Structural and functional synaptic plasticity induced by convergent synapse loss requires co-innervation in the Drosophila neuromuscular circuit

Abstract

Throughout the nervous system, the convergence of two or more presynaptic inputs on a target cell is commonly observed. The question we ask here is to what extent converging inputs influence each others structural and functional synaptic plasticity. In complex circuits, isolating individual inputs is difficult because postsynaptic cells can receive thousands of inputs. An ideal model to address this question is the Drosophila larval neuromuscular junction where each postsynaptic muscle cell receives inputs from two glutamatergic types of motor neurons (MNs), known as 1b and 1s MNs. Notably, each muscle is unique and receives input from a different combination of 1b and 1s motor neurons. We surveyed synapses on multiple muscles for this reason. Here, we identified a cell-specific promoter to ablate 1s MNs after innervation. Additionally, we genetically blocked 1s innervation. Then we measured 1b MN structural and functional responses using electrophysiology and microscopy. For all muscles, 1s MN ablation resulted in 1b MN synaptic expansion and increased basal neurotransmitter release. This demonstrates that 1b MNs can compensate for the loss of convergent inputs. However, only a subset of 1b MNs showed compensatory evoked activity, suggesting spontaneous and evoked plasticity are independently regulated. Finally, we used DIP- mutants that block 1s MN synaptic contacts; this eliminated robust 1b synaptic plasticity, raising the possibility that muscle co-innervation may define an activity "set point" that is referenced when subsequent synaptic perturbations occur. This model can be tested in more complex circuits to determine if co-innervation is fundamental for input-specific plasticity. SIGNIFICANCE STATEMENTIn complex neural circuits, multiple converging inputs contribute to the output of each target cell. Thus, each input must be regulated, but whether adjacent inputs contribute to this regulation is unclear. To examine input-specific synaptic plasticity in a structurally and functionally tractable system, we turn to the Drosophila neuromuscular circuit. Each muscle is innervated by a unique pair of motor neurons. Removal of one neuron after innervation causes the adjacent neuron to increase synaptic outgrowth and functional output. However, this is not a general feature since each MN differentially compensates. Also, robust compensation requires co-innervation by both neurons. Understanding how neurons respond to perturbations in adjacent neurons will provide insight into nervous system plasticity in both healthy and diseased states.