Immunohistochemical Observation on the Distribution and Morphological Changes of GAP-43 Positive Structures in the Formation of Experimental Apical Periodontitis of Rat Molars

Apical periodontitis is a disease frequently observed, inducing discomfort. Nerves in periodontal ligament may be involved in the appearance of this symptom. Nerve endings of the periodontal ligament are reportedly stained with anti-calcitonin gene-related peptide (CGRP) antibody or anti-growth-associated protein 43 (GAP-43) antibody. Previous studies investigated the kinetics of structures positive for various antibodies on the formation of apical periodontitis, but no study has examined any GAP-43 immunoreactive structure. The purpose of this study was to experimentally induce apical periodontitis of rat molars and examine changes in a GAP-43 immunoreactive structure in the periodontal ligament. Using 10-week-old male Wistar rats, the mesial root of the lower right first molar was investigated. After general anesthesia, the mesial root canal pulp was exposed, and kept open for 2, 4, and 6 weeks. The mandible involving the molars was decalcified, and paraffin-embedded according to conventional methods to prepare sections 5 µm thick. Hematoxylin and eosin (H-E) staining and immunohistochemistry with anti-GAP-43 antibody were conducted. The morphology of an immunopositive structure was examined, and its expression per unit was statistically investigated. In the control group, a GAP-43 positive reaction was observed in the 1/3 area on the apical side. Morphologically, thin, short structures and thick branches were noted. In the experiment group, GAP-43 immunoreactive structures were only a few structures with branches. Furthermore, a GAP-43 positive reaction was localized in the apical area. We compared the percent GAP-43 immunopositive area per unit between the control and experiment groups. There was no significant difference at any point. This study showed that the morphology and distribution of GAP-43 immunoreactive structures existing in the periodontal ligament changed in the presence of apical periodontitis, suggesting that these changes may be related to the discomfort associated with apical periodontitis.