Are you familiar with that feeling of being “hangry”? You know, when you’re low on energy, feeling cranky, and your brain is screaming at you to find food? Well, scientists have pinpointed a cluster of cells called AgRP neurons near the underside of the brain that may be responsible for this unpleasant sensation. These cells sit near the brain’s blood supply, giving them access to hormones arriving from the stomach and fat tissue that indicate energy levels. When energy is low, they act on a variety of other brain areas to promote feeding.
By studying AgRP neurons in mice, researchers have begun to unravel how these cells switch on and encourage animals to seek food when they’re low on nutrients, and how they sense food landing in the gut to turn back off. They’ve also found that the activity of AgRP neurons goes awry in mice with symptoms akin to those of anorexia, and that activating these neurons can help to restore normal eating patterns in those animals.
Understanding and manipulating AgRP neurons might lead to new treatments for both anorexia and overeating. “If we could control this hangry feeling, we might be better able to control our diets,” says Amber Alhadeff, a neuroscientist at the Monell Chemical Senses Center in Philadelphia.
AgRP neurons appear to be key players in appetite: Deactivating them in adult mice causes the animals to stop eating, and they may even die of starvation. Conversely, if researchers activate the neurons, mice hop into their food dishes and gorge themselves.
Experiments in 2015 helped to illustrate what AgRP neurons do. Researchers found that when mice hadn’t had enough to eat, AgRP neurons fired more frequently. But just the sight or smell of food was enough to dampen this activity, within seconds. From this, the scientists concluded that AgRP neurons cause animals to seek out food. Once food has been found, they stop firing as robustly.
One research team also showed that AgRP neuron activity appears to make mice feel bad. To demonstrate this, the scientists engineered mice so that the AgRP neurons would start firing when light was shone into the brain with an optical fiber. They placed these engineered mice in a box with two distinct areas: one colored black with a plastic grid floor, the other white with a soft, tissue paper floor. If the researchers activated AgRP neurons whenever the mice went into one of the two areas, the mice started avoiding that region.
AgRP activation felt “mildly unpleasant.” That makes sense in nature, says neuroscientist Scott Sternson: Any time a mouse leaves its nest, it’s at risk from predators, but it must overcome this fear in order to forage and eat. “These AgRP neurons are kind of the push that, in a dangerous environment, you’re going to go out and seek food to stay alive.”
Through additional experiments, Alhadeff and colleagues discovered that what turns the AgRP neurons off more reliably is calories landing in the gut. Check out the video below to learn more about AgRP neurons and their role in hunger.
A new study published in the journal Cell has identified two distinct sets of neurons in the brain which are responsible for ‘hangry’ sensations.
The researchers behind the study, led by Professor Andres Lozano of the University of Toronto, deployed a novel combination of experiments in order to uncover the underlying neural mechanisms of the ‘hangry’ feeling.
The sensation of ‘hangry’ is characterised by a usually sudden change of mood that is brought about by hunger, typically resulting in irritability and possibly even aggression.
The study involved inducing ‘hangry’ states in mice through the deprivation of food and electrophysiologically characterizing the resulting neuronal activity.
The researchers were able to identify two distinct sets of neurons in the brain’s hypothalamus that play crucial roles in the feeling of ‘hangry’. The first set is the arcuate nucleus, which mediates hunger and is generally activated during periods of food deprivation. The second set was the medial amygdala, which plays a key role in the perception of reward, and when activated contributes to reward-based behaviours even when hungry.
The findings could have implications for future treatments of mood disorders, such as depression and anxiety, as it provides valuable insight into the neural substrates of affective states.
In addition, the study ensures a better understanding of how hunger contributes to mood, and can be used to better inform dieting trends.
Overall, this study by Professor Lozano’s team is a promising advancement in our understanding of the brains mechanisms underlying the ‘hangry’ feeling. The findings also open up the possibility for new treatments for mood disorders, as well as providing a greater insight into how hunger can influence mood and behaviour.
