Kevin T. Ball

Acute effects of 3,4-methylenedioxymethamphetamine on striatal single-unit activity and behavior in freely moving rats: differential involvement of dopamine D1 and D2 receptors

3,4-Methylenedioxymethamphetamine (MDMA) is a widely abused amphetamine derivative that increases dopamine (DA) and serotonin release via a reverse transport mechanism. Changes in the activity of striatal neurons in response to increased DA transmission may shape the behavioral patterns associated with amphetamine-like stimulants. To determine how the striatum participates in MDMA-induced locomotor activation, we recorded the activity of >100 single units in the striatum of freely moving rats in response to a dose that increased motor activation (5.0 mg/kg). MDMA had a predominantly excitatory effect on neuronal activity that was positively correlated with the magnitude of locomotor activation. Categorizing neurons according to baseline locomotor responsiveness revealed that MDMA excited significantly more neurons showing movement-related increases in activity compared to units that were non-movement-related or associated with movement-related decreases in activity. Further analysis revealed that the drug-induced striatal activation was not simply secondary to the behavioral change, indicating a primary action of MDMA on striatal motor circuits. Prior administration of SCH-23390 (0.2 mg/kg), a D(1) antagonist, resulted in a late onset of MDMA-induced locomotion, which correlated positively with delayed neuronal excitations. Conversely, prior administration of eticlopride (0.2 mg/kg), a D(2) antagonist, completely abolished MDMA-induced locomotion, which paralleled its blockade of MDMA-induced excitatory neuronal responses. Our results highlight the importance of striatal neuronal activity in shaping the behavioral response to MDMA, and suggest that DA D(1) and D(2) receptors have distinct functional roles in the expression of MDMA-induced striatal and locomotor activation.

Reinstatement of MDMA (ecstasy) seeking by exposure to discrete drug-conditioned cues

The widely used recreational drug MDMA (ecstasy) supports self-administration in animals, but it is not known whether MDMA-associated cues are able to reinstate drug seeking in a relapse model of drug addiction. To assess this possibility, drug-naïve rats were trained to press a lever for MDMA infusions (0.30 mg/kg/infusion, i.v.) paired with a compound cue (light and tone) in daily 2 h sessions. Responding was reinforced contingent on a modified fixed-ratio 5 schedule of reinforcement. Conditioned cue-induced reinstatement tests were conducted after lever pressing was extinguished in the absence of MDMA and the conditioned cues. Conditioned cues reinstated lever pressing after extinction, and the magnitude of reinstatement was positively correlated with the level of responding during MDMA self-administration. These results show for the first time that conditioned cues can trigger reinstatement of MDMA-seeking behavior in rats, and that individual differences in the pattern of MDMA self-administration can predict the magnitude of reinstatement responding.

SENSITIZING REGIMENS OF (±)3, 4-METHYLENEDIOXYMETHAMPHETAMINE (ECSTASY) ELICIT ENDURING AND DIFFERENTIAL STRUCTURAL ALTERATIONS IN THE BRAIN MOTIVE CIRCUIT OF THE RAT

Repeated, intermittent exposure to the psychomotor stimulants amphetamine and cocaine induces a progressive and enduring augmentation of their locomotor-activating effects, known as behavioral sensitization, which is accompanied by similarly stable adaptations in the dendritic structure of cortico-striatal neurons. We examined whether repeated exposure to the increasingly abused amphetamine derivative 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) also results in long-lasting behavioral and morphological changes in mesocortical (medial prefrontal cortex) and ventral striatal (nucleus accumbens) neurons. Rats received two daily injections of either 5.0 mg/kg (+/-)-MDMA or saline vehicle, approximately 6 h apart, for 3 consecutive days, followed by 4 drug-free days for a total of 3 weeks. Following a 4-week drug-free period, MDMA-pretreated rats displayed behavioral sensitization, as well as large increases in spine density and the number of multiple-headed spines on medium spiny neurons in core and shell subregions of nucleus accumbens. In medial prefrontal cortex, the prelimbic subregion showed increased spine density on distal dendrites of layer V pyramidal neurons, while the anterior cingulate subregion showed a change in the distribution of dendritic material instead. Collectively, our results show that long-lasting locomotor sensitization to MDMA is accompanied by reorganization of synaptic connectivity in limbic-cortico-striatal circuitry. The differential plasticity in cortical subregions, moreover, suggests that drug-induced structural changes are not homogeneous and may be specific to the circuitry underlying long-term changes in drug-seeking and drug-taking behavior.

Context-dependent behavioural and neuronal sensitization in striatum to MDMA (ecstasy) administration in rats.

To investigate the neuronal mechanisms underlying the behavioural alterations that accompany repeated exposure to MDMA (ecstasy), we recorded the activity of > 200 striatal units in response to multiple, intermittent, locomotor‐activating doses (5.0 mg/kg) of MDMA. Rats were treated with once‐daily injections of either saline or MDMA for 5 days when housed in their home cage, followed by a challenge injection 3–5 days later when housed in a recording chamber. Because contextual drug associations might be particularly important to the expression of behavioural sensitization to chronic MDMA, a separate group of rats received repeated injections of MDMA alternately in the recording chamber or home cage, according to the above timeline. A sensitized locomotor response was observed only in rats that had previously experienced MDMA in the context of the recording chamber, and only on the challenge day. These sensitized animals also showed a decreased basal firing rate in neurons that were subsequently excited by MDMA when compared with the same category of neurons earlier in the treatment regimen. This resulted in a greater percentage increase from the baseline firing rate on the challenge day compared with the first and fifth days of treatment, even though this trend was not evident with an analysis of absolute firing rate. These results strongly support a role for context in the expression of MDMA‐induced locomotor sensitization, and implicate striatal involvement in the neurobehavioural changes associated with the repeated use of MDMA.

Opposing roles for dopamine D1- and D2-like receptors in discrete cue-induced reinstatement of food seeking.

We used a rat reinstatement model to test the involvement of dopamine D1- and D2-like receptors in discrete cue-induced reinstatement of food seeking. At a dose that did not alter responding during food self-administration, the D1-like antagonist SCH-23390 blocked reinstatement of food seeking, while the D2-like antagonist eticlopride significantly increased reinstatement responding. Thus, these results establish opposing roles for D1- and D2-like receptors in discrete cue-induced relapse to food seeking.

Electrophysiological and structural alterations in striatum associated with behavioral sensitization to (±)3,4-methylenedioxymethamphetamine (ecstasy) in rats: role of drug context

We examined whether repeated exposure to the increasingly abused amphetamine (AMPH) derivative 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) results in long-lasting neurobehavioral changes, and further, the ability of contextual cues to modulate these changes. We focused on dorsal striatum, a brain region implicated in the formation of persistent drug-related habits. Rats were transported to a novel recording chamber and treated with once-daily injections (s.c.) of (±)-MDMA (5.0 mg/kg) or saline for 5 days, followed by a challenge injection 14 days later either in the same (Experiment 1) or different context (Experiment 2). Chronically implanted micro-wire bundles were used to record from populations of striatal neurons on days 1, 5, and challenge. Twenty-four hours after the last injection, brains were removed and processed using a modified Golgi method to assess changes in neuronal morphology. A sensitized locomotor response was observed following MDMA challenge in 11 of 12 rats in Experiment 1 (same context), whereas only 58% of rats (7 of 12) displayed sensitization in Experiment 2 (different context). Furthermore, several alterations in striatal electrophysiology were apparent on challenge day, but only in rats that displayed sensitization. Conversely, structural changes in striatal medium spiny neurons, such as increases in spine density, were observed in MDMA-treated rats regardless of whether they displayed behavioral sensitization. Thus, it appears that reorganization of synaptic connectivity in dorsal striatum may contribute to long-lasting drug-induced behavioral alterations, but that these behavioral alterations are subject to modification depending on individual differences and the context surrounding drug administration.

Yohimbine reinstates extinguished 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) seeking in rats with prior exposure to chronic yohimbine

Although exposure to acute stress has been shown to reinstate extinguished responding for a wide variety of drugs, no studies have investigated stress-induced reinstatement in animals with a history of 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) self-administration. Thus, rats were trained to press a lever for MDMA (0.50 mg/kg/infusion) in daily sessions, and lever pressing was subsequently extinguished in the absence of MDMA and conditioned cues (light and tone). We then tested the ability of acute yohimbine (2.0 mg/kg), a pharmacological stressor, to reinstate lever-pressing under extinction conditions. Additionally, to model chronic stress, some rats were injected daily with yohimbine (5.0 mg/kg × 10 days) prior to reinstatement tests. To assess dopaminergic involvement, chronic yohimbine injections were combined with injections of SCH-23390 (0.0 or 10.0 μg/kg), a dopamine D1-like receptor antagonist. In a separate experiment, rats with a history of food self-administration were treated and tested in the same way. Results showed that acute yohimbine injections reinstated extinguished MDMA and food seeking, but only in rats with a history of chronic yohimbine exposure. Co-administration of SCH-23390 with chronic yohimbine injections prevented the potentiation of subsequent food seeking, but not MDMA seeking. These results suggest that abstinent MDMA users who also are exposed to chronic stress may be at increased risk for future relapse, and also that the effects of chronic stress on relapse may be mediated by different mechanisms depending on ones drug use history.

Effects of repeated yohimbine administration on reinstatement of palatable food seeking: involvement of dopamine D1 -like receptors and food-associated cues.

Acute exposure to the pharmacological stressor yohimbine induces relapse to both food and drug seeking in a rat model. However, no systematic studies on the effects of chronic stress on relapse have been conducted. Because chronic stress causes changes in dopamine D1‐like receptor‐mediated transmission in prefrontal cortex (a relapse node), we tested the hypothesis that chronic exposure to stress increases vulnerability to relapse via dopamine‐mediated mechanisms. Additionally, to determine the role of food‐conditioned cues in reinstatement of food seeking, we made discrete food‐paired cues either available (CS Present) or not available (CS Absent) during extinction and reinstatement testing. Rats responded for palatable food reinforcers in daily 3‐hour sessions, and the behavior was extinguished. To model chronic stress, rats were injected daily with yohimbine (0.0, 2.5, or 5.0 mg/kg; i.p.) during the first 7 days of extinction. Injections were combined with SCH‐23390 (0.0, 5.0, or 10.0 µg/kg; i.p.), a D1‐like receptor antagonist. Rats were then tested for reinstatement of food seeking triggered by acute yohimbine (0.0, 1.0, or 2.0 mg/kg; i.p.) and pellet priming. Rats treated previously with chronic yohimbine displayed increased responding following acute yohimbine priming relative to non‐chronically stressed rats, but in the CS Absent condition only. Conversely, the lower dose of chronic yohimbine caused an increase in pellet‐primed reinstatement, but this effect was more pronounced in the CS Present condition. Importantly, SCH‐23390 combined with repeated yohimbine injections attenuated these effects. Thus, chronic stress may increase vulnerability to relapse under specific circumstances via a dopamine D1‐like receptor‐mediated mechanism.

Low-dose oral caffeine induces a specific form of behavioral sensitization in rats

The present study assessed the effects of a low dose of orally administered caffeine on sensitization of open-field behavior in rats. Rats had free access to untreated water every day or water containing 0.2 mg/ml of caffeine every other day of the 14-day experiment. On alternate days discrete movements (horizontal and vertical) and ambulatory distance were measured in open-field activity monitors. Although caffeine intake significantly decreased across test sessions in caffeine-treated rats, the number of discrete horizontal movements significantly increased. These findings suggest that low doses of orally administered caffeine induce a specific form of behavioral sensitization in rats.

Effects of Fixed Ratio Schedules of Reinforcement on Exercise by College Students

Response-contingent reinforcement was expected to increase the time spent riding a stationary exercise cycle by college women. A single-subject experimental design was used. Three college-age women rode a stationary cycle until they were either too tired to continue or until 45 minutes elapsed. Video clips were used to reinforce cycling in the first part of the experiment. Under some conditions, no video clips were presented (baseline). In other conditions, the video was on for the entire session or 25 sec. of video was presented after the completion of FR 40 (40 rotations of the cycles pedals) or 80 (80 rotations of the cycles pedals) schedules. In the second part of the experiment, money was presenred following every 20 (FR 20) or 40 (FR 40) rotations of the cycles pedals. The time spent riding the cycle increased over the baseline when video clips were presented. No consistent differences were observed when the video clips were on for the entire session and when they were presented under FR 40 and FR 80 schedules. For one out of two participants, monetary reward increased the time spent riding the cycle. The data suggest that reinforcement schedules are a means of initiating and maintaining adherence to an exercise program.