Escalation of cocaine consumption in short and long access self-administration procedures

The Evolving Role of Animal Models in the Discovery and Development of Novel Treatments for Psychiatric Disorders

Historically, animal models have been routinely used in the characterization of novel chemical entities (NCEs) for various psychiatric disorders. Animal models have been essential in the in vivo validation of novel drug targets, establishment of lead compound pharmacokinetic to pharmacodynamic relationships, optimization of lead compounds through preclinical candidate selection, and development of translational measures of target occupancy and functional target engagement. Yet, with decades of multiple NCE failures in Phase II and III efficacy trials for different psychiatric disorders, the utility and value of animal models in the drug discovery process have come under intense scrutiny along with the widespread withdrawal of the pharmaceutical industry from psychiatric drug discovery. More recently, the development and utilization of animal models for the discovery of psychiatric NCEs has undergone a dynamic evolution with the application of the Research Domain Criteria (RDoC) framework for better design of preclinical to clinical translational studies combined with innovative genetic, neural circuitry-based, and automated testing technologies. In this chapter, the authors will discuss this evolving role of animal models for improving the different stages of the discovery and development in the identification of next generation treatments for psychiatric disorders.

Peri-adolescent exposure to (meth)amphetamine in animal models

Experimentation with psychoactive drugs is often initiated in the peri-adolescent period, but knowledge of differences in the outcomes of peri-adolescent- vs adult-initiated exposure is incomplete. We consider the existing animal research in this area for (meth)amphetamines. Established for a number of phenotypes, is lower sensitivity of peri-adolescents than adults to acute effects of (meth)amphetamines, including neurotoxic effects of binge-level exposure. More variable are data for long-term consequences of peri-adolescent exposure on motivational and cognitive traits. Moreover, investigations often exclude an adult-initiated exposure group critical for answering questions about outcomes unique to peri-adolescent initiation. Despite this, it is clear from the animal research that (meth)amphetamine exposure during the peri-adolescent period, whether self- or other-administered, impacts brain motivational circuitry and cognitive function, and alters adult sensitivity to other drugs and natural rewards. Such consequences occurring in humans have the potential to predispose toward unfortunate and potentially disastrous family, social and livelihood outcomes.

Methylenedioxymethamphetamine (MDMA): Serotonergic and dopaminergic mechanisms related to its use and misuse

Methylenedioxymethamphetamine (MDMA) is an amphetamine analogue that preferentially stimulates the release of serotonin (5HT) and results in relatively small increases in synaptic dopamine (DA). The ratio of drug-stimulated increases in synaptic DA, relative to 5HT, predicts the abuse liability; drugs with higher DA:5HT ratios are more likely to be abused. Nonetheless, MDMA is a drug that is misused. Clinical and preclinical studies have suggested that repeated MDMA exposure produces neuroadaptive responses in both 5HT and DA neurotransmission that might explain the development and maintenance of MDMA self-administration in some laboratory animals and the development of a substance use disorder in some humans. In this paper, we describe the research that has demonstrated an inhibitory effect of 5HT on the acquisition of MDMA self-administration and the critical role of DA in the maintenance of MDMA self-administration in laboratory animals. We then describe the circuitry and 5HT receptors that are positioned to modulate DA activity and review the limited research on the effects of MDMA exposure on these receptor mechanisms.

Acute cocaine treatment enhances the antagonistic allosteric adenosine A2A-dopamine D2 receptor-receptor interactions in rat dorsal striatum without increasing significantly extracellular dopamine levels

BACKGROUND

Antagonistic adenosine A2A receptor (A2AR)-dopamine D2 receptor (D2R) receptor-receptor interactions have previously been demonstrated in A2AR-D2R heteroreceptor complexes in the rat dorsal striatum. They mainly involve a reduction of affinity in the high-affinity component of the D2R agonist binding site upon activation in vivo of the A2AR by an A2AR agonist. Upon cocaine self-administration, this antagonistic A2AR-D2R interaction disappeared in the dorsal striatum.

METHODS

In the current experiments, it was tested whether such modifications in the antagonistic A2AR-D2R receptor-receptor interactions can develop also after an acute systemic injection of a low cocaine dose (1 mg/kg; sc).

RESULTS

Microdialysis experiments indicated that acute cocaine did not significantly alter the extracellular dopamine levels in the dorsal striatum of the awake Wistar rats. Competition dopamine receptor binding experiments demonstrated that in the acute cocaine group, the A2AR agonist CGS-21680 produced significantly larger increases in the D2R K_{i, High} values (reduction of high-affinity) versus the saline-injected (i.e. control) group. Furthermore, in the dorsal striatum membrane preparation from acute cocaine-injected rats, CGS-21680 also produced significant increases in the D2R K_{i, Low} values (reduction of low-affinity) and in the proportion of D2Rs in the high-affinity state (RH). Such significant effects were not observed with CGS-21680 in the control group.

CONCLUSIONS

The molecular mechanism involved in the acute cocaine-induced increase in the antagonistic allosteric A2AR-D2R receptor-receptor interactions may be an increased formation of higher-order complexes A2AR-D2R-sigma1R in which cocaine by binding to the sigma1R protomer also allosterically enhances the inhibitory A2AR-D2R interaction in this receptor complex.

Distinct relationships between risky decision making and cocaine self-administration under short- and long-access conditions

Substance use is strongly associated with impaired decision making, with cocaine use particularly linked to elevated risky and impulsive choice. It is not clear, however, whether such maladaptive decision making is a consequence of cocaine use or instead precedes and predisposes individuals to cocaine use. The current study was designed to specifically address the latter possibility with respect to risky choice in both male and female rats. Rats were first trained in a "Risky Decision-making Task" (RDT), in which they made discrete choices between a small, "safe" food reward and a large, "risky" food reward accompanied by increasing probabilities of mild footshock punishment. After reaching stable performance, rats underwent jugular catheter surgery followed by either short-access cocaine self-administration sessions (2 h, 0.5 mg/kg/infusion) for 5 days or long-access cocaine self-administration sessions (6 h, 0.5 mg/kg/infusion) for 14 days. Under short-access conditions, there was no relationship between risk preference and changes in cocaine intake over time, but greater risk aversion in females predicted greater overall cocaine intake. Under long-access conditions, heightened risk taking predicted greater escalation of cocaine intake over the course of self-administration, supporting the notion that pre-existing risk-taking behavior predicts cocaine intake. Collectively, results from these experiments have implications for understanding and identifying pre-existing vulnerabilities to substance use, which may lead to strategies to prevent development of substance use disorders.

Understanding Addiction Using Animal Models

Drug addiction is a neuropsychiatric disorder with grave personal consequences that has an extraordinary global economic impact. Despite decades of research, the options available to treat addiction are often ineffective because our rudimentary understanding of drug-induced pathology in brain circuits and synaptic physiology inhibits the rational design of successful therapies. This understanding will arise first from animal models of addiction where experimentation at the level of circuits and molecular biology is possible. We will review the most common preclinical models of addictive behavior and discuss the advantages and disadvantages of each. This includes non-contingent models in which animals are passively exposed to rewarding substances, as well as widely used contingent models such as drug self-administration and relapse. For the latter, we elaborate on the different ways of mimicking craving and relapse, which include using acute stress, drug administration or exposure to cues and contexts previously paired with drug self-administration. We further describe paradigms where drug-taking is challenged by alternative rewards, such as appetitive foods or social interaction. In an attempt to better model the individual vulnerability to drug abuse that characterizes human addiction, the field has also established preclinical paradigms in which drug-induced behaviors are ranked by various criteria of drug use in the presence of negative consequences. Separation of more vulnerable animals according to these criteria, along with other innate predispositions including goal- or sign-tracking, sensation-seeking behavior or impulsivity, has established individual genetic susceptibilities to developing drug addiction and relapse vulnerability. We further examine current models of behavioral addictions such as gambling, a disorder included in the DSM-5, and exercise, mentioned in the DSM-5 but not included yet due to insufficient peer-reviewed evidence. Finally, after reviewing the face validity of the aforementioned models, we consider the most common standardized tests used by pharmaceutical companies to assess the addictive potential of a drug during clinical trials.

Revisiting long-access versus short-access cocaine self-administration in rats: intermittent intake promotes addiction symptoms independent of session length

In rats, continuous cocaine access during long self-administration sessions (6 versus 1-2 hours) promotes the development of behavioral symptoms of addiction. This has led to the assumption that taking large amounts of drug during extended daily bouts is necessary to develop an addiction phenotype. Recent work shows that within-session intermittent access (IntA) to cocaine produces much less drug intake than continuous-access procedures (i.e. long-access sessions) but evokes addiction symptoms more effectively. IntA-sessions are also long, typically lasting 6 hours. It is not known whether IntA-sessions must be extended to promote addiction-relevant changes in drug use over time. Here, we determined the influence of IntA-session length on patterns of cocaine use relevant to addiction. Two groups of male Wistar rats self-administered cocaine (0.25 mg/kg/injection, injected over 5 seconds) during 18 daily IntA-sessions. One group had long 6-hour sessions (Long-IntA), the other group had shorter, 2-hour sessions (Short-IntA). Only Long-IntA rats escalated their cocaine intake over sessions, but both groups developed a burst-like pattern of drug use over time and similar levels of psychomotor sensitization. The two groups also showed robust and similar levels of both responding for cocaine under a progressive ratio schedule of reinforcement and cocaine-induced reinstatement of extinguished drug-seeking behavior. In summary, long IntA-sessions lead to greater cocaine intake than shorter IntA-sessions, but the two conditions are equally effective in evoking the patterns of drug-taking and drug-seeking that define addiction. This suggests that chronic intermittent cocaine use, even during short daily bouts, is sufficient to promote addiction symptoms.

Neurobiological substrates of persistent working memory deficits and cocaine-seeking in the prelimbic cortex of rats with a history of extended access to cocaine self-administration

Cocaine use disorder (CUD) is associated with prefrontal cortex dysfunction and cognitive deficits that may contribute to persistent relapse susceptibility. As the relationship between cognitive deficits, cortical abnormalities and drug seeking is poorly understood, development of relevant animal models is of high clinical importance. Here, we used an animal model to characterize working memory and reversal learning in rats with a history of extended access cocaine self-administration and prolonged abstinence. We also investigated immediate and long-term functional changes within the prelimbic cortex (PrL) in relation to cognitive performance and drug-seeking. Adult male rats underwent 6 days of short-access (1 h/day) followed by 12 days of long-access (6 h/day) cocaine self-administration, or received passive saline infusions. Next, rats were tested in delayed match-to-sample (DMS) and (non)match-to-sample (NMS) tasks, and finally in a single context + cue relapse test on day 90 of abstinence. We found that a history of chronic cocaine self-administration impaired working memory, though sparing reversal learning, and that the components of these cognitive measures correlated with later drug-seeking. Further, we found that dysregulated metabolic activity and mGlu5 receptor signaling in the PrL of cocaine rats correlated with past working memory performance and/or drug-seeking, as indicated by the analysis of cytochrome oxidase reactivity, mGlu5 and Homer 1b/c protein expression, as well as Arc mRNA expression in mGlu5-positive cells. These findings advocate for a persistent post-cocaine PrL dysfunction, rooted in ineffective compensatory changes and manifested as impaired working memory performance and hyperreactivity to cocaine cues. Considering the possible interplay between the neural correlates underlying post-cocaine cognitive deficits and drug-seeking, cognitive function should be evaluated and considered when developing neurobiologically-based treatments of cocaine relapse.

Contributions of prolonged contingent and non-contingent cocaine exposure to escalation of cocaine intake and glutamatergic gene expression

Similar to the pattern observed in people with substance abuse disorders, laboratory animals will exhibit escalation of cocaine intake when the drug is available over prolonged periods of time. Here, we investigated the contribution of behavioral contingency of cocaine administration on escalation of cocaine intake and gene expression in the dorsal medial prefrontal cortex (dmPFC) in adult male rats. Rats were allowed to self-administer intravenous cocaine (0.25 mg/infusion) under either limited cocaine-(1 h/day), prolonged cocaine-(6 h/day), or limited cocaine-(1 h/day) plus yoked cocaine-access (5 h/day); a control group received access to saline (1 h/day). One day after the final self-administration session, the rats were euthanized and the dmPFC was removed for quantification of mRNA expression of critical glutamatergic signaling genes, Homer2, Grin1, and Dlg4, as these genes and brain region have been previously implicated in addiction, learning, and memory. All groups with cocaine-access showed escalated cocaine intake during the first 10 min of each daily session, and within the first 1 h of cocaine administration. Additionally, the limited-access + yoked group exhibited more non-reinforced lever responses during self-administration sessions than the other groups tested. Lastly, Homer2, Grin1, and Dlg4 mRNA were impacted by both duration and mode of cocaine exposure. Only prolonged-access rats exhibited increases in mRNA expression for Homer2, Grin1, and Dlg4 mRNA. Taken together, these findings indicate that both contingent and non-contingent "excessive" cocaine exposure supports escalation behavior, but the behavioral contingency of cocaine-access has distinct effects on the patterning of operant responsiveness and changes in mRNA expression.

Modeling the development of drug addiction in male and female animals

An increasing emphasis has been placed on the development and use of animal models of addiction that capture defining features of human drug addiction, including escalation/binge drug use, enhanced motivation for the drug, preference for the drug over other reward options, use despite negative consequences, and enhanced drug-seeking/relapse vulnerability. The need to examine behavior in both males and females has also become apparent given evidence demonstrating that the addiction process occurs differently in males and females. This review discusses the procedures that are used to model features of addiction in animals, as well as factors that influence their development. Individual differences are also discussed, with a particular focus on sex differences. While no one procedure consistently produces all characteristics, different models have been developed to focus on certain characteristics. A history of escalating/binge patterns of use appears to be critical for producing other features characteristic of addiction, including an enhanced motivation for the drug, enhanced drug seeking, and use despite negative consequences. These characteristics tend to emerge over abstinence, and appear to increase rather than decrease in magnitude over time. In females, these characteristics develop sooner during abstinence and/or following less drug exposure as compared to males, and for psychostimulant addiction, may require estradiol. Although preference for the drug over other reward options has been demonstrated in non-human primates, it has been more difficult to establish in rats. Future research is needed to define the parameters that optimally induce each of these features of addiction in the majority of animals. Such models are essential for advancing our understanding of human drug addiction and its treatment in men and women.

Cocaine self-administration differentially affects allosteric A2A-D2 receptor-receptor interactions in the striatum. Relevance for cocaine use disorder

In the current study behavioral and biochemical experiments were performed to study changes in the allosteric A2AR-D2R interactions in the ventral and dorsal striatum after cocaine self-administration versus corresponding yoked saline control. By using ex vivo [(3)H]-raclopride/quinpirole competition experiments, the effects of the A2AR agonist CGS 21680 (100 nM) on the KiH and KiL values of the D2-like receptor (D2-likeR) were determined. One major result was a significant reduction in the D2-likeR agonist high affinity state observed with CGS 21680 after cocaine self-administration in the ventral striatum compared with the yoked saline group. The results therefore support the hypothesis that A2AR agonists can at least in part counteract the motivational actions of cocaine. This action is mediated via the D2-likeR by targeting the A2AR protomer of A2AR-D2-like R heteroreceptor complexes in the ventral striatum, which leads to the reduction of D2-likeR protomer recognition through the allosteric receptor-receptor interaction. In contrast, in the dorsal striatum the CGS 21680-induced antagonistic modulation in the D2-likeR agonist high affinity state was abolished after cocaine self-administration versus the yoked saline group probably due to a local dysfunction/disruption of the A2AR-D2-like R heteroreceptor complexes. Such a change in the dorsal striatum in cocaine self-administration can contribute to the development of either locomotor sensitization, habit-forming learning and/or the compulsive drug seeking by enhanced D2-likeR protomer signaling. Potential differences in the composition and stoichiometry of the A2AR-D2R heteroreceptor complexes, including differential recruitment of sigma 1 receptor, in the ventral and dorsal striatum may explain the differential regional changes observed in the A2A-D2-likeR interactions after cocaine self-administration.