SENSITIVITY OF RAT TEMPORALIS MUSCLE AFFERENT FIBERS TO PERIPHERAL N-METHYL-D-ASPARTATE RECEPTOR ACTIVATION X. D. DONG, a M. K. MANN, a B. J. SESSLE, b L. ARENDT-NIELSEN, c P. SVENSSON d AND B. E. CAIRNS a * a Faculty of Pharmaceutical Sciences, The University of British Colum- bia, 2146 East Mall, Vancouver, British Columbia, Canada V6T 1Z3 b Faculty of Dentistry, The University of Toronto, Toronto, Ontario, Canada M5G 1G6 c Laboratory for Experimental Pain Research, Center for Sensory- Motor Interaction, Aalborg University, DK-9220 Aalborg, Denmark d Department of Clinical Oral Physiology, Dental School, Århus Univer- sity, DK-8000 Århus C, Denmark Abstract—The temporalis muscle is a common source of pain in headache and chronic craniofacial pain conditions such as temporomandibular disorders, which have an in- creased prevalence in women. The characteristics of slowly conducting temporalis afferent fibers have not been investi- gated. Therefore, the aim of the present study was to examine the characteristics of slowly conducting temporalis muscle af- ferent 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 tempo- ralis 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 affer- ent 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-as- partate 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 de- creased 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 injec- tion of glutamate into the masseter muscle. These results suggest that unlike masseter afferent fibers, temporalis affer- ent fibers are relatively insensitive to peripheral N-methyl-D- aspartate receptor activation. © 2006 IBRO. Published by Elsevier Ltd. All rights reserved. Key words: nociception, craniofacial, mechanical, temporo- mandibular, jaw, trigeminal. The temporalis and masseter muscles are the principal jaw-closing muscles of mastication. The temporalis muscle can be a source of pain referral in headache and both muscles can be involved in chronic craniofacial pain con- ditions such as temporomandibular disorders (Dworkin et al., 1990; Rasmussen et al., 1991). The temporalis muscle is innervated by sensory afferent fibers whose cell bodies are contained in the caudal–lateral part of the tri- geminal ganglion as well as in the mesencephalic nucleus (Limwongse and Desantis, 1977; Yassin and Leong, 1979). Large diameter, fast conducting A (group Ia) and A (group II) afferent fibers whose cell bodies are found within the mesencephalic nucleus of the brainstem, appear to play a role in proprioception by innervating temporalis muscle spindles, whereas A (group III) and C (group IV) fibers whose cells of origin lie within the trigeminal ganglion are thought to mediate higher threshold mechanical sensa- tions, such as masticatory muscle pain (Capra and Des- sem, 1992; Yoshida et al., 1999). While the anatomical projections and response properties of spindle afferent fibers innervating the temporalis muscle have been exam- ined, the characteristics of slowly conducting temporalis afferent fibers have not been investigated. In contrast to the limited study of slowly conducting temporalis afferent fibers, the characteristics of putative masseter muscle nociceptors have been previously exam- ined. It has been found that slowly conducting mechano- receptive masseter afferent fibers project to subnucleus caudalis, the caudal-most subnucleus of the trigeminal sensory complex and a region believed to be critically important for the integration of craniofacial nociceptive signaling (Cairns et al., 2001a). A large proportion of these fibers are excited and sensitized by injection of glutamate into their mechanoreceptive fields (Cairns et al., 2002). Excitation of these afferent fibers by glutamate also exhib- its a sex-related difference in that glutamate-evoked affer- ent discharge is much greater in afferent fibers recorded in female rats than in male rats (Cairns et al., 2001a, 2002). A similar sex-related difference in glutamate-evoked tem- poromandibular (jaw) joint afferent fibers has also been found, which suggests that such sex-related differences may be a feature of deep craniofacial tissues (Cairns et al., 2001b). Further, since these sex-related differences in nociceptor function occur in the same tissues where pain occurs in temporomandibular disorders, they point to a possible biological factor contributing to the increased prevalence of temporomandibular disorders and other re- lated chronic craniofacial pain conditions among women (LeResche et al., 1997, 2003; Dao and LeResche, 2000). Multiple lines of evidence now implicate the peripheral N-methyl-D-aspartate (NMDA) receptor in mechanisms as- sociated with the transduction of noxious information from *Corresponding author. Tel: +1-604-822-7715; fax: +1-604-822-3035. E-mail address: brcairns@interchange.ubc.ca (B. E. Cairns). Abbreviations: CV, conduction velocity; NMDA, N-methyl-D-aspartate; PPT, pressure pain threshold. Neuroscience 141 (2006) 939 –945 0306-4522/06$30.00+0.00 © 2006 IBRO. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.neuroscience.2006.04.024 939