According to a study phasic firing of midbrain dopamine neurons helps the brain to differentiate between rewarding and aversive events.
The study was a collaborative effort by Carlos Paladini, assistant professor of neuroscience at The University of Texas at San Antonio (UTSA) and UTSA graduate student Collin Lobb, and researchers at The University of Washington at Seattle.
The researchers studied the firing patterns of midbrain dopamine neurons in mice during reward-based learning.
"Our research findings provide a direct functional link between the bursting activity of midbrain dopamine neurons and behavior. The research has significant applications for the improvement of health, because the dopamine neurons we are studying are the same neurons that become inactivated during Parkinson's Disease and with the consumption of psychostimulants such as cocaine and amphetamine," said Paladini.
Midbrain dopamine neurons fire in two characteristic modes, tonic and phasic, which are thought to modulate distinct aspects of behavior.
When an unexpected reward is presented to an individual, midbrain dopamine neurons fire high frequency bursts of electrical activity that allows the brain to learn to link the reward with cues in our environment, which may predict similar rewards in the future.
A protein called the NMDA receptor, which is expressed on the surface of the dopamine cells, is what controls the burst of electrical spikes observed in dopamine neurons.
For the study, the researchers removed the NMDA receptor from the dopamine cells only, leaving the dopamine neurons unable to fire bursts.
When researchers placed the mice in reward-based situations, it was found that the mice without the NMDA receptor in their dopaminergic neurons could not learn tasks that required them to link sensory cues with reward.
However, the same mice could learn tasks that did not involve an association with rewards.
"Now that we know NMDA receptors are required for burst firing in dopamine neurons, we need to explore the mechanisms by which NMDA receptor-mediated bursting is regulated or gated," said Lobb.
The study has been published in the online edition of the journal Proceedings of the National Academy of Sciences.