In a recent study, researchers investigated the effects of ADHD medication on the brain reward system using fMRI.
Attention deficit hyperactivity disorder (ADHD) is a neurobiological condition, characterized by increased levels of inattention, hyperactivity, and impulsiveness. Individuals with ADHD display symptoms in childhood, which often continue into adulthood. Methylphenidate is a drug commonly used to manage the symptoms of the condition. However, the effects of this ADHD drug on the brain reward system are not fully understood.
Analyzing the ADHD brain
Several behavioural and imaging research studies suggest that people with ADHD have different brain responses when expecting and receiving rewards. According to several studies, methylphenidate induces changes in dopamine availability, which enables individuals with ADHD to maintain focus. A recent study, published in Neuropharmacology, examined the effects of ADHD medication on the ventral striatum, a region in the brain reward system linked to primary dopamine-producing neurons.
For this study, researchers recruited participants from Brazil between 22 and 34 years old, with and without ADHD. Individuals were divided into two groups. Individuals with ADHD were placed in one group, while the participants without ADHD were assigned to the control group. Participants with ADHD were given the medication and placebo on two separate occasions and scanned at both times. Using functional magnetic resonance imaging (fMRI), the researchers examined the brain activity of participants as they played a computer game simulation of slot machines with two symbols. The symbols acted as either a reward-predicting cue or a non-reward-predicting cue. If the participants won money after spinning the slot machine’s reels, the symbol representing the reward-predicting cue was displayed.
The neuronal activity in the ventral striatum increased in response to the reward cue in comparison to the non-reward cue when the participants took methylphenidate. However, the neuronal activity was similar for both the reward and non-reward cues when the participants took the placebo. Methylphenidate increases levels of norepinephrine, a neurotransmitter released by neurons in the prefrontal cortex, and in turn, regulates dopamine levels in the striatum when rewards are given. The findings suggest this medication enables people with ADHD to discriminate between reward and non-reward cues.
Researchers also examined the association between neuronal activity in parts of the brain called the striatum and the medial prefrontal cortex, a region involved in decision-making and communicating with other brain areas. The neuronal activity in the striatum had a stronger correlation with the activity in the medial prefrontal cortex when the participants took a placebo than it was in those who took methylphenidate. This suggests that there is active communication between the striatum and the medial prefrontal cortex in those with ADHD.
A need to understand mechanisms
The researchers suggest that the effects of ADHD medication on the brain response to rewards are complex and further research will help better understand the mechanism by which methylphenidate acts on the brain. According to one of the authors, Dr. Furukawa, defining how methylphenidate functions may lead researchers to develop better and more targeted therapies for treating ADHD.
References
- Furukawa E, da Costa R, Bado P, et al. Methylphenidate modifies reward cue responses in adults with ADHD: An fMRI study. Neuropharmacology. November 2, 2019. https://www.sciencedirect.com/science/article/pii/S0028390819303995.
2. Okubo T. Rich rewards: Scientists reveal ADHD medication’s effect on the brain. EurekAlert! Published January 17, 2020. https://www.eurekalert.org/news-releases/591469.
