Abstract: The use of topical non-steroidal anti-inflammatory drugs, such as diclofenac, for the treatment of temporomandibular disorders-related myofascial pain is based on the premise that their analgesic effect is mediated by a local action on the excitability of muscle nociceptors, despite a lack of muscle inflammation in these patients. To investigate if diclofenac has an effect on muscle afferent fibers in the absence of inflammation, in vivo recordings of the response of masseter muscle afferent fibers to mechanical and noxious chemical (hypertonic saline) stimulation were made in anesthetized Sprague-Dawley rats. It was observed that injection of diclofenac (0.1 or 1 mg/ml) alone could elevate afferent mechanical threshold for a 10 min period post-injection. Hypertonic saline-evoked afferent discharge was also significantly attenuated by the higher concentration of diclofenac and lidocaine (20 mg/ml), but not by the lower concentration of diclofenac. Additional experiments were undertaken to investigate whether activation of ATP-sensitive potassium (K ATP) channels could contribute to the effects of diclofenac. The K ATP channel opener pinacidil (0.1 mg/ml) significantly enhanced potassium chloride-evoked afferent discharge consistent with the concept that masseter afferent fibers have functional K ATP channels, however, subsequent experiments indicated that diclofenac (1 mg/ml) significantly suppressed potassium chloride-evoked afferent discharge and that pinacidil did not affect hypertonic saline-evoked afferent discharge. These results indicate that diclofenac can exert a "local anesthetic-like" action on masseter afferent fibers in the absence of inflammation, but that this effect does not appear to involve the opening of K ATP channels.
Abstract: In the present study, we combined immunohistochemical experiments with in vivo single unit recordings to examine whether 5-HT(3) receptors are expressed by masticatory (masseter and temporalis) sensory ganglion neurons and to investigate the effects of intramuscular injection of 5-HT on the excitability and mechanical threshold of rat masticatory muscle afferent fibers. The expression of 5-HT(3) receptors by masticatory ganglion neurons was examined using immunohistochemical techniques. In vivo extracellular single unit recording techniques were used to assess changes in the excitability of individual masticatory muscle afferent fibers. Immunohistochemical experiments detected a relatively high frequency (52%) of 5-HT(3) receptor expression by masticatory ganglion neurons. Injection of 5-HT (10(-4), 10(-3), 10(-2)M) evoked concentration-related increases in the magnitude of afferent discharge, but did not significantly sensitize muscle afferent fibers to mechanical stimuli. No significant sex-related differences in 5-HT-evoked afferent discharge were identified. Afferent discharge evoked by 5-HT was significantly attenuated by co-injection with the selective 5-HT(3) receptor antagonist tropisetron (10(-3)M). Afferent discharge was also evoked by the selective 5-HT(3) receptor agonist 2-methyl-5-HT. Unexpectedly, a significant concentration-related decrease in median blood pressure in response to 5-HT injection was found. This 5-HT-induced decrease in blood pressure was not antagonized by tropisetron or mimicked by 2-methyl-5-HT, indicating that the drop in blood pressure was not 5-HT(3) receptor-mediated. The present results indicate that 5-HT excites slowly conducting masticatory muscle afferent fibers through activation of peripheral 5-HT(3) receptors, and suggest that similar mechanisms may contribute to 5-HT-evoked muscle pain in human subjects.
Abstract: In the present study, the hypothesis that sex-related differences in glutamate-evoked rat masseter muscle afferent discharge may result from estrogen-related modulation of peripheral N-methyl-d-aspartate (NMDA) receptor activity and/or expression was tested by examining afferent fiber discharge in response to masseter injection of NMDA and the expression of NR2A/B subunits by masseter ganglion neurons in male and female rats. The results showed that injection of NMDA into the masseter muscle evoked discharges in putative mechanonociceptive afferent fibers and increased blood pressure that was concentration-dependent, however, a systemic action of NMDA appeared responsible for increased blood pressure. NMDA-evoked afferent discharge was significantly greater in female than in male rats, was positively correlated with plasma estrogen levels in females and was significantly greater in ovariectomized female rats treated with a high dose (5 mug/day) compared with a low dose (0.5 mug/day) of estrogen. Pre-treatment of high dose estrogen-treated-ovariectomized female rats with the Src tyrosine kinase inhibitor PP2 did not affect NMDA-evoked afferent discharge. NMDA-evoked afferent discharge was attenuated by the antagonists ketamine and ifenprodil, which is selective for NR2B containing NMDA receptors. Fewer masseter ganglion neurons expressed the NR2A (16%) subunit as compared with the NR2B subunit (38%), which was expressed at higher frequencies in intact female (46%) and high dose estrogen-treated ovariectomized female (60%) rats than in male (31%) rats. Taken together, these results suggest that sex-related differences in NMDA-evoked masseter afferent discharge are due, at least in part, to an estrogen-mediated increase in expression of peripheral NMDA receptors by masseter ganglion neurons in female rats.
Abstract: 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.
Abstract: AIMS: To investigate whether local administration of nerve growth factor (NGF) decreases the mechanical threshold (MT) of putative nociceptive masseter afferent fibers as part of its mechanism of mechanical sensitization. METHODS: Electrophysiologic recordings were made from masseter afferents and a randomized, blinded approach was used to test the effects of intramuscular injection of NGF (2.5 or 25 microg/mL) into the rat masseter muscle on the MT of masseter afferents (n=65) and plasma protein extravasation. RESULTS: The plasma protein extravasation data and electrophysiological recordings indicated that rat NGF injection was not inflammatory and did not evoke afferent discharge or induce mechanical sensitization (as reflected in a decreased MT) in masseter afferents in either male or female rats. To investigate whether the lack of effect of NGF injection might be due to differences between human and rat NGF, additional experiments with human NGF injection (25 microg/mL) were undertaken. Intramuscular injection of human NGF into the rat masseter muscle also failed to evoke afferent discharges; however, it did decrease the MT of masseter afferent fibers. CONCLUSION: The finding that neither rat nor human NGF excited putative nociceptive masseter afferent fibers is consistent with a previous report that intramuscular NGF injections are not acutely painful in human subjects. The ability of human NGF injection into the rat masseter muscle to induce afferent mechanical sensitization suggests that this experimental approach may be useful for the study of peripheral mechanisms of myofascial pain and tenderness associated with temporomandibular disorders.
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 t 1/2 = 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 (t 1/2 = 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
Abstract: The human tooth structures should be understood clearly to improve clinically used restorative materials. The dentinoenamel junction (DEJ) plays a key role in resisting crack propagation in teeth. The aim of this study was to determine the fracture toughness of the enamel-DEJ-dentin complex and to investigate the influence of the DEJ on the fatigue crack propagation path across it by characterizing fatigue-fractured enamel-DEJ-dentin complexes using optical and scanning electron microscopy. The results of this study showed that the fracture toughness of the enamel-DEJ-dentin complex was 1.50 +/- 0.28 Mpa x m(1/2). Based on the results of this investigation, it was concluded that the DEJ complex played a critical role in resisting crack propagation from enamel into dentin. The DEJ complex is, approximately, a 100 to 150 microm broad region at the interface between enamel and dentin. The toughening mechanism of the DEJ complex may be explained by the fact that crack paths were deflected as cracks propagated across it. Understanding the mechanism of crack deflection could help in improving dentin-composite as well as ceramic-cement interfacial qualities with the aim to decrease the risk of clinical failure of restorations. Both can be viewed as being composed from a layer of material of high strength and hardness bonded to a softer but tougher substratum (dentin). The bonding agent or the luting cement layer may play the critical role of the DEJ in improving the strength of these restorations in clinical situations.
Abstract: OBJECTIVES: To understand better the clinically-relevant failure of the ceramic in ceramic-cement-substrate structures under Hertzian indentation, including the effects of supporting substrate modulus and ceramic thickness on the stress distribution in the ceramic. METHODS: Discs (thickness, T(c)=0.2, 0.6, 1.2, 1.6, 2.0, 2.4 mm) of a glass-ceramic material (IPS Empress 2, Ivoclar) were cemented (Variolink II, Vivadent) to flat polymer substrates with modulus of elasticity E(s) of 2, 6 and 10 GPa. The top surface of the ceramic-cement-substrate structure was loaded by a 20 mm radius spherical indenter until the initial failure of the ceramic occurred. The finite element method was used to analyse the stress distribution under such Hertzian indentation, varying E(s) and T(c), as well as calculating the maximum tensile stress based on the experimentally observed failure load and contact radius. The failure initiation site of the ceramic was identified by fractography using scanning electron microscopy. RESULTS: The tensile stress concentration at the cementation surface of the ceramic was the predominant factor controlling the ceramic failure. Failure load increased with increase of E(s), while the maximum tensile stress at the cementation surface of the ceramic clearly decreased. Failure load increased logarithmically with ceramic thickness, but the critical tensile stress increased linearly. SIGNIFICANCE: The failure mode observed clinically for ceramic restorations was reproduced in laboratory tests.
