A new study by Daniel Cardozo Pinto, now a Harvard Junior Fellow in MCB, has uncovered a neural circuit mechanism explaining how two key brain chemicals—dopamine and serotonin—work in opposition to regulate reward learning. The work, published in Nature Communications and conducted during Cardozo Pinto’s time at Stanford University, identifies a neural pathway in the brain’s reward circuitry by which the two neuromodulators exert opposing effects on behavior.
The work builds on Cardozo Pinto’s earlier research showing that dopamine and serotonin function together as a “gas–brake” system for reward in the mammalian brain.
“In previous work, we showed that dopamine and serotonin have an antagonistic relationship in reward learning,” said Cardozo Pinto. “In a key brain reward center, dopamine acts like the gas pedal, promoting reward learning and reinforcement, while serotonin acts more like a brake.” The new study set out to understand how this push–pull relationship is implemented within neural circuits.
“We wanted to know how these two neuromodulators shape activity in downstream neural circuits to produce opponent effects,” Cardozo Pinto said.
Mapping serotonin signaling in the striatum
The researchers focused on the striatum, a key brain region involved in motivation, decision-making, and reward learning. While dopamine signaling in this region has been extensively studied, the role of serotonin has been harder to define, in part because the serotonin system is far more complex.
“There are fourteen serotonin receptors, compared to five dopamine receptors,” Cardozo Pinto said. “So understanding how serotonin signals are organized in the brain has been a much more complicated problem.”
To address this, the researchers combined transcriptomic analysis with spatial transcriptomics to map where serotonin receptors are expressed in the striatum.
Much of this work was led by co–first author Michaela Guo, whom Cardozo Pinto mentored while she was an undergraduate and then a research assistant at Stanford. Guo carried out the spatial transcriptomics experiments that enabled the team to construct the first detailed molecular map of serotonin receptor distribution in this brain region.
The analysis revealed a striking pattern: neurons that express inhibitory dopamine receptors are strongly enriched for excitatory serotonin receptors.
“This result suggested that these neurons receive dopamine signals that suppress their activity, but also serotonin signals that excite them,” Cardozo Pinto said. “And this would create a natural push–pull relationship between the two neuromodulators.”
These cells, known as D2 receptor–expressing medium spiny neurons, are one of the primary neuron types in the striatum and play a central role in shaping behavioral responses to reward.
A push–pull circuit for reward signals
After identifying this receptor pattern, the researchers tested whether dopamine and serotonin indeed exert opposite physiological effects on these neurons.
Using electrophysiological recordings, they measured how the neurons responded when dopamine or serotonin was applied experimentally. The results confirmed the predicted antagonistic interaction.
“We found that dopamine inhibits these neurons, while serotonin excites the very same cells,” Cardozo Pinto said. “This provides the first circuit-level explanation for how dopamine and serotonin can oppose each other.”
A potential link to addiction vulnerability
The study also explored whether disrupting this dopamine–serotonin balance could influence sensitivity to drugs of abuse.
Previous studies had suggested that activating certain serotonin receptors—particularly the serotonin 2C receptor—can reduce addictive behaviors in lab animals, although the mechanism was unclear. The new receptor map revealed that these receptors are enriched on the same D2 neurons involved in the dopamine–serotonin push–pull circuit, suggesting that activating serotonin 2C receptors could be counteracting the effects of dopamine signaling on these cells.
To test this, the researchers reduced the expression of these receptors in D2 neurons and then administered cocaine to mice.
“When we reduced serotonin 2C receptor expression in these neurons, the animals became more sensitive to cocaine’s rewarding effects,” Cardozo Pinto said. “Removing that serotonin ‘brake’ allowed dopamine signaling to drive a stronger reward response.”
While the findings are based on experiments in mice, they suggest that disruptions in dopamine–serotonin balance could influence vulnerability to addiction.
“If you disturb the balance between dopamine and serotonin signaling, it may increase susceptibility to drug reward,” Cardozo Pinto said. “That raises the possibility that interindividual differences in this circuitry could help explain why some individuals are more vulnerable to substance use disorder than others.”
From Stanford to Harvard
The research was conducted primarily at Stanford while Cardozo Pinto was completing his PhD and a transitional postdoctoral year. The study was co-supervised by Robert Malenka, Cardozo Pinto’s doctoral advisor, and Neir Eshel, his postdoctoral advisor during the project.
“Daniel’s work at Stanford started from a systematic search for the locations where dopamine and serotonin interact in the brain by examining where axons of dopamine neurons and serotonin neurons project to, followed by functional investigations,” Naoshige Uchida added. “Now, his new study extends it by looking at receptors, gradually zooming in on where and how dopamine and serotonin interact. I very much look forward to seeing how his systematic approach leads to deeper understanding of how motivated behaviors, including those seen in addiction, are regulated.”
Cardozo Pinto continues to investigate how dopamine–serotonin interactions shape reward-related behavior. “We now have a circuit mechanism that links these two neuromodulatory systems,” Pinto Cardozo said. “The next step is understanding how differences in this circuitry influence behavior and vulnerability to disorders involving reward processing.”
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