Antinociceptive actions of different classes of excitatory amino acid receptor antagonists in mice

The utility of excitatory amino acid (EAA) antagonists as analgesic agents. I. Comparison of the antinociceptive activity of various classes of EAA antagonists in mechanical, thermal and chemical nociceptive tests

The present study was performed to assess the utility of excitatory amino acid (EAA) antagonists as analgesia agents. The antinociceptive activity of various classes of EAA antagonists was assessed in mechanical and thermal flexion reflexes tests, as well as in the formalin test. Additional testing assessed the motor dysfunction associated with antinociceptive dose levels of the agents used, by examining placing, grasping and righting reflexes, as well as occurrences of balance loss during locomotion. No antinociceptive activity was observed on any of the nociceptive measures for the non-NMDA receptor antagonists CNQX or L-AP-3. High doses of the non-competitive (PCP-site) NMDA receptor antagonist MK-801 and the allosteric-glycine receptor antagonist 7-CKA produced antinociception on both the mechanical and thermal flexion reflex measures, while a high dose of the competitive NMDA receptor antagonist CPP produced antinociception only on the thermal flexion reflex measure. Hyperalgesic effects on thermal flexion reflexes were obtained with all doses of the polyamine receptor antagonist ARCA, and with the highest dose of the allosteric-glycine receptor antagonist FICA. Formalin nociceptive behaviours were significantly reduced only by high doses of competitive (APV) and non-competitive (MK-801) NMDA receptor antagonists. The doses of EAA receptor antagonists which produced antinociceptive effects on any of the 3 nociceptive tests also produced evidence of motor dysfunction. Both competitive NMDA receptor antagonists (APV and CPP) produced disruptions of placing, grasping and righting reflexes, while 2 of the allosteric-glycine receptor antagonists (7-CKA and DCQX) significantly disrupted placing and righting reflexes.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Increased N-methyl-D-aspartate (NMDA) activity in the mouse spinal cord following morphine does not mediate opioid withdrawal

N-Methyl-D-aspartate (NMDA) receptors have been proposed to play a role in opioid tolerance and dependence. The present study was designed to determine whether the increased NMDA activity in the spinal cord, unmasked by naloxone in morphine-pretreated mice, reflects activity leading to opioid withdrawal. Behavioral responses to intrathecal injections of NMDA were inhibited by pretreatment (2 h) with morphine (10 mg/kg i.p.), but enhanced following morphine when naloxone was injected together with NMDA. Although injected at doses that inhibited NMDA activity, the excitatory effects of morphine on NMDA-induced behaviors were prevented by dizocilpine (MK-801), a phencyclidine (PCP) ligand, but not by 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1 phosphonic acid (CPP), a competitive NMDA antagonist. MK-801 also inhibited naloxone-induced withdrawal jumping, however, just as CPP failed to affect morphine-induced changes in MMDA-induced behaviors, CPP also failed to inhibit withdrawal jumping. Together these data indicated that withdrawal from acute opioid dependence correlates with, but is not mediated by enhanced NMDA activity.

Alteration by urethane of glutamatergic control of micturition

The i.v. administration of MK-801 (0.001-3 mg/kg), a non-competitive NMDA receptor antagonist, did not alter reflex bladder activity in unanesthetized decerebrate rat recorded during fast infusion (0.21 ml/min) cystometry or under isovolumetric conditions, but did depress reflex bladder contractions in doses between 0.1 and 3 mg/kg i.v. in the urethane-anesthetized (1.2 g/kg s.c.) intact rat during fast infusion cystometry. The ED50 and the dose to produce maximal inhibition in urethane-anesthetized intact rats were 0.25 mg/kg and 3 mg/kg i.v., respectively. During slow infusion (0.04 ml/min) cystometry, in unanesthetized decerebrate rats, MK-801 (0.1-1 mg/kg i.v. or 6-60 micrograms i.t.) decreased by 12-44% the micturition volume threshold (VT) but did not change the amplitude and duration of the bladder contractions. The administration of a larger i.t. dose (60 micrograms) of MK-801 produced no further decrease in VT but decreased the amplitude of bladder contractions by 24%. External urethral sphincter electromyogram activity was reduced or abolished by MK-801 (0.01-3 mg/kg i.v.) in both unanesthetized decerebrate and urethane-anesthetized intact rats with ED50 of 0.12 mg/kg and 0.05 mg/kg, respectively. These results indicate that NMDA receptors play an important role in both facilitatory and inhibitory central neural control of voiding function and that there is a significant interaction between urethane anesthesia and NMDA glutamatergic transmission. Thus, even though urethane anesthesia has been useful for studying the physiological characteristics of the micturition reflex, it seems inappropriate for analyzing the normal transmitter role of glutamic acid in reflex voiding.

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.

Chemically-diverse ligands at the glycine B site coupled to N-methyl-D-aspartate (NMDA) receptors selectively block the late phase of formalin-induced pain in mice

The glycine B receptor partial agonists L 687,414, D-cycloserine and (+)-HA 966, and the glycine B receptor antagonists MDL 29,951 and 5,7-dichloro-2,4 dihydroxy-3-phenyl-quinoline dione (DCPQ) dose-dependently inhibited the late phase (LP) of formalin-induced licking (FIL) elicited by intraplantar formalin in mice at doses exerting little motor disruption in the rotarod test. In distinction, the early phase (EP) of FIL and the writhing response to intra-abdominal acetic acid were little influenced and, irrespective of stimulus intensity, they failed to modify the tail-flick response to phasic, thermal or mechanical stimulation of the tail. In contrast to glycine B ligands, competitive antagonists at the NMDA receptor recognition site (CPP, CGS 19755, CGP 34879 and 39551) and blockers of the associated ion channel ((+)-MK 801, (-)-MK 801, memantine and ketamine) all blocked both the LP and EP of FIL and induced ataxia at comparable doses. In conclusion, normalization of transmission at NMDA receptors by inhibition of the coupled glycine B site preferentially elicits antinociception against prolonged (chemical) noxious stimulation in the absence of a marked influence upon motor coordination.

Role of excitatory amino acid receptors in the mediation of the nociceptive response to formalin in the rat

The relative contribution of the NMDA/glycine allosteric site and non-NMDA (AMPA) types of glutamate receptor to the acute and tonic phases of the behavioural nociceptive response to formalin has been studied in the rat. The AMPA receptor selective antagonist NBQX preferentially inhibited the acute phase indicating that AMPA receptors may be involved in mediating fast acute nociceptive transmission in the dorsal horn. In contrast, the strychnine-insensitive glycine site partial agonist (+)-HA-966 and the NMDA competitive antagonist CGS 19755 preferentially attenuated the tonic nociceptive phase. However, none of these compounds exhibited anti-inflammatory properties. Thus, both NMDA and non-NMDA antagonists can selectively block changes in neuronal excitability while tissue injury in the receptive field continues to evolve.

Morphine modulates excitatory amino acid-induced activity in the mouse spinal cord: short-term effects on N-methyl-D-aspartate (NMDA) and long-term effects on kainic acid

Excitatory amino acid (EAA) antagonists and phencyclidine (PCP) ligands inhibit the development of morphine tolerance and dependence. The present study tested the hypothesis that opioids increase EAA-induced activity by monitoring morphine's effect on the caudally-directed biting and scratching behaviors produced in mice by intrathecal (i.t.) injections of either N-methyl-D-aspartate (NMDA) or kainic acid (KA). A single injection of 10 mg/kg of morphine i.p. had no effect on the intensity of behaviors induced 2 h later by KA but inhibited NMDA-induced activity. Pretreatment with 100 mg/kg of morphine i.p. 24 h before testing did not alter NMDA-induced behaviors, but attenuated sensitization to repeated injections of KA, which is thought to reflect activation of primary afferent C-fibers. Coadministration of 0.1 microgram of naloxone with EAAs did not alter responses to either NMDA or KA in control mice, however, 2 h after 10 mg/kg of morphine, inclusion of naloxone potentiated NMDA-induced activity without altering responses to KA. 24 h after 100 mg/kg of morphine, naloxone, coadministered with KA, was also able to reverse the inhibitory effect of morphine pretreatment on KA-induced activity. In summary, morphine produces short-term inhibitory and excitatory effects on NMDA-induced activity, the latter of which is unmasked by naloxone. Morphine has no immediate effect on KA-induced activity but is able to bring about a long-term inhibition of sensitization to KA, an effect that is reversed by naloxone. Activity along pathways activated by NMDA and KA may, therefore, contribute to different aspects of opioid tolerance or withdrawal.

The role of nitric oxide in the development and maintenance of the hyperalgesia produced by intraplantar injection of carrageenan in the rat

Activation of the N-methyl-D-aspartate (NMDA) receptor has been reported to be involved in the mechanisms that underlie thermal hyperalgesia produced by the intraplantar injection of carrageenan. As NMDA-mediated thermal hyperalgesia produced in models of acute and persistent pain have been reported to involve production of nitric oxide, we examined the role of nitric oxide in both the development and maintenance of the thermal hyperalgesia produced by the intraplantar injection of carrageenan. In addition, we examined the role of nitric oxide in the maintenance of the mechanical hyperalgesia produced by intraplantar injection of carrageenan. Prior to the intraplantar injection of carrageenan (2 mg in 100 microliters) there was no significant difference in thermal withdrawal latencies or mechanical withdrawal thresholds between the left and right hindpaws. Three hours after injection of carrageenan into the left hindpaw, rats showed evidence of a significantly faster thermal withdrawal latency and lower mechanical withdrawal threshold of the left hindpaw compared to the right hindpaw. In addition, the left hindpaw was significantly increased in size (diameter) compared with the right hindpaw. In these same rats, the intrathecal administration of saline, NG-nitro-L-arginine methyl ester (L-NAME; 2-200 nmol) or the inactive enantiomer, NG-nitro-D-arginine methyl ester (D-NAME; 200 nmol) did not produce any significant change in thermal nociceptive withdrawal latencies in the non-injected paw. However, administration of L-NAME (2-20 nmol), but not saline or D-NAME produced a dose dependent and reversible block of the thermal hyperalgesia for a period of up to 3 h.(ABSTRACT TRUNCATED AT 250 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.

Involvement of glutamate receptors in allodynia induced by prostaglandins E2 and F2 alpha injected into conscious mice

In order to investigate the involvement of glutamate receptor systems in allodynia induced by prostaglandin (PG) E2 or F2 alpha, we co-administered antagonists for N-methyl-D-aspartate (NMDA), non-NMDA, or metabotropic glutamate receptors intrathecally with PGE2 or PGF2 alpha and examined their effects on the allodynia evoked in conscious mice by non-noxious brushing of the flanks. MK-801, a non-competitive NMDA receptor channel blocker, and D-AP-5, a selective NMDA receptor antagonist, dose-dependently blocked PGE2-induced allodynia with an IC50 of 1.60 and 0.52 microgram/mouse, respectively. A glycine binding-site antagonist for the NMDA receptor, 7-Cl-KYNA, did not influence it. None of these NMDA receptor antagonists inhibited PGF2 alpha-evoked allodynia. Non-NMDA receptor antagonists GAMS and CNQX inhibited both PGE2- and PGF2 alpha-induced allodynia. On the other hand, L-AP-3 and L-AP-4, putative metabotropic glutamate receptor antagonists, dose-dependently antagonized the allodynia induced by PGF2 alpha with an IC50 of 0.92 and 3.26 ng/mouse, respectively, but not that induced by PGE2. Intrathecal administration of L-glutamate produced allodynia over a wide range of low doses from 0.1 pg to 0.1 microgram/mouse, and the maximal effect was observed at 1 ng. Similar to allodynia induced by prostaglandins, the response lasted over a 50-min experimental period. These results demonstrate that both PGE2- and PGF2 alpha-evoked allodynia are mediated through a pathway that includes the glutamate receptor system but that subtypes of glutamate receptors involved and sites of action in the spinal cord may be different between them.

Antinociception induced by 3-((+-)-2-carboxypiperazin-4-yl)-propyl-1- phosphonic acid (CPP), an N-methyl-D-aspartate (NMDA) competitive antagonist, plus 6,7-dinitroquinoxaline-2,3-dione (DNQX), a non-NMDA antagonist, differs from that induced by MK-801 plus DNQX

Excitatory amino acid receptors have been implicated in mediating pain. 3-((+-)-2-Carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), a competitive N-methyl-D-aspartate (NMDA) antagonist and MK-801, a phencyclidine (PCP) ligand and non-competitive NMDA antagonist, were injected intrathecally in mice alone or in combination with 6,7-dinitroquinoxaline-2,3-dione (DNQX), a non-NMDA antagonist. When tested in the formalin model of pain, antinociception following CPP plus DNQX was greater than that after MK-801 plus DNQX in both the acute and tonic phases. These dissimilarities are not consistent with activity of CPP and MK-801 at the same sites in the spinal cord.

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.

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.

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.

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

We have previously reported that the response latency in the mouse hot-plate test is affected differently by spinal intrathecal (i.t.) injection of competitive and non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists, in that only the former produces an antinociceptive effect. Since the lipophilic non-competitive antagonists will redistribute rapidly from the spinal injection site, it is conceivable that they reach sites where they counteract the spinal antinociceptive effect. In the present study, we have tested this hypothesis by comparing the antinociceptive effect of the competitive NMDA receptor antagonist CGS 19755 and the non-competitive NMDA receptor antagonist MK-801 after i.t., intraperitoneal (i.p.) and intracerebroventricular (i.c.v.) administration as well as after combinations thereof. CGS 19755 injected i.p. or i.c.v. and MK-801 injected i.p. or i.t. attenuated the antinociceptive effect of i.t. injected CGS 19755. Both i.p. and i.c.v. administration of either CGS 19755 or MK-801 dose-dependently impaired motor function without producing antinociceptive effects. Thus, the effect of CGS 19755 and MK-801 on the motor system was found to be separate from their antinociceptive effect. In a separate experiment, changes in hind-paw skin temperature were excluded as a possible confounding factor. These findings demonstrate that supraspinal systems can limit the spinal antinociceptive effect of NMDA receptor antagonists.

Antinociceptive effects in the formalin and capsaicin tests after intrathecal administration of substance P analogues in mice

The antinociceptive effect of substance P- and substance P-(6-11) analogues containing D-histidine (D-His) in position 9 was examined in mice in the formalin and capsaicin tests. [D-Arg1,D-Trp7,9,Leu11]substance P (spantide) was used as reference drug. Intrathecal injections of the [D-His9]substance P and substance P-(6-11) analogues at 4.0 nmol resulted in no significant antinociception as measured in the 2.0% formalin test, although spantide was antinociceptive in the early and late phases. The early response induced by 0.0625% formalin was reduced significantly by the [D-His9]substance P and substance P-(6-11) analogues at 4.0 nmol, which were less potent than spantide. The antinociception induced by spantide and a few analogues of substance P and substance P-(6-11) containing D-His was reversed significantly by pretreatment with 2 mg/kg naloxone, an opioid antagonist. The nociceptive response to capsaicin was inhibited significantly by lower doses (2.0 nmol) of the analogues. The antinociception evoked by the analogues was not reversed by naloxone in the capsaicin test. Co-injection of the [D-His9]substance P and substance P-(6-11) analogues at 2.0 nmol selectively decreased substance P-induced licking, biting and scratching without affecting the behavioural responses to NK2 and NK3 receptor agonists. Spantide non-selectively inhibited the behavioural responses produced by not only substance P, but also neurokinin A, D-septide, neurokinin B and eledoisin. The data show that the capsaicin test may be a better method for evaluating neurokinin antagonists than the formalin test.

The NMDA antagonist Memantine blocks pain behavior in a rat model of formalin-induced facial pain

Recent studies have provided evidence that excitatory amino acid antagonists can exert analgesic effects in animals. These studies, however, have focused primarily on phasic pain or hyperalgesia rather than tonic pain. The present study evaluates the effects of systemic administration of Memantine (1-amino-3,5-dimethyl-adamantane), a clinically used N-methyl-D-aspartate (NMDA) receptor antagonist, on formalin-induced phasic and tonic pain behavior in the rat. Memantine (2.5, 5.0, 10.0 and 20.0 mg/kg) or normal saline was injected i.p. 1 h prior to a s.c. injection of formalin (5%, 50 microliters) into the vibrissal pad of adult rats (n = 5/group). Pain behavior was measured by the number of seconds of formalin-induced face grooming during a 42-min post-injection observation period. Saline-injected animals displayed a biphasic face-grooming response, consisting of an early, phasic phase (0-6 min) and a delayed, prolonged tonic phase (12-42 min). Memantine at doses of 2.5-10 mg/kg produced a significant dose-related inhibition of the second phase (65-93%) and a much smaller inhibition of the first phase (up to 52%). A higher dose (20 mg/kg) further inhibited both phases but also produced other motor effects (increased exploratory and decreased freezing behavior, hind-paw weakness and gait ataxia) which were not observed at the lower doses. These results suggest that the NMDA receptor antagonist Memantine can block formalin-induced tonic and, to a lesser extent, phasic pain, at doses that do not alter observed motor behaviors.

Regulation of sigma activity by the amino-terminus of substance P in the mouse spinal cord: involvement of phencyclidine (PCP) sites not linked to N-methyl-D-aspartate (NMDA) activity

Behavioral responses to kainic acid (KA) injected intrathecally in mice are enhanced by N-but not C-terminal fragments of substance P (SP). Repeated injections of KA result in sensitization to KA-induced activity, an effect that appears to be mediated by SP N-terminal activity and inhibited by PCP ligands. The present study was initiated to determine whether the ability of SP N-terminal fragments to enhance KA activity is also sensitive to PCP ligands. We compared the effect of a PCP ligand, dizocilpine (MK-801), to that of haloperidol, a sigma ligand and dopamine antagonist. MK-801 (1 nmol) failed to alter the enhancement of behavioral responses to KA (25 pmol) produced by SP(1-7) (22.5 pmol, 30 min). However, pretreatment with 1 nmol of either haloperidol or the N-terminal SP antagonist, [D-Pro2-D-Phe7]SP(1-7) [D-SP(1-7)], prevented potentiation of KA by SP(1-7). Like SP(1-7), 5 nmol of the sigma ligand 1,3-di(2-tolyl)guanidine (DTG) also enhanced behaviors elicited by KA, and this effect was also blocked by haloperidol or D-SP(1-7), but not spiperone (2.5 nmol), a dopamine antagonist. Together these data suggest that sigma receptors are involved in the potentiation of KA. A large dose of SP(1-7) (10 nmol) or DTG (20 nmol) did not alter the response to KA 24 hr later, yet further potentiated responses to KA 30 min after SP(1-7) (22.5 pmol) or DTG (5 nmol), suggesting sensitization to the effects of these compounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Spinal cord morphology and antinociception after chronic intrathecal administration of excitatory amino acid antagonists in the rat

Drugs that antagonize the action of excitatory amino acids on the NMDA receptor in the spinal cord are of interest in pain treatment. Before such drugs can be applied clinically, their potential toxicity should be studied. This study was performed in rats in order to reveal possible neurotoxicologic side effects following chronic intrathecal (i.t.) application of two NMDA receptor antagonists: 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) and kynurenic acid (KYN). Rats equipped with i.t. catheters were injected twice a day for 2 weeks with saline, 2 nmol (0.5 micrograms) CPP or 210 nmol (40 micrograms) KYN, where the doses of CPP and KYN were chosen on the basis of similar analgesic effects after one administration. Antinociception was tested daily using the tail-flick and hot-plate tests. The antinociceptive effect was similar in CPP- and KYN-treated rats on days 1 and 2. The effect of CPP decreased during the following days, whereas that of KYN persisted for the 12-day testing period. The spinal cord was then removed and prepared for light and electron microscopic examination, and a morphometric method using an unbiased stereological estimator of cell number and cell volume was applied as a sensitive variable of spinal cord neurotoxicity. Morphologic and ultrastructural analyses of the spinal cord segment adjacent to the tip of the catheter showed normal appearance with no differences between the groups. Furthermore, no differences in cell number or cell volume in the dorsal horn were found between the groups. In conclusion, chronic i.t. administration of pharmacologically active doses of CPP and KYN in rats did not produce neurotoxic effects in the spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)

Central hyperexcitability triggered by noxious inputs

Repetitive activity in unmyelinated sensory afferent neurones, arising from electrical stimuli, tissue injury or nerve damage, can induce long-lasting sensitization in dorsal horn neurones. This process can be blocked by antagonists of the NMDA receptor. In the past year it has emerged that sensory neuropeptides and nitric oxide are also essential mediators of this phenomenon.

(+)-HA 966, a partial agonist at the glycine site coupled to NMDA receptors, blocks formalin-induced pain in mice

(+)-(1-Hydroxy-3-aminopyrrolidine-2-one) ((+)-HA 966), a partial agonist at the glycine site coupled to N-methyl-D-aspartic acid (NMDA) receptors, abolished the late phase of licking induced by injection of formalin into the hind-paw of mice; inhibitory dose50 (ID50) = 1.6 mg/kg, s.c. In contrast, it was weakly active against the first phase; ID50 = 33.3 mg/kg, s.c. Further, (+)-HA 966 was inactive in the rotarod test of ataxia. These data support a role of NMDA receptors in the transmission of prolonged noxious stimulation and suggest that partial glycine receptor agonists may exert antinociceptive properties against persistent pain.

Blockade of morphine-induced analgesia and tolerance in mice by MK-801

The effect of MK-801 on morphine-induced analgesia, tolerance and opioid binding sites was examined in mice. In analgesia studies, mice received either naloxone or MK-801. Controls were injected with saline. Mice were then injected with morphine 10 or 30 min following naloxone or MK-801, respectively, and tested for analgesia (tail flick assay) 45 min later. Pretreatment with naloxone or MK-801 blocked morphine-induced analgesia. In tolerance studies, mice were pretreated with either saline or MK-801. Thirty minutes later, mice were injected with either saline or morphine (acutely or chronically) and tested for analgesia 24 h later. Pretreatment with MK-801 partially or completely blocked the development of acute and chronic tolerance, respectively. In binding studies, MK-801 displaced [3H]naloxone poorly compared to naloxone or morphine. Together, these data suggest a role for NMDA receptors in morphine-induced analgesia and tolerance. The poor inhibition of the [3H]naloxone binding sites by MK-801 supports the possibility that MK-801 might not act directly on the opioid receptors, but rather, inhibits morphine-induced analgesia and tolerance by some other mechanisms.

MK-801 and phencyclidine act at phencyclidine sites that are not linked to N-methyl-D-aspartate activity to inhibit behavioral sensitization to kainate

Sensitization to the behavioral effects of intrathecal kainate in mice depends on an accumulation of the N-terminus of substance P in the spinal cord and may reflect similar synaptic activity as that underlying pain transmission. The purpose of this study was to determine whether kainate sensitization, like pain, is sensitive to inhibition by phencyclidine ligands. Doses that selectively inhibit the behavioral response to a single injection of N-methyl-D-aspartate, but not kainate, were established for two non-competitive antagonists, dizocilpine (MK-801) and phencyclidine, as well as two competitive antagonists, D-amino-5-phosphonovaleric acid and (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid, of N-methyl-D-aspartate. Using these doses, we found that 1 nmol of MK-801 or 3 nmol of phencyclidine blocked sensitization to four injections of 25 pmol of kainate administered at 2 min intervals. In contrast, 1.48 nmol of D-amino-5-phosphonovaleric acid and 0.5 nmol of (+/-)-3)2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid failed to alter sensitization to kainate, indicating that activation of N-methyl-D-aspartate receptors is not necessary for kainate sensitization. Haloperidol (1 nmol), a sigma receptor ligand, also failed to inhibit sensitization to kainate, suggesting that the actions of MK-801 and phencyclidine were not produced by a non-selective effect at sigma sites. Together, these data suggest that MK-801 and phencyclidine inhibit behavioral sensitization to kainate via phencyclidine receptors that are not linked to the N-methyl-D-aspartate receptor complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of excitatory amino acid receptor antagonists on a capsaicin-evoked nociceptive reflex: a comparison with morphine, clonidine and baclofen

The rat isolated spinal cord-tail preparation has been employed to examine the effects of several antinociceptive drugs and excitatory amino acid (EAA) receptor antagonists on nociceptive reflexes (recorded in ventral roots) stimulated by peripheral application of capsaicin (CAP). Non-nociceptive monosynaptic and polysynaptic dorsal root-evoked ventral root potentials (DR-VRPs) were also examined. Morphine (0.01-3 microM) and clonidine (0.03-1 microM) inhibited CAP-stimulated activity, but not the non-nociceptive dorsal root-evoked monosynaptic reflex (MSR) or polysynaptic (PSR) activity. These effects were antagonized by naloxone and efaroxan, respectively. The AMPA/KA receptor antagonists CNQX (0.1-100 microM) and DNQX (0.1-30 microM) blocked nociceptive activity and were 4-fold selective for CAP-evoked potentials compared to the monosynaptic reflex. Kynurenate (1-300 microM), DL-AP-4 (3-300 microM), L-AP-4 (3-300 microM), and the GABAB receptor agonist baclofen (0.1-10 microM), inhibited all evoked potentials with relatively little selectivity between nociceptive and non-nociceptive responses. NMDA receptor antagonism by AP-5 (100 microM) reduced nociceptive and non-nociceptive potentials by a maximum of 30-33%. These data indicate that AMPA/KA receptor-mediated synapses are involved in acute spinal nociceptive transmission and suggest that AMPA/KA receptor subtypes could provide novel analgesic targets.

Tissue distribution of two NMDA receptor antagonists, [3H]CGS 19755 and [3H]MK-801, after intrathecal injection in mice

The tissue distribution of [3H]cis-4-phosphonomethyl-2-piperidine carboxylic acid (CGS 19755) and [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (NK-801) was investigated after a single IT injection into lumbar spinal cord of mice. The level of radioactivity was analyzed in the lumbar, thoracic, and cervical spinal cord, brainstem, frontal cortex, liver, lungs, kidneys, stomach, intestine, spleen, heart, and blood from 5 min up to 6 h after injection. Within the CNS, [3H]CGS 19755 redistributed slowly from the site of injection toward the brainstem and cortex, peaking in the cortex 3-4 h after IT injection. At no time, however, did the relative level per gram of tissue in the frontal cortex exceed 10% of the relative level in the lumbar region of the spinal cord. The highest peripheral level of [3H]CGS 19755 was found in the kidneys. [3H]MK-801 redistributed rapidly from the spinal cord injection site to the peripheral organs. The highest peripheral levels of [3H]MK-801 were found in the lungs and liver, where the radioactivity peaked at 10 and 30-60 min, respectively, after injection. The relative levels of [3H]CGS 19755 were consistently higher in CNS tissues (except for the first 15 min in the frontal cortex) and blood than the corresponding levels of [3H]MK-801. The opposite relationship was true in the liver, lungs, kidneys, stomach, intestine, spleen, and heart. The effect on the response latency in the hot-plate test was quantified in the same animals immediately prior to sacrifice for the distribution study.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrathecal co-administration of substance P and NMDA augments nociceptive responses in the formalin test

The effects of intrathecal administration of substance P and N-methyl-D-aspartate (NMDA) were studied in the formalin test in mice. Both substances were administered 5 min before injection of formalin into the hind paw. Co-administration of substance P and NMDA intensified the response in both the 1st (0-10 min) and the 2nd phase (20-30 min) of the formalin test, and increased the duration of the response. The increase in the response to formalin depended on the formalin concentration and was significant with 1% and 5% concentrations of formalin but not with a 0.05% concentration. No increase in the response was observed when NMDA or substance P was given alone. These findings indicate that concurrent activation of spinal NMDA and substance P receptors induces an enhancement of spinal transmission of nociception, and that this enhancement is dependent on the intensity or the quality of the peripheral stimulus.