Central Insulin Modulates Dopamine Signaling in the Human Striatum

Activity in the dopaminergic pathways of the brain is highly sensitive to body weight and metabolic states. Animal studies show that dopamine neurons are important targets for the metabolic hormone insulin with abolished effects in the insulin-resistant state, leading to increases in body weight and food intake. In humans, the influence of central acting insulin on dopamine and effects of their interplay are still elusive.

We investigated whether central administered insulin influences dopaminergic activity in striatal regions and whole-brain neural activity. Using a positron emission tomography (PET)/magnetic resonance imaging (MRI) hybrid scanner, we simultaneously performed [11C]-raclopride-PET and resting-state functional MRI in 10 healthy normal-weight men after application of intranasal insulin or placebo on 2 separate days in a randomized, placebo-controlled, blinded, crossover trial.

In response to central insulin compared with placebo administration, we observed greater [11C]-raclopride binding potential in the bilateral ventral and dorsal striatum. This suggests an insulin-induced reduction in synaptic dopamine levels. Resting-state striatal activity was lower 15 and 30 minutes after nasal insulin compared with placebo. Functional connectivity of the mesocorticolimbic circuitry associated with differences in dopamine levels: individuals with a stronger insulin-induced effect on dopamine levels showed a stronger increase in functional connectivity 45 minutes after intranasal insulin.

This study indicates that central insulin modulates dopaminergic tone in the striatum, which may affect regional brain activity and connectivity. Our results deepen the understanding of the insulin-dopamine interaction and the complex network that underlies the regulation of whole-body metabolism.