Common and uncommon behavioural effects of antagonists for different modulatory sites in the NMDA receptor/channel complex

GluN2B/N-methyl-D-aspartate Receptor Antagonists: Advances in Design, Synthesis, and Pharmacological Evaluation Studies

Selective GluN2B/N-methyl-D-aspartate receptor (NMDAR) antagonists have exposed their clinical effectiveness in a cluster of neurodegenerative diseases, such as epilepsy, Alzheimer's disease, Parkinson's disease, pain, and depression. Hence, GluN2B/NMDARs are considered to be a prospective target for the management of neurodegenerative diseases. Here, we have discussed the current results and significance of subunit selective GluN2B/NMDAR antagonists to pave the way for the establishment of new, safe, and economical drug candidates in the near future. By using summarized data of selective GluN2B/NMDAR antagonists, medicinal chemists are certainly a step closer to the goal of improving the therapeutic and side effect profile of selective antagonists. Outlined summary of designing strategies, synthetic schemes, and pharmacological evaluation studies reinvigorate efforts to identify, modify, and synthesize novel GluN2B/NMDAR antagonists for treating neurodegenerative diseases.

Exploring Hallucinogen Pharmacology and Psychedelic Medicine with Zebrafish Models

After decades of sociopolitical obstacles, the field of psychiatry is experiencing a revived interest in the use of hallucinogenic agents to treat brain disorders. Along with the use of ketamine for depression, recent pilot studies have highlighted the efficacy of classic serotonergic hallucinogens, such as lysergic acid diethylamide and psilocybin, in treating addiction, post-traumatic stress disorder, and anxiety. However, many basic pharmacological and toxicological questions remain unanswered with regard to these compounds. In this study, we discuss psychedelic medicine as well as the behavioral and toxicological effects of hallucinogenic drugs in zebrafish. We emphasize this aquatic organism as a model ideally suited to assess both the potential toxic and therapeutic effects of major known classes of hallucinogenic compounds. In addition, novel drugs with hallucinogenic properties can be efficiently screened using zebrafish models. Well-designed preclinical studies utilizing zebrafish can contribute to the reemerging treatment paradigm of psychedelic medicine, leading to new avenues of clinical exploration for psychiatric disorders.

Identification of potential Gly/NMDA receptor antagonists by cheminformatics approach: a combination of pharmacophore modelling, virtual screening and molecular docking studies

The Gly/NMDA receptor has become known as potential target for the management of neurodegenerative diseases. Discovery of Gly/NMDA antagonists has thus attracted much attention in recent years. In the present research, a cheminformatics approach has been used to determine structural requirements for Gly/NMDA antagonism and to identify potential antagonists. Here, 37 quinoxaline derivatives were selected to develop a significant pharmacophore model with good certainty. The selected model was validated by leave-one-out cross-validation, an external test set, decoy set and Y-randomization test. Applicability domain was verified by the standardization approach. The validated 3D-QSAR model was used to screen virtual hits from the ZINC database by pharmacophore mapping. Molecular docking was used for assessment of receptor-ligand binding modes and binding affinities. The GlideScore and molecular interactions with critical amino acids were considered as crucial features to identify final hits. Furthermore, hits were analysed for in silico pharmacokinetic parameters and Lipinski's rule of five, demonstrating their potential as drug-like candidates. The PubChem and SciFinder search tools were used to authenticate the novelty of leads retrieved. Finally, five different leads have been suggested as putative novel candidates for the exploration of potent Gly/NMDA receptor antagonists.

Impairment of the anterior thalamic head direction cell network following administration of the NMDA antagonist MK-801

Head direction (HD) cells, found in the rodent Papez circuit, are thought to form the neural circuitry responsible for directional orientation. Because NMDA transmission has been implicated in spatial tasks requiring directional orientation, we sought to determine if the NMDA antagonist dizocilpine (MK-801) would disrupt the directional signal carried by the HD network. Anterior thalamic HD cells were isolated in female Long-Evans rats and initially monitored for baseline directional activity while the animals foraged in a familiar enclosure. The animals were then administered MK-801 at a dose of .05 mg/kg or 0.1 mg/kg, or isotonic saline, and cells were re-examined for changes in directional specificity and landmark control. While the cells showed no changes in directional specificity and landmark control following administration of saline or the lower dose of MK-801, the higher dose of MK-801 caused a dramatic attenuation of the directional signal, characterized by decreases in peak firing rates, signal to noise, and directional information content. While the greatly attenuated directional specificity of cells in the high dose condition usually remained stable relative to the landmarks within the recording enclosure, a few cells in this condition exhibited unstable preferred directions within and between recording sessions. Our results are discussed relative to the possibility that the findings explain the effects of MK-801 on the acquisition and performance of spatial tasks.

Novel NMDA receptor modulators: an update

INTRODUCTION

The NMDA receptor is a ligand-gated ion channel that plays a critical role in higher level brain processes and has been implicated in a range of neurological and psychiatric conditions. Although initial studies for the use of NMDA receptor antagonists in neuroprotection were unsuccessful, more recently, NMDA receptor antagonists have shown clinical promise in other indications such as Alzheimer's disease, Parkinson's disease, pain and depression. Based on the clinical observations and more recent insights into receptor pharmacology, new modulatory approaches are beginning to emerge, with potential therapeutic benefit.

AREAS COVERED

The article covers the known pharmacology and important features regarding NMDA receptors and their function. A discussion of pre-clinical and clinical relevance is included, as well. The subsequent patent literature review highlights the current state of the art targeting the receptor since the last review in 2010.

EXPERT OPINION

The complex nature of the NMDA receptor structure and function is becoming better understood. As knowledge about this receptor increases, it opens up new opportunities for targeting the receptor for many therapeutic indications. New strategies and advances in older technologies will need to be further developed before clinical success can be achieved. First-in-class potentiators and subunit-selective agents form the basis for most new strategies, complemented by efforts to limit off-target liability and fine-tune on-target properties.

Role of altered structure and function of NMDA receptors in development of alcohol dependence

Long-term alcohol exposure gives rise to development of physical dependence on alcohol in consequence of changes in certain neurotransmitter functions. Accumulating evidence suggests that the glutamatergic neurotransmitter system, especially the N-methyl-D-aspartate (NMDA) type of glutamate receptors is a particularly important site of ethanol's action, since ethanol is a potent inhibitor of the NMDA receptors (NMDARs) and prolonged ethanol exposition leads to a compensatory "upregulation" of NMDAR mediated functions supposedly contributing to the occurrence of ethanol tolerance, dependence as well as the acute and delayed signs of ethanol withdrawal.Recently, expression of different types of NMDAR subunits was found altered after long-term ethanol exposure. Especially, the expression of the NR2B and certain splice variant forms of the NR1 subunits were increased in primary neuronal cultures treated intermittently with ethanol. Since NMDA ion channels with such an altered subunit composition have increased permeability for calcium ions, increased agonist sensitivity, and relatively slow closing kinetics, the abovementioned alterations may underlie the enhanced NMDAR activation observed after long-term ethanol exposure. In accordance with these changes, the inhibitory potential of NR2B subunit-selective NMDAR antagonists is also increased, demonstrating excellent potency against alcohol withdrawal-induced in vitro cytotoxicity. Although in vivo data are few with these compounds, according to the effectiveness of the classic NMDAR antagonists in attenuation, not only the physical symptoms, but also some affective and motivational components of alcohol withdrawal, novel NR2B subunit selective NMDAR antagonists may offer a preferable alternative in the pharmacotherapy of alcohol dependence.

Latent inhibition disruption by MK-801 in a conditioned taste-aversion paradigm

N-Methyl-D-aspartate (NMDA) receptors appear to be involved in CS processing and memory consolidation. The present paper analyzed the effect of the non-competitive NMDA receptor antagonist Dizocilpine maleate (MK-801) on Latent Inhibition (LI)-retarded learning of a CS-US association after to-be-CS preexposures at time of testing, using Wistar rats as experimental subjects. If NMDA receptors are involved in CS processing, MK-801 administration should affect LI. In fact, previous experiments revealed that a 2.0mg/kg MK-801 dose, administered 20 h before preexposure and conditioning, abolished LI in a conditioned taste-aversion paradigm. In the present paper, MK-801 (0.2 mg/kg) was either injected after preexposure, after conditioning, or after both preexposure and conditioning stages. LI was abolished when MK-801 was injected after preexposure, but not when it was injected after conditioning. These results support the role of NMDA receptors in CS processing and memory consolidation.

Evaluation of the NR2B-selective NMDA receptor antagonist Ro 63-1908 on rodent behaviour: evidence for an involvement of NR2B NMDA receptors in response inhibition

We have characterised the effects of the recently described NMDA NR2B subtype selective antagonist, Ro 63-1908, on spontaneous behaviour and in tasks sensitive to non-selective NMDA antagonists. In both rats and wild type mice, Ro 63-1908 (1-30mg/kg sc) produced a mild increase in motor activity of lesser magnitude than that elicited by dizocilpine. No signs of overt PCP-like stereotypy were seen in either species at equivalent doses. PPI was also unaffected. However, in mice lacking the NR2A subunit, Ro 63-1908 (3-30mg/kg) produced a profound hyperactivity of similar magnitude to dizocilpine but few other 'PCP-like' behaviours. In rats, Ro 63-1908 (1-10mg/kg) did not affect Morris water maze or delayed matching performance. In a 5-choice serial reaction time task, requiring rats to respond to a visual stimulus presented after a fixed time interval, Ro 63-1908 (0.3-3mg/kg) produced a dramatic increase in premature responses - accuracy was relatively unaffected. Finally in a DRL24 task, Ro 63-1908 (0.3-3mg/kg) reduced inter-response time, increased response rate, and consequently reduced efficiency. We conclude that the improved profile of Ro 63-1908 compared to NMDA channel blockers is due to both its selectivity for the NR2B vs. NR2A subunit containing receptors and its activity-dependent mechanism of action. However, in the 5-CSRT and DRL24 tasks, Ro 63-1908 produced behaviours suggestive of impaired response inhibition, implicating a critical role of NMDA NR2B transmission in this process.

Effect of multiple ifenprodil or spermidine treatment on social recognition in rats

We investigated the effects of multiple (21 x) ifenprodil (1.0 mg/kg, i.p.)-[IF] and spermidine (5.0 mg/kg, i.p.)-[SP] administration on short-term memory using the social recognition test in rats. The influence of a single (lx) injection of IF and SP was also established. 1x IF or SP treatment showed a statistically insignificant tendency to impair social memory task. In contrast, 21 x SP treatment facilitated short-term memory when compared with 1x SP administration. 21x IF had no affect on the memory paradigm. The results of the present study indicate that the prolonged systemic treatment of IF or SP in relatively low doses causes no impairment of short-term memory in rats.

Effects of acamprosate and some polyamine site ligands of NMDA receptor on short-term memory in rats

The aim of this study was to evaluate the effect of multiple acamprosate (500.0 mg/kg, p.o.) administration on short-term memory, using the social recognition test in rats. Ifenprodil (1.0 mg/kg, i.p.), arcaine (5.0 mg/kg, i.p.) and spermidine (20.0 mg/kg, i.p.) were chosen as polyamine ligands and their action or interaction with acamprosate was also studied. The doses used did not show any sedative activity, which was assessed by measuring locomotor activity and the hypnotic effect of ethanol. The findings suggest that acute acamprosate treatment did not impair short-term memory. Multiple acamprosate and a single spermidine or arcaine administration led to better performance in the memory test, whereas no significant difference was observed in ifenprodil-treated rats. Co-administration of a single arcaine or spermidine dose with multiple acamprosate produced worse results. This means that the effect of repeated acamprosate administration can be changed by the co-administration of other polyamine ligands, so that care should be taken in interpreting.

Attenuation of the acute amnestic effect of ethanol by ifenprodil: comparison with ondansetron and dizocilpine

The aim of the present study was to examine the influence of ifenprodil, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist which also blocks 5-HT3 receptors, on the amnestic effect of ethanol in a passive avoidance task in mice. The anti-amnestic action of ifenprodil was compared with the effects of the 5-HT3 receptor antagonist ondansetron and the non-competitive NMDA-receptor antagonist dizocilpine (MK-801). Ethanol, 2 g/kg and dizocilpine 0.1 mg/kg significantly impaired the passive avoidance response. In contrast, ifenprodil (0.1-10 mg/kg), ondansetron (0.03-0.3 mg/kg) and dizocilpine (0.01 and 0.03 mg/kg) did not alter passive avoidance by themselves. Dizocilpine did not diminish the amnestic action of ethanol when administered at doses of 0.03-0.1 mg/kg. However, the amnestic effect of ethanol was partially restored towards normal by ifenprodil 0.3 mg/kg and by ondansetron 0.03 mg/kg (alone or together with dizocilpine 0.01 mg/kg) but it was not affected by other doses of ifenprodil (0.1, 1 and 10 mg/kg) and ondansetron (0.1 and 0.3 mg/kg). In conclusion, ifenprodil at an appropriate dose reduced ethanol-induced amnesia in a step-through passive avoidance task. The results are compatible with the assumption that the anti-amnestic action of ifenprodil may be (at least partially) due to an antagonism at 5-HT3 receptors.

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

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

Ifenprodil influences changes in mouse behaviour related to acute and chronic ethanol administration

The aim of the present study was to examine the influence of ifenprodil (a non-competitive NMDA receptor antagonist which also blocks 5-HT3 receptors and alpha1-adrenoceptors) on the effects of ethanol in the mouse in vivo and to elucidate the role of various receptors in these actions. The ethanol (4 g/kg i.p.)-induced sleeping time was shortened by ifenprodil 1 mg/kg but was not affected by ifenprodil 0.3 mg/kg, the 5-HT3 receptor antagonist ondansetron 0.03 mg/kg and the non-competitive NMDA receptor antagonist MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cycloheptan-5,10-imine maleate) 0.01 mg/kg. Ifenprodil 10 mg/kg mimicked the alpha1-adrenoceptor antagonist prazosin 1 mg/kg in that it prolonged the hypnotic response to ethanol (no additive effect when both drugs were given in combination); this is compatible with an involvement of alpha1-adrenoceptors in this effect of ifenprodil. Chronic exposure to ethanol (7%) induced physical dependence. The severity of ethanol withdrawal was suppressed by ifenprodil 1 and 10 mg/kg. In conclusion, ifenprodil influences ethanol-related changes in mouse behaviour and may prove to be useful in the treatment of alcoholism.

GlycineB antagonists and partial agonists in rodent models of Parkinson's disease--comparison with uncompetitive N-methyl-D-aspartate receptor antagonist

Antiparkinsonian-like activity of glutamate receptor antagonists (mostly of N-methyl-D-aspartate (NMDA) receptors) has been demonstrated in animals and for uncompetitive agents, also in humans. In the present study we investigated the potential antiparkinsonian-like activity of compounds acting at the glycine site of the NMDA receptor complex in three animal models of Parkinson's disease and compared them with the new uncompetitive NMDA receptor antagonist MRZ 2/579. Haloperidol-induced catalepsy was inhibited by the Merz glycine site antagonists MRZ 2/570, MRZ 2/571 and MRZ 2/576 but not by another antagonist L-701,324 or the glycine site partial agonists ACPC and D-CS. None of the tested glycine site antagonists or partial agonists increased locomotor activity or potentiated L-DOPA responses in reserpine and alpha-MT treated rats. In rats with a unilateral 6-OHDA medial forebrain bundle lesion neither glycine site antagonists nor partial agonists affected rotations on their own or enhanced the contralateral rotations induced by L-DOPA. In contrast, the uncompetitive NMDA receptor antagonist MRZ 2/579 was active in all antiparkinsonian tests used in this study. Based on the present data the therapeutic potential of the glycine site antagonists and partial agonists tested for the treatment of Parkinson's disease is rather doubtful. Uncompetitive NMDA receptor antagonists seem to possess a better profile as antiparkinsonian agents.

Modulation of NMDA receptors by glycine--introduction to some basic aspects and recent developments

Glycine is a co-agonist at NMDA receptors and it's presence is a prerequisite for channel activation by glutamate or NMDA. Physiological concentrations reduce one form of NMDA receptor-desensitization. Interactions between the glycineB site and other domains of the NMDA receptor are complex and include the glutamate, Mg2+ and polyamines sites. Glycine shows different affinities at various NMDA receptor subtypes probably via to allosteric interactions between NMDA2 subunits and the glycine recognition site on the NMDAR1 subunit. There is still some debate whether the glycineB site is saturated in vivo but it seems likely that this depends on regional differences in receptor subtype expression, local glycine or D-serine concentrations and the expression of specific glycine transporters. GlycineB antagonists and partial agonists have been reported to have good therapeutic indices as neuroprotective agents against focal ischaemia and trauma, anti-epileptics, anxiolytics, anti-psychotomimetics and in models of chronic pain. They clearly lack two potentially serious side effects classically associated with NMDA receptor blockade, namely neurodegenerative changes in the cingulate/retrosplenial cortex and psychotomimetic-like effects. This improved therapeutic profile may be partially due to the ability of full glycineB antagonists to reveal glycine-sensitive desensitization and possibly also via functional and/or regional NMDA receptor subtype selectivity.

GlycineB antagonists as potential therapeutic agents. Previous hopes and present reality

It is not clear what therapeutic application is most likely for agents blocking glycine site of the NMDA receptors (glycineB). Majority of the studies to date used either glycineB antagonists with doubtful brain penetration or partial agonists. Following systemic administration to rats of our newly developed glycineB antagonists (MRZ 2/570; 2/571 and 2/576) and L-701,324 (MSD) as a reference agent the following behavioural effects were observed: weak (if any) antiparkinsonian-like effects, lack of anxiolytic activity, inhibition of physical and motivational aspects of morphine dependence and neuroprotective activity in global ischaemia. The side effects include: sedation, ataxia, and myorelaxation. We detected neither vacuolisation in the cingulate cortex nor impairment of pre-pulse inhibition indicating lack of psychotomimetic potential.

Dextromethorphan and dextrorphan in rats: common antitussives--different behavioural profiles

Dextromethorphan (DM), a widely used and well-tolerated centrally acting antitussive, has been tested in several clinical trials for its antiepileptic and neuroprotective properties. However, the use of DM in these new clinical indications requires higher doses than antitussive doses, which may therefore induce phencyclidine (PCP)-like side-effects (memory and psychotomimetic disturbances) through its metabolic conversion to the active metabolite dextrorphan (DX), a more potent PCP-like non-competitive antagonist at the N-methyl-D-aspartate (NMDA) receptor than DM. Thus, we compared the behavioural effects in rats of intraperitoneal administration of DM and DX on motor activity in an open field and on learning and memory in the Morris water maze. DM (20, 30, 40 mg/kg) produced a dose-dependent decrease in both locomotion and stereotyped behaviour with a slight ataxia for the highest dose. DX (20, 30, 40 mg/kg) induced a dose-dependent increase in locomotion and stereotypies (swaying, turning) with moderate ataxia. Assessments of learning and memory were performed with lower doses of DM (10, 20, 30 mg/kg) and DX (5, 10, 15 mg/kg) because of motivational deficits (40 mg/kg of DM, 20-40 mg/kg of DX) and motor disorders (30, 40 mg/kg of DX) in the cue learning procedure. DX (10, 15 mg/kg) impaired spatial learning with a long-lasting effect for the highest dose whereas 5 mg/kg of DX and DM (10-30 mg/kg) did not. Only 15 mg/kg of DX appeared to slightly impair working memory. DM (10-30 mg/kg) and DX (5-15 mg/kg) did not impair reference memory. Thus, the two antitussives DM and DX induced different behavioural effects suggesting sedative effects for DM and PCP-like effects for DX. However, PCP-like side-effects with DM remain possible through its metabolic conversion to DX, with very high doses and/or in extensive metabolizers and/or in aged subjects prone to cognitive dysfunction. Therefore, the identification of DM metabolism phenotype, an adapted prescription and a pharmacological modulation of the DM metabolism may avoid adverse effects.

Phencyclidine-like discriminative stimulus effects of polyamine modulators of N-methyl-D-aspartate receptor activity in rats

Excessive N-methyl-D-aspartate (NMDA) receptor activation has been implicated in many acute and chronic neuropathologies. NMDA antagonists might prove to be useful treatments, unfortunately, some can produce phencyclidine (PCP)-like side effects. The polyamine-site modulators, spermine (SPM) and spermidine (SPD), produce dose related biphasic modulation of NMDA channel currents while another polyamine, arcaine (ARC), produces only negative modulatory effects. The PCP-like effects of these compounds were tested in rats trained to discriminate PCP from saline in a standard two-lever drug discrimination paradigm under a fixed ratio schedule of food reinforcement. SPM, SPD and ARC occasioned little, if any, responding on the PCP-associated lever, even at response rate suppressing doses. The results provide further evidence that differences exist between the discriminative stimulus effects produced by drugs active at different sites on the NMDA receptor and suggest that the polyamine modulatory site should be a good target for development of NMDA antagonist medications with a reduced propensity for PCP-like acute behavioral effects.

Anxiolytic activity of glycine-B antagonists and partial agonists--no relation to intrinsic activity in the patch clamp

On the basis of animal models, anxiety was one of the first suggested clinical applications of partial agonists of the glycineB site coupled to the NMDA receptor. It is not certain, however, whether these findings can be extended to full glycineB antagonists and what is the relation between intrinsic activity (degree of NMDA receptor antagonism) and anxiolytic effect. In the present study several NMDA receptor antagonists, including several glycineB antagonists/partial agonists, were tested for anxiolytic activity in the Vogel conflict test and the elevated plus-maze. Additionally, the intrinsic activities of the glycineB partial agonists used [ACPC, (R,+)-HA-966 and D-cycloserine] were compared in patch-clamp experiments in cultured neurones. In the plus-maze the most striking increase in the time spent in open arms (index of anxiolytic effect) was seen after diazepam and D-cycloserine (at doses that did not change locomotion). Also reliable (dose-dependent), although weaker, anxiolytic activity was produced by the uncompetitive NMDA receptor antagonist (+)MK-801 and the competitive antagonist CGP 39551. Modest anxiolytic-like effect in the plus-maze was also observed after the glycineB antagonist L-701,324 and the partial agonist (+,R)-HA-966. Uncompetitive antagonists memantine and amantadine, the glycineB partial agonist ACPC (up to 600 mg/kg) or the full antagonists MRZ 2/570, MRZ 2/571 and MRZ 2/576 had no effect. In the Vogel conflict test neither memantine, nor any of the full glycineB antagonists tested (L-701,324 and MRZ 2/576), showed anxiolytic activity. Patch-clamp studies revealed that the intrinsic activity of (+,R)-HA-966, D-cycloserine and ACPC was 13, 57 and 92%, respectively, as compared to that of glycine itself (100%). In conclusion, for the agents tested there is no clear relation between the levels of intrinsic activity, i.e. degree of NMDA receptor inhibition, and anxiolytic activity. Moreover, L-701,324 and MRZ-type glycineB full antagonists do not exchibit anxiolytic activity in the elevated plus-maze and Vogel conflict test.

The influence of nicardipine and ifenprodil on the brain free arachidonic acid level and behavior in hypoxia-exposed rats

1. The effects of the calcium channel blockers, nicardipine and ifenprodil, on the brain free arachidonic acid level and learning ability in rats exposed to hypoxia were examined. 2. Adult rats were injected with 0.003; 0.01; 0.03; 0.1; 0.3 or 1.0 mg/kg of tested drugs i.p. Thirty min later the learning ability was tested in a passive avoidance task according to the step-through procedure. Immediately after the training trial, the animals were subjected to a period of oxygen deprivation hypoxia until the loss of the righting reflex. The retention trial was carried out 24 hr later. 3. The other groups of animals were pretreated with mentioned substances before hypoxia-exposure. Fifteen min after the loss of the righting reflex they were decapitated and brains were frozen in liquid nitrogen. The brain free arachidonic acid level was quantified by gas chromatography. 4. Both nicardipine and ifenprodil were effective in preventing a memory decline in hypoxia-exposed rats but did not prevent the accumulation of the brain free arachidonic acid in hypoxia-exposed rats. 5. The protective effects of both substances in behavioral studies during acute brain damage caused by hypoxia could not be explained by the prevention of the increase of the brain free arachidonic acid, but by some other mechanism.

Motor actions of eliprodil in the normal and monoamine-depleted mouse: a role in the treatment of Parkinson's disease?

The non-competitive NMDA polyamine site antagonist, eliprodil, was examined for its effects on exploratory activity in non-habituated mice and for its antiakinetic potential in reserpine-treated mice. A low dose of eliprodil (5 mg/kg) weakly stimulated locomotion in naive animals, whilst higher doses depressed rearing (20-40 mg/kg) and grooming (40 mg/kg), consistent with a sedative action. At no dose did eliprodil cause ataxia. In 24 h reserpine-treated mice, eliprodil (10-40 mg/kg) reversed akinesia, but this effect was subject to considerable inter-animal variation and was not statistically significant. Eliprodil did not alter the motor recovery elicited by the dopamine D1 agonist SKF 38393, or the dopamine D2 agonist RU 24213, and suppressed the motor stimulation induced by L-DOPA. These results indicate that eliprodil displays a far lower propensity than many other NMDA receptor antagonists for disturbing posture and gait, but lacks the essential motor stimulant action required to make it a safe and effective antiparkinsonian agent, at least in the reserpine-treated mouse model of Parkinson's disease.

Adverse effects of dextromethorphan on the spatial learning of rats in the Morris water maze

The effects of the non-competitive NMDA receptor antagonist dextromethorphan on spatial learning were assessed using the Morris water maze. Dextromethorphan was administered to 4 groups of rats in 10, 20, 30, and 40 mg/kg doses. An additional group of rats was administered saline to serve as a vehicle control group. Dextromethorphan impaired learning dose dependently in the initial training phase of the experiment. During the probe trial, dose-dependent performance deficits were noted in the first 15 s of the trial only. Search strategy differences between the lowest and highest dose groups were also observed during the probe trial. During the reversal training phase, when the platform was moved to a new location, the dose-dependent impairment was seen again, but the 40 mg/kg group perseverated to the former location longer than the other groups. A cued control trial indicated that in addition to the learning impairment produced, the highest dose of dextromethorphan may also impair sensory-motor coordination.

Antagonists of the NMDA receptor-channel complex and motor coordination

Many structurally different, centrally active antagonists of the NMDA receptor-channel complex induce phencyclidine-like side effects in mammals which include head weaving, body rolling, sniffing and disturbances of motor coordination. The ability of these compounds to cause disturbances of motor coordination correlates directly with their ability to antagonize the NMDA receptor-channel complex in vivo. Although noncompetitive antagonists increase motility in rodents, whereas competitive antagonists do not, both classes of compounds appear to induce schizophrenia-like psychosis in human beings, and cause similar changes in a variety of different biogenic amine neurotransmitter systems in the limbic and motoric areas of the brain. The complex spectrum of behavioural effects observed after the administration of antagonists of the NMDA receptor-channel complex probably reflects the intricate nature of the interaction with positive and negative feedback loops of the motor circuit. Recent research indicates that the site of integration of this interaction could be the striatal medium spiny GABAergic neuron.

Locomotor effects of amantadine in the mouse are not those of a typical glutamate antagonist

Amantadine has been shown to displace [3H]MK 801 from its binding site on the NMDA receptor. We have therefore studied the motor effects of amantadine in normal and 24 h reserpine-treated mice to determine whether the behavioural profile of this drug resembles that of other NMDA receptor antagonists (e.g. MK 801). In common with the latter, amantadine (5-40 mg/kg IP) produced a modest dose-dependent sedation in dopamine-intact mice, with a reduction in locomotion and other species-typical behaviours (e.g. rearing and grooming), but with no signs of the hyperactivity, stereotypy, ataxia or loss of muscle tone commonly seen with MK 801. Amantadine (5-80 mg/kg IP) effected a small increase in motility in akinetic reserpine-treated mice by itself, but this response was highly variable and not statistically significant. As with MK 801, amantadine significantly inhibited the locomotion induced by the selective D2 agonist RU 24213 (5 mg/kg SC) and the mixed D1/D2 agonist apomorphine (0.5 mg/kg SC) in monomine-depleted mice, without altering the animals' responsiveness to threshold doses of these drugs. However, amantadine did not facilitate the locomotion induced by threshold (3 mg/kg IP) or fully active doses (30 mg/kg IP) of the selective D1 agonist SKF 38393, which distinguishes amantadine from other NMDA receptor blockers. Since the potentiation of dopamine D1-dependent locomotion may be a major factor in the antiparkinson activity of MK 801 and other glutamate receptor antagonists, the inability of amantadine to potentiate SKF 38393 in this study suggests the mechanism of its anti-akinetic activity differs from that of conventional glutamate blocking drugs.

Excitatory amino acid receptor antagonist kynurenic acid attenuates rewarding potential of morphine

The effect of the non-selective antagonist of excitatory amino acid receptors kynurenic acid (50, 100 and 150 mg/kg, i.p.) on morphine-derived reward was studied in rats. Kynurenic acid dose dependently blocked the acquisition of morphine conditioned place preference when injected before conditioning. The expression of the previously established conditioned behavior was also blocked by the pretreatment with kynurenic acid (at the doses which do not reduce spontaneous locomotor activity) before testing. In the control experiments we failed to find that kynurenic acid alone exerts positive place preference conditioning or aversion. Kynurenic acid also attenuated the morphine-induced facilitation of responding in the intracranial self-stimulation test, causing a decrease in response rate and an increase in threshold current intensity. This effect was observed with doses at which kynurenic acid does not affect responding in the self-stimulation test by itself.