In the lab, mice made choices that would boost their dopamine levels.
Senior investigator and New York University Langone neuroscientist Margaret Rice, PhD, and her team claim that insulin plays a much stronger role than previously known in regulating release of dopamine.
Insulin is the hormone essential to all mammals for controlling blood sugar levels and giving a feeling of being full after eating; dopamine is a neurotransmitter that helps control the brain's reward and pleasure centers.
The findings reaffirm that insulin helps trigger the reuptake of dopamine when insulin levels rise; they also show for the first time that the net effect is a rise in dopamine levels.
The scientists may be the first to demonstrate that insulin's role in the dopamine pathway may affect and explain food choices.
Insulin boosts dopamine levels in reward centers
In one set of experiments on rodents, a rise in insulin led to 20-55% more dopamine being released in the striatal region of the brain. This is the area where dopamine's effects on the brain are felt, and which governs the body's response to getting a reward.
The rise coincided with an increase in insulin activity, as the insulin processed any food sugars the mice and rats ate. It occurred despite the reabsorption, or reuptake, of dopamine that in other regions of the brain tells an animal that its appetite is satisfied.
In separate experiments, rats that were fed low-calorie diets had a 10-fold greater sensitivity to increasing insulin levels in the brain. In other words, dopamine release occurred after only one tenth of a rise in insulin levels as seen in rats on a normal diet.
By contrast, rats on high-calorie diets lost all striatal-brain insulin responsiveness.
Role for insulin in the body's reward system
In addition, when offered a choice between a drink reward paired with either an insulin antibody injection to block hormone signaling, or a mock placebo injection, the rats always favored the drink-injection combination that led to intact insulin signaling, which in turn led to more dopamine.
This appears to indicate a new role for insulin as part of the brain's reward system. It suggests that rodents, and presumably people, may choose to consume high-carbohydrate or low-fat meals that release more insulin, in order to heighten dopamine release.
Rice says this finding is important because chronically elevated insulin levels and lowered insulin sensitivity in the brain are closely tied to obesity and type II diabetes, both very prevalent in the US.
The team plans further experiments on how insulin influences the mammalian brain's control over food motivation and reward pathways, and whether changes in insulin sensitivity brought about by obesity can be reversed or even prevented.
"If our future experiments prove successful, it could confirm our hypothesis that when people refer to an insulin-glucose rush, they may really be referring to a dopamine reward rush. And there are healthy ways to get that by making smart food choices."