Distribution and possible function of transient receptor potential melastatin-7 in the rat cranial sensory ganglia

The distribution of transient receptor potential melastatin 7 (TRPM7) was investigated in the trigeminal (TG), petrosal (PG), jugular (JG), and nodose ganglia (NG) in a rat model. Approximately half of the sensory neurons contained TRPM7-immunoreactivity in the TG, PG, JG, and NG. TRPM7-immunoreactive (-IR) sensory neurons had small- to medium-sized cell bodies. Triple immunofluorescence analysis revealed that TRPM7-IR sensory neurons showed mainly transient receptor potential vanilloid 1 (TRPV1) immunoreactivity, whereas less than 10% of TRPM7-IR neurons contained TRPV2-immunoreactivity. In the facial skin, some nerve fibers contained TRPM7-immunoreactivity and formed free nerve endings. Using a retrograde tracing method, 56% of cutaneous TG neurons contained TRPM7-immunoreactivity. Approximately half of TRPM7-IR cutaneous TG neurons contained TRPV1-immunoreactivity. Approximately two-thirds of PG, JG, and NG neurons innervating the external ear canal skin contained TRPM7-immunoreactivity. Most TRPM7-IR cutaneous PG, JG, and NG neurons contained TRPV1-immunoreactivity. However, TRPM7-IR neurons that innervate the tooth pulp and pharynx are relatively rare. This study suggests that the distribution of TRPM7 depends on the size of nerve cells and peripheral innervation. TRPM7 may be involved in pain sensation in the trigeminal, glossopharyngeal, and vagal ganglia sensory neurons.