They are part of the brain of almost all animal species, but they usually remain invisible even under an electron microscope. “Electrical synapses are like the dark matter of the brain,” says Alexander Borst, director of MPI for biological intelligence, in the (if) foundation. Today, a team from his department took a closer look at this rarely explored brain component: In the fruit fly brain Drosophilathey were able to show that electrical synapses occur in almost all areas of the brain and can influence the function and stability of individual nerve cells.
Neurons communicate via synapses, small contact points at which chemical messengers transmit a stimulus from one cell to another. We may remember that from biology class. However, that’s not the whole story. In addition to the commonly known chemical synapses, there is a second, little-known type of synapse: the electrical synapse. “Electrical synapses are much rarer and harder to detect with current methods. This is why they have hardly been studied so far”, explains Georg Ammer, who has long been fascinated by these hidden cellular connections. “In most animal brains, therefore, we don’t even know the basic things, such as exactly where electrical synapses occur or how they influence brain activity.”
An electrical synapse directly connects two neurons, allowing the electrical current that neurons use to communicate to flow from cell to cell without detours. Except in echinoderms, this particular type of synapse occurs in the brain of all animal species studied so far. “Electrical synapses must therefore have important functions: we just don’t know which ones!” Georg Ammer said.
Distribution in the brain
To track down these functions, Ammer and her two colleagues, Renée Vieira and Sandra Fendl, tagged an important protein component of electrical synapses. In the brain of Drosophila, they were able to show that electrical synapses do not occur in all nerve cells, but in almost all areas of the brain. By selectively turning off electrical synapses in the visual processing area, researchers could show that the reaction of affected neurons to certain stimuli is much weaker. Additionally, without electrical synapses, certain types of nerve cells became unstable and began to oscillate spontaneously.
“The results suggest that electrical synapses are important for various brain functions and can play very different functional roles, depending on the type of neuron,” summarizes Ammer. “These synapses should therefore also be incorporated into connectome studies.” The connectome is a map of all neurons and their connections in a brain or brain area. Often this information is reconstructed from electron microscope images, where electrical synapses are largely invisible. How these can be incorporated into investigations of the connectome and other secrets that electrical synapses hold is the subject of further study.
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