Heterogeneous responses to embryonic critical period perturbations within the Drosophila larval locomotor circuit

As developing neural circuits become functional, they undergo a phase of heightened plasticity that facilitates network tuning in response to intrinsic and/or extrinsic stimuli. These developmental windows are termed critical periods (CPs), because perturbations during, but not outside, the CP can lead to lasting and significant changes, such as the formation of sub-optimal or unstable networks. How separate, but connected elements, within a network might respond differently to a CP perturbation is not well understood. To study this, we used the locomotor network of the Drosophila larva as an experimental model. Using heat stress as a CP stimulus that has ecological relevance, we show that increasing ambient temperature elevates locomotor network activity. When embryos experience heat stress the subsequent development of their locomotor network is changed, creating larvae with reduced crawling speed and decreased network stability. Developing body wall muscles and central neurons are sensitive to heat stress perturbations during distinct, consecutive phases of embryogenesis. Selective manipulations showed that behavioural phenotypes caused by transient embryonic heat stress experience are caused by changes within the central locomotor circuitry, rather than at the peripheral neuromuscular junction. Within the CNS, we find transient embryonic CP perturbation leads to increased synaptic drive from premotor interneurons to motoneurons, which in turn adopt reduced excitability. In contrast, the peripheral neuromuscular junction maintains normal synaptic transmission, despite significant structural changes of synaptic terminal overgrowth and altered postsynaptic receptor field composition. Thus, working with this experimental insect model shows that temperature manipulations can be used as an ecologically relevant way to study CPs. Focusing on well-defined components of the locomotor network, from premotor interneurons to motoneurons to muscles, we show that connected elements respond differentially to a CP perturbation, suggesting a sequence, or hierarchy, of network adjustment during the embryonic CP.