Summary: Electrical synapses are found throughout the brain and can shape the stability and function of individual neurons.
Source: Max Planck Institute
Electrical synapses are widespread yet remain understudied. They exist across nearly every animal brain but are often invisible even in electron microscopy.
“Electrical synapses are like the dark matter of the brain,” says Alexander Borst, director at the Max Planck Institute for Biological Intelligence. A research team from his department has now published a study in Current Biology that examines these elusive connections more closely. Working in the brain of the fruit fly Drosophila, the researchers mapped electrical synapses across the central nervous system and demonstrated that these junctions can influence both neuronal function and intrinsic stability.
Neurons typically communicate at chemical synapses, where neurotransmitters cross the synaptic cleft to relay signals. But alongside these well-known chemical synapses, there is a second type: electrical synapses. These junctions create direct cytoplasmic continuity between cells via gap junctions, allowing ionic currents and small molecules to pass rapidly from one neuron to another.
“Electrical synapses are relatively rare and challenging to detect with standard techniques, which is why they have received far less attention,” explains Georg Ammer, one of the study’s lead authors. As a result, basic questions remain unanswered in many species: where exactly are electrical synapses distributed, and how do they alter brain activity?
Except for echinoderms, electrical synapses have been identified across the animal kingdom, suggesting they play important roles. To shed light on their distribution and function, Ammer and colleagues used an immunohistochemistry-based approach to label innexin proteins, the gap junction components that form electrical synapses in invertebrates. Their anatomical map reveals that while not every neuron contains electrical synapses, nearly all brain regions of Drosophila include innexin-based junctions.
Distribution and functional impact
The team found that different innexin types are expressed in distinct cellular populations: some localize primarily to glial cells, while others are predominantly neuronal. Among neuronal innexins, the shakB protein was the most broadly expressed. To probe function, the researchers selectively removed shakB from specific visual projection neurons known as VS and HS cells. Loss of shakB in these cells produced surprising results: many neurons became intrinsically unstable, exhibiting spontaneous voltage and calcium oscillations that were not present when electrical coupling remained intact.
In addition, disrupting electrical synapses within upstream visual circuitry reduced the responsiveness of certain neurons to visual motion stimuli. The effect differed across parallel visual pathways: ON and OFF motion pathways were impaired to different extents, while core computations such as direction selectivity were preserved. These findings indicate that electrical synapses can serve diverse roles depending on cell type and circuit context—supporting stability in some neurons and shaping sensory processing in others.
“Our results imply that electrical synapses contribute widely to brain function and can perform multiple, cell-specific roles,” Ammer summarizes. Given this diversity of effects, the authors argue that electrical synapses should be incorporated into connectome studies that aim to map brain wiring comprehensively.
Current connectomes are often reconstructed from electron microscopy datasets, where electrical synapses are frequently hard to detect. Integrating biochemical or light-microscopy markers for innexins could help reveal these hidden connections and refine our understanding of circuit architecture. How best to include electrical synapses in large-scale wiring maps, and what additional roles they play across behavior and development, remain promising avenues for future work.
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 atlas of innexin gap junctions across the Drosophila central nervous system
- VS and HS visual projection cells are electrically coupled to broad networks via shakB-containing gap junctions
- Removing electrical synapses from VS/HS cells triggers spontaneous voltage and calcium oscillations, revealing a role in maintaining cellular stability
- ShakB-type electrical synapses contribute to both ON and OFF visual motion pathways, affecting sensitivity without being strictly required for direction selectivity
Summary
Electrical synapses exist in virtually all animals with nervous systems, yet their distribution across the brain and the full scope of their contributions to neuronal function are poorly characterized in most species. This study presents a light-microscopy, immunohistochemistry-based map of innexin proteins—the invertebrate gap junction subunits—throughout the Drosophila central nervous system. The map shows cell-type-specific expression: some innexins are enriched in glia, while others are neuronal, with shakB being the most widespread neuronal innexin identified.
Focusing on shakB in VS and HS visual neurons uncovered an unexpected and essential role: electrical coupling via shakB stabilizes these neurons, preventing spontaneous intrinsic oscillations in membrane voltage and intracellular calcium. At the circuit level, ablating shakB in upstream visual pathways reduced neuronal responses to visual motion and differentially affected ON versus OFF processing streams, while leaving direction-selective computations largely intact.
Overall, the work demonstrates that electrical synapses are pervasive across the Drosophila nervous system and that they play critical, diverse roles in maintaining neuronal stability and shaping sensory information processing. Recognizing and mapping these connections will be important for building more complete connectomes and for understanding how electrical coupling contributes to neural computation and behavior.