Heterogeneous responses to embryonic critical period perturbations among different components of the Drosophila larval locomotor circuit

As developing neural circuits become functional, they undergo a phase of heightened plasticity in response to intrinsic and/or extrinsic stimuli. These developmental windows are termed critical periods (CPs), because perturbations during the CP can lead to lasting and significant change in subsequent development, such as sub-optimal and/or unstable networks. By contrast, the same manipulations before or after the CP does not create lasting changes. Here, we have used the Drosophila larval locomotor network to study how different identified, connected elements respond to a CP perturbation, from pre-motor interneuron to motoneuron, to neuromuscular junction. Using heat stress as an ecologically relevant stimulus, we show that increasing temperature causes increased network activity that, when applied during the CP, leads to larvae that crawl more slowly and that require longer to recover from electroshock-induced seizures, indicative of decreased network stability. Within the central nervous system, we find CP perturbation leads to pre-motor interneurons delivering increased synaptic drive to motoneurons, which in turn display reduced excitability. The peripheral neuromuscular junction, on the other hand, maintains normal synaptic transmission, despite significant structural changes of synaptic terminal overgrowth and altered postsynaptic receptor field composition. Overall, our data demonstrate that different connected elements within a network respond differentially to a CP perturbation. Our results suggest an underlying sequence, or hierarchy, of network adjustment during developmental CPs, and present the larval locomotor network as a highly tractable experimental model system with which to study CP biology.