Brian Cairns

Consumption of Monosodium Glutamate (MSG) in healthy people provokes headache, nausea and craniofacial tenderness that mimics that of an acute migraine attack in migraineurs. I hypothesize that systemic administration of MSG produces behavioural signs of headache, nausea and craniofacial tenderness through the activation of peripheral N-methyl-d-aspartate (NMDA) receptors in rats. The behavior of male and female Sprague Dawley rats was video recorded before and after intraperitoneal (i.p.) injections of either MSG (1-1000 mg/kg), nitroglycerin (GTN, 10 mg/kg) or normal saline. Behaviors (grimace score, head-flicks, rearing, head scratches, facial grooming, lying-on-belly, and temporalis muscle region mechanical withdrawal threshold (MT)) were evaluated. Facial cutaneous temperature of the nose and forehead was measured before and after i.p. injections via infrared thermography. Plasma glutamate and calcitonin-gene related peptide (CGRP) concentrations after administration of 1000 mg/kg MSG were measured in anesthetized rats. MSG induced headache- and nausea-like behaviors in a dose-related manner but had no effect on MT. MSG (1000 mg/kg) induced a significantly greater frequency of headache-like behavior in females, but a longer duration of nausea-like behavior in males. MSG produced a prolonged increase in plasma glutamate and CGRP concentrations. Co-administration of the median effective dose of MSG (350 mg/kg) with GTN (10 mg/kg) amplified headache-like behaviors, induced significant craniofacial sensitivity, and increased nausea-like behaviour. Co-administration of anti-migraine (sumatriptan and naproxen), anti-nausea (ondansetron and metoclopramide) drugs with MSG (1000 mg/kg) significantly attenuated MSG-induced headache and nausea-like behaviours respectively. Co-administration of the selective NMDA receptor antagonist 2R-amino-5-phosphonovaleric acid (APV 50 mg/kg), the non-NMDA receptor antagonist kynurenic acid (KYN 1 – 100 mg/kg) and the CGRP receptor antagonist (olcegepant 1 mg/kg), reduced headache-like behaviors evoked by MSG (1000 mg/kg). Only olcegepant when co-administered with MSG attenuated MSG-induced nausea-like behaviours. No alteration in motor function was observed by APV, KYN or olcegepant in rotarod experiments. My results suggest that systemic administration of MSG to rats induces behavioral correlates of headache and nausea that are mediated in part through peripheral NMDA receptor activation. Thus, systemic administration of MSG to rats may be used as a preclinical model for development and pre-clinical testing of new therapeutic targets in migraine.

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Intramuscular injections of nerve growth factor (NGF) into human masseter muscle induce a local mechanical sensitization that mimics the symptoms of myofascial pain in patients with temporomandibular disorders. I hypothesize that NGF induces the myofascial mechanical sensitization in part by increasing the expression of N-methyl-D-aspartate (NMDA) receptors in primary afferent neurons. In behavioral experiments, injection of NGF into rat masseter muscle induced a prolonged local mechanical sensitization that was greater in female rats than in male rats. This NGF-induced sensitization was partly attenuated by a local injection of the NMDA receptor antagonist APV at 3 days post NGF injection in the male rats but not in the female rats. Immunohistochemical studies found that this NGF-induced mechanical sensitization was accompanied by the increased expression of NMDA receptor subtype 2B (NR2B) in trigeminal ganglion neurons innervating the masseter muscle in both sexes, as well as an increase in the average soma size of NR2B-expressing neurons. An increase in the expression of neuropeptides (CGRP/SP) was also observed in the female rats but not in the male rats. In in vivo extracellular recordings of masseter trigeminal ganglion neurons, NGF increased NMDA-induced mechanical sensitization in the male rats but not in the female rats. However, in the female rats, this effect was greater in slow Aδ fibers (2-7 m/s) than fast Aδ fibers (>7-12 m/s). My results suggest that NGF-induced mechanical sensitization is mediated, in part, through an effect on peripheral NMDA receptors in a sexually dimorphic manner.

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Behavioral evidence in rats indicates that injection of tumor necrosis factor alpha (TNFalpha into skeletal muscle results in a prolonged mechanical sensitization without gross inflammation. The present series of studies were conducted to test the idea that injection of TNFalpha causes mechanical sensitization of skeletal muscle through a peripheral mechanism that involves lowering of the mechanical threshold (MT) of muscle nociceptors without inflammation. In- vivo extracellular electrophysiological recording was used to assess the effect of TNFalpha (1 or 0.1microgram) and other drugs on the excitability and MT of masseter muscle nociceptors. Expression of TNFR1 (P55) and TNFR2 (P75) receptors by the masseter muscle and trigeminal ganglion neurons that innervate that muscle was determined by Western blot and immunohistochemical methodologies, respectively. The Evans blue dye technique and thermal camera recordings were used to assess inflammation in muscle tissues. Enzyme-linked immunoassays and glutamate biosensor probes were used to measure muscle concentrations of prostaglandin (PG) E2 and nerve growth factor, and glutamate, respectively. Intramuscular injection of 1mg TNFalpha did not excite nociceptors, but did significantly decrease MT compared to vehicle control. There was no evidence of gross inflammation 3 hours after injection of TNFalpha. Co-injection of TNFalpha with P55 or P75 receptor antibodies attenuated TNFalpha-induced mechanical sensitization. P55 and P75 receptors were expressed by 29% and 62% of masseter nociceptors, respectively. PGE2 and glutamate concentrations were significantly changed 3 hours after TNFalpha injection into the masseter muscle. Injection of diclofenac, a cycloxygenase inhibitor that attenuates prostaglandin synthesis, partially reversed the TNFalpha-induced decreases in the MT of masseter muscle nociceptors, while vehicle control, DL-2-amino-5-phophonovaleric acid, a competitive NMDA receptor antagonist, and a tyrosine kinase A receptor antibody, which blocks NGF-induced masseter muscle nociceptor sensitization, did not significantly alter nociceptor MT. These findings indicate that TNFalpha-induced mechanical sensitization of masseter nociceptors is mediated, in part, by increased PGE2 levels through activation of peripheral P55 and P75 receptors. Over all, these results suggest that injection of TNFalpha into skeletal muscle could be used as a model of myofascial trigger points to study the peripheral pain mechanisms of masticatory muscle pain.

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