For decades, doctors have used methylphenidate, a stimulant drug sold as Ritalin and Concerta, to treat children with attention deficit/hyperactivity disorder (ADHD), making it one of the most commonly prescribed treatments aimed at the central nervous system.

Researchers might assume to know how methylphenidate works in the brain by now, yet nothing is known about the drug's mechanism of action.

A recent study published in the journal, Biological Psychiatry: Cognitive Neuroscience and Neuroimaging, aims to bridge this knowledge gap and better understand how methylphenidate interacts with cognitive control networks and attentional behaviour.

What researchers do know is that individuals with ADHD have lower dopamine signalling activity than neurotypical individuals in the interconnected brain networks that control attention and goal-directed behaviours. Specifically, methylphenidate is hypothesised to ameliorate ADHD symptoms by increasing dopamine levels in the nucleus accumbens (NAc), a hub for dopamine signalling.

In the new study, researchers led by Yoshifumi Mizuno, MD, PhD, Weidong Cai, PhD, and Vinod Menon, PhD, used brain imaging to explore the effects of methylphenidate on the NAc and a so-called triple network system that plays a key role in behaviours that require adaptive control of attention.

The three networks include the salience, frontoparietal, and default mode networks. The aberrant activity was detected in the NAc and in multiple brain networks in children with ADHD, suggesting that dysregulation in the system may underlie ADHD symptoms and that correcting the dysfunction might alleviate those symptoms.

"Our findings demonstrate in two independent cohorts that methylphenidate changes spontaneous neural activity in reward and cognitive control systems in children with ADHD. Medication-induced changes in cognitive control networks result in more stable sustained attention. Our findings reveal a novel brain mechanism underlying methylphenidate treatment in ADHD and inform biomarker development for evaluating treatment outcomes," noted Dr Menon, Department of Psychiatry & Behavioral Sciences, Stanford University School of Medicine.

The researchers used functional magnetic resonance imaging (fMRI) to measure the effects of methylphenidate on spontaneous brain activity in 27 children with ADHD and 49 typically developing controls.

Children with ADHD were scanned during two different visits one to six weeks apart -- once while receiving methylphenidate and once while receiving a placebo. (Typically developing children did not receive medication or placebo.) Outside the scanner, children with ADHD also performed a standardised task to assess sustained attention.

Additionally, the researchers tested the replicability of methylphenidate's effects on spontaneous brain activity in a second independent cohort.

Not surprisingly, children performed better on attention tasks when they were medicated. And as the researchers hypothesised, they also saw greater spontaneous neural activity in the NAc and the salience and default mode networks when methylphenidate was administered.

Children with ADHD who displayed enhanced changes in brain activity patterns in the default mode network with medication performed better on the attention tasks with medication.

Findings were replicated across two independent cohorts, providing further evidence that methylphenidate may alleviate ADHD symptoms by its actions on the NAc and the triple network cognitive system.

Cameron Carter, MD, editor of Biological Psychiatry: Cognitive Neuroscience and Neuroimaging, said of the study, "The findings, which used the widely available technique of resting-state functional MRI, confirm the positive effects of methylphenidate on attention in children with ADHD and reveal the likely mechanism of action, through improved coordinated brain network activity and a likely key role for enhanced dopamine effects in the NAc region of the brain."

The work advances researchers' understanding of how ADHD affects cognitive control networks in the brain and how methylphenidate interacts with these networks to shift behaviour. The findings could guide future work using brain imaging as a clinically useful biomarker of response to treatments.