Newly Identified Electrically Coupled Neurons Support Development of the Drosophila Giant Fiber Model Circuit

Abstract

The Drosophila Giant Fiber (GF) escape circuit is an extensively studied model for neuron connectivity and function. Researchers have long taken advantage of the simple linear neuronal pathway, which begins at peripheral sensory modalities, travels through the central GF Interneuron (GFI) to motor neurons, and terminates on wing/leg muscles. This circuit is more complex than it seems however, as there exists a complex web of coupled neurons connected to the GFI, which widely innervate the thoracic ganglion. Here, we define four new neuron clusters dye-coupled to the central GFI, which we name GF Coupled (GFC) 1-4. We identify new transgenic Gal4 drivers that express specifically in these neurons, and map both neuronal architecture and synaptic polarity. GFC1-4 share a central site of GFI connectivity, the Inframedial Bridge (IB), where the neurons each form electrical synapses. Targeted apoptotic ablation of GFC1 reveals a key role for proper development of the GF circuit, including the maintenance of GFI connectivity with upstream and downstream synaptic partners. GFC1 ablation frequently results in loss of one GFI, which is always compensated for by contralateral innervation from a branch of the persisting GFI axon. Overall, this work reveals extensively coupled interconnectivity within the GF circuit, and the requirement of coupled neurons for circuit development. Identification of this large population of electrically-coupled neurons in this classic model, and the ability to genetically manipulate these electrically synapsed neurons, expands the GF system capabilities for the nuanced, sophisticated circuit dissection necessary for deeper investigations into brain formation.

Significance Statement Genetic model neural circuits with individually identifiable neurons help us understand how nervous systems wire together during development, and then operate through coordinated chemical and electrical signaling. The Drosophila Giant Fiber circuit has long served as such a model, due to large neuron size, genetic malleability and easily visualized behavioral output: a jump in response to a threat. This study unveils new members of this circuit, all of which synapse with the circuit at one site on the central Giant Fiber Interneuron. We use new tools to identify and transgenically manipulate these neurons and show that these neurons are required for proper circuit development. This study provides a detailed circuit map for further dissection of neuronal connectivity and electrically-coupled communication.