Immunoelectron microscopic study on neuropeptide Y in the periodontal ligament of the incisor following peripheral nerve injury to the inferior alveolar nerve in the rat

Immunoelectron microscopic analysis was carried out to examine whether neuropeptide Y (NPY)-like immunoreactivity (-LI) is localized in mechanoreceptors in the lingual periodontal ligament of the rat incisor following peripheral nerve injury to the inferior alveolar nerve (IAN). In the lingual periodontal ligament of normal animals, no NPY-like immunoreactive (-IR) primary afferents were observed, except for a very few sympathetic perivascular nerve fibers which showed NPY-LI. Fourteen days following chronic constriction injury to the IAN combined with sympathectomy of the superior cervical ganglion, thick NPY-IR nerve fibers showing tree-like raminifications were detected in the shear zone between the tooth-related part and alveolus-related part as well as in the alveolus-related part. Immunoelectron microscopy revealed that expanded NPY-IR nerve terminals were covered with several Schwann sheaths and that a part of the axoplasm expanded to the surrounding tissues. These ultrastructural features of NPY-IR structures were identical to those of periodontal Ruffini endings, categorized as slowly adapting mechanoreceptors. Thick (6–8 μm in diameter) NPY-IR axons were also observed without any apparent myelin sheath. The present results provide further evidence that NPY is closely associated with thick axons, probably myelinated nerves and Ruffini endings, following peripheral nerve injury.