Parvalbumin-, substance P- and calcitonin gene-related peptide-immunopositive axons in the human dental pulp differ in their distribution of varicosities

Sook Kyung Park,Sang Hyeok Seo,Yong Chul Bae,Youn Gyung Kim,So Young Choi,Ji Hyun Lee,Seung Ki Choi,Jin Wook Kim

doi:10.1038/s41598-020-67804-x

Sook Kyung Park, Sang Hyeok Seo + Show 6 more

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https://doi.org/10.1038/s41598-020-67804-x

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Journal: Scientific Reports	Publication Date: Jun 30, 2020
Citations: 4	License type: open-access

Affiliation: Kyungpook National University

Abstract

Information on the frequency and spatial distribution of axonal varicosities associated with release of neurotransmitters in the dental pulp is important to help elucidate the peripheral mechanisms of dental pain, mediated by myelinated versus unmyelinated fibers. For this, we investigated the distribution of axonal varicosities in the human dental pulp using light- and electron-microscopic immunohistochemistry for the vesicular glutamate transporter 2 (VGLUT2), which is involved in the glutamatergic transmission, and syntaxin-1 and synaptosomal nerve-associated protein 25 (SNAP-25), combined with parvalbumin (PV), which is expressed mostly in myelinated axons, and substance P (SP) and calcitonin gene-related peptide (CGRP), which are expressed mostly in unmyelinated axons. We found that the varicosities of the SP- and CGRP-immunopositive (+) axons were uniformly distributed throughout the dental pulp, whereas those of PV+ axons were only dense in the peripheral pulp, and that the expression of PV, VGLUT2, syntaxin-1, SNAP-25, SP and CGRP was significantly higher in the varicosities than in the axonal segments between them. These findings are consistent with the release of glutamate and neuropeptides by axonal varicosities of SP+ and CGRP+ unmyelinated fibers, involved in pulpal pain throughout the human dental pulp, and by varicosities of PV+ fibers, arising from parent myelinated fibers, and involved in dentin sensitivity primarily in the peripheral pulp.

Highlights

The dental pulp is densely innervated by nociceptive neurons; the release of glutamate by their axons is a crucial part of the mechanism of signaling of acute nociceptive pain and of pathological pain[1,2,3,4]
Most axons in the dental pulp that were immunostained for PV, substance P (SP) or calcitonin gene-related peptide (CGRP) were within axonal bundles in the radicular pulp and occasionally in the core of the coronal pulp
Quantitative analysis revealed that the number of varicosities per unit length of the PV+ axons was significantly higher in the peripheral pulp than in the core of the coronal and the radicular pulp

Summary

Introduction

The dental pulp is densely innervated by nociceptive neurons; the release of glutamate by their axons is a crucial part of the mechanism of signaling of acute nociceptive pain and of pathological pain[1,2,3,4]. The pulpal axons frequently exhibit multiple varicosities along their length[8,9,10,11], short segments where the axon is enlarged and contains multiple vesicles and m itochondria[12,13] They express two classes of proteins, which are involved in the loading of glutamate into synaptic vesicles and its release, the vesicular glutamate transporters (VGLUT) and the soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNARE)[9,10,14,15,16]. These findings suggest that pulpal axons release glutamate from their varicosities. We used electron microscopic immunohistochemistry to examine the varicosities that expressed PV, SP, CGRP, VGLUT2, and the SNARE proteins syntaxin-1 and synaptosomal nerve-associated protein 25 (SNAP-25)

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