Expression of Calretinin Among Different Neurochemical Classes of Interneuron in the Superficial Dorsal Horn of the Mouse Spinal Cord

Maria Gutierrez-Mecinas,Olivia Davis,Erika Polgár,Mahvish Shahzad,Keila Navarro-Batista,Takahiro Furuta,Masahiko Watanabe,David I Hughes,Andrew J Todd

doi:10.1016/j.neuroscience.2018.12.009

Maria Gutierrez-Mecinas, Olivia Davis + Show 7 more

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https://doi.org/10.1016/j.neuroscience.2018.12.009

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Abstract

Around 75% of neurons in laminae I-II of the mouse dorsal horn are excitatory interneurons, and these are required for normal pain perception. We have shown that four largely non-overlapping excitatory interneuron populations can be defined by expression of the neuropeptides neurotensin, neurokinin B (NKB), gastrin-releasing peptide (GRP) and substance P. In addition, we recently identified a population of excitatory interneurons in glabrous skin territory that express dynorphin. The calcium-binding protein calretinin is present in many excitatory neurons in this region, but we know little about its relation to these neuropeptide markers. Here we show that calretinin is differentially expressed, being present in the majority of substance P-, GRP- and NKB-expressing cells, but not in the neurotensin or dynorphin cells. Calretinin-positive cells have been implicated in detection of noxious mechanical stimuli, but are not required for tactile allodynia after neuropathic pain. Our findings are therefore consistent with the suggestion that neuropathic allodynia involves the neurotensin and/or dynorphin excitatory interneuron populations. Around a quarter of inhibitory interneurons in lamina I-II contain calretinin, and recent transcriptomic studies suggest that these co-express substance P. We confirm this, by showing that inhibitory Cre-expressing cells in a Tac1Cre knock-in mouse are calretinin-immunoreactive. Interestingly, there is evidence that these cells express low levels of peptidylglycine alpha-amidating monooxygenase, an enzyme required for maturation of neuropeptides. This may explain our previous finding that although the substance P precursor preprotachykinin A can be detected in some inhibitory interneurons, very few inhibitory axonal boutons are immunoreactive for substance P.

Highlights

The superficial dorsal horn of the spinal cord is innervated by primary afferents that respond to a variety of sensory modalities (Todd, 2010, Braz et al, 2014)
Our identification of distinct excitatory interneuron populations that express neurotensin, neurokinin B (NKB), gastrinreleasing peptide (GRP) or substance P was based on identification of neuropeptides that showed a restricted laminar distribution and were known to be expressed by not among the neurotensin cells is in good agreement with the data reported by Haring et al, since their neurotensin population (Glut4) showed very low expression of the Calb2 gene, whereas this was more highly expressed in their Glut5-7 (NKB)
We previously reported that different subsets of PKCc-immunoreactive neurons could be recognized, based on expression of neurotensin and preprotachykinin B (PPTB), and this distinction is supported by the findings of Haring et al (2018), who detected PKCc mRNA in Glut4 and Glut57 (NKB), as well as in Glut2-3

Summary

Introduction

The superficial dorsal horn of the spinal cord (laminae I-II) is innervated by primary afferents that respond to a variety of sensory modalities (Todd, 2010, Braz et al, 2014). The high ratio of interneurons to projection cells indicates that a considerable amount of sensory processing occurs within the superficial dorsal horn, consistent with the suggestion that neuronal circuits in this region are involved in gating pain and suppressing itch (Melzack and Wall, 1965, Braz et al, 2014).

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