The researchers conducted their study using fruit flies and found a small region in their brains that became highly active when the flies were starved.
"Our experiments show that hungry individuals keep increasing their performance – they run up to nine meters per minute. Fruit flies which are full give up much faster," said co-author Ilona Grunwald Kadow of the Technical University of Munich and the Max Planck Institute of Neurobiology in Germany.
The study was published in the journal Neuron.
It's natural for almost all living organisms to be more persistent when it comes to obtaining food when they are hungry. But in the process, they expend more energy and overcome fear and exhaustion. But the mechanisms behind this response are poorly understood.
For their study, the researchers experimented on fruit flies by putting them on a fly treadmill. The fruit flies were then fixed in place so the flies cannot get away. Next, the researchers exposed the herb to a food odor so they can measure how much effort the fruit flies put into tracking food.
Their findings confirmed that the hungry flies worked harder and more persistently than those that were full. This demonstrates that need outweighs negative experience, as the hungry flies endured great exhaustion in pursuit of food.
"This proves that even simple organisms show stamina and perseverance. Up to now, these qualities were thought to be reserved for humans and other higher organisms," said Grunwald Kadow.
Next, the researchers examined the brain activity of hungry fruit flies. Using electron microscopy and in-vivo imaging, they found the neural circuit that influenced motivation-related behavior in flies.
This neural circuit is located in the learning and memory center of the brain and is controlled by the dopaminergic and octopamine neurons. Dopaminergic neurons are the main source of dopamine in the body, while octopamine neurons are a source of octopamine, a neurotransmitter present in fruit files but is similar to human noradrenaline.
The researchers found that dopaminergic neurons can boost motivation by increasing neural activity. On the other hand, octopamine neurons trigger reduced odor tracking.
According to Grunwald Kadow, these findings may apply to mammals because the neurotransmitters and the neural circuit also exist in their brains. She added that their study can be useful for understanding how neurons and neurotransmitters in the brain get out of control, such as in the case of addiction.
A similar study published in the journal Science also explored eating motivation among starved fruit flies. This time, however, researchers from Johns Hopkins University examined how fruit flies choose what they eat when hungry.
The researchers deprived the flies of protein and found a small set of dopaminergic neurons that predisposed the flies to food that they needed as opposed to what they normally prefer. These neurons released dopamine that triggered them to crave for their main protein source, yeast, and override their tendency to seek out sugar.
The researchers said that the neurons formed a circuit that is located in the learning and memory center of the brain of fruit flies. This neural circuit also controls persistent and motivated behavior in the insects. (Related: Scientists link chronic inflammation to low motivation and reduced dopamine levels.)
All in all, these findings can help advance the search for similar processes in mammals. Further studies can also help scientists develop a better understanding of human cravings and, ultimately, weight gain and obesity.