N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptor antagonists differentially suppress dorsal horn neuron responses to mechanical stimuli in rats with peripheral nerve injury

The dietary constituent resveratrol suppresses nociceptive neurotransmission via the NMDA receptor

Background

Although we have previously reported that intravenous resveratrol administration inhibits the nociceptive neuronal activity of spinal trigeminal nucleus caudalis neurons, the site of the central effect remains unclear. The aim of the present study was to examine whether acute intravenous resveratrol administration in the rat attenuates central glutamatergic transmission of spinal trigeminal nucleus caudalis neurons responding to nociceptive mechanical stimulation in vivo, using extracellular single-unit recordings and microiontophoretic techniques.

Results

Extracellular single-unit recordings using multibarrel electrodes were made from the spinal trigeminal nucleus caudalis wide dynamic range neurons responding to orofacial mechanical stimulation in pentobarbital anesthetized rats. These neurons also responded to iontophoretic application of glutamate, and the evoked neuronal discharge frequency was significantly increased in a current-dependent and reversible manner. The mean firing frequency evoked by the iontophoretic application of glutamate (30, 50, and 70 nA) was mimicked by the application of 10 g, 60 g, and noxious pinch mechanical stimulation, respectively. The mean firing frequency of spinal trigeminal nucleus caudalis wide dynamic range neurons responding to iontophoretic application of glutamate and N-methyl-D-aspartate were also significantly inhibited by intravenous administration of resveratrol (2 mg/kg) and the maximal inhibition of discharge frequency was observed within 10 min. These inhibitory effects lasted approximately 20 min. The relative magnitude of inhibition by resveratrol of the glutamate-evoked spinal trigeminal nucleus caudalis wide dynamic range neuronal discharge frequency was similar to that for N-methyl-D-aspartate iontophoretic application.

Conclusion

These results suggest that resveratrol suppresses glutamatergic neurotransmission of the spinal trigeminal nucleus caudalis neurons responding to nociceptive mechanical stimulation via the N-methyl-D-aspartate receptor in vivo, and resveratrol may be useful as a complementary or alternative therapeutic agent for the treatment of trigeminal nociceptive pain.

Paeoniflorin inhibits excitatory amino acid agonist-and high-dose morphine-induced nociceptive behavior in mice via modulation of N-methyl-D-aspartate receptors

BACKGROUND

Pain, the most common reasons for physician consultation, is a major symptom in many medical conditions that can significantly interfere with a person's life quality and general functioning. Almost all painkillers have its untoward effects. Therefore, seeking for a safe medication for pain relieve is notable nowadays. Paeonia lactiflora is a well-known traditional Chinese medicine. Paeoniflorin is an active component found in Paeonia lactiflora, which has been reported to inhibit formalin-induced nociceptive behavior in mice. Aims of this present study were to investigate effects of paeoniflorin on excitatory amino acid agonist- or high-dose morphine-induced nociceptive behaviors in mice.

RESULTS

Paeoniflorin (100, 200, 500 nmol, i.c.v.) alone and combined with glutamatergic antagonists (MK-801 14.8 pmol, or NBQX 5 nmol, i.t.) inhibited nociception. Those agents also inhibited the clonic seizure-like excitation induced by high-dose morphine (250 nmol, i.t) in mice. Antisense oligodeoxynucleotides of NMDA receptor subunits NR1, NR2A, NR2B significantly enhanced the inhibition of paeoniflorin on excitatory amino acid-and high-dose morphine-induced nociception. Docking energy data revealed that paeoniflorin had stronger binding activity in NR2A and NR2B than NR2C of NMDA receptors.

CONCLUSIONS

Results of this study indicate that paeoniflorin-induced inhibition of excitatory amino acid agonist- and high-dose morphine-induced nociceptive behaviors might be due to modulation of NMDA receptors, specifically the NR2B subunit.

Casein kinase II inhibition reverses pain hypersensitivity and potentiated spinal N-methyl-D-aspartate receptor activity caused by calcineurin inhibitor

Clinically used calcineurin inhibitors, including tacrolimus (FK506) and cyclosporine A, can induce calcineurin inhibitor-induced pain syndrome (CIPS), which is characterized as severe pain and pain hypersensitivity. Increased synaptic N-methyl-D-aspartate receptor (NMDAR) activity in the spinal dorsal horn plays a critical role in the development of CIPS. Casein kinase II (CK2), a serine/threonine protein kinase, can regulate synaptic NMDAR activity in the brain. In this study, we determined whether spinal CK2 is involved in increased NMDAR activity and pain hypersensitivity caused by systemic administration of FK506 in rats. FK506 treatment caused a large increase in the amplitude of NMDAR-mediated excitatory postsynaptic currents (EPSCs) evoked by primary afferent stimulation and in the frequency of miniature EPSCs of spinal dorsal horn neurons. CK2 inhibition with either 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole (DRB) or 4,5,6,7-tetrabromobenzotriazole (TBB) completely normalized the amplitude of evoked NMDAR-EPSCs of dorsal horn neurons in FK506-treated rats. In addition, DRB or TBB significantly attenuated the amplitude of NMDAR currents elicited by puff application of N-methyl-D-aspartate to dorsal horn neurons in FK506-treated rats. Furthermore, treatment with DRB or TBB significantly reduced the frequency of miniature EPSCs of spinal dorsal horn neurons increased by FK506 treatment. In addition, intrathecal injection of DRB or TBB dose-dependently reversed tactile allodynia and mechanical hyperalgesia in FK506-treated rats. Collectively, our findings indicate that CK2 inhibition abrogates pain hypersensitivity and increased pre- and postsynaptic NMDAR activity in the spinal cord caused by calcineurin inhibitors. CK2 inhibitors may represent a new therapeutic option for the treatment of CIPS.

Dorsal striatum metabotropic glutamate receptor 8 affects nocifensive responses and rostral ventromedial medulla cell activity in neuropathic pain conditions

The present study investigated the role of metabotropic glutamate receptor subtype 8 (mGluR8) in the dorsal striatum (DS) in modulating thermonociception and rostral ventromedial medulla (RVM) ON and OFF cell activities in conditions of neuropathic pain induced by spared nerve injury (SNI) of the sciatic nerve in rats. The role of DS mGluR8 on mechanical allodynia was also investigated. Intra-DS (S)-3,4-dicarboxyphenylglycine [(S)-3,4-DCPG], a selective mGluR8 agonist, did not modify the activity of the ON and OFF cells in sham-operated rats. In SNI rats, which showed a reduction of the mechanical withdrawal threshold, intra-DS microinjection of (S)-3,4-DCPG inhibited the ongoing and tail flick-evoked activity of the ON cells while increasing the activity of the OFF cells. AZ12216052, a selective mGluR8 positive allosteric modulator (PAM), behaved like (S)-3,4-DCPG in increasing tail flick latency and OFF cell activity and decreasing ON cell activity in SNI rats only but was less potent. VU0155041, a selective mGluR4 PAM, was ineffective in changing thermal nociception and ON and OFF cell activity in both sham-operated and SNI rats. (S)-3,4-DCPG did not change mechanical withdrawal threshold in sham-operated rats but increased it in SNI rats. Furthermore, a decreased level of mGluR8 gene and immunoreactivity, expressed on GABAergic terminals, associated with a protein increase was found in the DS of SNI rats. These results suggest that stimulation of mGluR8 inhibits thermoceptive responses and mechanical allodynia. These effects were associated with inhibition of ON cells and stimulation of OFF cells within RVM.

Calcineurin inhibitor induces pain hypersensitivity by potentiating pre- and postsynaptic NMDA receptor activity in spinal cords

Calcineurin inhibitors, such as cyclosporin A and tacrolimus (FK506), have played a pivotal role in the preservation of allograft function. However, these drugs can cause unexplained severe pain in patients, often referred to as calcineurin inhibitor-induced pain syndrome (CIPS). Although calcineurin can regulate NMDA receptor (NMDAR) activity, the causal relationship between spinal synaptic plasticity and CIPS remains unknown. In this study, we showed that systemic administration of FK506 (1.5 mg kg(-1) day(-1)) for 7 days in rats led to long-lasting nociceptive and mechanical hypersensitivity. Whole-cell patch-clamp recordings in spinal cord slices revealed that FK506 treatment caused a large increase in the amplitude of NMDAR-mediated excitatory postsynaptic currents (EPSCs) of dorsal horn neurons evoked by dorsal root stimulation. The amplitude of NMDAR currents elicited by puff NMDA application to dorsal horn neurons was also significantly greater in FK506-treated than in vehicle-treated rats. The frequency of spontaneous and miniature EPSCs in most dorsal horn neurons was profoundly increased in FK506-treated rats and was reduced by blocking NMDARs. Furthermore, blocking GluN2A or GluN2B subunits similarly reduced the amplitude of evoked EPSCs and the frequency of miniature EPSCs in dorsal horn neurons of FK506-treated rats. In addition, intrathecal injection of an NMDAR antagonist or systemic administration of memantine effectively reversed nociceptive and mechanical hypersensitivity in FK506-treated rats. Our findings indicate that calcineurin inhibition increases glutamate-mediated nociceptive input by potentiating presynaptic and postsynaptic NMDAR activity in spinal cords. NMDAR antagonists may represent a new therapeutic option for the treatment of CIPS.

Sprouting of A-fibre primary afferents into lamina II in two rat models of neuropathic pain

Following peripheral nerve section, injured sensory A-fibres into lamina II of the dorsal horn and form aberrant functional synapses. Such structural changes may underlie some of the sensory abnormalities observed in nerve-injured patients, including neuropathic pain. This study compared the ability of intact and injured A-fibres to sprout in two experimental models of neuropathic pain, where the onset and presence of abnormal behaviours indicative of neuropathic pain have been well described. Rats received either a unilateral chronic constriction injury of the sciatic nerve (CCI) or lesion of the L5 spinal nerve (SNL). The central distribution of the injured and uninjured afferents labelled with choleragenoid conjugated to horseradish peroxidase (B-HRP) was examined at different postoperative survival times. In both models, the contralateral uninjured side, used for control nerve or ganglion injections, showed labelling of the L3-6 spinal segments in laminae I, III-V, leaving lamina II unlabelled. In CCI rats, injured sciatic afferents sprouted in lamina II of the L4-5 dorsal horn by 10 days postinjury. In SNL rats, injured L5 afferents sprouted into lamina II of the L4-5 dorsal horn by 24 h postinjury and were robust from 3 to 10 days. In both models, the labelling in lamina II was absent by 4 months. Labelling of the adjacent uninjured saphenous or intact L4 spinal nerve afferents did not reveal A-fibre sprouting. As the time-course of sprouting of injured A-fibres parallels the previously described behaviour interpreted as neuropathic pain in these models, this may be a phenomenon that contributes to sensory abnormalities such as ongoing pain and mechanical hypersensitivity.

Synaptic Plasticity and Pain: Role of Ionotropic Glutamate Receptors

Pain hypersensitivity that develops after tissue or nerve injury is dependent both on peripheral processes in the affected tissue and on enhanced neuronal responses in the central nervous system, including the dorsal horn of the spinal cord. It has become increasingly clear that strengthening of glutamatergic sensory synapses, such as those established in the dorsal horn by nociceptive thin-caliber primary afferent fibers, is a major contributor to sensitization of neuronal responses that leads to pain hypersensitivity. Here, the authors review recent findings on the roles of ionotropic glutamate receptors in synaptic plasticity in the dorsal horn in relation to acute and persistent pain.

Ionotropic glutamate receptors in spinal nociceptive processing

Glutamate is the predominant excitatory transmitter used by primary afferent synapses and intrinsic neurons in the spinal cord dorsal horn. Accordingly, ionotropic glutamate receptors mediate basal spinal transmission of sensory, including nociceptive, information that is relayed to supraspinal centers. However, it has become gradually more evident that these receptors are also crucially involved in short- and long-term plasticity of spinal nociceptive transmission, and that such plasticity have an important role in the pain hypersensitivity that may result from tissue or nerve injury. This review will cover recent findings on pre- and postsynaptic regulation of synaptic function by ionotropic glutamate receptors in the dorsal horn and how such mechanisms contribute to acute and chronic pain.

Modality-specific hyperexcitability of dorsal horn neurons to mechanical stimuli in herpetic mice

Percutaneous inoculation of mice with herpes simplex virus type-1 produces marked dynamic allodynia in the zosteriform dermatome. In this study, we examined the electrophysiological excitability of the wide-dynamic range neuron in the spinal dorsal horn and the tibial nerve in response to mechanical (brush, punctum, and pinch) stimuli in mice with herpetic allodynia. The excitatory response of wide-dynamic range neurons to brush, but neither punctum nor pinch, stimulation of the zosteriform dermatome was increased in herpetic mice. The responses of the tibial nerve to all kinds of mechanical stimuli examined were decreased. These results suggest that dynamic allodynia in the affected dermatome is because of the increased excitability of wide-dynamic range neurons, but not primary afferents, to brush stimulation.

Plasticity and emerging role of BKCa channels in nociceptive control in neuropathic pain

Large-conductance Ca(2+)-activated K(+) (BK(Ca), MaxiK) channels are important for the regulation of neuronal excitability. Peripheral nerve injury causes plasticity of primary afferent neurons and spinal dorsal horn neurons, leading to central sensitization and neuropathic pain. However, little is known about changes in the BK(Ca) channels in the dorsal root ganglion (DRG) and spinal dorsal horn and their role in the control of nociception in neuropathic pain. Here we show that L5 and L6 spinal nerve ligation in rats resulted in a substantial reduction in both the mRNA and protein levels of BK(Ca) channels in the DRG but not in the spinal cord. Nerve injury primarily reduced the BK(Ca) channel immunoreactivity in small- and medium-sized DRG neurons. Furthermore, although the BK(Ca) channel immunoreactivity was decreased in the lateral dorsal horn, there was an increase in the BK(Ca) channel immunoreactivity present on dorsal horn neurons near the dorsal root entry zone. Blocking the BK(Ca) channel with iberiotoxin at the spinal level significantly reduced the mechanical nociceptive withdrawal threshold in control and nerve-injured rats. Intrathecal injection of the BK(Ca) channel opener [1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl)phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one] dose dependently reversed allodynia and hyperalgesia in nerve-ligated rats but it had no significant effect on nociception in control rats. Our study provides novel information that nerve injury suppresses BK(Ca) channel expression in the DRG and induces a redistribution of BK(Ca) channels in the spinal dorsal horn. BK(Ca) channels are increasingly involved in the control of sensory input in neuropathic pain and may represent a new target for neuropathic pain treatment.

The interaction of gabapentin and N6-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA) on mechanical allodynia in rats with a spinal nerve ligation

We examined the antiallodynic interaction between gabapentin and adenosine A1 receptor agonist, N(6)-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA), in a rat model of nerve ligation injury. Rats were prepared with ligation of left L5-6 spinal nerves and intrathecal catheter implantation for drug administration. Mechanical allodynia was measured by applying von Frey filaments. Gabapentin and R-PIA were administered to obtain the dose-response curve and the 50% effective dose (ED(50)). Fractions of ED(50)s were administered concurrently to establish the ED(50) of the drug combination. The drug interaction between gabapentin and R-PIA was analyzed using the isobolographic method. Adenosine A1 receptor antagonist was administered intrathecally to examine the reversal of the antiallodynic effect. Locomotor function changes were evaluated by rotarod testing. Intrathecal gabapentin and R-PIA and their combination produced a dose-dependent antagonism against mechanical allodynia without severe side effects. Intrathecal gabapentin synergistically enhanced the antiallodynic effect of R-PIA when coadministered. There were no significant changes in rotarod performance time, except gabapentin 300 microg. In the combination group, the maximal antiallodynic effect was reversed by A1 adenosine receptor antagonist. These results suggest that activation of adenosine A1 receptors at the spinal level is required for the synergistic interaction on the mechanical allodynia.

Pain mechanisms and their implication for the management of pain in farm and companion animals

Over the last two decades there has been a dramatic increase in the literature relating to the mechanisms and management of pain in domestic animals. Understanding the mechanisms of pain is crucial for its effective management. This review highlights the current understanding of the neurophysiology of nociception and the plastic changes involved in chronic pain states. Additionally, we describe a range of novel molecules and pathways that offer opportunities for the development of mechanism-based analgesic therapies. Pain management in animals is limited by pain assessment which remains highly subjective, with clinicians relying on indirect measures of pain, using rating scales and (less frequently) quantifiable physiological and behavioural parameters. The need for a systematic approach which would assess different pain components is well justified. Species-specific issues on pain assessment and management in mammalian companion and farm animals are addressed in the later part of this review.

Increased nociceptive input rapidly modulates spinal GABAergic transmission through endogenously released glutamate

Stimulation of nociceptive primary afferents elicits pain by promoting glutamatergic transmission in the spinal cord. Little is known about how increased nociceptive input controls GABAergic tone in the spinal dorsal horn. In this study, we determined how increased nociceptive inflow affects GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs) of lamina II neurons by using whole cell recordings in rat spinal cord slices. Bath application of capsaicin for 3 min induced a long-lasting inhibition of sIPSCs in 50% of the neurons tested. In the other half of the neurons, capsaicin either increased the frequency of sIPSCs (34.6%) or had no effect on sIPSCs (15.4%). The GABA(A) current elicited by puff application of GABA was not altered by capsaicin. Capsaicin did not inhibit sIPSCs in rats treated with intrathecal pertussis toxin. Also, capsaicin failed to inhibit sIPSCs in the presence of ionotropic glutamate receptor antagonists or in the presence of both LY341495 and CPPG (group II and group III metabotropic glutamate receptor antagonists, respectively). However, when LY341495 or CPPG was used alone, capsaicin still decreased the frequency of sIPSCs in some neurons. Additionally, bradykinin significantly inhibited sIPSCs in a population of lamina II neurons and this inhibitory effect was also abolished by LY341495 and CPPG. Our study provides novel information that stimulation of nociceptive primary afferents rapidly suppresses GABAergic input to many dorsal horn neurons through endogenous glutamate and activation of presynaptic group II and group III metabotropic glutamate receptors. These findings extend our understanding of the microcircuitry of the spinal dorsal horn involved in nociception.

Intrathecal midazolam regulates spinal AMPA receptor expression and function after nerve injury in rats

Spinal gamma-aminobutyric acid (GABA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors have been implicated in the mechanisms of neuropathic pain after nerve injury; however, how these two receptors interact at the spinal level remains unclear. Here we show that intrathecal midazolam through activation of spinal GABAA receptors attenuated the expression and function of spinal AMPA receptors in rats following peripheral nerve injury. Thermal hyperalgesia and mechanical allodynia induced by chronic constriction nerve injury (CCI) in rats were attenuated by the short-acting benzodiazepine midazolam (20=10>5 mug>vehicle) administered intrathecally once daily for 7 postoperative days. CCI-induced upregulation of AMPA receptors within the spinal cord dorsal horn was also significantly reduced by the intrathecal midazolam (10, 20 mug) treatment. The inhibitory effects of midazolam (10, 20 mug) on neuropathic pain behaviors and AMPA receptor expression were prevented by co-administration of midazolam with the GABAA receptor antagonist bicuculline (3 mug), whereas intrathecal treatment with bicuculline (1 or 3 mug) alone in naive rats induced the upregulation of spinal AMPA receptor expression and nociceptive responses, indicating a tonic regulatory effect from endogenous GABAergic activity on the AMPA receptor expression and spinal nociceptive processing. These results indicate that modulation of spinal AMPA receptor expression and function by the GABAergic activity may serve as a mechanism contributory to the spinal nociceptive processing.

NMDA receptor antagonist treatment at the time of nerve injury prevents injury-induced changes in spinal NR1 and NR2B subunit expression and increases the sensitivity of residual pain behaviours to subsequently administered NMDA receptor antagonists

Spinal NMDA receptors (NMDA R) are important in neuropathic sensitisation and acute administration of antagonists can provide temporary attenuation of sensitisation. If establishment of the chronic pain state could be prevented by brief administration of such agents at or around the time of nerve injury (pre-emptive analgesia) it might be possible to avoid many of the unacceptable side effects associated with repeated administration of these or other antagonists. Several reports describe aspects of effective pre-emptive analgesia from NMDA R antagonists in animal models of neuropathic pain. The first aim of the present study was to make a direct comparison of changes in mechanical allodynia, cold allodynia and thermal hyperalgesia following nerve injury, demonstrating their increasing degree of susceptibility to pre-emptive NMDA R antagonist treatment. Secondly, we used immunoblotting and immunohistochemistry to investigate the effects of nerve injury on NMDA receptor subunit expression, revealing increased expression of NR2B, but not NR2A and reduced NR1 in the superficial dorsal horn. These changes were attenuated following NMDA receptor antagonist pre-treatment. Thirdly, we investigated the pharmacological properties of residual mechanical allodynia and cold allodynia that remained after pre-emptive treatment and revealed a greater sensitivity to NMDA R antagonists. These findings indicate that in addition to a marked suppression of thermal hyperalgesia and cold allodynia, pre-emptive treatment with NMDA R antagonist causes a lasting change in spinal NMDA R complexes such that remaining mechanical allodynia should be more effectively targeted by NMDA R antagonists.

Evaluation of interaction between intrathecal adenosine and MK801 or NBQX in a rat formalin pain model

Adenosine and excitatory amino acids have been known to be involved in modulating nociceptive transmission at the spinal level. The authors assessed the characteristics of the interaction of the adenosine-excitatory amino acid antagonist combinations in the spinal cord of rats on the formalin-induced nociception. Intrathecal NMDA antagonist ((5R, 10S)-(+)-5-methyl-10,11-dihydro-(5)H-dibenzo[a[,]d]cyclohepten-5,10-imine hydrogen maleate, MK801, 30 microg) and AMPA antagonist (2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[F]quinoxaline-7-sulfonamide, NBQX, 3 microg) decreased the total number of flinches during both phases in the formalin test. Intrathecal adenosine (300 microg) had little effect on the phase 1 flinching response, but decreased the phase 2 response. The fixed dose analysis and the isobolographic analysis revealed that adenosine interacts additively with MK801 and NBQX in the spinal cord.

Attenuation of morphine tolerance by intrathecal gabapentin is associated with suppression of morphine-evoked excitatory amino acid release in the rat spinal cord

This study was designed to investigate the effect of acute and chronic intrathecal (i.t.) injection of gabapentin (GBP) on the antinociceptive effect of morphine and tolerance development using a tail-flick latency test. Levels of excitatory amino acids (EAA) in i.t. CSF dialysates were also measured by high performance liquid chromatography. Male Wistar rats were implanted with either one or two i.t. catheters for drug injection or pump infusion and with a microdialysis probe for CSF dialysate collection. The effect of acute GBP (10 microg i.t.) injection on the morphine dose response was examined in both naïve rats and rats made tolerant by continuous infusion of morphine (15 microg/h i.t.) for 5 days. At such a low dose (10 microg i.t.), GBP did not enhance morphine's antinociception in naïve rats. In morphine-tolerant rats, however, acute GBP (10 microg i.t.) injection potentiated morphine's antinociception and yielded a 14.6-fold shift in morphine's dose-response curve. When GBP (10 microg/h i.t.) was co-infused with morphine (15 microg/h i.t.) to examine its effect on the development of morphine tolerance, GBP attenuated the development of morphine tolerance. The effect of GBP and morphine on CSF glutamate and aspartate levels was examined in naïve rats, and the effect of morphine challenge on CSF glutamate and aspartate levels was examined in rats previously infused for 5 days with morphine alone or morphine plus GBP. Acute injection of GBP (10 microg i.t.), morphine (50 microg i.t.), or GBP (10 microg i.t.) followed by morphine (50 microg i.t.) 30 min later had no significant effect on CSF EAA concentration in naïve rats; however, in tolerant rats, morphine challenge (50 microg i.t.) increased aspartate and glutamate levels to 221 +/- 22% and 296 +/- 43%, respectively, of those before morphine challenge, and this phenomenon was inhibited by GBP co-infusion. Our results show that GBP, at a dose without enhanced effect on morphine's antinociception in naïve rats, not only potentiates morphine's antinociceptive effect in morphine-tolerant rats but also attenuates the development of morphine tolerance. The mechanism of the effect of GBP on morphine tolerance might be via suppression of the EAA concentration in spinal CSF dialysate.

Spinal administration of MK-801 and NBQX demonstrates NMDA-independent dorsal horn sensitization in incisional pain

Surgery commonly causes pain and neural plasticity that are unique compared to other persistent pain problems. To more precisely study central sensitization and plasticity, we examined the role of ionotropic EAA receptors in dorsal horn neuron sensitization early after incision. Sensitization, in the form of increased background activity, increased mechanosensitivity or pinch receptive field expansion, was induced by plantar incision 1 h later in 30 neurons. (+)-5-Methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine (MK-801) or 1 mM 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo[f]quinoxaline-7-sulfonamide (NBQX) was administered through a microdialysis fiber to block NMDA and nonNMDA EAA receptors, respectively. Dorsal horn neuron sensitization was reexamined 1 h later. Spinal administration of NBQX blocked AMPA-induced excitation but did not affect excitation by NMDA. NBQX decreased background activity in the neurons that developed sustained increased activity after incision. The median decrease caused by NBQX was from 2.3 to 0.0 imp/s. Spinal administration of 5 mM MK-801 blocked NMDA-induced excitation but did not affect excitation by AMPA. The median change (from 2.6 to 1.1 imp/s) in background activity increased by incision was not significantly affected by MK-801. The responses to mechanical stimuli were enhanced after incision in wide dynamic range (WDR) neurons. NBQX eliminated these responses but MK-801 had no effect. The pinch receptive field (RF) expansion into uninjured areas of the paw and hindquarters occurred after incision. Only 1 of 13 neurons exhibited RF expansion after spinal NBQX administration; 9 of 12 neurons had RF expansion remaining after MK-801. Thus, nonNMDA receptors are critical and NMDA-independent factors influence the increased responsiveness of dorsal horn neurons that occur early after incision.

Antinociceptive interactions between intrathecal gabapentin and MK801 or NBQX in rat formalin test

Antagonists for spinal N-methyl-D-aspartate (NMDA) and amino-hydroxy-methtyl-isoxazolepropionate (AMPA) receptors are effective in attenuating acute nociception or injury-induced hyperalgesia. The antinociception of spinal gabapentin is developed in injury-induced hyperalgesia without affecting acute nociception. The authors evaluated the effects of intrathecal gabapentin, NMDA antagonist (MK801) and AMPA antagonist (NBQX) in the formalin test which shows injury-induced hyperalgesia as well as acute pain. We further assessed the interactions between gabapentin and either MK801 or NBQX. Male Sprague-Dawley rats were implanted with intrathecal catheters. To evoke pain, 50 microL of 5% formalin solution was injected into the hindpaw. The interaction was investigated by a fixed dose analysis or an isobolographic analysis. MK801 and NBQX suppressed flinching responses during phase 1 of the formalin test, while gabapentin had little effect on phase 1. All three agents decreased the phase 2 flinching response. A fixed dose analysis in phase 1 showed that gabapentin potentiated the antinociceptive effect of MK801 and NBQX. Isobolographic analysis in phase 2 revealed a synergistic interaction after coadministration of gabapentin-MK801 or gabapentin-NBQX. Correspondingly, spinal gabapentin with NMDA or AMPA antagonist may be useful in managing acute pain and injury-induced hyperalgesia.

Distinct roles of group III metabotropic glutamate receptors in control of nociception and dorsal horn neurons in normal and nerve-injured Rats

Increased glutamatergic input to spinal dorsal horn neurons constitutes an important mechanism for neuropathic pain. However, the role of group III metabotropic glutamate receptors (mGluRs) in regulation of nociception and dorsal horn neurons in normal and neuropathic pain conditions is not fully known. In this study, we determined the effect of the group III mGluR specific agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) on nociception and dorsal horn projection neurons in normal rats and a rat model of neuropathic pain. Tactile allodynia was induced by ligation of L5/L6 left spinal nerves in rats. Allodynia was determined by von Frey filaments in nerve-injured rats. The nociceptive threshold was tested using a radiant heat and a Randall-Selitto pressure device in normal rats. Single-unit activity of ascending dorsal horn neurons was recorded from the lumbar spinal cord in anesthetized rats. An intrathecal (5-30 microg) L-AP4 dose-dependently attenuated allodynia in nerve-injured rats but had no antinociceptive effect in normal rats. Topical spinal application of 5 to 50 microM L-AP4 also significantly inhibited the evoked responses of ascending dorsal horn neurons in nerve-ligated but not normal rats. Furthermore, blockade of spinal group III mGluRs significantly decreased the withdrawal threshold and increased the evoked responses of dorsal horn neurons in normal but not nerve-injured rats. These data suggest that group III mGluRs play distinct roles in regulation of nociception and dorsal horn neurons in normal and neuropathic pain states. Activation of spinal group III mGluRs suppresses allodynia and inhibits the hypersensitivity of dorsal horn projection neurons associated with neuropathic pain.

Suppression of neuropathic pain by peripheral electrical stimulation in rats: μ-opioid receptor and NMDA receptor implicated

Peripheral electrical stimulation (PES) has been utilized to manage chronic pain associated with nerve injury. However, the data on clinical effectiveness are conflicting and the neurophysiological mechanism is not well known. This study was designed to assess whether PES relieved neuropathic pain and its possible mechanisms. The neuropathic pain model was made with lumbar 5th (L5) and 6th (L6) spinal nerve ligations in rats. Nociceptive responses of the rats were assessed by the cold plate test (the number and duration of paw lifts that occurred in 5 min on a 5 +/- 1 degrees C cold plate). PES with a frequency of 2 Hz and at increasing strengths was given for 30 min via stainless-steel needles inserted into standard acupoints on the leg and back, respectively. Immunochemistry was used to examine the immunoreactivity of the NMDA receptor 1 (NR1) subunit in the spinal cord dorsal horn. The results are as follows: (1) PES relieved neuropathic pain and the effect was blocked by 1.0 mg/kg naloxone. (2) The effect of one session of PES lasted up to 12 h. (3) Repetitive PES showed a cumulative effect and no tolerance was observed. (4) There was a significant increase of NR1 immunoreactivity in the superficial laminae of the spinal cord of neuropathic pain rats as compared with naive rats. This increase could be reversed by repetitive 2 Hz PES. These results suggest that PES can relieve neuropathic pain, and that mu-opioid receptors and NMDA receptors are involved in the effect of PES.

Nociceptive response to innocuous mechanical stimulation is mediated via myelinated afferents and NK-1 receptor activation in a rat model of neuropathic pain

Peripheral nerve injury in humans can produce a persistent pain state characterized by spontaneous pain and painful responses to normally innocuous stimuli (allodynia). Here we attempt to identify some of the neurophysiological and neurochemical mechanisms underlying neuropathic pain using an animal model of peripheral neuropathy induced in male Sprague-Dawley rats by placing a 2-mm polyethylene cuff around the left sciatic nerve according to the method of Mosconi and Kruger. von Frey hair testing confirmed tactile allodynia in all cuff-implanted rats before electrophysiological testing. Rats were anesthetized and spinalized for extracellular recording from single spinal wide dynamic range neurons (L(3-4)). In neuropathic rats (days 11-14 and 42-52 after cuff implantation), ongoing discharge was greater and hind paw receptive field size was expanded compared to control rats. Activation of low-threshold sensory afferents by innocuous mechanical stimulation (0.2 N for 3 s) in the hind paw receptive field evoked the typical brief excitation in control rats. However, in neuropathic rats, innocuous stimulation also induced a nociceptive-like afterdischarge that persisted 2-3 min. This afterdischarge was never observed in control rats, and, in this model, is the distinguishing feature of the spinal neural correlate of tactile allodynia. Electrical stimulation of the sciatic nerve at 4 and at 20 Hz each produced an initial discharge that was identical in control and in neuropathic rats. This stimulation also produced an afterdischarge that was similar at the two frequencies in control rats. However, in neuropathic rats, the afterdischarge produced by 20-Hz stimulation was greater than that produced by 4-Hz stimulation. Given that acutely spinalized rats were studied, only peripheral and/or spinal mechanisms can account for the data obtained; as synaptic responses from C fibers begin to fail above approximately 5-Hz stimulation [Pain 46 (1991) 327], the afterdischarge in response to 20-Hz stimulation suggests a change mainly in myelinated afferents and a predominant role of these fibers in eliciting this afterdischarge. These data are consistent with the suggestion that peripheral neuropathy induces phenotypic changes predominantly in myelinated afferents, the sensory neurons that normally respond to mechanical stimulation. The NK-1 receptor antagonist, CP-99,994 (0.5 mg/kg, i.v.), depressed the innocuous pressure-evoked afterdischarge but not the brief initial discharge of wide dynamic range neurons, and decreased the elevated ongoing rate of discharge in neuropathic rats. These results support the concept that following peripheral neuropathy, myelinated afferents may now synthesize and release substance P. A result of this is that tonic release of substance P from the central terminals of these phenotypically altered neurons would lead to ongoing excitation of NK-1-expressing nociceptive spinal neurons. In addition, these spinal neurons would also exhibit exaggerated responses to innocuous pressure stimulation. The data in this study put forth a possible neurophysiological and neurochemical basis of neuropathic pain and identify substance P and the NK-1 receptor as potential neurochemical targets for its management.

Peripheral nerve injury induces trans-synaptic modification of channels, receptors and signal pathways in rat dorsal spinal cord

Peripheral tissue injury-induced central sensitization may result from the altered biochemical properties of spinal dorsal horn. However, peripheral nerve injury-induced modification of genes in the dorsal horn remains largely unknown. Here we identified strong changes of 14 channels, 25 receptors and 42 signal transduction related molecules in Sprague-Dawley rat dorsal spinal cord 14 days after peripheral axotomy by cDNA microarray. Twenty-nine genes were further confirmed by semiquantitative RT-PCR, Northern blotting, in situ hybridization and immunohistochemistry. These regulated genes included Ca2+ channel alpha1E and alpha2/delta1 subunits, alpha subunits for Na+ channel 1 and 6, Na+ channel beta subunit, AMAP receptor GluR3 and 4, GABAA receptor alpha5 subunit, nicotinic acetylcholine receptor alpha5 and beta2 subunits, PKC alpha, betaI and delta isozymes, JNK1-3, ERK2-3, p38 MAPK and BatK and Lyn tyrosine-protein kinases, indicating that several signal transduction pathways were activated in dorsal spinal cord by peripheral nerve injury. These results demonstrate that peripheral nerve injury causes phenotypic changes in spinal dorsal horn. Increases in Ca2+ channel alpha2/delta1 subunit, GABAA receptor alpha5 subunit, Na+ channels and nicotinic acetylcholine receptors in both dorsal spinal cord and dorsal root ganglia indicate their potential roles in neuropathic pain control.

Effects of ketamine on electroencephalographic and autonomic arousal and segmental reflex responses in the cat

OBJECTIVE

To provide evidence concerning doses of ketamine that affect electroencephalographic (EEG) and autonomic signs of arousal during nociceptive stimulation.

STUDY DESIGN

Prospective psychophysical test in people. Single injection or progressively increasing infusions of ketamine in cats. ANIMALS AND PEOPLE: Seven people (20-60 years old) and three cats (3-5 kg) for EEG recording and six cats for EMG recordings.

METHODS

In order to define innocuous and nociceptive stimulus intensities which could be applied to cats to evaluate arousal, psychophysical evaluations of sensations elicited by compression of the skin overlying phalangeal bones of the hand were obtained from human subjects. Then, following administration of ketamine, recordings of EEG frequency and of autonomic responses (heart rate, respiratory rate and arterial blood pressure) were obtained before and during stimulation of the tails of cats at pressures identified by human observers as either innocuous or nociceptive. Observations of withdrawal reflexes of the hindlimbs following interdigital skin stimulation were interposed between recording periods. In separate sessions, stretch reflex activity was assessed during awake and anesthetic conditions by recording electromyographic activity from soleus muscles and resistive force to dorsiflexion of the tibiotarsal joint.

RESULTS

There were no changes in either total EEG (0.5-30.0 Hz), low-frequency (0.6-7.5 Hz) or high-frequency (7.5-30.0 Hz) power produced by nociceptive stimulation for a period of 18-24 minutes following an intramuscular bolus dose of ketamine (33.0 mg kg-1), although withdrawal reflexes were present. Thereafter, nociceptive stimulation produced EEG arousal responses in the low-frequency and total power range and increased systolic blood pressure and respiration rate. In tests after intravenous infusion of ketamine (10.0-22.2 mg kg-1 hour-1), total and low-frequency EEG power and autonomic responses to nociceptive stimulation were eliminated. Organized motor responses were never elicited during IV infusion, but withdrawal reflexes were observed at each dosage. Also, stretch reflexes were shown by quantitative analysis to be retained at all doses of ketamine infusion.

CONCLUSIONS AND CLINICAL RELEVANCE

These results show that testing of withdrawal reflexes does not reveal the adequacy of ketamine anesthesia. Segmental stretch and withdrawal reflexes are preserved and can be investigated during infusion of ketamine at doses that eliminate arousal from brief periods of nociceptive stimulation.

The challenge of chronic pain

Chronic pain is a complex problem with staggering negative health and economic consequences. The complexity of chronic pain is presented within Cervero and Laird's model that describes three phases of pain, including pain without tissue damage, pain with tissue damage and inflammation, and neuropathic pain. The increased afferent input in phases 2 and 3 of chronic pain produces marked changes in primary afferents, dorsal root ganglia, and spinal cord dorsal horn. These changes promote the symptoms of chronic pain, including spontaneous pain, hyperalgesia, and allodynia. Increased afferent input also evokes supraspinal input to the dorsal horn, including biphasic innervation from the ventromedial medulla and A7 catecholamine cell group, that promotes hyperalgesia and allodynia. More rostral brain structures, such as the lateral hypothalamus, amygdala, and hippocampus, may also play a role in chronic pain. Although much has been discovered about the multiple pathological mechanisms involved in chronic pain, further research is needed to fully comprehend these mechanisms.

A dissociative change in the efficacy of supraspinal versus spinal morphine in the neuropathic rat

The efficacy of spinally versus supraspinally administered morphine was studied in rats with a spinal nerve ligation-induced neuropathy. Behavioural assessment indicated that the effect of intrathecally administered morphine on pain-related responses was attenuated when compared with unoperated controls. The decreased efficacy of spinal morphine was associated with neuropathic symptoms, since sham ligation or nerve ligation without accompanying tactile allodynia did not lead to spinal inefficacy of morphine. In contrast, the pain attenuating effect of morphine in the periaqueductal gray (PAG) was enhanced in neuropathic animals. The effect of systemically administered morphine on pain-related behavior of neuropathic rats was in the same range as in controls or decreased, depending on the test. Coadministration of lidocaine or MK-801, a N-methyl-D-aspartate (NMDA) receptor antagonist, into the rostroventromedial medulla enhanced the tactile antiallodynic but not the thermal antinociceptive effect of intrathecally administered morphine in neuropathic animals. Supraspinal administration of MK-801 or lidocaine did not influence efficacy of spinal morphine in sham-operated animals. Electrophysiological recordings of nociceptive wide-dynamic range (WDR) neurons in the deep spinal dorsal horn of pentobarbitone-anesthetized animals corresponded to a large extent with behavioral results. The inhibitory effect of spinally and systemically administered morphine on WDR neuron responses was attenuated whereas that induced by morphine in the PAG was enhanced in neuropathic animals. The results indicate that in spinal nerve ligation-induced neuropathy the efficacy of spinal morphine is decreased whereas that of supraspinal morphine is increased. Descending influence from brainstem-spinal pathways, involving NMDA receptors in the rostroventromedial medulla, may contribute to the selective reduction in tactile antiallodynic efficacy of spinal morphine.

Powerful antinociceptive effects of the cone snail venom-derived subtype-selective NMDA receptor antagonists conantokins G and T

Subunit non-selective N-methyl-D-aspartate (NMDA) receptor antagonists reduce injury-induced pain behavior, but generally produce unacceptable side effects. In this study, we examined the antinociceptive and motor effects of cone snail venom-derived peptides, conantokins G and T (conG and conT), which are selective inhibitors of the NR2B or NR2A and NR2B subtypes of the NMDA receptor, respectively. We tested the effects of conG and conT in models of tissue (formalin test), nerve injury (partial sciatic nerve ligation) and inflammation-induced (intraplantar Complete Freund's Adjuvant; CFA) pain in mice. In the formalin test, intrathecal (i.t.) conG or conT suppressed the ongoing pain behavior (ED(50) and 95% confidence intervals (CI), 11 (7-19) and 19 (11-33), respectively) at doses that were 17-27 times lower than those required to impair motor function (accelerating rotarod treadmill test: ED(50) and 95% CI, 300 (120-730) and 320 (190-540) pmol, respectively). By comparison, SNX-111, an N-type voltage-sensitive calcium channel antagonist that is also derived from cone snail venom, produced significant motor impairment at a dose (3.0 pmol, i.t.) that was only partially efficacious in the formalin test. Furthermore, conG reversed the allodynia produced by nerve injury, with greater potency on thermal (ED50 and 95% CI, 24 (10-55) pmol) than on mechanical allodynia (59 (33-105) pmol). Finally, a single dose of conG (100 pmol, i.t.) also reduced CFA-evoked thermal and mechanical allodynia. Taken together, these results demonstrate that conantokins exhibit potent antinociceptive effects in several models of injury-induced pain. The study supports the notion that drugs directed against subtypes of the NMDA receptor, by virtue of their reduced side-effect profile, hold promise as novel therapeutic agents for the control of pain.

Alteration in the voltage dependence of NMDA receptor channels in rat dorsal horn neurones following peripheral inflammation

1. It has been proposed that the activation of NMDA receptors and upregulation of protein kinase C (PKC) underlie the exaggerated and persistent pain experienced in the inflammatory state. However, there is no direct evidence to show that inflammation alters the function of NMDA receptors. 2. We examined the voltage-dependent properties of NMDA receptor channels in rat dorsal horn neurones that receive sensory inputs from an inflamed hindpaw. 3. Peripheral inflammation was induced by injections of complete Freund's adjuvant (CFA). Membrane currents were measured using the perforated patch-clamp technique. 4. After CFA treatment, the current-voltage relationship of NMDA receptor channels was shifted in the hyperpolarized direction. This resulted in enhanced NMDA responses at negative potentials. 5. The change was mediated by PKC because the voltage shift was blocked by the selective PKC inhibitors chelerythrine and bisindolylmaleimide I. 6. Furthermore, the Mg(2+) blockade of NMDA receptors was reduced. This reduction could account for the shift in the voltage dependence of NMDA receptor channels. 7. These results indicate that NMDA receptor channel characteristics in the dorsal horn are altered by inflammation, and that the changes observed could contribute to the hyperalgesia and allodynia associated with tissue injury.

Pain behavior and response properties of spinal dorsal horn neurons following experimental diabetic neuropathy in the rat: modulation by nitecapone, a COMT inhibitor with antioxidant properties

We attempted to characterize a spinal neuronal correlate of painful neuropathy induced by diabetes mellitus (DM). Pain behavior and response properties of spinal dorsal horn neurons were determined in rats with a streptozocin-induced DM. A catechol-O-methyltransferase inhibitor with potent antioxidant properties, nitecapone, was used in an attempt to attenuate neuropathic symptoms. Behaviorally DM induced mechanical hypersensitivity that was markedly attenuated by oral treatment with nitecapone. The antihyperalgesic effect of nitecapone was not reversed by naloxone, an opioid antagonist, or atipamezole, an alpha-2-adrenoceptor antagonist. Electrophysiological recordings performed in pentobarbitone-anesthetized animals revealed that the most distinct abnormality in response properties of spinal dorsal horn wide-dynamic range (WDR) neurons was the increase in their spontaneous activity observed in untreated but not in nitecapone-treated DM rats. Conditioning electrical stimulation and a lidocaine block of the rostroventromedial medulla (RVM) had a similar modulatory effect on evoked responses of spinal dorsal horn WDR neurons in all experimental groups. The response properties of spinal dorsal horn nociceptive-specific or low-threshold mechanoreceptive neurons were not markedly different between the experimental groups. The results indicate that increased spontaneous activity in spinal dorsal horn WDR neurons may be causally related to behaviorally observed mechanical hypersensitivity in DM. Attenuation of the increased spontaneous activity in WDR neurons may explain the antihyperalgesic effect by nitecapone, due to naloxone- and alpha-2-adrenoceptor-insensitive mechanisms. DM or nitecapone treatment did not produce significant changes in phasic or tonic descending pain regulation originating in the RVM.

Cellular mechanisms of hyperalgesia and spontaneous pain in a spinalized rat model of peripheral neuropathy: changes in myelinated afferent inputs implicated

Various hypotheses have been proposed to account for the mechanical hyperalgesia and spontaneous pain seen in animal models of peripheral neuropathy. The purpose of the present study was to determine whether there exists a spinal neuronal correlate to these properties. An experimental neuropathy was induced in male Sprague-Dawley rats by placing a 2-mm PE-90 polyethylene cuff around the sciatic nerve. All rats were subsequently confirmed to exhibit mechanical allodynia in the von Frey test. After induction of anaesthesia with pentobarbital and acute spinalization at T9, electrophysiological experiments were performed, recording extracellular single unit activity from ipsi- and contralateral wide dynamic range dorsal horn neurons in spinal segments L1-4. On-going activity was greater in short-term (11-22 days after cuff implantation) and long-term (42-52 days) cuff-implanted rats; 38 spikes/s in short-term versus 19 spikes/s in controls; 29 spikes/s in long-term ipsi- and contralateral neurons. Receptive fields in controls were always restricted, but in almost all cuff-implanted rats extended over the whole hind paw. Responses to noxious mechanical (pinch) and noxious heat stimulation of the cutaneous receptive field in controls consisted of the typical fast initial discharge followed by an afterdischarge. In all neurons from cuff-implanted rats the initial discharge resembled that in controls. However, the afterdischarge, particularly that in response to noxious pinch, was markedly greater in both magnitude and duration. It is suggested that the greater on-going discharge is the cellular correlate of spontaneous pain, and the potentiation of the afterdischarge in response to noxious stimulation is the correlate of hyperalgesia. Given that acutely spinalized rats were tested, only peripheral and/or spinal mechanisms can be considered to explain these data. Considering all the data, it can be concluded that there is a greater change in fibres mediating noxious mechanical than noxious thermal inputs. Among different hypotheses, the one with which the present data are most compatible is that which proposes that chronic nerve injury or inflammation induces phenotypic changes predominantly in myelinated afferents. There may be a redistribution of membrane-bound ion channels, predominantly sodium channels, which leads to ectopic activity and thus spontaneous discharge of dorsal horn neurons. With regard to mechanical stimulation-evoked synaptic input, the central terminals of myelinated afferents expand into regions of the spinal cord which normally receive their predominant input from unmyelinated nociceptive afferents. This may be coupled with a change in these myelinated afferents so that they now synthesize and release peptides, primarily substance P, from their central terminals with the result that the effects of their chemical mediators of synaptic transmission add to the effects of nociceptive inputs leading to exaggerated responses to painful stimuli, thus the basis of clinical hyperalgesia.

Spinal plasticity of acute opioid tolerance

Spinal acute opioid tolerance remains mechanistically undercharacterized. Expanded clinical use of direct spinal administration of opioids and other analgesics indicates that studies to further understand spinal mechanisms of analgesic tolerance are warranted. Rodent models of spinal administration facilitate this objective. Specifically, acute spinal opioid tolerance in mice presents a plasticity-dependent, rapid, and efficient opportunity for evaluation of novel clinical agents. Similarities between the pharmacology of acute and chronic spinal opioid tolerance, neuropathic pain, and learning and memory suggest that this model may serve as a high through-put predictor of bioactivity of novel plasticity-modifying compounds.

The kainate receptor antagonist 2S,4R-4-methylglutamate attenuates mechanical allodynia and thermal hyperalgesia in a rat model of nerve injury

Opioids and receptor antagonists of excitatory amino acids attenuate mechanical allodynia and thermal hyperalgesia in animal models of neuropathic pain. Recently, a kainate receptor antagonist, 2S,4R-4-methylglutamate, has been developed but has not been tested for antinociceptive effects in animal models of neuropathic pain. We evaluated whether 2S,4R-4-methylglutamate attenuated responses to mechanical and thermal stimuli in uninjured (control) rats and increased responsiveness in rats with chronic constriction injury. Rats were tested for a number of withdrawal responses using a calibrated von Frey filament (mechanical stimulus) and withdrawal latencies from a radiant heat source (thermal stimulus). In control rats, 2S,4R-4-methylglutamate produced a small but significant decrease in responses from the mechanical stimulus (25 mg/kg) and significantly increased withdrawal latencies from the thermal stimulus at the highest dose administered (100 mg/kg). In addition, 2S,4R-4-methylglutamate greatly attenuated increased responsiveness in rats with chronic constriction injury. At four to eight days following chronic constriction injury, animals that displayed increased responsiveness to mechanical and thermal stimuli were injected intraperitoneally with either dizocilpine maleate (0.1 mg/kg), morphine (4 mg/kg), vehicle as controls, or 2S,4R-4-methylglutamate (25, 50, 75 or 100 mg/kg). 2S,4R-4-Methylglutamate (25, 50, 75 and 100 mg/kg) significantly attenuated the frequency of responses to mechanical stimuli (Wilcoxon, P < 0.05) and the latency of responses to thermal stimuli (analysis of variance and Duncan's, P < 0.05). Dizocilpine maleate and morphine, as expected, also reduced these responses. These results suggest that, in addition to opioid and N-methyl-D-aspartate receptors, kainate receptors may play a role in the maintenance of mechanical allodynia and thermal hyperalgesia associated with peripheral nerve injury.

Pharmacological characterization of N-methyl-D-aspartate receptors in spinal cord of rats with a chronic peripheral mononeuropathy

N-Methyl-D-aspartate (NMDA) receptor antagonists, acting in the spinal cord, are analgesic. However, the clinical utility of these antagonists is diminished by their adverse effects on cognition and behavior. To facilitate the development of spinal cord-selective NMDA receptor antagonists, we characterized ligand interactions at NMDA receptors in spinal cord of normal rats and rats with a chronic peripheral neuropathy. NMDA receptors in spinal cord were distinguished from those in cerebral cortex on the basis of differences in the potencies of competitive and noncompetitive antagonists and on the basis of differences in their response to spermidine. D(-)-2-Amino-5-phosphonopentanoic acid (AP-5) and (+)-(1-hydroxy-3-aminopyrrolidine-2-one) (HA-966) were more potent in inhibiting NMDA-dependent [3H]TCP binding in spinal cord while, conversely, MK-801 was more potent in inhibiting [3H]TCP binding to NMDA receptors in cerebral cortex. Spermidine increased [3H]TCP binding to NMDA receptors in cerebral cortex (39+/-8%) but not spinal cord (2+/-1%). Based on these properties, NMDA receptors in spinal cord more closely resembled those in cerebellum than those in cerebral cortex. Generation of a chronic neuropathy had no effect on the density of NMDA receptors in lumbar spinal cord. There were also no major changes in the potencies of competitive antagonists or channel blocking ligands, although there was a trend for kynurenic acid and D-CPP to be more potent in the spinal cords of neuropathic animals. These findings indicate that, in both normal and neuropathic pain states, NMDA receptors in spinal cord can be distinguished pharmacologically from those in cerebral cortex. These findings underscore the feasibility of developing spinal cord-selective NMDA receptor antagonists as novel analgesics.