Striatal D1- and D2-type dopamine receptors are linked to motor response inhibition in human subjects
In the paper “Striatal D1- and D2-type Dopamine Receptors Are Linked to Motor Response Inhibition in Human Subjects,” written by Chelsea L. Robertson, Kenji Ishibashi, Mark A. Mandelkern, Amira K. Brown, Dara G. Ghahremani, Fred Sabb, Robert Bilder, Tyrone Cannon, Jacqueline Borg, and Edythe D. London, researchers used Positron Emission Tomography (PET) imaging to test the role of dopamine receptors in human response inhibition. Motor response inhibition is the essential ability to suppress inappropriate or premature actions. This mechanism is guided by complex neural circuits involving dopaminergic transmission. Additionally, deficits in this inhibitory control show hallmarks of psychological disorders like ADHD and substance use/addiction. While prior animal studies point to distinct roles for D1- and D2-type receptors, human studies hadn’t yet evaluated D1-type receptors in this specific stopping mechanism. Taking this into account, this study analyzes how the individual variability in D1- and D2-receptors leads to varying levels of behavioral control.
The purpose of this study is to determine whether individual differences in D1- and D2-type dopamine receptor availability in the striatum are related to motor response inhibition in humans, and whether these receptors function in opposing ways across different brain regions. This research is highly important because understanding how dopamine influences behavioral inhibition could lead to improved treatments for disorders involving impulsive behavior, such as ADHD and substance addiction. To investigate this, the researchers recruited 31 healthy adult volunteers to participate in the study. Each participant completed two specialized neurobehavioral computer tasks: the Stop-Signal Task (SST) and the Continuous Performance Task (CPT), which were used to measure the different aspects of motor response inhibition. Participants also underwent structural MRI scans to identify and outline specific regions of the brain, specifically dividing the striatum into its functional subdivisions: the limbic, associative, and sensory-motor regions. Furthermore, PET imaging was performed using the targeted radioligands [11C]NNC-112 (for D1-type) and [18F]fallypride (for D2-type) to measure the exact binding potential and availability of these dopamine receptors.
The study demonstrated that Stop-Signal Reaction Time (SSRT) had a significantly negative correlation with both D1- and D2-type receptors in the whole striatum. Because a lower SSRT indicates a faster stopping speed, this negative correlation means that a higher receptor availability directly predicts faster, more efficient stopping speeds. Crucially, the researchers discovered that the link between the movement and the D1- and D2-type receptors was highly localized only in the dorsal striatum (the sensory-motor and associative sub-regions) and was not found in the ventral striatum (the limbic sub-region). Furthermore, they found that there was no significant correlation between dopamine receptor abundance and performance on the Continuous Performance Task (CPT). This reveals that higher receptor abundance specifically in the dorsal striatum predicts better stopping metrics, while different tasks measuring “inhibition” actually utilize entirely separate neurological and neurochemical pathways.
In this experiment, there were notable time lapses averaging 17 months between when participants completed their initial behavioral tests (SST, CPT) and when they underwent the PET imaging procedures. During this time, participants’ D1- and D2-receptor abundance may have increased or decreased, potentially weakening the accuracy of the study. However, despite this limitation, the researchers conducted multiple re-testings on a subset of the participants 40 months later to prove that their observations and the participants’ neurobehavioral metrics were highly reliable and stable over time. Furthermore, the relatively small sample size of 31 healthy volunteers may have been too small to draw large-population conclusions. Nevertheless, a noteworthy strength of the paper is that both receptor subtypes contribute similarly to the stopping metric. This suggests that motor control depends on a delicate balance between phasic (D1) and tonic (D2) dopaminergic activity, rather than acting in strictly opposing ways.
In conclusion, both D1-type and D2-type dopamine receptors are explicitly linked to motor response inhibition in humans. Dopamine mainly controls your ability to stop an action in the brain’s movement center (the dorsal striatum) rather than its reward center (the ventral striatum). This happens because the two different receptor types showed similar relationships with stopping tasks, proving that a balance between D1 and D2 activity pathways is crucial for behavioral control. While response inhibition capacity can vary heavily across individuals, these tasks capture completely distinct neurological profiles. Therefore, researchers must selectively and carefully choose behavioral tasks when assessing clinical impulsivity in patients. Ultimately, this study is important because identifying this specific dopamine receptor distribution allows localized neurochemical pharmaceutical therapies to be developed to aid individuals struggling with clinical impulsivity and behavioral disorders, like ADHD or Parkinson’s Disease.