Parenterally administered kainic acid induces a persistent hyperalgesia in the mouse and rat

Glutaminase Increases in Rat Dorsal Root Ganglion Neurons after Unilateral Adjuvant-Induced Hind Paw Inflammation

Glutamate is a neurotransmitter used at both the peripheral and central terminals of nociceptive primary sensory neurons, yet little is known concerning regulation of glutamate metabolism during peripheral inflammation. Glutaminase (GLS) is an enzyme of the glutamate-glutamine cycle that converts glutamine into glutamate for neurotransmission and is implicated in producing elevated levels of glutamate in central and peripheral terminals. A potential mechanism for increased levels of glutamate is an elevation in GLS expression. We assessed GLS expression after unilateral hind paw inflammation by measuring GLS immunoreactivity (ir) with quantitative image analysis of L4 dorsal root ganglion (DRG) neurons after one, two, four, and eight days of adjuvant-induced arthritis (AIA) compared to saline injected controls. No significant elevation in GLS-ir occurred in the DRG ipsilateral to the inflamed hind paw after one or two days of AIA. After four days AIA, GLS-ir was elevated significantly in all sizes of DRG neurons. After eight days AIA, GLS-ir remained elevated in small (<400 µm²), presumably nociceptive neurons. Western blot analysis of the L4 DRG at day four AIA confirmed the elevated GLS-ir. The present study indicates that GLS expression is increased in the chronic stage of inflammation and may be a target for chronic pain therapy.

Glutamate pharmacology and metabolism in peripheral primary afferents: physiological and pathophysiological mechanisms

In addition to using glutamate as a neurotransmitter at central synapses, many primary sensory neurons release glutamate from peripheral terminals. Primary sensory neurons with cell bodies in dorsal root or trigeminal ganglia produce glutaminase, the synthetic enzyme for glutamate, and transport the enzyme in mitochondria to peripheral terminals. Vesicular glutamate transporters fill neurotransmitter vesicles with glutamate and they are shipped to peripheral terminals. Intense noxious stimuli or tissue damage causes glutamate to be released from peripheral afferent nerve terminals and augmented release occurs during acute and chronic inflammation. The site of action for glutamate can be at the autologous or nearby nerve terminals. Peripheral nerve terminals contain both ionotropic and metabotropic excitatory amino acid receptors (EAARs) and activation of these receptors can lower the activation threshold and increase the excitability of primary afferents. Antagonism of EAARs can reduce excitability of activated afferents and produce antinociception in many animal models of acute and chronic pain. Glutamate injected into human skin and muscle causes acute pain. Trauma in humans, such as arthritis, myalgia, and tendonitis, elevates glutamate levels in affected tissues. There is evidence that EAAR antagonism at peripheral sites can provide relief in some chronic pain sufferers.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Automated evaluation of sensitivity to foot shock in mice: inbred strain differences and pharmacological validation

Assessing foot shock sensitivity in rodents can be useful in identifying analgesic or hyperalgesic drugs, and phenotyping inbred or genetically altered mice. Furthermore, as foot shock is an integral part of several rodent behavioral models, sensitivity should also be assessed to accurately interpret behavioral measures from these models. To eliminate variability and increase the efficiency of manually scored shock sensitivity paradigms, we utilized a startle reflex system to automatically quantify responses to varying levels of foot shock. Eight inbred mouse strains were tested for reactivity to foot shock in this system, as well as inherent startle response activity to loud noise bursts. Strain rank order for shock reactivity differed from that for acoustic startle, suggesting that pathways activated in response to each differed. Analgesic doses of morphine and acetaminophen specifically reduced foot shock responses without affecting motor reflexive responses to loud noises in each strain tested. We also tested diazepam and scopolamine, which are often used to disrupt behavior in shock-related paradigms to illustrate the usefulness of this assay. Overall, these results demonstrate that our automated method is a quick and simple way to accurately assess potential foot shock sensitivity differences owing to strain, genotype or drug treatments.

Kainate-induced excitation and sensitization of nociceptors in normal and inflamed rat glabrous skin

This study investigates contributions of peripheral kainate receptors to acute nociception and persistent inflammatory pain in rat. Immunohistochemical analysis of kainate receptor expression using antibodies recognizing glutamate receptor subunits 5, 6, and 7 demonstrates that 28% of unmyelinated axons in normal digital nerve are positively labeled. Following intraplantar injection of complete Freund's adjuvant, a significant increase in glutamate receptor subunits 5, 6, and 7-labeled axons occurs at 2 days (40%), but not 7 (31%) or 14 days (28%) post-complete Freund's adjuvant. In behavioral studies, we confirm an increased mechanical sensitivity in complete Freund's adjuvant-injected hind paws. Furthermore, activation of kainate receptors following intraplantar injection of 1.0 mM kainate in normal animals results in a mechanical sensitivity similar to that observed in inflamed animals. A 1.0 mM kainate injection into inflamed hind paws further enhances the mechanical sensitivity. Injection of the non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (0.1 mM) reverses complete Freund's adjuvant-induced mechanical sensitivity through a local effect. In single unit recordings from nociceptors in a glabrous skin-nerve preparation, mechanical sensitization is present in inflamed skin evidenced by a decrease in mechanical threshold and an increase in discharge rate during a suprathreshold, constant force stimulus. Thermal sensitization is also present evidenced by a decrease in heat threshold. There is a dose-dependent increase in kainate-induced nociceptor activity in both normal and inflamed skin but the kainate required to induce activation is reduced in inflamed skin. Although proportions of kainate-activated nociceptors are the same in normal and inflamed skin, the kainate-induced mean discharge rate is significantly enhanced in inflamed skin. Exposure of normal and inflamed nociceptors to 0.3 mM kainate sensitizes fibers to re-application of kainate and heat. This sensitization is blocked in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione or the glutamate receptor subunit 5 selective antagonist 3S,4aR,6S,8aR-6-[4-carboxy-phenyl] methyl-1,2,3,4,4a,5,6,7,8,8a-deca-hydroisoquinoline-3-carboxylic acid. The data indicate that peripheral kainate receptors not only play an important role in normal nociception but also contribute to mechanical sensitivity and heat sensitization accompanying inflammatory pain.

Physiological and behavioral evidence for focal nociception induced by epidural glutamate infusion in rats

STUDY DESIGN

Blinded animal study.

OBJECTIVES

To determine if an increased concentration of epidural glutamate can cause a focal nociceptive response in the lower extremities that is consistent with sciatica.

SUMMARY OF BACKGROUND DATA

It is believed that the origin of sciatic pain is related to more than physical pressure on the nerve roots. Recently, it was determined that disc material may be a significant source of free glutamate, resulting from the enzymatic degradation of matrix aggrecan proteins. We believe that this free glutamate acts as a neurotransmitter at glutamate receptors on the dorsal root ganglion (DRG) cell bodies, thereby initiating a nociceptive response.

METHODS

Rats were subject to a 72-hour epidural glutamate infusion via a mini osmotic pump. Von Frey behavioral testing was performed 24 hours before, and 24 and 72 hours after the onset of the infusion. DRG and dorsal horn tissues were analyzed for changes in receptor expression, which have been previously shown to correlate with a nociceptive state.

RESULTS

Von Frey behavioral tests showed focal hyperalgesia that was maximal at the 0.02 mmol/L glutamate concentration. Significant changes in DRG glutamate receptor expression were seen for alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid, kainite, and N-methyl-D aspartate receptors. Analysis of dorsal horn glutamate receptors also showed patterns in alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid and kainate receptor expression that were consistent with a nociceptive state.

CONCLUSIONS

Epidural glutamate elicits a focal nociceptive response. Free glutamate that has been liberated from the disc material may be an important factor in the development of sciatic pain.

Indomethacin, alone and combined with prochlorperazine and caffeine, but not sumatriptan, abolishes peripheral and central sensitization in in vivo models of migraine

Recently it has been proposed that the throbbing pain of migraine is mediated by sensitization of peripheral trigeminovascular neurons, and that cutaneous allodynia of migraine is mediated by sensitization of central trigeminovascular neurons, and, moreover, that the triptans are less effective in aborting a migraine attack if the central sensitization is already established. The combination of indomethacin, prochlorperazine, and caffeine (IndoProCaf) is a drug of well-established use in the acute treatment of migraine. The aim of this study was to investigate whether the 3 active principles of IndoProCaf, alone and combined, compared to sumatriptan, were able to abolish the peripheral sensitization induced by kainic acid and the central sensitization induced by N-methyl-D-aspartate (NMDA) in in vivo models of hyperalgesia. The study showed that indomethacin or IndoProCaf is able to abolish both the kainic acid-induced and the NMDA-induced hyperalgesia. If administered at different times, IndoProCaf was always effective in reversing the kainic acid-induced hyperalgesia. Sumatriptan was not able to reverse either the kainic acid-induced or the NMDA-induced hyperalgesia. The efficacy of indomethacin, alone and combined with prochlorperazine and caffeine, in abolishing peripheral and central sensitization in in vivo models of hyperalgesia is a further explanation of the clinical efficacy of IndoProCaf in the treatment of migraine.

PERSPECTIVE

This study suggests that, although triptans were shown to be able to abort migraine attacks only if given before the establishment of cutaneous allodynia and central sensitization, IndoProCaf should be able to abort migraine attacks independently from the time of administration, because it is able to abolish an already established peripheral and central sensitization.

Thrombin inhibits NMDA-mediated nociceptive activity in the mouse: possible mediation by endothelin

The CNS expresses many components of an extracellular protease signalling system, including the protease-activated receptor-1 (PAR-1) whose tethered ligand is generated by thrombin. Activation of PAR-1 potentiates NMDA receptor activity in hippocampal neurons. Because NMDA activity mediates hyperalgesia, we tested the hypothesis that PAR-1 receptors also regulate pain processing. In contrast to the potentiating effect of thrombin in the hippocampus, NMDA-induced behaviours and the transient mechanical hyperalgesia (von Frey fibres) induced by intrathecally injected NMDA in mice were inhibited by thrombin in a dose-related fashion. This anti-hyperalgesic effect was mimicked by SFLLRN, the natural ligand at PAR-1 binding sites, but not SLIGRL-amide, a PAR-2 agonist. The effects of SFLLRN were less potent and shorter in duration than that of thrombin, consistent with its more transient effect on PAR-1 sites. Both thrombin and SFLLRN inhibited acetic acid-induced abdominal stretch (writhing) behaviours, which were also sensitive to NMDA antagonism, but not hot plate or tail flick latencies, which were insensitive to NMDA antagonists. TFLLR-amide, a selective ligand for PAR-1 sites, mimicked the effects of thrombin while RLLFT-amide, an inactive, reverse peptide sequence, did not. In addition, the effect of TFLLR-amide was prevented by RWJ-56110, a PAR-1 antagonist. Thrombin and TFLLR-amide produced no oedema (Evans Blue extravasation) in the spinal cord that would account for these effects. Based on the reported ability of thrombin to mobilize endothelin-1 from astrocytes, we tested the role of this compound in thrombin's activity. BQ123, an endothelin A receptor antagonist, prevented thrombin's inhibition of writhing and NMDA-induced behaviours while BQ788, an endothelin B receptor antagonist, did not. Thus, activation of PAR-1 sites by thrombin in the CNS appears to inhibit NMDA-mediated nociception by a pathway involving endothelin type A receptors.

SYM 2081, an agonist that desensitizes kainate receptors, attenuates capsaicin and inflammatory hyperalgesia

Excitatory amino acids acting at non-NMDA receptors contribute to transmission of nociceptive information. SYM 2081 ((2S,4R)-4-methyl glutamic acid) desensitizes kainate receptors, one subtype of non-NMDA receptors, to subsequent release of excitatory amino acids and thus may attenuate transmission of nociceptive information. To determine if SYM 2081 can prevent development of hyperalgesia, SYM 2081 (10, 50 or 100 mg/kg, i.p.) was administered prior to injection of capsaicin into the hindpaw of rats, which produces mechanical and heat hyperalgesia. To determine if SYM 2081 can reduce ongoing inflammatory hyperalgesia, SYM 2081 (10 or 100 mg/kg, i.p.) was administered after development of carrageenan-evoked hyperalgesia. Intraplantar injection of capsaicin produced an increase in hindpaw withdrawal frequency to mechanical stimuli (from 4+/-2 to 41+/-7%; mean+/-S.E.M.) and a decrease in withdrawal latency to heat (from 12.3+/-0.3 to 5.9+/-0.4 s) in rats that received vehicle. In contrast, rats that received SYM 2081 (100 mg/kg) prior to injection of capsaicin exhibited a lower hindpaw withdrawal frequency (18+/-4%) and a longer withdrawal latency (7.7+/-0.5 s). Intrathecal (1-100 microg/5 microl), but not intraplantar (10 or 100 microg/50 microl), injection of SYM 2081 attenuated the development of capsaicin-evoked heat hyperalgesia suggesting that SYM 2081's antihyperalgesic effects were due to its central effects. Furthermore, SYM 2081 completely reversed ongoing carrageenan-evoked mechanical hyperalgesia and partially (approximately 50%) reversed ongoing heat hyperalgesia. The present study demonstrates that administration of a high-potency ligand that selectively desensitizes kainate receptors attenuates the development of mechanical and heat hyperalgesia and attenuates ongoing inflammatory hyperalgesia.

The role of neuropeptides and capsaicin-sensitive fibres in glutamate-induced nociception and paw oedema in mice

This study sought to establish whether sensory neuropeptides and the capsaicin-sensitive fibres are involved in the nociception and oedema formation caused by intraplantar (i.pl.) injection of glutamate into the mouse paw. The i.pl. co-injection of the selective neurokinin (NK) NK(2) (SR 48968, 0.05-0.5 nmol/paw), and to a lesser extent the selective NK(1) (FK 888, 0.25-1.0 nmol/paw) receptor antagonists, resulted in a significant inhibition of glutamate-induced nociception. The percentages of inhibition were 82 and 37%, respectively. In contrast, the selective NK(3) receptor antagonist (SR 142801, 0.25-1.0 nmol/paw) failed to significantly affect glutamate-induced nociception. SR 48968, but not FK 888 or SR 142801, significantly inhibited (36%) glutamate-induced paw oedema formation. The i.pl. injection of kinin B(1) receptor antagonist des-Arg(9)-[Leu(8)]-BK (0.2-0.8 nmol/paw), but not the B(2) receptor antagonist HOE 140 (1.0-4.0 nmol/paw), together with glutamate, also inhibited glutamate-induced nociception (53%) in a graded manner, without affecting glutamate-induced paw oedema. The i.pl. co-injection of the calcitonin gene-related peptide (CGRP) receptor antagonist CGRP(8-37) (1 nmol/paw) failed to significantly inhibit glutamate-induced nociception or oedema. Finally, neonatal-capsaicin (50 mg/kg, s.c.) treatment inhibited glutamate-induced nociception by 69% and to a lesser extent glutamate-mediated oedema formation (30%). Collectively, the current results indicate that the nociception caused by i.pl. injection of glutamate in mice is clearly mediated by capsaicin-sensitive fibres and by release of neurokinins from sensory neurones that activate NK(2) receptors and to a lesser extent NK(1) receptors. Furthermore, kinins acting at B(1) (but not at B(2)) receptors also largely account for glutamate-mediated nociceptive behaviour response. In contrast, glutamate-induced paw oedema seems to be primarily mediated via activation of NK(2) receptors and stimulation of capsaicin-sensitive C-fibres. CGRP receptors do not seem to be involved in either of the glutamate responses.

Vagal afferents are necessary for the establishment but not the maintenance of kainic acid-induced hyperalgesia in mice

Systemic administration of a single, sub-convulsive dose (20mg/kg) of kainic acid (KA) produces long-term hyperalgesia. The robustness and reproducibility of this effect makes this a valuable model of chronic pain. However, the mechanism by which KA produces hyperalgesia remains unknown. We evaluated the role of vagal afferents on KA-induced hyperalgesia in mice by assessing the influence of bilateral subdiaphragmatic vagotomy and of direct application of KA to vagal afferents on the development of hyperalgesia. The hot plate and tail flick tests were used to assess pain behavior. Central nervous system (CNS) activity evoked by acute administration of KA or exposure to a nociceptive stimulus was also determined by the immunocytochemical detection of Fos and of phosphorylated extracellular signal-regulated protein kinases 1 and 2 (pErk). Mice exhibited a persistent hyperalgesia after either systemic application of KA or topical treatment with KA on vagal afferents. Vagotomy performed 2 weeks before the application of KA was able to prevent the establishment of hyperalgesia, but vagotomy performed 2 weeks after the application of KA was unable to reverse the already established hyperalgesia. This result establishes that vagal afferents are pivotal to the onset of hyperalgesia. Consistent with this, KA evoked the expression of Fos in vagal related areas of the brainstem, including the nucleus tractus solitarius (NTS) and area postrema (AP), as well as widespread areas of the forebrain. Vagotomy selectively decreased KA-evoked Fos in the NTS while sparing that in other brain areas. In addition to hyperalgesia, weeks after KA treatment, stimulus induced pErk was increased in spinal nociceptive neurons and the medial hypothalamus, a phenomenon that was prevented by prior vagotomy. No signs of cell death were detected using in situ nick end-labeling (TUNEL) assay and Nissl staining at 1, 5, 24, 36 h and 12 days post-KA. These findings suggest that the mechanism underlying KA-induced hyperalgesia is a long-term dysfunction of CNS areas that are activated by vagal afferents and involved in descending control of spinal nociceptive neurons.

Activation of spinal kainate receptors after inflammation: behavioral hyperalgesia and subunit gene expression

We determined whether neural responses to inflammation and hyperalgesia involve activation of kainate receptors, a subgroup of glutamate receptors. Inflammation was introduced into the hind paw by intraplantar injection of complete Freund's adjuvant. The inflammation-induced thermal hyperalgesia was attenuated by intrathecal administration of a non-selective alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor antagonist, 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium (NBQX), as well as by selective kainate receptor antagonists, 6,7,8,9-tetrohydro-5-nitro-1H-benz[g]indole-2,3-dione 3-oxime (NS-102) and 3S,4aR,6S,8aR-6-(4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-deca-hydroisoquinoline-3-carboxylic acid (LY382884). Reverse transcription-polymerase chain reaction (RT-PCR) indicated that the GluR5 and GluR6, but not the GluR7, KA1 and KA2 subunits, exhibited increased mRNA expression at 2 h to 3 days following inflammation (P<0.05). Western blot showed an increase in GluR6 protein levels (P<0.01) with a time course consistent with the changes in its mRNA levels. cDNA sequence and BbvI endonuclease digestion of the GluR6 PCR product revealed that the upregulated GluR6 mRNAs were predominantly the unedited form (Q). These results suggest that a selective upregulation of kainate receptor subunit expression contributes to inflammatory hyperalgesia.

Vocalization responses after intrathecal administration of ionotropic glutamate receptor agonists in rats

Inotropic glutamate receptors in the spinal cord (N-methyl-D-aspartic acid [NMDA], alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA], and kainate receptors) seem to play a key role in acute pain transmission and the neuronal plasticity in chronic pain states. Vocalization responses produced by activation of these receptors on the pain pathways can be quantified semiautomatically and thus could be used as a research tool. We studied vocalization responses induced by intrathecal administration of various agonists acting at the glutamate receptors in normal rats and in the presence of peripheral inflammation and a chronic constriction injury model of neuropathic pain. The nonselective endogenous agonist, glutamate, and the NMDA receptor glycine site agonist D-serine did not produce vocalization, whereas selective agonists acting at AMPA, NMDA, and kainate receptors produced dose-related vocalization responses. The vocalization response evoked by the administration of AMPA was significantly increased in the neuropathic pain model. In conclusion, spinal administration of ionotropic glutamate receptor agonists produce short-lasting, dose-related vocalization responses that can be used as a basic research and screening tool for analgesic studies. However, peripheral inflammation or nerve injury did not substantially alter vocalization responses overall, possibly indicating that the vocalization test is not a good tool for studying the role of excitatory amino acids in these pathological pain conditions.

IMPLICATIONS

Vocalization responses evoked by spinal administration of ionotropic glutamate receptor agonists can be used for experimental analgesic studies. However, pathological pain models did not substantially alter vocalization responses, possibly indicating that this test is not suitable for studying the role of spinal excitatory amino acids in central sensitization.

Peripheral excitatory amino acids

Glutamate receptors (GluRs) are localized in the periphery on nociceptive primary afferent terminals. Studies in animal models of pain demonstrate that peripheral glutamate is involved in nociceptive transmission in the normal and the inflamed state and that modulation of peripheral GluRs reduces pain behaviors and nociceptor activity. These data provide strong motivation to develop new pharmacological agents that will target peripheral GluRs, offering novel approaches to treatment of pain of peripheral origin.

Zinc in the extracellular area of the central nervous system is necessary for the development of kainic acid-induced persistent hyperalgesia in mice

Kainic acid produces a persistent hyperalgesia when injected intraperitoneally (i.p.) in the rat or mouse. At higher doses than those needed to influence nociception, kainic acid induces seizures and translocation of histologically reactive zinc in the hippocampus. We tested the hypothesis that zinc, localized in a population of small diameter primary afferent neurons, plays a role in kainic acid-induced hyperalgesia similar to that in the hippocampus where zinc translocation accompanies kainic acid-induced seizures. The importance of zinc in the extracellular area was assessed by the influence of compounds that chelate divalent cations (disodium calcium ethylene diaminetetraacetate (CaEDTA)) or zinc (dipicolinic acid (DPA)) on kainic acid-induced hyperalgesia. When measured using the tail flick assay, thermal hyperalgesia was blocked by pretreatment intrathecally (i.t.) with either 10 nmol of NaCaEDTA or 1 nmol of DPA, drugs whose distribution is limited to the extracellular area. Injection of 10 ng zinc chloride i.t. had no long-term effect on nociception or on kainic acid-induced hyperalgesia. Whether zinc is translocated in response to a hyperalgesic dose of kainic acid was determined using the zinc-selective dye, N-(6-methoxy-8-quinolyl)-para-toluenensulfonamide (TSQ), which produces a delicate stain in the neuropil of the mouse spinal cord as well as a dense stain in the hippocampus. Injection of a hyperalgesic dose of kainic acid failed to alter TSQ fluorescence in either the spinal cord or hippocampus, in contrast to the distinct bleaching of TSQ in the hippocampus 24 h after a convulsant dose of kainic acid. Together these data suggest that, while not translocated, zinc in the extracellular area is necessary but not sufficient for the development of kainic acid-induced hyperalgesia.