Abstract: Brain injury induced by fluid percussion in rats caused a marked elevation in extracellular glutamate and aspartate adjacent to the trauma site. This increase in excitatory amino acids was related to the severity of the injury and was associated with a reduction in cellular bioenergetic state and intracellular free magnesium. Treatment with the noncompetitive N-methyl-D-aspartate (NMDA) antagonist dextrophan or the competitive antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid limited the resultant neurological dysfunction; dextrorphan treatment also improved the bioenergetic state after trauma and increased the intracellular free magnesium. Thus, excitatory amino acids contribute to delayed tissue damage after brain trauma; NMDA antagonists may be of benefit in treating acute head injury.

Abstract: The ion current induced by the glutamate receptor agonist N-methyl-D-aspartate (NMDA) in voltage-clamped hippocampal neurons was inhibited by ethanol (EtOH). Inhibition increased in a concentration-dependent manner over the range 5 to 50 mM, a range that also produces intoxication. The amplitude of the NMDA-activated current was reduced 61 percent by 50 mM EtOH; in contrast, this concentration of EtOH reduced the amplitude of current activated by the glutamate receptor agonists kainate and quisqualate by only 18 and 15 percent, respectively. The potency for inhibition of the NMDA-activated current by several alcohols is linearly related to their intoxicating potency, suggesting that alcohol-induced inhibition of responses to NMDA receptor activation may contribute to the neural and cognitive impairments associated with intoxication.

Abstract: This paper reports a series of 5 experiments concerned with a possible role for N-methyl-D-aspartate (NMDA) receptors in certain types of learning. The results show that chronic intraventricular infusion of the NMDA receptor antagonist D,L-2-amino-5-phosphonopentanoic acid (D,L- AP5) caused an impairment of spatial but not of visual discrimination learning in rats. Such selectivity of the learning impairment occurred despite widespread distribution of the drug throughout the CNS. AP5 sometimes caused a disturbance of sensorimotor function during learning, but one experiment addressing whether this disturbance could be responsible for the spatial learning impairment established that it was statistically independent. Another experiment showed that AP5 did not affect the retention of previously acquired spatial information. These behavioral effects were all obtained with a concentration of AP5 that, in a final study, was found to be sufficient to block hippocampal long-term potentiation (LTP) in vivo without affecting normal synaptic transmission. Taken together, these observations (1) implicate NMDA receptors in certain types of learning, and (2) extend recent work showing that saturation of LTP causes an anterograde spatial amnesia (McNaughton et al., 1986). A preliminary report of parts of this work has been published (Morris et al., 1986a).

Abstract: Membranes from rat telencephalon contain a single class of strychnine-insensitive glycine sites. That these sites are associated with N-methyl-D-aspartic acid (NMDA) receptors is indicated by the observations that [3H]glycine binding is selectively modulated by NMDA receptor ligands and, conversely, that several amino acids interacting with the glycine sites increase [3H]N-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) binding to the phencyclidine site of the NMDA receptor. The endogenous compound kynurenate and several related quinoline and quinoxaline derivatives inhibit glycine binding with affinities that are much higher than their affinities for glutamate binding sites. In contrast to glycine, kynurenate-type compounds inhibit [3H]TCP binding and thus are suggested to form a novel class of antagonists of the NMDA receptor acting through the glycine site. These results suggest the existence of a dual and opposite modulation of NMDA receptors by endogenous ligands.

Abstract: A CENTRAL assumption about long-term potentiation in the hip-pocampus is that the two classes of glutamate-receptor ion channel, the N-methyl-D-aspartate (NMDA) and the kainate/quisqualate (non-NMDA) subtypes, are co-localized at individual excitatory synapses1,2. This assumption is important because of the perceived interplay between NMDA and non-NMDA receptors in the induc-tion and expression of long-term potentiation: the NMDA class, by virtue of its voltage-dependent channel block by magnesium3,4 and calcium permeability5,6, provides the trigger for the induction of long-term potentiation, whereas the actual enhancement of synaptic efficacy is thought to be provided by the non-NMDA class7,9. If both receptor subtypes are present at the one synapse, such cross-modulation could occur rapidly and locally through diffusible factors. By measuring miniature synaptic currents in cultured hippocampal neurons we show that the majority (∼70%) of the excitatory synapses on a postsynaptic cell possess both kinds of receptor, although to different extents. Of the remaining excita-tory synapses, ∼20% contain only the non-NMDA subtype and the rest possess only NMDA receptors. This finding provides direct evidence for co-localization of glutamate-receptor subtypes at individual synapses, and also points to the possibility that long-term potentiation might be differentially expressed at each synapse according to the mix of receptor subtypes at that synapse.

Abstract: Measurements of calcium uptake and cyclic GMP production by cerebellar granule cells grown in primary culture demonstrated that ethanol preferentially inhibited N-methyl-D-aspartate (NMDA) receptor-gated cation channel function. Concentrations of ethanol as low as 10 mM inhibited NMDA-stimulated Ca2+ uptake by greater than 30%, and ethanol also inhibited NMDA-stimulated (Ca2+-dependent) cyclic GMP accumulation in a similar, dose-dependent manner. Responses to kainate were significantly less sensitive to ethanol. Studies using various concentrations of NMDA, as well as phencyclidine (PCP) and glycine, suggested that ethanol affected the "coagonist" binding site of the NMDA receptor-channel complex, rather than the PCP recognition site.

Abstract: Long-term potentiation (LTP) in the hippocampus is widely studied as the mechanisms involved in its induction and maintenance are believed to underlie fundamental properties of learning and memory in vertebrates. Most synapses that exhibit LTP use an excitatory amino-acid neurotransmitter that acts on two types of receptor, the N-methyl-D-aspartate (NMDA) and quisqualate receptors. The quisqualate receptor mediates the fast synaptic response evoked by low-frequency stimulation, whereas the NMDA receptor system is activated transiently by tetanic stimulation, leading to the induction of LTP. The events responsible for maintaining LTP once it is established are not known. We now demonstrate that the sensitivity of CA1 neurons in hippocampal slices to ionophoretically-applied quisqualate receptor ligands slowly increases following the induction of LTP. This provides direct evidence for a functional post-synaptic change and suggests that pre-synaptic mechanisms also contribute, but in a temporally distinct manner, to the maintenance of LTP.

Abstract: Responses to the excitatory amino acid N-methyl-D-aspartate (NMDA) are markedly potentiated by nanomolar concentrations of glycine. This is due to the action of glycine at a novel strychnine-resistant binding site with an anatomical distribution identical to that for NMDA receptors, suggesting that the NMDA receptor channel complex contains at least two classes of amino-acid recognition site. Antagonists at the glycine-binding site associated with NMDA receptors act as potent non-competitive antagonists, but do not alter the mean open time or conductance, as estimated by fluctuation analysis. The mechanisms by which glycine acts on NMDA receptors are unknown, but single-channel recording experiments show an increase in opening frequency with no change in mean open time or conductance, suggesting that glycine could regulate transitions to states that are intermediate between binding of NMDA receptor agonists and ion-channel gating. It has been suggested that glycine acts as a co-agonist at the NMDA receptor, and that responses to NMDA cannot be obtained in the complete absence of glycine, but in these experiments the response to NMDA was measured at equilibrium, and it is unlikely that sufficient temporal resolution was achieved to detect rapid alterations in receptor gating. Using a fast perfusion system we find that glycine regulates desensitization at NMDA receptors; this has a major effect on the response to NMDA measured at equilibrium, as would occur with slower applications of agonist. Reduction of NMDA receptor desensitization by glycine provides an example of a novel mechanism for regulation of ion-channel activity.

Abstract: It was shown in the present study that the selective non-competitive N-methyl-D-aspartate (NMDA) antagonist MK-801 [(+)-5-methyl-10,11-dihydroxy-5H-dibenzo(a,d)cyclohepten-5,10-imin e] caused a pronounced and dose-dependent increase in locomotion in mice pretreated with a combination of reserpine and alpha-methyl-para-tyrosine. Haloperidol pretreatment did not antagonize the MK-801-induced stimulation of locomotion. The findings are discussed in relation to the concept of a corticostriatothalamocortical negative feedback loop serving to protect the cortex from an overload of information and hyperarousal. Such a feedback loop would encompass i.a. corticostriatal glutamatergic neurons and it would be modulated by mesencephalostriatal dopaminergic neurons.

Abstract: Activation of NMDA (N-methyl-D-aspartate) receptors by neurotransmitter glutamate stimulates phospholipase A2 to release arachidonic acid. This second messenger facilitates long-term potentiation of glutamatergic synapses in the hippocampus, possibly by blocking glutamate uptake. We have studied the effect of arachidonic acid on glutamate uptake into glial cells using the whole-cell patch-clamp technique to monitor the uptake electrically. Micromolar levels of arachidonic acid inhibit glutamate uptake, mainly by reducing the maximum uptake rate with only small effects on the affinity for external glutamate and sodium. On removal of arachidonic acid a rapid (5 minutes) phase of partial recovery is followed by a maintained suppression of uptake lasting at least 20 minutes. Surprisingly, the action of arachidonic acid is unaffected by cyclo-oxygenase or lipoxygenase inhibitors suggesting that it inhibits uptake directly, possibly by increasing membrane fluidity. As blockade of phospholipase A2 prevents the induction of long-term potentiation (LTP), inhibition of glutamate uptake by arachidonic acid may contribute to the increase of synaptic gain that occurs in LTP. During anoxia, release of arachidonic acid could severely compromise glutamate uptake and thus contribute to neuronal death.

Abstract: Administration of convulsant doses of Metrazole (pentylenetetrazol) and picrotoxin, as well as maximal electroshock, results in a rapid but transient increase in c-fos mRNA in mouse brain. Elevation of c-fos mRNA is followed by the accumulation and subsequent disappearance of Fos, the protein encoded by c-fos. In addition, immunoblots reveal the induction of two additional proteins that are antigenically related to Fos (Fra, Fos-related antigens). Fos and the various Fra appear and disappear in a staggered manner over an 8 hour period, such that at longer times after stimulation the brain contains no Fos but relatively large amounts of Fra (the latter being designated here by their apparent molecular weights, Fra-46K and Fra-35K). Previous studies have established that Fos, as well as several Fra, contribute to transcription factor AP-1 nucleoprotein complexes along with Jun, the product of the jun proto-oncogene. The appearance in brain of Fos and Fra coincides with a protracted increase in total AP-1 DNA binding activity, indicating that all the Fos-like proteins can participate in AP-1 complexes. Furthermore, the molecular composition of these complexes alters with time after stimulation. The induction of c-fos by Metrazole is blocked or attenuated by known anticonvulsants such as diazepam and valproate as well as the N-methyl-D-aspartate (NMDA) receptor antagonists, 2-amino-5-phosphonovaleric acid (APV) and MK-801. This suggests that fos induction might involve stimulation of a glutamate receptor. This conclusion was strengthened by the observations that two glutamate receptor agonists, kainic acid and NMDA, induced c-fos expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Abstract: The role of N-methyl-D-aspartate (NMDA) receptors in cat visual cortex was studied as a function of both layer and age by iontophoresis of the NMDA antagonist (D)-2-amino-5-phosphonovaleric acid (APV). Effects on both visual responses and spontaneous activity were observed. In superficial layers (II and III), D-APV reduced visual responses substantially at all ages. Iontophoresis of D-APV with 10 nA of ejecting current for 2–3 min was sufficient to reduce the response to approximately one third of control levels. The magnitude of the reduction did not vary with age. In granular and deep layers (IV, V, and VI), D-APV affected the visual response in young animals but only spontaneous activity in older animals. On average, visual responses were reduced to about half at 20–23 days of age and to about 75% at 4 weeks of age but in most cases were not significantly affected in adults. The rapid change in the functional effect of NMDA receptors over the third and fourth week in granular and deep layers suggests a role in development. There was a reasonable age correlation between the change in effect and the period of geniculocortical afferent segregation. Further experiments will be necessary to determine whether NMDA receptors are necessary for segregation to occur. The presence of an NMDA component to the visual response in the adult in layers II and III argues either that these layers retain some form of plasticity in the adult or that NMDA receptors play a role in the transmission of normal visual input to these layers.

Abstract: 1. Responses to the excitatory amino acids kainate, quisqualate, N-methyl-D-aspartate (NMDA), L-glutamate and L-aspartate were recorded in mouse hippocampal neurones in cell culture, using the whole-cell configuration of the patch clamp technique. Agonists were applied rapidly from an array of flow pipes each of 250 microns diameter, positioned within 100 microns of the nerve cell body. 2. Responses to NMDA, L-aspartate and to low concentrations of L-glutamate, recorded with glycine in the extracellular fluid, were strongly antagonized by 50 microM-zinc. Responses to kainate, quisqualate, and in glycine-free solution, responses to L-glutamate, were potentiated by 50 microM-zinc, but partially antagonized by 1 mM-zinc. On average, with 50 microM-zinc, responses to NMDA were reduced to 0.19 times control, while responses to kainate and quisqualate were increased to 1.09 and 1.14 times control. With 1 mM-zinc responses to kainate and quisqualate were reduced to 0.54 and 0.42 times control. 3. Cadmium had a similar, though less potent action, and at 50 microM antagonized responses to NMDA but potentiated responses to kainate and quisqualate. On average, with 50 microM-cadmium, responses to NMDA were reduced to 0.39 times control, while responses to kainate and quisqualate were increased to 1.08 and 1.15 times control. With 1 mM-cadmium responses to NMDA were reduced to 0.04 times control while responses to kainate and quisqualate were reduced to 0.79 and 0.60 times control. Mercury was neurotoxic and increased the leakage current; however, no reduction of the response to NMDA was produced by 5 microM-mercury. 4. The equilibrium dissociation constant (Kd) for zinc antagonism of responses to NMDA, estimated from fit of a single binding site adsorption isotherm, was 13 microM; cadmium was about 4 times less potent than zinc. These effects of zinc and cadmium were nearly voltage independent. In contrast the antagonism of responses to NMDA by 150 microM-magnesium was highly voltage dependent, such that the Kd for magnesium increased e-fold per 17.6 mV depolarization. 5. The potency of zinc as an NMDA antagonist did not vary with the concentration of NMDA, and was not greatly influenced by a 1000-fold variation in the concentration of the NMDA-modulator glycine. This suggests that zinc acts as a non-competitive antagonist, and does not directly interfere with the binding of NMDA to the agonist recognition site nor with the binding of glycine to an allosteric site on the NMDA receptor complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Abstract: Antagonists of the N-methyl-D-aspartate (NMDA) subclass of glutamate receptors may offer a new approach for the treatment of ischemic brain injury. This strategy is supported by a well-developed scientific foundation and encouraging results in a variety of in vivo and in vitro experimental models. Several specific antagonists, including MK-801, dextrorphan, dextromethorphan, and ketamine, have already been used at low doses in humans for other indications and are potential candidates for Phase I clinical trials.

Abstract: L-Glutamate, the natural agonist of quisqualate- and N-methyl-D-aspartate (NMDA)-sensitive excitatory amino acid receptors, elicits a rapid, transient, dose-dependent increase of the steady state level of c-fos mRNA followed by an accumulation of c-fos protein immunostaining in cell nuclei. This induction is prevented by 2-amino-5-phosphonovalerate, an isosteric glutamate receptor antagonist, and by Mg2+ ion and phencyclidine, two noncompetitive allosteric antagonists of NMDA-sensitive glutamate receptors. Kainate and quisqualate (up to 150 microM) failed to alter the basal expression of c-fos mRNA. Furthermore, glycine, a positive allosteric modulator of NMDA-sensitive glutamate receptors, potentiated the glutamate response in a strychnine-insensitive manner. Activation of other transmitter receptors present in these cells (gamma-aminobutyric acid(A), gamma-aminobutyric acid(B), and muscarinic) failed to increase c-fos mRNA expression. Our results provide evidence that activation of NMDA-sensitive glutamate receptors plays an exclusive role in the induction of c-fos mRNA expression and translation in primary cultures of granule cells. It can be inferred that, by this mechanism, glutamate can initiate a transcriptional program that may result in changes in the simultaneous expression of a set of target genes involved in neuron-specific responses.

Abstract: In the present study the effect of L-glutamic acid (L-Glu), N-methyl-D-aspartic acid (NMDA), kainic acid (KA) and quisqualic acid (QUIS) on the release of endogenous dopamine (DA) from slices of the rat caudate-putamen was investigated. DA was measured by high-performance liquid chromatography coupled to an electrochemical detector. L-Glu, NMDA, KA and QUIS, in the absence of Mg++, produced a dose-related, Ca++-dependent increase in DA release. The order of agonist efficacy was L-Glu greater than NMDA greater than KA = QUIS. D-2-amino-7-phosphonoheptanoic acid (0.5 mM), but not L-2-amino-7-phosphonoheptanoic acid, antagonized the action of L-Glu and NMDA, but did not modify the effect of KA or QUIS. Tetrodotoxin (0.1 microM) partially inhibited the stimulatory effect of KA and QUIS, but not that of L-Glu or NMDA. Mg++ (1.2 mM) abolished the excitatory effect of NMDA, significantly reduced the action of L-Glu, but did not influence the action of KA or QUIS. The inhibitory action of Mg++ on the L-Glu-induced DA release was reversed when L-Glu was coupled to high concentrations of K+. N-allylnormetazocine (SKF-10,047), a benzmorphan agent, produced a stereospecific inhibition of L-Glu-induced DA release. This inhibition was also produced by 1-[1-(2-thienyl)cyclohexyl]piperidine, a phencyclidine receptor ligand, but not by 1,3-di-O-tolylguanidine, a sigma receptor-selective ligand. The results of this study show that L-Glu increases DA release predominantly by activation of the NMDA receptor located presynaptically on dopaminergic afferents.(ABSTRACT TRUNCATED AT 250 WORDS)

Abstract: A heat-stable factor of low molecular weight that increases the binding of [3H]MK-801 to rat brain membranes in the presence of maximally effective concentrations of L-glutamate and glycine was purified from bovine brain by reverse phase and ion-exchange high pressure liquid chromatography. The stimulatory activity was due to the presence of spermidine in the active fractions. Polyamines including spermine and spermidine are found in high concentrations in mammalian tissue. These compounds increase the affinity of N-methyl-D-aspartate (NMDA) receptors for [3H]MK-801 when assays are carried out in the presence of 100 microM L-glutamate and 100 microM glycine. At concentrations of 1 to 300 microM, a number of di- and triamines, including NH2(CH2)3NH2, NH2(CH2)3NH(CH2)2NH2, and NH2(CH2)3NH(CH2)3NH2, have partial or full agonist-like activity similar to that of spermidine. Other polyamines, including putrescine, cadaverine, NH2(CH2)2NH(CH2)2NH2, and CH3NH(CH2)3NHCH3, at concentrations of 1 to 100 microM, inhibited the binding of [3H]MK-801 in the presence of spermine, L-glutamate, and glycine but not in the presence of only L-glutamate and glycine. It is concluded that these compounds are selective antagonists of the effects of spermine at the NMDA receptor. These results suggest that there may be a polyamine recognition site on the NMDA receptor complex.

Abstract: One class of excitatory amino-acid receptors, the N-methyl-D-aspartate (NMDA) receptors, mediates transmission at a small, but important, group of synapses in the neocortex. These receptors are implicated in neuronal plasticity during development in young mammals and in memory acquisition in adults. Recently, responses of isolated membrane patches to NMDA were shown to be greatly enhanced by glycine. This, together with the demonstration that the strychnine-insensitive glycine-binding site is distinct from, but linked to, the NMDA receptor has excited intense interest in glycine as a synaptic modulator. Before proposing a physiological function, however, it is important to determine whether glycine could enhance synaptic responses to NMDA receptor activation in intact, adult tissue. An earlier study failed to demonstrate enhancement of NMDA responses when glycine was applied and it was proposed that in intact tissue the high-affinity glycine site was already saturated by endogenous glycine. It remained possible that glycine concentrations can be maintained at low levels close to synaptic receptors. We have examined responses of neurons in slices of adult neocortex to focal applications of excitatory amino acids and glycine and report enhancement by glycine of NMDA receptor-mediated excitatory postsynaptic potentials.

Abstract: We have investigated the interactions of polyamines and the N-methyl-D-aspartate (NMDA) receptor antagonist ifenprodil with the binding of [3H]MK801 to the NMDA receptor. Spermine and spermidine but not putrescine substantially increase [3H]MK801 binding to well washed rat brain membranes in the absence or presence of saturating concentrations of glutamate and glycine. Spermine also increased the association and dissociation of [3H]MK801 from its binding site, suggesting that polyamines activate the NMDA receptor in a similar manner to glycine. Ifenprodil inhibited the binding of [3H]MK801 in a biphasic fashion. The high affinity phase of binding (Ki of approximately 15 nM) accounted for 50-60% of total [3H]MK801 binding in the nominal absence of glutamate, glycine, and polyamines or in the presence of 100 microM glutamate. This fraction was reduced to 20% by the addition of 30 microM glycine and could be abolished by the addition of 50 microM spermine. However, ifenprodil apparently did not act by binding to the polyamine recognition site. The low affinity phase (Ki of 20-40 microM) was insensitive to the presence of positive modulators and may represent binding to the Zn2+ regulatory site. Ifenprodil decreased NMDA and glycine-induced Ca2+ influx into cultured rat brain neurons. The potency of ifenprodil suggests that spermine may activate NMDA receptors in vivo. These data indicate that ifenprodil may bind to the NMDA receptor in a state-dependent fashion and preferentially stabilize an inactivated form of the channel.

Abstract: 1. The whole-cell patch clamp recording technique was employed to investigate the intracellular regulation of N-methyl-D-aspartate (NMDA) receptors in cultured murine hippocampal neurones. Excitatory amino acids were repeatedly applied at regular intervals during intracellular dialysis with solutions of various composition. 2. Currents evoked by L-aspartate, an agonist of NMDA receptors, gradually 'washed out' to approximately 50% of their initial amplitude during dialysis with an intracellular solution containing CsCl and EGTA as a calcium buffer. In contrast, responses to kainate did not wash out. The wash-out of L-aspartate currents followed an exponential time course with a time constant of about 150 s. Wash-out did not appear to be related to desensitization of NMDA receptors. 3. Following wash-out, L-aspartate responses were blocked by Mg2+, ketamine or D-2-amino-5-phosphonovalerate indicating that these responses were still mediated by NMDA receptors. Furthermore, responses to NMDA itself showed wash-out to the same extent and with a time course similar to that for L-aspartate responses. 4. Neither the time course nor the extent of the wash-out of responses to L-aspartate was affected when the Ca2+ concentration of the dialysate was varied from zero to 1.5 x 10(-5) M. In addition, wash-out was unaffected by substitution of BAPTA for EGTA, indicating that wash-out was not a consequence of changes in intracellular pH related to the binding of Ca2+ to the buffer or to the kinetics of this binding. Therefore, the wash-out of NMDA currents could not be attributed to a gradual elevation of the concentration of intracellular Ca2+. 5. The extent of the wash-out of L-aspartate currents was similar for cells held at +40 versus -60 mV although the rate of wash-out was slower at the depolarized potential. In addition, the reversal potential of these currents was not altered, demonstrating that a change in driving force did not account for a component of the wash-out. 6. Inclusion of an ATP regeneration solution (Forscher & Oxford, 1985) in the dialysate prevented the wash-out of L-aspartate currents. ATP alone was less effective in preventing wash-out whereas phosphocreatine and creatine phosphokinase were ineffective by themselves. Wash-out also occurred when ATP was replaced with the non-hydrolysable analogue, beta, gamma-methyleneATP, or with GTP. In cells where wash-out of L-aspartate currents had been established, subsequent dialysis with the ATP regenerating solution partially reversed this wash-out.(ABSTRACT TRUNCATED AT 400 WORDS)

Abstract: 1. Some possible molecular mechanisms of action of the anxiolytic, anticonvulsant and neuroprotective agent MK-801 have been examined in 'whole-cell' voltage clamp recordings performed on rat hippocampal and cortical neurones, bovine adrenomedullary chromaffin cells and N1E-115 neuroblastoma cells maintained in cell culture. 2. Transmembrane currents recorded from rat hippocampal and cortical neurones in response to locally applied N-methyl-D-aspartate (NMDA) were antagonized by MK-801 (0.1-3.0 microM). Blockade was use-dependent, and little influenced by transmembrane potential. MK-801 (3 microM) had no effect on currents evoked by kainate (100 microM). 3. The antagonism of NMDA-induced currents by MK-801 was only slowly and incompletely reversed when the cell membrane potential was clamped at -60 mV during washout. Prolonged applications of NMDA at +40, but not -60 mV during washout, markedly accelerated recovery from block. 4. In contrast to MK-801, ketamine (10 microM) blocked NMDA-induced currents in a voltage-dependent manner. Blockade increased with membrane hyperpolarization and was completely reversible upon washout. 5. MK-801 (1-10 microM) produced a voltage- and concentration-dependent block of membrane currents elicited by ionophoretically applied acetylcholine (ACh) recorded from bovine chromaffin cells. The block was readily reversible upon washout. 6. gamma-Aminobutyric acidA (GABAA) receptor-mediated chloride currents of chromaffin cells were unaffected by MK-801 (1-100 microM). In contrast, such currents were potentiated by diazepam (1 microM). MK-801 (100 microM) had no effect on currents evoked by GABA on hippocampal neurones. 7. MK-801 (10 microM) had little effect on membrane currents recorded from N1E-115 neuroblastoma cells in response to ionophoretically applied 5-hydroxytryptamine (5-HT). Such currents were antagonized by the 5-HT3 receptor antagonist GR 38032F (1 nM) and also by MK-801 at high concentration (100 microM). 8. Voltage-activated, tetrodotoxin-sensitive, sodium currents of chromaffin cells were unaffected by 10 microM MK-801. However, at a relatively high concentration (100 microM), MK-801 reduced the amplitude of such currents to approximately 77% of control. 9. The relevance of the present results to the central actions of MK-801 is discussed.