Deciphering neural networks in the somatosensory system using single-cell transcriptomics and rabies tracing

Sammanfattning: Itch has evolved to protect us from malicious parasites keen to suck our blood or lay their eggs in our skin. We can detect both the movement of these parasites and the toxins they introduce with specialized neurons called pruriceptors. When we feel an itch, we get a desire to scratch it. Scratching an itch eases the itch sensation, and this is thought to be regulated by neuronal circuits in the spinal cord. This reactive aspect of itch makes is an interesting system to study as it involves both sensory and motor circuitry. The spinal cord hosts a vast number of different neuronal cell types, and better understanding of these are needed to efficiently delineate the circuitry between them. To find these cell types, we sequence the transcriptome of thousands of individual neurons in the dorsal horn of the spinal cord and identified 15 excitatory and 15 inhibitory neuronal populations (Paper I). Furthermore, we found that cell types expressing neuropeptide Y (NPY) contributed to the inhibition of chemically induced itch via the NPY receptor 2 (Paper II) and inhibition of somatostatin-induced itch via NPY receptor 1 (Paper III). We are currently mapping the neurons presynaptic to the NPY neurons using a Npy-Cre mouse line combined with monosynaptic rabies tracing and find inputs from the dorsal root ganglions, the spinal cord, and the brain (Paper IV). To help decipher circuit connectivity, we developed a method that links cell types expressing matching ligand and receptor pairs in single cell RNA-sequencing (scRNA-seq) datasets (Paper V). We furthermore used scRNA-seq to identify differences and similarities of locomotor circuitry related cells expressing doublesex and mab-3 related transcription factor 3 in zebrafish and mouse (Paper VI).In this thesis, we used a combination of powerful and novel tools to investigate questions that were previously difficult to address. It is my belief that spatial transcriptomics, now poised with the knowledge gained from scRNA-seq, will transform how we think about cell types in the central nervous system, since the location of a neuron is critical for its role in a circuit.