A taxonomy of transcriptomic cell types across the isocortex and hippocampal formation Graphical abstract Highlights d Single-cell transcriptomics from >1.3 million cells in the mouse cortex and hippocampus d Many neuron types specific to associational cortex and hippocampal regions are identified d Parallel cell-type and laminar organization between isocortex and hippocampal formation d Large-scale continuous neuron-type variation in isocortex and hippocampus/subiculum

SUMMARY Itch is the least well understood of all the somatic senses, and the neural circuits that underlie this sensation are poorly defined. Here we show that the atonal-related transcription factor Bhlhb5 is transiently expressed in the dorsal horn of the developing spinal cord and appears to play a role in the formation and regulation of pruritic (itch) circuits. Mice lacking Bhlhb5 develop self-inflicted skin lesions and show significantly enhanced scratching responses to pruritic agents. Through genetic fate-mapping and conditional ablation we provide evidence that the pruritic phenotype in Bhlhb5 mutants may be due to selective loss of a subset of inhibitory interneurons in the dorsal horn. Our findings suggest that Bhlhb5 is required for the survival of a specific population of inhibitory interneurons that regulate pruritis and provide evidence that the loss of inhibitory synaptic input results in abnormal itch.

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