Candidate neural substrates for off-edge motion detection in Drosophila

Area of Science:

• Neuroscience
• Visual Processing
• Insect Vision

Background:

• T4 and T5 neurons are key relay cells in the fly's visual motion detection pathway, responding to small-field motion.
• Previous work identified columnar neurons relaying ON-channel input to T4, suggesting a substrate for elementary motion detectors (EMDs).
• The interneurons relaying the OFF-channel input to T5 from the L2 neuron remained unknown.

Purpose of the Study:

• To identify the interneurons that provide OFF-channel input to T5 neurons.
• To elucidate the circuit mechanisms underlying motion detection in T5 cells.
• To investigate the potential substrates for an elementary motion detector (EMD) in the T5 pathway.

Main Methods:

• Investigated inputs from transmedulla (Tm) neurons to T5 neurons in the lobula.
• Analyzed synaptic connections and spatial segregation of inputs onto T5 dendrites.
• Performed transcript profiling of T5 neurons for cholinoceptor expression.
• Examined choline acetyltransferase (ChAT) expression in presynaptic Tm neurons.

Main Results:

• Multiple Tm neuron types (Tm1, Tm2, Tm4, Tm9) provide complex, multi-columnar inputs to T5.
• Tm9 relays input from the L3 interneuron, suggesting combinatorial processing with the L2 pathway.
• Spatially segregated inputs from Tm2 and Tm9, and from Tm1 and Tm2, provide candidate substrates for T5 EMD circuits.
• T5 neurons express nicotinic and muscarinic cholinoceptors, and Tm2 and Tm9 express ChAT, indicating cholinergic signaling.

Conclusions:

• T5 neurons receive diverse inputs from multiple Tm neurons, forming a complex circuit.
• The identified inputs and T5's cholinoceptor expression support a model where T5 computes motion signals.
• A hypothesized T5 EMD circuit integrates multiple cholinergic inputs via nicotinic and muscarinic receptors.