Intracellular and extracellular changes of [Ca2+] in hypoxia and ischemia in rat brain in vivo

Changes in intra- and extracellular free calcium concentration were evaluated with ion-selective microelectrodes during periods of anoxia and ischemia in three different regions of intact rat brain. Recordings stable for at least 2 min and in most cases for 4-6 min were chosen for analysis. Under normoxic conditions neuronal [Ca2+]i varied between less than 10(-8) and 10(-7) M from cell to cell but no systematic regional differences were observed. Elimination of O2 or interruption in blood flow caused, within 30-60 s, slight intracellular alkalinization followed by a small rise in [Ca2+]i, a mild degree of hyperpolarization, and disappearance of electrical activity in the cortex, in that order. It is postulated that a decline in cellular energy levels, as manifested by H+ uptake associated with creatine phosphate hydrolysis, leads to an increase in [Ca2+]i, which activates Ca2(+)-dependent K+ channels and consequently enhances gK. 2-4 min later there was a sudden, large rise in [K+]e, a fall in [Ca2+]e and a rapid elevation of [Ca2+]i. The magnitude of the latter was greatest in a high proportion of hippocampal neurons in area CA1 and some cortical cells, while it was smallest and relatively delayed in thalamic neurons. In the hippocampus area CA1 increases in [Ca2+]i to as much as 6-8 x 10(-4) were observed; some of these could be reversed when O2 or blood flow were restored to normal. Pretreatment of animals with ketamine and MK-801, antagonists of excitatory amino acid transmitters, markedly slowed and decreased the rises in [Ca2+]i. The effects of the two agents were most pronounced in the hippocampus. It is concluded that the receptor-operated channels are largely responsible for Ca2+ entry into certain cells during hypoxia/ischemia. This pathway may be of primary importance in parts of the hippocampus and cortex, regions of the brain that are particularly vulnerable to O2 deprivation and which receive high glutamatergic input and have an abundance of excitatory amino acid receptors.

Protective effects of the glutamate antagonist MK-801 on pyrithiamine-induced lesions and amino acid changes in rat brain

An acute bout of pyrithiamine-induced thiamine deficiency (PTD) produces pathologic lesions within thalamus, mammillary body, and periventricular regions of rat brain. The biological bases for these pathologic changes and their selective distribution within the brain are unclear. The type of tissue damage observed within the thalamus of PTD rats closely resembles that observed following anoxic-ischemic insults and suggests the involvement of excitotoxic amino acids in its pathogenesis. The effects of the N-methyl-D-aspartate receptor antagonist MK-801 (3 mg/kg, i. p.) on brain lesions and amino acid changes have been assessed in rats killed during the late acute stages of PTD. A marked loss of neurons within midline intralaminar nuclei and the posterior nuclear group of the thalamus were observed in the early acute stage of PTD treatment. In the late acute stage, these changes were present throughout the entire thalamus and extended caudally to the periacqueductal gray and mesencephalic tegmentum. Hemorrhagic lesions were observed only in the late acute group and were the primary lesion within the mammillary body and medial and lateral geniculates. No pathologic changes were observed in hippocampus, amygdala, and cortex. MK-801 administered during the late stages resulted in a marked attenuation of necrotic damage to thalamus and periacqueductal gray and a reduction in the number and size of hemorrhagic lesions. Significant reductions of aspartate and glutamate and increases of glycine were observed in 5 regions of thalamus, the hippocampus, hypothalamus, and mammillary bodies of both the early and late acute PTD groups. Levels of GABA and taurine in caudal areas were significantly elevated in the early acute stage but were unchanged from controls in the late acute group. These amino acid changes were reduced in the MK-801 treated late acute group. These observations suggest that NMDA receptors are involved the pathogenesis of PTD-induced brain lesions and that nuclei of the intralaminar and posterior nuclear groups are most vulnerable to PTD effects.

The noncompetitive N-methyl-D-aspartate antagonists, MK-801, phencyclidine and ketamine, increase the potency of general anesthetics

The potency of general anesthetics from different chemical classes was tested after pretreatment with subanesthetic doses of noncompetitive N-methyl-D-aspartate (NMDA) antagonists in mice. Changes in general anesthetic potency were assessed by determination of alteration of duration of loss of righting reflex for ethanol and pentobarbital and changes in the minimum alveolar concentration (MAC) for the volatile anesthetics, halothane and diethyl ether. The ability of the noncompetitive NMDA antagonists, MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclo-hepten-5,10-imine ], phencyclidine (PCP) and ketamine, to increase the potency of general anesthetics paralleled their potency as NMDA antagonists and their affinity for the PCP receptor site of the NMDA receptor-ionophore complex (MK-801 greater than PCP greater than ketamine). These results indicate that block of central NMDA receptors may contribute to the production of anesthesia by a variety of agents.

Interfering with glutamatergic neurotransmission by means of NMDA antagonist administration discloses the locomotor stimulatory potential of other transmitter systems

In the present paper it is shown that when either of the noncompetitive NMDA antagonists MK-801 or ketamine are combined with the alpha-adrenergic agonist clonidine, a pronounced stimulation of locomotion is produced in monoamine-depleted mice. Likewise, when a subthreshold dose of MK-801 is combined with the muscarinic antagonist atropine, a forceful synergism with regard to locomotor activity in monoamine-depleted mice is observed. Furthermore, the present study shows that also in monoamine-depleted rats MK-801, as well as the competitive NMDA antagonist AP-5 (DL-2-amino-5-phosphonovaleric acid), interact synergistically with clonidine to enhance locomotor activity. Taken together, our findings suggest that central glutamatergic systems exert a powerful inhibitory influence on locomotion. Interfering with this inhibitory force by administration of an NMDA antagonist promotes locomotion and discloses the activational potential of other transmitter systems. The results are discussed in relation to 1) the pathophysiology of schizophrenia, with emphasis on the glutamate hypothesis of schizophrenia, and 2) implications for the treatment of Parkinson's disease.

NMDA receptor antagonists inhibit kindling epileptogenesis and seizure expression in developing rats

N-Methyl-D-aspartate (NMDA) receptor antagonists inhibit both the kindling process and the expression of seizures in previously kindled adult rats. Experimental seizures are more readily produced in infant than adult rats, possibly related to a developmental predominance of NMDA receptor-mediated effects. If so, reduction of seizure susceptibility by NMDA receptor antagonists should be more dramatic in infant rats than in adults. We studied the effect of ketamine and MK-801 on kindling epileptogenesis and seizure expression in 15-day-old rats. Ketamine (5, 10, and 20 mg/kg) and MK-801 (0.033 and 0.1 mg/kg) both significantly increased the latency to stage 3 or 4 seizures in dose-dependent fashion. These results were similar to those found in adults but occurred at slightly lower doses. Ketamine 20 mg/kg and MK-801 0.33 mg/kg totally eliminated clinical seizure activity and nearly abolished afterdischarge in previously kindled infant rats, effects exceeding those reported in adults using doses up to 6 times as great. These results support the hypotheses that NMDA receptor-mediated neurotransmission plays an important role in seizure production and the increased seizure susceptibility in immature brain and raise the possibility that NMDA receptor antagonists could be useful antiepilepsy agents in young children.

MK-801 reduced cerebral ischemic injury by inducing hypothermia

The non-competitive N-methyl-D-aspartate (NMDA) antagonist, MK-801, has been reported to prevent or attenuate ischemic brain damage in various animal models. In halothane-anesthetized gerbils it was found that an optimal dose of MK-801 (3.0 mg/kg) for providing cerebral protection also produced hypothermia (31.1 +/- 0.62 degrees C) relative to control animals (34.2 +/- 0.77 degrees C, P less than 0.01). This degree of hypothermia alone was sufficient to provide complete histological and functional protection (spatial memory) against 5 min of carotid artery occlusion. In gerbils made ischemic, but maintained at normal body temperature, a dose of 3.0 mg/kg of MK-801 provided no protection against hippocampal cell loss or spatial memory impairment. These data suggest that the protective actions of MK-801 may be due entirely to drug-induced hypothermia.

Photoaffinity labeling and binding studies reveal the existence of two types of phencyclidine receptors in the NCB-20 cell line

The mouse neuroblastoma-Chinese hamster brain hybrid cell line NCB-20 is the only clonal cell line in which binding studies indicate the presence of phencyclidine (PCP) receptor-like sites. We report here that polypeptide components of NCB-20 cell PCP sites were identified with the photolabile PCP derivative [3H]N-[1-(3-azidophenyl)cyclohexyl]piperidine ([3H]AZ-PCP). The pharmacological selectivity of [3H]AZ-PCP binding (under reversible conditions) was similar to that observed for [3H]N-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) binding to NCB-20 cell membranes. Inhibition of [3H]TCP binding by AZ-PCP, dexoxadrol or MK-801 was biphasic, suggesting the presence of two types of PCP sites on NCB-20 cells. Photolysis of NCB-20 cell membranes pre-equilibrated with [3H]AZ-PCP, followed by SDS-polyacrylamide gel electrophoresis (SDS-PAGE), revealed the presence of 5 major labeled bands (Mr 90,000, 68,000, 49,000, 40,000 and 33,000), a pattern similar to that observed for rat brain membranes. MK-801 and D-2-amino-5-phosphonovaleric acid (D-(-)-AP5) selectively inhibited the labeling of Mr 68,000 and 90,000 polypeptides. These results indicate that the labeled bands represent constituents of at least two different PCP binding proteins. The Mr 68,000 and 90,000 components appear to correspond to a high-affinity site, which comprises approximately 20% of total [3H]TCP sites in these cells, and exhibits the pharmacology expected for the PCP receptor of the N-methyl-D-aspartate (NMDA)-gated channel.

Differential effects of sigma and phencyclidine receptor ligands on learning

Several phencyclidine (PCP) and sigma receptor ligands were examined for their effects on a single trial passive avoidance test in rats. Rats were administered the PCP receptor ligands (+)-5-methyl-10,11-dihydro-5Hdibenzo[a,d]cyclohepten-5,10-im ine maleate (MK-801), PCP, ketamine or the sigma receptor ligands (+)-N-allylnormetazocine ((+)-NANM), (+)-pentazocine, (+)-3-(3-hydroxyphenyl)-N-n-propylpiperidine ((+)-3-PPP) or 1,3-Di(2-[5-3H]tolyl)guanidine (DTG) subcutaneously prior to acquisition of the passive avoidance response, and tested 24 h later for retention. MK-801 (0.1-0.3 mg/kg), PCP (0.54-1.7 mg/kg), ketamine (10.0-17.2 mg/kg) and (+)-N-allylnormetazocine (5.4-10.0 mg/kg) produced significant memory deficits. (+)-Pentazocine (54 mg/kg) and (+)-3-PPP (30 mg/kg) also produced retention deficits, but at significantly higher doses. DTG (0.3-3.0 mg/kg s.c.) had no effect on retention. There was a positive correlation between production of retention deficits and the compounds' PCP receptor binding affinity. The results suggest that the sigma receptor is not involved in learning the passive avoidance response.

Evidence that NMDA receptors contribute to synaptic function in the guinea pig medial vestibular nucleus

Single medial vestibular nucleus neurons were recorded from guinea pig brainstem slices in vitro while superfusing with the selective N-methyl-D-aspartate (NMDA) antagonists, MK801 and CPP. The majority of neurons tested showed a decrease in firing rate in response to these NMDA antagonists, suggesting that NMDA receptors may contribute to the resting activity of MVN neurons.

Long-term D1-dopamine receptor sensitization in neonatal 6-OHDA-lesioned rats is blocked by an NMDA antagonist

Repeated administration of the D1-dopamine agonist SKF-38393 to adult rats having had dopaminergic neurons destroyed early in development results in an increasing enhancement of the behavioral response to SKF-38393 with each dose until a maximum is reached. This increased sensitivity lasts for at least 6 months. In the present study, this long-lasting change in behavioral responsiveness to repeated treatment with SKF-38393, referred to as D1-dopamine receptor priming, was shown to be dose dependent with smaller doses requiring an increased number of administrations to produce a maximal response when compared to higher doses. In addition, priming occurred equally well when treatment intervals ranged from 1 day to 14 days. These latter data reinforced the view that activation of D1-dopamine receptors results in a prolonged change in neural function. In subsequent experiments, D1-dopamine receptor priming was blocked by pretreatment with the NMDA-receptor antagonist MK-801. This antagonism of priming could not be attributed to a blockade of D1-dopamine receptors by MK-801 or to the induction of interfering behaviors. Because an NMDA antagonist interfered with D1-receptor priming as it does with other long-term neural messages, a common requirement for these diverse forms of neuronal plasticity appears to involve activation of the NMDA receptor. This functional link between NMDA receptors and dopaminergic function and its relationship to neuronal plasticity could have relevance to the biochemical mechanisms involved in learning and to symptoms in central disorders during development that worsen over time, particularly those proposed to involve malfunctioning dopaminergic mechanisms.

NMDA receptor agonist and antagonists alter retinal ganglion cell arbor structure in the developing frog retinotectal projection

The development of sensory maps is thought to require an activity-dependent structural rearrangement of afferent terminal arbors within the CNS which recreates the topographic relations of sensory somata present in the periphery. In the frog retinotectal projection, activation of the NMDA receptor plays a role in this structural plasticity. Exposure of the optic tectum of tadpoles to NMDA receptor antagonists results in a rearrangement of retinal ganglion cell arbors so that their organization into a topographic projection and eye-specific stripes is disrupted (Cline et al., 1987; Cline and Constantine-Paton, 1989). Exposure of the optic tectum to the receptor agonist, NMDA, increases the eye-specific segregation of these arbors (Cline et al., 1987). We examined the projection of the supernumerary retina and the morphology of individual retinal afferent arbors of untreated, NMDA-treated, APV-treated, MK801-treated, and MK801/NMDA-treated 3-eyed tadpoles and young postmetamorphic frogs in an effort to understand how NMDA receptor activation is involved in the growth and ordering of retinal arbors. Treatments with MK801 in combination with NMDA resulted in a desegregation of eye-specific stripes, whereas treatments with MK801 or NMDA alone did not. As reported previously, APV treatment resulted in stripe desegregation without increasing the tangential area (measured from 2-dimensional drawings) of the terminal arbors. However, a detailed analysis revealed that the APV-treated tadpole arbors have 35% reduction in branch density (branch tips/area) compared to untreated 3-eyed tadpole arbors. We treated the optic tectum with a range of concentrations of NMDA prepared in the slow-release plastic Elvax. NMDA at 10(-4) M in Elvax was the optimal concentration to produce the sharpening of stripe borders. Exposure of the tectum to NMDA at 10(-6) M in Elvax produced no change in the stripe pattern, while 10(-2) M NMDA in Elvax resulted in beading of the arbors. At the optimal concentration NMDA treatment results in a 75% reduction in the number of axons crossing from a stripe to an interstripe zone. Drawings of individual HRP-labeled, NMDA-treated arbors demonstrate that they have fewer branch points and fewer branch tips. NMDA treatment reduced arbor density by approximately 50%. Arbors drawn from untreated postmetamorphic frogs have twice the branch density of arbors from untreated tadpoles. NMDA treatment in these animals reduced the branch density by 55%, comparable to the reduction seen in tadpole branch density. Our data support a specific hypothesis for NMDA receptor involvement in the activity-dependent structural refinement process within the developing retinotectal projection.(ABSTRACT TRUNCATED AT 400 WORDS)

Interactions of phencyclidine receptor agonist MK-801 with dopaminergic system: regional studies in the rat

Interactions of the potent phencyclidine receptor agonist MK-801 with the dopaminergic system were examined in various brain regions in the rat. MK-801 increased dopamine (DA) metabolism in the pyriform cortex, entorhinal cortex, prefrontal cortex, striatum, olfactory tubercle, amygdala, and septum without affecting DA metabolism in the cingulate cortex and nucleus accumbens. In pyriform cortex and amygdala, MK-801 was more potent than phencyclidine at increasing DA metabolism. Local injections of MK-801 into ventral tegmental area and into the amygdala/pyriform cortex interface indicated that MK-801 may act at the cell body as well as the nerve terminal level to increase DA metabolism and that ongoing dopaminergic neuronal activity is a prerequisite for full drug action.

Competitive and noncompetitive N-methyl-D-aspartate antagonists modify hypoxia-induced membrane potential changes and protect rat hippocampal slices from functional failure: a quantitative comparison

Hippocampal slice survival after hypoxia was improved by exposure to competitive and noncompetitive N-methyl-D-aspartate (NMDA) antagonists. The rapid blockade and reappearance of synaptic transmission during hypoxia was not altered by these antagonists. However, [(+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d] cyclohepten-5, 10-imine maleate (MK-801) (50 microM) and 2-amino-5-phosphonopentanoic acid (dl-AP5) (25 microM) delayed the disappearance of these reactivated orthodromic population spikes. Intracellular recordings showed that MK-801 and dl-AP5 delayed the late, but not early, hypoxic depolarization of pyramidal cells, and improved recovery of membrane potential and input resistance (Rin) in many cells after reoxygenation. No untreated cells showed recovery. Thus, both competitive and noncompetitive NMDA antagonists decreased the hypoxia-induced depolarization and Rin. With the return of evoked population spikes as an index for recovery from hypoxia, the ED50 values for dl-AP5, MK-801, phencyclidine and dextromethorphan were 7.7, 4.5, 7.1 and 75 microM, respectively. Thus, in contrast to their higher affinities for the NMDA receptor MK-801 and phencyclidine were not significantly more potent than dl-AP5 in protecting hippocampal slices from hypoxia, and isobolographic analysis revealed dose-additive interactions. The unexpectedly low potency of the noncompetitive NMDA antagonists in protecting against hypoxia is attributed to magnesium in the buffer and the hypoxia-induced membrane depolarization, both of which have been shown to reduce the effectiveness of noncompetitive NMDA antagonists.

Electrically stimulated rapid gene expression in the brain: ornithine decarboxylase and c-fos

A single electroconvulsive shock (ECS) resulted in a major induction of cerebral ornithine decarboxylase (ODC) mRNA and a rapid and transient elevation of ODC enzyme activity. The proto-oncogene c-fos was also transiently induced under the same conditions. Following a rapid rise in mRNA levels, the messages for these proteins take different courses. The c-fos mRNA fell to below control levels by 1 h, while the ODC mRNA remained elevated beyond 24 h. The ECS-induced elevation of ODC enzyme activity was not abolished by adrenalectomy but was attenuated significantly by the anti-convulsant MK-801. These results imply that the induction of cerebral ODC may be neuronal activity dependent, and suggest that the ODC/polyamine system may be linked to the proposed third messenger cascade, involving c-fos, which couples cell stimulation to gene expression, resulting in long-term adaptive responses.

Ketamine and MK801 attenuate paired pulse inhibition in the olfactory bulb of the rat

We have investigated the effects of the phencyclidine like-compounds ketamine and MK801 on the evoked field potentials of rat olfactory bulb. Low doses of ketamine (3-6 mg/kg) blocked the inhibition of mitral cells by granule cells evoked by stimulation of lateral olfactory tract fibres or by stimulation of olfactory nerve. This blockade was not accompanied by a decrease in granule cell excitation as revealed by field potential recording. MK801 had a similar effect on the inhibition of mitral cells evoked by stimulation of the lateral olfactory tract. As ketamine does not influence the inhibitory action of GABA (Anis et al. 1983) these results suggest that both ketamine and MK801 block inhibition by an action on intrinsic excitatory feed-back circuits in the olfactory bulb.

Sex differences in the inhibitory effects of the NMDA antagonist, MK-801, on morphine and stress-induced analgesia

We examined the effects of intraperitoneal administrations of the noncompetitive NMDA receptor antagonist, (+) MK-801, its inactive enantiomer, (-) MK-801, and the prototypic opiate antagonist, naloxone, on restraint- and morphine-induced analgesia in male and female deer mice, Peromyscus maniculatus. Both restraint (30 min) and morphine (1.0 mg/kg) induced significant analgesic responses with male mice displaying significantly greater levels of opioid-induced analgesia than female animals. These analgesic responses were completely blocked by, naloxone (1.0 mg/kg), significantly reduced by (+) MK-801 (0.25 mg/kg) and unaffected by (-) MK-801 (0.25 mg/kg) pretreatments. There were significant male-female differences in the inhibitory effects of (+) MK-801; the higher levels of morphine- and restraint-induced analgesia of the males were completely blocked, while the lower level analgesic responses of the females were significantly reduced, but not blocked, by (+) MK-801. These observations provide further evidence that NMDA receptors are involved in the mediation of endogenous and exogenous opioid analgesia and show that there are significant male-female differences in the inhibitory effects of (+) MK-801 on opioid-mediated analgesia.

Effects of the NMDA antagonists CPP and MK-801 on radial arm maze performance in rats

The dose- and time-dependent effects of N-methyl-D-aspartate receptor/channel antagonists on radial 8-arm maze performance were examined in rats. Both CPP (1.0-30 mg/kg), a competitive NMDA antagonist, and MK-801 (0.1-1.0 mg/kg), a noncompetitive NMDA antagonist, produced dose-dependent increases in the number of errors made to sample all 8 baited arms. The effective doses of both drugs produced maximal performance impairments 2 hr after IP injection, and no effects after 24 hr. In a second radial arm maze task where only 4 arms were baited, CPP (10 mg/kg) had a somewhat greater effect on the number of working memory errors than on reference memory errors. MK-801 (0.1, 0.33 mg/kg) had no effects on either this task or on a task involving a 1-hr delay between correct choices 4 and 5 on the 8 choice task. CPP (10 mg/kg), however, impaired performance on this latter task. These results indicate that doses of NMDA antagonists, sufficient to block hippocampal long-term potentiation, also disrupt radial arm maze performance.

Inhibitory influence of excitatory amino acid antagonists on penicillin-induced epileptiform bursting in rat hippocampal slices

The inhibitory influence of excitatory amino acid (E.A.A.) antagonists such as kynurenic acid, 2-amino-5-phosphonopentanoic acid (AP5), cis-2,3-piperidine dicarboxylic acid (cis-2,3 PDA) and (+)-5-methyl-10,11,-dihydro-5H-dibenzo(a,d)cyclo-hepten-5,10-imine maleate (MK 801), has been studied on the epileptiform activity elicited in rat hippocampal slices, bathed in penicillin (1 mM). The rank of the inhibitory potency was: MK 801 greater than kynurenic acid greater than cis 2,3 PDA greater than AP5. Moreover, only MK 801 was able to block the last population spike of the penicillin-induced epileptiform bursting in 100% of the experiments. The data indicate that the antiepileptic activity of E.A.A. antagonists on the penicillin epileptiform bursting in CA1 pyramidal cells is low and limited, indicating that the hippocampal area is not the primary site of the anticonvulsant activity of E.A.A. antagonists.

The effect of the NMDA receptor antagonist, MK-801, on the course and outcome of kindling

A rapid kindling procedure was used to distinguish between the anticonvulsant activity of drugs and their ability to retard the kindling process. MK-801 is a specific ligand at the phencyclidine (PCP) recognition site, and acts as a noncompetitive antagonist of NMDA-type glutamate/aspartate receptors. Intraperitoneal injections of MK-801 (0.5-4.0 mg/kg IP) significantly reduced the cumulated effect of 12 2-hr kindling stimulations, as determined from behavioral measures of seizure activity in immediately ensuing 24-hr drug-free kindling sessions; however, the corresponding electrographic effects did not reach significance. MK-801 also showed significant anticonvulsant activity when injected in fully kindled rats. Higher doses tested were accompanied by locomotor and postural effects. The anticonvulsant benzodiazepine, clonazepam, formulated with a proprietary diluent (as Rivotril, Roche), injected in anticonvulsant doses during the first 12 kindling sessions (0.64 mg/kg IP, repeated after 9 hr) did not significantly affect the course of subsequent sessions of drug-free kindling. Systemic injections of kynurenic acid (300-600 mg/kg IP 4 hours), a nonspecific antagonist of glutamate receptors in vitro, were without significant anticonvulsant or antikindling activity. Activity of NMDA-sensitive glutamate/aspartate receptors associated with the PCP recognition site may induce lasting facilitation of neural transmission; this facilitation may be responsible for the remote propagation and progressive enhancement of seizure activity kindled in the amygdala. The facilitatory process appears to be antagonised by MK-801.

Induction of ornithine decarboxylase mRNA in cerebral cortex in response to kainate lesion of nucleus basalis: involvement of NMDA receptors

Excitotoxin lesion of nucleus basalis with kainate or ibotenate results in degeneration of cholinergic neurones and a subsequent cholinergic deficit in cerebral cortex ipsilateral to the lesion. This lesion is accompanied by a massive increase in ornithine decarboxylase (ODC) activity in ipsilateral cortex, which was further investigated in this study by assay of levels of ODC mRNA in cerebral cortex after lesion. Injection of kainate into the nucleus basalis induced a significant increase in ODC mRNA in ipsilateral cerebral cortex which was maximal at 8 h after lesion and declined to control levels by 72 h. This induction showed clear regional specificity and was completely prevented by injection of an N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801, at a dose of 1 mg/kg, 2 h after excitotoxin lesion. This study indicates that excitotoxin lesion causes an early and transient increase in ODC mRNA that is mediated by NMDA receptors and may represent a physiological response to injury.

Degeneration of hippocampal CA1 neurons following transient ischemia due to raised intracranial pressure: evidence for a temperature-dependent excitotoxic process

Degeneration of hippocampal CA1 neurons occurs following transient complete ischemia produced by raised intracranial pressure. Both systemic injection of MK-801 and profound cerebral hypothermia produced by cisternal infusion of room temperature (22-25 degrees C) fluids protect vulnerable CA1 neurons from degeneration. Hypothermia appears to decrease hippocampal extracellular levels of glutamate during and after ischemia but provides only relative protection from ischemia as CA1 degeneration does occur with prolonged (30 min) periods of ischemia. Elevated intracranial pressure appears to produce ischemic degeneration in the hippocampus via an NMDA receptor mediated excitotoxic process which is highly temperature dependent.

MK-801 induces c-fos protein in thalamic and neocortical neurons of rat brain

MK-801, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, leads to a dramatic induction of c-fos-like protein in neurons in deep layers of the neocortex, in dorsal and ventral midline thalamic nuclei and in neurons in the central grey of rat brain. This induction of c-fos by MK-801 is dose-and time-dependent occurring within 2 h and dissipating by 24 h after injection (0.5-8.0 mg/kg, i.p.). The mechanism of this paradoxical induction of c-fos by MK-801 is unclear; however, the pattern of induction appears to follow the distribution of the antagonist-preferring NMDA receptor site.

The effects of N-methyl-D-aspartate receptor blockade with MK-801 upon the relationship between cerebral blood flow and glucose utilisation

The local cerebral circulatory and metabolic effects of MK-801, a selective non-competitive N-methyl-D-aspartate receptor antagonist have been examined in conscious rats with quantitative autoradiographic techniques using [14C]iodoantipyrine and [14C]2-deoxyglucose as tracers. Local cerebral blood flow (CBF) and local cerebral glucose utilisation (GU) were measured in 41 discrete neuroanatomical loci using identical criteria for region of interest localisation. Animals received either saline or MK-801 (0.5 mg/kg in saline) intravenously 10 min prior to the start of GU determination, or 15 min before the CBF measurement. MK-801 effects an immediate transient, elevation in mean arterial pressure (elevated by 30% from baseline) which returned rapidly to preinjection levels and a sustained moderate hypercapnia (arterial carbon dioxide tension increased by 16%) which persisted throughout the measurement periods. Statistically significant changes in GU were observed in 13 brain region structures after MK-801 administration. Glucose utilisation was significantly and markedly elevated with MK-801 in some limbic structures (particularly the hippocampus, posterior cingulate and entorhinal cortices), the inferior colliculus and most of the neocortex displayed moderate reductions in GU after MK-801 treatment. In the majority of brain regions (28 or the 41 studied), MK-801 minimally altered GU. There were widespread alterations in local CBF with MK-801 although in the majority of brain regions (24 of the 41 studied) there was no statistically significant alteration in CBF with MK-801. With one exception (the anterior thalamic nucleus), CBF was increased with MK-801 in all regions in which glucose use was elevated with the drug.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the neuroprotective effects of the N-methyl-D-aspartate antagonist MK-801 and the opiate-receptor antagonist nalmefene in experimental spinal cord ischemia

Both N-methyl-D-aspartate (NMDA)-receptor antagonists and opiate-receptor antagonists have been shown to limit tissue damage after ischemic central nervous system injury. We compared the neuroprotective effects of the noncompetitive NMDA-receptor antagonist MK-801 and the opiate-receptor antagonist nalmefene in a model of global spinal cord ischemia and reperfusion in unanesthetized rabbits. MK-801 (1 mg/kg) or nalmefene (0.1 mg/kg) was administered intravenously 5 minutes after reperfusion. MK-801 treatment and nalmefene treatment each significantly improved the neurologic and histologic outcome compared with saline controls. Differences in these outcome measures between MK-801 treatment and nalmefene treatment did not reach statistical significance. Our results are consistent with the hypothesis that multiple factors, including endogenous opioids and excitatory amino acids, contribute to the secondary tissue injury after central nervous system ischemia. These data also provide further evidence that therapeutic interventions with opiate-receptor antagonists or NMDA antagonists may be beneficial in limiting neurologic dysfunction after ischemic brain or spinal cord injury.

Role of glycine in the N-methyl-D-aspartate-mediated neuronal cytotoxicity

Current evidence indicates that glutamate acting via the N-methyl-D-aspartate (NMDA) receptor/ion channel complex plays a major role in the neuronal degeneration associated with a variety of neurological disorders. In this report the role of glycine in NMDA neurotoxicity was examined. We demonstrate that NMDA-mediated neurotoxicity is markedly potentiated by glycine and other amino acids, e.g., D-serine. Putative glycine antagonists HA-966 and 7-chlorokynurenic acid were highly effective in preventing NMDA neurotoxicity, even in the absence of added glycine. The neuroprotective action of HA-966 and 7-chlorokynurenic acid, but not that of NMDA antagonists 3-(2-carboxypiperazine-4-yl)propylphosphonate and MK-801, could be reversed by glycine. These results indicate that glycine, operating through a strychinine-insensitive glycine site, plays a central permissive role in NMDA-mediated neurotoxicity.

Selective activation of dopaminergic pathways in the mesocortex by compounds that act at the phencyclidine (PCP) binding site: tentative evidence for PCP recognition sites not coupled to N-methyl-D-aspartate (NMDA) receptors

Several lines of evidence suggest a tight functional coupling between N-methyl-D-aspartate (NMDA) and phencyclidine (PCP) receptors. The effects of PCP receptor agonists (PCP, dexoxadrol, ketamine and MK-801) and NMDA receptor antagonists, cis-4-phosphonomethyl-2-piperidine carboxylic acid (CGS-19755) and 3-(2-carboxypiperizin-4-yl)-propyl-1-phosphonic acid (CPP), have been examined on the metabolism of dopamine in the mesocortex, with a view of studying the coupling between these two receptor systems. Phencyclidine receptor agonists selectively increased the metabolism of dopamine in the mesocortex without affecting the metabolism of dopamine in the striatum. N-Methyl-D-aspartate and the competitive antagonists of NMDA receptors did not effect the metabolism of dopamine, neither did the sigma receptor ligands, 1,3-di-(2-tolyl)guanidine (DTG) and rimcazole. Rimcazole also did not affect the increases in the metabolism of dopamine in the mesocortex, seen after MK-801. These data indicate that dopaminergic neurons in the mesocortex are positively modulated by PCP receptors but tentatively suggest that those recognition sites for PCP are not coupled to NMDA receptors.

Structure and activity of hydrogenated derivatives of (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801)

Several hydrogenated derivatives of the potent NMDA antagonist 1 have been prepared and evaluated as competitive inhibitors of [3H]-1 binding. These compounds were also tested for their ability to act as noncompetitive antagonists of NMDA in vitro. These studies indicate that two aromatic rings are not strictly required for high-affinity binding or NMDA antagonism.

MK801 prevents quinolinic acid-induced behavioral deficits and neurotoxicity in the striatum

Bilateral intrastriatal injections of quinolinic acid (QA) (180 nmoles) induced weight loss and neurologic and behavioral deficits including convulsions, decreased catalepsy response to haloperidol, increased nocturnal locomotor activity, and abnormal feeding behavior in adult male Sprague-Dawley rats. Pretreatment with the noncompetitive N-methyl-D-aspartate (NMDA) antagonist, MK801 (4 mg/kg IP) 30 min prior to stereotaxic surgery prevented the appearance of all QA-induced behavioral abnormalities and prevented weight loss. Twelve weeks after surgery the QA-lesioned animals recovered to sham levels on feeding behavior and nocturnal locomotor activity, but showed persistent reductions in haloperidol-induced catalepsy. Histological examination of the QA-lesioned brains showed extensive lesions of the dorsolateral striatum and frontoparietal cortex. MK801 pretreatment protected against these lesions. These results confirm that MK801 treatment prevents the appearance of neuropathological damage after QA neurotoxicity, and further show that neuronal protection with MK801 is correlated with the absence of QA-induced behavioral deficits.

Regional neuroprotective effects of the NMDA receptor antagonist MK-801 (dizocilpine) in hypoglycemic brain damage

Current evidence points to an important role of N-methyl-D-aspartate (NMDA) receptor activation in the pathogenesis of hypoglycemic neuronal death. MK-801 [dizocilpine maleate, (+)-5-methyl-10,11-dihydro-5H-di[a,d]cyclohepten-5,10-imine] is an anticonvulsant compound also known to be a potent noncompetitive antagonist at NMDA receptors, readily crossing the blood-brain barrier after parenteral administration. Treatment of rats with dizocilpine (1.5-5.0 mg/kg) injected intravenously during profound hypoglycemia (blood glucose levels 1.5-2.0 mM) at the stage of delta-wave (1-4 Hz) slowing of the EEG mitigated selective neuronal necrosis in the hippocampus and striatum, assessed histologically after 1-week survival. The degree of neuroprotection in the striatum and in the CA1 pyramidal cells of the hippocampus was dose dependent. Because of concern for a possible hypothermic mechanism of brain protection by MK-801, core temperature was closely monitored and was found not to decrease significantly. Since CBF is normal or increased in hypoglycemia, a fall in brain temperature during hypoglycemia is unlikely to play a role in the mechanism of the neuroprotection seen with the drug. The findings indicate that in profound hypoglycemia, intravenous administration of the NMDA antagonist dizocilpine, even after the appearance of delta-wave EEG slowing, can reduce the number of necrotic neurons in several brain regions and suggest that the neuroprotective effect of MK-801 is not related to hypothermia.

NMDA receptor activation mediates the loss of GABAergic inhibition induced by recurrent seizures

Previously we have shown that delivery of rapidly recurring hippocampal seizures (RRHS) to awake rats causes a rapid kindling and that RRHS in urethane-anesthetized rats leads to a progressive lengthening of afterdischarges and diminution of paired pulse inhibition. The present experiments examined the relationship between the changes in afterdischarge durations and inhibition. Pre-treatment before RRHS with the non-competitive NMDA receptor antagonists MK-801 and ketamine blocked afterdischarge lengthening. MK-801 also prevented RRHS-induced changes in paired pulse inhibition. For pharmacodynamic and pharmacokinetic reasons the ability of ketamine to counteract RRHS-induced changes of paired pulse inhibition was not examined. MK-801 also blocked the rightward shift of stimulus intensity vs. population spike curves which RRHS caused. We suggest that RRHS leads to an enduring diminution of GABAergic inhibition and that this accounts, at least in part, for the lengthening of afterdischarges seen with recurrent hippocampal seizures. In addition, NMDA receptor activation appears to play a role in this decrease of the potency of GABAergic inhibition. However, mechanisms which are not dependent on NMDA receptor activation also play a critical role in hippocampal epileptogenesis.

The effect of NMDA antagonists in the radial arm maze task with an interposed delay

N-Methyl-d-aspartate (NMDA) receptors mediate the triggering of hippocampal long-term potentiation (LTP), a current physiological model of memory. This model was tested in the rat through the effect of (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801, a novel noncompetitive NMDA antagonist) on the radial arm maze (RAM) task with a 15-minute delay interposed at the midpoint choice. In two separate experiments, substereotypical drug doses of MK-801 and phencyclidine (a dissociative anesthetic with NMDA antagonist properties) were given intraperitoneally, before the trial or at the start of the delay. "Efficiency" was impaired in both tasks, but near-instantaneous use of encoded information seemed to be unaffected. This evidence would support a proposed role for NMDA-mediated pathways (and possibly LTP) in delayed stages of memory formation or use.

A role for N-methyl-D-aspartate (NMDA) receptors in the control of LH secretion and initiation of female puberty

The physiological role of N-methyl-D-aspartate (NMDA) receptors in controlling LH secretion and the initiation of puberty was investigated using two specific antagonists, MK-801 and DL-2-amino-5-phosphono valeric acid (AP-5). Single daily sc injections of MK-801 (0.1-0.2 mg/kg BW), a noncompetitive NMDA receptor antagonist, given to prepubertal rats significantly delayed but did not prevent the timing of puberty, as determined by the age at vaginal opening and first ovulation. Infusion of MK-801 (5 micrograms/h) via osmotic minipumps for 4 days inhibited the postovariectomy rise of LH secretion in prepubertal rats. Both MK-801 (0.2 mg/kg BW, sc) and AP-5 (4 x 30 mg, iv), a competitive NMDA receptor antagonist, blocked the estradiol-induced LH surge in prepubertal ovariectomized rats. These results demonstrate that blockade of NMDA receptors can prevent the development of enhanced LH secretion in female rats undergoing sexual maturation. Moreover, they support the view that activation of NMDA receptors significantly contributes to the physiological initiation of the pubertal process.

Synthesis and structure-activity relationship of C5-substituted analogues of (+-)-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine [(+-)-desmethyl-MK801]: ligands for the NMDA receptor-coupled phencyclidine binding site

A series of eight C5-substituted analogues of (+-)-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (1) have been prepared by the directed lithiation-alkylation (and acylation) of its (+-)-N-tert-butylformamidinyl derivative 2 followed by formamidine solvolysis. An additional 10 analogues were prepared by elaboration of the C5-ethyl ester derivative. Analogues possessing large (e.g. propyl and larger) lipophilic substituents displace [3H]-1-(1-thienylcyclohexyl)piperidine [( 3H]TCP) from the high-affinity phencyclidine (PCP) binding site in rat brain homogenates only at high concentrations (Ki greater than 1000 nM); however, the presence of a polar amino functionality (e.g. 2-aminoethyl) offsets this effect (Ki = 20 nM). Thus, the boundary condition for lipophilic substituents larger than ethyl appears to be polar in nature. Interaction of the 11 relatively small (MR less than 14) C5-substituted analogues of 1 with the high-affinity PCP binding site associated with the N-methyl-D-aspartate (NMDA) receptor is best described by the equation log (1/Ki) = -5.83F + 0.64 pi + 7.41 (r = 0.90).

Application of potassium chloride to the brain surface induces the c-fos proto-oncogene: reversal by MK-801

The proto-oncogene c-fos is activated in the brain by a variety of stimuli including brain injury. In unilateral brain injury, c-fos immunoreactivity is confined to the damaged hemisphere, an effect reminiscent of spreading depression. Here we show that topical application of KCl (3 M) to the brain surface (which induces spreading depression) is accompanied by ipsilateral increase in c-fos immunolabeling. The activation of c-fos, like spreading depression, is markedly reduced by the non-competitive NMDA antagonist MK-801 (3 mg/kg i.p.).

Kainate-induced functional deficits are not blocked by MK-801

Male, Fischer-344 rats were pretreated with MK-801 (0.1, 1.0 or 10.0 mg/kg, i.p.) prior to bilateral injection of kainate (0.33 micrograms/site) into the dorsal and ventral hippocampus. Kainate impaired the acquisition of a water maze acquisition task 4 weeks after surgery, an effect not attenuated by pretreatment with MK-801. However, higher doses (1.0 and 10.0 mg/kg) of MK-801 reduced the amount of kainate-induced granule cell and to some extent CA1 pyramidal cell damage in the hippocampus. Kainate-induced CA3/CA4 damage was not affected by MK-801 pretreatment. MK-801 (10 mg/kg) also reduced the amount of thalamic damage produced by kainate. These data support the conclusion that intrahippocampal kainate-induced destruction of CA3/CA4 pyramidal cells is mediated by non-N-methyl-D-aspartate (non-NMDA) receptors and that kainate-induced loss of these cells is associated with the neurobehavioral effects of intrahippocampally administered kainate.

Anticonvulsant effects of antagonists of the N-methyl-D-aspartate receptor complex in a genetic model of epilepsy: the quaking mouse

Tonic-clonic convulsions of mutant quaking mice were antagonized by the intracerebroventricular injection of N-methyl-D-aspartate receptor antagonists. The competitive antagonists, CPP (3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid) and CGS 19755 (cis-4-(phosphonomethyl)-2-piperidine carboxylic acid), exerted a partial anticonvulsant action, with ED50S of 0.115 and 0.076 nmol, respectively. The non-competitive antagonists, TCP (1-(1-(2-thienyl)cyclohexyl)piperidine) and MK-801 [+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine), provided full protection, with ED50s of 4.49 and 2.67 nmol, respectively. The competitive antagonists elicited a marked ataxia whereas the non-competitive antagonists did not have side-effects. These results might reflect the involvement of glutamatergic neurotransmission in the convulsions of the quaking mutants.

Ethanol withdrawal seizures and the NMDA receptor complex

Prior biochemical and electrophysiological studies have shown that low doses of ethanol inhibited calcium influx through the N-methyl-D-aspartate (NMDA) receptor/ionophore. The present data show that chronic ethanol treatment results in an increase in the number of NMDA receptor/ionophore complexes in the hippocampus, a brain area known to be associated with ethanol withdrawal seizure activity. Treatment during withdrawal with NMDA-exacerbated handling induced withdrawal seizures in the ethanol-dependent mice, while administration of the NMDA receptor-associated calcium channel antagonist MK-801 decreased the occurrence and severity of the withdrawal seizures in a dose-dependent manner. The results are consistent with the hypothesis that the up-regulation of the NMDA receptor systems following chronic ethanol treatment may mediate the seizures associated with ethanol withdrawal in dependent animals.

Chronic treatment with MK-801 increases the quinolinic acid-induced loss of D-1 dopamine receptors in rat striatum

Following withdrawal from short-term treatment with the non-competitive N-methyl-d-aspartate receptor antagonist MK-801 (1 mg/kg per day) there was no significant change in the Bmax or KD of [3H]SCH23390 binding to dopamine D-1 receptors in rat striatum. Intrastriatal injection of the excitotoxin quinolinic acid (100 nmol) produced a significant decrease in [3H]SCH23390 binding. In rats withdrawn from chronic MK-801 treatment quinolinic acid produced a significantly greater loss of [3H]SCH23390 binding sites than in rats not treated with MK-801. These data indicated that striatal neurons are hypersensitive to the neurotoxic actions of quinolinic acid following withdrawal from chronic MK-801 treatment.

The neuroprotective actions of kynurenic acid and MK-801 in gerbils are synergistic and not related to hypothermia

The broad spectrum excitatory amino acid antagonist, kynurenic acid was evaluated in a transient forebrain ischaemia model in gerbils. When administered i.p., 15 min prior to a 5 min period of ischaemia, a dose-related neuroprotective effect was seen with 800 mg/kg of kynurenic acid showing very good protection. A combination of kynurenic acid (200 or 400 mg/kg) and MK-801 (0.1 mg/kg) gave a synergistic neuroprotective effect. Neither kynurenic acid (200 or 400 mg/kg) nor MK-801 (0.3 mg/kg) was neuroprotective when administered by itself 30 min post-ischaemically, but when co-administered significant protection of the CA1 pyramidal neurones of the hippocampus was seen. In addition, we examined the effect of maintaining core body temperature on the neuroprotective action of MK-801 and kynurenic acid following the suggestion that it was a hypothermic effect of MK-801 which resulted in neuroprotection in gerbils. When the body temperature of the gerbils was maintained at 37 degrees C for a period of 24 h it did not affect the neuroprotective action of MK-801 (0.1 or 10 mg/kg) or kynurenic acid (200 mg/kg).

MK-801, an antagonist of NMDA receptors, inhibits injury-induced c-fos protein accumulation in rat brain

Unilateral lesions of the rat hippocampus produced by needle insertion lead to ipsilateral accumulation of c-fos protein in dentate granule cells and neurons in the piriform cortex, as well as in glial-like cells in the corpus callosum and in ependymal cells lining the lateral ventricle adjacent to the lesion site. C-fos protein was detected immunocytochemically using two different antibodies in formalin-fixed brain sections. The N-methyl-D-aspartate (NMDA) antagonist MK-801 produced a dose- and time-dependent inhibition of c-fos protein accumulation in dentate granule cells and in neurons in the piriform cortex, but did not affect glial or ependymal c-fos protein accumulation. MK-801 at 4 mg/kg injected two hours before lesion inhibited c-fos accumulation. Thus, c-fos protein accumulation in hippocampal neurons and in neurons in the piriform cortex induced after traumatic brain injury involves activation of NMDA receptors.

Effects of N-methyl-D-aspartate (NMDA) antagonist MK-801 on breathing pattern in rats

In anaesthetized, bivagotomized and artificially ventilated rats, the respiratory effects of systemic injection of MK-801, a non-competitive N-methyl-D-aspartate antagonist, were studied. In all the experiments (n = 11), the injection increased the inspiratory duration and decreased the expiratory duration. In 4 experiments, the inspiratory duration was drastically lengthened, resulting in an apneustic-like breathing pattern. These results demonstrate that apneusis is difficult but possible to induce in rats and suggest that termination of inspiration is controlled via central mechanisms in which NMDA-like receptors are involved.

Effects of hypoxia-ischemia and MK-801 treatment on the binding of a phencyclidine analogue in the developing rat brain

The phencyclidine analogue [3H](1-[2-thienyl]cyclohexyl)piperidine (3H-TCP) binds to the ion channel associated with the N-methyl-D-aspartate receptor channel complex. In vitro autoradiography indicates that the distribution of 3H-TCP binding in brain closely parallels that of [3H]glutamate binding to the N-methyl-D-aspartate receptor. In nine 7-day-old rats, an acute focal hypoxic-ischemic insult produced by unilateral carotid artery ligation and subsequent exposure to 8% oxygen acutely reduced 3H-TCP binding ipsilateral to the ligation by 30% in the CA1, by 27% in the CA3, by 26% in the dentate gyrus, and by 17% in the striatum compared with values from the contralateral hemisphere. In 10 littermates that received 1 mg/kg of the neuroprotective noncompetitive N-methyl-D-aspartate antagonist MK-801 immediately before hypoxic exposure, the regional distribution of 3H-TCP binding in hypoxic-ischemic brain was relatively preserved and there were no interhemispheric asymmetries in 3H-TCP binding densities. In addition, in three unoperated rats decapitated 24 hours after MK-801 treatment, 3H-TCP binding was reduced by 15-35%; similar bilateral suppression of 3H-TCP binding was detected in MK-801-treated ligates. Our data indicate that 3H-TCP autoradiography can be used to assay the efficacy of neuroprotective agents in this experimental model of perinatal stroke.

Serum and depolarizing agents cause acute neurotoxicity in cultured cerebellar granule cells: role of the glutamate receptor responsive to N-methyl-D-aspartate

The life span of neonatal rat cerebellar granule cells, grown in basal minimal Eagle's medium containing 10% (vol/vol) fetal calf serum, was extended to 21-30 days by weekly supplementation with glucose. Addition of 1% fetal calf serum to the culture at 14 days killed 85% of the cells within 1 hr. This lethal effect could be prevented by the N-methyl-D-aspartate (NMDA) receptor antagonists dibenzocyclohepteneimine (MK-801) and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP). These findings suggested that the glutamate in the serum caused the dramatic neuronal death through action on the NMDA receptor. Indeed, a 5-min incubation in a Locke physiological salt solution containing 20 microM glutamate and 5 microM glycine killed 55-90% of the cells. This acute toxicity could be prevented by a lyso-GM1 ganglioside with N-acetylated sphingosine. The relatively low glutamate content of the sera analyzed suggests that factors in addition to glycine potentiate serum neurotoxicity. The above noted antagonists of the NMDA receptor also greatly reduced the lethal effect of depolarization by 90 mM KCl or 10 microM veratridine. Therefore, it is likely that the toxicity of the depolarizing agents is mediated by glutamate released from the cells. It is concluded that survival of cerebellar neurons in primary culture may be strongly affected by unsuspected neurotoxic phenomena elicited by brief action of a rather low glutamate concentration.

MK-801 potently inhibits alcohol withdrawal seizures in rats

The ability of MK-801, an N-methyl-D-aspartate (NMDA)-channel antagonist, to suppress alcohol withdrawal seizures generated audio-genically was studied in adult male rats using a cross-over experimental design. MK-801 treatment reduced overall seizure score and proportion of rats seizing. In comparison to other seizure models, alcohol withdrawal seizures seem to be particularly sensitive to MK-801, suggesting that mechanisms which result in seizure susceptibility after withdrawal of chronic ethanol exposure may be dependent upon sensitization or upregulation of NMDA processes.

Activation of NMDA receptors induces dephosphorylation of DARPP-32 in rat striatal slices

In the caudate-putamen the glutamatergic cortical input and the dopaminergic nigrostriatal input have opposite effects on the firing rate of striatal neurons. Although little is known of the biochemical mechanisms underlying this antagonism, one action of dopamine is to stimulate the cyclic AMP-dependent phosphorylation of DARPP-32 (dopamine and cAMP-regulated phospho-protein, of relative molecular mass 32,000 (32K]. This phosphorylation converts DARPP-32 from an inactive molecule into a potent inhibitor of protein phosphatase-1. Here we show that activation of the NMDA (N-methyl-D-aspartate) subclass of glutamate receptors reverses the cAMP-stimulated phosphorylation of DARPP-32 in striatal slices through NMDA-induced dephosphorylation of DARPP-32. Thus, the antagonistic effects of dopamine and glutamate on the excitability of striatal neurons are reflected in antagonistic effects of these neurotransmitters on the state of phosphorylation of DARPP-32. Our results indicate that stimulation of NMDA receptors leads to the activation of a neuronal protein phosphatase, presumably the calcium-dependent phosphatase calcineurin, and show, in an intact cell preparation, that signal transduction in the nervous system can be mediated by protein dephosphorylation.

MK-801 ameliorates delayed amnesia, but potentiates acute amnesia induced by CO

The effects of non-competitive N-methyl-D-aspartate receptor antagonists on amnesia induced by carbon monoxide (CO) were investigated, since they have neuroprotective effects on delayed degeneration induced by ischemia. In the mice exposed to CO, acute and delayed amnesia were induced. (+)-MK-801 and (-)-MK-801 improved the delayed amnesia, but the effects of phencyclidine (PCP) were weak. (+)-MK-801 and PCP potentiated the acute amnesia. From these results, it is suggested that there is a stereoselectivity in the effects of MK-801 on CO-induced amnesia and that CO-induced delayed amnesia animals could be used as an ischemic amnesia model.

Behavioral effects of MK-801 in the rat

Several experiments were conducted to study the effects of the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801, on learning and memory in the rat. Rats displayed impaired performance on several sensorimotor tests and appeared grossly intoxicated when treated IP with 0.2 mg/kg MK-801, but not when treated with lower doses (0.05 or 0.1 mg/kg). Postacquisition performance on two spatial learning tasks involving working memory protocols (reinforced alternation and radial arm maze) was impaired by MK-801 at intoxicating doses (greater than or equal to 0.2 mg/kg) but not at lower doses (0.05 or 0.1 mg/kg). Using a position habit reversal task, we found that rats could learn to reverse a position habit while under the influence of a nonintoxicating dose of MK-801 (0.1 mg/kg), but when tested on the following day performed as if they did not recall what they had learned. Thus, acute administration of a nonintoxicating dose of MK-801 disrupts the retention of new information learned under the influence of the drug but does not interfere with the performance of tasks that are well learned before the drug is administered. Whether the performance deficits on the spatial learning tasks observed only following intoxicating doses of MK-801 reflect an effect on memory is not clear.

Attenuation of febrile seizures in epileptic chicks by N-methyl-D-aspartate receptor antagonists

Experimental febrile seizures can be evoked in epileptic chicks by elevation of their body temperature. Both competitive N-methyl-D-aspartate (NMDA) receptor antagonists [(3-(+/- )2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), DL-2-amino-7-phosphosphonoheptanoic acid (APH), DL-2-amino-5-phosphonovaleric acid (APV), D-alpha-aminoadipic acid (AAA), and DL-alpha, epsilon-diaminopimelic acid (DAP)] and the noncompetitive NMDA antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5, 10-imine maleate (MK-801) produced dose-dependent increases in latency to the onset of seizures. Of the drugs tested, MK-801 had the highest potency followed in order by CPP = APH greater than APV much greater than AAA greater than DAP. There was a high correlation (r = 0.995) between the dose capable of doubling seizure latency and the affinity of the competitive NMDA antagonists for the NMDA receptor as determined by in vitro binding assays. These data suggest that NMDA receptor mediated mechanisms may be involved in the production of seizures in response to hyperthermia.

On the possible involvement of glutamate receptors in conditioning of behavioural effects of apomorphine

It was shown previously that behavioural effects of apomorphine (locomotor activation and stereotyped behaviour) can be conditioned when they are associated with well-defined environmental stimuli. In the present study, the hypothesis was tested that glutamatergic mechanisms play an important role either in formation of conditioned responses to apomorphine or in the expression of previously established conditioned responses. For this purpose, two blockers of glutamate receptors were applied, either MK-801 (dizocilpine), a non-competitive, but selective blocker of NMDA-type receptors or MLV-6976, a non-selective blocker of glutamate receptors. MK-801 produced some locomotor activation by itself in a dose-dependent way (0.125-0.50 mg/kg ip). The locomotor activation produced by 0.25 mg/kg could not be conditioned. When rats were conditioned 9 times with 2 mg/kg apomorphine after pretreatment with 0.25 mg/kg of MK-801, this pretreatment did not prevent the development of apomorphine-conditioned locomotor activity or stereotypies which appeared when the rats were treated with saline in presence of the conditioned stimuli. Similar results were obtained when rats were conditioned 7 times with the same dose of apomorphine after pretreatment with 20 mg/kg ip MLV-6976, which drug did not induce any visible alterations in motility by itself. When rats were conditioned 7 times with 2 mg/kg apomorphine alone and tested with MK-801 (0.25 mg/kg) in the presence of the conditioned stimuli, neither locomotor activity nor stereotypies appeared as conditioned responses. When rats were conditioned with the same dose of apomorphine alone and tested with MLV-6976 (20 mg/kg ip), stereotypies did not appear as conditioned responses, but some locomotor activity occurred.(ABSTRACT TRUNCATED AT 250 WORDS)