Blood glucose levels are controlled by the autonomous nervous system. Parasympathetic axons directly influence islet function, leading to higher insulin release and lower levels of blood glucose. The results are published in December 2012 in PNAS.
The autonomous nervous system responsible for the unconscious regulation consists of parasympathetic and sympathetic parts, managing the calm and stressful conditions respectively. The scientific community has under many years speculated that the autonomous nervous system controls levels of blood glucose:
- To prepare for flight, the sympathetic nervous system is anticipated to lower levels of insulin production and leave more glucose in the blood to provide more energy for the body.
- During a relaxed period, the parasympathetic system stimulates islets to produce more insulin and reduce the levels of glucose.
Recently, researchers in PO Berggren’s group in Miami have for the first time proven that autonomic nerves influence blood sugar levels.
To study the direct interaction between the axons and the insulin-producing beta cells, the researchers transplanted islets into the anterior chamber of the mouse eye using the InSight Technology. The islets became fully innervated by axons in a manner similar to that in the pancreas.
The researchers then exposed the mice to light and darkness to stimulate the parasympathetic and sympathetic nervous system respectively. The pupil contracts on exposure to light, sending a signal to the parasympathetic system. In contrast, when in darkness, the pupil opens, signalling to the sympathetic system.
The experiments showed the following:
- When mice were in bright light, transplanted islets released more insulin, lowering the levels of blood sugar. This effect was inhibited by blocking the parasympathetic pathway with a drug applied as an eye drop.
- When the animals were put into darkness, the islets released less insulin, resulting in higher blood glucose levels.
- There is a connection between the density of innervation and how levels of glucose are controlled: The stronger the innervation, the better it compensates for increased insulin demands.
These findings are fundamental for understanding the dependency of insulin secretion on the autonomous nervous system. In the longer run, they may explain the development and progress of diabetes and potentially new ways to treat the disease.