NMR IN BIOMEDICINE NMR Biomed. 2005;18:345–351 Published online 11 July 2005 in Wiley InterScience (www.interscience.wiley.com). DOI:10.1002/nbm.962 MRS assessment of glutamate clearance in a novel masticatory muscle pain model G. Gambarota, 1 * M. Philippens, 2 B. E. Cairns, 3 X. D. Dong, 3 W. K. J. Renema 1 and A. Heerschap 1 1 Department of Radiology, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands 2 Radiation Oncology, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands 3 Faculty of Pharmaceutical Sciences, The University of British Columbia, Vancouver, Canada Received 23 August 2004; Revised 21 February 2005; Accepted 21 February 2005 ABSTRACT: The injection of 1.0 M glutamate into the masseter (jaw-closer) muscle results in a short period of muscle pain (5–10 min) and a prolonged period of mechanical sensitization (> 30 min). It is unclear, however, whether there is a temporal relationship between intramuscular glutamate concentration and either muscle pain or mechanical sensitization. In the present study, 1 H MRS and electrophysiological recording of masticatory muscle nerve fibers were performed in order to monitor glutamate clearance and nerve fiber activity, respectively, after injection of glutamate into rat masticatory muscles. Glutamate signal amplitude was found to decay rapidly (half-life t½ ¼ 108  42 s), and became indistinguishable from the baseline 10 min after the injection. Glutamate-evoked nerve fiber activity was also found to decay rapidly (t½ ¼ 76  28 s). These results suggest that glutamate clearance correlates well with the time course of glutamate-evoked muscle pain fiber discharge. Copyright # 2005 John Wiley & Sons, Ltd. KEYWORDS: glutamate; muscle; nociceptor; proton MR spectroscopy; pain model; drug clearance INTRODUCTION Intramuscular injection of glutamate is a novel experi- mental model of acute, non-inflammatory muscle pain. Injection of 1.0 M glutamate into the human masseter muscle evokes aching muscle pain that lasts for 5–10 min and a prolonged period of mechanical sensitization, which lasts more than 30 min. 1–3 Parallel animal experi- ments have revealed that a single injection of 1.0 M glutamate into the masseter muscle also results in short- and long-lasting effects. Short-lasting effects (5–10 min) include the excitation of small-diameter sensory nerve fibers which are thought to be responsible for the trans- duction of muscle pain signals. 1,4,5 On the other hand, long-lasting effects include the decrease of the mechan- ical threshold of masseter muscle nerve fibers as well as an increase of both muscle blood flow and extracellular water content for periods of up to 60 min. 4,6 The excita- tion and sensitization of muscle sensory nerve fibers by glutamate results from the activation of peripheral gluta- mate receptors. 4,5 In previous studies on the same pain model, MRI was used to investigate the distribution and biophysical properties of water within in vivo muscle after intra- muscular injections of glutamate or isotonic saline solutions. In particular, the edema volumes and intra- cellular vs extracellular water fraction in rat masseter muscle following injection of glutamate or saline were assessed. 6,7 It was found that glutamate injection pro- duced an edema which was significantly larger than that produced by isotonic saline. Moreover, measurements of T 2 relaxation decays revealed that glutamate injec- tion resulted in a higher extracellular water fraction in muscle when compared with isotonic saline injection. Both these effects were indicative of the muscle re- sponse to the osmotic stress induced by hypertonic glutamate solutions. No measurements of glutamate concentration, however, were performed and therefore it is not known what tissue level of glutamate is achieved or how long glutamate levels remain elevated in the muscle tissue after injection of 1.0 M glutamate. These questions are important ones, since it has been proposed that elevated tissue glutamate levels contri- bute to localized pain and sensitization of tissues in certain human pain conditions. 5,8 It is not clear whether there is a temporal relationship between intramuscular glutamate concentration and either glutamate-evoked muscle pain or glutamate-induced mechanical sensiti- zation. In order to answer these questions, in the present study localized 1 H MRS was used to measure glutamate clearance from the rat masseter muscle and single unit Copyright # 2005 John Wiley & Sons, Ltd. NMR Biomed. 2005;18:345–351 *Correspondence to: G. Gambarota, Department of Radiology 430, Radboud University Nijmegen Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands. E-mail: g.gambarota@rad.umcn.nl Contract/grant sponsor: Dutch Cancer Society; contract grant num- ber: KUN 2000–2307. Contract/grant sponsor: US National Institute of Dental and Cranio- facial Research; contract grant number: 1 R01 DE15420-01. Contract/grant sponsors: Canada Foundation for Innovation; British Columbia Knowledge Development Fund. Abbreviations used: CV, conduction velocity; VOI, voxel of interest.