Masseter Ganglion Neurons Express Acid Sensing Ion Channels (ASIC)

Objectives: In humans, intramuscular infusion of low pH solutions evokes muscle pain (Eur J Pain 2000;4:195-209). These responses are thought to be mediated in part by activation of ASIC. Several different ASIC subtypes have been described which include the ASIC1, that are rapidly desensitizing cation channels activated around pH 6.9, and the ASIC3, which has a sustained cation current but is activated around pH 6.0. The present study examined whether trigeminal ganglion neurons that innervate the masseter muscle express ASIC1 or ASIC3 in rats.

Methods: The fluorescent tracer dye fast blue was injected bilaterally into the masseter muscle of 2 male and 2 female Sprague Dawley rats to identify masseter muscle trigeminal ganglion neurons. Rats were euthanized seven days after injection with an overdose of pentobarbital and the trigeminal ganglia removed and cut into thin sections with a vibratome. The expression of ASIC1 and ASIC3 by masseter ganglion neurons was investigated using commercially available antibodies (Sigma).

Results: Masseter ganglion neuron expression of ASIC3 (55%) was significantly greater than ASIC1 (43%) (p<0.05, Mann-Whitney test). Expression of both receptors was greatest in medium to large (>500 µm²) ganglion neurons. In female rats expression of both channels (ASIC1-47%, ASIC3-60%) was greater than in male rats (ASIC1-36%, ASIC3-49%).

Conclusions: Both ASIC are expressed by medium to large masseter ganglion neurons, which suggests that many masseter nociceptors are likely to be excited by decreased muscle pH. The extent of expression of both ASIC channels by masseter ganglion neurons may also depend on biological sex.