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

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.