Summary: Electrical synapses occur in almost every brain region and can influence the stability and function of individual neurons.
Source: Max Planck Institute
Electrical synapses are omnipresent and yet hardly explored. They are part of the brain of almost every animal species, yet they remain usually invisible even under the electron microscope.
“Electrical synapses are like the dark matter of the brain,” says Alexander Borst, director at the MPI for Biological Intelligence, in foundation (i.f). Now a team from his department has published a study in Current Biology that has taken a closer look at this rarely explored brain component: In the brain of the fruit fly Drosophila, they were able to show that electrical synapses occur in almost all brain areas 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 the next. We may remember this from biology class. However, that is 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 are hard to detect with current methods. That’s why they have hardly been researched so far,” explains Georg Ammer, who has long been fascinated by these hidden cell connections.
“In most animal brains, we therefore don’t know even basic things, such as where exactly electrical synapses occur or how they influence brain activity.”
An electrical synapse connects two neurons directly, allowing the electrical current that neurons use to communicate, to flow from one cell to the next without a detour. Except in echinoderms, this particular type of synapse occurs in the brain of every animal species studied so far. “Electrical synapses must therefore have important functions; we just do not know which ones,” says Georg Ammer.
Distribution in the brain
To track down these functions, Ammer and his two colleagues, Renée Vieira and Sandra Fendl, labeled an important protein building block of electrical synapses. In the brains of fruit flies, they were thus able to show that electrical synapses do not occur in all nerve cells, but in almost all areas of the brain.
By selectively switching off the electrical synapses in the area of visual processing, the researchers could show that the affected neurons’ reaction to certain stimuli is much weaker. Furthermore, without electrical synapses, some nerve cell types became unstable and began to oscillate spontaneously.
“The results suggest that electrical synapses are important for diverse brain functions and can play very different functional roles, depending on the type of neuron,” Ammer summarizes.
“These synapses should therefore also be integrated in 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 integrated into connectome investigations and what other secrets electrical synapses hold is a subject for further studies.
About this neuroscience research news
Author: Press Office
Source: Max Planck Institute
Contact: Press Office – Max Planck Institute
Image: The image is credited to MPI for Biological Intelligence, i.f. / Julia Kuhl
Original Research: Open access.
“Anatomical distribution and functional roles of electrical synapses in Drosophila” by Georg Ammer et al. Current Biology
Abstract
Anatomical distribution and functional roles of electrical synapses in Drosophila
Highlights
- An immunohistochemistry-based map of innexin gap junctions in the Drosophila CNS
- VS/HS cells are electrically coupled to large cell networks via shakB gap junctions
- Loss of electrical synapses from VS/HS cells induces voltage and calcium oscillations
- Electrical synapses play functional roles in both ON and OFF vision pathways
Summary
Electrical synapses are present in almost all organisms that have a nervous system. However, their brain-wide expression patterns and the full range of contributions to neural function are unknown in most species.
Here, we first provide a light-microscopic, immunohistochemistry-based anatomical map of all innexin gap junction proteins—the building blocks of electrical synapses—in the central nervous system of Drosophila melanogaster.
Of those innexin types that are expressed in the nervous system, some localize to glial cells, whereas others are predominantly expressed in neurons, with shakB being the most widely expressed neuronal innexin.
We then focus on the function of shakB in VS/HS cells—a class of visual projection neurons—thereby uncovering an unexpected role for electrical synapses. Removing shakB from these neurons leads to spontaneous, cell-autonomous voltage and calcium oscillations, demonstrating that electrical synapses are required for these cells’ intrinsic stability.
Furthermore, we investigate the role of shakB-type electrical synapses in early visual processing. We find that the loss of shakB from the visual circuits upstream of VS/HS cells differentially impairs ON and OFF visual motion processing pathways but is not required for the computation of direction selectivity per see.
Taken together, our study demonstrates that electrical synapses are widespread across the Drosophila nervous system and that they play essential roles in neuronal function and visual information processing.
