Immunocytochemical localization of substance P neurokinin-1 receptors in rat gingival tissue

The distributions of substance P (SP) and the neurokinin-1 receptor (NK₁-R), the receptor preferentially activated by SP, were examined in rat gingiva by immunocytochemical methods with light and electron microscopy. SP-immunoreactive nerve fibers were located preferentially in the junctional epithelium (JE) but few in the other oral and oral sulcular epithelia. NK₁-R immunoreactivity was found in the endothelial cells (capillaries and postcapillary venules underlying the JE). NK₁-R-labeled and -unlabeled unmyelinated nerve fibers were located close to the blood vessels and partially or completely covered by a Schwann cell sheath. In the JE, labeled naked axons without Schwann cell sheaths were observed. Neutrophils and macrophages in the connective tissue underlying the JE and in the JE were also labeled with NK₁-R. Furthermore, NK₁-R was found in the JE cells. Basically, immunoreaction products for NK₁-R were found throughout various cells (endothelial cells, neutrophils, and JE cells) at invaginations of the plasma membrane and in vesicular and granular structures that are probably endosomes and are found close to both the plasma membrane and the nucleus. This is a first report, demonstrating the presence of NK₁-R in the gingival tissue in the normal nonstimulated condition. Furthermore, it is thought that SP may modulate the permeability of blood vessels beneath the JE, the production of antimicrobial agents in neutrophils, and the proliferation and endocytotic ability of JE cells through NK₁-R.