Interstitial Glutamate Concentration Research Articles

Nerve Growth Factor Increases Glutamate in Sensory Fibres Innervating the Masseter Muscles of Female Rats.

To investigate whether nerve growth factor (NGF) alters glutamate expression in sensory fibres and glutamate concentration in the masseter muscle of female rats. Ten female rats were injected with NGF (25 µg/ml, 10 μl) and vehicle into the right and left masseter muscles, respectively. Immunohistochemistry and microdialysis were performed after 3 days to evaluate glutamate expression and concentration in the muscle. The frequency of expression of glutamate in the nerve fibres innervating the masseter muscle was significantly greater 3 days after NGF (56 ± 5%) than after vehicle (39 ± 5%) injection. The majority of fibres co-expressed the neuropeptide substance P (SP); a marker for sensory afferent fibres. There was no effect of NGF on the expression of the excitatory amino acid transporter type 2 (EAAT2). In the microdialysis experiment, mean interstitial glutamate concentration on the vehicle side (21.6 ± 9.8 µM) was not significantly different from that on the NGF side (16.2 ± 9.2 µM). These results suggest that, in part, NGF increases the mechanical sensitivity of the masseter muscle by increasing glutamate expression in the sensory nerve endings in the muscle. This effect was local to the site of the NGF injection, as it was only detectable through immunohistochemistry, but not by microdialysis.

Increased pain and muscle glutamate concentration after single ingestion of monosodium glutamate by myofascial temporomandibular disorders patients.

A randomized, double-blinded, placebo-controlled study was conducted to investigate if single monosodium glutamate (MSG) administration would elevate muscle/serum glutamate concentrations and affect muscle pain sensitivity in myofascial temporomandibular disorders (TMD) patients more than in healthy individuals. Twelve myofascial TMD patients and 12 sex- and age-matched healthy controls participated in two sessions. Participants drank MSG (150mg/kg) or NaCl (24mg/kg; control) diluted in 400mL of soda. The concentration of glutamate in the masseter muscle, blood plasma and saliva was determined before and after the ingestion of MSG or control. At baseline and every 15min after the ingestion, pain intensity was scored on a 0-10 numeric rating scale. Pressure pain threshold, pressure pain tolerance (PPTol) and autonomic parameters were measured. All participants were asked to report adverse effects after the ingestion. In TMD, interstitial glutamate concentration was significantly greater after the MSG ingestion when compared with healthy controls. TMD reported a mean pain intensity of 2.8/10 at baseline, which significantly increased by 40% 30min post MSG ingestion. At baseline, TMD showed lower PPTols in the masseter and trapezius, and higher diastolic blood pressure and heart rate than healthy controls. The MSG ingestion resulted in reports of headache by half of the TMD and healthy controls, respectively. These findings suggest that myofascial TMD patients may be particularly sensitive to the effects of ingested MSG. WHAT DOES THIS STUDY ADD?': Elevation of interstitial glutamate concentration in the masseter muscle caused by monosodium glutamate (MSG) ingestion was significantly greater in myofascial myofascial temporomandibular disorders (TMD) patients than healthy individuals. This elevation of interstitial glutamate concentration in the masseter muscle significantly increased the intensity of spontaneous pain in myofascial TMD patients.

Differential effects of repetitive oral administration of monosodium glutamate on interstitial glutamate concentration and muscle pain sensitivity

ObjectiveThe aim of this study was to determine the relationship of high daily monosodium glutamate (MSG) consumption with glutamate concentrations in jaw muscle, saliva, and serum, and muscle pain sensitivity in healthy participants. MethodsA randomized, double-blinded, placebo-controlled study was conducted to investigate the effect of repetitive consumption of high-dose MSG on glutamate concentration in the masseter muscles measured by microdialysis and muscle pain sensitivity. In five contiguous experimental daily sessions, 32 healthy participants drank MSG (150 mg/kg) or NaCl (24 mg/kg) diluted with a 400 mL soda. The concentrations of glutamate before and after the ingestion were assessed in dialysate and plasma samples on the first and last days. Saliva glutamate concentration was assessed every day. Pressure pain threshold, pressure pain tolerance, autonomic parameters (heart rate, systolic and diastolic blood pressures) and reported side effects also were assessed. ResultsNo significant change was noted in the baseline concentration of glutamate in the masseter muscle, blood, or saliva, but the peak concentration in the masseter muscle increased significantly between day 1 and 5. A statistically significant increase in systolic and diastolic blood pressures after MSG administration was observed, as well as a significantly higher frequency of reports of nausea and headache in the MSG group. No robust effect of MSG on muscle sensitivity was found. ConclusionInterstitial glutamate concentration in the masseter muscle is not highly disturbed by excessive repetitive intake of MSG in healthy man.

TNFα Mechanically Sensitizes Masseter Muscle Nociceptors by Increasing Prostaglandin E2 Levels

TNFα induces mechanical sensitization of rat masseter muscle nociceptors, which takes 2-3 h to manifest and is mediated through activation of P55 and P75 receptors. This study was undertaken to determine whether TNFα induces nociceptor mechanical sensitization through the release of other algogenic substances such as glutamate, prostaglandin E(2) (PGE(2)), and/or nerve growth factor (NGF), which have been shown to induce mechanical sensitization of muscle nociceptors. Masseter muscle homogenate levels of PGE(2) and NGF were measured 3 h after injection of TNFα (1 μg) or vehicle control using commercially available kits. Interstitial glutamate concentration was measured after injection of TNFα or vehicle control using a glutamate-selective biosensor probe. Diclofenac, a cyclooxygenase inhibitor that blocks the synthesis of PGE(2), D-2-amino-5-phosphonovaleric acid (APV), a competitive N-methyl-D-aspartate (NMDA) receptor antagonist, and a tyrosine kinase A (TrkA) receptor antibody, which blocks NGF-induced masseter muscle nociceptor sensitization, were used to assess the contribution of PGE(2), glutamate, and NGF to TNFα-induced nociceptor sensitization. PGE(2) and glutamate concentrations were significantly elevated 3 h after TNFα injection into the masseter muscle. Injection of diclofenac partially reversed the TNFα-induced decreases in the mechanical threshold (MT) of masseter muscle nociceptors, whereas vehicle control, APV, and TrkA antibody did not significantly alter nociceptor MT. These results suggest that TNFα-induced mechanical sensitization of masseter muscle nociceptors is mediated in part by increased PGE(2) levels. The findings of this study support the hypothesis that TNFα induces a delayed mechanical sensitization of masseter muscle nociceptors indirectly by the release of PGE(2).

Botulinum neurotoxin type A (BoNTA) decreases the mechanical sensitivity of nociceptors and inhibits neurogenic vasodilation in a craniofacial muscle targeted for migraine prophylaxis

The mechanism by which intramuscular injection of BoNTA into the craniofacial muscles decreases migraine headaches is not known. In a blinded study, the effect of BoNTA on the mechanical and chemical responsiveness of individual temporalis muscle nociceptors and muscle neurogenic vasodilation was investigated in female rats. Mechanical threshold was measured for 3h following intramuscular injection of BoNTA or vehicle, and for 10min after a subsequent injection of the algogen glutamate. Injection of BoNTA significantly increased the mechanical threshold of muscle nociceptors without altering the muscle surface temperature and blocked glutamate-induced mechanical sensitization and neurogenic vasodilation. None of these effects were reproduced by pancuronium-induced muscle paralysis. Western blot analysis of temporalis muscles indicated that BoNTA significantly decreased SNAP-25. Measurement of interstitial glutamate concentration with a glutamate biosensor indicated that BoNTA significantly reduced glutamate concentrations. The mechanical sensitivity of muscle nociceptors is modulated by glutamate concentration through activation of peripheral NMDA receptors. Immunohistochemical experiments were conducted and they indicated that half of the NMDA-expressing temporalis nerve fibers co-expressed substance P or CGRP. Additional electrophysiology experiments examined the effect of antagonists for NMDA, CGRP and NK1 receptors on glutamate-induced effects. Glutamate-induced mechanical sensitization was only blocked by the NMDA receptor antagonist, but muscle neurogenic vasodilation was attenuated by NMDA or CGRP receptor antagonists. These data suggest that injection of BoNTA into craniofacial muscles acts to decrease migraine headaches by rapidly decreasing the mechanical sensitivity of temporalis muscle nociceptors through inhibition of glutamate release and by attenuating the provoked release of CGRP from muscle nociceptors.

Long-Term Gliosis and Molecular Changes in the Cervical Spinal Cord of the Rhesus Monkey after Traumatic Brain Injury

Recovery of fine motor skills after traumatic brain injury (TBI) is variable, with some patients showing progressive improvements over time while others show poor recovery. We therefore studied possible cellular mechanisms accompanying the recovery process in a non-human primate model system, in which the lateral frontal motor cortex areas controlling the preferred upper limb were unilaterally lesioned, and the animals eventually regained fine hand motor function. Immunohistochemical staining of the cervical spinal cord, the site of compensatory sprouting and degeneration of corticospinal axons, showed profound increases in immunoreactivities for major histocompatibility complex class II molecule (MHC-II) and extracellular signal-regulated kinases (ERK1/2) up to 12 months post lesion, particularly within the lateral corticospinal tract (LCST). Double immunostaining demonstrated that phosphorylated ERK1/2 colocalized within the MCH-II + microglia, suggesting a trophic role of long-term microglia activation after TBI at the site of compensatory sprouting. Active sprouting was observed in the LCST as well as in the spinal gray matter of the lesioned animals, as illustrated by increases in growth associated protein 43. Upregulation of Nogo receptor and glutamate transporter expression was also observed in this region after TBI, suggesting possible mechanisms for controlling aberrant sprouting and/or synaptic formation en route and interstitial glutamate concentration changes at the site of axon degeneration, respectively. Taken together, these changes in the non-human primate spinal cord support a long-term trophic/tropic role for reactive microglia, in particular, during functional and structural recovery after TBI.

Astrocyte-neuron interactions

AQP4= : aquaporin-4; ATP= : adenosine triphosphate; EAAT= : excitatory amino acid transporter; GABA= : γ-aminobutyric acid; Pg= : prostaglandin; TNF= : tumor necrosis factor. Astrocytes provide metabolic support to neurons and are critical for homeostasis of the neuronal microenvironment, including maintenance of a low interstitial glutamate concentration and control of the volume of brain interstitial volume, extracellular potassium (K+) concentration, and local blood flow. Bidirectional communication between neurons and astrocytes at the synapses may have a critical role in regulating chemical neurotransmission. Whereas the initial concepts on astrocyte signaling were based on studies on cultured astroglia, new imaging and electrophysiologic methods applied to brain slices and the development of genetically modified mice models have provided further insight into mechanisms of the reciprocal neuron-astrocyte interactions. Given their critical role in maintenance of neuronal microenvironment and modulation of excitatory synapses, astrocytes have been implicated in mechanisms of neuronal and neuronal network hyperexcitability underlying epilepsy. These topics have been covered in several recent excellent reviews.1–9,24 ### Structural interactions among astrocytes, synapses, and blood vessels. Astrocytes are the predominant type of glial cell in the CNS. There are many distinct subsets of astrocytes that can be distinguished on the basis of their morphology, biochemical, physiologic, and pharmacologic characteristics, and interactions with the local environment.1,10 Individual astrocytes occupy distinct, nonoverlapping domains and extend thousands of fine membranous processes that ensheathe synapses and fine blood vessels. It has been estimated that a process from a single astrocyte can envelop approximately 140,000 synapses and that more than 99% of cerebrovascular surface is ensheathed by astrocytic processes.11,12 This close coupling among neurons, glia, and blood vessels has been termed the neurovascular unit and is critical for regulation of local blood flow.12–14 Astrocytic processes are coupled through gap junctions formed by connexins 43 and 32 to form large intercellular networks. The extent of gap junction coupling between astrocytes depends on the type of astrocyte and the level of neuronal activity.15 Perivascular astroglial metabolic networks allow activity-dependent …

The Role of Peripheral Glutamate and Glutamate Receptors in Muscle Pain

Objectives: To review evidence that associates elevated muscle interstitial glutamate concentration and the activation of peripheral glutamate receptors with the development and maintenance of muscle pain in conditions such as myofascial temporomandibular disorders [TMDs].Findings: In certain noninflammatory musculoskeletal pain conditions, an association between pain, elevated glutamate concentrations and the expression of peripheral N-methyl-d-aspartate receptors [NRs] has been made. Myofascial TMDs are a noninflammatory muscle pain disorder characterized by pain and localized mechanical sensitivity in the masticatory muscle and a greater prevalence in young women than in men. Artificial elevation of glutamate concentrations in the masticatory muscles of healthy subjects evokes intense, short duration pain that is significantly greater in woman than in men and induces a more prolonged, localized mechanical sensitization that is similar in both sexes. Glutamate-evoked pain and sensitization may be attenu...

Systemic administration of monosodium glutamate elevates intramuscular glutamate levels and sensitizes rat masseter muscle afferent fibers

There is evidence that elevated tissue concentrations of glutamate may contribute to pain and sensitivity in certain musculoskeletal pain conditions. In the present study, the food additive monosodium glutamate (MSG) was injected intravenously into rats to determine whether it could significantly elevate interstitial concentrations of glutamate in the masseter muscle and whether MSG administration could excite and/or sensitize slowly conducting masseter afferent fibers through N-methyl- d-aspartate (NMDA) receptor activation. The interstitial concentration of glutamate after systemic injection of isotonic phosphate-buffered saline (control) or MSG (10 and 50 mg/kg) was measured with a glutamate-selective biosensor. The pre-injection baseline interstitial concentration of glutamate in the rat masseter muscle was 24 ± 11 μM. Peak interstitial concentration after injection of 50 mg/kg MSG was 63 ± 18 μM and remained elevated above baseline for ∼18 min. In vivo single unit recording experiments were undertaken to assess the effect of MSG (50 mg/kg) on masseter afferent fibers. Injection of MSG evoked a brief discharge in one afferent fiber, and significantly decreased (∼25%) the average afferent mechanical threshold ( n = 10) during the first 5 min after injection of MSG. Intravenous injection of ketamine (1 mg/kg), 5 min prior to MSG, prevented the MSG-induced decreases in the mechanical threshold of masseter afferent fibers. The present results indicate that a 2- to 3-fold elevation in interstitial glutamate levels in the masseter muscle is sufficient to excite and induce afferent mechanical sensitization through NMDA receptor activation. These findings suggest that modest elevations of interstitial glutamate concentration could alter musculoskeletal pain sensitivity in humans.

No release of interstitial glutamate in experimental human model of muscle pain

Glutamate may be released from muscle nociceptors and thereby contribute to mechanisms underlying acute and chronic muscle pain. In vivo concentration of glutamate during muscle pain has not previously been studied in either animals or humans. In the present study, we aimed to study the in vivo concentration of glutamate before, during and after acute pain of trapezius muscle in humans using the microdialysis technique. In addition, we examined the nutritive skeletal muscle blood flow and the interstitial concentrations of lactate, glucose, glycerol, pyruvate and urea. Experimental pain and tenderness were induced by intramuscular infusion of a chemical mixture consisting of bradykinin, prostaglandin E 2, histamine and serotonin. One EMG-needle and one microdialysis catheter were inserted into non-dominant and dominant trapezius muscles on a standard anatomical point in 19 healthy subjects. Dialysates were collected at rest, during infusion and 60 and 120 min after stop of infusion. Local tenderness was recorded at baseline and at the end of experiment. Local pain was recorded during infusion. The infusion of chemical mixture was more painful than infusion of placebo ( p < 0.05) and resulted in significantly higher local tenderness score than placebo ( p = 0.007). There was no difference in change in interstitial concentrations of glutamate, lactate, glucose, glycerol, pyruvate and urea from baseline to infusion and post-infusion periods between chemical mixture and placebo ( p > 0.05). Muscle blood flow increased significantly over time in response to infusion of chemical mixture and placebo ( p = 0.001). However, we found no difference in changes in muscle blood flow between chemical mixture and placebo ( p > 0.05). In conclusion, the present study demonstrates no signs of increased release of glutamate from myofascial nociceptors during and after acute experimentally induced muscle pain and tenderness.

Simultaneous glutamate and perfusion fMRI responses to regional brain stimulation.

Functional magnetic resonance imaging (fMRI) rests on the assumption that regional brain activity is closely coupled to regional cerebral blood flow (rCBF) in vivo. To test the degree of coupling, cortical brain activity was locally stimulated in rats by reversed microdialysis infusion of picrotoxinin, alphagamma-aminobutyric acid-A antagonist. Before and during the first 30 minutes of infusion, simultaneous fMRI (rCBF) and neurochemical (interstitial glutamate concentration) measures of brain activity were highly correlated (r = 0.83). After 30 minutes of picrotoxinin-induced stimulation, glutamate levels decreased but rCBF remained elevated, suggesting that additional factors modulate the relationship between neuronal neurotransmitters and hemodynamics at these later stages.

Nitric oxide synthase inhibition and extracellular glutamate concentration after cerebral ischemia/reperfusion.

Transient cerebral ischemia in rats results in selective loss of neuronal viability, eg, hippocampal CA1 neurons. The neurochemical variables responsible for this selective vulnerability to ischemia/reperfusion (IR) appear to involve excitatory amino acids. In brain IR, excitatory amino acid toxicity may be modulated by endogenous nitric oxide (NO.) gas. To investigate NO. in global brain IR, we measured the effects of NO. synthase (NOS) inhibition on interstitial excitatory amino acids in rats. Changes in postischemic cerebral blood flow and blood-brain barrier function also were evaluated. Forebrain ischemia was produced by systemic hypotension and occlusion of both carotid arteries for 15 minutes. Blood flow was restored for 60 minutes by unclamping the carotids and reinfusing with blood. A microdialysis probe was placed into the cortex and hippocampus using a stereotaxic device. Interstitial glutamate concentration was measured during IR with high-performance liquid chromatography. A competitive NOS inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME), was given intraperitoneally 30 minutes before ischemia in doses of 1, 4, and 20 mg/kg. Changes in cerebral blood flow and blood-brain barrier during IR were determined using laser-Doppler flowmetry and microdialysis with sodium fluorescein. Glutamate in the dialysate during IR increased transiently 10-fold and returned to baseline levels by 30 minutes of reperfusion. Animals treated with L-NAME 30 minutes before ischemia also showed increases in glutamate concentration during ischemia, but glutamate remained elevated during reperfusion. The increase in glutamate concentration during reperfusion caused by L-NAME was prevented by L-arginine. The administration of L-arginine and L-NAME together decreased extracellular glutamate concentration during ischemia. Cerebral blood flow decreased to about 5% of baseline values during ischemia but increased approximately fourfold relative to control values on reperfusion. The hyperemic responses after ischemia were not different between IR groups treated with or without L-NAME. Brain ischemia increased the permeability of the blood-brain barrier to fluorescein; however, this change was attenuated by L-NAME administration at 20 mg/kg. NOS inhibition did not attenuate extracellular glutamate accumulation during ischemia and increased its concentration on reperfusion. The elevated glutamate concentration after IR in L-NAME-treated rats did not appear to be due to either a decrease in cerebral blood flow response after ischemia or increases in local blood-brain barrier permeability. For the most part, the blood-brain barrier was spared in the immediate postischemic period by L-NAME treatment. These data suggest that NO. production may oppose synaptic excitatory amino acid accumulation and presumably excitotoxicity during IR.