Competitive N-methyl-D-aspartate Research Articles

Behavioral and molecular evidence for psychotropic effects in l-theanine

l-Theanine (N-ethyl-l-glutamine) is an amino acid uniquely found in green tea and historically considered to be a relaxing agent. It is a glutamate derivative and has an affinity for glutamatergic receptors. However, its psychotropic effects remain unclear. To elucidate effects of l-theanine on psychiatric disease-related behaviors in mice and its molecular basis focusing on brain-derived neurotrophic factor (BDNF) and N-methyl-d-aspartate (NMDA) receptor. We examined the effects of l-theanine on behaviors in mice by using the open-field test (OFT), forced swim test (FST), elevated plus-maze test (EPMT), and prepulse inhibition (PPI) of acoustic startle. By western blot analysis, we looked at the effect of l-theanine on the expression of BDNF and related proteins in the hippocampus and cerebral cortex. To determine whether l-theanine has agonistic action on the NMDA receptor, we performed Fluo-3 intracellular Ca2+ imaging in cultured cortical neurons. Single administration of l-theanine significantly attenuated MK-801-induced deficits in PPI. Subchronic administration (3-week duration) of l-theanine significantly reduced immobility time in the FST and improved baseline PPI. Western blotting analysis showed increased expression of BDNF protein in the hippocampus after subchronic administration of l-theanine. In cultured cortical neurons, l-theanine significantly increased the intracellular Ca2+ concentration, and this increase was suppressed by competitive and non-competitive NMDA receptor antagonists (AP-5 and MK-801, respectively). Our results suggest that l-theanine has antipsychotic-like and possibly antidepressant-like effects. It exerts these effects, at least in part, through induction of BDNF in the hippocampus and the agonistic action of l-theanine on the NMDA receptor.

Chronic administration of MK-801 and the NMDA receptor: Further evidence for reduced sensitivity of the primary acceptor site from studies with the cortical wedge preparation

Cortical slices removed from rats pre-treated with MK-901 0.5 mg kg-1 twice a day for 7 days had reduced responses to N-methyl-D-aspartate (NMDA) relative to quisqualate and glutamate compared with control animals. Potencies of competitive (CPMP) and non-competitive (ketamine) NMDA antagonists appeared unchanged. These changes are consistent with a reduced density of NMDA receptors.

NMDA-gated ion channel research and its therapeutic potentials in neurodegenerative diseases: a review

NMDA-gated ion channel research and its therapeutic potentials in neurodegenerative diseases: a review Maryam Majdi1, Huei-Sheng Vincent Chen1,21Center for Neuroscience, Aging and Stem Cell Research, Burnham Institute for Medical Research, La Jolla, California, USA; 2Division of Cardiology, University of California, San Diego, San Diego, California, USAAbstract: The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor (NMDAR) is essential for normal function of the central nervous system (CNS). Classical NMDARs, activated by glycine and glutamate, are heteromultimers comprising NR1 and NR2 subunits. Nonetheless, excessive activation of NMDARs by excitatory amino acids such as glutamate is thought to mediate neuronal damage in many neurological disorders. The dual role of NMDARs in normal and abnormal functioning of the CNS imposes significant constraints on possible therapeutic strategies aimed at ameliorating neurodegenerative diseases. To create safe NMDAR-based therapies, blockade of excessive NMDAR activity must therefore be achieved with minimal interference on its normal neuronal function. In general, NMDAR antagonists can be classified pharmacologically according to the site of action on the receptor-channel complex. These include drugs acting at the agonist sites (NMDA and glycine), channel pore, and modulatory sites. Both competitive NMDA and glycine antagonists result in generalized inhibition of NMDAR activities and have, thus, failed in clinical trials. Open-channel blockers with uncompetitive antagonism and drugs modulating NMDAR activities are appealing therapeutic strategies because, in theory, these properties could decrease neurotoxicity due to excessive levels of glutamate while sparing physiological neurotransmission. We review here NMDAR-related research that may lead to future therapeutic intervention against neurotoxicity.Keywords: excitotoxicity, open-channel block, uncompetitive antagonism, Alzheimer disease, memantine

Werner J. Schmidt (1950–2007) pushing the boundary of neuroscience: a true academician and a complete gentleman

was truly 'a man for all seasons'. Character, not temperament, decency, not easy triumphs, integrity, rather than cheap compromise, and respectful love of fellow humankind, not popularity, guided him as he placed family, friends and colleagues, and his science and work at the centre of his life. Throughout our lives we shall never meet a finer human being or friend. Werner was an honest man. He died on April 16, 2007 in Tubingen. Werner contributed importantly to many areas within the broad field of neuroscience. We will mention a few examples that demonstrate the unique impact that his work has had upon our understanding of brain disease processes. Thus, Werner studied the functional and neurobiological role of N-methylD-aspartate (NMDA) receptors in different brain regions and their contribution to health and disease. For example, in "Behavioural effects of NMDAreceptor antagonists" published in the J. Neural Transm. (Vol. 43:63-69, 1994), he reviewed the effects of NMDA receptor antagonists in anticonvulsant and anticataleptic activity in a number of behavioural paradigms. In particular he showed that glutamate receptor antagonists produced behavioural effects similar to those of dopamine receptor agonists. In subsequent work with TM Tzschentke he expanded on these basic findings to investigate dopamine-glutamate interactions in reward-related learning (Neurosci. Lett. 193:37-40; 1995; Behav. Brain Res. 84:99-107, 1997) and he combined behavioural and neurochemical testing using intracerebral micordialysis of multiple brain regions to detail the differential mechanisms of action of competitive and non-competitive NMDA receptor antagonists (with W Hauber in Behav. Brain Res. 41:161-166, 1990; with M Bubser, U Keseberg, and PK Notz in Eur. J. Pharmacol. 229:75-82, 1992); these papers collectively produced over 400 citations. Werner integrated his own findings and those of many researchers from European and North American countries in his recent book co-edited with ME Reith, Dopamine and Glutamate in Psychiatric Disorders (Humana Press, 2005). This volume contains multiple detailed perspectives on the interactive role of these two neurotransmitter systems in psychiatric diseases, that sets a new standard in understanding the links between brain neuochemical systems and neuropsychiatric disorders. This volume is a good indicator of Werner's major impact on the field of neuroscience. Werner developed animal models to investigate and understand neurological and psychiatric disease states that afflict humans. For example, in "LowF.P. Graham Publishing Co.

A glycine site antagonist ZD9379 reduces number of spreading depressions and infarct size in rats with permanent middle cerebral artery occlusion.

Spreading depressions (SDs) occur in experimental focal ischemia and contribute to lesion evolution. N-methyl-D-aspartate (NMDA) antagonists inhibit SDs and reduce infarct size. The glycine site on the NMDA receptor complex offers a therapeutic target for acute focal ischemia, potentially devoid of many side effects associated with competitive and non-competitive NMDA antagonists. We evaluated the effect of the glycine antagonist, ZD9379, on SDs and brain infarction. Male Sprague-Dawley rats (n = 18) weighing 290 to 340 g undergoing permanent middle cerebral artery occlusion (MCAO) were randomly and blindly assigned to receive drug or placebo: Group 1 (pre-MCAO treatment group, n = 5) a 5 mg/kg bolus of ZD9379 over 5 minutes followed by 5 mg/kg/hour drug infusion for 4 hours beginning 30 minutes before MCAO; Group 2 (post-MCAO treatment group, n = 7) a 5 mg/kg bolus of ZD9379 30 minutes after MCAO followed by 5 mg/kg/hour drug infusion for 4 hours; and Group 3 (control group, n = 6) vehicle for 5 hours beginning 30 minutes before MCAO. SDs were monitored electrophysiologically for 4.5 hours following MCAO by continuous recording of cortical direct current (DC) potentials and electrocorticogram (ECoG). Infarct volume was measured 24 hours after MCAO by 2,3,5 triphenyltetrazolium chloride (TTC) staining. Corrected infarct volume was 90 +/- 72 mm3 (mean +/- standard deviation) in Group 1, 105 +/- 46 mm3 in Group 2, and 226 +/- 41 mm3 in Group 3 (P < .001). The corresponding numbers of SDs in the 3 groups were 8.2 +/- 5.8, 8.1 +/- 2.5, and 16.0 +/- 5.1, respectively (P < .01). When all animals (n = 18) were analyzed, infarct volumes and the number of SDs were significantly correlated (r = .68, P = .002). This study demonstrated that ZD9379 initiated before or after MCAO significantly reduced the number of SDs and infarct volume in a permanent focal ischemia model, implying that ZD9379 is neuroprotective and its neuroprotective effect may be related to inhibiting ischemia-related SDs.

Association between the low threshold calcium spike and activation of NMDA receptors in guinea-pig substantia nigra pars compacta neurons.

The aim of this study was to examine the interaction between N-methyl-D-aspartate (NMDA) receptor activation and the low threshold calcium spike (LTS) of phasically firing neurons in the rostral part of the substantia nigra pars compacta (SNpc) in mid-brain slices. Bath perfusion of 10 microM NMDA gradually increased the LTS area and the effect reached a maximum after 6 min of perfusion. This enhancement of the LTS by NMDA was blocked both by a competitive and non-competitive NMDA receptor antagonist, 50 microM D-AP5 and 10 microM MK801, respectively, demonstrating that this effect of NMDA was mediated through NMDA receptors. Prolonged exposure to increasing concentrations of NMDA (0.1-100 microM) progressively decreased the LTS area. The higher doses led to an irreversible marked depolarization and decrease of the membrane resistance. These results suggest that the LTS of SNpc neurons can trigger a NMDA receptor-dependent response which may have physiological and pathological roles.

Treatment with the competitive NMDA antagonist GPI 3000 does not improve outcome after cardiac arrest in dogs.

We previously showed that treatment with a competitive N-methyl-D-aspartate (NMDA) receptor antagonist GPI-3000 (GPI) improved short-term physiological recovery after incomplete global cerebral ischemia complicated by dense acidosis. We tested the hypothesis that GPI administered after resuscitation from cardiac arrest would improve a more long-term recovery as measured by neurobehavioral assessment and neuropathology 4 days after resuscitation. Anesthetized dogs were subjected to 7 minutes of cardiac arrest followed by vest cardiopulmonary resuscitation. Neurobehavioral outcomes were scored daily on a score ranging from 0 (normal) to 500 (worst). On the fourth day, the animals were killed, and neuropathology was evaluated in a blinded manner in the hippocampus and the neocortex by hematoxylin and eosin staining and by determination of percentage of injured neurons. Three groups of animals were treated in a randomized, blinded protocol with either saline (SAL), low-dose GPI (5 mg/kg followed by 1 mg/kg per hour for 2 hours), or high-dose GPI (25 mg/kg, followed by 5 mg/kg per hour for 2 hours). The mortality rate was higher in animals receiving GPI than in saline-treated control animals (4 of 15 deaths in SAL, 6 of 15 in the low-dose GPI group, and 9 of 18 in the high-dose GPI group). Neurobehavioral scores were depressed in GPI-treated animals compared with saline-treated control animals in a dose-dependent manner, with 96-hour scores of essentially normal (9+/-2) in saline-treated animals compared with those animals with significant impairment (181+/-47) treated with high-dose GPI. Neuropathological damage in the neocortex was most severe in GPI-treated animals, with the percentage of injured neurons dependent on the dose: 8.3%+/-2.7% SAL, 13.2%+/-6.4% low-dose GPI, and 39.4%+/-10.1%, high-dose GPI. CA1 neuronal damage was severe regardless of treatment. Contrary to results seen in experimental global and focal cerebral ischemia, in which NMDA receptor antagonism may improve responses to injury, receptor antagonism with GPI does not improve brain outcome after cardiac arrest and resuscitation in the dog. Behavioral and histological outcomes both were worsened by GPI treatment at two doses, and mortality was higher relative to saline control treatment. We speculate that systemic drug effects, as well as potential neurotoxicity of the drug under ischemic conditions, may be responsible for the deleterious outcomes observed in our cardiac arrest model.

Weak anticonvulsant effects of two novel glycineB receptor antagonists in the amygdala-kindling model in rats.

In the present work we evaluated the anticonvulsant effects of two novel antagonists of the glycine co-agonist site (glycineB receptor) within the N-methyl-D-aspartate (NMDA) receptor complex, MRZ 2/576 (a tricyclic pyrido-phtalazin dione derivative) and L-701,324 (7-chloro-4-hydroxy-3-(3-phenoxy)phenyl-2(H)quinoline). As a model of epilepsy we used amygdala-kindled rats, which are considered as a model to study the efficacy of drugs against human complex partial seizures. MRZ 2/576 (2.5-10 mg/kg i.p. 15 min before testing) did not influence afterdischarge threshold, which is believed to be the most subtle indicator of efficacy against kindled seizures, nor did it affect other measures of seizure activity such as seizure severity, seizure duration and afterdischarge duration. However, MRZ 2/576 produced dose-dependent ataxia as measured in the open field and rotarod test. The highest dose tested (10 mg/kg) also markedly reduced rectal temperature (by about 1.5 degrees C). L-701,324 (2.5-10 mg/kg i.p. 30 min before testing) dose dependently and significantly increased afterdischarge threshold, but other seizure parameters remained unchanged. The ataxia produced by lower doses of L-701,324 (2.5 and 5 mg/kg) was more pronounced than that caused by MRZ 2/576. However, the ataxia observed following the higher dose of L-701,324 (10 mg/kg) was less intense than that elicited by MRZ 2/576. The behavioral alterations produced by the two drugs did not resemble those characteristic for classical competitive and uncompetitive NMDA receptor antagonists. In conclusion, our data indicate that glycineB receptor antagonists are not promising candidates for the treatment of complex partial seizures in humans, at least as monotherapy.

The neuroprotective effect of the forebrain-selective NMDA antagonist CP101,606 upon focal ischemic brain damage caused by acute subdural hematoma in the rat.

The neuroprotective effects of drugs that act against excitotoxic damage, caused by glutamate, are well described in focal ischemia, but behavioral effects, and apparent failure in clinical trials of "first-generation" competitive N-methyl D-aspartate (NMDA) antagonists, such as Selfotel (CGS19755), has led to interest in evaluating newer NMDA antagonists with fewer behavioral effects. We have therefore evaluated the neuroprotective effect of a new forebrain-selective polyamine site NMDA antagonist, CP101,606 in a rat subdural hematoma (SDH) model. An SDH was produced by slow injection of 0.4 ml autologous blood into the parietal subdural space. Brain damage was assessed histologically at eight coronal planes, in animals sacrificed 4 h after induction of hematoma. The drug was infused 30 min after induction of SDH. The reductions of ischemic brain damage achieved by CP101,606, was 29% for the low dose and 37% for the high dose. This novel glutamate antagonist has shown a magnitude of neuroprotection which is comparable with that seen with "first-generation" NMDA antagonists such as MK801, D-CPP-ene and CGS19755, in this same model. This new agent is claimed to have fewer psychomotor and behavioral effects than MK801, D-CPP-ene, and CGS19755.

Glutathione is an endogenous ligand of rat brain N-methyl-D-aspartate (NMDA) and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors.

A study was made of the effects of reduced (GSH) and oxidized (GSSG) glutathione on the Na(+)-independent and N-methyl-D-aspartate (NMDA) displaceable bindings of glutamate, on the binding of kainate, 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), and ligand of the brain NMDA receptor-ionophore complex: glycine, dizocilpine (MK-801) and (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP). GSH and GSSG strongly inhibited the binding of glutamate, CPP and AMPA, kainate and glycine binding being less affected. Both peptides enhanced the binding of dizocilpine in a time- and concentration-dependent manner. This activatory effect was not additive to that of saturating concentrations of glutamate or glutamate plus glycine. The activation of dizocilpine binding by GSH and GSSG was prevented by the competitive NMDA and glycine antagonists DL-2-amino-5-phosphonovalerate and 7-chlorokynurenate. GSH and GSSG may be endogenous ligands of AMPA and NMDA receptors, binding preferably to the glutamate recognition site via their gamma-glutamyl moieties. In addition to this, at millimolar concentrations they may regulate the redox state of the NMDA receptor-ionophore complex.

Parallel antioxidant and antiexcitotoxic therapy improves outcome after incomplete global cerebral ischemia in dogs.

We have previously shown that incomplete global cerebral ischemia complicated by dense acidosis produces a profound secondary deterioration of energy metabolism and cerebral blood flow. Antioxidant treatment only partially averts this deterioration, suggesting that parallel or sequential mechanisms are involved in cerebral ischemic injury. We tested the hypothesis that a novel competitive N-methyl-D-aspartate (NMDA) receptor antagonist GPI 3000 (GPI) ameliorates metabolic injury and that the effectiveness of the iron-chelator and antioxidant deferoxamine (DFO) is augmented by combined therapy with GPI after incomplete global cerebral ischemia. Anesthetized dogs were treated with 30 minutes of global incomplete cerebral ischemia. Preischemic plasma glucose was raised to approximately 500 mg/dL to exaggerate lactic acidosis. Brain ATP, phosphocreatine, and pH were measured by 31P MR spectroscopy for 180 minutes of reperfusion. Neurophysiological outcomes were assessed by evoked potential monitoring. Five groups were treated with either saline; 75 mg/kg DFO preischemia plus 75 mg/kg at reperfusion onset, followed by 27.5 mg/kg per hour for the remainder of reperfusion (DFO group); 25 mg/kg GPI pretreatment, followed by 5 mg/kg per hour (GPI-pre group); 25 mg/kg GPI at reperfusion, followed by 5 mg/kg per hour (GPI-post group); or DFO and GPI-pre at the same doses (Combined group). Ischemic cerebral blood flow (microspheres: 5 to 8 mL/min per 100 g) was similar among the groups. End-ischemic pHi was also similar; 5.9 in saline, 6.1 in DFO, 6.2 in GPI-pre, 6.2 in Combined, and 6.1 in GPI-post groups. Progressive hypoperfusion was observed in all groups except Combined during reperfusion. Metabolic recovery was improved relative to saline in all drug-treated groups. Phosphocreatine recovery was improved in Combined compared with DFO and GPI-pre groups. Somatosensory evoked potential recovery was not observed in the saline group and incomplete in all treatment groups. At 60 and 90 minutes of reperfusion, DFO, GPI-pre, and Combined groups demonstrated improved recovery relative to the saline group. Pretreatment and posttreatment with GPI ameliorated postischemic metabolic failure, suggesting that NMDA-mediated mechanisms are more important in global cerebral ischemia complicated by dense, acidosis than early studies indicated. Combined treatment with GPI and DFO improved cerebral blood flow during reperfusion and one indicator of energy recovery. These data support the hypothesis that parallel therapy aimed at antioxidant and antiexcitotoxic mechanisms of ischemic brain injury augment recovery compared with the individual agents.

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

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

NMDA-mediated activation of the NO/cGMP pathway: characteristics and regulation in cultured neocortical neurones.

The linkage of the N-methyl-D-aspartate (NMDA) subtype of L-glutamate receptor to the nitric oxide (NO)/3, 5'-cyclic guanosine monophosphate (cGMP) intracellular signalling system was investigated in murine neocortical cultures by examining the effects of NMDA antagonists, NO synthase inhibitors, and drugs targeting second messenger systems on NMDA-stimulated synthesis of cGMP. NMDA-stimulated synthesis of cGMP was time- and concentration-dependent, and inhibited by competitive (LY 274614, 100 mu M) and non-competitive NMDA antagonists (MK-801 30 mu M, 7-chlorokynurenate 100 mu M, and ifenprodil 100 mu M). NO synthase inhibitors (NG-nitro-L-arginine, KN-62, diphenyleneiodonium) and LY 83583, an inhibitor of guanylate cyclase, all inhibited NMDA-stimulated cGMP synthesis in a concentration-dependent manner, demonstrating its dependence on the two enzymes. Phorbol 12-myristyl 13-acetate (0.1 mu M), arachidonic acid (1 mu M), and thapsigargin (10 mu M) produced approximately 50% inhibition of NMDA-induced cGMP synthesis. These observations demonstrate that all domains of the NMDA receptor-complex and of NO synthase are active in neocortical neuronal cultures, and that the essential NO/cGMP signalling system has complex interactions with other second messengers.

Failure of carbamazepine to prevent behavioural and histopathological sequels of experimentally induced status epilepticus

Sustained electrical stimulation of the perforant pathway was used to induce long-lasting hippocampal seizures in conscious rats. One hour prior to stimulation, rats were given i.p. injections of either saline or a commonly used antiepileptic drug, carbamazepine (5 H-dibenz[ b,f]azepine-5-carboxamide; CBZ; 20 mg/kg). When tested 2 weeks later in a water maze, both the saline- and the carbamazepine-pretreated rats showed similarly a severe impairment in spatial learning compared to non-stimulated controls. Histological evaluation revealed that the pyramidal cell damage was ( P < 0.05) milder in the carbamazepine-pretreated group in the CA1, but not the CA3c subfield. However, the number of somatostatin-immunoreactive neurons in both stimulated groups was reduced equally. Thus, at the dose of 20 mg/kg, which is a usual anticonvulsive dose in humans, carbamazepine seems to offer only partial protection against pyramidal cell damage, but no protection against the hilar somatostatin-immunoreactive neuron loss or the spatial learning deficit after perforant pathway stimulation in rats. The result clearly differs from that obtained either with a GABA (γ-aminobutyric acid)-enhancing drug and a novel antiepileptic, vigabatrin (4-amino-hex-5-enoic acid) or with a competitive NMDA ( N-methyl- d-aspartate) receptor antagonist, CGP 39551 ( dl-[ E]-2-amino-4-methyl-5-phosphono-3-pentenoic acid carboxyethylester) in the same test situation.

The effects of N-methyl-D-aspartate agonists and antagonists on isolated bovine cerebral arteries.

This pharmacologic study examines the direct cerebrovascular effects of N-methyl-D-aspartate (NMDA) receptor agonists and antagonists to determine whether large cerebral arteries have NMDA receptors. Bovine middle cerebral arteries were cut into rings to measure isometric tension development in vitro. Two competitive agonists, L-glutamate and NMDA, each had negligible effects on ring tension in the absence of exogenous vasoconstrictors. L-glutamate (in high concentrations) produced direct relaxation of potassium (K+)-constricted arteries, but the relaxation was not selective for L-glutamate, D-glutamate, or mannitol. Relaxation with L-glutamate was abolished when it was isosmotically substituted in the K(+)-rich medium. NMDA (in the absence or presence of glycine) and two competitive antagonists, 2-amino-5-phosphopentanoic acid (AP5) and (+/-)-3-(s-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), each had little effect on the tone of arteries preconstricted with potassium or the stable thromboxane A2 analog U-46,619. Three noncompetitive antagonists (S(+)-ketamine, dizocilpine, and dextrorphan) and their steroeisomers (R(-)-ketamine, (-)MK-801, and levorphanol) each produced dose-dependent relaxation of K(+)- or U-46,619-constricted arteries; relaxation was not selective for the (+) or (-) stereoisomers. These results suggest that large cerebral arteries lack NMDA receptors mediating constriction or relaxation. All noncompetitive antagonists dilated cerebral arteries, but by mechanisms that were not stereospecific.

Chronic treatment with MK-801 affects the behavioral response to both D1 and D2 dopamine agonist in the one-trial inhibitory avoidance

Post-training administration of the N-methyl-D-aspartate (NMDA) antagonists CPP (0.5 and 1.0 mg/kg) and MK-801 (0.25 and 0.5 mg/kg) impaired, in a dose dependent fashion, the one-trial inhibitory avoidance response in NMRI mice. The D1 dopamine (DA) agonist SKF 38393 (10 and 20 mg/kg) and the D2 agonist quinpirole (0.5 and 1.0 mg/kg) instead facilitate the response in the same behavioral paradigm. Sub-chronic blockade of NMDA receptors with MK-801 (0.25 mg/kg once a day for 14 days) did not change the response to both competitive (CPP) and non-competitive (MK-801) NMDA antagonists. The same chronic treatment with MK-801 induced an increased response to both SKF 38393 and quinpirole. These data suggest that repeated administration of MK-801 induce an upregulation of both D1 and D2 DA receptors without affecting NMDA receptors.

Effects of phencyclidine and dizocilpine on NMDA-, kainate-, and water deprivation-induced drinking in pigeons

The excitatory amino acid (EAA) agonists, N-methyl-D-aspartate (NMDA) and kainate, elicit a copious drinking response in pigeons. NMDA-induced drinking, as compared with kainate- and water deprivation-induced drinking, is selectively antagonized by the competitive, NMDA receptor antagonist CGS 19755, and appears to be mediated by NMDA receptors located in brain. There have been several studies which have reported differences between competitive and non-competitive (PCP-like) NMDA antagonists in blocking various behavioral effects of NMDA, such as discriminative stimulus effects. The present studies examined the effects of the non-competitive antagonists, phencyclidine (PCP) and dizocilpine, on drinking elicited by NMDA, kainate, and water deprivation. PCP and dizocilpine were effective antagonists of NMDA-induced drinking, resulting in surmountable shifts to the right in agonist dose-response functions. These compounds had little effect on drinking evoked by either kainate or water deprivation. These results support the notion that the dipsogenic effects of NMDA are mediated by NMDA-type receptors, and also provide important information as to the characteristics of non-competitive NMDA antagonists. EAA-induced drinking provides a useful tool for the examination of the behavioral pharmacology of EAA agonists and antagonists.

Estradiol selectively regulates agonist binding sites on the N-methyl-D-aspartate receptor complex in the CA1 region of the hippocampus.

Estradiol alters cognitive function and lowers the threshold for seizures in women and laboratory animals. Both of these activities are modulated by the excitatory neurotransmitter glutamate in the hippocampus. To assess the hypothesis that estradiol increases the sensitivity of the hippocampus to glutamate activation by increasing glutamate binding sites, the densities of N-methyl-D-aspartate (NMDA) agonist sites (determined by NMDA displaced glutamate), competitive antagonist sites (CGP 39653), noncompetitive antagonist sites (MK801) as well as the non-NMDA glutamate receptors for kainate and AMPA (using kainate and CNQX, respectively) were measured using autoradiographic procedures. Two days of estradiol treatment increased the density of NMDA agonist, but not of competitive nor noncompetitive NMDA antagonist binding sites exclusively in the CA1 region of the hippocampus. The density of noncompetitive NMDA antagonist sites, however, was decreased in the dentate gyrus by estradiol treatment. Ovarian steroids had no effect on the density of kainate or AMPA receptors in any region of the hippocampus examined. These data indicate that the agonist and antagonist binding sites on the NMDA receptor/ion channel complex are regulated independently by an as yet unidentified mechanism, and that this regulation exhibits regional specificity in the hippocampus. The increase in NMDA agonist sites with ovarian hormone treatment should result in an increase in the sensitivity of the hippocampus to glutamate activation which may mediate some of the effects of estradiol on learning and epileptic seizure activity.

Polyamines inhibit [formula omitted] antagonist-induced darting behavior in the rat prefrontal cortex

The competitive NMDA ( N- methyl- d-aspartate ) receptor antagonist, CPP (3(2-carboxypiperazin-4-yl)-propyl-1-phophonic acid), microinjected into the medial prefrontal cortex of rats, induces a unique behavioral syndrome termed ‘darting’, characterized by rapid leaping across an open field arena 15. In addition, CPP induces generalized hyperactivity when microinjected into the medial prefrontal cortex, nucleus accumbens, and caudate nucleus. Polyamine modulation of the NMDA receptor was tested at the medial prefrontal cortex microinjection site in this behavioral paradigm. The polyamine spermidine, and its diamine precursor, blocked CPP-induced darting behavior, as well as CPP-induced hyperactivity, at doses which did not decrease locomotor activity when administered alone. The putative polyamine antagonists, ifenprodil and diethylenetriamine, did not prevent spermidine from inhibiting CPP-induced darting. These results suggest that polyamines, presumably by acting as positive allosteric modulators of the NMDA receptor, can inhibit the CPP-induced behavioral syndrome at the prefrontal cortex site.

Inhibition of high-affinity [ 3H]L-proline binding to rat brain membranes by 2-amino-7-phosphonoheptanoic acid

L-Glutamate and 2-amino-5-phosphonoheptanoic acid (AP-7) (10 −9 to 10 −6 M), a competitive NMDA (N-methyl-D-aspartate) antagonist, inhibit ≈ 60% of [ 3H]L-proline binding to rat membranes from midbrain. In hippocampal membranes. AP-7 inhibits proline binding by 80%, while in cerebellar membranes AP-7 had little effect. These results are indicative of the possible neuromodulatory role of proline in the central nervous system, possibly through the NMDA receptor(s).