Semi-intact ex vivo approach to investigate spinal somatosensory circuits.

Junichi Hachisuka,Sarah E Ross,Lindsey M Snyder,H Richard Koerber,Yu Omori,Kyle M Baumbauer

doi:10.7554/elife.22866

Abstract

The somatosensory input that gives rise to the perceptions of pain, itch, cold and heat are initially integrated in the superficial dorsal horn of the spinal cord. Here, we describe a new approach to investigate these neural circuits in mouse. This semi-intact somatosensory preparation enables recording from spinal output neurons, while precisely controlling somatosensory input, and simultaneously manipulating specific populations of spinal interneurons. Our findings suggest that spinal interneurons show distinct temporal and spatial tuning properties. We also show that modality selectivity - mechanical, heat and cold - can be assessed in both retrogradely labeled spinoparabrachial projection neurons and genetically labeled spinal interneurons. Finally, we demonstrate that interneuron connectivity can be determined via optogenetic activation of specific interneuron subtypes. This new approach may facilitate key conceptual advances in our understanding of the spinal somatosensory circuits in health and disease.

Highlights

Noxious chemical, mechanical and thermal stimuli are conveyed to the dorsal horn of the spinal cord where they are integrated by excitatory and inhibitory interneurons
This preparation comprises of a large portion of spinal cord (~C2 – S3), together with lumbar 2 (L2) and L3 roots, ganglia, saphenous nerve and hindlimb skin, dissected in continuum (Figure 1A and B)
Spinal segments L2 and L3, which are targeted by the saphenous nerve, are regions of the spinal cord that are well-suited for visually-guided whole cell recordings because they contain regions of grey matter that are clearly visible between gaps in the roots (Figure 1C, dotted ovals)

Summary

Introduction

Mechanical and thermal stimuli are conveyed to the dorsal horn of the spinal cord where they are integrated by excitatory and inhibitory interneurons. With the advent of a new generation of molecular genetic tools, it has been possible to overcome this constraint by recording from genetically defined subsets of neurons (Bourane et al, 2015a, 2015b; Cui et al, 2016; Duan et al, 2014; Hantman et al, 2004; Heinke et al, 2004; Kardon et al, 2014; Peirs et al, 2015; Petitjean et al, 2015; Zeilhofer et al, 2005) Though, these experiments have been performed within the context of a spinal slice, which suffers from the shortcoming that key components of the nociceptive circuit, such as the input, are severed.

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