Immunohistochemical detection of ENaCβ in the terminal Schwann cells associated with the periodontal Ruffini endings of the rat incisor

Epithelial sodium channels (ENaCs) are a subfamily of ion channels within the degenerin/ENaC (DEG/ENaC) superfamily. Previous studies have shown the immunolocalization of ENaC in the neural elements of the Cutaneous mechanoreceptors as well as dorsal root and trigeminal ganglion neurons, indicating the involvement of this molecule in mechanotransduction. The present study examined the expression of ENaC beta, a major component of ENaC protein, in the mechanoreceptive Ruffini endings in the periodontal ligament of the rat incisors by immunohistochemistry. The expression of ENaC beta in the trigeminal ganglion-which innervates the periodontal Ruffini endings-was also investigated at the mRNA and protein levels. Furthermore, double staining and a nerve injury experiment were applied to clarify its detailed localization in the periodontal Ruffini endings. ENaC beta immunoreaction in the trigeminal ganglion was recognizable in the comparatively large neurons which have been considered to mediate mechanotransduction. Immunohistochemistry for ENaC beta demonstrated dendritic ramifications of the Ruffini endings as well as the rounded cells in the periodontal ligament. Double staining with ENaC beta and either PGP9.5 or S-100 protein showed immunoreaction for ENaC beta in both the axonal and glial elements in the periodontal ligament. Some ENaC beta positive cells with rounded profiles were reactive to non-specific cholinesterase activity. Furthermore, a transection of the inferior alveolar nerve failed to eliminate the rounded cells with ENaC beta reaction, indicating that they were the terminal Schwann cells associated with the periodontal Ruffini endings. These findings suggest that ENaC beta is a key mechanotransducing channel in the periodontal Ruffini endings. Probably, the terminal Schwann cells together with the axon terminals regulate mechanotransduction in the periodontal endings.