Senior author Michael J. Krashes, of the National Institutes of Diabetes and Digestive and Kidney Diseases (NIDDK) at the National Institutes of Health (NIH), and colleagues recently published their findings in the journal Neuron.
Put simply, motivation is the reason for acting in a particular way or making a certain choice over another.
In the 1940s, American psychologist Abraham H. Maslow created the "hierarchy of needs" - a set of five "needs" that he believed explained human motivation.
These range from physiological needs - such as food, water, and other requirements for human survival - to self-actualization, the desire for personal growth and success.
Over the years, researchers have either acknowledged, criticized, or amplified Maslow's theory. With regard to the latter, neurologists have increasingly investigated the role of the human brain in motivation.
Hungry and thirsty mice opted for food over water
According to Krashes and team, most neurological studies of motivation are conducted in tightly controlled conditions and have focused on investigating one motivational state at a time, which has made it difficult to determine if some states are stronger drivers than others and what brain circuits are involved.
With a view to addressing this knowledge gap, the researchers conducted a series of mouse experiments in which they assessed a variety of motivational states, including hunger, fear, anxiety, and social needs.
For the study, the team used optogenetics - a technique that uses light to control cells - to govern nerve cells in the brain known as agouti-related peptide (AgRP) neurons.
AgRP neurons are situated in the brain's hypothalamus. They are known to regulate appetite and are crucial for survival.
For one experiment, the researchers either deprived mice of food for 24 hours or activated their AgRP neurons in order to make them hungry. These mice were also deprived of water, making them thirsty. A control group was deprived of water but not food.
When presented with food and water, the mice that were both hungry and thirsty opted for food over water, while the control group chose the water. This indicates that hunger is a stronger motivational force than thirst.
"We interpret this as a unique ability of hunger-tuned neurons to anticipate the benefits of searching for food, and then alter behavior accordingly," says Krashes.
Overcoming fear in the face of hunger
In another experiment, the researchers induced hunger in the mice by activating AgRP neurons, before exposing them to a chamber scented with a chemical produced by foxes - an environment that triggers anxiety and fear for the rodents.
When food was placed in the chamber, the team found that the hungry mice overcame their fear to get food, while control mice that were not hungry chose to stay in the "safe" zones, suggesting that hunger trumps fear and anxiety as a motivational force.
A further experiment revealed hunger is also a greater motivation than social needs; AgRP activation led to socially isolated mice opting for a chamber containing food, rather than a chamber with another mouse. The reverse was true for control mice.
Interestingly, AgRP activity increased when another mouse was close by, which suggests AgRP neurons respond to potential competition for food.
"We think that the presence of another mouse could be viewed as competition for limited resources, increasing the motivation to seek food, which is a finding that no other studies have indicated thus far," says Krashes.
Overall, Krashes says the team's findings suggest motivational forces are more deeply connected than previously thought.
"Therefore, studying isolated motivated behaviors may not accurately demonstrate how the big picture nervous system works," he adds. "Our study is one of the first steps to investigating feeding behavior in a more complicated, naturalistic setting."
Additionally, the researchers believe the findings shed light on how animals and humans have evolved.
Could the findings advance treatments for obesity?
Medical News Today asked Krashes whether their findings might pave the way for new treatments for obesity - for example, by reducing motivation for food.
"It should be stated that we, and others, have demonstrated the necessity of these neurons in regulating feeding and body weight homeostasis," he replied. "I am of the opinion that no individual study puts us in a position to pave the way in treating obesity, but collectively, as we link and evaluate everyone's findings, I believe we are on the right track."
"I do think it's important to study feeding in a more ethological framework, as we have done here, because the neural circuits underlying food acquisition evolved during a time when we were constantly bombarded with external stimuli and had to make complex decisions regarding our foraging behavior," he added.
Krashes told MNT that the researchers hope to expand their findings by creating an assay that enables them to study foraging behavior.
"Specifically, we'd like to determine how searching for and eating food is influenced by social structure, distance traveled, predation, thirst etc.," he explained. "We're also prioritizing why AgRP neural activity is influenced by the presence of another animal." [source]