A new study on the hormone ghrelin shows that hunger and decision-making do not mix.
Impulsivity affects everyone to differing degrees, and each individual can be more or less impulsive depending on the situation.
Forgoing something pleasurable now, in favor of something better later on, shows control. This so-called delayed gratification is regarded as the opposite of impulsive behavior.
Impulsivity can be broken down into two types: Impulsive action, in other words, the inability to stop one's self from making a physical action; and impulsive choice, an inability to delay gratification.
Although most people can control their impulses sufficiently, impulsivity is a major factor in a number of conditions, including attention deficit hyperactivity disorder (ADHD), obsessive-compulsive disorder (OCD), eating disorders, and substance abuse.
This connection to various psychiatric conditions makes impulsivity an important area of study.
Earlier studies have uncovered a relationship between food reward behavior and impulsivity. However, a mechanism has not yet been proven.
A new study, published recently in Neuropsychopharmacology, aimed to fill this gap. The researchers investigated impulsivity in rats, specifically in relation to the hormone ghrelin.
What is ghrelin?
Ghrelin is a hormone, produced in the gastrointestinal tract, that acts on the central nervous system. It is released when the stomach is empty. Once the stomach has become filled, production of ghrelin ceases. Ghrelin readies the body for food, and it also works on cells of the hypothalamus to induce the feeling of hunger.
The role of ghrelin is not limited to the hunger response alone. It has also been implicated in the reward behavior associated with drugs, alcohol, and food intake.
Researchers at the Sahlgrenska Academy, led by Karolina Skibicka, set out to investigate ghrelin's potential role in impulsive behavior.
The team trained rats to perform a variety of tasks that allowed them to measure impulsive behavior. The first, referred to as the "go/no-go" test, measured the rats ability to restrain a response.
Rats were trained to either press a lever to get a reward - referred to as a "go" signal - or they were rewarded for not pressing a lever - a "no-go" signal. The rats were taught to either "go," or "no-go," dependent on an auditory signal (a light or buzzer).
A second trial, called the "differential reinforcement of low rate," provided rats with a food pellet reward only if they were able to withhold their response for a set period of time.
The third leg, called "delay discount," measured the rats' ability to delay gratification. The rats were presented with two levers, one of which would dispense one food pellet as soon as it was pressed, while the other would dispense four food pellets, but only after a significant delay.
If the first lever was pressed, the second was blocked. In this way, the rats were taught to reject their initial impulse in order to receive the maximum reward later on.
Ghrelin and impulsivity
During the experiment, ghrelin was injected directly into the rats' brains, replicating how the hormone would normally behave when the animals were hungry.
As expected, the injection made the rats unable to resist pressing the lever in all three trials. In other words, impulsivity had increased.
In fact, in the "go/no-go" trial, the rats were almost three times more likely to press the lever during a "no-go" period when their brains were infused with ghrelin.
Further to this, the researchers found that just a short period of fasting gave the same impulsive results in the rats.
Skibicka and her team managed to pinpoint the area of the brain that appears to be involved in this impulsive behavior.
"Our results showed that restricting ghrelin effects to the ventral tegmental area, the part of the brain that is a crucial component of the reward system, was sufficient to make the rats more impulsive. Importantly, when we blocked ghrelin, the impulsive behavior was greatly reduced."
Karolina Skibicka, associate professor
These findings are the first to demonstrate that ghrelin increases impulsivity in rats. The researchers hope that the findings might assist in the development of new psychoactive drugs.
Ghrelin antagonists (drugs that block ghrelin) are already being studied for their potential use as anti-obesity medications, and to help manage drug intake in addicts.
As researchers uncover more details about the actions of this fascinating hormone, other therapeutic avenues look sure to open up. Skibicka hopes that, eventually, the brain's ghrelin receptors could be a target for the "treatment of psychiatric disorders that are characterized by problems with impulsivity."