Summary: Researchers uncovered that dopamine, the “feel-good” hormone typically associated with rewards, plays a vital role in learning even without immediate incentives.
The study discovered that dopamine and acetylcholine, both crucial for memory and learning, maintain a dynamic balance in the brain irrespective of external rewards. This “ebb and flow” of the hormones creates an environment conducive to continuous learning.
The findings challenge the traditional understanding of how the brain processes information and learns without external cues.
- Dopamine and acetylcholine levels in the brain naturally oscillate, even without external rewards, fostering an environment for ongoing learning.
- The research observed dopamine and acetylcholine cycles in mice approximately twice every second, regardless of activity.
- The findings could offer new insights into neuropsychiatric conditions linked to dopamine imbalances, such as schizophrenia and depression.
Researchers have long thought that rewards like food or money encourage learning in the brain by causing the release of the “feel-good” hormone dopamine, known to reinforce storage of new information.
Now, a new study in rodents describes how learning still occurs in the absence of an immediate incentive.
Led by researchers at NYU Grossman School of Medicine, the study explored the relationship between dopamine and the brain chemical acetylcholine, also known to play a role in learning and memory.
Past research had shown that these two hormones compete with one another, so that a boost in one causes a decline in the other. Rewards were thought to promote learning by simultaneously triggering an increase in dopamine and a decrease in acetylcholine.
This sudden hormone imbalance is believed to open a window of opportunity for brain cells to adjust to new circumstances and form memories for later use. Known as neuroplasticity, this process is a major feature of learning as well as recovery after injury.
However, the question had remained whether food and other external rewards are the only drivers for this memory system, or whether our brains instead are able to create the same conditions that are favorable to learning without outside help.
To provide some clarity, the study authors focused on when and under what circumstances dopamine levels are high at the same time as acetylcholine levels are low.
They found that this situation occurs frequently, even in the absence of rewards. In fact, it turns out that the hormones constantly ebb and flow in the brain, with dopamine levels regularly raised while acetylcholine levels are low, setting the stage for continual learning.
“Our findings challenge the current understanding of when and how dopamine and acetylcholine work together in the brain,” said study lead author Anne Krok, PhD.
“Rather than creating unique conditions for learning, rewards take advantage of a mechanism that is already in place and is constantly at work,” added Krok, who is also a medical student at NYU Grossman School of Medicine.
For the research, publishing online Aug. 9 in the journal Nature, the study team gave dozens of mice access to a wheel on which they could run or rest at will.
On occasion, the researchers offered the animals a drink of water. Then they recorded rodent brain activity and measured the amount of dopamine and acetylcholine released at different moments.
As expected, the drink treats created the typical patterns of dopamine and acetylcholine release that are prompted by rewards.
However, the team also observed that well before receiving water treats, dopamine and acetylcholine already followed “ebb and flow” cycles approximately twice every second, during which the levels of one hormone dipped while the other surged.
Krok notes that this pattern continued regardless of whether the rodents were running or standing still. Similar brain waves have been observed in humans during periods of introspection and rest, she adds.
“These results may help explain how the brain learns and rehearses on its own, without the need for external incentives,” said study senior author and neuroscientist Nicolas Tritsch, PhD. “Perhaps this pulsing circuit triggers the brain to reflect on past events and to learn from them.”
That said, Tritsch, an assistant professor in the Department of Neuroscience and Physiology at NYU Langone Health, cautions that their research was not designed to tell whether mouse brains process information the same way as human brains do during this “self-driven” learning, as he describes it.
Nevertheless, he says, the results of the study may also offer insight into new ways of understanding neuropsychiatric conditions that have been tied to incorrect levels of dopamine, such as schizophrenia, attention-deficit/hyperactivity disorder (ADHD), and depression.
In schizophrenia, for example, patients often experience delusions that contradict reality. If the dopamine-acetylcholine circuit is constantly strengthening connections in the brain, says Tritsch, then problems with this mechanism might lead to the formation of too many, and incorrect, connections, causing them to “learn” of events that did not really occur.
Similarly, lack of motivation is a common symptom of depression, making it challenging to perform basic tasks such as getting out of bed, brushing teeth, or going to work. It is possible that a disruption in the internal-drive system might be contributing to these issues, the authors say.
As a result, Tritsch says the research team next plans to examine how dopamine-acetylcholine cycles behave in animal models of such mental illnesses, as well as during sleep, which is important for memory consolidation.
Funding: Funding for the study was provided by National Institutes of Health grants DP2NS105553, R01MH130658, T32NS086750, T32GM007308, and T32GM136573. Further funding was provided by the Alfred P. Sloan Foundation, the Danna Foundation, the Whitehall Foundation, the Feldstein Medical Foundations, and the Vilcek Scholars Award.
In addition to Krok and Tritsch, other investigators involved in the study were Marta Maltese, PhD; and Pratik Mistry, MS; at NYU Langone, and Xiaolei Miao, PhD; and Yulong Li, PhD, at Peking University School of Life Sciences in Beijing.
About this dopamine and learning research news
Original Research: Closed access.
“Intrinsic dopamine and acetylcholine dynamics in the striatum of mice” by Anne Krok et al. Nature
Intrinsic dopamine and acetylcholine dynamics in the striatum of mice
External rewards such as food and money are potent modifiers of behaviour. Pioneering studies established that these salient sensory stimuli briefly interrupt the tonic discharge of neurons that produce the neuromodulators dopamine (DA) and acetylcholine (ACh): midbrain DA neurons (DANs) fire a burst of action potentials that broadly elevates DA in the striatum at the same time that striatal cholinergic interneurons (CINs) produce a characteristic pause in firing. These phasic responses are thought to create unique, temporally limited conditions that motivate action and promote learning.
However, the dynamics of DA and ACh outside explicitly rewarded situations remain poorly understood. Here we show that extracellular DA and ACh levels fluctuate spontaneously and periodically at a frequency of approximately 2 Hz in the dorsal striatum of mice and maintain the same temporal relationship relative to one another as that evoked by reward.
We show that this neuromodulatory coordination does not arise from direct interactions between DA and ACh within the striatum. Instead, we provide evidence that periodic fluctuations in striatal DA are inherited from midbrain DANs, while striatal ACh transients are driven by glutamatergic inputs, which act to locally synchronize the spiking of CINs. Together, our findings show that striatal neuromodulatory dynamics are autonomously organized by distributed extra-striatal afferents.
The dominance of intrinsic rhythms in DA and ACh offers new insights for explaining how reward-associated neural dynamics emerge and how the brain motivates action and promotes learning from within.