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A new study finds dopamine neurons are more diverse than previously thought, in that the neuromodulatory molecule has been found to controls movement, not just rewards. This has come after researchers from Northwestern University have recorded responses from three genetic subtypes of dopamine neurons.
While dopamine neurons have been thought to only respond to rewards or reward-predicting cues, the researchers an unusual genetic subtype. This discovery, based on a mouse model, shows how this one subtype accounts for about 30 percent of dopamine neurons in a subpart of the midbrain. This region fires during movement and is unresponsive to rewards.
The researchers noted that whenever a mouse accelerated they observed activity; and in contrast, they did not see activity in response to a rewarding stimulus.
This discovery builds on a previous study, which found a population of dopamine neurons associated with movement in mice. This suggested that some dopamine neurons had motor signals.
To examine the responses, the researchers used genetic tools to isolate and label populations of neurons based on their gene expression. From this the scientists were able to tag neurons in the brains of a genetically modified mouse model with fluorescent sensors. This enabled the researchers to see which neurons glowed during behaviour.
This assessment revealed which neurons control different specific functions. In the experiments, about 30 percent of dopamine neurons only glowed when the mice moved. These neurons were one of the genetic subtypes. The other populations of dopamine neurons responded to aversive stimuli (causing an avoidance response) or to rewards.
This finding shed new light on the mysterious nature of the brain. In addition, it opens up new research directions for further understanding and potentially treating Parkinson’s disease. For decades, researchers were confounded by why patients with Parkinson’s disease lose dopamine neurons yet have difficulties moving.
Parkinson’s disease is characterized by the loss of dopamine neurons and it affects the motor system. When the dopamine neurons die, people have trouble with movement. There could be a genetic subtype that is more susceptible to degradation over time as people age.
The 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.
Hence, the discovery leads to a new hypothesis to be explored in the future, around the preservation of an anti-movement signal that is active when animals decelerate. It could be a signal imbalance that strengthens the signal to stop moving.
The research appears in the journal Nature Neuroscience. The research paper is titled “Unique functional responses differentially map onto genetic subtypes of dopamine neurons.”
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