Competitive Noncompetitive N-methyl-D-aspartate Receptor Antagonist Research Articles

Metaplastic effects of subanesthetic ketamine on CA1 hippocampal function

Ketamine is a non-competitive N-methyl-d-aspartate receptor (NMDAR) antagonist of interest in neuropsychiatry. In the present studies, we examined the effects of subanesthetic, low micromolar ketamine on excitatory postsynaptic potentials (EPSPs), population spikes (PSs) and synaptic plasticity in the CA1 region of rat hippocampal slices. Ketamine acutely inhibited NMDAR-mediated synaptic responses with half-maximal effects near 10 μM. When administered for 15–30 min at 1–10 μM, ketamine had no effect on baseline dendritic AMPA receptor-mediated EPSPs, but persistently enhanced somatic EPSPs in the pyramidal cell body layer and augmented PS firing. Acute low micromolar ketamine also had no effect on the induction of long-term potentiation (LTP) but blocked long-term depression (LTD). Following 30 min administration of 1–10 μM ketamine, however, a slowly developing and persistent form of LTP inhibition was observed that took two hours following ketamine washout to become manifest. This LTP inhibition did not result from prolonged or enhanced NMDAR inhibition during drug washout. Effects of low ketamine on somatic EPSPs and LTP were not mimicked by a high ketamine concentration that completely inhibited NMDARs, and both of these effects were blocked by co-administration of low ketamine with a low concentration of the competitive NMDAR antagonist, 2-amino-5-phosphonovalerate or inhibitors of nitric oxide synthase. These results indicate that concentrations of ketamine relevant to psychotropic and psychotomimetic effects have complex metaplastic effects on hippocampal function that involve activation of unblocked NMDARs during ketamine exposure.

Reduction of food intake by cholecystokinin requires activation of hindbrain NMDA-type glutamate receptors

Intraperitoneal injection of CCK reduces food intake and triggers a behavioral pattern similar to natural satiation. Reduction of food intake by CCK is mediated by vagal afferents that innervate the stomach and small intestine. These afferents synapse in the hindbrain nucleus of the solitary tract (NTS) where gastrointestinal satiation signals are processed. Previously, we demonstrated that intraperitoneal (IP) administration of either competitive or noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonists attenuates reduction of food intake by CCK. However, because vagal afferents themselves express NMDA receptors at both central and peripheral endings, our results did not speak to the question of whether NMDA receptors in the brain play an essential role in reduction of feeding by CCK. We hypothesized that activation of NMDA receptors in the NTS is necessary for reduction of food intake by CCK. To test this hypothesis, we measured food intake following IP CCK, subsequent to NMDA receptor antagonist injections into the fourth ventricle, directly into the NTS or subcutaneously. We found that either fourth-ventricle or NTS injection of the noncompetitive NMDA receptor antagonist MK-801 was sufficient to inhibit CCK-induced reduction of feeding, while the same antagonist doses injected subcutaneously did not. Similarly fourth ventricle injection of d-3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphoric acid (d-CPPene), a competitive NMDA receptor antagonist, also blocked reduction of food intake following IP CCK. Finally, d-CPPene injected into the fourth ventricle attenuated CCK-induced expression of nuclear c-Fos immunoreactivity in the dorsal vagal complex. We conclude that activation of NMDA receptors in the hindbrain is necessary for the reduction of food intake by CCK. Hindbrain NMDA receptors could comprise a critical avenue for control and modulation of satiation signals to influence food intake and energy balance.

The entry of manganese ions into the brain is accelerated by the activation of N-methyl-d-aspartate receptors

Manganese-enhanced magnetic resonance imaging (MEMRI) is receiving increased interest as a valuable tool for monitoring the physiological functions in the animal brain based on the ability of manganese ions to mimic calcium ions entering to excitable cells. Here the possibility that in vivo MEMRI can detect the entry of manganese ions (Mn2+) in the brain of rats behaving without intended stimulation is tested. This hypothesis was a result of the unexpected observation that Mn2+-dependent signal enhancement was dramatically suppressed in ketamine-anesthetized rats compared with other anesthetics, such as urethane, pentobarbital and isoflurane. The effects of noncompetitive N-methyl-d-aspartate receptor (NMDAR) antagonists, ketamine and MK-801, on MEMRI for MnCl2 injected rats were examined. Treatment with MK-801 suppressed the signal enhancement more effectively than with ketamine. NMDAR agonists, glutamate (100 mg/kg) and N-methyl-d-aspartate (NMDA) (35 mg/kg), enhanced the signal intensities on MEMRI, and this signal enhancement was completely antagonized by MK-801. The systemic administration of the competitive NMDAR antagonist, D-2-amino-5-phosphono-pentanoate (D-AP5), which does not cross the blood-brain barrier (BBB), showed no effects on the signal enhancement induced by NMDA and glutamate. A selective AMPA receptor (AMPAR) antagonist, 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX), did not block the signal enhancement. These data indicated that the Mn2+-dependent signal enhancement took place as a result of the activation of glutamatergic neurons through NMDAR, but not through AMPAR in the brain.

Hindbrain administration of NMDA receptor antagonist AP-5 increases food intake in the rat

Hindbrain administration of MK-801, a noncompetitive N-methyl-D-aspartate (NMDA) channel blocker, increases meal size, suggesting NMDA receptors in this location participate in control of food intake. However, dizocilpine (MK-801) reportedly antagonizes some non-NMDA ion channels. Therefore, to further assess hindbrain NMDA receptor participation in food intake control, we measured deprivation-induced intakes of 15% sucrose solution or rat chow after intraperitoneal injection of either saline vehicle or D(-)-2-amino-5-phosphonopentanoic acid (AP5), a competitive NMDA receptor antagonist, to the fourth ventricular, or nucleus of the solitary tract (NTS). Intraperitoneal injection of AP5 (0.05, 0.1, 1.0, 3.0, and 5.0 mg/kg) did not alter 30-min sucrose intake at any dose (10.7 +/- 0.4 ml, saline control) (11.0 +/- 0.8, 11.2 +/- 1.0, 11.2 +/- 1.0, 13.1 +/- 2.2, and 11.0 +/- 1.9 ml, AP5 doses, respectively). Fourth ventricular administration of both 0.2 mug (16.7 +/- 0.6 ml) and 0.4 mug (14.9 +/- 0.5 ml) but not 0.1 and 0.6 mug of AP5 significantly increased 60-min sucrose intake compared with saline (11.2 +/- 0.4 ml). Twenty-four hour chow intake also was increased compared with saline (AP5: 31.5 +/- 0.1 g vs. saline: 27.1 +/- 0.6 g). Furthermore, rats did not increase intake of 0.2% saccharin after fourth ventricular AP5 administration (AP5: 9.8 +/- 0.7 ml, vs. saline: 10.5 +/- 0.5 ml). Finally, NTS AP5 (20 ng/30 nl) significantly increased 30- (AP5: 17.2 +/- 0.7 ml vs. saline: 14.6 +/- 1.7 ml), and 60-min (AP5: 19.4 +/- 0.6 ml vs. saline: 15.5 +/- 1.4 ml) sucrose intake, as well as 24-h chow intake (AP5: 31.6 +/- 0.3 g vs. saline: 26.1 +/- 1.2 g). These results support the hypothesis that hindbrain NMDA receptors participate in control of food intake and suggest that this participation also may contribute to control of body weight over a 24-h period.

N‐methyl‐d‐aspartate Receptor Responses Are Differentially Modulated by Noncompetitive Receptor Antagonists and Ethanol in Inbred Long‐Sleep and Short‐Sleep Mice: Behavior and Electrophysiology

Short-sleep (SS) mice exhibit higher locomotor activity than do long-sleep (LS) mice when injected with low doses of ethanol or the noncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonist MK-801 (dizocilpine). SS mice also have higher densities of brain NMDARs. However, two strains of LS X SS recombinant inbred (RI) mice also show differential activation to ethanol and MK-801, but have similar numbers of NMDARs. Here we used inbred LS (ILS) and SS (ISS) mice to investigate further the relationship between NMDARs and sensitivity to the stimulant effects of low doses of ethanol. Open field activity and spontaneous alternations were measured after saline or drug injection. [3H]MK-801 binding parameters were determined in hippocampus, cortex, striatum, and nucleus accumbens. Extracellular field excitatory postsynaptic potentials (fEPSPs) were recorded in the CA1 region of hippocampal slices. Systemic injection of either ethanol or MK-801 increased locomotor activity to a greater extent in ISS mice than in ILS mice. The competitive NMDAR antagonist 2-carboxypiperazin-4-yl-propyl-1-lphosphonic acid (+/- CPP) depressed activity of ILS, but not ISS, mice. No strain differences were observed in spontaneous alternations or in the number or affinity of NMDARs in the brain regions examined. Likewise, the magnitudes of hippocampal NMDAR-mediated fEPSPs were similar in ILS and ISS mice and were inhibited to the same extent by a competitive NMDAR antagonist. However, both ethanol and the NMDAR NR2B receptor antagonist ifenprodil inhibited the late component of hippocampal NMDAR fEPSPs to a greater extent in ISS, than in ILS, mice. Differential ethanol- and MK-801-induced behavioral activation in ILS and ISS mice was not associated with differences in NMDAR number. Nonetheless, pharmacological differences in hippocampal NMDAR responsiveness suggest that ISS mice express NMDARs that have a greater sensitivity to noncompetitive, but not competitive, NMDAR antagonists. These differences, which may reflect differences in NMDAR subunit composition, could underlie the differential responsiveness to low doses of ethanol in ILS and ISS mice.

Excitotoxicity Plays a Role in the Death of Tyrosine Hydroxylase- Immunopositive Nigral Neurons Cultured in Serum-Free Medium

We studied the effects of different amino acid receptor antagonists and a calcium (Ca2+) channel blocker on the survival of embryonic tyrosine hydroxylase (TH)-immunopositive nigral neurons grown under serum-free culture conditions. Ventral mesencephalic neurons were cultivated for 2 or 7 days. Following serum withdrawal on day 2, some cultures were treated with different concentrations of the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine hydrogen maleate (MK-801), the competitive NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid, the competitive kainate/quisqualate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione, and the Ca2+ channel blocker flunarizine. Treatment with MK-801 or flunarizine increased the survival of TH-positive neurons after serum deprivation. These findings suggest a possible role for excitotoxicity in dopaminergic cell death which can be prevented by blocking the NMDA receptor or by inhibiting Ca2+ entry through voltage-gated channels.

NMDA receptor antagonists inhibit apomorphine-induced climbing behavior not only in intact mice but also in reserpine-treated mice

The present study showed that the noncompetitive N-methyl- d-aspartate (NMDA) receptor antagonists, MK-801 {(+)-5-methyl-10,11-dihydroxy-5H-dibenzo-[a,d]-cyclohepten-5,10-imine hydrogen maleate}, ketamine, dextrorphan and dextromethorphan attenuated apomorphine-induced climbing behavior in reserpine-treated mice. In addition, the competitive NMDA receptor antagonists, d(−)-2-amino-5-phosphonopentanoic acid (AP-5) and d(−)-3-(2-carboxypipera-zine-4-yl)-propyl-1-phosphonic acid (CPP), also inhibited the apomorphine-induced climbing behavior in reserpine-treated mice as well as in intact mice. Previous work in our laboratory had shown that the noncompetitive NMDA receptor antagonists, MK-801, ketamine, dextrorphan and dextromethorphan cause a pronounced inhibition of apomorphine-induced cage climbing behavior in intact mice, suggesting the involvement of NMDA receptors in the glutamatergic modulation of dopaminergic function at the postsynaptic dopamine (DA) receptors. Therefore, the present results strongly support our previous conclusion that the NMDA receptors play important roles in the glutamatergic modulation of dopaminergic function at the postsynaptic DA receptors.

The non-competitive NMDA receptor blocker dizocilpine potentiates serotonergic function.

Forepaw treading induced in rats by the 5HT1A agonist 8-OH-DPAT, and head shakes caused by the administration of the 5HT2A receptor against DOI, and by the 5HT precursor (-)5HTP, were significantly increased by pretreatment with the non-competitive N-methyl-D-aspartate (NMDA) antagonist dizocilpine. Dizocilpine administration also significantly increased the locomotor activity induced by the serotonin agonists. The competitive NMDA receptor antagonist CGP 43487 increased only the head shakes induced by DOI, but did not alter the behavior elicited by 8-OH-DPAT, or (-)5HTP, and did not modify locomotor responses to any of the agonists used. The dizocilpine-induced potentiation of head shakes elicited by DOI and (-)5HTP was inhibited by the 5HT2 agonist ketanserin, but was not modified by the selective dopamine D1 and D2 receptor blockers SCH 23390 and (-)sulpiride. The dopamine receptor antagonists did, however, counteract the dizocilpine facilitation of both forepaw treading induced by 8-OH-DPAT, and the locomotor response to all the serotonergic agonists. The results indicate that, unlike competitive NMDA receptor antagonists, the non-competitive antagonists enhanced the expression of serotonergic stimulation, and suggest that a glutamate deficiency could contribute to the pathogenesis of schizophrenia, not only through dopaminergic, but also through serotonergic, hyperactivity.

Effect of MK-801 on Seizures Induced by Exposure to Hyperbaric Oxygen: Comparison with AP-7

The effect of the noncompetitiveN-methyl-d-aspartate (NMDA)-receptor antagonist MK-801 on seizures induced by hyperbaric oxygen in relation to changes in cerebral blood flow (CBF) was investigated. Rats were injected with MK-801 (0.005–8 mg/kg) 30 min before exposure to 100% O2at 5 atm (gauge pressure). MK-801 administration resulted in a biphasic response in seizure latency. Doses of 0.1–4 mg/kg significantly decreased time to EEG and motor seizures, while 8 mg/kg had no effect on seizure latency. MK-801 had no effect on seizure duration. In a dose range 0.1–8 mg/kg MK-801 increased CBF in awake animals, which might be responsible for the decreased seizure latency. The gradual increase in seizure latency with increasing MK-801 doses suggests involvement of an additional factor probably related to the drug's anticonvulsive effect. Unlike MK-801, a competitive NMDA receptor antagonist, AP-7, at a dose 250 mg/kg had no effect on latency to seizures or CBF.

Treatment of a chronic allodynia-like response in spinally injured rats: effects of systemically administered excitatory amino acid receptor antagonists

A chronic allodynia-like response to mechanical stimulation was observed in rats after severe spinal cord ischemia. This allodynia-like response was not relieved by most conventional analgesics used for treating chronic neuropathic pain. The present experiments evaluated the effects of systemiccally administered excitatory amino acid receptor antagonists, including the non-competitive N-methyl- d-aspartate (NMDA) receptor/channel blockers MK-801 and dextromethorphan, the competitive NMDA receptor antagonist CGS 19755 and a competitive antagonist of the α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA) receptor NBQX, on the chronic allodynia-like response in spinally injured rats. Systemic MK-801, dextromethorphan and CGS 19755 dose-dependently relieved the mechanical allodynia-like response. Systemic MK-801 and CGS 19755, but not dextromethorphan, also induced severe motor impairment at analgesic doses. All three NMDA antagonists increased spontaneous motor activity. Systemic NBQX reduced muscle tone and caused sedation. The mechanical allodynia was only relieved by NBQX at a sedative dose. It is concluded that systemic NMDA, but not AMPA, receptor antagonists may have an analgesic effect upon the chronic allodynia-like response. However, the analgesic effect of all NMDA antagonists was associated with side effects. Dextromethorphan, which is clinically tolerated and produced less side effects, may be useful for treating chronic pain associated with central nervous system injury.

A dose-response analysis of the behavioral effects of (+)MK-801 in guinea pig: Comparison with CPP

The noncompetitive N- methyl- d- aspartate (NMDA) antagonist, dizocilpine maleate (+)-5-methyl-10,11-dihydro-5 H-dibenzo[ a, d] -cyclohepten-5, 10-imine maleate [(+)MK-801], has attracted considerable interest because of its potential use as an anticonvulsant and neuroprotectant. However, its cognitive side effects in humans have limited its use in human pharmacotherapy. Although the behavioral effects of (+)MK-801 have been documented in mouse, rat, pigeon, and rhesus monkey, there are no available data on its effects in guinea pig. The objective of this study was to conduct a dose-response analysis of the effects of (+)MK-801 on stereotyped behavior, ataxia, locomotor activity, and righting reflex latency in guinea pig. In the dose range used (0.0625–0.5 mg/kg, IP), we found no significant differences between (+)MK-801 and vehicle in terms of stereotyped behavior and locomotor activity; however, at higher doses, (+)MK-801 caused significant ataxia and impairment of the righting reflex. By comparison, the competitive NMDA receptor antagonist, 3-((±)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP, 5 mg/kg, IP), produced neither stereotyped behavior nor impairment of the righting reflex; at higher doses (10 mg/kg, IP), it produced only ataxia and an increase in locomotor activity.

807 Cloning of rat S182 cDNA

Caffeine (100 mg/kg, i.p.) induces a rapid increase in the expression of mRNA for the immediate early genes (IEGs) c-fos and NGFI-A in rat striatum. We have examined how this response is affected by pretreatment with either the noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist MK-801 (1 and 3 mg/kg, i.p.), the competitive NMDA receptor antagonist D-CPP (6 mg/kg, i.p.), or the non-selective excitatory amino acid receptor antagonist kynurenic acid (300 mg/kg, i.p.). The two NMDA receptor antagonists significantly reduced the caffeine-induced expression of both c-fos mRNA and NGFI-A mRNA in the medial part of the caudate putamen. The effect was less pronounced in the lateral part of the caudate putamen. MK-801 caused an enancement of c-fos and NGFI-A mRNA expression in nucleus accumbens. Pretreatment with kynurenic acid caused no marked alterations in the caffeine-induced expression of c-fos mRNA and NGFI-A mRNA in any brain region. These findings suggest that glutamatergic transmission via NMDA receptors contributes to the induction of c-fos mRNA and NGFI-A mRNA by caffeine in striatum. In addition we show that MK-801 can either increase or decrease the caffeine effect of IEGs depending on the region studied.

Synergism between diltiazem and MK-801 but not APV in protecting hippocampal slices against hypoxic damage

In the present study, we investigated the possibility that MK-801 (dizocilpine), a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, owes its potent neuroprotective properties to calcium channel blocking ability rather than to its NMDA receptor antagonism. Rat hippocampal slices were exposed to a long hypoxic period (20 min) from which only 13.8% recovered their neuronal function after 30 min of reoxygenation. The recovery rate of neuronal function from 20-min hypoxia was increased to 100% when slices were pretreated with 5 μM MK-801.DL-2-amino-5-phosphonovalerate (APV), a competitive NMDA receptor antagonist, even at relatively high concentration (100 μM), provided only marginal protection against a such severe hypoxic insult. The L-type calcium channel blocker diltiazem (DILT) was more effective than APV in protecting hypoxic slices against neuronal damage. Combining suboptimal concentrations of DILT and MK-801 produced a neuroprotective effect which significantly exceeded the calculated additive effect of the two drugs. Such synergism could not be demonstrated between DILT and APV, a combination that produced only the expected additive neuroprotective effect. The observed synergy between the calcium channel blocker (DILT) and MK-801, along with other studies that demonstrated interaction between these two drugs, led us to postulate that MK-801 possesses calcium channel blocking properties through which its neuroprotective effect is exerted. These calcium channels could either be of the L-type or otherwise, channels which are being activated only under stressful conditions, such as hypoxia or ischemia.

Inhibition of protein synthesis by the NMDA channel blocker MK-801.

MK-801 is a selective non-competitive N-methyl-D-aspartate (NMDA) channel blocker which has been extensively used in order to determine the properties of NMDA receptors and their role in epilepsy and ischaemic cell death. We now report that MK-801 (10 microM) inhibits [35S]methionine incorporation into polypeptides by 45 +/- 7.5% in adult rat hippocampal slices and by 35 +/- 10% in cultured glial cells. This effect is NMDA-receptor independent since (i) only high doses of MK-801 induced such an effect and (ii) APV (50 microM), a competitive NMDA receptor antagonist, or Mg2+ (10 mM), a NMDA channel blocker, did not reduce protein synthesis.

Comparison of the effects of pretreatment with competitive or noncompetitive NMDA antagonists on vestibular compensation

Unilateral labyrinthectomy (UL) results in a syndrome of ocular motor and postural disorders which abates over time in a process of behavioural recovery known as vestibular compensation. We have previously reported that a single systemic pre-UL injection of the organic Ca 2+ channel antagonist verapamil or the noncompetitive N-methyl- d-aspartate (NMDA) receptor antagonist MK-801 reduces the behavioural effects of UL in guinea pigs. The present study was conducted to determine if similar effects would be obtained with single injections of the competitive NMDA receptor antagonists 3-[(±)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP) or cis-4-(phosphonomethyl)-piperidine-2-carboxylic acid (CGS 19755). Guinea pigs received an IP injection of 5 mg/kg CPP 2.5 h pre-UL, 5 or 10 mg/kg CPP 1 h pre-UL, 10 or 20 mg/kg CGS 19755 1 h pre-UL, or 1 ml/kg vehicle (saline) 1 h pre-UL, and the effects on the compensation of spontaneous nystagmus were measured over the following 52 h post-UL. Pretreatment with CPP had no significant effect on spontaneous nystagmus frequency or its compensation over 52 h post-UL. However, pretreatment with CGS 19755 resulted in a significant decrease in spontaneous nystagmus frequency without any acceleration of the rate of compensation.

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-competitiveN-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.) adminsstration 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.

Differential behavioural and neurochemical effects of competitive and non-competitive NMDA receptor antagonists in rats

The behavioural and biochemical effects of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists, dizocilpine and memantine, and the competitive NMDA receptor antagonist, CGP 39551, were investigated in rats. Systemic injections of dizocilpine (0.33 mg/kg) increased locomotion and rearing in an open field, whereas memantine (20 mg/kg) increased only locomotor activity. CGP 39551 (10 and 20 mg/kg) did not change open field activity. Dopamine (DA) metabolism - as measured by the ratio of dihydroxyphenylacetic acid/dopamine (DOPAC/DA) - increased in response to dizocilpine in the prefrontal cortex and the nucleus accumbens. Memantine enhanced DOPAC/DA in the prefrontal cortex, the nucleus accumbens and to a lesser degree in the posterior striatum. In contrast to non-competitive NMDA receptor antagonists, CGP 39551 did not increase DA metabolism of subcortical structures and even decreased DOPAC/DA in the prefrontal cortex. These results indicate that competitive and non-competitive NMDA receptor antagonists affect spontaneous locomotion differentially in rats. The biochemical data imply that the stimulant actions of non-competitive NMDA receptor antagonists are at least partially due to activation of ascending dopaminergic systems. Potential mechanisms involved in the differential effects of both types of NMDA receptor antagonists are discussed.

Competitive and non-competitive N-methyl-D-aspartate antagonists fail to prevent the induction of methamphetamine-induced sensitization

In order to elucidate the possible roles of the glutamate system in the mechanisms underlying behavioral sensitization, which is used as an animal model for human psychosis, we investigated the effects of 3-((±)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) and MK-801 ((+)-dizocilpine), a competitive and non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, respectively, on methamphetamine-induced behavioral sensitization in rats. Administration of 0.5 mg/kg MK-801 enhanced 2 mg/kg methamphetamine-induced hyperactivity, whereas it reduced 6 mg/kg methamphetamine-induced stereotyped behavior markedly. CPP (10 mg/kg) reduced 2 mg/kg methamphetamine-induced stereotypy slightly. Repeated treatment with 2 and 6 mg/kg methamphetamine alone induced progressive augmentation of stereotypy, whereas combining either MK-801 or CPP with methamphetamine treatment abolished or attenuated this augmentation. However, when rats were challenged with methamphetamine after a 7-day period of abstinence, the intensity of stereotypy among the rats pretreated with repeated doses of methamphetamine alone or in combination with MK-801 or CPP did not differ significantly. These results indicate that competitive and non-competitive NMDA receptor antagonists modulate acute methamphetamine-induced abnormal behavior and sensitization expression, but they failed to prevent the induction of the neural mechanisms underlying behavioral sensitization.

Antidystonic effects of the NMDA receptor antagonists memantine, MK-801 and CGP 37849 in a mutant hamster model of paroxysmal dystonia

The effects of competitive and noncompetitive N- methyl- d-aspartate (NMDA) receptor antagonists were evaluated in an inbred line of Syrian golden hamsters, in which sustained dystonic postures of limbs and trunk can be initiated by handling or mild environmental stimuli. In this model of paroxysmal dystonia, the noncompetitive NMDA receptor antagonists memantine and MK-801 (dizocilpine) delayed the progression of dystonic attacks in a dose-dependent fashion. The novel competitive NMDA receptor antagonist CGP 37849 ( dl-[E]-2-amino-4-methyl-5-phosphono-3-pentenoic acid) was more effective than memantine and MK-801, because it retarded not only the progression but also reduced the severity of the dystonic movements. All compounds exhibited antidystonic effects at doses which did not cause marked ataxia or sedation. The data indicate that NMDA receptor antagonists might be interesting candidates for treatment of dystonia.

Electrophysiological evidence for [formula omitted] excitatory amino acid receptors in the rat supraoptic nucleus in vitro

The responses of neurons in slices of the rat supraoptic nucleus (SON) to afferent stimulation were recorded under current-clamp conditions. In magnesium (Mg 2+-free incubation medium, synaptic responses were prolonged and were partially antagonized by the non-competitive N- methyl- d-aspartate (NMDA) receptor antagonist (+)-5-methyl-10,11-dihydro-5 H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801). During blockade of non-NMDA excitatory amino acid (EAA) receptors, the synaptic responses in Mg 2+-free medium were blocked by the competitive NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (AP5). The results of these experiments provide electrophysiological evidence for the existence of NMDA receptors in the rat SON.