Research has shown that dopamine (DA) plays a key role in how the body regulates the production of insulin. Typically, insulin is secreted by beta-cells in the pancreas in response to glucose—a process that is aptly called glucose-stimulated insulin secretion (GSIS). DA negatively regulates GSIS, leading to transient changes in the body’s levels of insulins. But the mechanism behind this regulation was unknown—until now.

A team led by scientists from the Tokyo Institute of Technology (Tokyo Tech) says it has uncovered the precise mechanism through which DA regulates insulin secretions. Using total internal reflection fluorescence microscopy, they were able to reveal that DA receptors D1 and D2 (proteins on cells that DA can bind to) act in concert to achieve the transient regulation of insulin. Their study, “Dopamine negatively regulates insulin secretion through the activation of D1-D2 receptor heteromer,” appears in Diabetes.

“There is increasing evidence that dopamine (DA) functions as a negative regulator of glucose-stimulated insulin secretion (GSIS); however, the underlying molecular mechanism remains unknown. Using total internal reflection fluorescence microscopy, we monitored insulin granule exocytosis in primary islet cells to dissect the effect of DA. We found that D1 receptor antagonists rescued the DA-mediated inhibition of glucose-stimulated calcium (Ca2+) flux, thereby suggesting a role of D1 in the DA-mediated inhibition of insulin secretion,” write the investigators.

“Overexpression of D2 but not D1 alone exerted an inhibitory and toxic effect that abolished the glucose-stimulated Ca2+ influx and insulin secretion in beta-cells. Proximity ligation and western blot assays revealed that D1 and D2 form heteromers in beta-cells. Treatment with a D1-D2 heteromer agonist, SKF83959, transiently inhibited glucose-induced Ca2+ influx and insulin granule exocytosis.

“Co-expression of D1 and D2 enabled beta-cells to bypass the toxic effect of D2 overexpression. DA transiently inhibited glucose-stimulated Ca2+ flux and insulin exocytosis by activating the D1-D2 heteromer.

“We conclude that D1 protects beta-cells from the harmful effects of DA by modulating D2 signaling.The finding will contribute to our understanding of the DA signaling in regulating insulin secretion and improve methods for preventing and treating diabetes.”

“We found that D1 receptor antagonists—drugs that block D1 receptors from activation—decreased the dopamine-mediated inhibition of insulin secretion. We also saw that overexpression of only D2 receptors on beta cells exerted an inhibitory and toxic effect and abolished insulin secretion in beta-cells. This gave us a clue to the mechanism of down-regulation,” explains Tokyo Tech’s Shoen Kume, PhD, who led the study.

The research team then performed proximity ligation and Western blot assays to study the receptors further. They found that D1 and D2 bound to each other to form a complex (heteromer). When activated by DA, this heteromer transiently inhibited insulin secretion. They also saw that when D1 and D2 were co-expressed on beta-cells, the cells were able to bypass the toxic effects of D2 overexpression.

According to Kume, “From these findings it can be concluded that D1 modulates D2 signaling to protect beta-cells from the harmful effects of DA. This study greatly improves our understanding of DA signaling in diabetes.”

Understanding the mechanism of DA signaling in the regulation of insulin secretion is sure to provide new therapeutic targets for the prevention, treatment, and management of diabetes.

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