Skip to main content
NCBI home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2022 Dec 15.
Published in final edited form as: Eur J Pharmacol. 2021 Nov 17;913:174646. doi: 10.1016/j.ejphar.2021.174646

Lack of Evidence for Positive Reinforcing and Prosocial Effects of MDMA in Pair-Housed Male and Female Rats

Mark A Smith 1, Karl T Schmidt 1, Jessica L Sharp 1, Tallia Pearson 1, Anna L Davis 1, Abi Gibson 1, Kenzie M Potter 1
PMCID: PMC8675402  NIHMSID: NIHMS1759607  PMID: 34800468

Abstract

3,4-Methylenedioxymethamphetamine (MDMA) is classified as an entactogen, producing feelings of emotional openness and relatedness. One unique feature of MDMA is that people tend to selectively take this drug in social and/or intimate situations. Although MDMA is recognized as having abuse liability, preclinical studies report that it has weak reinforcing effects in animals. The objective of this study was to characterize the positive reinforcing and prosocial effects of MDMA in a translational model of the social environment in which two rats have simultaneous and contingent access to MDMA in close physical proximity. To this end, MDMA self-administration was examined on both fixed and progressive ratio schedules of reinforcement in six groups of rats: (1) isolated males, (2) isolated females, (3) male-male dyads, (4) female-female dyads, (5) male-female dyads, and (6) female-male dyads. For pair-housed rats, data from both rats were analyzed. Next, social preferences were examined in a partner preference test. MDMA failed to produce positive reinforcing effects under all conditions examined. Across a 30-fold dose range (0.01 – 1.0 mg/kg/infusion), MDMA did not maintain higher responding than saline on both schedules of reinforcement and in all groups tested. In partner preference tests, a history of shared exposure to MDMA did not establish a social preference, and acute administration of MDMA failed to establish a preference for another MDMA-treated rat. These data suggest that social contact does not increase the positive reinforcing effects of MDMA in rats, and that neither contingent nor noncontingent MDMA administration establishes a social preference in rats.

Keywords: ecstasy, molly, partner preference, self-administration, social behavior, social choice

1. Introduction

3,4-Methylenedioxymethamphetamine (MDMA) is a monoamine transporter substrate that stimulates the presynaptic release of dopamine, serotonin, and norepinephrine (de la Torre et al., 2004). Its combination of dopaminergic and serotonergic actions contributes to its stimulant- and hallucinogenic-like effects (Halpin et al., 2014). It is commonly misused/abused in human populations in social situations (e.g., parties, “raves”) and in intimate settings between sexual partners (Gahlinger, 2004; McElrath, 2005; Smith et al., 2002; Theall et al., 2006). Human laboratory studies report that MDMA produces several positive subjective effects that contribute to its abuse liability. Similar to psychomotor stimulants, MDMA increases subjective ratings of stimulation and euphoria (Cami et al., 2000; Peiró et al., 2013; Tancer and Johanson, 2001); however, it also increases ratings of extroversion, friendliness, openness, lovingness, emotional concern, and trusting of others (Bedi et al., 2010; Hysek et al., 2014; Kirkpatrick and de Wit, 2015; Tancer and Johanson, 2003; Wardle and de Wit, 2014). These latter effects have led some investigators to classify MDMA as an entactogen, a pharmacological class of drugs that produces feelings of oneness, relatedness, and emotional openness, as well as empathy and sympathy for others (Saez-Briones and Hernandez, 2013). The entactogenic effects of MDMA are believed to mediate its abuse liability in human populations and contribute to its popularity as an intoxicant in social and intimate settings (Kamilar-Britt and Bedi, 2015; Morgan et al., 2013; Sumnall et al., 2006).

Despite having significant abuse liability in humans, MDMA functions as a weak positive reinforcer in laboratory animals. Rodent studies reveal that drug-naïve subjects are slow to acquire MDMA self-administration, and that a significant number of subjects fail to acquire even with extended training (Aronsen et al., 2016; Bradbury et al., 2014; Schenk et al., 2007; van de Wetering and Schenk, 2017; Schenk et al., 2012). Once self-administration is established, animals maintain lower rates of self-administration on both fixed ratio (FR) and progressive ratio (PR) schedules of reinforcement relative to other drugs (Creehan et al. 2015; Vandewater et al., 2015; Ratzenboeck et al., 2001), and temporal patterns of MDMA self-administration do not resemble those of other drugs with stimulant-like effects (Schenk et al., 2003). Collectively, these data suggest that existing animal models are limited in their ability to detect the positive reinforcing effects of MDMA that contribute to its abuse liability. One potential explanation for the disparity between human and animal studies is that animal studies have not sufficiently considered the social context in which MDMA is self-administered in humans.

We recently developed custom-built, operant conditioning chambers that permit two rats to self-administer drugs simultaneously, side-by-side, in the same chamber. Using these chambers, we have shown that cocaine self-administration could either be facilitated or inhibited by social contact depending on the behavior of the partner. Specifically, cocaine self-administration was increased in rats with a social partner that was also self-administering cocaine, but cocaine self-administration was decreased in rats with a partner that did not have access to cocaine (Smith, 2012; Smith et al., 2014; Robinson et al., 2016; 2017). Our findings obtained with cocaine emphasize the critical role that social context plays in drug intake and addictive behavior. Consequently, these chambers may be uniquely suited to examine the abuse-related effects of MDMA and other drugs from the entactogen class.

In this study, we examined the positive reinforcing and prosocial effects of MDMA in an ecologically relevant model of the social environment in which a social partner was immediately present at the time of drug self-administration. To this end, we examined MDMA self-administration in isolated males, isolated females, male-male dyads, female-female dyads, male-female dyads, and female-male dyads. At the completion of drug self-administration testing, we examined the effects of MDMA on social preference in a partner preference test.

2. Materials and Methods

2.1. Animals

Male and female Long-Evans rats were obtained on postnatal day 62 from Charles River Laboratories (Raleigh, NC, USA). Rats were initially housed individually in transparent polycarbonate cages until the commencement of drug self-administration. At that time, rats were transferred to custom-built, operant conditioning chambers that also served as home cages (see Section 2.2 Apparatus). Throughout the study, rats lived in a temperature- and humidity-controlled colony room on a 12-hr light/dark cycle (lights on: 0500; lights off: 1700). Rats had free access to food and water throughout the study, except during the brief period of lever-press training (see Section 2.3. Lever-Press Training). All subjects were maintained in accordance with the guidelines established in the Guide for the Care and Use of Laboratory Animals (Institute for Laboratory Animal Research), and all procedures were approved by the Davidson College Animal Care and Use Committee (Protocol 7-19-56: Social Contact and MDMA Use).

2.2. Apparatus

All drug self-administration sessions took place in custom-built, operant conditioning chambers manufactured by Faircloth Machine Shop (Winston-Salem, NC, USA) described previously (Lacy et al., 2014; Smith, 2012). Briefly, operant chambers for isolated rats were cubic in design with a single response lever on the rear wall. Operant chambers for pair-housed rats were constructed from two isolated chambers separated by a 14-guage, wire-screen panel. The wire screen allowed rats full visual, auditory, and olfactory contact, as well as limited tactile contact with one another (Figure 1A). Each rat had access to one response lever mounted on the rear wall. The response levers were positioned 13 cm apart from one another and 6.5 cm on either side of the wire screen. Each of these levers were active during training and testing sessions, and no inactive levers were used. Drug infusions were delivered via Tygon tubing protected by a stainless-steel spring and connected to a counter balanced swivel suspended above the chamber. An infusion pump (3.33 rpm) was mounted behind the cage and connected to interfacing equipment provided by Med Associates, Inc. (St Albans, VT, USA). Fresh food was placed inside the cage daily, and water dispensers were continuously available inside the cage. Cotton pads were provided for bedding and replaced biweekly.

Figure 1.

Figure 1.

Open in a new tab

A. Photograph of interior of operant chamber serving as home cages for pair-housed rats. Rats are separated by a 14-guage, wire-screen panel. The wire screen allows rats full visual, auditory, and olfactory contact, as well as limited tactile contact with one another. Each rat has access to one response lever mounted on the rear wall. The response levers are positioned 13 cm apart from one another and 6.5 cm on either side of the wire screen. White arrow indicates one response lever. B. Photograph of the partner preference apparatus. The chamber is functionally divided into three equally sized zones measuring 1080 cm2 each. A solid partition (46 cm) partially bisects the chamber to physically separate two of the three zones. During testing, two rats are restrained in wire-screen containers, one on each side of the partition. One test rat can move freely between the two sides of the chamber, thus maintaining physical proximity with one partner relative to the other partner. When on a given side, the test rat has full visual, auditory, and olfactory contact, as well as limited tactile contact, with the rat restrained on that side of the partition. The remaining zone corresponds to the area of the chamber that is physically distant from both rats, but it is not separated from the other zones by a physical barrier. White broken lines indicate zone boundaries.

Lever-press training using food reinforcement was conducted in commercially available operant conditioning chambers from Med Associates. Each chamber contained a houselight, two response levers, two white stimulus lights located above the two levers, a food receptacle located between the two levers, a pellet dispenser located behind the forward wall, and an infusion pump located outside the chamber.

Partner preference testing was conducted in a custom-built chamber described previously (Smith et al., 2015). Briefly, the chamber (60 × 54 cm) is functionally divided into three equally sized zones measuring 1080 cm2 each. A solid partition (46 cm) partially bisects the chamber to physically separate two of the three zones. During testing, two rats are restrained in wire-screen containers, one on each side of the partition. One test rat can move freely between the two sides of the chamber, thus maintaining physical proximity with one partner relative to the other partner. When on a given side, the test rat has full visual, auditory, and olfactory contact, as well as limited tactile contact, with the rat restrained on that side of the partition. The remaining zone corresponds to the area of the chamber that is physically distant from both rats, but it is not separated from the other zones by a physical barrier (Figure 1B).

2.3. Lever-Press Training

A 30-day pilot study that aimed to examine the acquisition of MDMA self-administration in the absence of any prior operant training failed to establish MDMA-reinforced responding in all 14 rats tested (isolated rats: n = 8; pair-housed rats: n = 6). Consequently, all rats used in this study were trained to lever-press using food reinforcement and then trained to self-administer cocaine before transitioning to MDMA self-administration.

Approximately one week after arrival, rats were food-restricted to no less than 90% of their free-feeding body weight and trained to lever press using food reinforcement. During training sessions, each lever press was reinforced with one 45-mg grain pellet (BioServ F0165; Flemington NJ) on a fixed ratio (FR1) schedule of reinforcement. Each session lasted until 40 reinforcers were delivered or until 2 hr elapsed, whichever occurred first. Training continued in this manner until a rat earned 40 reinforcers during any four training sessions. All rats reached this criterion within six sessions and then returned to free feeding conditions for the remainder of the study.

2.4. Surgery

One week following lever press training, rats were anesthetized with a combination of xylazine HCl (8.0 mg/kg, ip) and ketamine (100 mg/kg, ip), and an intravenous catheter was inserted into the right jugular vein that exited the body on the dorsal surface of the scapulae. Each rat was administered Ketoprofen (3.0 mg/kg, sc) as a post-operative analgesic immediately following surgery and once again 18–24 hr later. A topical antibiotic was applied to all incisions immediately after surgery and reapplied daily for three consecutive days. For seven days after surgery, a solution of heparinized saline and ticarcillin (20 mg/kg, iv) was infused through the catheter daily to maintain patency and prevent infection. After the initial seven days, ticarcillin administration was discontinued and catheter patency was maintained by biweekly infusions of heparinized saline for the remainder of the study.

2.5. Group Assignment

Following recovery from surgery (5–7 days later), rats were moved to custom-built operant conditioning chambers that also served as home cages for the remainder of the study (see Section 2.2. Apparatus). At this time, rats were randomly assigned to either isolated, same-sex paired, or opposite-sex paired housing conditions to yield six groups of rats: (1) isolated males, (2) isolated females, (3) male-male dyads, (4) female-female dyads, (5) male-female dyads, and (6) female-male dyads. In rats assigned to pair-housed conditions, both rats received the same daily experimental conditions as its partner for the duration of the study (i.e., drug, dose, and schedule of reinforcement were matched between partners), and data from both rats were analyzed. The estrous cycle of all female rats was determined via vaginal lavage 30 min prior to each session, followed by cell typing using light microscopy (see description in Lacy et al., 2016).

2.6. Self-Administration Training

Drug self-administration training began approximately 24 hr following transfer to the custom-built, operant conditioning chambers. Each training session began promptly at the beginning of the dark phase of the light-dark cycle. During training, drug self-administration was established using cocaine. Each session began with a noncontingent infusion of 0.5 mg/kg cocaine and insertion of the retractable lever into the chamber. Each lever press produced an infusion of cocaine (0.5 mg/kg/infusion) on an FR1 schedule and retraction of the response lever for 20 s. Sessions continued for 4 hr and no limit was placed on the maximum number of reinforcers that could be earned. Training continued in this manner for 7 consecutive days before transitioning to MDMA.

2.7. MDMA Self-Administration

After 7 days of cocaine self-administration, cocaine was replaced with MDMA, but all other experimental conditions were identical to those described for cocaine. Specifically, each session began promptly at the beginning of the dark phase of the light-dark cycle with a noncontingent infusion of 0.5 mg/kg (±)-MDMA (MDMA) and insertion of the retractable lever into the chamber. Each lever press produced an infusion of MDMA (0.5 mg/kg/infusion) on an FR1 schedule and retraction of the response lever for 20 s. Sessions continued for 4 hr and no limit was placed on the maximum number of reinforcers that could be earned. Training continued in this manner for 7 consecutive days.

Dose-response testing on the FR1 schedule commenced 7 days after initial exposure to MDMA. A different dose of MDMA was tested each day, such that four doses of MDMA spanning a 30-fold dose range was evaluated (0.03 – 1.0 mg/kg/infusion). In addition, a saline substitution test was performed in which saline was substituted for MDMA. Doses were tested in a pseudorandom order with the stipulation that no more than two ascending or descending doses could be tested in a row. All other parameters were identical to those used during self-administration training.

Immediately following dose-response testing on the FR1 schedule, contingencies were changed and responding was reinforced on a PR schedule. On this schedule, the number of responses required for an infusion of MDMA initially doubled (1, 2, 4, 8) and then subsequently increased by 8 (16, 24, 32...) until a breakpoint was reached, with breakpoint defined as the number of infusions obtained before one hour elapsed with no infusions. All rats received one day of training on the PR schedule in which responding was reinforced with 0.5 mg/kg MDMA. Afterwards, a different dose of MDMA was tested each day as described for the FR1 dose-response analysis. As before, four doses of MDMA and saline were tested in a pseudorandom order.

2.8. Partner Preference Testing

2.8.1. Shared History of MDMA Self-Administration in Pair-Housed Rats

One aim of this study was to determine whether a shared history of MDMA self-administration enhances the preference for a familiar partner. Following MDMA self-administration testing, a rat from a dyad group assignment was placed in a partner preference apparatus for a 15-min habituation period in the absence of other rats. After 15 min, this rat was removed and placed into an isolation cage for 15 min. During this 15-min interval, its social partner was restrained in a wire-screen container on one side of the partner preference apparatus and an unfamiliar partner (of the same sex as the familiar partner) was restrained in an identical wire-screen container on the opposite side. The position of the two partners was counterbalanced. The original rat was then returned to the partner preference apparatus for a 15-min test, and the time spent in each of the three zones was measured (Familiar, Unfamiliar, Alone; also see Section 2.2. Apparatus). No drugs were administered to any rat during this partner preference test. The primary outcome variable was the time spent in each of the three zones during the 15-min test.

2.8.2. Acute Effects of MDMA in Isolated Rats

An additional aim of this study was to determine whether simultaneous MDMA administration influences the choice of two unfamiliar partners. Following MDMA self-administration testing, a rat from an isolation group assignment was placed into a partner preference apparatus for a 15-min habituation period in the absence of other rats. After 15 min, this rat was removed from the apparatus, administered (±)-MDMA (MDMA: 5.0 mg/kg, ip), and placed into an isolation cage for 15 min. During this 15-min interval, one unfamiliar partner of the same sex as the test rat was injected with the same dose and restrained in a wire-screen container on one side of the partner preference apparatus. A second unfamiliar partner of the same sex was injected with saline and restrained in an identical wire-screen container on the opposite side of the apparatus. The position of the two partners was counterbalanced. The original rat was then returned to the partner place apparatus for a 15-min test, and the time spent in each of the three zones was measured (MDMA-Treated, Saline-Treated, Alone; also see Section 2.2. Apparatus).

2.9. Data Analysis

Rats that lost catheter patency (and their partners, if housed socially) prior to advancing to testing on the FR schedule of reinforcement were removed from the study, their data were discarded, and no further testing was conducted. Rats that lost catheter patency during testing on the FR or PR schedules of reinforcement (and their partners, if housed socially) were removed from further self-administration testing, their self-administration data were discarded, but they advanced to partner preference testing with the remaining rats. One isolated male rat died during testing on the PR schedule for reasons unrelated to drug administration; its data from the FR schedule (but not the PR schedule) were included in the final analysis.

To determine the effects of sex of subject (male, female), partner condition (isolated, same-sex, opposite-sex), and dose (0.03, 0.1, 0.3., 1.0 mg/kg/infusion) on MDMA self-administration, dose-response data were examined using a three-way, mixed-factor ANOVA, with sex of subject and partner condition serving as between-subject factors and dose serving as the repeated measure. To determine whether MDMA served as a positive reinforcer (i.e., maintained greater levels of responding than saline), the number of infusions obtained at each dose of MDMA was compared to the number of infusions obtained during the saline substitution test via paired-samples t-tests using the Holms-Bonferroni correction for multiple comparisons. Data obtained on both the FR and PR schedules of reinforcement were examined.

The partner preference apparatus was divided into three zones of equal size corresponding to (1) an area in physical proximity to one partner, (2) an area in physical proximity to a second partner, and (3) an area physically distant from both partners. Data were expressed as the time spent in each zone, with values greater than 300 s (i.e., 1/3 of the 900-s test session) indicating a positive preference for that zone. Preferences for and against an area were considered significant when the 95% confidence limits (95% CL) did not overlap 300 s, the theoretical point of indifference. Comparisons between areas were made via paired-samples t-tests using the Holms-Bonferroni correction for multiple comparisons. Note that an ANOVA was not used to compare the three zones because data from this test are perfectly ipsative (i.e., all possible values for the three zones sum to the same amount).

3. Results

3.1. Drug Self-Administration

Cocaine maintained high levels of responding that did not vary significantly by sex of subject (male, female) or partner condition (same-sex, opposite-sex, isolated: Figure 2). Responding decreased significantly when MDMA was substituted for cocaine (comparison between last day of cocaine and first day of MDMA revealed a main effect of drug: F(1, 52) = 84.928, p < .001), and these decreases did not vary by sex of subject or by partner condition. The change in responding was large and abrupt, decreasing approximately 75% on the first day in which MDMA was substituted for cocaine, and remained low thereafter.

Figure 2.

Figure 2.

Open in a new tab

Drug self-administration in isolated male rats (top left; n = 9), isolated female rats (top right; n = 5), male rats partnered with another male (middle left; n = 12), female rats partnered with another female (middle right; n = 14), male rats partnered with a female (bottom left; n = 12), and female rats partnered with a male (bottom right; n = 12). Left axis depicts the number of infusions obtained during 4-hr sessions. Bottom axis depicts sessions conducted during cocaine training (open triangles: 0.5 mg/kg, FR1, 5 days), MDMA training (circles: 0.5 mg/kg, FR1, 7 days), MDMA testing on a fixed ratio schedule (circles: saline (S), 0.03, 0.1, 0.3, 1.0 mg/kg/infusion), and MDMA testing on a progressive ratio schedule (circles: saline (S), 0.03, 0.1, 0.3, 1.0 mg/kg/infusion). Data depict mean (+/− SEM). No significant main effects or interactions were observed for sex of subject (male, female), partner condition (isolated, same-sex, opposite-sex) or dose of MDMA (0.03, 0.1, 0.3, 1.0 mg/kg) in either FR or PR schedules of reinforcement.

During testing on the FR1 schedule, responding maintained by MDMA decreased significantly as a function of dose (main effect of dose: F(3, 156) = 7.333, p < .001), and this did not vary as a function of sex of subject or partner condition. The two highest doses of MDMA significantly decreased responding relative to saline (p = .001 and p < .001, respectively), and no dose of MDMA maintained responding greater than saline.

During testing on the PR schedule, responding did not vary as a function of dose, and no dose of MDMA maintained responding greater than saline. Responding during the PR test varied as a function of partner condition (main effect of partner condition: F(2, 51) = 5.295, p = .008), which was driven by lower responding in rats housed with an opposite-sex partner versus a same-sex partner (p = .007).

Analysis of individual data failed to identify a single rat in which MDMA served as a positive reinforcer. No rat demonstrated prototypical dose-effect curves in which MDMA produced dose-related elevations in responding that markedly differed from saline. Indeed, a subgroup analysis taking only the top three responders in each group (n = 3 rats × 6 groups = 18 rats) on the FR1 schedule revealed a mean of 11.2 infusions per dose of MDMA versus a mean of 12.0 infusions of saline. Similarly, a subgroup analysis taking only the top three responders in each group on the PR schedule revealed a mean of 3.3 infusions per dose of MDMA versus 3.6 infusions of saline (data not shown).

Although estrous cycle was monitored in female subjects, responding was so low in all rats (and variance was so low in all groups) that meaningful statistical comparisons could not be made, but no visual trends were evident (data not shown).

3.2. Partner Preference

3.2.1. Effects of Shared History of MDMA Administration on Partner Preference

After completing drug self-administration testing, socially housed rats were tested in a partner preference procedure to test the hypothesis that a shared history of MDMA exposure would establish a social preference for that partner. Contrary to our hypothesis, a shared history of MDMA exposure failed to establish a preference for a familiar partner regardless of the sex of the subject or the sex of the partner (Figure 3).

Figure 3.

Figure 3.

Open in a new tab

Social preferences in male rats partnered with another male (top left; n = 21), female rats partnered with another female (top right; n = 15), male rats partnered with a female (bottom left; n = 12), and female rats partnered with a male (bottom right; n = 12). Data depict mean (+/− 95% CL) time (s) spent in three equally sized zones during a 900-s partner preference test. The three zones were defined by areas in physical proximity to a familiar rat (Familiar), in physical proximity to an unfamiliar rat (Unfamiliar), and physically distant from both rats (Alone). Dashed line indicates the theoretical point of indifference (900 s / 3 = 300 s). Filled symbols reflect significant preference for an area, open symbols reflect significant preference against an area, half-filled symbols reflect no significant preference for or against an area. Asterisks and linkage lines indicate significant difference between two areas.

In same-sex groups, male rats exhibited a preference for being alone and a preference against being with a familiar male. Male rats in this group spent significantly more time alone than in proximity to a familiar male (p = .001) or an unfamiliar male (p = .006). Female rats exhibited a preference for being with an unfamiliar female, but pairwise comparisons in this group were not significant. In opposite-sex groups, male rats exhibited a preference for being with an unfamiliar female and a preference against being alone, a difference that was statistically significant (p = .017). Female rats did not develop a preference for or against either partner.

3.2.2. Effects of Acute MDMA Administration on Partner Preference

After completing drug self-administration testing, isolated rats were tested in a partner preference procedure to test the hypothesis that acute administration of MDMA would establish a social preference for another rat of the same sex that was also administered MDMA. Contrary to our hypothesis, acute administration of MDMA did not establish a preference for a same-sex rat also administered MDMA (Figure 4). Male rats administered MDMA exhibited a preference for being alone and a preference against being with both MDMA- and saline-treated partners, spending significantly more time alone than with either the MDMA-treated (p = .015) or saline-treated (p = .009) partner. Female rats exhibited a preference for being alone and a preference against being with a saline-treated partner, and this difference was statistically significant (p = .011).

Figure 4.

Figure 4.

Open in a new tab

Social preferences in isolated, MDMA-treated male rats (top left; n = 7) and isolated, MDMA-treated female rats (right; n = 8). Data depict mean (+/− 95% CL) time (s) spent in three equally sized zones during a 900-s partner preference test. The three zones were defined by areas in physical proximity to an unfamiliar MDMA-treated rat (MDMD-Treated), in physical proximity to an unfamiliar saline-treated rat (Saline-Treated), and physically distant from both rats (Alone). Dashed line indicates the theoretical point of indifference (900 s / 3 = 300 s). Filled symbols reflect significant preference for an area, open symbols reflect significant preference against an area, half-filled symbols reflect no significant preference for or against an area. Asterisks and linkage lines indicate significant difference between two areas.

4. Discussion

The principal findings of this study are that (1) MDMA did not produce positive reinforcing effects under conditions in which two rats had simultaneous visual, auditory, olfactory, and limited tactile contact and (2) MDMA failed to engender a social preference for a similarly treated partner or a familiar partner with a shared history of MDMA exposure. These effects were observed in models previously used to demonstrate social facilitation of cocaine self-administration (Smith, 2012, Smith et al., 2014; Robinson et al., 2016; 2017) and cocaine-facilitated preferences for a familiar partner (Smith et al., 2015). To our knowledge, this is the first study to make direct comparisons between the positive reinforcing and prosocial effects of MDMA in laboratory rats or to make both same-sex and opposite-sex comparisons of MDMA in both males and females.

Pilot tests revealed that naïve rats would not acquire lever pressing using intravenous MDMA as a reinforcing stimulus; consequently, rats in this study were trained to press a response lever using food reinforcement, and then later trained to self-administer cocaine on an FR1 schedule of reinforcement. Previous studies suggest a prior history of cocaine self-administration may facilitate the acquisition of MDMA self-administration (Ratzenboeck et al. 2001; Schenk et al., 2003) but that was not observed in the present study. All rats responded on the first day of intravenous cocaine availability, maintaining consistent and high rates of cocaine intake over 7 consecutive days; however, responding decreased abruptly on the first day of MDMA availability in all groups and remained low for the following 7 days. Collectively, these data reveal that MDMA is devoid of positive reinforcing effects in both experimentally naïve rats and in rats transitioning directly from cocaine self-administration under the conditions used in this study.

MDMA dose-dependently decreased responding during testing on the FR1 schedule. The two highest doses of MDMA decreased responding relative to saline control, thus functioning as a positive punisher under the conditions tested. Responding remained low when the contingencies were changed to a PR schedule of reinforcement, with no doses maintaining responding greater than saline. Previous studies report that MDMA functions as a weak positive reinforcer, with approximately 50% of rats failing to acquire self-administration (Oakley et al., 2014; Schenk et al., 2007; Schenk et al., 2012); however, responding may be relatively high in those rats that do acquire (e.g., ~ 50 infusions / 6 hr: Schenk et al., 2008). Despite extensive operant training in the present study, MDMA failed to function as a positive reinforcer in all 58 rats tested in this study, including those rats tested in the presence of a social partner with simultaneous access to MDMA to better model the human condition. These null effects were observed for both males and females and for both same-sex and opposite-sex partners.

There is not a consistent set of parameters that predict when MDMA will and will not maintain self-administration. Although procedural details differ across studies, we choose parameters that were generally consistent across studies reporting positive effects (e.g., establishing cocaine self-administration before transitioning to MDMA, extended duration test sessions, maintenance dose of 0.5 mg/kg/infusion MDMA). One difference between this study and prior studies is that all rats were trained and tested in modified operant conditioning chambers that also served as home cages. This difference is relevant given data indicating that drug self-administration for some drugs is significantly attenuated with testing occurs in the home cage rather than a distinct context (e.g., cocaine: de Luca et al., 2019). Although we believe drug self-administration procedures are more translationally relevant when they occur in a familiar environment, this aspect of the experimental design may have reduced the sensitivity of the model to capture the positive reinforcing effects of MDMA.

An additional difference between this study and prior studies is that an explicit audio and/or visual (a/v) stimulus was not used as a conditioned stimulus (CS) to signal MDMA self-administration. Although each infusion was coupled to the retraction of the response lever, experiments were carried out in the dark, and our infusion pumps are functionally silent (i.e., the sound produced by their operation is less than the ambient sounds within the room). This is potentially significant, given that most studies reporting high levels of MDMA-maintained responding have incorporated an a/v CS into the experimental design (e.g., Frankowska et al., 2020; Highgate and Schenk, 2018), and the removal of an MDMA-paired a/v CS significantly decreases MDMA self-administration (Daniela et al., 2006). Interestingly, many studies examining MDMA self-administration operationally define its reinforcing effects as a difference between active and inactive lever responding, rather than as a difference between MDMA and vehicle-maintained responding. In this scenario, responding on an active lever (MDMA + CS) may be greater than responding on an inactive lever (not [!] MDMA + ! CS), even though active lever responding is greater when the consequence is vehicle (saline + CS) than MDMA (MDMA + CS; see Aarde et al., 2017 for an example). These effects are similar to those of other drugs with weak primary reinforcing effects (e.g., nicotine), which maintain low or null levels of responding unless accompanied by a distinct audiovisual CS (Caggiula et al., 2002; Palmatier et al., 2006; 2007).

A shared history of MDMA self-administration failed to produce a preference for a familiar partner with that same shared history. In partner preference tests, rats either preferred to be alone (males housed with males), to be with an unfamiliar partner (males housed with females, females housed with females), or exhibited no preference (females housed with males). Likewise, acute administration of MDMA failed to produce a preference for an MDMA-treated, same-sex partner. Both male and female rats exhibited a preference for being alone following acute MDMA administration, technically producing asocial effects under the conditions examined. In rodent models, MDMA produces prosocial effects on some but not all measures. For instance, MDMA increases a preference for a restrained partner over an empty restrainer in mice, and this effect is enhanced if both the test mouse and restrained mouse are administered MDMA (Heifets et al., 2019). In rats, 5.0 mg/kg MDMA (the dose used in the present study) increases adjacent lying (Morley et al., 2005; Morley and McGregor, 2000; Ramos et al., 2013; Thompson et al., 2007) and increases social investigation under some conditions (Morley et al., 2005; Morley and McGregor, 2000; Thompson et al., 2009). Under other conditions, this dose decreases social play, social exploration, and social grooming (Homberg et al., 2007). Importantly, repeated administration of MDMA decreases social interaction in a drug-free state and reduces its prosocial effects (Thompson et al., 2008), and it is likely that both phenomena were present in this study. Finally, MDMA facilitates the establishment of a conditioned preference for a partner-paired chamber (Heifets et al., 2019; Ramos et al., 2015), with the caveat that it also facilitates the establishment of a conditioned preference for a nonsocial (tennis ball)-paired chamber (Ramos et al., 2015). We are not aware of any studies examining the effects of MDMA on social choice (i.e., a preference for one partner over another partner) as was examined in this present study.

5. Conclusions

The primary conclusion of this study is that an animal model that permits two social partners to have simultaneous access to MDMA does not capture either the positive reinforcing or prosocial effects of MDMA described in humans. The model was chosen because it was previously used to demonstrate social-induced facilitation of cocaine self-administration and cocaine-induced facilitation of a partner preference. The use of both drug self-administration and partner preference assays was initially considered an advantage because it permits direct comparisons between the positive reinforcing and prosocial effects of MDMA in individual animals. Such a comparison was not possible because MDMA did not function as a positive reinforcer in any of the rats tested. Regardless, one important implication of these findings is that MDMA may often fail to produce positive reinforcing effects in rats because it fails to produce reliable prosocial (i.e., entactogenic; empathogenic) effects in this species.

Acknowledgements

The authors thank the National Institute on Drug Abuse for supplying the study drugs.

Funding

This work was supported by the National Institutes of Health [grant numbers DA045364, DA031725, and DA045714].

Role of Funding Source

The NIH had no role in study design; in the collection, analysis, and interpretation of data; in the writing of the report; and in the decision to submit the article for publication.

Footnotes

Credit Author Statement

MAS: Conceptualization, Methodology, Funding Acquisition, Writing.

KTS, JLS, TP, ALD, AG, KMP: Investigation.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

We have no declarations of interests to report.

Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

References

  1. Aarde SM, Huang PK, Taffe MA. High ambient temperature facilitates the acquisition of 3,4-methylenedioxymethamphetamine (MDMA) self-administration. Pharmacol Biochem Behav. 2017; 163: 38–49. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Aronsen D, Bukholt N, Schenk S. Repeated administration of the 5-HT(1)B/(1)A agonist, RU 24969, facilitates the acquisition of MDMA self-administration: role of 5-HT(1)A and 5-HT(1)B receptor mechanisms. Psychopharmacology (Berl). 2016; 233: 1339–47. [DOI] [PubMed] [Google Scholar]
  3. Bedi G, Hyman D, de Wit H. Is ecstasy an “empathogen”? Effects of +/−3,4-methylenedioxymethamphetamine on prosocial feelings and identification of emotional states in others. Biol Psychiatry. 2010; 68: 1134–40. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Bradbury S, Bird J, Colussi-Mas J, Mueller M, Ricaurte G, Schenk S. Acquisition of MDMA self-administration: pharmacokinetic factors and MDMA-induced serotonin release. Addict Biol. 2014; 19: 874–84. [DOI] [PubMed] [Google Scholar]
  5. Caggiula AR, Donny EC, White AR, Chaudhri N, Booth S, Gharib MA, Hoffman A, Perkins KA, Sved AF. Environmental stimuli promote the acquisition of nicotine self-administration in rats. Psychopharmacology (Berl). 2002; 163: 230–7 [DOI] [PubMed] [Google Scholar]
  6. Cami J, Farré M, Mas M, Roset PN, Poudevida S, Mas A, San L, de la Torre R. Human pharmacology of 3,4-methylenedioxymethamphetamine (“ecstasy”): psychomotor performance and subjective effects. J. Clin. Psychopharmacol 2000; 20: 455–66 [DOI] [PubMed] [Google Scholar]
  7. Creehan KM, Vandewater SA, Taffe MA. Intravenous self-administration of mephedrone, methylone and MDMA in female rats. Neuropharmacology. 2015; 92: 90–7. [DOI] [PMC free article] [PubMed] [Google Scholar]
  8. Daniela E, Gittings D, Schenk S Conditioning following repeated exposure to MDMA in rats: Role in the maintenance of MDMA self-administration. Behavioral Neuroscience, 2006; 120: 1144–50. [DOI] [PubMed] [Google Scholar]
  9. de la Torre R, Farré M, Roset PN, Pizarro N, Abanades S, Segura M, Segura J, Camí J Human pharmacology of MDMA: pharmacokinetics, metabolism, and disposition. Ther Drug Monit. 2004; 26: 137–44. [DOI] [PubMed] [Google Scholar]
  10. de Luca MT, Montanari C, Meringolo M, Contu L, Celentano M, Badiani A. Heroin versus cocaine: opposite choice as a function of context but not of drug history in the rat. Psychopharmacology (Berl). 2019; 236: 787–98. [DOI] [PMC free article] [PubMed] [Google Scholar]
  11. Frankowska M, Miszkiel J, Pomierny-Chamiolo L, Pomierny B, Celeste Borelli A, Suder A, Filip M. The impact of 3,4-methylendioxymetamphetamine (MDMA) abstinence on seeking behavior and the expression of the D2-like and mGlu5 receptors in the rat brain using saturation binding analyses. J Physiol Pharmacol. 2020; 71: 537–46 [DOI] [PubMed] [Google Scholar]
  12. Gahlinger PM. Club drugs: MDMA, gamma-hydroxybutyrate (GHB), Rohypnol, and ketamine. American family physician. 2004; 69: 2619–26. [PubMed] [Google Scholar]
  13. Halpin LE, Collins SA, Yamamoto BK. Neurotoxicity of methamphetamine and 3,4-methylenedioxymethamphetamine. Life sciences. 2014; 97: 37–44. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Heifets BD, Salgado JS, Taylor MD, Hoerbelt P, Pinto DFC, Steinberg EE, and Malenka RC. Distinct neural mechanisms for the prosocial and rewarding properties of MDMA. Science translational medicine. 2019; 11: 522. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. Highgate Q, Schenk S. Comparison of the effects of abstinence on MDMA and cocaine self-administration in rats. Psychopharmacology (Berl). 2018; 235: 3233–41 [DOI] [PubMed] [Google Scholar]
  16. Homberg JR, Schiepers OJ, Schoffelmeer AN, Cuppen E, Vanderschuren LJ. Acute and constitutive increases in central serotonin levels reduce social play behaviour in peri-adolescent rats. Psychopharmacology. 2007; 195: 175–182. [DOI] [PMC free article] [PubMed] [Google Scholar]
  17. Hysek CM, Schmid Y, Simmler LD, Domes G, Heinrichs M, Eisenegger C, Preller K, Quednow BB, Liechti ME. MDMA enhances emotional empathy and prosocial behavior. Social cognitive and affective neuroscience. 2014; 9: 1645–52. [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Kamilar-Britt P, Bedi G. The prosocial effects of 3,4-methylenedioxymethamphetamine (MDMA): Controlled studies in humans and laboratory animals. Neuroscience and biobehavioral reviews. 2015; 57: 433–46. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Kirkpatrick MG, de Wit H. MDMA: a social drug in a social context. Psychopharmacology. 2015; 232: 1155–63. [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Lacy RT, Strickland JC, Feinstein MA, Robinson AM, Smith MA. The effects of sex, estrous cycle, and social contact on cocaine and heroin self-administration in rats. Psychopharmacology. 2016; 233: 3201–3210. doi: 10.1007/s00213-016-4368-9. [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Lacy RT, Strickland JC, Smith MA. Cocaine self-administration in social dyads using custom-built operant conditioning chambers. Neurosci Methods. 2014; 236: 11–18 [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. McElrath K MDMA and sexual behavior: ecstasy users’ perceptions about sexuality and sexual risk. Substance use & misuse. 2005; 40: 1461–77. [DOI] [PubMed] [Google Scholar]
  23. Morgan CJ, Noronha LA, Muetzelfeldt M, Feilding A, Curran HV. Harms and benefits associated with psychoactive drugs: findings of an international survey of active drug users. Journal of psychopharmacology (Oxford, England). 2013; 27: 497–506. [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Morley KC, Arnold JC, McGregor IS. Serotonin (1A) receptor involvement in acute 3, 4-methylenedioxymethamphetamine (MDMA) facilitation of social interaction in the rat. Progress in Neuro-Psychopharmacology and Biological Psychiatry. 2005; 29: 648–57. [DOI] [PubMed] [Google Scholar]
  25. Morley KC, McGregor IS. (+/−)-3,4-methylenedioxymethamphetamine (MDMA, ‘Ecstasy’) increases social interaction in rats. Eur J Pharmacol. 2000; 408: 41–9. [DOI] [PubMed] [Google Scholar]
  26. Oakley A, Brox B, Schenk S, Ellenbroek A. A genetic deletion of the serotonin transporter greatly enhances the reinforcing properties of MDMA in rats. Mol Psychiatry. 2014; 19: 534–5. [DOI] [PubMed] [Google Scholar]
  27. Palmatier MI, Evans-Martin FF, Hoffman A, Caggiula AR, Chaudhri N, Donny EC, Liu X, Booth S, Gharib M, Craven L, Sved AF. Dissociating the primary reinforcing and reinforcement-enhancing effects of nicotine using a rat self-administration paradigm with concurrently available drug and environmental reinforcers. Psychopharmacology (Berl). 2006; 184: 391–400. [DOI] [PubMed] [Google Scholar]
  28. Palmatier MI, Liu X, Caggiula AR, Donny EC, Sved AF. The role of nicotinic acetylcholine receptors in the primary reinforcing and reinforcement-enhancing effects of nicotine. Neuropsychopharmacology. 2007; 32: 1098–108. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Peiró AM, Farré M, Roset PN, Carbó M, Pujadas M, Torrens M, Camí J, de la Torre R. Human pharmacology of 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) after repeated doses taken 2 h apart. Psychopharmacology. 2013; 225: 883–93. [DOI] [PubMed] [Google Scholar]
  30. Ramos L, Hicks C, Caminer A, Goodwin J, McGregor IS. Oxytocin and MDMA (‘Ecstasy’) enhance social reward in rats. Psychopharmacology. 2015; 232: 2631–2641. [DOI] [PubMed] [Google Scholar]
  31. Ramos L, Hicks C, Kevin R, Caminer A, Narlawar R, Kassiou M, McGregor IS. Acute prosocial effects of oxytocin and vasopressin when given alone or in combination with 3, 4-methylenedioxymethamphetamine in rats: involvement of the V1 A receptor. Neuropsychopharmacology. 2013; 38: 2249–2259. [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. Ratzenboeck E, Saria A, Kriechbaum N, Zernig G. Reinforcing effects of MDMA (“ecstasy”) in drug-naive and cocaine-trained rats. Pharmacology. 2001; 62: 138–44. [DOI] [PubMed] [Google Scholar]
  33. Robinson AM, Fronk GE, Zhang H, Tonidandel S, Smith MA. The effects of social contact on cocaine intake in female rats. Drug Alcohol Depend. 2017; 177 :48–53 [DOI] [PMC free article] [PubMed] [Google Scholar]
  34. Robinson AM, Lacy RT, Strickland JC, Magee CP, Smith MA. The effects of social contact on cocaine intake under extended-access conditions in male rats. Exp Clin Psychopharmacol. 2016; 24: 285–96. [DOI] [PMC free article] [PubMed] [Google Scholar]
  35. Saez-Briones P, Hernandez A. MDMA (3,4-Methylenedioxymethamphetamine) Analogues as Tools to Characterize MDMA-Like Effects: An Approach to Understand Entactogen Pharmacology. Curr Neuropharmacol. 2013; 11: 521–34. [DOI] [PMC free article] [PubMed] [Google Scholar]
  36. Schenk S, Colussi-Mas J, Do J, Bird J. Profile of MDMA Self-Administration from a Large Cohort of Rats: MDMA Develops a Profile of Dependence with Extended Testing. Journal of Drug and Alcohol Research. 2012; 1: 1–6 [Google Scholar]
  37. Schenk S, Gittings D, Johnstone M, Daniela E. Development, maintenance and temporal pattern of self-administration maintained by ecstasy (MDMA) in rats. Psychopharm. 2003; 169: 21–27. [DOI] [PubMed] [Google Scholar]
  38. Schenk S, Hely L, Gittings D, Lake B, Daniela E. Effects of priming injections of MDMA and cocaine on reinstatement of MDMA- and cocaine-seeking in rats. Drug Alcohol Depend. 2008; 96:249–55. [DOI] [PubMed] [Google Scholar]
  39. Schenk S, Hely L, Lake B, Daniela E, Gittings D, Mash DC. MDMA self-administration in rats: acquisition, progressive ratio responding and serotonin transporter binding. Eur J Neurosci. 2007; 26: 3229–36. [DOI] [PubMed] [Google Scholar]
  40. Smith KM, Larive LL, Romanelli F. Club drugs: methylenedioxymethamphetamine, flunitrazepam, ketamine hydrochloride, and gamma-hydroxybutyrate. Am. J. Health-Syst. Pharm. 2002; 59: 1067–76. [DOI] [PubMed] [Google Scholar]
  41. Smith MA, Lacy RT, Strickland JC. The effects of social learning on the acquisition of cocaine self-administration. Drug Alcohol Depend. 2014; 141:1–8. [DOI] [PMC free article] [PubMed] [Google Scholar]
  42. Smith MA, Strickland JC, Bills SE, Lacy RT. The effects of a shared history of drug exposure on social choice. Behav Pharmacol. 2015; 26: 631–5. [DOI] [PMC free article] [PubMed] [Google Scholar]
  43. Smith M Peer influences on drug self-administration: social facilitation and social inhibition of cocaine intake in male rats. Psychopharm. 2012; 224: 81–90 [DOI] [PMC free article] [PubMed] [Google Scholar]
  44. Sumnall HR, Cole JC, Jerome L. The varieties of ecstatic experience: an exploration of the subjective experiences of ecstasy. J Psychopharmacol. 2006; 20: 670–82. [DOI] [PubMed] [Google Scholar]
  45. Tancer M, Johanson CE. Reinforcing, subjective, and physiological effects of MDMA in humans: a comparison with d-amphetamine and mCPP. Drug and alcohol dependence. 2003; 72: 33–44. [DOI] [PubMed] [Google Scholar]
  46. Tancer ME, Johanson CE. The subjective effects of MDMA and mCPP in moderate MDMA users. Drug and alcohol dependence. 2001; 65: 97–101. [DOI] [PubMed] [Google Scholar]
  47. Theall KP, Elifson KW, Sterk CE. Sex, touch, and HIV risk among ecstasy users. AIDS Behav. 2006; 10: 169–78. [DOI] [PubMed] [Google Scholar]
  48. Thompson MR, Callaghan PD, Hunt GE, McGregor IS. Reduced sensitivity to MDMA-induced facilitation of social behavior in MDMA pre-exposed rats. Progress in Neuro Psychopharmacology and Biological Psychiatry. 2008; 32: 1013–1021 [DOI] [PubMed] [Google Scholar]
  49. Thompson MR, Callaghan PD, Hunt GE, Cornish JL, McGregor IS. A role for oxytocin and 5-HT(1A) receptors in the prosocial effects of 3,4 methylenedioxymethamphetamine (“ecstasy”). Neuroscience. 2007;146:509–14. [DOI] [PubMed] [Google Scholar]
  50. Thompson MR, Hunt GE, McGregor IS. Neural correlates of MDMA (“Ecstasy”)-induced social interaction in rats. Social Neuroscience. 2009; 4: 60–72 [DOI] [PubMed] [Google Scholar]
  51. van de Wetering R, Schenk S. Repeated MDMA administration increases MDMA-produced locomotor activity and facilitates the acquisition of MDMA self-administration: role of dopamine D2 receptor mechanisms. Psychopharmacology. 2017; 234: 1155–64. [DOI] [PubMed] [Google Scholar]
  52. Vandewater SA, Creehan KM, Taffe MA. Intravenous self-administration of entactogen-class stimulants in male rats. Neuropharmacology. 2015; 99: 538–45. [DOI] [PMC free article] [PubMed] [Google Scholar]
  53. Wardle MC, de Wit H. MDMA alters emotional processing and facilitates positive social interaction. Psychopharmacology. 2014; 231: 4219–29. [DOI] [PMC free article] [PubMed] [Google Scholar]

RESOURCES