Scientists from Nagoya University in Japan have discovered how a specific group of brain cells helps maintain determination when achieving a reward requires increasing effort.
Research published in the journal Proceedings of the National Academy of Sciences has revealed the key role of orexin neurons. These cells, known for their regulation of sleep, appetite, and energy expenditure, have also proven important for maintaining motivated behavior.
The team, led by Associate Professor Hiroyuki Mizoguchi and Professor Emeritus Kiyofumi Yamada, created genetically modified "orexin-Cre" rats. This allowed for precise targeting of orexin-producing neurons and the study of their influence in complex behavioral tests.
In the experiments, rats performed a progressive ratio task where the number of screen touches required to obtain food gradually increased. Activation of orexin neurons increased the "breakpoint," a measure of how far an animal is willing to go for a reward. Conversely, their degeneration or suppression decreased this measure.
What allows humans to continue striving for a goal as difficulties mount? Real-time activity monitoring revealed a clear pattern: neurons activated in anticipation of a reward, especially when required effort increased, and remained active if the reward did not materialize.
Suppressing activity at the moment of reward anticipation using optogenetics caused rats to perform tasks for longer periods and give up more quickly. Enhancing activity did not provide an additional boost in motivation, indicating the necessity of these neurons but not their sufficiency for super-motivation.
The findings shed light on the mechanisms underlying motivation loss in depression, addiction, and ADHD. Future research will focus on the brain circuits interacting with these neurons to better understand how reward anticipation translates into sustained action.
"Our study demonstrated significant changes in the activity of orexin neurons depending on expected rewards and required efforts, suggesting a potential mechanism for translating expectations into sustained action," concluded Mizoguchi.
