N-methyl aspartate activates voltage-dependent calcium conductance in rat hippocampal pyramidal cells

N-methylaspartate receptors mediate epileptiform activity evoked in some, but not all, conditions in rat neocortical slices

Using intracellular recordings from pyramidal neurons in isolated slices of rat cerebral cortex epileptiform discharges evoked (1) in the presence of gamma-aminobutyric acid antagonists, and (2) in the absence of Mg2+ were compared. Depolarization shift responses recorded in the presence of bath applied picrotoxin, or electrophoretically applied picrotoxin or bicuculline, were similar in many respects to depolarization shifts reported previously, except that they could be evoked by stimuli subthreshold for evoking discernible postsynaptic potentials in these experiments. Large depolarizations evoked by repetitive activation of an N-methylaspartate receptor mediated synapse in the absence of Mg2+, displayed several properties similar to those of depolarization shifts evoked in the presence of gamma-aminobutyric acid antagonists, i.e. similar shape, latency, inability to follow high repetition rates and a similar voltage relation, suggesting activation of the same cellular mechanism. "Slow spikes" evoked as part of the response to electrophoretically applied N-methylaspartate were augmented, i.e. they were replaced by larger, longer, more complex events, when gamma-aminobutyric acid antagonists were applied. The potentiated response, evoked in the absence of Mg2+, was dependent on the activation of an N-methylaspartate receptor mediated synapse and was blocked by N-methylaspartate antagonists. In contrast, depolarization shifts could be evoked in the presence of large doses of N-methylaspartate antagonists, when gamma-aminobutyric acid antagonists were applied. Spontaneous depolarizations similar to depolarization shifts were recorded when cells were exposed to low, tonic, electrophoretic applications of excitatory amino acids under control conditions. In addition, some potentiation of the N-methylaspartate receptor mediated excitatory postsynaptic potential was achieved in the presence of Mg2+ when cells were depolarized by 10-20 mV. Depolarization shifts evoked when bicuculline was applied electrophoretically to different parts of the dendritic field, some hundreds of microns from the soma, differed in shape, latency and time course and the depolarization shift evoked when bicuculline was applied at one site summed with the depolarization shift evoked when it was applied elsewhere. We conclude that different inputs are required to activate the responses evoked in the presence of gamma-aminobutyric acid antagonists and in the absence of Mg2+. The possibility that both involve activation of dendritic Ca2+ currents and that the magnitude of the response depends on the proportion of the dendritic field activated, is discussed.

Involvement of N-methyl-D-aspartate receptors in epileptiform bursting in the rat hippocampal slice

The effects of the N-methyl-D-aspartate (NMDA) receptor antagonist, D-2-amino-5-phosphonovaleric acid (D-APV), and other excitatory amino acid antagonists, were studied on CA1 pyramidal neurones treated with picrotoxin or bicuculline to reduce synaptic inhibition mediated by gamma-aminobutyric acid (GABA). Under these conditions epileptiform burst firing is readily produced by orthodromic stimulation of the pyramidal cell population. D-APV reduced the plateau amplitude and duration of the depolarization underlying evoked and spontaneous bursts without affecting membrane potential, input resistance or the ability of the cell to fire a Ca2+ spike or a short train of Na+ spikes. A late component of the subthreshold excitatory post-synaptic potential (e.p.s.p.) was voltage dependent, being reduced in amplitude on membrane hyperpolarization. D-APV selectively removed this component of the e.p.s.p. in disinhibited slices. In contrast, in the absence of GABA antagonists, D-APV had no noticeable effect on the e.p.s.p. as studied with field potential recordings. The concentration-response relationship of the inhibitory effect of D-APV and L-APV on population spike bursts was studied. The action of APV was highly stereoselective; the EC50 of D-APV was approximately 700 nM, whereas a similar inhibition required 540 microM-L-APV. A number of other excitatory amino acid antagonists were tested at a fixed concentration (100 microM). Among them, the quisqualate antagonist gamma-D-glutamylaminomethyl sulphonic acid was ineffective against epileptiform bursts. In the low nanomolar concentration range both D- and L-APV potentiated bursting. These results suggest that in the absence of GABAergic inhibition, a significant component of the slow depolarization underlying burst firing is voltage dependent, synaptic in origin and mediated by NMDA receptors. We propose that, under normal (non-epileptic) physiological conditions, the balance between synaptic inhibition mediated by GABA receptors and synaptic excitation mediated by NMDA receptors may modulate the excitability of pyramidal cell dendrites.

Ionic channels: II. Voltage- and agonist-gated and agonist-modified channel properties and structure

This article reviews the different forms of ionic channels: voltage-gated, agonist-gated, and agonist- and second messenger-modified channels. The recent advances in our knowledge of the amino acid sequence of the sodium channel and the nicotinic acetylcholine receptor and the relationship of the primary structure to the channels' quarternary structure and function are discussed.

The action of valproate on spontaneous epileptiform activity in the absence of synaptic transmission and on evoked changes in [Ca2+]o and [K+]o in the hippocampal slice

The effects of valproate (VPA) on neuronal excitability and on changes in extracellular potassium ([K+]o) and calcium ([Ca2+]o) were investigated with ion selective-reference electrode pairs in area CA1 of rat hippocampal slices. Field potential responses to single ortho- and antidromic stimuli were unaltered by VPA (1-5 mM). The afferent volley evoked in the Schaffer-commissural fibers was also unaffected. In contrast, VPA (1 mM) depressed frequency potentiation and paired pulse facilitation markedly. Decreases in [Ca2+]o induced either by repetitive stimulation or by application of the excitatory amino acids N-methyl-D-aspartate and quisqualate were reduced, and the latter results suggest that VPA interferes with postsynaptic Ca2+ entry. When synaptic transmission was blocked by lowering [Ca2+]o (0.2 mM) and elevating [Mg2+]o (7 mM), prolonged afterdischarges elicited by antidromic stimulation were blocked by VPA. VPA also suppressed the spontaneous epileptiform activity seen when [Ca2+]o was lowered to 0.2 mM, without elevating [Mg2+]o. The amplitudes of the rises in [K+]o induced by repetitive orthodromic stimulation were only slightly depressed and those elicited by antidromic stimulation were generally unaltered by VPA, as were laminar profiles of stimulus-evoked [K+]o signals. These results indicate that VPA has membrane actions in addition to known effects on excitatory and inhibitory transmitter pools.

Calcium-induced long-term potentiation in the hippocampal slice: characterization of the time course and conditions

A transient increase in extracellular calcium concentration causes a long-lasting enhancement of radiatum fibers evoked excitatory postsynaptic potential and population spike responses of CA1 pyramidal neurons which resembles long-term potentiation (LTP). The duration of this potentiation is much longer than described previously and is probably limited by the survival of the preparation itself (greater than 8 hr). Therefore, Ca-induced LTP can be used for the investigation of a postulated late phase of LTP. Ca effects were activity-independent, since the subsequently evoked responses were facilitated even when the presynaptic fibers were not concurrently stimulated during or immediately after superfusion with the high Ca medium. In contrast, if too frequent testing of the synaptic input was done during the high Ca pulse, a short lasting depression instead of potentiation was observed. A lower extracellular magnesium concentration in the standard medium (1.3 instead of 2.0 mM MgSO4) prevents the potentiation of the EPSP at least for the first few hours. Presumably, both tetanus- and Ca-induced LTP share some common mechanisms, since an additional tetanization after Ca induction was not followed by an additional LTP. Compared to the potentiation following tetanization, the Ca-induced LTP was, however, not accompanied by a potentiation of the EPSP/spike ratio within the range of the population spike threshold intensity.

Effects of GABA and bicuculline on N-methyl-D-aspartate- and quisqualate-induced reductions in extracellular free calcium in area CA1 of the hippocampal slice

Decreases in extracellular free calcium ([Ca2+]o) and concomitant field potentials were recorded from the dendritic and cell body layers of the CA1 field in transverse hippocampal slices. They were elicited by tetanic stimulation of Schaffer collaterals and commissural fibers or by iontophoretic application of the excitatory amino acids N-methyl-D-aspartate (NMDA) and quisqualate (Quis). Under control conditions, decreases in [Ca2+]o were found to be maximal in stratum pyramidale (SP). In stratum radiatum (SR), 100 micron away from SP, decreases in [Ca2+]o were half the size of those observed in SP. Bicuculline methiodide, bath-applied at concentrations of 10-100 microM, enhanced the reductions in [Ca2+]o, increased the field potentials in all layers and also induced "spontaneous" epileptiform activity. In the presence of bicuculline, the decreases in [Ca2+]o were particularly enhanced in SR and were often greater than those recorded in SP. This was the case for changes in [Ca2+]o induced either by repetitive electrical stimulation or by application of NMDA and Quis. When synaptic transmission was blocked by perfusing the slices with a low Ca2+ medium, all NMDA and Quis-induced changes in [Ca2+]o were predictably reduced but there was a relative enhancement of changes in [Ca2+]o in SR with respect to those in SP. We propose that, under normal conditions, an inhibitory control mediated by GABA limits the reductions of [Ca2+]o particularly in SR. In support of this proposal, we found that bath-applied GABA had a depressant action on changes in [Ca2+]o.

Ketamine selectively suppresses synchronized afterdischarges in immature hippocampus

The role of excitatory amino acid neurotransmission in epileptogenesis was investigated in the developing hippocampus. Bath application of ketamine blocked penicillin-induced, synchronized afterdischarges in immature rat CA3 hippocampal neurons. Ketamine also decreased the duration of the preceding intracellularly recorded depolarization shift but had no measurable effect on the resting membrane potential or input impedance of pyramidal cells. Concentrations of ketamine that blocked afterdischarge generation dramatically depressed intracellular depolarizations produced by iontophoretic application of N-methyl-D-aspartate (NMDA) but not quisqualate. The effects of the NMDA antagonist 2-amino-7-phosphonoheptanoic acid on epileptiform discharges were identical to those of ketamine. These results suggest that an endogenous excitatory amino acid acting on an NMDA receptor plays a key role in the pronounced capacity of immature hippocampus for seizures.

Ineffectiveness of organic calcium channel blockers in antagonizing long-term potentiation

Evidence has accumulated suggesting that the presence of calcium is critical for development of hippocampal long-term potentiation (LTP). However, there is a paucity of information about whether calcium's role in LTP is pre- or postsynaptic. In the present study, we examined the effectiveness of nitrendipine, verapamil, flunarizine and the benzodiazepine diazepam in: blocking voltage-dependent calcium channels; blocking synaptic transmission; and preventing development of LTP. Using the in vitro slice preparation, we obtained intracellular and extracellular recordings from guinea pig hippocampal CA1 pyramidal cells. At the cellular level, all 4 drugs were ineffective in blocking voltage-dependent calcium spikes (TTX resistant) and the calcium-dependent afterhyperpolarization. Verapamil and diazepam appeared to antagonize synaptic transmission, as reflected in smaller population spike amplitudes. Development of long-term potentiation was not affected by the presence of verapamil, flunarizine and diazepam. Nitrendipine appeared to reduce the percentage of slices exhibiting LTP; however, ethanol, the vehicle used to dissolve nitrendipine, was shown in separate experiments to reduce the percentage of slices exhibiting LTP. These results suggest that neither the organic calcium channel blockers--nitrendipine, verapamil, and flunarizine--nor micromolar concentrations of diazepam are potent blockers of extrasynaptic voltage-sensitive calcium channels in hippocampus. They thus cannot be used to demonstrate a specific pre- or postsynaptic calcium role in LTP.

Frequency-dependent involvement of NMDA receptors in the hippocampus: a novel synaptic mechanism

Acidic amino acids, such as l-glutamate, are believed to be excitatory neurotransmitters in the mammalian brain and exert effects on several different receptors named after the selective agonists kainate, quisqualate and N-methyl-D-aspartate (NMDA). The first two receptors collectively termed non-NMDA receptors, have been implicated in the mediation of synaptic transmission in many excitatory pathways in the central nervous system (CNS), whereas NMDA receptors, with few exceptions do not appear to be involved; this is typified in the hippocampus where there is a high density of NMDA receptors yet selective NMDA receptor antagonists, such as D-2-amino-5-phosphonovalerate (APV), do not affect synaptic potentials. NMDA receptors have, however, been shown to be involved in long-term potentiation (LTP) in the hippocampus, a form of synaptic plasticity which may be involved in learning and memory. NMDA receptors have also been found to contribute to epileptiform activity in this region. We now describe how NMDA receptors can participate during high-frequency synaptic transmission in the hippocampus, their involvement during low-frequency transmission being greatly suppressed by Mg2+. A frequency dependent alleviation of this blockade provides a novel synaptic mechanism whereby a single neurotransmitter can transmit very different information depending on the temporal nature of the input. This mechanism could account for the involvement of NMDA receptors in the initiation of LPT and their contribution, in part, to epileptic activity.

NMDA-receptor activation increases cytoplasmic calcium concentration in cultured spinal cord neurones

Excitatory amino acids act via receptor subtypes in the mammalian central nervous system (CNS). The receptor selectively activated by N-methyl-D-aspartic acid (NMDA) has been best characterized using voltage-clamp and single-channel recording; the results suggest that NMDA receptors gate channels that are permeable to Na+, K+ and other monovalent cations. Various experiments suggest that Ca2+ flux is also associated with the activation of excitatory amino-acid receptors on vertebrate neurones. Whether Ca2+ enters through voltage-dependent Ca2+ channels or through excitatory amino-acid-activated channels of one or more subtype is unclear. Mg2+ can be used to distinguish NMDA-receptor-activated channels from voltage-dependent Ca2+ channels, because at micromolar concentrations Mg2+ has little effect on voltage-dependent Ca2+ channels while it enters and blocks NMDA receptor channels. Marked differences in the potency of other divalent cations acting as Ca2+ channel blockers compared with their action as NMDA antagonists also distinguish the NMDA channel from voltage-sensitive Ca2+ channels. However, we now directly demonstrate that excitatory amino acids acting at NMDA receptors on spinal cord neurones increase the intracellular Ca2+ activity, measured using the indicator dye arsenazo III, and that this is the result of Ca2+ influx through NMDA receptor channels. Kainic acid (KA), which acts at another subtype of excitatory amino-acid receptor, was much less effective in triggering increases in intracellular free Ca2+.

Induction of synaptic potentiation in hippocampus by patterned stimulation involves two events

Electrical stimulation of axons in the hippocampus with short high-frequency bursts that resemble in vivo activity patterns produces stable potentiation of postsynaptic responses when the bursts occur at intervals of 200 milliseconds but not 2 seconds. When a burst was applied to one input and a second burst applied to a different input to the same target neuron 200 milliseconds later, only the synapses activated by the second burst showed stable potentiation. This effect was observed even when the two inputs innervated completely different regions of the postsynaptic cells; but did not occur when the inputs were stimulated simultaneously or when the second burst was delayed by 2 seconds. Intracellular recordings indicated that the first burst extended the decay phase of excitatory postsynaptic potentials evoked 200 milliseconds later. These results suggest that a single burst of axonal stimulation produces a transient, spatially diffuse "priming" effect that prolongs responses to subsequent bursts, and that these altered responses trigger spatially restricted synaptic modifications. The similarity of the temporal parameters of the priming effect and the theta rhythm that dominates the hippocampal electroencephalogram (EEG) during learning episodes suggests that this priming may be involved in behaviorally induced synaptic plasticity.

Mode of action of excitatory amino acid receptor antagonists on hippocampal long-lasting potentiation

The effects of the N-methyl-D-aspartate receptor antagonists 2-amino-5-phosphonovalerate and gamma-D-glutamylglycine on the induction of long-lasting potentiation in the CAl and dentate areas of the hippocampal slice preparation have been examined. Synaptic activity was recorded extracellularly in the dendritic layer as a field excitatory postsynaptic potential, and the amount of long-lasting potentiation produced was measured from the change in slope of the rising phase of this potential. Experiments were generally performed with the gamma-aminobutyric acid antagonist picrotoxin in the solution. It is shown that 2-amino-5-phosphonovalerate prevents the induction of long-lasting potentiation following afferent tetanization of an input, without any effect on other inputs projecting to the same postsynaptic neurons. This result makes it unlikely that the preventive action of 2-amino-5-phosphonovalerate is related to any unspecific depressive action. Instead, 2-amino-5-phosphonovalerate was observed to block a postsynaptic depolarizing process appearing during the tetanus, likely related to current through synaptically activated N-methyl-D-aspartate receptor channels. It is suggested that 2-amino-5-phosphonovalerate prevents the induction of long-lasting potentiation by blockade of these currents through its antagonistic action on the N-methyl-D-aspartate receptors. Application of gamma-D-glutamylglycine similarly prevented the induction of long-lasting potentiation. No potentiation appeared following wash-out of the drug. The results exclude the possibility that the preventive action of this drug is related to a mere masking action on long-lasting potentiation induced in presynaptic terminals. It is suggested that gamma-D-glutamylglycine blocks the induction of long-lasting potentiation by its antagonistic action on the N-methyl-D-aspartate receptors, i.e. in a manner similar to that of 2-amino-5-phosphonovalerate.

Effect of calcium ions on the sensitivities of cultured cerebellar neurons to glutamate and aspartate

Iontophoretically applied glutamate and aspartate induced depolarizations in immature (6-13 days in culture) and mature (25-45 days) cultured chick cerebellar neurons, immature neurons being less sensitive. The input resistances of the neurons were variously changed by these amino acids. Reversal potentials of the depolarizations induced by both amino acids were similar in either immature or mature neurons. The population of amino acid-sensitive neurons increased with maturation. In mature neurons, the amplitude of glutamate- or aspartate-induced depolarization was decreased by addition of 10 mM Ca2+ to normal Tyrode's solution, aspartate responses being decreased more greatly. In low-Na+ solution (2.7 mM), however, high Ca2+ significantly enhanced amino acid-induced depolarizations. In immature neurons, on the other hand, the amplitude of glutamate- or aspartate-induced depolarization was drastically and consistently increased when 10 mM Ca2+ was added either to normal solution or to the low-Na+ solution. These enhancing actions of Ca2+ were abolished by Mn2+, but only partially by 10 mM glutamic acid diethylester or 1 mM D-alpha-aminoadipate, though responses to both amino acids in normal solution were blocked by these antagonists at the same concentrations. These results suggest that calcium ions enhance the effect of glutamate and aspartate in immature neurons, possibly by interacting with the ionophores involved in amino acid responses.

The role of specific ions in glutamate neurotoxicity

When the chick embryo retina is incubated in balanced salt solution containing glutamate (Glu) in 1 mM concentration, a neurodegenerative reaction occurs within 30 min. Here we report that the neurotoxic action of Glu on retinal neurons is dependent on the presence of Na+ and Cl-, but not Ca2+, in the incubation medium. Also, we report that depolarizing concentrations of K+ can induce a severe cytotoxic reaction in chick retina which, like the depolarization-linked neurotoxicity of Glu, is a Cl- dependent phenomenon.

"Desensitization" of excitatory amino acid responses in the rat olfactory cortex

Repeated application of the excitatory amino acid transmitter candidates, L-aspartate and L-glutamate and of N-methyl-D-aspartate, kainate and quisqualate to slices of olfactory cortex evoked progressively smaller depolarizations. These "desensitizations" were concentration-dependent, essentially irreversible and non-selective, although responses to gamma-aminobutyric acid (GABA) and to potassium ions were not significantly depressed. The specific N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonopentanoic acid, partially blocked the reduction in responses to amino acids which accompanied "desensitization" by N-methyl-D-aspartate, suggesting that activation of receptors is an obligatory step in provoking the phenomenon. "Desensitization" of responses was not prevented by the lectin concanavalin A but was potentiated by ouabain, an inhibitor of the sodium-potassium pump. It is proposed that the phenomenon does not reflect a true desensitization of receptors but is possibly the result of accumulation of intracellular sodium because of overloading the sodium pump. Under circumstances where responses to N-methyl-D-aspartate, quisqualate and kainate were "desensitized" by approx. 96%, depolarizations evoked by L-aspartate and L-glutamate were reduced by only 55%: these residual responses were not antagonized by the excitatory amino acid receptor blockers, (+/-)cis-2,3-piperidine dicarboxylate and 2-amino-4-phosphonobutyrate or by dihydrokainate, an inhibitor of the uptake of glutamate and aspartate. One possibility is that the residual responses reflect an interaction between L-aspartate and L-glutamate and an as yet unknown category of receptors.

2-Amino-5-phosphonovalerate and Co2+ selectively block depolarization and burst firing of rat hippocampal CA1 pyramidal neurones by N-methyl-D-aspartate

Intracellular recordings from pyramidal neurones during microiontophoretic ejection of N-methyl-D-aspartate and quisqualate into the pyramidal cell layer of the CA1 region of the rat hippocampal slice showed that both amino acids caused depolarization and evoked spike activity. Whereas quisqualate evoked tetrodotoxin-sensitive spikes, those produced by N-methyl-D-aspartate consisted of bursts of tetrodotoxin-sensitive action potentials superimposed on an underlying depolarizing shift of membrane potential. Both membrane depolarization and the superimposed depolarizing shifts associated with N-methyl-D-aspartate excitation were selectively and reversibly antagonized by the D(-) isomer of 2-amino-5-phosphonovalerate and Co2+. Both amino acids caused an increase in membrane conductance when small ejection currents were used, and the depolarizing response to these compounds was prevented by current injection. However, only the increase by N-methyl-D-aspartate was blocked by 2-amino-5-phosphonovalerate and Co2+. These results provide evidence to support the suggestion that different mechanisms underlie the excitatory response to N-methyl-D-aspartate and quisqualate in CA1 pyramidal neurones.

N-methylaspartate-activated calcium channels in rat brain cortex slices. Effect of calcium channel blockers and of inhibitory and depressant substances

N-Methyl-DL-aspartate, L-glutamate, kainate and DL-homocysteate were found to increase the initial rate and the maximal uptake of 45Ca into the non-inulin space of rat brain cortex slices incubated in vitro. The N-methylaspartate-stimulated calcium uptake was blocked by cadmium and cobalt ions, but not by the organic calcium channel blocker nifedipine or by tetrodotoxin, both of which stimulated the N-methylaspartate-independent calcium influx. gamma-Aminobutyrate increased the spontaneous calcium influx, and also reduced that stimulated by N-methylaspartate to the same level, as found with gamma-aminobutyrate alone. Adenosine (1-100 microM), ethanol (0.1 M), pentobarbital (10-100 microM) and morphine (0.2 mM), were unable to inhibit the N-methylaspartate-activated calcium influx. Ethanol (0.1 M), had no effect on the glutamate- or kainate-activated calcium influx. These findings suggest that the excitatory amino acids, because of their neuronal depolarizing action in brain cortex, lead to the opening of voltage-sensitive calcium channels, which may be blocked by cadmium, but not by the organic calcium channel antagonist, nifedipine. The activation of calcium channels by the excitatory amino acid N-methylaspartate, was entirely unaffected by the depressants ethanol, pentobarbital or morphine, or by the endogenous inhibitory substance, adenosine, thus suggesting that their inhibitory or depressant effects occur through interference with a neuronal mechanism unrelated to the one studied here. gamma-Aminobutyrate, on the other hand, considerably inhibited N-methylaspartate-induced calcium uptake, an effect interpreted as due to a gamma-aminobutyrate-induced increase in chloride conductance, that "clamps" the membrane potential and does not allow further depolarization by N-methylaspartate.

Actions of phencyclidine on rat locus coeruleus neurones in vitro

Intracellular recordings were made from neurones in the rat locus coeruleus in a brain slice maintained in vitro. Phencyclidine and related psychotomimetic drugs, applied in known concentrations in the fluid bathing the slice, depressed responses to N-methyl-D-aspartic acid noradrenaline (in the presence of the uptake inhibitor desmethylimipramine) and [D-Ala2,MePhe4,Gly-ol5]enkephalin and also prolonged the action potential. The sensitivities of these responses to depression by phencyclidine was N-methyl-D-aspartic acid (IC50 0.4 microM) greater than noradrenaline (IC50 3.9 microM) greater than [D-Ala2,MePhe4,Gly-ol5]enkephalin (IC50 8.5 microM) greater than prolongation of the action potential (41% increase by 30 microM). Stereoselectivity was observed only in the depression of responses to N-methyl-D-aspartic acid where (+)-1-(1-phenylcyclohexyl)-3-methyl piperidine was 3.3-fold more potent in suppressing N-methyl-D-aspartic acid depolarizations than its (-) isomer. The responses to N-methyl-D-aspartic acid were also depressed by the structurally unrelated psychotomimetic (+/-)-N-allyl-N-normetazocine (IC50 0.9 microM). All of the effects of the psychotomimetic drugs examined were slow in onset and difficult to reverse following washout. No effect of phencyclidine (0.03-100 microM) or related drugs was observed on membrane potential, input resistance or spontaneous action potential firing rate of locus coeruleus neurones. The depression of responses to N-methyl-D-aspartic acid by phencyclidine was the most potent and the only stereoselective effect of those studied. The importance of this effect and of those not showing stereoselectivity in relation to the phencyclidine behavioural syndrome is discussed.

The induction and modification of voltage-sensitive responses in cat neocortical neurons by N-methyl-D-aspartate

The actions of the excitatory amino acid, N-methyl-D-aspartate (NMDA), on layer V neurons of cat sensorimotor cortex were examined in an in vitro slice preparation using current clamp, single electrode voltage clamp (SEVC), and ionic substitution techniques. Low doses of NMDA evoked a slow depolarization with a net decrease of input conductance. Larger doses additionally evoked repetitive firing, rhythmic depolarization shifts (DSs), low-threshold calcium spikes (in the presence of TEA+) and bistable membrane potential behavior. Ionic substitution experiments suggested that entry of both Ca2+ and Na+ ions contributed to the NMDA responses. Attention was focused on the NMDA response with Ca2+ entry blocked. Examination by SEVC revealed that, in both normal cells and in the presence of several blocking agents, NMDA induced a highly voltage-dependent inward ionic current which could result in a region of negative slope conductance on the cell's current-voltage relation. The development of this current seems capable of accounting for all aspects of the observed response, including the DSs and low-threshold Ca2+ spikes. Substitution of TEA+ for most external Na+ (with Ca2+ entry blocked) largely eliminated the NMDA responses and corresponding ionic current. Our results in neocortical neurons are compared to those recently obtained in cultured murine neurons.

Comparison of responses to transmitter candidates at an N-methylaspartate receptor mediated synapse, in slices of rat cerebral cortex

Intracellular recordings from pyramidal neurones in isolated slices of rat cerebral cortex allowed a comparison of postsynaptic potentials and responses to electrophoretic application of excitatory amino acids. The responses of all neurones to N-methylaspartate displayed an unusual voltage relation and were associated with an apparent increase in membrane resistance, properties that were dependent on the presence of extracellular Mg2+. Responses to N-methylaspartate could be elicited only when the electrophoretic pipette was positioned close to the cell soma and were associated with generation of slow spikes which triggered bursts of fast spikes. In contrast, in the majority of neurones, responses to glutamate, aspartate, cysteate and cysteine sulphinate demonstrated a conventional voltage relation, were associated with a decrease in membrane resistance, evoked no slow spikes, were insensitive to extracellular Mg2+ concentrations between 1 and near 0 mM and could be evoked with small currents of amino acids when the electrophoretic pipette was greater than 100 micron from the cell soma. Responses to the five amino acids were tested with the N-methylaspartate antagonist 2-amino-5-phosphonovaleric acid. In the majority of cells, this antagonist blocked responses to N-methylaspartate at doses that had only a small effect on responses to the other amino acids. In these experiments it was also possible to confirm previous reports that ketamine and cyclazocine act as selective N-methylaspartate antagonists. In 4/63 neurones, responses to glutamate and aspartate and in 1/5 neurones one component of the response to cysteate, displayed properties similar to those of responses to N-methylaspartate. In one other neurone, large applications of cysteine sulphinate or glutamate could evoke slow spikes and fast spike bursts, a firing pattern that was sensitive to 2-amino-5-phosphonovalerate. The present experiments therefore demonstrate that all four naturally occurring amino acids tested can activate N-methylaspartate receptors and indicate that any one of these could be the transmitter at the N-methylaspartate receptor-mediated synapse on cortical pyramidal neurones. However, in the majority of neurones, these putative transmitters activated other receptor types preferentially. The possibility that this may result from the greater accessibility of non-N-methylaspartate receptors to electrophoretically applied agonists, is discussed.

A magnesium-sensitive post-synaptic potential in rat cerebral cortex resembles neuronal responses to N-methylaspartate

In isolated slices of rat cerebral cortex, intracellular recordings were obtained from pyramidal cells that were predominantly in layers II/III. These cells could be antidromically activated from the underlying white matter and had resting potentials of greater than -75 mV, action potentials with amplitudes of greater than 70 mV (measured from threshold), overshoots of 20-30 mV, and thresholds 20-30 mV positive to the resting potential. The responses of these cells to short (1-2 s) pulses of electrophoretically applied N-methylaspartate (NMA) decreased in amplitude with membrane hyperpolarization between -40 and -120 mV, and were associated with an apparent increase in membrane resistance when recorded in the presence of 1 mM-Mg2+. However, in the absence of Mg2+, responses to NMA increased progressively in amplitude with hyperpolarization and were associated with a decrease in membrane resistance. In addition to conventional excitatory post-synaptic potentials (e.p.s.p.s) and inhibitory post-synaptic potentials (i.p.s.p.s), electrical stimulation of the underlying white matter evoked a novel e.p.s.p. This e.p.s.p. displayed a similar voltage relation to the response evoked by NMA and was associated with an apparent increase in membrane resistance. When Mg2+ was removed from the bathing medium, the properties of the novel e.p.s.p. changed, and it displayed a conventional voltage relation and was associated with a decrease in membrane resistance. In the absence of Mg2+, novel e.p.s.p.s showed potentiation on low frequency repetitive stimulation (0.5-2 Hz). A fully potentiated response could evoke bursts of slow potentials each of which could evoke a burst of fast spikes. In contrast, the more conventional e.p.s.p.s and i.p.s.p.s evoked in pyramidal neurones were unaffected by reducing the Mg2+ concentration from 1.0 to near 0 mM and conventional e.p.s.p.s showed no potentiation on repetitive, low frequency repetition, even after several hours exposure to Mg2+-free medium. The NMA antagonists: 2-amino-5-phosphonovaleric acid, ketamine and cyclazocine, applied electrophoretically at doses that blocked responses to NMA, but which had little effect on responses to glutamate, blocked the novel e.p.s.p. and its potentiation.

Long-term alterations in amino acid-induced ionic conductances in chronic epilepsy

Extracellular free sodium (Na+)o and calcium (Ca2+)o concentration changes were measured in the rat motor cortex, using ion-selective microelectrodes. During ionophoretic applications of excitatory amino acids, decreases in (Na2+)o and in (Ca2+)o were observed. Ca2+ signals were not or very little modified by applications of tetrodotoxin while Na+ signals were slightly depressed, up to 20%. Laminar profile analysis revealed that, while the magnitude of Na+ signals was rather constant throughout the cortex, Ca2+ signals were largest in upper cortical layers. Lesioning and pharmacological experiments indicated that the corresponding permeabilities were most probably located on apical dendrites of pyramidal tract neurons. The relative amplitude of Na+ and Ca2+ signals induced by the release of the glutamate agonists N-methyl-D-aspartate, quisqualate and kainate and the shape of the laminar profile of such responses indicated that different ionic permeabilities located on different neurons underlie such responses. Similar experiments performed on chronic epileptogenic motor foci in rats indicated that the amino acid-induced ionic responses were altered. The significance of such alterations for epileptogenesis is discussed.

Action of excitatory amino acids and their antagonists on hippocampal neurons

Intracellular recordings were obtained from guinea pig hippocampal neurons maintained in vitro. Current- and voltage-clamp techniques were used to study the effect of microiontophoresis of excitatory amino acid agonists. Modification of agonist responses by bath application of known concentrations of antagonist agents was also examined. All agonists used, glutamate, aspartate, N-methyl-D-aspartic acid (NMDA), and quisqualate, depolarized hippocampal neurons and caused repetitive firing. NMDA was also noted to induce burst-firing in some neurons. Quisqualate and NMDA were more potent than glutamate or aspartate. In slices perfused with a nominally calcium-free saline containing tetrodotoxin and manganese, quisqualate application produced a depolarization associated with a conductance increase. Under those conditions, NMDA-induced depolarizations caused apparent decreases as well as increases in conductance. The apparent decreases in conductance were observed in the voltage range of -40 to -70 mV, whereas increases in conductance were observed at membrane potentials more positive than -35 mV. Under voltage-clamp conditions, quisqualate produced an inward current whose amplitude increased with hyperpolarization and decreased upon depolarization, reversing near 0 mV. The conductance change induced by quisqualate was independent of voltage. NMDA application resulted in an inward current that was maximal around the resting potential and decreased with both hyperpolarization and depolarization. Response reversal was not observed with hyperpolarization to -100 mV but was apparent with depolarization beyond 0 mV. Conductance changes induced by NMDA were voltage dependent, and the application of this agent was associated with the appearance of a region of negative slope conductance in the current-voltage relationship. Apparent decreases in conductance in response to NMDA were reduced when the extracellular magnesium concentration was lowered. Response amplitudes were not affected. The NMDA receptor antagonist DL-2-amino-5-phosphonovalerate (DL-APV) was a potent and selective blocker of NMDA responses, whereas the antagonist DL-2-amino-4-phosphonobutyric acid (DL-APB) was less potent and did not select between NMDA and quisqualate responses. Analysis of iontophoretic dose-response curves indicated that DL-APV was a competitive antagonist. The results of these experiments indicate that hippocampal CA1 pyramidal neurons possess separate receptors for quisqualate and NMDA, with different pharmacological and electrophysiological profiles.

Cobalt ions block L-glutamate and L-aspartate-induced currents in cultured neurons from embryonic chick spinal cord

The effects of Co2+ on L-glutamate and L-aspartate responses wee studied in cultured spinal cord neurons of the embryonic spinal cord of the chick by employing the patch-clamp technique in whole cell mode [9]. It was found that Co2+ blocks the responses at negative membrane potentials for both amino acids, while only partial inhibition was observed at positive membrane potentials. Co2+ alone decreases the resting membrane current which exhibits reversal close to zero. It is suggested that the effects of Co2+ are produced by non-specific interaction with negative charges on the outer side of the membrane.

N-Methylaspartate-evoked liberation of taurine and phosphoethanolamine in vivo: site of release

The effect of N-methyl-D,L-aspartic acid (NMA) on extracellular amino acids was studied in the rabbit hippocampus with the brain dialysis technique. Administration of 0.5 or 5 mM NMA caused a concentration-dependent liberation of taurine and phosphoethanolamine (PEA). Taurine increased by 1,200% and PEA by 2,400% during perfusion with 5 mM NMA whereas most other amino acids rose by 20-100%. The effect of NMA appeared to be receptor-mediated, as coperfusion with D-2-amino-5-phosphonovaleric acid curtailed the NMA response by some 90%. The NMA-stimulated release of taurine and PEA was suppressed when Ca2+ was omitted and further inhibited when Co2+ was included in the perfusion medium. The effect of NMA was mimicked by the endogenous NMA agonist quinolinic acid and the partial NMA agonist D,L-cis-2,3-piperidine dicarboxylic acid. Although the NMA-evoked release of taurine and PEA was Ca2+-dependent in vivo, NMA had no effect on Ca2+ accumulation in hippocampal synaptosomes. The previously reported NMA-induced activation of dendritic Ca2+ spikes and the lack of effect on synaptosomal Ca2+ uptake suggest that taurine and PEA are released from sites other than nerve terminals, possibly from dendrosomatic sites. This notion was strengthened by the absence of an effect of NMA on the efflux of radiolabelled taurine from hippocampal synaptosomes. In contrast, high K+ stimulated synaptosomal uptake of Ca2+ and release of taurine.

Cholinergic agonists suppress a potassium current in freshly dissociated smooth muscle cells of the toad

Single micro-electrode voltage-clamp and current-clamp techniques were used to study cholinergic responses in single freshly isolated gastric smooth muscle cells from the toad Bufo marinus. Acetylcholine (ACh) or muscarine caused membrane depolarization, which sometimes gave rise to action potentials and contractions. The agonist-induced depolarization is due to the suppression of a voltage-dependent K+ conductance, a conclusion based on the following observations. Depolarization was accompanied by an apparent membrane conductance decrease, seen as the increased size of voltage deflexions in response to constant current pulses. The conductance decrease was confirmed under voltage clamp, where current deflexions in response to constant voltage jumps were smaller in the presence of cholinergic agonists. Muscarine induced net inward currents at potentials positive to the K+ equilibrium potential (EK), and net outward currents at potentials negative to EK. In experiments where external K+ concentration ([K+]o) ranged from 20 to 90 mM the reversal potentials shifted 58 mV positive per tenfold elevation of [K+]o, as expected for a K+ current. The steady-state current-voltage relationship revealed that the K+ current inhibited by muscarine was larger at more positive potentials than expected from driving force considerations alone. Therefore, the underlying conductance suppressed by cholinergic agonists was voltage dependent, with almost complete deactivation at potentials more negative than approximately -70 mV and exhibiting a sigmoidal activation curve upon depolarization. The deactivation of this voltage-dependent K+ conductance caused slow current relaxations to occur in response to hyperpolarizing voltage commands from depolarized holding potentials. In experiments where [K+]o ranged from 3 to 30 mM, these current relaxations reversed direction at potentials near EK and the reversal potential shifted 52 mV positive per tenfold elevation of [K+]o, indicating that K ions carry most of the charge. The current relaxations that occurred in response to hyperpolarizing voltage commands were suppressed by ACh, muscarine and oxotremorine. The effects of muscarine persisted in nominally Ca2+-free solutions containing Mn2+. Ba2+ mimicked the effects of muscarinic agonists. Thus, isolated smooth muscle cells exhibit a K+ current resembling the M-current of sympathetic and other neurones, which is reversibly suppressed by cholinergic agonists. The existence of a cholinergic K+ conductance decrease is of interest because it has not previously been demonstrated in smooth muscle.

Synaptic transmission at N-methyl-D-aspartate receptors in the proximal retina of the mudpuppy

The effects of excitatory amino acid analogues and antagonists on retinal ganglion cells were studied using intracellular recording in the superfused mudpuppy eyecup preparation. Aspartate, glutamate, quisqualate (QA), kainate (KA) and N-methylaspartate (NMA) caused depolarization and decreased input resistance in all classes of ganglion cells. The order of sensitivity was QA greater than or equal to KA greater than NMA greater than aspartate greater than or equal to glutamate. All of these agonists were effective when transmitter release was blocked with 4 mM-Co2+ or Mn2+, indicating that they acted at receptor sites on the ganglion cells. At a concentration of 250 microM, 2-amino-5-phosphonovalerate (APV) blocked the responses of all ganglion cells to NMA, but not to QA or KA, indicating that NMA acts at different receptor sites from QA or KA. Responses to bath-applied aspartate and glutamate were reduced slightly or not at all in the presence of APV, indicating that they were acting mainly at non-NMDA (N-methyl-D-aspartate) receptors. In all ganglion cells 250 microM-APV strongly suppressed the sustained responses driven by the 'on'-pathway but not those driven by the 'off'-pathway. In most on-off ganglion cells the transient excitatory responses at 'light on' and 'light off' were not reduced by 500 microM-APV. APV-resistant transient excitatory responses were also present in some on-centre ganglion cells. APV did not block the transient inhibitory responses in any class of ganglion cells. At concentrations which blocked the sustained responses of ganglion cells, APV did not affect the sustained responses of bipolar cells, indicating that it acted at sites which were post-synaptic to bipolar cells. The simplest interpretation of these results is that the transmitter released by depolarizing bipolar cells acts at NMDA receptors on sustained depolarizing amacrine and ganglion cells. It may act at non-NMDA receptors at synapses which produce transient excitatory responses, but this could not be proved. The transmitter released by hyperpolarizing bipolar cells does not appear to act at NMDA receptors on any post-synaptic cells.

Effects of bath-applied excitatory amino acids and their analogs on spinal interneurons of the lamprey

Depolarizations, conductance increases and time courses of the responses to bath application of glutamate, aspartate, DL-homocysteate, N-methyl-DL-aspartate (NMDLA), quisqualate and kainate were determined in interneurons of the isolated spinal cord of the lamprey, one of the most primitive vertebrates. Conductance increases produced by these excitants in perfusate containing tetrodotoxin (0.5 microgram/ml), 4-aminopyridine (1 mM) and without Ca2+ were very small in comparison with those produced by glycine or GABA. NMDLA-induced depolarizations were associated with conductance decreases and rhythmic oscillations in membrane potentials in this perfusate. Quisqualate was strongest among these amino acids in producing depolarizations and conductance increases. Responses induced by analogs were slower than those produced by glutamate and aspartate. Phylogenetic distribution of N-methyl-D-aspartate receptors on neurons and muscles is discussed.

Intracellular demonstration of an N-methyl-D-aspartate receptor mediated component of synaptic transmission in the rat hippocampus

Rat hippocampal CA1 pyramidal neurones were monosynaptically activated via stimulation of the Schaffer collateral-commissural pathway. On changing from a 1 mM Mg2+-containing to a Mg2+-free medium there was a pronounced prolongation of the intracellularly recorded excitatory postsynaptic potential. This effect was reversibly abolished by the selective N-methyl-D-aspartate (NMDA) antagonist, D-2-amino-5-phosphonovalerate (APV). We propose that Mg2+ normally prevents expression of NMDA receptor-mediated responses during low-frequency stimulation. During a period of tetanic stimulation, however, cells may depolarize sufficiently to allow a significant NMDA component of the response to be manifest. This could then initiate long-term potentiation.

An intracellular study of the interactions of N-methyl-DL-aspartate with ketamine in the mouse hippocampal slice

Intracellular recordings were made from dentate and CA1 pyramidal cells of the mouse hippocampal slice preparation. N-methyl-DL-aspartate (NMDLA), quisqualate and kainate and the anaesthetic agent, ketamine, were applied by microelectrophoresis. Excitation by NMDLA but not by the other amino acids, was associated with increased outward rectification. Ketamine had no effect on the resting potential or current/voltage relation of the cells, but selectively antagonised the responses to NMDLA. Action potentials evoked by NMDLA were characteristically broader than those evoked by the other amino acids or by the passage of depolarising current through the electrode.

Facilitation of hippocampal long-lasting potentiation by GABA antagonists

Long-lasting potentiation (LLP) of synaptic transmission in the CAI region of the hippocampal slice preparation has been examined. The effects of reduced postsynaptic inhibition given by application of gamma-aminobutyric acid (GABA) antagonists (mainly picrotoxin) on the generation of LLP were investigated. It was first demonstrated that picrotoxin had little effect on excitatory synaptic transmission itself as judged by the rising phase of the field EPSP. Moreover, there were largely no actions on short-lasting synaptic effects such as paired pulse facilitation and frequency potentiation. On the other hand, following drug application, much fewer afferent volleys were needed to generate a given amount of LLP. Long-lasting potentiation could be produced by trains containing as few as 2-5 impulses, trains that normally give rise to only short-lasting effects. There was no apparent difference in the maximal amount of LLP that could be produced for a given input, suggesting that the GABA antagonists do not operate by enhancing the capacity for LLP production but by facilitating its induction. As in normal solution, the LLP in the presence of the drugs was confined to the tetanized pathway. Tetanization in the treated slices was associated with enhanced somatic firing as well as an increase of the negative extracellular potential recorded in the dendritic layer. It is proposed that part of this increased negativity represents current through synaptically opened N-methyl-D-aspartate (NMDA) receptor channels. Furthermore, it is suggested that the facilitated induction of LLP in the presence of GABA antagonists is related to a facilitated activation of these NMDA receptor channels which is secondary to the higher levels of dendritic depolarization attained during tetanization under conditions of reduced postsynaptic inhibition.

Repetitive firing of CA1 hippocampal pyramidal cells elicited by dendritic glutamate: slow prepotentials and burst-pause pattern

In order to compare responses to dendritic vs. somatic depolarization, CA1 pyramidal cells in rat hippocampal slices were stimulated by iontophoresis of glutamate to sensitive spots in the dendrites, and by somatic current injection. Low intensities of either stimulus elicited slow repetitive firing. Each action potential was preceded by a slow depolarizing prepotential (SPP), lasting 50-300 ms and was followed by fast (3-5 ms) and slow (more than 100 ms) afterhyperpolarizations (AHPs). The SPPs, and AHPs were indistinguishable for the two types of stimuli. In response to strong depolarizations, most cells showed an initial burst of spikes, followed by a pause before the steady discharge. This pattern was elicited by both glutamate and current. The input resistance usually increased 5-20% during subthreshold depolarizations by glutamate or current. In contrast, large doses of glutamate caused a slow decline in the resistance (up to 40%), which was larger than during comparable current-induced discharge, and the response was followed by a longer AHP. It is concluded that both dendritic and somatic depolarization, induced by glutamate and current, respectively, can elicit sustained repetitive firing with SPPs, fast and slow AHPs and burst-pause pattern, thus, increasing the likelihood that these phenomena play a role during natural activation of CA1 cells.

Pharmacological evidence for L-aspartate as the neurotransmitter of cerebellar climbing fibres in the guinea-pig

Climbing fibre responses (c.f.r.s) evoked by white matter stimulation and the depolarizations induced by iontophoretically applied L-glutamate and L-aspartate were recorded intracellularly from the proximal dendrites of Purkinje cells in in vitro slice preparations of the guinea-pig cerebellum. Short pulses of L-glutamate and L-aspartate dose-dependently depolarized the Purkinje cell dendrite. Even small doses of these amino acids reduced the input resistance. The maximum decrease in input resistance induced by L-glutamate was 36% and that by L-aspartate was 38%. Intracellular injection of Cs+ allowed Purkinje cell dendrites to be depolarized to a range of -15 to +30 mV. The mean reversal potential for the c.f.r. (Ec) was found to be +10.2 mV (n = 4). The mean reversal potentials obtained for L-glutamate (Eg) and for L-aspartate (Ea) were +7.3 mV (n = 7) and +5.6 mV (n = 7) respectively. When external Na+ concentration was reduced, Ec, Ea and Eg were linearly and similarly shifted in the negative direction, indicating that all these reversal potentials are determined primarily by a Na+ conductance. The effects of the glutamate antagonists 2-amino-5-phosphonovaleric acid (APV), gamma-D-glutamylglycine (gamma-DGG), N-methyl-DL-aspartic acid (NMDLA) and glutamic acid diethylester (GDEE) were compared as to the responses to L-glutamate and L-aspartate and Ca2+-activated focal climbing fibre responses (c.f.c.f.r.s) in order to investigate the receptor type at the synapses formed by the climbing fibres with Purkinje cell dendrites. The order of antagonistic potency to the c.f.c.f.r. was : APV (mean percentage blockade = 99%) greater than gamma-DGG (87%) greater than NMDLA (71%) greater than GDEE (28%). The order of antagonistic potency to the response to L-aspartate was: gamma-DGG (69%) greater than APV (66%) greater than NMDLA (60%) greater than GDEE (31%), and that to the response to L-glutamate was: GDEE (63%) greater than NMDLA (22%) greater than gamma-GDD (15%) greater than APV (14%). APV was found to be the most effective anatagonist of the c.f.c.f.r. Its action was reversible, selective for L-aspartate-induced depolarization and had no effect on the responses to L-glutamate. NMDLA, which has no activity as an agonist, was a greater suppressant of the responses to L-aspartate than those to L-glutamate. These electrophysiological and pharmacological findings suggest that the receptor for the transmitter at the synapses formed by climbing fibres with Purkinje cell dendrites is of the L-aspartate-preferring type, and are thus consistent with the bio-and histochemical findings that L-aspartate may be the endogenous transmitter at this synapse.

Effects of excitatory amino acids on calcium transport by brain membranes

The effect of excitatory amino acids on the uptake of 45Ca was studied in crude mitochondrial (P2) fractions prepared from mouse brain. L-Glutamate stimulated calcium uptake, but this action was not shared by other amino acids including D-glutamate, L-aspartate, N-methyl-aspartate or kainate. The glutamate-stimulated calcium uptake was, however, blocked by inhibitors of glutamate dehydrogenase, such as D-glutamate, glutarate and triiodothyronine. Subcellular fractionation demonstrated that the uptake was enriched in the mitochondrial fraction. Furthermore, most of the uptake found in the synaptosomal fraction was inhibited by triiodothyronine. These results indicate that the glutamate-stimulated uptake of calcium by brain membranes is due mainly to mitochondrial uptake of calcium that is driven by the metabolism of glutamate by glutamate dehydrogenase. Previous suggestions of coupling of glutamate receptors to calcium channels based on uptake of 45Ca by brain membranes must now be reevaluated.

Activation of NMDA receptors elicits fictive locomotion and bistable membrane properties in the lamprey spinal cord

The motor pattern underlying locomotion in the lamprey can be elicited in the spinal cord in vitro by applying excitatory amino acids that activate NMDA receptors. When this is done oscillatory membrane potentials phase-linked with the locomotory rhythm can be recorded in different types of neurones. In some spinal neurones large amplitude oscillation continues after elimination of synaptic input with application of TTX. This oscillatory pacemaker-like activity is dependent on an activation of NMDA receptors, and is probably important in the generation of locomotion.

Binding sites for L-[3H]glutamate on hippocampal synaptic membranes: three populations differentially affected by chloride and calcium ions

The effects of Cl- and Ca2+ were studied on the specific binding of L-[3H]glutamate to multiple sites on rat hippocampal synaptic membranes. Quisqualate (5 microM) or DL-2-amino-4-phosphonobutyrate (2-APB) (300 microM) was used to discriminate two previously identified classes of binding sites. Saturation isotherms and displacement curves constructed under different ionic conditions suggested that the effects of Cl- and Ca2+ could best be explained by postulating the existence of three major binding site populations in this preparation rather than two. The binding of L-glutamate to Glu A sites exhibits an absolute dependence on Cl-, and Ca2+ markedly increases the maximum density of these sites. Glu A sites bind quisqualate and 2-APB with relatively high affinity. Cl- (47 mM) more than doubles the maximum density of Glu B sites, but Ca2+ appears to have no effect. Glu B sites can be discriminated from the other classes by their relatively low affinity for quisqualate and 2-APB. There is reason to think that the Glu B population is heterogeneous. The novel Glu C population can be virtually selectively labeled by exposing 2-APB-sensitive binding sites to radioligand in Tris-HOAc buffer with Ca2+. Binding of L-[3H]glutamate to these sites is enhanced by both Cl- and Ca2+, but requires neither ion. Ca2+ appears to increase both the affinity of Glu C sites for L-glutamate and their maximum binding site density. In the presence of Ca2+ and Cl-, Glu C sites bind the radioligand with micromolar affinity (KD approximately 2 microM) and high capacity (Bmax approximately 160 pmol/mg protein).(ABSTRACT TRUNCATED AT 250 WORDS)

Two different mechanisms control inhibition of spike discharge in neurons of cat motor cortex after stimulation of the pyramidal tract

Intracellular recordings were made from neurons of the motor cortex of awake cats while the pyramidal tract (PT) was stimulated at the level of the facial nucleus. In some neurons IPSPs of 35-120 ms peak latency were recorded that diminished in size or reversed with hyperpolarizing current. During these IPSPs a decrease in input resistance reflective of a conductance increase was measured. More often, however, PT stimulation produced IPSPs with comparable latencies that increased in size with hyperpolarizing current. These IPSPs diminished with depolarizing current, and in some instances they appeared to reverse with strong depolarization. During these IPSPs an increased input resistance reflective of a decreased conductance was measured. The results indicate that two different mechanisms control rapid inhibition of spike discharge in neurons of the motor cortex after PT stimulation.

Postsynaptic firing during repetitive stimulation is required for long-term potentiation in hippocampus

Long-term potentiation (LTP) in the hippocampus is a long lasting enhancement of the postsynaptic evoked response following high frequency, repetitive stimulation of afferents. The extracellularly recorded action potential (population spike) can be reversibly blocked, without affecting the extracellularly recorded excitatory postsynaptic potential, by focal application of gamma-aminobutyric acid, tetrodotoxin, or pentobarbital, to the CA1 pyramidal cells of the hippocampal slice. When the population spike is blocked during repetitive stimulation, LTP does not occur. It appears that postsynaptic firing of action potentials during repetitive stimulation is necessary to produce LTP.

Calcium influx accompanies but does not cause excitotoxin-induced neuronal necrosis in retina

Several authors have recently proposed that excessive calcium (Ca++) influx into postsynaptic cells may be the mechanism by which excitotoxins such as glutamate (Glu), N-methylaspartate (NMA) and kainic acid (KA) cause neuronal necrosis. Here we have undertaken both in vivo and in vitro studies to explore this hypothesis. Our findings indicate that Ca++ does accumulate selectively in neural elements undergoing degeneration in the in vivo mouse hypothalamus following subcutaneous administration of NMA. However, pretreatment with the putative Ca++ channel blocker nimodipine resulted in augmentation rather than suppression of the toxic action of NMA and Glu on the mouse hypothalamus and eliminating Ca++ from the incubation medium did not interfere with the toxic action of Glu, NMA or KA on the chick embryo retina in vitro. We conclude, therefore, that Ca++ influx is an unlikely explanation for excitotoxin-induced degeneration of retinal or hypothalamic neurons.

The action of N-methyl-D-aspartic acid on mouse spinal neurones in culture

Neurones from the ventral half of mouse embryo spinal cord were grown in dissociated culture and voltage clamped. The current-voltage relation of responses evoked by N-methyl-D-aspartic acid (NMDA), L-glutamic acid and kainic acid was recorded in media of different ionic composition. On removal of Mg2+ from the extracellular solution, responses to NMDA and L-glutamate became less voltage sensitive, such that NMDA responses were no longer associated with a region of negative slope conductance. The antagonism of NMDA responses produced by application of Mg2+ to neurones bathed in nominally Mg2+-free solutions shows voltage dependence and uncompetitive kinetics. Voltage-jump experiments showed that the voltage-dependent action of Mg2+ occurred rapidly, and with complex kinetics. Ni2+ and Cd2+, two potent blockers of calcium currents in spinal cord neurones, had significantly different potencies as NMDA antagonists, Ni2+ being of greater potency than Mg2+, and Cd2+ considerably weaker. The voltage-dependent block of NMDA responses produced by physiological concentrations of Mg2+ is sufficient to explain the apparent increase in membrane resistance produced by NMDA in current-clamp experiments, and the ability of NMDA to support repetitive firing. Substitution of choline for Na+ produced a hyperpolarizing shift in the reversal potential for responses evoked by kainic acid consistent with an increase in permeability to Na+ and K+. In choline-substituted solutions, the reversal potential of NMDA responses was more positive than that recorded for kainic acid, and in addition NMDA responses showed enhanced desensitization.

Excitatory amino acid-induced responses of frog motoneurones bathed in low Na+ media: an intracellular study

Motoneurones of the frog spinal cord slice preparation were impaled with microelectrodes and superfused at 7 degrees C with the excitatory amino acids glutamate, quisqualate or N-methyl-D-aspartate. The role of Na+ in the action of these amino acids was studied by comparing amplitude matched depolarizations obtained in control Ringer solution with the responses recorded from the same cells after replacing (86-100%) Na+ by choline or glucosamine. Effective replacement of extracellular Na+ proved to be a rather slow process requiring 30-60 min. In glucosamine solution depolarizations evoked by glutamate, N-methyl-D-aspartate or quisqualate were abolished or strongly reduced with recovery following return to control Ringer. In choline solution, glutamate and N-methyl-D-aspartate effects were blocked whereas the quisqualate response was surprisingly unaffected. Mn2+ (2 mM) added to choline solution strongly diminished the action of quisqualate. These results suggest that Na+ was important in mediating amino acid responses and that quisqualate activated an additional conductance mechanism (perhaps to Ca2+) unmasked only in choline-containing solution.

An N-methylaspartate receptor-mediated synapse in rat cerebral cortex: a site of action of ketamine?

It has been proposed that three major receptor subtypes subserve the putative transmitter role of glutamate and aspartate in the mammalian central nervous system. One subtype is classified by the specific agonist N-methylaspartate (NMA) and the specific antagonist 4-amino-2-phosphonovaleric acid. It has been shown recently that excitation of neurones by NMA is also selectively reduced by dissociative anaesthetics such as ketamine and phencyclidine and by sigma opiates, drugs of abuse with common psychotomimetic properties. Responses to NMA have an unusual voltage relation which may result from a voltage-dependent block of the activated channel by physiological concentrations of magnesium. No synaptic potential with properties similar to those of responses to NMA, however, has yet been reported. We describe here an excitatory postsynaptic potential (e.p.s.p.) evoked by electrical stimulation of the white matter and recorded intracellularly from pyramidal cells in slices of rat somatosensory cortex. This e.p.s.p. has the appropriate voltage relation and sensitivity to Mg2+ and ketamine to be an NMA receptor-mediated synapse and a potential central site for the psychotomimetic actions of ketamine.