Gambling Rats: Insight into Impulsive and Addictive Behavior

Although nothing more than a harmless recreation for most, gambling can become a ‘behavioural addiction' comparable to drug dependency (Potenza, 2008). The incidence of such pathological gambling (PG) in the USA is estimated at 1–5%, yet current treatments are limited and often ineffective. Animal models of substance use have made a significant contribution to our understanding of drug addiction. Modeling gambling processes in non-human subjects could likewise facilitate our understanding of the drive to gamble, and stimulate the development of pharmacotherapies.

Clinically, one of the most common tests of gambling-related decision-making is the Iowa Gambling Task (IGT), in which subjects choose from four decks of cards to win money or points (Bechara et al, 1994). Two of the decks are associated with bigger immediate rewards but disproportionately large losses. The correct approach is therefore to favour decks which deliver smaller amounts but also lower penalties, thereby leading to greater long-term gain. Using the IGT as a template, novel rodent models of gambling behaviour have been developed in which rats play against the clock to earn as many sugar pellets as possible by sampling between four different options. The amount of reward available on each option varies in size (1–4 pellets). On a given trial, the animal receives either the set reward or a punishing ‘time-out' during which reward cannot be earned. The reinforcement schedules are fixed such that the larger pellet options are associated with a greater frequency or duration of time-outs, decreasing their net worth. Rats must therefore learn to avoid risky options associated with larger rewards, analogous to the optimal strategy on the IGT. Recent data indicate that rats are capable of ‘playing the odds' in this way. Furthermore, such choice behaviour can be modulated by drugs which target the serotonin and dopamine systems, with a D₂ receptor antagonist enhancing performance, while amphetamine and a serotonin 1A receptor agonist impair choice (Zeeb et al, 2009). Animals which prefer the maladaptive ‘high-risk high-reward' options also show elevated preference for exposed or novel environments, indicating this choice pattern may be a valid marker for risk-prone behaviour (Rivalan et al, 2009).

Human and animal studies indicate that highly impulsive individuals, particularly those showing elevated levels of motor impulsivity, are more vulnerable to both cocaine dependency and PG (Verdejo-Garcia et al, 2008). Whether this form of impulsivity represents an endophenotype for chemical and behavioural addictions remains an interesting hypothesis now open to empirical verification. Demonstrating that rodents can engage in a decision-making process reminiscent of gambling, and that such decision-making can be pharmacologically modified, is certainly a step forward. As with research into substance abuse, however, it will be important to dissociate the act of performing a potentially-addictive behaviour and actually exhibiting symptoms of dependence. Some of the DSM-IV criteria for drug dependency have been translated into behavioural measurements applied to rats self-administering cocaine, theoretically demarcating dependent subjects (Belin et al, 2008). A future challenge for the field will be to create a similar standard for gambling models to help identify subjects exhibiting a PG-like phenotype.

The author declares no conflict of interest.