Comprehensive phenotyping of sensory neurons using an ex vivo somatosensory system

In adults, primary sensory neurons exhibit pronounced diversity in many phenotypic traits including peripheral response properties, somal spike shape, neurochemical content, and laminar distribution of projections in the spinal dorsal horn. While these traits are correlated with sensory submodality type, single traits are not sufficient to accurately characterize individual neurons. We have recently developed a novel mouse ex vivo somatosensory preparation that allows the examination of multiple traits for individual cutaneous sensory neurons. Here we describe the results from our initial studies of adult and neonatal mice employing this preparation. Adult mice were anesthetized and perfused with chilled oxygenated artificial cerebrospinal fluid (aCSF). The spinal cord, dorsal root ganglia (DRGs), several attached dorsal cutaneous nerves (DCNs), and skin were isolated and placed in a chamber with a circulating bath of oxygenated aCSF. Sensory neuron somata were impaled, their cutaneous mechanical response properties were determined, and then one soma/ganglion was injected with Neurobiotin (NB). Spinal cord sections containing the cell's central projections were reacted with horseradish peroxidase-conjugated avidin and visualized using diaminobenzidene. DRG sections were reacted with primary antisera for calcitonin gene-related peptide (CGRP), and the NB-stained somata were visualized using avidin-FITC. In these initial studies, neonatal cutaneous sensory neurons are found to be virtually miniature replicas of the their adult counterparts. These findings challenge long-held beliefs that the laminar distribution of the central projections and somal spike properties of cutaneous low-threshold mechanoreceptors (LTMRs) undergo delayed maturation during the first 2-3 weeks of postnatal development.