Dopamine is a type of neurotransmitter that can provide an intense feeling of reward. It has been a long-standing assumption that most, if not all, dopamine neurons solely respond to rewards or reward-predicting cues. However, a study in mice led by researchers at Northwestern University reveals dopamine may also control movements. The researchers uncovered that one genetic subtype fires when the body moves and that these neurons do not respond to rewards at all.
The findings are published in Nature Neuroscience in an article titled, “Unique functional responses differentially map onto genetic subtypes of dopamine neurons,” and shed new light on the brain which may lead to new research on Parkinson’s disease, which is characterized by the loss of dopamine neurons yet affects the motor system.
“Dopamine neurons are characterized by their response to unexpected rewards, but they also fire during movement and aversive stimuli,” the researchers wrote. “Dopamine neuron diversity has been observed based on molecular expression profiles; however, whether different functions map onto such genetic subtypes remains unclear. In this study, we established that three genetic dopamine subtypes within the substantia nigra pars compacta, characterized by the expression of Slc17a6 (Vglut2), Calb1, and Anxa1, each have a unique set of responses to rewards, aversive stimuli, and accelerations and decelerations, and these signaling patterns are highly correlated between somas and axons within subtypes.”
“When people think about dopamine, they likely think about reward signals,” said Northwestern’s Daniel Dombeck, PhD, who co-led the study. “But when the dopamine neurons die, people have trouble with movement. That’s what happens with Parkinson’s disease, and it’s been a confusing problem for the field. We found a subtype that are motor signaling without any reward response, and they sit right where dopamine neurons first die in Parkinson’s disease. That’s just another hint and clue that seems to suggest that there’s some genetic subtype that’s more susceptible to degradation over time as people age.”
“This genetic subtype is correlated with acceleration,” added Northwestern’s Rajeshwar Awatramani, PhD, who co-led the study with Dombeck. “Whenever the mouse accelerated, we saw activity, but in contrast, we did not see activity in response to a rewarding stimulus. This goes against the dogma of what most people think these neurons should be doing. Not all dopamine neurons respond to rewards. That’s a big change for the field. And now we found a signature for that dopamine neuron that does not show reward response.”
The new study builds on a previous study from Dombeck’s lab, which found a population of dopamine neurons associated with movement in mice.
“At the time, we thought it was just a tiny fraction of neurons,” Dombeck said. “And others continued to assume that all dopamine neurons were still reward neurons. Maybe some of them just had motor signals too.”
To explore further, Dombeck teamed with Awatramani, who used genetic tools to isolate and label populations of neurons based on their gene expression. Using this information, Dombeck’s team then tagged neurons in the brains of a genetically modified mouse model, which was generated at the Northwestern Transgenic and Targeted Mutagenesis Lab, with fluorescent sensors. This allowed the researchers to see which neurons glowed during behavior—ultimately revealing which neurons control different specific functions.
About 30% of dopamine neurons only glowed when the mice moved. These neurons were one of the genetic subtypes that Awatramani’s team identified. The other populations of dopamine neurons responded to aversive stimuli (causing an avoidance response) or to rewards.
“It’s not like people with Parkinson’s disease only lose their drive to be happy because their dopamine response is damaged,” Dombeck said. “Something else is going on that affects motor skills.”
Dombeck and Awatramani’s new study might provide the missing piece to the puzzle.
In their work, the researchers noted that dopamine neurons correlated with acceleration in mice appear to be in the same location of the midbrain as those that tend to die in patients with Parkinson’s disease. But the dopamine neurons that survive are correlated with deceleration. The discovery leads to a new hypothesis that Dombeck and Awatramani plan to explore in the future.
“We’re wondering if it’s not just the loss of the motor-driving signal that’s leading to the disease—but the preservation of the anti-movement signal that’s active when animals decelerate,” Dombeck said. “It could be this signal imbalance that strengthens the signal to stop moving. That might explain some of the symptoms. It’s not just that patients with Parkinson’s can’t move. It could also be that they are being driven to stop moving.”
“We’re still trying to figure out what this all means,” Awatramani said. “I would say this is a starting point. It’s a new way of thinking about the brain in Parkinson’s.”