Systemic or intracerebroventricular injection of NMDA receptor antagonists attenuates the antinociceptive activity of intrathecally administered NMDA receptor antagonists

The L-kynurenine signalling pathway in trigeminal pain processing: A potential therapeutic target in migraine?

Introduction: In recent years the kynurenine family of compounds, metabolites of tryptophan, has become an area of intensive research because of its neuroactive properties. Two metabolites of this family have become of interest in relation to migraine and pain processing.

Discussion: Experimental studies have shown that kynurenic acid (KYNA) plays an important role in the transmission of sensory impulses in the trigeminovascular system and that increased levels of KYNA decrease the sensitivity of the cerebral cortex to cortical spreading depression. Furthermore, another metabolite of the kynurenine family, L-kynurenine, exerts vasodilating effects similar to nitric oxide by increasing cyclic guanosine monophosphate.

Conclusion: This review summarizes current knowledge of the role of kynurenine signalling in trigeminal and central pain processing, including its therapeutic prospects in migraine treatment.

Dopamine and NMDA systems modulate long-term nociception in the rat anterior cingulate cortex

The anterior cingulate cortex (ACC) plays a key role in pain processing. It has been reported that increased activity of glutamatergic projections into the ACC intensifies nociception; whereas dopaminergic projections inhibit it. The aim of this study was to evaluate the role of dopaminergic and NMDA systems of the ACC in the modulation of long-term nociception elicited by sciatic denervation in the rat. Score, onset and incidence of long-term nociception were measured by the autotomy behavior. The effects of a single microinjection into the ACC of different doses of dopamine (100 nM, 100 microM and 100 mM), a NMDA receptor antagonist (MK801 200 nM and 9.34 mM) and amantadine, a dopamine agonist and NMDA receptor antagonist (10, 100 and 1000 microM) were tested on long-term nociception. Dopamine diminished autotomy behavior in an inverse dose-dependent manner, with dopamine 100 nM as most effective concentration. MK801 and amantadine elicited a significant reduction on autotomy score. Prior injections of D1 and D2 receptor antagonists blocked the antinociceptive effects of amantadine on long-term nociceptive behavior. The present study suggests an interaction between dopaminergic and glutamatergic systems within the ACC in the genesis and maintenance of long-term nociception.

The anesthetic interaction between adenosine triphosphate and N-methyl-D-aspartate receptor antagonists in the rat

Modulation of synaptic neurotransmission through the ligand-gated ion channel is probably involved in the mechanisms of analgesic and anesthetic actions. In the central nervous system, adenosine triphosphate and glutamate are fast excitatory neurotransmitters through their effects on P2X and N-methyl-D-aspartate (NMDA) receptors respectively. To examine the anesthetic interaction between adenosine triphosphate and NMDA receptor antagonists, we studied the effect of intracerebroventricular administration of P2 and/or NMDA antagonists on the minimum alveolar concentration (MAC) of sevoflurane in rats. Intracerebro- ventricular administration of phosphonopentanoic acid azophenyl-2',4'-disulfonate and D (-)-2-anino-5-phophonopentanoic acid, P2 and NMDA antagonists, significantly reduced the MAC of sevoflurane. The reduction of the MAC by both phosphonopentanoic acid azophenyl-2',4'-disulfonate and D (-)-2-anino-5-phophonopentanoic acid was dose-dependent. The effect of coadministration of both antagonists was additive in the reduction of sevoflurane minimum alveolar concentration. These results suggest that P2 and NMDA receptors mediate nociceptive/anesthetic processing as inhibition of these receptors resulted in analgesic and anesthetic effects. However the pathway mediated through each receptor may be different postsynaptically and/or one of these presynaptic receptors may modulate the neurotransmitter release of the other.

Non-opioid antinociceptive effects of supraspinal histogranin and related peptides: possible involvement of central dopamine D(2) receptor

The antinociceptive effects of intracerebroventricular (ICV) administration of histogranin (HN) and related peptides were assessed in the mouse writhing and tail-flick assays. In the writhing test, the peptides displayed dose-dependent analgesic effects with an AD(50) of 23.9 nmol/mouse for HN and the following order for other peptides: HN-(7-15)<histone H4-(86-100) approximately HN approximately HN-(7-10)<[Ser(1)]HN<osteogenic growth peptide (OGP) approximately HN-(1-10). HN-(6-9) and HN-(8-10) did not show any significant analgesic activity at 50 nmol/mouse. The importance of the C- and N-terminal amino acids in the analgesic activity of the peptides was demonstrated by the prolonged effects of HN and [Ser(1)]HN ( approximately 30 min) compared with those of HN fragments (HN-(7-15), HN-(1-10) and HN-(7-10): 5-10 min). The analgesic activity of [Ser(1)]HN (50 nmol/mouse) was not affected by the coadministration of opioid (naloxone, 1 nmol/mouse), NMDA (CPP, 0.3 and MK-801, 0.3 nmol/mouse) and D(1) (SCH-23390, 0.5 nmol/mouse) receptor antagonists, but it was significantly antagonized by the coinjection of the D(2) receptor antagonist raclopride (0.5 nmol/mouse). In the mouse tail-flick assay, HN and related peptides (50 nmol/mouse) also showed significant analgesic activity (15-35% MPE). The analgesic effect of [Ser(1)]HN was dose-dependent and, at 75 nmol/mouse, lasted for up to 45 min, and was partially blocked by the coadministration of raclopride (1 nmol/mouse), but not naloxone (2 nmol/mouse). In the mouse rotarod assay, relative high doses (75-100 nmol/mouse) of HN and related peptides did not significantly affect motor coordination. These results indicate that supraspinal administration of HN and related peptides induce significant non-opioid analgesic effects devoid of motor activity by a mechanism that involves the participation of central dopamine D(2) receptors.

Increased immunoreactive labeling of the spinal N-methyl-D-aspartate R1 receptors after dorsal root ganglionectomy in the rats

The N-methyl-D-aspartate (NMDA) receptor plays an important role in the development of the autotomy after dorsal root ganglionectomy (DRGn). In this study, we further investigated the expression of the NMDAR1 in the spinal cord of the rats after right DRGn by immunohistochemical analyses. Computerized densitometric analysis of the NMDAR1 immunoreactivity was done and the integrated optical density (IOD) of the superficial laminae of the dorsal horn of the spinal cord was measured. The immunoreactive labeling of the NMDAR1 was increased in the cervical spinal cord ipsilateral to the DRGn from day 5 to 14 after DRGn. The ratio of the right/left IOD of the rats receiving DRGn was significantly higher than the rats in the sham-operated group and the control group (P<0.05). The expression of the NMDAR1 increased gradually to reach the peak at day 7 after DRGn (mean right/left IOD ratio=1.52), then decreased thereafter. The increased expression of the NMDAR1 at day 7 was suppressed by MK-801 (NMDA receptor antagonist) administered immediately after DRGn, but not by normal saline or 1,2,3,4-tetrahydro-6-nitro-2, 3-dioxo-benzo[f] quinoxaline-7-sulfonamide (NBQX, non-NMDA receptor antagonist). The results indicated that the expression of the NMDAR1 in the superficial laminae of the dorsal horn of the spinal cord was increased after DRGn and the time course was compatible with the onset and development of the autotomy induced by DRGn.

Modulation of nociceptive transmission by NMDA/glycine site receptor in the ventroposterolateral nucleus of the thalamus

NMDA-type glutamate receptors are involved in the generation and maintenance of altered pain states. In the present study, we examined the effect of an NMDA-glycine site antagonist, GV196771A [E-4, 6-dichloro-3-(2-oxo-1-phenyl-pyrrolidin-3-ylidenemethyl)-1H- indole-2- carboxylic acid sodium salt], on responses to noxious stimuli both in normal rats and during peripheral mononeuropathy induced by chronic constriction injury (CCI) of the sciatic nerve. In one series of experiments, activity of nociceptive neurons in the ventroposterolateral (VPL) nucleus of the thalamus was recorded in response to pressure stimuli to the contralateral hindpaw. Intravenous injection (iv) of the glycine antagonist had no effect on these cells in normal rats. When tested in rats with CCI induced 2-3 weeks previously, however, GV196771A (0.125, 0.5 and 2.0mg/kg) blocked responses to noxious stimulation in a dose-dependent and reversible manner. Morphine (0.5mg/kg, iv) and the NMDA channel blocker MK801 (0.1mg/kg, iv) suppressed noxious stimulus-evoked activity of VPL neurons in both normal and CCI-treated rats. MK801 also decreased the responses of non-nociceptive neurons to brush stimulation in both sets of animals, in contrast to the glycine antagonist which did not alter the responses of these cells. Similar results were obtained from a series of behavior experiments in which the latency for paw withdrawal from heat stimulation was measured in normal and CCI-treated rats. GV196771A (3 and 10mg/kg) injected orally, reduced the hyperalgesic response in the treated rats but did not change the withdrawal latency in normal rats. Taken together, these findings suggest that block of the NMDA receptor decreases nociceptive transmission in the thalamus and can modulate hyperalgesic states. GV196771A and glycine antagonists in general may represent innovative and safe agents for the treatment of neuropathic pain.

Substantia nigra lesion suppresses the antagonistic effects of N-methyl-D-aspartate receptor antagonist (MK-801) on the autotomy in the rat

Rats received right dorsal root ganglionectomy (DRGn) to induce autotomy, and were treated with MK-801 and/or left substantia nigra (SN) lesion after DRGn. The behavior was quantified using an autotomy grading scale. All the rats in the control groups manifested autotomy from 4 to 19 days after DRGn and attained the highest autotomy score. The group treated with MK-801 immediately after DRGn showed suppression of the development of autotomy. The groups receiving left SN lesion with 6-hydroxydopamine immediately, 2, or 4 days after DRGn showed similar patterns of autotomy as the control groups. However, when combined with the administration of MK-801 immediately after DRGn, SN lesion done immediately or 2 days after DRGn suppressed the antagonistic effect of MK-801 (P < 0.01). When the SN lesion was delayed by 4 days, the suppression effect disappeared. These data suggest that the action of the NMDA receptor antagonist on the autotomy within 4 days after DRGn depend on the integrity of the dopaminergic system.

Suppression of autotomy by N-methyl-D-aspartate receptor antagonist (MK-801) in the rat

The effects of different doses and time of administration of the N-methyl-D-aspartate (NMDA) receptor antagonist (MK-801) on the development of the autotomy (self-mutilation) were studied in the rats receiving dorsal root ganglionectomy (DRGn). The rats without any treatment and those treated with normal saline immediately after DRGn were the control groups. Three groups of rats were treated with 0.1, 0.5 or 1.0 mg/kg of MK-801 immediately after DRGn, and another three were treated with 1.0 mg/kg of MK-801 2, 4, or 7 days after DRGn. The behavioral observations of these rats were quantified using an autotomy grading scale ranging from 0 to 19, and the scores were compared among these groups. The rats in the control groups manifested autotomy from 5 to 17 days after DRGn and all of them (100%) attained the highest autotomy score. Lower doses (0.1 or 0.5 mg/kg) of MK-801 had no effect on the development of the autotomy. In contrast, higher dose (1.0 mg/kg) of MK-801 administered immediately after DRGn significantly suppressed the autotomy as compared to the control groups (P < 0.01) and only 17% of the rats in this group attained the highest score. The antagonistic effect was retained when the treatment of MK-801 was delayed to 2 days after DRGn, however, it disappeared when the treatment was delayed to 4 or 7 days after DRGn. Thus, the antagonistic effect of MK-801 on the autotomy induced by DRGn was dose-related and time-dependent. The main role of the NMDA receptor in the development of the autotomy was within several days after DRGn.

Antinociceptive effects of NMDA and non-NMDA receptor antagonists in the tail flick test in mice

Inhibition of spinal glutamate receptors induces antinociceptive effects in numerous animal models of pain. The present study compares the effects of intrathecally administered N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptor antagonists on nociceptive responses in the tail flick test. Potency of antagonists at NMDA and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors was first measured by electrical assays in Xenopus oocytes expressing rat cerebral cortex poly(A)+ RNA. Subsequently, Swiss Webster mice were injected intrathecally with the antagonists and tested for antinociception. The drugs tested were: NBQX and GYKI-52466, selective AMPA receptor antagonists, ketamine, MK-801, R(+) HA-966 and ACEA-0762, selective NMDA receptor antagonists, and ACEA-1031, ACEA-1328 and ACEA-0593, NMDA receptor antagonists that also show inhibition of non-NMDA receptors. Selective NMDA receptor antagonists induced essentially no antinociceptive effects in the tail flick test. Antinociceptive activity generally correlated with inhibition of AMPA receptors. The exception was the non-competitive AMPA receptor antagonist GYKI-52466, which was unexpectedly weak. This may be due to inadequate dosing, because the compound has limited solubility, or may be due to differences in the non-NMDA receptor subtype-selectivity profile of GYKI-52466 as compared to competitive antagonists such as NBQX. Overall, our results suggest that inhibition of spinal non-NMDA receptors is the primary, and necessary, mechanism of antinociception by these drugs in the tail flick test in mice.

Attenuation of nociceptive responses by ACEA-1021, a competitive NMDA receptor/glycine site antagonist, in the mice

Intrathecal administration of N-methyl-D-aspartate (NMDA) and non-NMDA receptor antagonists has been shown to produce antinociception in various animal models of pain. In the present study we examined the effect of 5-nitro-6,7-dichloro-1,4-dihydro-2,3-quinoxalinedione (ACEA-1021), a competitive NMDA receptor/glycine site antagonist, on nociceptive responses in the tail flick and formalin tests in mice. Swiss Webster mice were injected with ACEA-1021 intraperitoneally (1-60 mg/kg), or intrathecally (1-40 micrograms/mouse), and tested for antinociception. Systemic administration of ACEA-1021 attenuated the nociceptive responses solely in the formalin test. Nevertheless, intrathecal administration of ACEA-1021 showed equally potent attenuation of nociceptive responses in both animal models of pain. The effect of ACEA-1021 was also examined on caudally directed biting and scratching (CDBS) behaviors induced by intrathecal administration of NMDA. Microinjections of NMDA (1-1000 microM) in the spinal cord produced dose-dependent CDBS behaviors. Mice pretreated with ACEA-1021 (0.5-40 micrograms/mouse) showed dose-dependent protection against CDBS behaviors induced by intrathecal NMDA. Taken together, the results suggest that ACEA-1021 may block spinal NMDA receptors to attenuate nociceptive responses, however, our data cannot exclude the involvement of non-NMDA receptors.

Systemic ketamine attenuates nociceptive behaviors in a rat model of peripheral neuropathy

The efficacy of ketamine (KET), a non-competitive NMDA receptor-channel blocker, was assessed in relieving nociceptive behaviors in neuropathic rats with tight ligations of the L5 and L6 spinal nerves. The antinociceptive effects of KET were dose- and time-dependent. A systemic injection of 0.01 mg/kg KET transiently (15-30 min) attenuated several nociceptive behaviors, including mechanical allodynia and hyperalgesia, cold allodynia, spontaneous pain, and cold stress-induced pain. Treatment with 1.0 mg/kg KET consistently decreased all nociceptive behaviors for 45-75 min, without noticeable side effects. Higher doses (25 and 50 mg/kg) provided longer lasting relief: however, these doses resulted in transient motor impairment which lasted for 15-30 min post-injection. Systemic KET was most effective in decreasing the behavioral signs of mechanical allodynia and hyperalgesia, followed by cold allodynia, cold stress-induced pain, and spontaneous pain. The present results demonstrate that blockade of NMDA receptors effectively alleviates nociceptive behaviors in a rat model of peripheral neuropathy, substantiating the important role of these receptors in the central sensitization that underlies the maintenance of neuropathic pain. In addition, the ability of KET to reduce significantly a variety of nocifensive behaviors suggests that this clinically safe drug could be used in pain management for neuropathic patients.

Excitatory amino acid antagonists in the rostral ventromedial medulla inhibit mesencephalic morphine analgesia in rats

Supraspinal opioid analgesia is mediated in part by connections between the periaqueductal gray (PAG) and rostral ventromedial medulla (RVM). Morphine analgesia elicited from the PAG is respectively decreased by selective serotonergic and opioid receptor antagonists administered into the RVM, and increased by RVM neurotensin antagonists. Since glutamate and excitatory amino acid (EAA) receptors are also active in the RVM, the present study evaluated whether either competitive (AP7) or non-competitive (MK-801) N-methyl-D-aspartate (NMDA) antagonists or a kainate/AMPA (CNQX) antagonist microinjected into the RVM altered morphine (2.5 micrograms) analgesia elicited from the PAG as measured by the tail-flick and jump tests. Mesencephalic morphine analgesia was markedly reduced on both tests after RVM pretreatment with either AP7 (0.01-1 microgram, 0.08-7.8 nmol) or MK-801 (0.03-3 micrograms, 0.04-4.4 nmol). In contrast, small but significant reductions in mesencephalic morphine analgesia occurred on the jump test following CNQX (0.5 microgram, 2.2 nmol) in the RVM. NMDA antagonists did not markedly alter either basal nociceptive thresholds following RVM administration, or mesencephalic morphine analgesia following administration into medullary placements lateral or dorsal to the RVM. These data implicate EAA and particularly NMDA receptors in the RVM in modulating the transmission of opioid pain-inhibitory signals from the PAG.

An attempt to attenuate experimental pain in humans by dextromethorphan, an NMDA receptor antagonist

Dextromethorphan (100 mg, orally), an NMDA receptor antagonist, did not significantly attenuate pain intensity or unpleasantness induced by experimental ischemia or by topical capsaicin in healthy human subjects, nor did it increase the threshold for heat pain or mechanical pain. A dose of 200 mg produced marked side effects. Thus, systemically administered dextromethorphan does not attenuate pain at clinically applicable doses in humans.

Peripheral and central mechanisms of NGF-induced hyperalgesia

Mechanisms underlying the hyperalgesia induced by a single systemic injection of nerve growth factor (NGF) in adult rats were studied in vivo. A single dose of NGF initiated a prolonged thermal hyperalgesia to a radiant heat source within minutes that lasted for days. Animals which had been pretreated with the mast cell degranulating compound 48/80 or either one of two specific 5-hydroxytryptamine receptor antagonists (ICS 205-930 and methiothepin) also developed an NGF-induced thermal hyperalgesia, but onset was delayed by more than 3 h. In the presence of ICS 205-930 or methiothepin the early component NGF-induced hyperalgesia was reversed and the animals responded with an initial hypoalgesia to the thermal stimuli. Whereas these results indicate a peripheral mechanism for the initial thermal hyperalgesia, the later phase (7 h-4 days after NGF) appeared to be centrally maintained, since it could be selectively blocked by the non-competitive NMDA receptor antagonist MK-801. In contrast to the almost immediate thermal hyperalgesia following a single injection of NGF, a significant mechanical hyperalgesia began only after a 7 h latency. This NGF-induced mechanical hyperalgesia was not blocked by any of the treatments that attenuated the thermal hyperalgesia, indicating that a separate mechanism may be involved. Additional electrophysiological experiments showed that NGF-induced hyperalgesia was not maintained by an increased amount of spontaneous activity in C-fibres. A final result showed that endogenous release of NGF in a model of acute inflammation (complete Freund's adjuvant-induced inflammation) may be involved in the development of thermal hyperalgesia, since it could be blocked by concomitant treatment with anti-NGF antisera. These data indicate that NGF-induced thermal and mechanical hyperalgesia are mediated by different mechanisms. The rapid onset component of thermal hyperalgesia is due to a peripheral mechanism involving the degranulation of mast cells, whereas the late component involves central NMDA receptors. In contrast, the NGF-induced mechanical hyperalgesia seems to be independent of mast cell degranulation or central NMDA receptor sites.

The NMDA antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) has antinociceptive effect after intrathecal injection in the rat

This behavioral study was performed in order to delineate the antinociceptive effects of and the influence on motor function of a highly potent, competitive NMDA receptor antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP). After intrathecal (i.t.) administration of CPP to chronically catheterized rats, antinociception was studied in 3 different nociceptive tests: the tail-flick test, the hot-plate test, and the formalin test. The lowest dose producing visible motor dysfunction was 1 nmol, with 2 of 8 animals showing slight ataxia. Dose-related motor dysfunction and apparent sedation was present after 5 and 10 nmol. Dose-related antinociception was evident in the thermal tests following doses that produced little or no motor dysfunction. In the tail-flick test, the antinociceptive effect was attenuated at higher doses, resulting in a bell-shaped dose-response relationship. Dose-related antinociception was found in both the first and second phase of the formalin test following doses from 0.25 up to 1 nmol. The present study shows that the competitive NMDA antagonist CPP has an antinociceptive effect in doses that do not affect motor function. Furthermore, antinociception was evident in both phasic and tonic nociceptive tests. Finally, the dose-response relationship in the tail-flick test was bell-shaped. As discussed this indicates that NMDA receptors may be involved in functionally divergent nociceptive systems.