Sensitivity of rat temporalis muscle afferent fibers to peripheral N-methyl-D-aspartate receptor activation.

The temporalis muscle is a common source of pain in headache and chronic craniofacial pain conditions such as temporomandibular disorders, which have an increased prevalence in women. The characteristics of slowly conducting temporalis afferent fibers have not been investigated. Therefore, the aim of the present study was to examine the characteristics of slowly conducting temporalis muscle afferent fibers and to determine whether these fibers are excited by activation of peripheral N-methyl-D-aspartate receptors. The response properties of a total of 117 temporalis afferent fibers were assessed in male and female rats. A majority of these fibers had high mechanical thresholds and slow conduction velocities (<10 m/s). The mechanical threshold of the temporalis afferent fibers was inversely correlated with afferent conduction velocity, however, no sex-related differences in mechanical threshold were identified. There were also no sex-related differences in N-methyl-D-aspartate-evoked afferent discharge. Indeed, injection of a high concentration (1600 mM) of N-methyl-D-aspartate into the temporalis muscle was necessary to evoke significant afferent discharge. Thirty minutes after the initial injection of N-methyl-D-aspartate into the temporalis muscle, a second injection of N-methyl-D-aspartate produced a response only about 50% as large as the initial injection. Co-injection of ketamine (20 mM) with the second injection of N-methyl-D-aspartate significantly decreased N-methyl-D-aspartate-evoked afferent discharge in both sexes. This concentration of ketamine is greater than that needed to attenuate afferent discharge evoked by injection of glutamate into the masseter muscle. These results suggest that unlike masseter afferent fibers, temporalis afferent fibers are relatively insensitive to peripheral N-methyl-D-aspartate receptor activation.