The researchers believe that an understanding of the factors that control impulsive behaviour could provide new research areas for impulsive-linked neurodegenerative disorders, such as eating disorders and substance abuse.
Ghrelin, a hormone produced in the stomach, not only controls basic food intake but has also been linked to increases in reward behaviour for food, alcohol, and other substances of abuse.
Researchers from the University of Gothenburg enrolled adult male rats to test a hypothesis that ghrelin increased impulsivity.
The rats were trained in three complementary tests of impulsive behaviour and choice. One of the tests resulted in rats being rewarded with sugar when a lever was activated (“go”). Another test rewarded the rats only when they resisted pressing the lever (“no-go”).
The rats understood this by repeatedly being given a signal such as a flash of light or a buzzing sound that indicated which action should be executed in order to receive a reward.
Ghrelin injections
When these subjects were injected with ghrelin rats were unable to resist pressing the lever, when the “no-go” signal is given. The researchers interpreted this as a sign of impulsivity.
Researchers then took it to the next stage by injected ghrelin directly into the rat’s brains. Here, the rats were more likely to press the lever instead of waiting, despite it causing them loose their reward.
“For the first time, we have been able to show that increasing ghrelin to levels that are seen prior to meals or during fasting, causes the brain to act impulsively and also affects the ability to make rational decisions,” says Karolina Skibicka, docent at Sahlgrenska Academy, University of Gothenburg.
“Our results showed that restricting ghrelin affects the ventral tegmental area, the part of the brain that is a crucial component of the reward system, which was sufficient to make the rats more impulsive. Importantly, when we blocked ghrelin, the impulsive behaviour was greatly reduced,” she added.
The researchers pointed towards dopamine and opioid signalling as the main control mechanisms that determine the impact of ghrelin on reward.
Mechanism of action
The impact of ghrelin on reward is thought to be mediated by dopamine and opioid signalling. Previous studies have identified food reward behaviour as being positively linked to impulsive behaviour.
“The impact of ghrelin on neurotransmitters critical for regulation of impulsive behaviour, taken together with the regulatory role for ghrelin on food reward behaviour (a behaviour that correlates with impulsivity), led us to hypothesize that ghrelin's neurobiological role could extend to the regulation of impulsivity,” the authors wrote.
Impulsivity is a characteristic of many neuropsychiatric disorders and behaviour disorders such as ADHD, obsessive compulsive disorder (OCD), autism spectrum disorder (ASD), drug abuse and eating disorders.
“The therapeutic implications of our discovery are considerable,” the study noted. “This is especially considering that few substances currently available on the market have an ability to target all forms of impulsive responding.”
Source: Neuropsychopharmacology
Published online ahead of print, doi: 10.1038/npp.2015.297
“The Stomach-Derived Hormone Ghrelin Increases Impulsive Behavior.”
Authors: Rozita Anderberg et al.