Substance P-expressing Neurons in the Superficial Dorsal Horn of the Mouse Spinal Cord: Insights into Their Functions and their Roles in Synaptic Circuits

Erika Polgár,Andrew M Bell,Maria Gutierrez-Mecinas,Allen C Dickie,Oğuz Akar,Miruna Costreie,Masahiko Watanabe,Andrew J Todd

doi:10.1016/j.neuroscience.2020.06.038

Erika Polgár, Andrew M Bell + Show 6 more

Open Access

https://doi.org/10.1016/j.neuroscience.2020.06.038

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Journal: Neuroscience	Publication Date: Jul 4, 2020
Citations: 13	License type: cc-by

Affiliation: Hokkaido University, University of Glasgow

Abstract

The tachykinin peptide substance P (SP) is expressed by many interneurons and some projection neurons in the superficial dorsal horn of the spinal cord. We have recently shown that SP-expressing excitatory interneurons in lamina II correspond largely to a morphological class known as radial cells. However, little is known about their function, or their synaptic connectivity. Here we use a modification of the Brainbow technique to define the excitatory synaptic input to SP radial cells. We show that around half of their excitatory synapses (identified by expression of Homer) are from boutons with VGLUT2, which are likely to originate mainly from local interneurons. The remaining synapses presumably include primary afferents, which generally have very low levels of VGLUT2. Our results also suggest that the SP cells are preferentially innervated by a population of excitatory interneurons defined by expression of green fluorescent protein under control of the gene for gastrin-releasing peptide, and that they receive sparser input from other types of excitatory interneuron. We show that around 40% of lamina I projection neurons express Tac1, the gene encoding substance P. Finally, we show that silencing Tac1-expressing cells in the dorsal horn results in a significant reduction in reflex responses to cold and radiant heat, but does not affect withdrawal to von Frey hairs, or chloroquine-evoked itch.

Highlights

The neuropeptide substance P (SP) was identified in a plexus of axons in the superficial dorsal horn (SDH; laminae I-II) by Hokfelt et al (1975), who showed that some of these axons originated from a population of SP-expressing primary afferent neurons
In a series of recent studies (Gutierrez-Mecinas et al, 2017; GutierrezMecinas et al, 2018; Dickie et al, 2019), we have shown that SP-positive excitatory interneurons are numerous in lamina II, that many of them correspond to a population that had previously been defined by Grudt and Perl (2002) as radial cells, and that they can give rise to long propriospinal projections that travel in the dorsolateral fasciculus and target the lateral spinal nucleus (LSN)
Consistent with the transcriptomic data, we showed that they are distinct from other neurochemical populations of excitatory interneurons, which can be defined by expression of neurotensin, neurokinin B (NKB), cholecystokinin (CCK) and neuropeptide FF (NPFF) (GutierrezMecinas et al, 2019a; Gutierrez-Mecinas et al, 2019b)

Summary

INTRODUCTION

The neuropeptide substance P (SP) was identified in a plexus of axons in the superficial dorsal horn (SDH; laminae I-II) by Hokfelt et al (1975), who showed that some of these axons originated from a population of SP-expressing primary afferent neurons. Consistent with the transcriptomic data, we showed that they are distinct from other neurochemical populations of excitatory interneurons, which can be defined by expression of neurotensin, neurokinin B (NKB), cholecystokinin (CCK) and neuropeptide FF (NPFF) (GutierrezMecinas et al, 2019a; Gutierrez-Mecinas et al, 2019b). They do not overlap significantly with cells that contain enhanced green fluorescent protein (eGFP) in the GRP::eGFP mouse line, in which eGFP is expressed under control of the promoter for gastrin-releasing peptide (GRP) (Dickie et al, 2019; Bell et al, 2020). Our final aim was to test the effect of silencing spinal Tac1expressing cells on nocifensive reflexes

EXPERIMENTAL PROCEDURES

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