Methamphetamine increases risky choice in rats, but only when magnitude and probability of reinforcement are manipulated within a session.

Risky choice is associated with maladaptive behaviors, particularly substance use disorders. Current animal models of risky choice are often confounded by other constructs like behavioral flexibility and suboptimal choice. The purpose of the current experiment was to determine if the psychostimulant methamphetamine, a drug whose popularity has increased in recent years, increases risky choice in an equivalent expected value (EEV) task. In the EEV task, rats are given a choice between two reinforcer alternatives that differ in magnitude and probability of delivery, but have equivalent expected value. Forty-eight Sprague Dawley rats were tested in three versions of the EEV task. In the first version of the EEV task, both reinforcer magnitude and probability were adjusted across blocks of trials for both alternatives. In the second and the third versions of the EEV task, reinforcer magnitude was held constant across each block of trials (either 1 vs. 2 pellets or 4 vs. 5 pellets). We found that male rats preferred the "riskier" option, except when reinforcer magnitudes were held constant at 4 and 5 pellets across each block of trials. Methamphetamine (0.5 mg/kg) increased preference for the risky option in both males and females, but only when both reinforcer magnitude and probability were manipulated across blocks of trials for each alternative. The current results demonstrate that both magnitude of reinforcement and probability of reinforcement interact to influence risky choice. Overall, this study provides additional support for using reinforcers with expected value to measure risky choice.

Rats have low motivation to self-administer oral methamphetamine across increasing response requirements.

Methamphetamine (METH) is a psychostimulant drug that has become increasingly popular in recent years, with overdose deaths more than doubling during the second half of the 2010s. As methamphetamine use disorder rates continue to increase, finding effective treatment strategies to decrease METH dependence is important. Animal studies are well-suited for studying the neurobiological mechanisms underlying addiction-like behaviors. Although individuals can ingest METH orally, few studies have examined oral METH self-administration in animals. Mice show decreased responding for oral METH as the response requirement increases across sessions. The purpose of the current study was to determine if rats show a similar decrease in motivation to earn oral METH across increasing response requirements. Sixteen Sprague Dawley rats were trained to emit a response in an aperture to receive a 0.1-ml METH solution (40 mg/l) according to an FR 1 schedule. The FR requirement increased across sessions to a terminal FR 10. Responses for METH decreased significantly when an FR 10 schedule was used. These results suggest that rats, similarly to mice, have low motivation to self-administer oral METH.