Sensory integration and neuromodulatory feedback facilitate Drosophila mechanonociceptive behavior

Abstract

Nociception is an evolutionary conserved mechanism to encode and process harmful environmental stimuli. Like most animals, Drosophila larvae respond to a variety of nociceptive stimuli, including noxious touch and temperature, with a stereotyped escape response through activation of multimodal nociceptors. How behavioral responses to these different modalities are processed and integrated by the downstream network remains poorly understood. By combining transsynaptic labeling, ultrastructural analysis, calcium imaging, optogenetic and behavioral analyses, we uncovered a circuit specific for mechano- but not thermo-nociception. Interestingly, integration of mechanosensory input from innocuous and nociceptive sensory neurons is required for robust mechano-nociceptive responses. We further show that neurons integrating mechanosensory input facilitate primary nociceptive output via releasing short Neuropeptide F (sNPF), the Drosophila Neuropeptide Y (NPY) homolog. Our findings unveil how integration of somatosensory input and neuropeptide-mediated modulation can produce robust modality-specific escape behavior.

Maintenance of cell type-specific connectivity and circuit function requires Tao kinase

“…Functional network analyses by these and additional studies have revealed a hierarchical network organization, multisensory integration, and modality and positionspecific network functions suggesting extensive processing and modulation of nociceptive inputs 18,24,29 . This system thus offers a unique opportunity to probe how CNS circuit growth is regulated while preserving specific connectivity and functional output.…”

“…Recent studies have described a hierarchical circuit architecture and multisensory integration in the larval somatosensory network, which integrates the activity of distinct sensory subsets and determines specific behaviors by inhibition or facilitation 18,24,25,27,48 . While individual larval sensory neuron subsets have been shown to respond to specific modalities like vibration (cho) 70 , noxious cold (cho, C3da) 45,47 , gentle (C2da, C3da) 21,46 and harsh touch (C4da) 21,71 , overlap in 2 nd order neuron innervation also results in extensive sensory integration (Fig.…”

“…To evaluate the extent of synapses formed by neurons in the larval nociceptive circuit, we focused our efforts on establishing methods to visualize and quantify connections between C4da and A08n neurons, which display extensive synaptic contact along the entire ventral nerve cord (VNC) 24 . To this end, we used three independent methods to assess synaptic connectivity by i) employing synapse-specific GFP reconstitution across synaptic partners (Syb-GRASP 30 ), ii) measuring the apposition of pre-and postsynaptic marker proteins 31 and iii) performing immuno-electron microscopy (EM) of synaptic markers labeling C4da-A08n neuron synapses 24 . We first quantified the number of synaptic GRASP puncta from C4da-A08n neuron synapses in 3 rd instar larvae at 96 h after egg laying (AEL) using blind analysis of deconvolved 3D image stacks with automatic thresholding of synaptic puncta (details in methods).…”

“…Indeed, most larval behaviors including nociceptive responses are conserved throughout all stages suggesting that the majority of larval circuits maintain their function during animal growth 23 . In recent years, a subset of C4da 2 nd order neurons has been studied in greater detail including A08n, DnB, Basin and mCSI neurons, which have been shown to be sufficient for nociceptive rolling behavior when activated by opto-or thermogenetic means 18,[24][25][26][27][28] .…”

“…We and others have previously characterized A08n interneurons, which are major postsynaptic partners of C4da neurons required for nociceptive behavior 24,27,28 . Here we characterize the developmental changes of the C4da-A08n circuit during larval growth at the synaptic level.…”