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

Possible Role of N-Methyl-D-Aspartate Receptors in Physiology and Pathophysiology of Cardiovascular System

N-methyl-D-aspartate (NMDA) receptors belong to ionotropic glutamate receptor family, together with α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, kainite receptors and δ-receptors. All of these receptors are tetramers composed of four subunits. NMDA receptors have several unique features in relation to other ionotropic glutamate receptors: requirement for simultaneous action of two coagonists, glutamate and glycine; dual control of receptor activation, ligand-dependent (by glutamate and glycine) and voltage-dependent (Mg2+ block) control; and influx of considerable amounts of Ca2+ following receptor activation. Increasing number of researches deals with physiological and pathophysiological roles of NMDA receptors outside of nerve tissues, especially in the cardiovascular system. NMDA receptors are found in all cell types represented in cardiovascular system, and their overstimulation in pathological conditions, such as hyperhomocysteinemia, is related to a range of cardiovascular disorders. On the other hand we demonstrated that blockade of NMDA receptors depresses heart function. There is a need for the intensive study of NMDA receptor in cardiovascular system as potential theraputical target both in prevention and treatment of cardiovascular disorders.

Modulation of N-methyl-d-aspartate receptors in isolated rat heart

Considering the limited data on the role of NMDA-Rs in the cardiovascular system, the aim of the present study was to examine the effects of NMDA and DL-Hcy TLHC, alone and in combination with glycine, memantine, and ifenprodil, in the isolated rat heart. The hearts of Wistar albino rats were retrogradely perfused according to the Langendorff technique at a constant perfusion pressure. The experimental protocol for all experimental groups included the stabilization period, application of estimated substance for 5 min, followed by a washout period of 10 min. Using a sensor placed in the left ventricle, we registered the following parameters of myocardial function: dp/dt_max, dp/dt_min, SLVP, DVLP, HR; CF was measured using flowmetry). We estimated the following oxidative stress biomarkers in the coronary venous effluent using spectrophotometry: TBARS, NO₂^-, O₂^-, and H₂O₂. NMDA alone did not induce any change in any of the observed parameters, while DL-Hcy TLHC alone, as well as a combined application of NMDA and DL-Hcy TLHC with glycine, induced a reduction of most cardiodynamic parameters. Memantine and ifenprodil induced a reduction of cardiodynamic parameters and CF, as well as some oxidative stress biomarkers.

The potential of targeting NMDA receptors outside the CNS

INTRODUCTION

NMDA receptor (NMDAR) is an ionotropic glutamate receptor with a high permeability to calcium and a unique feature of controlling numerous calcium-dependent processes. Apart from being widely distributed in the CNS, the presence of NMDAR and its potential significance in a variety of non-neuronal cells and tissues has become an interesting research topic.

AREAS COVERED

The current review summarizes prevailing knowledge on the role of NMDARs in the kidney, bone and parathyroid gland, three main organs responsible for calcium homeostasis, as well as in the heart, an organ whose function is highly dependable on balanced intracellular calcium concentrations. The review also examines studies that have advanced our understanding of the therapeutic potential of NMDAR agonists and antagonists in renal, cardiovascular and bone pathologies.

EXPERT OPINION

NMDARs have a preeminent role in many physiological and pathological processes outside the CNS. In certain organs and/or disease conditions, activating the NMDAR leads to beneficial effects for the target organ, whereas in other diseases cell signaling downstream of NMDAR activation can exacerbate their pathology. Therefore, targeting NMDARs therapeutically is rather intricate work, and surely requires more extensive investigation in order to properly tune up the diverse NMDAR's actions translating them into beneficial cellular responses.

The effects of the modulation of NMDA receptors by homocysteine thiolactone and dizocilpine on cardiodynamics and oxidative stress in isolated rat heart

In light of the limited data concerning the role of N-methyl-D-aspartate (NMDA) receptors in cardiac function, the aim of the present study was to determine the role of NMDA receptors in cardiac function, as well as the possible role played by the oxidative stress induced by the overstimulation of NMDA receptors in isolated rat heart. The hearts of male, Wistar albino rats (n = 24, 12 in each experimental group, BM 180-200 g) were retrogradely perfused at a constant perfusion pressure (70 cm H₂O₂), using the Langendorff technique, and cardiodynamic parameters were determined during the subsequent administration of DL-homocysteine thiolactone (DL-Hcy TLHC) alone, the combination of DL-Hcy TLHC and dizocilpine (MK-801), and MK-801 alone. In the second experimental group, the order of the administration of each of the substances was reversed. The oxidative stress biomarkers, including thiobarbituric acid reactive substances (TBARS), NO(2)(-), O(2)(-) and H₂O₂, were each determined spectrophotometrically. DL-Hcy TLHC and MK-801 depressed cardiac function. DL-Hcy TLHC decreased oxidative stress, a finding that contrasted with the results of the experiments in which MK-801 was administered first. The findings of this study were suggestive of the likely role played by NMDA receptors in the regulation of cardiac function and coronary circulation in isolated rat heart.

Functional NMDA receptors in rat erythrocytes

N-methyl-d-aspartate (NMDA) receptors are ligand-gated nonselective cation channels mediating fast neuronal transmission and long-term potentiation in the central nervous system. These channels have a 10-fold higher permeability for Ca2+compared with Na+or K+and binding of the agonists (glutamate, homocysteine, homocysteic acid, NMDA) triggers Ca2+uptake. The present study demonstrates the presence of NMDA receptors in rat erythrocytes. The receptors are most abundant in both erythroid precursor cells and immature red blood cells, reticulocytes. Treatment of erythrocytes with NMDA receptor agonists leads to a rapid increase in intracellular Ca2+resulting in a transient shrinkage via Gardos channel activation. Additionally, the exposure of erythrocytes to NMDA receptor agonists causes activation of the nitric oxide (NO) synthase facilitating either NO production in l-arginine-containing medium or superoxide anion (O2·−) generation in the absence of l-arginine. Conversely, treatment with an NMDA receptor antagonist MK-80, or the removal of Ca2+from the incubation medium causes suppression of Ca2+accumulation and prevents attendant changes in cell volume and NO/O2·−production. These results suggest that the NMDA receptor activity in circulating erythrocytes is regulated by the plasma concentrations of homocysteine and homocysteic acid. Moreover, receptor hyperactivation may contribute to an increased incidence of thrombosis during hyperhomocysteinemia.

Identification of a homocysteine receptor in the peripheral endothelium and its role in proliferation

BACKGROUND

Homocysteine, a risk factor for atherosclerosis, increases intimal hyperplasia after carotid endarterectomy with associated smooth muscle cell proliferation and modulation of cytokines. The N-methyl-D-aspartate receptor (NMDAr), a glutamate-gated ion channel receptor, is associated with homocysteine-induced cerebrovascular injury; however, the receptor has not been identified in peripheral vascular cells, nor has any interaction with homocysteine been clarified. Our objectives were first, to identify NMDAr in rat carotid artery and rat aorta endothelial cells (RAEC); and second, to determine whether homocysteine activates NMDAr in the endothelium.

METHODS

NR1 and NR2A, two NMDAr subunits, were probed in rat carotid arteries by immunohistochemistry. RNA was isolated from RAECs, and expression of all NMDAr subunits (NR1, 2A, 2B, 2C, and 2D) were examined by RT-PCR and sequencing. For receptor protein expression, RAEC were incubated with different homocysteine concentrations and incubation times and also were treated with 50 microM homocysteine and/or preincubated with 50 microM dizocilpine MK-801, an NMDAr inhibitor.

RESULTS

Both NR1 and NR2A were expressed in rat carotid arteries. All NMDAr subunits were expressed in the RAECs, and there was 92% to 100% similarity compared with rat NMDAr from the National Center for Biotechnology Information (NCBI) GenBank. Homocysteine upregulated NR1 expression and increased cell proliferation. RAEC pretreatment with MK-801 reduced homocysteine-mediated cell proliferation.

CONCLUSION

This study is the first to show that NMDAr exists in the peripheral vasculature, and that homocysteine may act via NMDAr to increase intimal hyperplasia.

CLINICAL RELEVANCE

Our objectives included the identification of a homocysteine receptor in the peripheral vasculature. The possible inhibition of a homocysteine receptor to prevent intimal hyperplasia rather than treat established stenosis would make a significant clinical impact. This will open further avenues of study in determining the role of homocysteine in the pathogenesis of intimal hyperplasia.

Altered NMDA receptor expression in renal toxicity: Protection with a receptor antagonist

BACKGROUND

The N-methyl-d-aspartate (NMDA) receptor is expressed in the kidney. The receptor plays a major role in gentamicin ototoxicity. We assessed the role of the renal NMDA receptor subunits NR1 and NR2C in a model of gentamicin nephrotoxicity.

METHODS

Rats were exposed to either saline (control), high-dose, short-term gentamicin, or short-term gentamicin plus the NMDA antagonist MK-801 (short-term gentamicin + MK-801) for 3 days.

RESULTS

Real-time reverse transcription-polymerase chain reaction (RT-PCR) revealed that NR1 mRNA expression was significantly higher (P= 0.03) in the renal cortex of short-term gentamicin rats. NR2C subunit mRNA expression was unaltered in short-term gentamicin rats. Western blot analysis revealed that NR1 (P= 0.009) and NR2C (P= 0.003) protein abundance was significantly higher in the renal cortex short-term gentamicin rats. We assessed two potential intracellular pathways that may mediate short-term gentamicin/NMDA. Calpain I and II expression was similar in short-term gentamicin and control rats. Endothelin type B receptor (ETBR) expression was significantly increased in the renal cortex of short-term gentamicin rats (P= 0.0003), and urinary nitrite concentration (reflecting nitric oxide) was significantly increased in short-term gentamicin rats (P= 0.03). Serum creatinine was significantly elevated in short-term gentamicin animals (P= 0.03), and this increase was attenuated in short-term gentamicin + MK-801 rats. Blood pressure was higher in short-term gentamicin rats; this was attenuated in short-term gentamicin + MK-801 rats. Urine pH was significantly lower in short-term gentamicin (P < 0.0001) rats; this was reversed in short-term gentamicin + MK-801 (P= 0.005) rats. Urinary nitrite was significantly higher in short-term gentamicin rats; this was normalized in short-term gentamicin + MK-801 rats. MK-801 alone had no effect on clinical parameters.

CONCLUSION

NMDA receptor subunit expression is increased in short-term gentamicin animals, and the receptor likely mediates cell damage via the endothelin-ETBR-nitric oxide pathway. NMDA antagonism ameliorated renal damage after exposure to short-term gentamicin.

Expression and developmental regulation of the NMDA receptor subunits in the kidney and cardiovascular system

Antagonists to the N-methyl-D-aspartate (NMDA) receptor bind to various extraneuronal tissues. We therefore assessed the expression of the main NMDA subunit, NR1, in various tissues. We demonstrate that NR1 appears to be most abundant in the rat kidney and heart. NR1 is present in total rat kidney, cortex, and medulla. Of the NR2 subunits, only the NR2C subunit protein is present in the kidney. The abundance of the NR1 subunit protein increases with kidney development. Both NR1 and NR2C are present in opossum kidney, Madin-Darby canine kidney, and LLC-PK(1) cells. Immunohistochemistry studies show that the NR1 subunit is present in the renal proximal tubule. NR1 is abundant in the atrium and ventricle but is also expressed in the aorta and pulmonary artery. The NR2 subunits are not expressed in the heart. NR1 subunit protein expression is constant throughout heart development. Finally, the NR1 subunit protein is expressed in heart cells (H9c2) grown in culture. These studies reveal the presence of the NMDA receptor in the kidney and the cardiovascular system.

Epithelium-dependent effect of L-glutamate on airways: involvement of prostaglandins

We investigated the effect of the excitatory amino acid (EAA) receptor agonists L-glutamate, N-methyl-D-aspartate (NMDA), (RS)-a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainic acid on KCl-induced contractions of rabbit tracheal smooth muscle, as well as the role of epithelium and endogenously produced nitric oxide and prostaglandins on these responses. L-Glutamate decreased KCI-induced contractions up to 30%. This effect was attenuated by epithelium removal, tetrodotoxin, methylene blue and indomethacin but not by NG-nitro-L-arginine methyl ester. While NMDA, AMPA and kainic acid had no effect, the combination of NMDA + kainic acid decreased KCI-induced contractions. These results suggest that, in rabbit trachea, L-glutamate has, at least in part, an epithelium-dependent effect mediated via prostaglandin formation and that the EAA receptors involved are non-classical.

Glutamate signalling in non-neuronal tissues

Since the discovery of its role in the CNS, glutamate, together with its involvement in signalling at synapses, has been the subject of a vast amount of research. More recently, it has become clear that glutamate signalling is also functional in non-neuronal tissues and occurs in sites as diverse as bone, pancreas and skin. These findings raise the possibility that glutamate acts as a more widespread 'cytokine' and is able to influence cellular activity in a range of tissue types. The impact of these discoveries is significant because they offer a rapid way to advance the development of therapeutics. Agents developed for use in neuroscience applications might be beneficial in the modulation of pathology peripherally, impacting on conditions such as osteoporosis, diabetes and wound healing.

Hypothermia prevents biphasic glutamate release and corresponding neuronal degeneration after transient spinal cord ischemia in the rat

1. Spinal cord ischemia evoked a biphasic increase in CSF-Glu during 20 min of ischemia (40%) and at 2 hr after reperfusion (70%) in the nontreated group that was attenuated by all treated groups. But MK-801 (15 micrograms i.t.) did not affect the increased Glu at 2 hr (80%). 2. The argyrophilia observed in laminae II-V at 8 hr after reperfusion was attenuated by hypothermia (33 degrees C) and combination with MK-801, but the attenuation was less with MK-801. 3. Mild hypothermia attenuated the biphasic increase in CSF-Glu and corresponding development of neuronal damage after spinal cord ischemia. 4. Mild hypothermia with NMDA antagonism did not yield any further effects, suggesting that hypothermia itself plays a pivotal role in the protection.

The effect of intrathecal magnesium sulphate on nociception in rat acute pain models

We examined the antinociceptive effect of intrathecally administered magnesium sulphate (MgSO4) in rats, using acute pain models including mechanical pressure, heat and subcutaneous formalin injection. According to the locomotion test 10 microliters of 6.2% MgSO4 did not produce motor paralysis. At the same dose, responses to pressure and heat were intact, compared with controls given saline. MgSO4 produced depression of pain responses only after the first 10 min in the formalin test. Our studies indicated that MgSO4 did not show remarkable antinociceptive effects in acute pain models.

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

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

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.

Association of nerve growth factor mRNA levels with MK-801-induced explosive behaviors in mice

MK-801, a noncompetitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, stimulated an outbred strain of NIH Swiss mice to display discrete episodes of explosive jumping behavior, designated as "popping." The rapid onset of the MK-801-induced "popping" seems to follow the rapid distribution of the drug to the frontal cortex, the area that contains high levels of NMDA receptors. We examined the effect of this drug on the levels of mRNA coding for nerve growth factor (NGF) in the frontal cortex in relation to the exhibited "popping" episodes. Mice treated with 1 mg/kg MK-801 could be split into two groups based on the total number of "popping" episodes in a 30 min post-injection period. These groups also differed in the steady-state levels of frontal cortex NGF mRNA. Animals that exhibited low numbers of "popping" had levels of NGF mRNA significantly higher than saline treated controls or mice that exhibited high numbers of "popping." Mice treated with 10 mg/kg MK-801 had a high frequency of "popping" that was impossible to separate into episodes. In addition, these mice had levels of frontal cortex NGF mRNA that were significantly lower than either group of mice treated with 1 mg/kg MK-801. These data indicated that there was an increased level of NGF mRNA under conditions where MK-801 induced a low level of "popping" behavior. However, when "popping" intensified, NGF mRNA levels were decreased, suggesting a possible behavioral antagonism of the NGF response.

Neuroprotection by N-methyl-D-aspartate antagonists in focal cerebral ischemia is dependent on continued maintenance dosing

While N-methyl-D-aspartate antagonists have been shown to attenuate neuronal damage in focal cerebral ischemia, few studies have examined whether continuous or multiple dose treatment is necessary for maximum efficacy. We studied the effect of a loading dose only or load plus maintenance infusion using several non-competitive N-methyl-D-aspartate antagonists (dextromethorphan, dextrorphan, MK-801) and the levorotatory enantiomer of dextromethorphan (levomethorphan) in a rabbit model of focal cerebral ischemia. Forty-seven anesthetized rabbits underwent occlusion of the left internal carotid, anterior cerebral and middle cerebral arteries for 2 h followed by 4 h of reperfusion. Drugs were administered 10 min after occlusion. Dextromethorphan and dextrorphan protected against ischemic edema only when given as load plus maintenance (29% and 31% reduction, respectively), while both load only and load plus maintenance of MK-801 protected against edema (26% and 31% reduction, respectively). Levomethorphan load plus maintenance also protected against ischemic edema (25% reduction). However, dextromethorphan and dextrorphan both required maintenance infusion to protect against ischemic neuronal damage (24% and 27% reduction in area of ischemic neuronal damage, respectively), while levomethorphan failed to protect against neuronal injury even when given as load plus maintenance. Administration of MK-801 as load plus maintenance reduced ischemic neuronal damage by 23%, but this difference was not quite statistically significant. These results suggest that processes of ischemic damage, such as excitotoxic injury, continue for several hours beyond the initial period of focal ischemia, and that non-competitive N-methyl-D-aspartate antagonists require more prolonged administration to achieve neuroprotection.(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.

Effects of competitive and non-competitive N-methyl-D-aspartate (NMDA) antagonists in squirrel monkeys trained to discriminate D-CPPene (SDZ EAA 494) from vehicle

Drug discrimination studies have proven useful for comparing and contrasting the behavioral effects of site-selective N-methyl-D-aspartate (NMDA) antagonists. This study examined the effects of competitive and non-competitive NMDA antagonists in squirrel monkeys trained to discriminate 1 mg/kg D-CPPene [D-3-(2-carboxypiperazine-4-yl)-1- propenyl-1-phosphonic acid; SDZ EAA 494] from vehicle in a two-lever drug discrimination procedure. Results show that D-CPPene and several other competitive NMDA antagonists (NPC 17742, CGS 19755, and CGP 37849) completely substituted for D-CPPene in a dose-dependent manner. In contrast, phencyclidine (PCP) and ketamine produced only partial substitution at doses that severely suppressed response rates. These results are consistent with results of earlier studies with rats and monkeys showing differences in the discriminative stimulus effects of competitive and PCP-like non-competitive NMDA antagonists. The data support the predictions (1) that D-CPPene and the other competitive NMDA antagonists tested would have similar subjective effects in humans and (2) that some differences would be found in the subjective effects of competitive NMDA antagonists and PCP-like non-competitive antagonists.

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.

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.