A research team led by the Netherlands Institute for Neuroscience has recently explored the role of the neurotransmitter dopamine in learning and motivation. Traditionally, there have been two contrasting perspectives on dopamine’s function, one suggesting that it serves as a learning signal and the other proposing that it drives motivation. However, a series of experiments has now revealed that dopamine may in fact be involved in both of these processes.
The role of the dopamine system in signaling reward-related information and facilitating rewarding actions is widely recognized. Researchers commonly investigate this phenomenon through two types of conditioning experiments.
Pavlovian conditioning involves the formation of associations between previously unrelated situations or stimuli in the brain. A famous illustration is Pavlov’s experiment with a dog, where a sound (initially unrelated to food) was repeatedly paired with food delivery. As a result, the dog eventually learned to associate the sound alone with the impending arrival of food, leading to a salivation response.
By contrast, operant conditioning – also known as instrumental learning – focuses on an individual’s behavior to earn a reward. In this scenario, the individual must engage in a specific action, referred to as an operant response, following the presentation of a stimulus (such as a sound) to obtain the reward. For instance, in animal experiments, pressing a lever is a common operant response used to receive a food reward.
By employing both Pavlovian and operant conditioning experiments, researchers can gain insights into the intricate workings of the dopamine system, its role in processing rewards, and its influence on motivated behavior.
In the new study, the scientists used two groups of male rats to compare the two conditioning paradigms while measuring dopamine release in their nucleus accumbens, a key brain region involved in processing reward-related information.
In the Pavlovian group, a cue light was illuminated for five seconds, followed by the delivery of a food pellet into the reward magazine after the light turned off. In the operant conditioning group, turning off the cue light triggered the extension of a lever below the light into the operant box. Pressing the lever once immediately resulted in the delivery of a food pellet reward into the food magazine. If no lever press occurred within five seconds, the lever was retracted, and no reward was given.
The experiments revealed that rats in both groups released the same amount of dopamine when the reward-predictive cue was presented. However, only the operant-conditioning group exhibited a sustained plateau of dopamine concentration throughout the entire five-second cue presentation, which persisted even before the lever press. This sustained dopamine release was consistently observed across various experimental parameters and behavioral training manipulations.
The researchers suggest that sustained dopamine levels may serve as an intermediary between learning and action, closely linked to the motivation to perform actions that lead to reward attainment.
“Our results bring together the two camps of scientists that often battle with each other: one says that dopamine is a so-called reward-prediction error signal, meaning that dopamine is released when something better than expected happens, and is suppressed when something worse than expected happens. It is a learning (or teaching) signal,” said study senior author Ingo Willuhn, a neuroscientist at the Netherlands Institute for Neuroscience.
“The other camp says that this is not true. They say that dopamine has something to do with motivation. Increased dopamine release will invigorate the subjects and they work harder to get the reward. There have been a few attempts in the past to bring these two camps together, but there is still need for more knowledge on the subject.”
“What we saw in our study is that only in the operant-learning task dopamine levels stayed high. It seems that the motivation is encoded in this plateau. Reward prediction is the initial dopamine peak, but how much the signal stays up, reflects motivation. Thus, our paper suggests that there is a possibility that dopamine is involved in both, learning and motivation. The next steps will be to get more details out of this. We need to replicate the experiments and make them more sophisticated. The more sophisticated you make it, the more precise our predictions have to be. We are going to build on it and see whether it still holds up.”
Since dopamine is not only involved in everyday life, but also in disorders such as addiction, schizophrenia, or Parkinson’s disease, these findings could have significant clinical implications. For instance, prescribed drugs could influence both the learning and motivation systems at the same time, making most of them problematic for treating various conditions.
“If you give schizophrenic patients classic antipsychotic medication, they become slow and cannot act much because their motivation system is down. Parkinson’s patients take pro-dopamine drugs essentially because they lost their dopamine, but some patients start to gamble because their dopamine system is on overdrive suddenly. We cannot influence learning and motivation components separately. As soon as you give a drug it is going to hit all of it, so it is good to keep that in mind,” Willuhn explained.
Thus, better understanding the complex role dopamine plays in the brain could potentially lead to the development of more efficient treatments and positively contribute to individual and public health. The study is published in The Journal of Neuroscience.