An experimental arthritis in rats: Dorsal horn aspartate and glutamate increases

A novel transverse push-pull microprobe: in vitro characterization and in vivo demonstration of the enzymatic production of adenosine in the spinal cord dorsal horn

Adenosine produces analgesia in the spinal cord and can be formed extracellularly through enzymatic conversion of adenine nucleotides. A transverse push-pull microprobe was developed and characterized to sample extracellular adenosine concentrations of the dorsal horn of the rat spinal cord. Samples collected via this sampling technique reveal that AMP is converted to adenosine in the dorsal horn. This conversion is decreased by the ecto-5'-nucleotidase inhibitor, alpha,beta-methylene ADP. Related behavioral studies demonstrate that AMP administered directly to the spinal cord can reverse the secondary mechanical hyperalgesia characteristic of the intradermal capsaicin model of inflammatory pain. The specific adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT) inhibits the antihyperalgesia produced by AMP. This research introduces a novel microprobe that can be used as an adjunct sampling technique to microdialysis and push-pull cannulas. Furthermore, we conclude that AMP is converted to adenosine in the dorsal horn of the spinal cord by ecto-5'-nucleotidase and subsequently may be one source of adenosine, acting through adenosine A(1) receptors in the dorsal horn of the spinal cord, which produce antihyperalgesia.

Differential effects of calcitonin gene-related peptide and calcitonin gene-related peptide 8-37 upon responses to N-methyl-D-aspartate or (R, S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate in spinal nociceptive neurons with knee joint input in the rat

Calcitonin gene-related peptide is involved in the spinal processing of nociceptive input from the knee joint and in the generation and maintenance of joint inflammation-evoked hyperexcitability of spinal cord neurons. The present study examined whether this peptide influences the excitation of nociceptive spinal cord neurons by agonists at the N-methyl-D-aspartate and the non-N-methyl-D-aspartate [(R, S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate] receptors, both of which are essential for the excitation and hyperexcitability of spinal cord neurons. In anaesthetized rats extracellular recordings were made from dorsal horn neurons with knee input, and compounds were administered ionophoretically close to the neurons recorded. When calcitonin gene-related peptide was administered the responses of the neurons to the application of both N-methyl-D-aspartate and AMPA were increased. The coadministration of the antagonist calcitonin gene-related peptide 8-37 had no effect on the responses to N-methyl-D-aspartate, but it prevented the enhancement of the responses to N-methyl-D-aspartate by calcitonin gene-related peptide. By contrast, the administration of calcitonin gene-related peptide 8-37 enhanced the responses of the neurons to AMPA, and it did not antagonize but rather increased the effects of calcitonin gene-related peptide on these responses. The data suggest that the facilitatory role of calcitonin gene-related peptide on the development and maintenance of inflammation-evoked hyperexcitability is caused at least in part by the modulation of the activation of the dorsal horn neurons through their N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors. The different effects of calcitonin gene-related peptide 8-37 on the respones to N-methyl-D-aspartate and AMPA suggest that different intracellular pathways may facilitate the activation of N-methyl-D-aspartate and ionotropic non-N-methyl-D-aspartate receptors.

Enhanced phosphorylation of NMDA receptor 1 subunits in spinal cord dorsal horn and spinothalamic tract neurons after intradermal injection of capsaicin in rats

The functional enhancement of NMDA receptors after peripheral tissue injury is proposed to contribute to the sensitization of spinothalamic tract (STT) cells and hyperalgesia. Protein phosphorylation is a major mechanism for the regulation of NMDA receptor function. In this study, Western blots, immunofluorescence double labeling, and the retrograde tracing method were used to examine whether phosphorylation of NMDA receptor 1 (NR1) subunits increases in spinal cord tissue and spinal dorsal horn neurons, especially in STT cells, after injection of capsaicin (CAP) into the glabrous skin of one hindpaw of anesthetized rats. Western blots showed that phosphorylated NR1 protein in spinal cord tissue was increased 30 min after CAP injection. Immunofluorescence double-labeling staining showed no significant difference in the number of the NR1-like immunoreactive neurons in laminae I-VII in the lumbosacral segments (L(4)-S(1)) on the ipsilateral and the contralateral sides 30 min after CAP or vehicle injection. However, the numbers of phospho-NR1-like immunoreactive neurons were significantly increased on the ipsilateral side compared with the vehicle injection group. STT cells were labeled by bilateral microinjections of the retrograde tracer fluorogold into the lateral thalamus, including the ventral-posterior lateral nucleus. Immunofluorescence staining was performed at 30, 60, and 120 min after CAP injection or at 30 min after vehicle injection. There was a significant increase in the proportion of STT cells with phosphorylated NR1 subunits compared either with the contralateral side 30 and 60 min after CAP injection or either side of animals after intradermal injection of vehicle. These results provide direct evidence that NMDA receptors in STT cells are phosphorylated after CAP injection.

Expression of metabotropic glutamate receptors mRNA in the thalamus and brainstem of monoarthritic rats

Evidence for the involvement of metabotropic glutamate receptors (mGluR) in sensory processing has been emerging. Additionally, the differential distribution of distinct mGluR subtypes mRNA in particular thalamic nuclei of normal rats suggests that they could be involved in the processing of somatosensory information. In the present study, mGluR1, 3, 4 and 7 mRNAs expression was investigated by in situ hybridisation in selected brainstem and thalamic nuclei of adult monoarthritic rats at different time points of the disease (2, 4 and 14 days). Monoarthritic rats displayed behavioural and physical signs of painful arthritis at all time points. At 2 days of monoarthritis the mGluR1 mRNA expression was decreased mainly in the ventrobasal complex (VB) and in the posterior thalamic nuclei (Po) contralateral to the inflamed joint. The mGluR4 mRNA expression was also reduced, but minimum values were found at 4 days of monoarthritis, when no changes could be found in mGluR1 mRNA expression. At 14 days, mGluR4 mRNA expression was similar to controls, while mGluR1 mRNA was again reduced. Similar decreases of mGluR7 mRNA expression in the VB and Po were found at all time points, while mGluR3 mRNA expression was bilaterally increased in the reticular thalamic nucleus (Rt). In the brainstem no changes could be found in the expression of any mGluR subtype mRNA. The reduced expression of mGluR1, 4 and 7 transcripts in VB and Po, and the increases of mGluR3 mRNA in the Rt may contribute to counteract the increased noxious input arising from the periphery.

Amino acid release into the knee joint: key role in nociception and inflammation

This study examined the release of several amino acids after induction of knee joint inflammation in rats using kaolin and carrageenan. During the initial 10-min collection after knee joint injection with the irritants, the concentration of glutamate and the nitric oxide metabolites, arginine and citrulline, doubled. This increase persisted for at least two hours. During the same time period aspartate concentrations remained unchanged. Direct knee joint administration of lidocaine prevented the increases in amino acid concentration measurable by microdialysis probe inserted into the joint. These data suggest the possibility that glutamate may be released by neuronal endings in the joint.

Excitatory amino acid agonists and antagonists: pharmacology and therapeutic applications

Glutamic acid is the major excitatory neurotransmitter in the mammalian central nervous system (CNS). Specific receptors bind glutamate and some of these when activated open an integral ion channel and are thus known as ionotropic receptors. Within the ionotropic family of glutamate receptors, three major subtypes have been identified using classical specific agonist activation, selective competitive antagonists together with their structural heterogeneity. These receptors have thus been named N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate receptors. The NMDA receptor has sites in addition to its agonist-binding site and these seem to either positively or negatively modulate the agonist effect. The NMDA receptor also is unique in that another amino acid, glycine, acts as a co-agonist with glutamate. Changes in glutamate transmission have been associated with a number of CNS pathologies; these include, acute stroke, chronic neurodegeneration, chronic pain, depression, drug dependency, epilepsy, Parkinson's Disease and schizophrenia.

Effects of antagonists to high-threshold calcium channels upon spinal mechanisms of pain, hyperalgesia and allodynia

High-threshold voltage-dependent calcium channels enable calcium ions to enter neurons upon depolarization and thereby influence synaptic mediator/receptor systems, membrane excitability levels, second and third messenger concentration, and gene expression. These phenomena underlie several processes including those of normal nociception and of hyperalgesia and allodynia. The present article deals with the role of spinal L-, N- and P/Q-type calcium channels in short-lasting nociception as well as in the hyperalgesia and allodynia elicited by chemical irritants of peripheral nociceptors, inflammatory and mechanical lesions of peripheral tissues, and lesions of peripheral nerves. The studies summarized herein are based on the spinal delivery of specific antagonists to high-threshold calcium channels, and reveal that blockade of L-type, P/Q-type and, particularly, N-type channels can prevent, attenuate, or both, subjective pain as well as primary and/or secondary hyperalgesia and allodynia in a variety of experimental and clinical conditions.

The Role of Type 1 Metabotropic Glutamate Receptors in the Generation of Dorsal Root Reflexes Induced by Acute Arthritis or the Spinal Infusion of 4-Aminopyridine in the Anesthetized Rat

Antidromically propagated action potentials can be recorded in the proximal end of the severed medial articular nerve (MAN) on mechanical stimulation of an inflamed knee in rats and are referred to as dorsal root reflex (DRR) activity. The absence of DRR activity in normal rats suggests that the activity could be the result of hyperexcitability of spinal neurons induced by inflammation. In this study, the role of spinal type 1 metabotropic glutamate (mGlu(1)) receptors in the generation of DRR activity in the MAN during acute knee inflammation was investigated. Four hours after an injection of a mixture of kaolin and carrageenan (k/c) into a knee joint, DRR activity could be evoked in the ipsilateral MAN by mechanical stimulation of the inflamed limb. Spinal application of a selective mGlu(1) receptor antagonist, [RS]-1-Aminoindan-1,5-dicarboxylic acid/UPF 523 (AIDA), or a potent, but less specific mGlu(1) receptor antagonist, LY393053, both depressed the DRR activity significantly. AIDA and LY39053 had no effect on recordings in the MAN from noninflamed control animals. However, spinal administration of AIDA did suppress DRR activity generated by infusion of 4-aminopyridine (4-AP), a K(+) channel blocker, into the dorsal horn of noninflamed animals. These observations suggest that mGlu(1) receptors support the generation of DRR activity in the MAN following sensitization of spinal cord neurons.

The role of non-N-methyl-D-aspartate ionotropic glutamate receptors in the spinal transmission of nociception in normal animals and animals with carrageenan inflammation

The role of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and kainate receptors in spinal nociceptive transmission in both normal animals and animals with carrageenan inflammation was investigated using the AMPA/kainate receptor antagonist 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (NBQX) and the selective GluR5 kainate receptor antagonist LY382884 [3S,4aR,6S,8aR-6-(4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8, 8a-deca-hydroisoquinoline-3-carboxylic acid]. In normal animals, spinal administration of 100 microg of LY382884 produced a significant inhibition of both the C-fibre-evoked response and post-discharge of dorsal horn neurons, with the wind-up of the neurons being reduced by both 50 and 100 microg of LY382884. The spinal actions of LY382884 were enhanced following 3 h of carrageenan inflammation, such that doses of 20 microg and above were able to produce significant inhibitions of the noxious-evoked response of the neurons. Spinal administration of NBQX in normal animals (5-50 microg) inhibited the C-fibre-evoked response of the dorsal horn neurons, but only 50 microg of NBQX was able to inhibit the wind-up and post-discharge of the neurons. Following 3 h of carrageenan inflammation, the ability of NBQX to inhibit the wind-up and post-discharge of the neurons was markedly enhanced. These data suggest that both AMPA and kainate GluR5 receptors play an enhanced role in spinal nociceptive processing following the development of peripheral inflammation, as antagonists at both receptors are more effective against nociceptive responses, including wind-up under these inflammatory conditions.

Extracellular glutamate in the dorsal horn of the lumbar spinal cord in the freely moving rat during hindlimb stepping

The capacity to reestablish locomotor function after complete spinal cord transection in the adult mammal is now well documented. Further studies have shown different neurotransmitters to be involved in the initiation and maintenance of these locomotor patterns. However, there has been no in vivo evidence of the changes in glutamate or any other neurotransmitter in the extracellular space of the dorsal horn during an alternating motor pattern such as hindlimb stepping. This study describes an in vivo microdialysis technique to measure extracellular glutamate in the dorsal horn of the spinal cord in the fully awake intact rat. A concentric microdialysis probe was placed in the dorsal horn at L5, and 18 h later dialysate samples were collected at 20-min intervals before, during, and after 20 min of hindlimb stepping. During stepping, extracellular glutamate rose 150% above resting levels and returned to resting levels 40 min later. This increase may have occurred either as a result of primary afferent depolarization or modulation by the descending and ascending supraspinal pathways. In another series of experiments extracellular glutamate was, therefore, measured in the dorsal horn of the chronic spinally transected rat during 20 min of hindlimb stepping. Although the spinal group did not take as many steps as the intact group, those taking more than 40 steps showed a significant rise in extracellular glutamate, and the number of steps taken by the individual spinal rats correlated positively with the individual values of extracellular glutamate (r2 = 0.63). These results are consistent with glutamate being an important neurotransmitter in the spinal cord in normal locomotion.

Effects of omega-agatoxin IVA, a P-type calcium channel antagonist, on the development of spinal neuronal hyperexcitability caused by knee inflammation in rats

Both N- and P-type high-threshold calcium channels are located presynaptically in the CNS and are involved in the release of transmitters. To investigate the importance of P-type calcium channels in the generation of inflammation-evoked hyperexcitability of spinal cord neurons, electrophysiological recordings were made from wide-dynamic-range neurons with input from the knee joint in the anesthetized rat. The responses of each neuron to innocuous and noxious pressure onto the knee and the ankle were continuously assessed before and during the development of an inflammation in the knee joint induced by the injections of K/C into the joint cavity. The specific antagonist at P-type calcium channels omega-agatoxin was administered into a 30-microl trough on the spinal cord surface above the recorded neuron. In most neurons the application of omega-agatoxin before induction of inflammation slightly enhanced the responses to pressure onto the knee and ankle or left them unchanged. Two different protocols were then followed. In the control group (13 rats) only Tyrode was administered to the spinal cord during and after induction of inflammation. In these neurons the responses to mechanical stimuli applied to both the inflamed knee and to the noninflamed ankle showed a significant increase over 4 h. In the experimental group (12 rats) omega-agatoxin was applied during knee injection and in five 15-min periods up to 180 min after kaolin. This prevented the increase of the neuronal responses to innocuous pressure onto the knee and to innocuous and noxious pressure onto the ankle; only the responses to noxious pressure onto the knee were significantly enhanced during development of inflammation. Thus the development of inflammation-evoked hyperexcitability was attenuated by omega-agatoxin, and this suggests that P-type calcium channels in the spinal cord are involved in the generation of inflammation-evoked hyperexcitability of spinal cord neurons. Finally, when omega-agatoxin was administered to the spinal cord 4 h after the kaolin injection, i.e., when inflammation-evoked hyperexcitability was fully established, the responses to innocuous and noxious pressure onto the knee were reduced by 20-30% on average. The shift in the effect of omega-agatoxin, from slight facilitation or no change of the responses before inflammation to inhibition in the state of hyperexcitability, indicates that P-type calcium channels are important for excitatory synaptic transmission involved in the maintenance of inflammation-evoked hyperexcitability.

Unilateral joint inflammation induces bilateral and time-dependent changes in neuropeptide FF binding in the superficial dorsal horn of the rat spinal cord: implication of supraspinal descending systems

Using quantitative autoradiography, the effects of acute and chronic inflammation on specific 125I-1DMethyl-FLFQPQRFamide binding were investigated in the rat spinal cord dorsal horn superficial layers, at 6 and 24 h and 2, 4, 6 and 12 weeks after induction of monoarthritis produced by injection of killed Mycobacterium butyricum suspended in Freund adjuvant in one tibio-tarsal joint. Six hours after monoarthritis induction, no modification in specific 125I-1DMethyl-FLFQPQRFamide binding was observed, whereas a significant bilateral increase occurred after 24 h and 2 weeks in L4/L5 dorsal horns, with a return to control values at 4, 6 and 12 weeks. Specific 125I-1DMethyl-FLFQPQRFamide binding was also investigated 24 h after monoarthritis induction in rats submitted 4 days before the induction to spinal cord lesions at the thoracic level (T9-T10). Hemisection of the spinal cord contralateral to the affected ankle prevented the transient bilateral increase in specific 125I-1DMethyl-FLFQPQRFamide binding, whereas total spinal cord section induced a significant bilateral decrease. All of these modifications were restricted to the spinal segments receiving afferent input from the arthritic ankle (L4/L5); no modifications were found at the levels L1 or C6-C8. These data suggest that FLFQPQRFamide is involved in spinal nociceptive processing during sustained peripheral nociceptor activation. The effects of spinal cord lesions in monoarthritic rats indicate that the modifications seen in the FLFQPQRFamide system activity, during sustained peripheral inflammation, depend on afferent fiber activation as well as on supraspinal controls.

Considerations in the determination by microdialysis of resting extracellular amino acid concentrations and release upon spinal cord injury

The following issues are further addressed: (1) Is there considerable leakage of amino acids from the circulation into the space around microdialysis probes, or are amino acid concentrations naturally much higher in the interstitial space than is generally thought? (2) Do observed high interstitial concentrations or depletion of substances in the intracellular space by microdialysis affect release measurements upon spinal cord injury? Amino acid concentrations around microdialysis fibres in the spinal cord of rats were found to approach those in the circulation and to be much higher than interstitial concentrations previously estimated in the CNS. However, much lower concentrations of amino acids were derived in the hippocampus by analogous experiments. Considerable Evans Blue/albumin leaked from the circulation into the interstitial space in the spinal cord immediately after fibre insertion. However, this movement diminished considerably by 4 h later, demonstrating substantial resealing of the blood-brain barrier, at least to large molecules. There is either substantial damage-induced movement of amino acids from the circulation into the dialysis zone after insertion of a microdialysis probe, or there is much less impediment to movement of amino acids across the blood-brain barrier in the spinal cord than in the brain. At low flow rates through the fibre, adding concentrations of amino acids to the inside of the fibre equal to the concentrations around the fibre to prevent their depletion by removal through the microdialysis fibre did not affect increases in concentrations of amino acids in microdialysates following injury. Thus the high concentrations of amino acids present around microdialysis fibres following their insertion do not seem to disturb measurements of amino acid release upon spinal cord injury.

Increased spinal release of excitatory amino acids following intradermal injection of capsaicin is reduced by a protein kinase G inhibitor

Second messengers have been shown to play a role in the release of neurotransmitters presynaptically in several brain regions and cell types. This study was designed to test the hypothesis that the increased release of aspartate and glutamate that occurs after injection of capsaicin is dependent on activation of the cAMP and the cGMP transduction cascades. A microdialysis fiber was implanted into the dorsal horn of the spinal cord for collection of extracellular fluid and for administration of drugs to the spinal cord. Dialysate samples were collected before and after injection of capsaicin and after infusion of inhibitors of protein kinase G (PKG; KT5823) or protein kinase A (PKA; H89). KT5823, H89, or artificial cerebrospinal fluid (ACSF; control) were administered after injection of capsaicin to reduce the increased release of aspartate and glutamate. At the time of injection of capsaicin, there is an increase in release of aspartate (191+/-21%) and glutamate (194+/-14%). This increased release is maintained through 2.5 h for both glutamate and aspartate at approximately 125% to 150%. The increase in aspartate and glutamate concentrations that occurs after capsaicin injection was reduced back to baseline after spinal infusion of the PKG inhibitor, KT5823. Blockade of PKA had no effect on the increased release of aspartate and glutamate. Thus, the current data support a role for the cGMP-PKG pathway in the control of neurotransmitter release in vivo.

Activation of the cAMP transduction cascade contributes to the mechanical hyperalgesia and allodynia induced by intradermal injection of capsaicin

1. The spinal role of the cAMP transduction cascade in nociceptive processing was investigated in awake behaving rats (male, Sprague-Dawley) by activating or inhibiting this pathway spinally. Microdialysis fibres were implanted into the dorsal horn to infuse drugs directly to the spinal cord. 2. Animals, without peripheral tissue injury, were tested for responses to repeated applications (10 trials) of von Frey filaments and threshold to mechanical stimulation before and after infusion of 8-bromo-cAMP. In this group of animals treated spinally with 8-br-cAMP (1-10 mM) a dose-dependent hyperalgesia and allodynia were produced. This was manifested as an increased number of responses to 10 trials of von Frey filaments (10, 50, 150, 250 mN) and a decrease in mechanical threshold. 3. A second series of experiments studied the manipulation of the cAMP pathway spinally in a model of tissue injury induced by intradermal injection of capsaicin. Animals were either pre- or post-treated spinally with the adenylate cyclase inhibitor, tetrahydrofuryl adenine (THFA) or the protein kinase A inhibitor, myrosilated protein kinase (14-22) amide (PKI). Injection of capsaicin resulted in an increased number of responses to repeated applications of von Frey filaments and a decrease in threshold to mechanical stimuli outside the site of injection, secondary mechanical hyperalgesia and allodynia. 4. Pre-treatment with either THFA (1 mM) or PKI (5 mM) had no effect on the capsaicin-evoked secondary hyperalgesia and allodynia. 5. In contrast, post-treatment spinally with THFA (0.01-1 mM) or PKI (0.05-50 mM) dose-dependently reduced the mechanical hyperalgesia and allodynia produced by capsaicin injection. Furthermore, the mechanical hyperalgesia and allodynia blocked by the adenylate cyclase inhibitor, THFA (1 mM), was reversed by infusion of 8-bromo-cAMP (0.01-10 mM) in a dose-dependent manner. 6. Thus, this study demonstrates that activation of the cAMP transduction cascade at the spinal cord level results in mechanical hyperalgesia and allodynia and that the secondary mechanical hyperalgesia and allodynia following intradermal injection of capsaicin is mediated by this same transduction cascade.

Excitatory amino acid receptor involvement in peripheral nociceptive transmission in rats

The involvement of excitatory amino acid receptors in peripheral nociceptive processing was assessed in two separate experiments. In the first, one knee joint cavity of rats was injected with 0.1 ml of L-glutamate (0.001 mM; 0.1 mM; 1.0 mM), L-aspartate (0.001 mM; 0.1 mM: 1.0 mM), L-arginine (0.1 mM) or different combinations of these amino acids. The animals tested for paw withdrawal latency to radiant heat and withdrawal threshold to von Frey filaments at different time points. Combinations of glutamate/aspartate, aspartate/arginine or glutamate/aspartate/arginine when injected into the joint, in the absence of any other treatment, reduced the paw withdrawal latency and withdrawal threshold immediately after the injection and persisting up to 5 h indicating the development of hyperalgesia and allodynia. Subsequent intra-articular injection of either an NMDA or a non-NMDA glutamate receptor antagonist ((+/-)-2-amino-7-phosphonoheptanoic acid (AP7), 0.2 mM) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 0.1 mM) attenuated the thermal hyperalgesia and the mechanical allodynia produced by glutamate/aspartate/arginine. On the other hand, in a second experiment intra-articular injection of AP7, ketamine or CNQX reversed the hyperalgesia and allodynia produced by injection of a mixture of kaolin and carrageenan into the joint. These receptor antagonists, however, did not have an effect on the joint edema. These findings provide evidence for a potential role of peripheral NMDA and non-NMDA receptors in nociceptive transmission.

Knee joint inflammation attenuates spinal FOS expression after unilateral paw formalin injection in rat

Carrageenan-induced knee joint inflammation evoked a transient spinal FOS protein expression in neurons localized in the apical region of laminae I-III with peak activity observed 2 h after inflammation. Consistent with previously published observations, paw formalin injection evoked a distinct pattern of FOS protein expression in L3-L5 spinal segments. The majority of FOS-positive neurons were localized in the superficial dorsal horn (laminae I-II). Laminae V and VI contained moderate numbers of labeled neurons and only a few labeled nuclei were visible in laminae VII-X. In contrast, ipsilateral paw formalin injection, if administered 4 h after carrageenan-induced knee inflammation, evoked significantly fewer FOS positive neurons in all laminar and segmental levels analyzed as compared with formalin injected animals but without previous knee joint inflammation. These data indicate that primary acute or subacute nociceptive input may evoke central processes that are characterized by an inducible form of central inhibition which then may serve to modulate the subsequent spinal effect of superimposed nociceptive peripheral stimulation.

Differential roles of neurokinin 1 and neurokinin 2 receptors in the development and maintenance of heat hyperalgesia induced by acute inflammation

1. Following induction of acute inflammation by intraarticular injection of kaolin and carrageenan into the knee joint in rats, there was a significant decrease in the withdrawal latency to radiant heat applied to the paw (i.e. heat hyperalgesia), an increased joint circumference and increased joint temperature. 2. A neurokinin1 (NK1) receptor antagonist (CP-99,994, 10 mM) had no effect on the paw withdrawal latency when it was administered spinally through a microdialysis fibre before the induction of inflammation. Pretreatment with a NK2 receptor antagonist (SR48968, 1 mM) administered spinally through the microdialysis fibre prevented the heat hyperalgesia from developing in the early stages of the inflammation. 3. Post-treatment through the microdialysis fibre with the NK1 receptor antagonist (0.01-10 mM) was effective in reversing the heat hyperalgesia. In contrast, post-treatment spinally with the NK2 receptor antagonist (0.01-1 mM) had no effect on the heat hyperalgesia. The inactive stereoisomers of the NK1 receptor antagonist, CP100,263, or the NK2 receptor antagonist, SR48965, administered at the same doses, had no effect on the joint inflammation or the heat hyperalgesia. 4. Pretreatment systemically with the NK1 receptor antagonist (30 mg kg-1) had no effect on the heat hyperalgesia or pain-related behaviour ratings where 0 is none and 5 is non weight bearing and complete avoidance of limb contact. Pretreatment with a NK2 receptor antagonist (10 mg kg-1) systemically prevented the heat hyperalgesia and pain-related behaviour ratings from developing in the early stages of the inflammation. The inactive stereoisomers of NK1 receptor antagonist, CP100,263, or the NK2 receptor antagonist, SR48965, administered at the same doses, had no effect on the joint inflammation or the heat hyperalgesia. 5. Post-treatment systemically with either the NK1 (0.1-30 mg kg-1) or the NK2 (0.1-10 mg kg-1) receptor antagonist resulted in a dose-dependent reversal of the heat hyperalgesia. Pain-related behaviour ratings were reduced by post-treatment only with the NK1 receptor antagonist. The inactive stereoisomers of the NK1 receptor antagonist, CP100,263, or the NK2 receptor antagonist, SR48965, administered at the same doses, had no effect on the behavioural responses. 6. Direct pretreatment of the knee joint with either the NK1 (30 mg) or the NK2 (10 mg) receptor antagonist prevented the heat hyperalgesia from developing without affecting joint swelling. The inactive stereoisomers of the NK1 receptor antagonist, CP100,263, or the NK2 receptor antagonist, SR48965, administered at the same doses, had no effect on the joint inflammation or the heat hyperalgesia. 7. There appears to be a differential role for the spinal tachykinin receptors in the development and maintenance of the heat hyperalgesia associated with acute joint inflammation. The NK2 receptors appear to be activated early in the development of the heat hyperalgesia and NK1 receptors are involved in the maintenance of the heat hyperalgesia. 8. Peripherally, both NK1 and NK2 receptors are involved in the development of heat hyperalgesia and pain-related behaviour ratings induced by acute inflammation.

Spinal cord adenosine receptor stimulation in rats inhibits peripheral neutrophil accumulation. The role of N-methyl-D-aspartate receptors

The effect of spinal adenosine receptor ligation on peripheral leukocyte accumulation was studied in two rat models of inflammation. Neutrophil infiltration into dermal inflammatory sites was signficantly reduced by adenosine A1 receptor agonists injected through intrathecal catheters. These effects were reversed by N-methyl-D-aspartate (NMDA), and were mimicked by (+/-)-2-amino-5-phosphonopentanoic acid (AP-5), a glutamate NMDA receptor antagonist. Peripheral adenosine levels, as measured in air pouch exudates, decreased markedly in inflamed pouches but remained near normal after intrathecal treatment with AP-5. Moreover, the antiinflammatory effects of intrathecal A1 receptor agonists and AP-5 were reversed by an adenosine A2 receptor antagonist administered intraperitoneally. Hence, central NMDA receptor activity can regulate neutrophil accumulation in peripheral inflammatory sites by reducing local levels of adenosine, an antiinflammatory autacoid which inhibits neutrophil function through A2 receptor activation. This represents a previously unknown pathway by which the central nervous system influences inflammatory responses.

Involvement of increased excitatory amino acids and intracellular Ca2+ concentration in the spinal dorsal horn in an animal model of neuropathic pain

Neuropathic pain following nerve injury is believed to involve excitatory amino acids (EAAs) and Ca2+-mediated neuronal plastic changes in the central nervous system (CNS). This study was designed to investigate the changes in glutamate and aspartate contents in the dorsal half of the spinal cord following chronic constrictive injury (CCI) of the rat common sciatic nerve. We also examined the changes in intracellular calcium ion concentration ([Ca2+]i) of the spinal dorsal horn in transverse spinal slices in the same animal model. Thermal and mechanical hyperalgesia were observed on day 2 and thereafter following CCI (P < 0.0001). In the CCI rats to which 0.5 mg/kg of i.p. MK-801 was given 30 min prior to CCI and subsequently three daily treatments with 0.5 mg/kg of i.p. MK-801, the development of thermal and mechanical hyperalgesia was suppressed for a period of up to 7 days; however, hyperalgesia appeared on day 10 and day 14 (P < 0.001). In CCI rats, significant increases were observed in glutamate and aspartate contents on the ipsilateral side of the dorsal horn to nerve ligation on days 4, 7 and 14 (P < 0.001). Moreover, significant increases in [Ca2+]i in the spinal dorsal horn were also observed in the superficial (lamina I-II) and deep layers (lamina V-VI) on the ipsilateral side to nerve ligation on days 4, 7 and 14 after nerve ligation in the spinal slices (P < 0.0001). The treatment with i.p. MK-801 suppressed the increases in the contents of glutamate and aspartate and in [Ca2+]i on days 4 and 7. However, the ipsilateral contents of glutamate and aspartate significantly increased on day 14 (P < 0.001 and 0.003, respectively); the increased [Ca2+]i was also observed on day 14 (P < 0.001), and the spatial pattern of the increased regions was similar to untreated CCI rats. We interpret these results to indicate that neuropathic hyperalgesia induced by CCI in the rat is associated with an increase in glutamate and aspartate contents and the subsequent activation of NMDA receptors, followed by an increase in [Ca2+]i within dorsal horn of the spinal cord.

Mechanisms of touch-evoked pain (allodynia): a new model

In this paper we review the current neurophysiological models of touch-evoked pain and present a new proposal that addresses the mechanisms of allodynia. The new model is based on the notion that A-beta mechanoreceptors can gain access to nociceptive neurones by means of a presynaptic link, at central level, between low threshold mechanoreceptors and nociceptors. We propose that the excitation of nociceptors provoked by a peripheral injury activates the spinal interneurones that mediate primary afferent depolarization (PAD) between low threshold mechanoreceptors and nociceptors. As a consequence of the increased and persistent barrage driving these neurones their excitability is increased such that, when activated by low threshold mechanoreceptors from areas surrounding the injury site, they produce a very intense PAD in the nociceptive afferents which is capable of generating spike activity. This activation would be conducted antidromically in the form of dorsal root reflexes (DRRs) but would also be conducted forward activating the second order neurones normally driven by nociceptors. The sensory consequence of this mechanism is pain evoked by the activation of low threshold mechanoreceptors from an area surrounding an injury site (allodynia).

Characterization of time course of spinal amino acids, citrulline and PGE2 release after carrageenan/kaolin-induced knee joint inflammation: a chronic microdialysis study

Pharmacological studies have implicated the spinal activation of excitatory amino acids, nitric oxide, and prostaglandins systems in the development of tactile and thermal hypersensitivity and central sensitization after peripheral inflammation. In the present study, using a chronically placed loop dialysis catheter, we examined in the unanesthetized rat the effect of carrageenan/kaolin (C/K)-induced knee joint inflammation on the time course of spinal release of several active factors including excitatory amino acids (glutamate, aspartate), citrulline (a marker of nitric oxide formation), and prostaglandin E2 (PGE2) as well as the concomitant development of tactile and thermal hypersensitivity. Infection of C/K in the knee evoked a significant release of glutamate, with an initial peak seen immediately after knee C/K injection (179 +/- 22%) and with a progressive and consistent increase over a period of 24 h (153-186%). Comparable changes in the concentration of aspartate (123-179%) were observed. Citrulline was constantly above baseline for the 24-h period (121-158%). PGE2 was significantly increased at 10 min (146 +/- 11%) with no change observed between 3-5 h. At 24 h, PGE2 was again significantly (143 +/- 18%) increased. Behaviorally, a prominent thermal and tactile allodynia developed after injection with the peak seen by 1-3 h after induction of the inflammation. This hypersensitivity state, while diminished in its intensity, persisted for the entire observation period. These data suggest that increased spinal release of excitatory amino acids (EAA), nitric oxide and/or PGE2 is involved in the maintenance of the pain state initiated by acute peripheral inflammation.

Effect of acute stimulation on Fos expression in spinal neurons in the presence of persisting C-fiber activity

Expression of the inducible transcription factor c-Fos has been examined in the lumbar spinal cord following noxious chemical stimulation (injection of 2% formalin) of the ankles or the ventral skin of the hindpaws of either normal rats, or monoarthritic rats during the chronic phase of the disease. In normal animals the basal expression of c-Fos was low. One day after induction of monoarthritis by an intra-articular injection of killed Mycobacterium butyricum (in complete Freund's adjuvant) there were numerous c-Fos labelled cells in the ipsilateral dorsal horn, and bilaterally in lamina VIII and in other areas of the ventral horn. Four weeks after induction of the arthritis, although marked inflammation of the ankle was still present, all the expression of c-Fos had returned to the basal levels. One hour after formalin stimulation of the ankle or hindpaw skin of normal rats expression of c-Fos was observed throughout the ipsilateral, but not contralateral dorsal horn. Formalin stimulation of the inflamed ankle in four-week arthritic rats induced a 3-to-6 fold increase in c-Fos expression in the ipsilateral dorsal horn compared to formalin stimulation of the ankle in normal rats. In addition, c-Fos expression was induced in the contralateral deep, but not superficial laminae, at a density similar to that produced ipsilaterally by formalin stimulation of the ankle of normal rats. Formalin stimulation of the contralateral ankle in monoarthritic rats (i.e. the non-inflamed ankle) induced an ipsilateral expression of c-Fos which was similar to that observed after stimulation of the arthritic ankle. This stimulation of the normal ankle also resulted in an expression of c-Fos in the contralateral deep, but not superficial laminae, that was similar to that induced ipsilaterally by stimulation of the arthritic ankle. Finally, formalin stimulation of the hindpaw skin (which was not inflamed) of the arthritic limb induced the same number of c-Fos labelled cells in the superficial laminae as did formalin stimulation of the skin of normal rats; but in the deep laminae there was a 1.6-fold increase in the number of labelled cells. These different observations show that the down-regulation of c-Fos expression observed in chronic monoarthritis is in fact associated with a sensitization and an extension of the field of its expression in response to an acute nociceptive stimulation.

Calcitonin gene-related peptide is involved in the spinal processing of mechanosensory input from the rat's knee joint and in the generation and maintenance of hyperexcitability of dorsal horn-neurons during development of acute inflammation

In an electrophysiological study in anaesthetized rats, the involvement of calcitonin gene-related peptide in the spinal processing of mechanosensory information from the normal and inflamed knee joint was investigated. Calcitonin gene-related peptide(8-37), a specific antagonist at calcitonin gene-related peptide 1 receptors was administered ionophoretically close to nociceptive neurons with input from the knee joint before, during, and after development of acute inflammation in the knee induced by the intra-articular injections of kaolin and carrageenan. Calcitonin gene-related peptide (8-37) selectively antagonized the effects of ionophoretically applied calcitonin gene-related peptide but not those of ionophoretically applied substance P, neurokinin A, and (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid. Before inflammation, calcitonin gene-related peptide (8-37) reduced the responses to noxious pressure applied to the knee in 22 of 23 neurons; in 14 of 22 neurons, the responses to innocuous pressure were also reduced. In eight neurons calcitonin gene-related peptide (8-37) was administered during induction and in three periods within the first 90 min of inflammation. In these neurons the developing inflammation evoked a significantly smaller increase of the responses to innocuous and noxious pressure applied to the injected knee than in 13 control neurons which were not treated by the antagonist during induction of inflammation. In 16 of 16 neurons, calcitonin gene-related peptide (8-37) reduced the responses to innocuous and noxious pressure once inflammation and hyperexcitability of the spinal cord neurons were established. These data show that calcitonin gene-related peptide is involved in the spinal processing of mechanosensory input from the normal joint. Furthermore, this peptide and its spinal receptors significantly contribute to the generation and expression of inflammation-evoked hyperexcitability of spinal cord neurons during the development of inflammation. Finally, calcitonin gene-related peptide is involved in the maintenance of inflammation-evoked hyperexcitability. By these effects calcitonin gene-related peptide receptors may significantly contribute to the neuronal basis of hyperalgesia and allodynia associated with inflammation.

The role of spinal neurokinin-2 receptors in the processing of nociceptive information from the joint and in the generation and maintenance of inflammation-evoked hyperexcitability of dorsal horn neurons in the rat

In spinal cord neurons in anesthetized rats, the role on neurokinin A and neurokinin-2 receptors in the processing of nociceptive information from the knee joint was studied. The specific non-peptide antagonist at the neurokinin-2 receptor, SR48968, its inactive R-enantiomer, SR48965, neurokinin A, substance P and (RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), were administered ionophoretically close to neurons with input from the knee joint. SR48968 reduced the effects of exogenous neurokinin A, but not those of exogenous substance P and AMPA, indicating selective blockade of neurokinin-2 receptors. In most neurons with input from the normal knee joint, SR48968 reduced dose-dependently the responses to noxious pressure with applied to the knee, and in approximately 50% of the neurons the responses to innocuous pressure. The administration of SR48968 during the induction of an experimental joint inflammation markedly attenuated the development of inflammation-evoked hyperexcitability. In hyperexcitable neurons with input from the inflamed joint, SR48968 reduced the responses to noxious and innocuous pressure. The relative reduction of the responses was more pronounced than in neurons with input from the normal joint. None of the effects of SR48968 was mimicked by SR48965. These data show that neurokinin-2 receptors are involved in the spinal processing of nociceptive information from the normal joint. Furthermore, neurokinin-2 receptors must be coactivated at an early stage of inflammation, to allow the generation of hyperexcitability. Finally, neurokinin-2 receptors are involved in maintenance of hyperexcitability during inflammation. In summary, spinal neurokinin-2 receptors are important in the generation of pain in the normal and inflamed joint.

Role of peri-axonal inflammation in the development of thermal hyperalgesia and guarding behavior in a rat model of neuropathic pain

Loose ligation of a rat's sciatic nerve produces hyperalgesia to thermal stimuli and elicits guarding behavior directed at the afflicted paw. The present experiments test whether localized inflammation induced by the suture material used to ligate the nerve is critical to the development of hyperalgesia. Daily injections of dexamethasone reduced the inflammatory response induced by the sutures and blocked the development of guarding behavior and thermal hyperalgesia. In a second experiment inflammation associated with cotton sutures was enhanced by soaking the sutures in Freund's adjuvant prior to ligation. This caused an augmentation of thermal hyperalgesia and guarding behavior. These results suggest that inflammation around the nerve is critical for the development of guarding behavior and thermal hyperalgesia in this model of neuropathic pain.

Differential effects of N-methyl-D-aspartate (NMDA) and non-NMDA receptor antagonists on spinal release of amino acids after development of acute arthritis in rats

Following induction of acute knee joint arthritis in rats, an increase in the release of amino acids in the spinal dorsal horn occurs in two phases: (1) at the time of injection for all amino acids tested; and (2) a late prolonged phase for aspartate (Asp) and glutamate (Glu) (3.5-8 h). In the present study, the increased late phase release of Glu was reversed by posttreatment of the spinal cord with the N-methyl-D-aspartate (NMDA) receptor antagonist, AP7, but not with the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Asp late phase release in arthritic animals was unaffected by posttreatment of the spinal cord with either AP7 or CNQX. Arthritic animals became hyperalgesic to radiant heat stimuli by 4 h and this hyperalgesia was reversed by both CNQX and AP7. During the paw withdrawal latency (PWL) test for heat hyperalgesia, there was an increase in the glycine (Gly) and serine (Ser) concentrations in the dorsal horn. This increase in Gly and Ser was blocked by both CNQX and AP7. Indications of inflammation in arthritic animals posttreated with AP7, including increased joint circumference and temperature, were similar to animals that did not receive antagonists. Arthritic animals posttreated with CNQX, however, showed a reduction in the degree of joint swelling. Thus, both non-NMDA and NMDA receptors appear to play a role in the processing of the information evoked by stimuli in the periphery. The arthritis-induced release of Gly and Ser during the PWL test for heat hyperalgesia appears to be dependent on activation of both non-NMDA and NMDA receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction in joint swelling and hyperalgesia following post-treatment with a non-NMDA glutamate receptor antagonist

The experimental arthritis of the knee joint used in the present study leads to joint swelling, increased joint temperature, limping, guarding, and a decrease in paw withdrawal latency (PWL) to radiant heat (hyperalgesia) within hours in rats. Unexpectedly, administration of the non-NMDA receptor antagonist, CNQX, in the spinal cord 4 h after initiation of the arthritis significantly reduced the degree of joint inflammation and returned PWL times to baseline. Therefore, the present results indicate that established joint swelling and hyperalgesia can be reduced significantly by CNQX.

Conditioned place preference paradigm: a novel approach for analgesic drug assessment against chronic pain

In response to concerns over the clinical relevance of analgesic testing paradigms which involve acute nociceptive stimuli, the present research examined the utility of the conditioned place preference (CPP) paradigm as a novel approach for determination of analgesic drug efficacy against chronic nociception. Rats display preferences for environments that have been previously paired with positively reinforcing drugs; whether place preference to the negatively reinforcing effects of analgesic drugs in an animal model of chronic pain occurs is yet unknown. The present research sought to determine whether animals experiencing chronic pain would display a place preference for an environment paired with analgesic drug treatment. Persistent inflammatory nociception was induced by unilateral injections of complete Freund's adjuvant (0.1 ml) into the rat hind paw. Place preference to the opiate agonist morphine, the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 and the non-steroidal anti-inflammatory drug (NSAID) indomethacin was examined in 3 separate experiments. Rats received 8 counter-balanced conditioning trials (4 drug, 4 no-drug) of 60 min each with various drug doses (morphine: 3.0 and 10.0 mg/kg; indomethacin: 2.5 and 5.0 mg/kg; MK-801: 0.03, 0.1 and 0.3 mg/kg, i.p.) or vehicle serving as the reinforcing stimuli in a 3 compartment (2 stimuli, 1 neutral) place preference apparatus. In general, morphine place preference was observed in both inflamed and non-inflamed groups; inflamed groups exhibited enhanced morphine place preference than non-inflamed groups. MK-801 produced a low-dose place preference in inflamed animals; higher doses of MK-801 produced a place aversion in both inflamed and non-inflamed groups. Indomethacin failed to produced place preference in either inflamed or non-inflamed groups. These data demonstrate that the negatively reinforcing properties of analgesic drugs can be assessed via the CPP paradigm. In addition, this paradigm offers greater clinical relevance as animals determine drug efficacy without the involvement of high-intensity, phasic nociceptive stimulation.

Intrathecal excitatory amino acid (EAA) agonists increase tail flick latencies (TFLs) of spinal rats

The facilitation of spinal nociceptive reflexes that occurs after spinal transection reveals the existence of descending, supraspinally mediated inhibition. Substantial evidence indicates that the excitatory amino acids (EAAs) are involved in these spinal circuits. Therefore, it was hypothesized that reflex facilitation in the spinal animal might be due to the removal of inhibitory input normally exerted on the spinal action of EAAs. If so, the facilitatory decrease in reflex latency, observed in the spinal preparation, might be potentiated by intrathecal (IT) administration of EAA agonists. This was tested by comparing the effect of IT injections of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) on the thermally elicited tail flick (TF) response of Intact and acute spinal rats. In intact rats, a low (0.25 nM) dose of NMDA produced a hyperalgesic decrease in latency, relative to saline, whereas higher doses produced an overall increase in latency. A large dose (0.5 microM) produced overt signs of toxicity (crippling, self-mutilation, and loss of the reflex). Only the highest (1.0 nM) dose of AMPA affected the response, resulting in a significant increase. After spinal transection, the hyperalgesic reaction to 0.25 nM of NMDA was absent, and latencies were significantly increased by 1.0 nM. The toxic reaction to 0.5 microM appeared to be potentiated. Tail flick responses to AMPA were also significantly increased in spinal rats. Contrary to the prediction, reflex latencies were significantly increased by these drugs after spinal transection. It was suggested that, although the spinal action of EAAs appears to be supraspinally modulated, this influence may be facilitatory rather than inhibitory.

Regulation of the second-messenger systems in the rat spinal cord during prolonged peripheral inflammation

Unilateral intraplantar injection of Freund's complete adjuvant (FCA) into 1 hind paw of rats was used as a model of peripheral inflammation and persistent pain in order to examine time course effects of a continuous barrage of nociceptive input on the second-messenger transducing systems in the spinal cord. cAMP, cGMP and inositol 1,4,5-trisphosphate (insP3) were extracted from the lumbosacral cord at days 1, 7, 14, 21 and 42 following FCA injection and quantified by either radioreceptor-assay (RRA) or radioimmunoassay (RIA). The lumbosacral contents of cAMP and cGMP when quantified in whole lumbosacral cord segment were not significantly changed by FCA treatment at all time points. InsP3 accumulation was significantly increased on days 14, 21 and 42 following FCA injection relative to sham-treated time-matched controls. However, cGMP and insP3 contents were significantly increased in the left longitudinal half of the lumbar enlargement ipsilateral to the injected paw on day 21 following FCA treatment, but not in the sham-treated time-matched controls. With [3H]insP3 as a ligand, Scatchard (Rosenthal) analyses of the concentration-dependent saturation curves showed that the densities (Bmax) of insP3 receptors (insP3R) were significantly increased throughout the time course of adjuvant-induced peripheral inflammation. The binding affinities (KD) for insP3R were significantly decreased on days 7, 14 and 21 following FCA injection corresponding to the times of most stable and peak inflammation. InsP3R from the cerebelli of the same rats as used in the lumbosacral insP3R characterization was used as a positive control in this study and did not show any change in both Bmax and KD as a result of FCA treatment, thus demonstrating that the changes in lumbosacral insP3R characteristics might be specific to the nociceptive sensory pathway such as the spinal cord. Thus it appears that sustained afferent nociceptive input induced by FCA injection increased the accumulation of cGMP, insP3 and insP3R density in the spinal cord through increased neuronal activities of functional receptors coupled to major classes of chemical mediators of nociception including neuropeptides and excitatory aminoacids. Changes in insP3 accumulation in the lumbosacral cord following FCA injection were significantly correlated with changes in insP3R density. Changes in the ratios of lumbosacral insP3 contents and insP3R density were also significantly correlated with changes in body weight and hind paw size induced by FCA injection.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence of a role for N-methyl-D-aspartate (NMDA) receptors in the facilitation of tail withdrawal after spinal transection

Peripheral injury produces a characteristic excitation of spinal cord dorsal horn cells (wind-up) which is associated with a facilitation of spinal nociceptive reflexes (hyperalgesia). These phenomena are believed to be mediated by a trauma-induced increase in the release of excitatory amino acids (EAAs). A similar increase in the activity of dorsal horn neurons and spinal reflexes occurs after spinal transection. Therefore, the present studies examined the possibility that EAAs, acting through the NMDA receptor, might also be involved in behavioral hyperalgesia produced by central injury. The first experiment assessed the effect of pretreatment with the NMDA antagonist, ketamine, on the facilitated tail flick (TF) response of spinally transected rats. Separate groups of animals were spinalized under isoflurane anesthesia alone, intramuscular ketamine anesthesia alone, or a combination of isoflurane and intrathecal ketamine. The TF was examined 24 h later, before and 30 min after an intrathecal injection of morphine. In the second experiment, the effect of intraperitoneal or intrathecal ketamine on the TF was assessed to separate groups of rats that underwent spinal transection or sham surgery under isoflurane anesthesia. Pretreatment with either systemic or intrathecal ketamine did not alter TF facilitation or morphine-induced antinociception in spinal rats. However, both systemic and intrathecal ketamine significantly increased TF latencies in spinal, relative to intact rats. These results indicate that ketamine did not prevent the development of spinal reflex facilitation, but it selectively reduced this reaction once it was established in spinal rats. The data support an involvement of EAAs in reflex facilitation produced by spinal transection.

Intrathecal co-administration of morphine and excitatory amino acid agonists produce differential effects on the tail-flick of intact and spinal rats

Previous reports, that intrathecal morphine is more potent on the tail-flick test of acute spinal rats than intact rats, suggested that spinal opiate analgesia was attenuated by neurotransmitter release from descending pathways. To determine if this phenomenon involved excitatory amino acids (EAAs), 0.25 nm of N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) were i.t. co-administered with morphine to Intact and Spinal rats. NMDA potentiated morphine antinociception in Intact but not Spinal rats; AMPA had no effect in Intact rats, but significantly reduced morphine antinociception in Spinal rats. The data suggest a reciprocal descending, modulatory influence on the spinal interaction between EAAs and morphine.

Pre treatment with MK-801, a non-competitive NMDA antagonist, prevents development of mechanical hyperalgesia in a rat model of chronic neuropathy, but not in a model of chronic inflammation

In the rat, loose ligation of the sciatic nerve results in behavioural signs of hyperalgesia reminiscent of neuropathy in man. A further rat model, of chronic inflammatory hyperalgesia, is produced by intraplantar administration of Freund's complete adjuvant (FCA). We report here that preemptive administration of a non-competitive antagonist of the glutamate N-methyl-D-aspartate (NMDA) receptor, MK-801 (0.3 mg kg-1, s.c.) 30 min prior to and twice daily for a further 8 days following loose ligation of the sciatic nerve, blocks the development of mechanical hyperalgesia measured 27 days later. In contrast, MK-801 administration using the same dosing regimen did not significantly inhibit the hyperalgesia apparent 15 days following i.pl. administration of FCA. Our results suggest that the mechanisms responsible for the development of mechanical hyperalgesia associated with chronic nerve injury and chronic inflammation differ.

Behavioral and immunohistochemical changes in an experimental arthritis model in rats

An experimental arthritis induced by injection of kaolin and carrageenan into the knee joint resulted in a temporal relationship between glutamate dorsal horn content and paw withdrawal latency (PWL) which was positively correlated. Limping, guarding, increased response to heat stimuli (hyperalgesia) and altered staining patterns for glutamate (GLU), substance P (SP), and calcitonin gene-related peptide (CGRP) were monitored in the awake behaving arthritic rat over a 1 week time course. A decrease in PWL occurred on the side ipsilateral to the inflamed knee as early as 4 h after the induction of arthritis indicating the animals are hyperalgesic. The PWL remained decreased through the first 24 h. Computer-assisted quantification of the density of immunohistochemical staining indicated the content of GLU, SP and CGRP was altered differentially throughout the time course of the arthritis. The changes observed for all three substances occurred across the entire superficial dorsal horn. There was an initial depletion of SP followed by an increase in both SP and CGRP content which was maintained through 1 week. The GLU content was increased during the hyperalgesic period. The GLU changes followed the same time course and were positively correlated with the changes in PWL. In a small group of animals injected with kaolin and carrageenan, hyperalgesia did not develop. In this group of animals, no change in dorsal horn GLU or SP content occurred. Rather, there was an increase in CGRP content in the middle portion of the superficial dorsal horn which is the termination site of knee joint afferents. These data indicate that the development of heat hyperalgesia is dependent on GLU and possibly SP. Since inflammation of the knee joint does not involve the foot pad, the heat hyperalgesia observed during the first 24 h following induction of arthritis represents a central neuronal sensitization.

Spinal cord amino acid release and content in an arthritis model: the effects of pretreatment with non-NMDA, NMDA, and NK1 receptor antagonists

An experimental arthritis, induced by injection of the knee joint with kaolin and carrageenan, results in guarding of and decreased weight bearing on the limb. At the time of injection, a transient increased release of all amino acids examined is measurable in samples collected by microdialysis. A second and prolonged increase of aspartate (ASP), glutamate (GLU), and glutamine (GLN) concentrations follows after 3 h. The increased release at time of injection is blocked by microdialysis application of a non-N-methyl-D-aspartate (non-NMDA) or an NMDA receptor antagonist, and the release of ASP, GLU, and GLN in the late phase is blocked by pretreatment with a non-NMDA (CNQX), an NMDA (AP7) or a neurokinin 1 (NK1; CP-96,345) antagonist. Dorsal horn immunoreactive staining of GLU, substance P (SP), and calcitonin gene-related peptide (CGRP) is reflective of the events occurring in the late phase of amino acid release since GLU release is positively correlated with GLU staining density. Increased immunoreactivity for GLU, SP, and CGRP at 8 hr in the arthritic animals is differentially altered by pretreatment of the spinal cord dorsal horn with non-NMDA, NMDA, or NK1 receptor antagonists. The differential staining pattern for GLU, SP, and CGRP, the differential release of ASP and GLU, and the differential activation of the EAA and NK1 receptors implies that ASP, GLU, SP, and CGRP are each involved in the processing of sensory information and that their roles in the central sensitization occurring with the inflammatory process, are unique.

Centrally administered non-NMDA but not NMDA receptor antagonists block peripheral knee joint inflammation

An experimental arthritis of the knee joint resulted in limping, guarding, and an increased response to heat stimuli (heat hyperalgesia). Spinal administration of the non-N-methyl-D-aspartate (non-NMDA) antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), significantly reduced the degree of peripheral inflammation, thermal and behavioral manifestations of arthritis. NMDA antagonists had no effect on the inflammation but did prevent the development of the heat hyperalgesia. Thus, central non-NMDA receptors play a major role in the development of peripheral inflammation while both non-NMDA and NMDA receptors are involved in the development of heat hyperalgesia.

An experimental arthritis model in rats: the effects of NMDA and non-NMDA antagonists on aspartate and glutamate release in the dorsal horn

Release of excitatory amino acids (EAA's) in the dorsal horn of awake rats was monitored by microdialysis during the development of arthritis induced by injection of 3% kaolin and 3% carrageenan into the knee joint. Concentrations of EAA's in the dialysate samples were measured by high performance liquid chromatography at baseline, during delivery of EAA antagonists, and for the first 8 h of arthritis. An initial increase in aspartate (ASP) and glutamate (GLU) was observed on injection of the knee joint in rats made arthritic. Subsequently, there was a prolonged release phase after 3 h which persisted at least 8 h. Specific EAA antagonists to non-N-methyl-D-aspartate (non-NMDA; CNQX) and to NMDA (AP7) receptors were used to block the effects seen in the untreated arthritic animals. The increase in ASP and GLU release seen at the time of injection in untreated arthritic animals did not occur in arthritic animals treated with EAA receptor antagonists (CNQX or AP7). In arthritic animals treated with CNQX, the prolonged release phase was delayed and attenuated for GLU and decreased below baseline for ASP. In the AP7-treated arthritic animals, no change from baseline concentration was observed for ASP until 7 h, and GLU decreased minimally. The data indicate that this arthritis model is accompanied by an initial increased release of EAA's at the time of injection which is dependent on the activation of both non-NMDA and NMDA receptors. Subsequent development of arthritis, manifested as an inflamed joint and a delayed and prolonged release of ASP and GLU, is dependent on the initial activation of these EAA receptors.(ABSTRACT TRUNCATED AT 250 WORDS)