Robert Ranaldi

NMDA receptor antagonism in the ventral tegmental area impairs acquisition of reward-related learning

Mechanisms underlying reward-related learning presumably involve neural plasticity integrating signals representing unconditioned and conditioned stimuli in regions mediating reward. The ventral tegmental area (VTA) receives such signals and shows synaptic plasticity which is NMDA receptor-dependent. To test the hypothesis that NMDA receptor stimulation in the VTA is necessary for the acquisition of food-reinforced appetitive learning, Long-Evans male rats were prepared with bilateral VTA cannulae and tested in operant chambers with the opportunity to lever press for food for 10 sessions. Animals received microinjections of AP-5 or vehicle immediately before sessions 1-4 and 10. AP-5 impaired acquisition of lever pressing during sessions 1-4 (but not when injected dorsal to the VTA). All groups increased lever pressing across sessions 5-9. On session 10, lever pressing was not affected regardless of treatment. In separate experiments, AP-5 failed to reduce free feeding, food reward or motor activity, suggesting that impairment in acquisition was not due to reduced food motivation or activity. NMDA transmission in the VTA thus appears to be necessary for the acquisition, but not expression, of reward-related learning.

Microinjections of SCH 23390 in the ventral tegmental area reduce operant responding under a progressive ratio schedule of food reinforcement in rats

We recently demonstrated that dopamine D1 receptors in the ventral tegmental area (VTA) are involved in intravenous cocaine reward. Here, we investigated whether VTA D1 receptors also are involved in food reward by testing the hypothesis that blockade of dopamine D1 receptors in the VTA attenuates the rewarding effects of food. Eighteen rats, with bilateral cannulae positioned to allow for microinjections in or just dorsal to the VTA, were trained to lever press under a progressive ratio schedule of reinforcement. After stable break points (BPs) were established, the rats received bilateral microinjections of SCH 23390, a D1 receptor antagonist. In Experiment 1, where the reward consisted of 1 food pellet, injections of SCH 23390 (0, 1, 2, or 4 microg/0.5 microl) in the VTA (N=9) significantly decreased BPs (P <0.001), while bilateral microinjections dorsal to the VTA (N=9) did not. In Experiment 2 (N=6), where the reward consisted of 1 or 2 food pellets, intra-VTA injections of SCH 23390 (0 and 4 microg/0.5 microl) decreased BPs at the 1 food pellet level (P <0.05), but not at the 2 food pellet level. Thus, the data showed that intra-VTA microinjections of SCH 23390 reduced the rewarding effects of food. This effect was surmountable by increasing food reward, ruling out motoric effects, and did not occur when injections were made dorsal to VTA, eliminating the possibility that the effect was caused by the dorsal diffusion of drug. These data suggest that dendritically released dopamine in the VTA plays a significant role in food reward.

The effects of VTA NMDA receptor antagonism on reward-related learning and associated c-fos expression in forebrain

The mechanisms whereby reward-associated stimuli come to function as conditioned stimuli and acquire the capacity to activate the same neural regions activated by primary rewards (i.e., dopamine terminal regions) is not fully understood. We hypothesized that NMDA receptor stimulation in the VTA is necessary for the acquisition by a CS to both produce conditioned approach and activate dopamine terminal regions. Rats were tested in a conditioned approach protocol that consisted of light stimulus-food conditioning sessions (30 randomly presented light stimulus-food pellet pairings), a session with no stimuli or food and 1 session with only light stimulus (CS-only) presentations. Food trough head entries during the CS and just prior to the CS were recorded and a CS/pre-CS ratio indicating the conditioned approach response was calculated. Brain tissue was harvested after the CS-only session and processed for c-fos expression in prefrontal cortex area 2, nucleus accumbens core and shell and medial and lateral caudate. When bilateral intra-VTA microinjections of AP-5 (0, 0.25 or 0.5 μg) were made prior to each of the conditioning sessions the 0.5 μg AP-5 dose prevented the acquisition of conditioned approach; when 0.5 μg AP-5 injections were made prior to the CS-only test they failed to affect expression of the response. Also, 0.5 μg AP-5 prior to conditioning significantly reduced c-fos expression in response to the CS in nucleus accumbens core. These results suggest that VTA NMDA receptor stimulation is necessary for both the acquisition of reward-related learning and acquisition by the CS to activate dopamine terminal regions.

Drug abstinence: exploring animal models and behavioral treatment strategies

Background and rationaleAn enormous amount of resources has been devoted to the development of pharmacotherapies for drug addiction, with relatively little or no long-term success reported. The current review argues that a successful drug addiction treatment program will likely be one that focuses on both the neural mechanisms and the environmental contingencies that mediate drug use. Further, because the neural mechanisms and environmental factors that support abstinence in humans are similar in laboratory animals, several animal models of abstinence and relapse have been developed. Thus, this review also compares the similarities in the mechanisms that lead to abstinence between animals and humans.ObjectiveWe evaluate the construct and face validities of the behavioral strategies that help support human drug abstinence. Further, we crucially evaluate animal models by assessing their validity and utility in addressing human behavior that leads to long-term abstinence.ConclusionsWe found that the behavioral strategies with the greatest likelihood of supporting long-term abstinence are those that are carried out in drug addicts’ natural setting(s) and while drug is readily available. Further, the behavioral strategies that may be most successful in supporting abstinence in humans are those that employ both positive consequences for abstinent related behavior and negative consequences for continued drug seeking or taking. Moreover, the animal models of abstinence and relapse that more closely represent the factors that support long-term abstinence in humans are those that limit their use of extinction or forced abstinence and present negative consequences for drug seeking and taking.

The novel dopamine D3 receptor antagonist, SR 21502, reduces cocaine conditioned place preference in rats

Research has shown that dopamine (DA) D3 receptors play a crucial role in cocaine addiction. Recently, there has been a strong focus on the development of DA D3 receptor antagonists as potential pharmacological treatments for cocaine addiction. We investigated the ability of a novel selective D3 receptor antagonist SR 21502 to block the expression of cocaine-induced conditioned place preference (CPP) in rats. CPP was determined using a two-chamber apparatus. All of the animals had free access to both chambers on day 1, followed by 4 alternating conditioning days of cocaine injection (paired chamber) and 4 alternating non-conditioning days with saline (non-paired chamber). On the test day, animals were systemically treated with 0, 3.75, 7.5 or 15mg/kg of SR 21502, 10min prior to being placed in the CPP apparatus, and the time spent in each chamber was recorded for 15min. The amount of time spent in the cocaine-paired chamber on the test and pre-exposure days was analyzed. Vehicle-treated animals spent significantly more time in the cocaine-paired side during the test than during the pre-exposure session, indicating a cocaine CPP. SR 21502 produced a dose-related significant reduction in the time spent in the cocaine-paired side compared to vehicle. The DA D3 receptor antagonist SR 21502 blocks the rats preference for the cocaine-paired chamber, thereby attenuating the rewarding effect of the cocaine cues. This suggests that this compound may be an effective pharmacological treatment against cocaine addiction.

Impulsive choice, as measured in a delay discounting paradigm, remains stable after chronic heroin administration

Heroin addicts display poorer impulse control than non-addicts, however it is not known if high impulsivity is a function of chronic heroin intake or a pre-disposing vulnerability for heroin addiction. Using animal models, relatively few studies have examined changes in impulsive choice as a function of chronic drug. The objective of this study was to measure alterations in impulsive choice through a delay discounting paradigm, as a function of chronic heroin administration. Animals were trained on a series of delay discounting sessions. Each session contained 5 blocks of trials. Blocks started with 2 forced, followed by 6 free choice trials. Pressing one lever resulted in the delivery of a small immediate (1 food pellet) reward and another lever in a large delayed (5 pellets) reward. Sessions consisted of the 3 ascending delay sequences in seconds. On the terminal sequence (0, 10, 20, 40, and 60s) animals exhibited a reversal of reward choice pattern of responding that allowed for the calculation of an indifference point (IP). After animals showed stable IPs they were treated with either heroin or saline for 12 days. Three days after the last injection animals were again placed in operant chambers and experienced the terminal delay discounting sequence at which time IPs were reassessed. Heroin-treated animals exhibited significant progressive increases in locomotor activity. Groups did not differ in IPs or performance across delay conditions during either before or after chronic treatment periods. These results indicate that chronic heroin intake does not impact later impulsive responding for natural (food) reward.

Environmental enrichment induces early heroin abstinence in an animal conflict model.

RATIONALE AND OBJECTIVES Heroin addiction is a significant health and societal problem for which there is no highly effective long-term behavioral or pharmacological treatment. Therefore, strategies that support heroin abstinence should be a primary focus of heroin treatment research. To this end, the current study used an animal conflict model that captures the aversive consequences of drug seeking (as are typical in humans, e.g., incarceration and job loss) to induce abstinence. Using this abstinence model, we examined the capacity of environmental enrichment (EE) to facilitate abstinence in heroin seeking rats. METHODS The procedure consisted of two phases: drug self-administration (phase 1) and electric barrier application (phase 2) that resulted in abstinence. For phase 1, male rats were trained to self-administer intravenous heroin under a fixed-ratio schedule of reinforcement. After self-administration was acquired, animals were housed either in EE or standard cages (non-EE control). During abstinence in phase 2, the electric barrier was introduced in the operant conditioning chambers by electrifying the floor area near the levers. RESULTS We found that EE rats achieved abstinence (zero active lever presses for 3 consecutive sessions) in significantly fewer sessions than NEE rats. Further, EE rats abstained at significantly lower electric currents than NEE rats. CONCLUSIONS EE facilitated abstinence in the conflict model. The current use of the abstinence-conflict model to investigate EE as a behavioral strategy to facilitate abstinence will help in the development of effective treatments for human addicts by bringing together the positive consequences of abstinent behavior in an enriched environment with the aversive consequences of drug seeking.

A food-associated CS activates c-Fos in VTA DA neurons and elicits conditioned approach.

Neutral stimuli associated with unconditioned stimuli (USs) acquire the ability to act as conditioned stimuli (CSs), which can elicit behaviors similar to the US with which they are associated. The neural mechanisms by which this occurs are not fully known. We have previously proposed a model stipulating CSs function as such because they acquire the capacity to activate dopamine (DA) neurons at the level of the ventral tegmental area (VTA). In the present experiments we hypothesized that a food-associated CS (light), which demonstrably functions as such by eliciting conditioned responses (CRs), comes to acquire the capacity to activate VTA DA neurons. In Experiment 1, rats were allowed to eat or not eat food (food being the US). In Experiment 2, rats were trained to retrieve food pellets after light presentations (the CS) and then tested for the expression of the food checking response (the CR) with only CS presentations. In Experiment 1, eating food (exposure to the US) caused a significantly greater number of VTA DA (TH-labeled) cells to express c-Fos than not eating. In Experiment 2, CS (light) presentations caused a significant amount of conditioned approach and a significantly greater number of VTA TH-labeled (DA) cells to express c-Fos. These findings support our model stipulating that conditioned approach learning occurs when CSs acquire the capacity to cause conditioned activation of VTA DA neurons.

Microinjections of a dopamine D1 receptor antagonist into the ventral tegmental area block the expression of cocaine conditioned place preference in rats

Stimulation of dopamine (DA) D1 receptors in the ventral tegmental area (VTA) is involved in primary rewards. In the current study we investigated whether VTA D1 receptor stimulation likewise plays a role in mediating the rewarding effects of cocaine-associated stimuli, using the cocaine conditioned place preference (CPP) paradigm. Rats were prepared with cannulae so as to allow microinjections in the VTA and later conditioned to a cocaine-associated environment using the CPP paradigm. Prior to each conditioning session rats were injected with either saline or cocaine (10mg/kg, intraperitoneally) and then placed in one of the two sides of the CPP apparatus. Sessions lasted 30min a day over a period of eight days, such that rats alternated daily between consistently experiencing cocaine in one side and saline in the other. On the test day, which was conducted one day after conditioning, rats were given bilateral microinjections of one of four doses of the D1 antagonist, SCH 23390, (0, 2, 4 or 8μg/0.5μl) directly into the VTA and allowed free access to both sides of the apparatus. Preference for either side was measured as time spent in each side and compared to the same measures taken before conditioning. The D1 antagonist produced a dose-related, significant reduction in the preference for the cocaine-paired side compared to vehicle. These data suggest that the expression of cocaine conditioned place preference requires stimulation of VTA D1 receptors and, as such, are the first to suggest a role for VTA dendritically released DA in cocaine-, or other reward-, related learning.

The selective dopamine D3 receptor antagonist, SR 21502, reduces cue-induced reinstatement of heroin seeking and heroin conditioned place preference in rats

BACKGROUND Because the role of dopamine (DA) D3 receptors has been investigated primarily in relation to cocaine-related behaviors little is known of the role of these receptors in heroin seeking. PURPOSES To investigate the effect of the selective DA D3 receptor antagonist, SR 21502, on cue-induced reinstatement of heroin seeking and heroin conditioned place preference (CPP). METHODS In experiment 1, rats were trained to self-administer intravenous heroin for 15 days followed by extinction. Following extinction animals were treated with one of several SR 21502 doses (0, 7.5, 10 or 15mg/kg) and a cue-induced reinstatement test was conducted. In experiment 2, animals were conditioned to experience heroin in one compartment of a CPP apparatus and saline in the other. On the test day animals were treated with 0, 3.75, 7.5, 10 or 15mg/kg of SR 21502 and tested for their CPP. RESULTS The results from experiment 1 showed a significant dose-related reduction in cue-induced reinstatement of active lever pressing in the 7.5 and 10mg groups and an absence of the reinstatement effect in the 15mg group. In experiment 2, animals treated with vehicle or 3.75mg of SR 21502 showed significant heroin place preferences but those treated with the higher doses showed no CPP. CONCLUSIONS Our findings suggest that DA D3 receptors play a significant role in heroin approach behaviors driven by conditioned stimuli. As such, we propose that SR 21502 holds potential as an effective pharmacotherapeutic agent for relapse prevention and should be studied further.