A recent study published in Nature Communications has identified a process involving the brain chemicals dopamine and serotonin that may help distinguish essential tremor from Parkinson’s disease. Researchers found that while dopamine-related processes are known to be disrupted in Parkinson’s disease, serotonin also plays a critical role in differentiating between the two conditions.
Distinguishing Symptoms
Essential tremor primarily presents with uncontrollable shaking of the hands. In contrast, Parkinson’s disease—a neurodegenerative condition associated with ageing—causes tremors in the limbs, difficulty maintaining balance, and other movement-related challenges. Previous studies have shown that Parkinson’s is linked to reduced dopamine production, which is vital for regulating motivation and pleasure.
The Dopamine–Serotonin Interaction
Senior author William Howe, Assistant Professor at Virginia Polytechnic Institute and State University’s School of Neuroscience, explained that the clearest difference between the two conditions lies in the interaction of dopamine and serotonin. In patients with Parkinson’s disease, the normal rise-and-fall pattern of these chemicals was absent. “It wasn’t just that dopamine was disrupted. It was that the normal back-and-forth between dopamine and serotonin was gone,” Howe said.
Testing Brain Responses Through Decision-Making
To explore this further, researchers monitored brain activity in the caudate of the striatum—a region linked to decision-making and reward processing. Patients with Parkinson’s disease and essential tremor played a game involving fair and unfair monetary offers. The researchers then used a computational model to analyze how patients formed and adjusted expectations during the game.
Key Findings
Among essential tremor patients, offers that violated expectations triggered a “seesaw” response—dopamine levels rose while serotonin levels dropped. By contrast, Parkinson’s patients did not show this simultaneous fluctuation. Instead, both the serotonin dip and dopamine rise were missing. This absence of dynamic interaction, researchers concluded, was the strongest indicator distinguishing Parkinson’s from essential tremor.
Broader Implications
As reported by The Week, the findings challenge the traditional view that only dopamine disruption defines Parkinson’s disease. Since serotonin is not usually considered central to the disease, the study opens up new perspectives and potential clinical insights. The researchers noted: “Violations in the expected value of monetary offers are encoded by opponent patterns of dopamine and serotonin release in essential tremor, but not in Parkinson’s disease.”