Study on learning in rats finds MDMA does not reduce sensitivity to reinforcement

MDMA does not reduce sensitivity to reinforcement, according to a study published this April in Behavioral Neuroscience. This is despite evidence of it causing disruptions to learning and memory, along with reductions in sensitivity to reinforcement reported in similar drugs.

MDMA, also called ecstasy when in pill form, is a psychoactive drug used primarily for recreational purposes. People usually take the drug because of its effects on increasing empathy and euphoria, and heightening sensations. The main ways that MDMA can cause this is by increasing the amount of the neurotransmitter serotonin (related to mood and happiness) and dopamine (related to a sense of reward and pleasure) in the brain.

Despite this, MDMA can produce a variety of adverse health effects, some of which are psychological. Most research into these effects have been conducted using animals, with research finding impairments in learning and memory – including in short-term memory, spatial learning and memory, reference memory and working memory.

Research into similar drugs has found that they cause reduced “sensitivity to reinforcement”. Sensitivity to reinforcement is the awareness of the relationship between recent events, the individual’s own behavior, and the effect this has on their future behavior. Short-term administration of d-amphetamine, which is used to treat attention deficit hyperactivity disorder (ADHD) and narcolepsy, has been shown to reduce sensitivity to reinforcement. Given the effect d-amphetamine has on increasing the availability of dopamine in the brain, it has been suggested that similar drugs that interact with dopamine levels may affect sensitivity to reinforcement.

The study, by Celia Lie (University of Otago), Anne Macaskill (Victoria University of Wellington) and David Harper (Victoria University of Wellington), used rats to assess the short-term and long-term effects of MDMA on sensitivity to reinforcement. Rats were trained to press two levers and then schedules of reinforcement (receiving 0.01ml of sweetened milk) were arranged on each lever throughout the session. A task called the “rapidly changing concurrent choice procedure” was used – this involves the rats having to constantly make decisions based on changing rules.

Results revealed that both short-term and long-term administration of MDMA produced reductions in the number of times they pressed the lever, but did not reduce sensitivity to reinforcement. In fact the study found that some doses of short-term MDMA increased sensitivity to reinforcement, but these effects did not remain.

The findings suggest that MDMA does not affect long-term sensitivity to reinforcement unlike similar drugs, despite disrupting other aspects of learning and memory. Therefore, it provides evidence that it is not the effect on dopamine, but possibly a drug’s other affects (such as on serotonin), that produces an increase in sensitivity to reinforcement.