine from saline vehicle. Tests of stimulus generalization were conducted with cocaine, (±)MDMA, S(+)MDMA, and R(−)MDMA; results for saline vehicle are represented by S. Drug doses are plotted using a logarithmic scale.
rate = 4.4 response per min), Five doses of R(−)MDMA were examined; 2.0 mg/kg of R(−)MDMA elicited 14% cocaine-appropriate responding with three of five animals responding. Administration of higher doses of R(−)MDMA resulted in disruption of behavior with no animals responding at 2.25 mg/kg and only two of six animals responding at 2.5 mg/kg (i.e. 0 and 100% drug-appropriate responding; response rate = 2.8 and 3.2 response per min, respectively). 3.3. Radioligand binding (±)MDMA and cocaine were examined in about 40 different radioligand binding assays to determine if they shared any binding similarities (Table 1). Both agents lacked appreciable affinity for most receptor populations examined. Whereas one of the two agents might have displayed low micromolar affinity for a particular receptor population, the other did not. In other words, there was no common receptor population where both agents displayed affinities (i.e. Ki values) of <2000 nM.
Previous drug discrimination studies have reported that animals trained to distinguish racemic MDMA from vehicle generalize to both MDMA optical isomers and that S(+)MDMA is about two to three times more potent than R(−)MDMA (Glennon et al., 1986; Oberlender and Nichols, 1988; Schechter, 1987). In the present study, using animals trained to discriminate (±)MDMA from saline vehicle, S(+)MDMA (ED50 = 0.8 mg/kg) was found to be 1.5 times more potent than R(−)MDMA (ED50 = 1.2 mg/kg). Only once before has cocaine been administered to rats trained to discriminate (±)MDMA from vehicle; in Fawn-Hooded rats, drug-appropriate responding never exceeded 40% (Schechter, 1998). In the present investigation, cocaine substituted for (±)MDMA in a fairly orderly fashion (Fig. 1). The potency of cocaine (ED50 = 4.6 mg/kg) was several-fold less than that of either MDMA isomer. The apparent inconsistency in the results obtained in the current study, as compared to that of the Schechter (1998) study, might be related to differences in methodology or, more likely, to the different strains of animals used in the two studies (see Section 1). Nonetheless, the present findings suggest that there is some similarity of stimulus effect produced by the two agents; this action might be related to the effect that the two agents have on increasing synaptic levels of 5-HT, DA, and/or NE. However, at cocaine doses of 10 and 11 mg/kg (where cocaine produced 75 and 88% MDMA-appropriate responding, respectively), the animals’ response rates were markedly reduced (Fig. 1). Thus, it is also tempting to speculate that the reduced response rates observed at the higher doses of cocaine are the result of some degree of differential action(s) of the two agents on one or more of the three neurotransmitter systems mentioned above. In addition, studies with the serotonergically-dysfunctional Fawn-Hooded rats suggest that serotonergic systems might play an important role in the stimulus actions of MDMA or in the ability of MDMA-trained animals to recognize cocaine. In an attempt to determine if (±)MDMA and cocaine produce their stimulus effects by a common but as yet unidentified receptor mechanism, the binding of both agents was examined at about 40 receptor populations. Table 1 shows that both MDMA and cocaine share some affinity for ␣2 - and 1 -adrenergic receptors; but, in general, their affinities are relatively low (Ki values are in the 2000–20,000 nM range). MDMA (Ki = 220 nM) and cocaine (Ki = 2, 800 nM) also bind at imidazoline I1 receptors (Table 1) with low affinity. Nevertheless, because both agents bind at these receptor populations, a contributory role for one or more of these populations cannot be excluded at this time. Racemic MDMA has not been previously examined in cocaine-trained animals. However, it has been reported that R(−)- but not S(+)MDMA substitutes in cocaine-trained animals (Broadbent et al., 1989). The current results show that (±)MDMA produced a maximum
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of 36% cocaine-appropriate responding (Fig. 2). At first, this result seems inconsistent with that of the earlier study. However, the possibility exists that the presence of the S(+)isomer in the racemic mixture of MDMA could account for the behaviorally disruptive actions of the racemate. Specifically, in the Broadbent et al. (1989) study, doses of S(+)MDMA ≥ 1.0 mg/kg reduced the animals’ lever-pressing behavior; similar dose levels of (±)MDMA impaired lever-pressing in the present animals. Thus, it could be argued that the S(+)MDMA component of racemic MDMA doses might have produced a disruptive effect and, consequently, precluded the possibility that the animals would have been able to respond to the reported cocaine-like effect of R(−)MDMA. But, the results presented here also show that neither R(−)- nor S(+)MDMA substituted in rats trained to discriminate a dose of 8 mg/kg of cocaine from vehicle. However, it should be noted that in the Broadbent et al. (1989) study, R(−)MDMA substituted only in animals trained to discriminate 20 mg/kg of cocaine, and produced incomplete generalization in animals trained to discriminate either 3.5 or 10 mg/kg of cocaine. This present findings with R(−)MDMA are, therefore, not inconsistent with the idea proposed by other investigators that results of a stimulus generalization test in cocaine-trained animals may be highly dependent on the training dose employed for cocaine (Broadbent et al., 1989; Kantak et al., 1999; Schechter, 1997; Spealman, 1995; Terry et al., 1994). Could the asymmetric generalization that occurred between (±)MDMA and cocaine be consistent with the findings that pre-exposure to (±)MDMA elevates locomotor activity, cocaine self-administration, and extracellular dopamine levels in the nucleus accumbens following a cocaine challenge in animals (Fletcher et al., 2001)? In the drug discrimination procedure, the (±)MDMA-trained animals obviously have been pre-exposed to MDMA as the training stimulus, and the administration of cocaine to these animals produced MDMA-like responding (Fig. 1). The cocaine-trained animals have been pre-exposed to cocaine as their training drug, but the administration of (±)MDMA did not produce cocaine-like responding. Finally, might the pre-exposure to MDMA followed by the administration of cocaine help explain the phenomenon referred to as “bumping” and “cloud nine” (see Section 1)? Users of MDMA usually ingest the agent orally, in tablet or capsule form. The psychoactive effects of MDMA are thought to be related, at least in part, to its action of releasing neuronal monoamine neurotransmitters, including dopamine, and increasing synaptic activity in areas of the brain such as the nucleus accumbens. The behavioral effects of MDMA last approximately 4–6 h. Inhalation of cocaine powder produces a euphoric effect lasting, typically, from 15 to 30 min. However, administration of cocaine following administration of MDMA might be able to intensify or “bump” a state of euphoria because it can function to block the reuptake process of dopamine which likely exists at an
increased level due to the neurochemical actions of MDMA. Similarly, smoking of cocaine in its free base or “crack” form allows a relatively high dose of cocaine to reach the brain very quickly and brings about a very immediate and intense bump that might last only 5–10 min, but may form the basis for a “cloud nine” euphoric effect. These effects might be the result of acute drug co-administration and, in the present study, the effects of acute cocaine and MDMA co-administration were not examined. However, MDMA is capable of producing long term neurotransmitter-related effects in animals lasting several weeks following a single administration of drug (Mayerhofer et al., 2001); hence, it is possible that pre-exposure to MDMA (e.g. as training drug) might influence the actions of subsequently administered cocaine doses. In summary, the results of the present study show for the first time that (±)MDMA-trained rats recognize the stimulus effects of cocaine (at the specific drug doses employed); stimulus similarity might be explained on the basis of mechanistic similarities reported for the two agents with respect to their ability to increase synaptic concentrations of serotonin, dopamine, and/or norepinephrine. Generalization is asymmetric, however, in that cocaine-trained animals failed to recognize (±)MDMA or either MDMA optical isomer. Asymmetric generalization might be related to differences in the extent to which each agent increases synaptic levels of serotonin, dopamine, and/or norepinephrine, and to which of these mechanisms predominates in the respective cuing properties of MDMA and cocaine. Studies by Schechter (1998) showing lack of substitution by cocaine in MDMA-trained Fawn-Hooded rats have particular significance with respect to this concept. Extended radioligand binding studies identified no other common receptor population where both MDMA and cocaine displayed high affinity. Pre-exposure to MDMA followed by administration of cocaine is another aspect of the asymmetric relationship that might aid our understanding of the reason individuals combine the effects of MDMA and cocaine, but this remains to be further investigated.
Acknowledgements The present study was supported in part by DA-01642. We also wish to acknowledge the NIMH Psychoactive Drug Screening Program for providing us with much of the radioligand binding data presented in Table 1. NK, a Royal Thai Fellow, was supported in part by a scholarship from the government of Thailand.
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