Dr. Mark Smith: Establishing Preclinical Models to Investigate the Role of Social Learning on the Acquisition of Cocaine Self-Administration
By Ziva Cooper, Ph.D., Animals in Research Committee Member
Epidemiological studies have shown repeatedly that one of the best prognosticators of adolescent or young adult drug use is whether an individual’s peers use drugs. The high concordance rates of drug use within peer groups can be explained by either selection theories or social learning theories. Selection theories propose that individuals choose or select their peers based on shared attitudes and behaviors regarding drug use. On the other hand, social learning theories propose that individuals learn drug use behaviors (or abstinent-related behaviors) by observing and imitating the behavior of their peers. Experimental studies addressing these theories have been limited because of a lack of preclinical models that allow laboratory animals to observe and mimic the drug use of others.
To circumvent this problem, Dr. Mark Smith and his colleagues constructed custom-built, ‘social learning’, drug-administration chambers that enabled two animals to intravenously self-administer drugs at the same time and in the same chamber. Thus, these chambers allowed these animals to observe (and mimic) one another’s behavior. A wire screen prevented one rat from accessing the intravenous line and response lever of its partner.
In order to determine the impact of observing a peer self-administer cocaine on the acquisition of cocaine use, Dr. Smith and colleagues assessed the behavior of three groups of rats. The first two groups were pair-housed and the third group was reared in isolation. For the first pair-housed group, one rat from each pair was trained to self-administer cocaine in isolation prior to reintroduction to its partner in the ‘social learning’ operant chamber where both rats had access to cocaine. For the second pair-housed group, neither had cocaine self-administration experience, and one rat from each pair did not have access to cocaine when placed in the ‘social learning’ operant chamber. The rats reared in isolation self-administered cocaine in standard (non-social-learning) test chambers.
Dr. Smith and his colleagues found acquisition of cocaine self-administration was fastest among rats partnered with peers that had cocaine-self-administration experience, whereas rats partnered with peers that were not self-administering cocaine were the slowest to acquire cocaine self-administration. These findings demonstrate that observing a peer using cocaine has a profound impact on the development of drug use, whereas exposure to an abstinent peer is protective in delaying drug self-administration. As such, these findings provide evidence to support social learning theories underlying the development of drug use.
The use of an animal model was an essential feature to this study that allowed the investigators to determine the cause-and-effect role that social contact with drug-using and abstinent peers have on the acquisition of drug use. These types of questions cannot be directly addressed in human populations, thus emphasizing the importance of animal models of human behavior in biomedical research.
Dr. Smith has been a member of CPDD since 1994. He currently serves on the Board of Directors and as Chair of the Animals in Research Committee. He previously served as Chair of the Awards Committee (2013-2017). Dr. Cooper has been a member of CPDD since 2013. She is a member-elect of the Board of Directors and currently serves as a member on the Animals in Research Committee. She has previously served on the Program Committee (2014-2017) and Nominating Committee (2016-2017).
Dr. Greg Collins: Preclinical Investigations of Synthetic Cathinones (i.e., “Bath Salts”)
By Brett Ginsburg, Ph.D., Animals in Research Committee Chair
Dr. Greg Collins is using procedures in laboratory animals to understand aspects related to designer psychostimulant abuse. The rise of designer recreational drugs has added further complication to our efforts to reduce drug addiction and abuse. These drugs are synthesized to mimic effects of existing recreational drugs, but often skirt detection and laws due to constantly changing, subtle alterations in their chemical structure. Of these new designer drugs, up to 20% are modified cathinones, often sold as ‘bath salts’ and marketed as alternatives to traditional psychostimulants like cocaine, methamphetamine, or MDMA. These drugs are commonly combined with other active ingredients in retail preparations; caffeine being among the most prominent co-constituents. Initially Dr. Collins expected that this adulteration simply represented a cost cutting measure by adding ‘filler’ to retail products. However, recent research from his preclinical laboratory indicates that the addition of caffeine to these products may impact their abuse liability and toxicity.
The prototypical synthetic cathinones are MDPV and methylone, though over 140 related compounds have been identified in illicit use. Unlike traditional psychostimulants that act by inhibiting dopamine and serotonin reuptake back into neurons similarly, MDPV is more potent at inhibiting dopamine reuptake compared to serotonin. Based on our understanding of the critical role dopamine plays in the abuse liability of recreational drugs and the modulatory role serotonin plays, Dr. Collins hypothesized that MDPV might have even greater abuse liability than cocaine or methamphetamine.
To explore this possibility, Dr. Collins uses intravenous self-administration procedures. He also uses these techniques to understand how the addition of caffeine to products containing synthetic cathinones impacts their abuse liability. In one procedure, laboratory rats press a lever a fixed number of times for each delivery of the drug or drug combination. In another, rats must make progressively more lever presses for each subsequent drug delivery. Using these techniques, Dr. Collins can assess the reinforcing potency and effectiveness of synthetic cathinones alone or combined with caffeine, and then compare these results to more traditional psychostimulants such as cocaine and methamphetamine.
In his studies, Dr. Collins has found that synthetic cathinones maintain substantially higher amounts of behavior than cocaine or methamphetamine, indicating potentially greater abuse liability. Using dose-addition analysis, he also found that the addition of caffeine further enhances the effectiveness of synthetic cathinones to maintain behavior, and also increases their potency to do so. This suggests that caffeine may be an important constituent in these products that may further increase their misuse. This is consistent with clinical reports that cathinone products can engender prolonged bouts of use that exceed the patterns of use of more traditional psychostimulants.
Dr. Collins is beginning to investigate other aspects of this interaction, particularly the potential that combinations of synthetic cathinones and caffeine are more toxic than either drug alone. To this end, he has begun assessing cardiopulmonary effects of these drugs and their combination. If these combinations are both more liable to abuse and produce greater toxicity, it would focus efforts on increasing awareness about these potential problems among psychostimulant users.
Dr. Collins’ preclinical research is important in several ways. It is illuminating an often under-appreciated aspect of substance abuse; that other constituents in a product can interact with the primary ingredient to produce unexpected and often detrimental effects. It also represents an attempt to better model patterns of human substance use in animal models, by examining pharmacological interactions that occur in polysubstance abuse, the most common pattern seen in humans.
Dr. Collins has regularly attended the CPDD annual meeting since 2004 and has been a regular member since 2011. He is active as a member of the Programming Committee. Dr. Ginsburg has been a member of CPDD since 2006 and currently serves on the Animals in Research Committee.
Dr. Cassie Gipson-Reichardt: Neuroinflammation as a novel mechanism underlying nicotine relapse
By August Holtyn, Ph.D., Animals in Research Committee Chair
Tobacco use is the greatest preventable cause of death, cancer, and illness in the United States. Some smoking cessation interventions are effective, but the majority of individuals who stop smoking relapse within a year. A better understanding of the mechanisms underlying vulnerability to relapse can help pave the way for more effective treatments.
Mark Namba and Dr. Cassie Gipson-Reichardt in the Neurobiology and Behavior in Addiction Lab at Arizona State University use an animal model of cue-induced relapse to study neurobiological substrates underlying relapse vulnerability. Under this model, rats are trained to self-administer nicotine, the primary active alkaloid in tobacco. Then, rats are placed into extinction training for several days, in which self-administration responses no longer produce nicotine or nicotine-paired cues. Following extinction, cue-induced nicotine relapse is tested by allowing the rats to respond in the presence of cues that had previously been paired with nicotine delivery.
In an ongoing study, Namba and Dr. Gipson-Reichardt are using this model to examine whether drug-induced neuroinflammation may underlie vulnerability to nicotine relapse. Chronic nicotine self-administration is associated with enduring alterations in glutamatergic plasticity within the nucleus accumbens core, including dysregulation of glial glutamate transport. The nuclear factor kappa B (NF-κB) pathway mediates drug-induced neuroinflammation and is a key regulator of synaptic plasticity. Notably, little is known about NF-κB’s role in cue-induced nicotine relapse.
To address this limitation, Namba and Dr. Gipson-Reichardt are using a herpes virus gene transfer strategy to express several forms of IκB kinase (IKK) to bi-directionally modulate activation of downstream NF-κB. Additionally, they and others have demonstrated that N-acetylcysteine, an antioxidant and glutamatergic agent, inhibits cue-induced nicotine relapse and are currently examining whether NF-κB signaling underlies its therapeutic efficacy. Preliminary results suggest that NF-κB signaling mediates cue-induced nicotine relapse. Specifically, inhibition of NF-κB inhibits cue-induced reinstatement, whereas activation of NF-κB drives reinstatement and blocks the attenuating effect of N-acetylcysteine on reinstatement.
Overall, these findings are significant because they highlight the potential role for neuroimmune signaling as a modulatory mechanism that regulates glutamatergic synaptic plasticity and subsequent drug-seeking behavior, thus revealing a new avenue for the development of novel pharmacotherapeutics aimed at reducing the risk of nicotine relapse.
Dr. Gipson-Reichardt has been a member of CPDD since 2013; she currently serves on the Program Committee. Dr. Holtyn has been a member of CPDD since 2013; she currently serves on the Animals in Research Committee.
*This work was supported by the National Institutes of Health Grant DA036569 and -S1.
Dr. Wendy Lynch: Discovering Novel Interventions for Cocaine Addiction Using an Animal Model of Relapse
By Mark Smith, Ph.D., Animals in Research Committee Chair
Although cocaine use is a leading cause of overdose deaths in the US, second only to opioids, there are currently no FDA-approved medications for its treatment. Exercise shows promise as a treatment option given its potential to reduce drug craving and vulnerability to relapse, but studies conducted in clinical populations have yield variable findings.
Dr. Wendy Lynch and her colleagues at the University of Virginia use an animal model of relapse to examine behavioral and pharmacological treatments that might reduce relapse vulnerability. Dr. Lynch trains laboratory rats to self-administer cocaine by pressing a response lever. Later, cocaine is removed and an exercise wheel is introduced during either early or late abstinence. Relapse vulnerability is then tested by allowing the rats to respond on the lever in the presence of cues that had previously been predictive of cocaine delivery.
Using this model, Dr. Lynch and her colleagues recently reported that exercise introduced early, but not late, during abstinence is critical for its efficacy to reduce relapse vulnerability. They then determined the mechanism by which exercise introduced early during abstinence may reduce relapse to cocaine use.
Dr. Lynch and her colleagues found that the protective effects of exercise were associated with metabotropic glutamate receptor 5 expression (mGluR5) in the prefrontal cortex. Glutamate signaling in the prefrontal cortex is known to be involved in the development of compulsive drug use, and this finding suggests that exercise may be producing its protective effects by altering signaling in this area.
To examine this possibility, they injected a mGluR5 receptor agonist directly into the prefrontal cortex during early abstinence and then tested the rats in their model of relapse. As they predicted, stimulation of mGluR5 receptors in the prefrontal cortex simulated the protective effect of early-initiated exercise and reduced relapse vulnerability.
These results are the first to show that exercise initiated during early abstinence functions as an effective treatment intervention for cocaine addiction by normalizing dysregulated mGluR5 signaling in the prefrontal cortex. The use of an animal model of relapse allowed Dr. Lynch to determine the causal mechanisms by which exercise was producing its protective effects on relapse vulnerability. These findings are important for the development of treatment interventions in human populations by showing that relapse can be prevented through exercise and/or medications that upregulate glutamatergic signaling in the prefrontal cortex during early abstinence.
Dr. Lynch has been a member of CPDD since 1997; she has previously served on the Program Committee (2011-2013) and the Animals in Research Committee (2009-2012). Dr. Smith has been a member of CPDD since 1994. He currently serves on the Board of Directors and as Chair of the Animals in Research Committee. He previously served as Chair of the Awards Committee (2013-2017).
Dr. Sarah Withey: Concurrent Assessment of the Antinociceptive and Behaviorally Disruptive Effects of Opioid Analgesics in a Preclinical Model
By Roger Spealman, Ph.D., Animals in Research Committee Member
Opioid analgesics remain the primary treatment for moderate to severe pain, but often cause behavioral impairment that constrains their therapeutic utility. Preclinical antinociception assays in animals have been indispensable in the development of analgesic medications, but have not typically provided concurrent assessment of behaviorally disruptive effects.
To address this shortcoming, Sarah Withey and colleagues Carol Paronis and Jack Bergman of the Preclinical Pharmacology Laboratory of McLean Hospital, Harvard Medical School modified a previously validated warm-water tail withdrawal assay to measure tail withdrawal latencies (antinociception) concurrently with disruption of food-reinforced operant responding (behavioral impairment).
A total of six different opioids were tested, and each resulted in dose-dependent antinociception and varying degrees of behavioral impairment. The researchers used a preclinical therapeutic ratio as a measure of a drug’s behavioral selectivity, which was defined as the ED50 for behavioral impairment divided by the ED50 for antinociception. The preclinical therapeutic ratio differed widely among the drugs. Nalbuphine had the highest therapeutic ratio (4.88) indicating significant antinociception at doses that did not cause behavioral impairment, whereas butorphanol had the lowest therapeutic ratio (0.17) indicating significant behavioral impairment at doses that were not sufficient to produce antinociception. Other opioids, including oxycodone, heroin, buprenorphine and methadone, had therapeutic ratios approaching 1.0, indicating that the antinociceptive and behaviorally impairing effects were produced by similar doses.
These results are important because they demonstrate the utility of this animal model to concurrently measure antinociception and behavioral impairment, which may provide a useful technique for predicting the selectivity with which novel analgesics exert their therapeutic effects.
Dr. Withey is new to the CPDD family; Dr. Bergman has been a member of CPDD since 1993 and currently serves as the Treasurer of the organization; Dr. Spealman has been a member of CPDD since 1992 and served as a Board of Director from 2010-2013.
This research is published in The Journal of Pain.
Dr. Michael Nader: Using Animal Models to Evaluate the Relationship Between Social Status and Social Stress
By Justin C. Strickland, M.S., Animals in Research Committee Member
Stress is an important predictor of substance use and misuse. For example, early-life or chronic stress can contribute to the development of disordered patterns of substance use. Similarly, stressful events may precipitate relapse and continued substance use among recovering individuals. However, not everyone reacts to the effects of stress equally. Animal models can help provide insight into the behavioral, physiological, and neural correlates underlying differential responses to environmental events, such as stress.
New research by Dr. Michael Nader and colleagues at Wake Forest School of Medicine uses a nonhuman primate model of cocaine use to show that social rank is an important factor related to the behavioral and biological response to social stress. This study is a part of Dr. Nader’s broader research program evaluating the neurobiological and behavioral consequences of social status on measures relevant to cocaine and other substance use disorders.
The first experiment in this study began by examining cocaine self-administration in socially housed, male cynomolgus macaques using a food-drug choice procedure. This food-drug choice procedure provides a translationally relevant measure of substance use by modeling the decision to use drugs in the presence of a competing non-drug reinforcer (i.e., food). Subjects were also divided into a dominant or subordinate group based on social behaviors observed in the home cage. Dominant and subordinate monkeys were tested under conditions such that similar rates of self-administration were observed under these “baseline” conditions.
Following baseline testing, subjects were exposed to social stress using a resident/intruder procedure. In this procedure, subjects were placed into another social group as an “intruder” for 30 minutes (note that the resident monkeys could not physically contact the intruder). The effects of this social stress on cocaine self-administration were then determined using the same food-drug choice paradigm. A majority of subjects were affected by the social stress manipulation, however the direction of this effect depended on the monkey’s social rank. Specifically, being an intruder in another social group produced increases in sensitivity to the reinforcing effects of cocaine in subordinate monkeys, but produced decreases in sensitivity to the reinforcing effects of cocaine in dominant monkeys. The former effects resemble the consequences of stress, while the latter outcome is what would be hypothesized to occur following an enriching event. Importantly, the same environmental manipulation had different consequences depending on the social rank of the monkey.
A second experiment in this study attempted to reveal what neurobiological mechanisms mediated this social rank-stress relationship. To this end, PET imaging was used to study brain glucose metabolism in dominant and subordinate monkeys at baseline and following intruder stress. Clear differences between the dominant and subordinate monkeys were observed under both conditions. At baseline, dominant monkeys showed greater activity in regions associated with visual processing, attentional control, and vigilance, whereas subordinate monkey showed greater activity in regions associated with emotional processing, fear, and anxiety. Social stress increased activity along the HPA-axis in both groups with enhanced activity in the posterior cingulate cortex (a region associated with risk to cocaine relapse) in subordinate monkeys.
Dr. Nader and colleagues’ study highlights the importance of preclinical animal models for identifying individual phenotypes conferring resilience or susceptibility to environmental events relevant to substance use. Although food-drug choice behavior was similar between dominant and subordinate monkeys under baseline conditions, acute social stress produced marked differences in responding based on a subject’s social rank. Animal models, such as these, can help detect clinically relevant factors underlying different patterns of behavior observed in the human clinical condition. Such an approach can also reveal behavioral, physiological, and neural markers to promote individualized approaches in treating substance use disorder.
Michael Nader has been a member of CPDD since 1988 and served on the Board of Directors from 2006-2010, the Drug Testing and Evaluation Liaison Committee (2000-2003; 2007-2010), being Chair of that committee from 2007-2009, and is a past member of the Nomination Committee (2010). Justin Strickland has been a member of CPDD since 2014 and currently serves on the Animals in Research and Travel Award committees.
The study was supported by the National Institute on Drug Abuse.
Gould, R. W., Czoty, P. W., Porrino, L. J., & Nader, M. A. (2017). Social status in monkeys: effects of social confrontation on brain function and cocaine self-administration. Neuropsychopharmacology, 42(5), 1093-1102.