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

Action of externally applied adenosine triphosphate on single smooth muscle cells dispersed from rabbit ear artery

1. Adenosine triphosphate (ATP), applied in the bathing solution or ionophoretically, depolarized freshly dispersed single arterial smooth muscle cells obtained by collagenase and elastase treatment of the rabbit ear artery. 2. Ionophoretic application of ATP evoked an inward current with a latency of about 70 ms and a time to peak of about 230 ms in cells held under voltage clamp using whole-cell patch-pipette techniques. 3. Bath application of 10 microM-ATP evoked a transient inward current at negative holding potentials. The amplitude of the ATP-induced current was linearly related to the clamp potential with a reversal potential near 0 mV. Removal of extracellular calcium, buffering intracellular calcium with high EGTA concentration, or depleting calcium stores with caffeine or noradrenaline treatment did not affect the ATP-evoked current. 4. Changing the chloride concentration gradient by decreasing extracellular or intracellular chloride concentration, or using the chloride channel blocker, frusemide, had no effect on the currents. 5. Replacing sodium with Tris shifted the reversal potential to more negative potentials. The reversal potential was not affected by exchanging intracellular potassium for caesium or sodium. Replacing extracellular sodium with 89 mM-barium also had little effect on the reversal potential. 6. These results are consistent with ATP activating a conductance that is cation selective but allows both monovalent and divalent cations to pass across the membrane.

Brain spectrin, calpain and long-term changes in synaptic efficacy

This chapter discusses the possibility that proteolytic digestion of cytoskeletal proteins, in particular spectrin, is part of the mechanisms through which physiological activity elicits structural and chemical changes in brain synapses. Recent work from several laboratories has produced a description of the initial events that trigger the long-term potentiation (LTP) of synaptic responses that appears in hippocampus after brief episodes of high frequency electrical stimulation. A likely sequence is as follows: suppression of IPSPs, prolongation of EPSPs, activation of N-methyl-D-aspartate (NMDA) receptors, influx of calcium into target cells. After briefly describing the evidence for this triggering sequence, the review takes up the question of what types of calcium sensitive chemistries are available to synaptic region that could produce functional changes lasting for weeks (i.e., for LTP). It is argued that the partial degradation of spectrin by a calcium-activated protease (calpain) provides a mechanism of this type. Spectrin is a substrate for calpain and both it and a breakdown product comparable to that produced by calpain are found in postsynaptic densities. Moreover, there is substantial evidence that spectrin regulates the surface chemistry and morphology of cells and thus its partial degradation would be expected to produce pronounced and persistent modifications in synapses. To reinforce this point, the review discusses recent findings suggesting that calpain mediated proteolysis of spectrin and other cytoskeletal proteins produces substantial changes in the shape of blood-borne cells and the distribution of their surface receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Zinc selectively blocks the action of N-methyl-D-aspartate on cortical neurons

Large amounts of zinc are present in synaptic vesicles of mammalian central excitatory boutons and may be released during synaptic activity, but the functional significance of the metal for excitatory neurotransmission is currently unknown. Zinc (10 to 1000 micromolar) was found to have little intrinsic membrane effect on cortical neurons, but invariably produced a zinc concentration-dependent, rapid-onset, reversible, and selective attenuation of the membrane responses to N-methyl-D-aspartate, homocysteate, or quinolinate. In contrast, zinc generally potentiated the membrane responses to quisqualate or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate and often did not affect the response to kainate. Zinc also attenuated N-methyl-D-aspartate receptor-mediated neurotoxicity but not quisqualate or kainate neurotoxicity. The ability of zinc to specifically modulate postsynaptic neuronal responses to excitatory amino acid transmitters, reducing N-methyl-to-aspartate receptor-mediated excitation while often increasing quisqualate receptor-mediated excitation, is proposed to underlie its normal function at central excitatory synapses and furthermore could be relevant to neuronal cell loss in certain disease states.

Activation of NMDA receptors blocks GABAergic inhibition in an in vitro model of epilepsy

The application of tetanic electrical stimuli to the stratum radiatum fibre pathway in the hippocampus in vitro produces an NMDA (N-methyl-D-aspartate) receptor-dependent enhancement of synaptic efficacy. Repeated application of such stimuli produces a progressive enhancement of synaptic efficacy leading to the genesis of spontaneous and stimulation-evoked epileptiform discharges. We have used this in vitro approach to explore the cellular mechanisms which underlie the kindling model of epilepsy. Kindling of the stratum radiatum fibre pathway in vitro induced a progressive, long-lasting reduction of both spontaneous and stimulation-evoked GABAergic (gamma-aminobutyric acid-mediated) inhibitory postsynaptic potentials (i.p.s.ps). The reduction of i.p.s.ps by kindling was associated with a profound decrease in the sensitivity of CA1 pyramidal neurons to ionophoretically applied GABA and an increase in sensitivity to NMDA. The reduction of i.p.s.ps and GABA sensitivity was prevented by kindling in the presence of the NMDA receptor antagonist D-2-amino-5-phosphonovalerate (D-APV). These results demonstrate that kindling-like stimulus patterns produce a reduction of GABAergic inhibition in the hippocampus resulting from a stimulus-induced postsynaptic activation of NMDA receptors. The modulation of GABAergic inhibition by NMDA receptors may cause the synaptic plasticity which underlies the kindling model of epilepsy.

NMDA receptors of dentate gyrus granule cells participate in synaptic transmission following kindling

In the mammalian central nervous system, receptors for the excitatory amino-acid neurotransmitters are divided into three subtypes depending on their sensitivity to three specific agonists: kainate, quisqualate and N-methyl-D-aspartate (NMDA). The ionophores operated by NMDA are gated by Mg2+ in a voltage-dependent manner and allow passage of several cations, including Ca2+ which may be important in plastic alterations of neuronal excitability. Indeed, specific antagonists of NMDA receptors effectively block spatial learning, long-term potentiation and some animal models of chronic epilepsy. Despite their abundance on central neurons, NMDA receptors, with a few noteworthy exceptions, do not generally seem to be involved in low-frequency synaptic transmission. Here we report for the first time that NMDA receptors of the dentate gyrus, where they do not normally contribute to the generation of synaptic potentials, become actively involved in synaptic transmission following long-lasting neuronal changes induced by daily electrical stimulation (kindling) of the amygdala or hippocampal commissures. In contrast to controls, the excitatory postsynaptic potentials (e.p.s.ps) of granule cells in hippocampal slices obtained from kindled animals displayed characteristics typical of an NMDA-receptor-mediated component. The involvement of NMDA receptors in synaptic transmission may underlie the long-lasting changes in neuronal function induced by kindling.

Evidence for a magnesium-insensitive membrane resistance increase during NMDA-induced depolarizations in rat neocortical neurons in vitro

The responses of rat neocortical neurons in vitro to iontophoretically applied N-methyl-D-aspartate (NMDA) were investigated by means of intracellular recording in the presence and absence of extracellular magnesium ions (Mg2+). At Mg2+-concentrations of 1.3 mM the neurons responded with a depolarization accompanied by an increase in membrane resistance. Upon removal of Mg2+ the NMDA-induced depolarization was markedly potentiated. However, even in neurons recorded from slices which were incubated in a Mg2+-free solution for 3-7 h, the NMDA response was still associated with a resistance increase, suggesting that the voltage-dependence of the NMDA-activated conductance is not exclusively determined by Mg2+.

Calcium channels and calcium channel antagonists

Changes in free intracellular Ca2+ levels provide signals that allow nerve and muscle cells to respond to a host of external stimuli. A major mechanism for elevating the level of intracellular Ca2+ is the influx of extracellular Ca2+ through voltage-dependent channels in the cell membrane. Recent research has yielded new insights into the physiological properties, molecular structure, biochemical regulation, and functional heterogeneity of voltage-dependent Ca2+ channels. In addition, Ca2+ channel antagonist drugs have been developed that are valuable both as probes of channel structure and function and as therapeutic agents. Preliminary evidence suggests that these drugs may be useful in the treatment of diverse neurological disorders, including headache, subarachnoid hemorrhage, stroke, and epilepsy.

Responses of pyriform cortex neurons to excitatory amino acids: voltage dependence, conductance changes, and effects of divalent cations

The actions of ionophoretically applied N-methyl aspartate (NMA), quisqualate, and kainate, thought to activate three different types of excitatory amino acid receptors, were studied on pyramidal neurons of the rat pyriform cortex, maintained in an isolated, submerged, and perfused brain slice. Intracellular recordings were made with either K acetate or CsCl electrodes. In most neurons all three agonists elicited monophasic responses which could be evoked at 20-sec intervals. Some neurons showed biphasic responses, most commonly to kainate but, on occasion, also for quisqualate. The slower component appeared to be correlated with excitotoxicity and, consequently, was difficult to study. As a result the kainate responses studied were from neurons selected for having a single component. In neurons selected for having a linear current-voltage relationship or neurons loaded with Cs to suppress K conductance and linearize the current-voltage relationship, the average changes in resistance recorded during ionophoretic responses at resting potential were as follows: NMA, 131.2 +/- 6.7% of control; kainate, 104.7 +/- 5.8% of control; and quisqualate, 92.8 +/- 2.8% of control. The magnitude and direction of the conductance change were very reproducible in any one neuron, but especially for kainate some cells showed clear conductance increases, while others showed clear conductance decreases. Using CsCl electrodes it was possible to reduce K+ conductance and depolarize the neurons over a wider range. By passing depolarizing current it was possible to reverse the responses. The response to all three agonists reversed at the same depolarized potential. This observation indicates that while there are differences in the ionic channels associated with the three agonists at resting potential, the channels have similar properties at more depolarized potentials. Responses to all three agonists were influenced by the concentrations of divalent cations in the perfusion medium. The NMA responses were most sensitive to Mg, increasing in amplitude in the absence of Mg and being depressed by Mg elevation. All responses were sensitive to Ca, with discharges being greatly increased by low Ca and depressed by high Ca. The kainate response was most sensitive to Ca concentration changes. Unlike reports from other preparations the apparent conductance decreases to NMA were not altered by the perfusion of solutions with either no added Mg or no added Ca. The NMA response was very much reduced in either Co (1-2 mM) or Zn (100-200 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Dextrorphan and dextromethorphan attenuate glutamate neurotoxicity

The dextrorotatory morphinan opioid, dextrorphan, which has recently been reported to block the excitation of cortical neurons by N-methyl-D-aspartate, was found at 10-100 microM concentrations to attenuate both morphological and chemical evidence of glutamate neurotoxicity in murine neocortical cell cultures; a similar effect was found with its methyl ester derivative, dextromethorphan. Given other data suggesting that glutamate neurotoxicity may participate in the pathogenesis of the central neuronal loss associated with certain human neurological diseases, the present observations raise the possibility that these clinically tested opioids, or related compounds, may eventually prove to have some clinical therapeutic utility.

Multiple-conductance channels activated by excitatory amino acids in cerebellar neurons

In the mammalian central nervous system amino acids such as L-glutamate and L-aspartate are thought to act as fast synaptic transmitters. It has been suggested that at least three pharmacologically-distinguishable types of glutamate receptor occur in central neurons and that these are selectively activated by the glutamate analogues N-methyl-D-aspartate (NMDA), quisqualate and kainate. These three receptor types would be expected to open ion channels with different conductances. Hence if agonists produce similar channel conductances this would suggest they are acting on the same receptor. Another possibility is suggested by experiments on spinal neurons, where GABA (gamma-amino butyric acid) and glycine appear to open different sub-conductance levels of one class of channel while acting on different receptors. By analogy, several types of glutamate receptor could also be linked to a single type of channel with several sub-conductance states. We have examined these possibilities in cerebellar neurons by analysing the single-channel currents activated by L-glutamate, L-aspartate, NMDA, quisqualate and kainate in excised membrane patches. All of these agonists are capable of opening channels with at least five different conductance levels, the largest being about 45-50 pS. NMDA predominantly activated conductance levels above 30 pS while quisqualate and kainate mainly activated ones below 20 pS. The presence of clear transitions between levels favours the idea that the five main levels are all sub-states of the same type of channel.

Glutamate activates multiple single channel conductances in hippocampal neurons

There is considerable evidence that glutamate is the principal neurotransmitter that mediates fast excitatory synaptic transmission in the vertebrate central nervous system. This single transmitter seems to activate two or three distinct types of receptors, defined by their affinities for three selective structural analogues of glutamate, NMDA (N-methyl-D-aspartate), quisqualate and kainate. All these agonists increase membrane permeability to monovalent cations, but NMDA also activates a conductance that permits significant calcium influx and is blocked in a voltage-dependent manner by extracellular magnesium. Fast synaptic excitation seems to be mediated mainly by kainate/quisqualate receptors, although NMDA receptors are sometimes activated. We have investigated the properties of these conductances using single-channel recording in primary cultures of hippocampal neurons, because the hippocampus contains all subtypes of glutamate receptors and because long-term potentiation of synaptic transmission occurs in this structure. We find that four or more distinct single-channel currents are evoked by applying glutamate to each outside-out membrane patch. These conductances vary in their ionic permeability and in the agonist most effective in causing them to open. Clear transitions between all the conductance levels are observed. Our observations are compatible with the model that all the single channel conductances activated by glutamate reflect the operation of one or two complex molecular entities.

A new type of glutamate receptor linked to inositol phospholipid metabolism

Receptors for excitatory amino acids in the mammalian central nervous system are classified into three major subtypes, ones which prefer N-methyl-D-aspartate (NMDA), quisqualate (QA), or kainate (KA) as type agonists respectively. These receptors are considered to mediate fast postsynaptic potentials by activating ion channels directly (ionotropic type). Recently it was reported that exposure of mammalian brain cells to glutamate (Glu) or its analogues causes enhanced hydrolysis of inositol phospholipids, but it is not clear whether the enhanced hydrolysis is the cause or effect of physiological responses. Membrane depolarization or Ca2+ influx, which can result from Glu receptor activation, can induce enhanced hydrolysis of inositol phospholipids. We have characterized the functional properties of two types of excitatory amino-acid responses, those activated by QA (or Glu) and those activated by KA, induced in Xenopus oocytes injected with rat-brain messenger RNA. We report evidence for a new type of Glu receptor, which prefers QA as agonist, and which directly activates inositol phospholipid metabolism through interaction with GTP-binding regulatory proteins (Gi or Go), leading to the formation of inositol 1,4,5-trisphosphate (InsP3) and mobilization of intracellular Ca2+. This QA/Glu reaction is inhibited by islet-activating protein (IAP, pertussis toxin), but was not blocked by Joro spider toxin (JSTX), a specific blocker of traditional ionotropic QA/Glu receptors.

Excitatory amino acids increase glycogen phosphorylase activity in the rat spinal cord

Glycogen phosphorylase is present in nervous tissue in an active and inactive form. Using a histochemical technique, an investigation into which putative neurotransmitters have the capacity to modify the activity of the enzyme, has been performed on the rat spinal cord. Intrathecal injections of L-glutamate and L-aspartate elevate glycogen phosphorylase activity in the dorsal horn, while substance P has no effect and only high doses of adenosine triphosphate (ATP) increase the enzyme activity. In addition the N-methyl-D-aspartate receptor antagonist, 5-amino-phosphonovaleric acid was found to block the elevation of glycogen phosphorylase activity in the dorsal horn produced by the peripheral activation of chemo-sensitive primary afferents. Excitatory amino-acid neurotransmitters can therefore, acting via second messengers and protein kinases, modify glycogen metabolism in the spinal cord.

Multiple calcium channels and neuronal function

Recent investigations have demonstrated that neurons have a number of different types of calcium channels, each with their own unique properties and pharmacology. These calcium channels may be important in the control of different aspects of nerve activity. Some of the possibilities can now be discussed.

Effects of Ca2+ entry blockers on kainate-induced changes in extracellular amino acids and Ca2+ in vivo

The effect of organic Ca2+ channel blockers and Co2+ on kainate-induced changes in 45Ca2+ efflux and amino acid release was studied in the rabbit hippocampus with the dialysis-perfusion technique. Administration of 1 mM kainate caused a transient, 50% drop of extracellular Ca2+. This effect was insensitive to 100 microM flunarizine or verapamil, 10 microM nimodipine, and 6 mM CoCl2. The organic Ca2+ entry blockers did not significantly influence kainate-induced changes in extracellular amino acids, whereas Co2+ affected both basal and kainic acid stimulated release of amino acids. These results indicate that kainate-regulated Ca2+ ionophores differ from Ca2+ channels in peripheral tissues in terms of sensitivity to Ca2+ entry inhibitors.

Excitatory amino acids and intracellular pH in motoneurons of the isolated frog spinal cord

Double-barrelled pH-sensitive micro-electrodes were used to measure changes of intracellular and extracellular pH in and around motoneurons of the isolated frog spinal cord during application of excitatory amino acids. It was found that N-methyl-D-aspartate, quisqualate and kainate produced a concentration-dependent intracellular acidification. Extracellularly, triphasic pH changes (acid-alkaline-acid going pH transients) were observed during the action of these amino acids. The possible significance of such pH changes for the physiological and pathophysiological effects of excitatory amino acids are discussed.

Amino acid neurotoxicity: intracellular sites of calcium accumulation associated with the onset of irreversible damage to rat cerebellar neurones in vitro

Electron microscopy and the combined oxalate-pyroantimonate technique were used to locate calcium in intracerebellar nucleus neurones of rat cerebellar slices subjected to a neurotoxic concentration of N-methyl-D-aspartate. After a sub-lethal exposure period (5 min) calcium pyroantimonate deposits were found in swollen cisterns of the Golgi apparatus and, in lesser amounts, in the nuclei. Deposits were more prominent in the nuclei after a just-lethal exposure (10 min) when they were additionally observed within a population of swollen mitochondria and also apparently free in the dendritic and somatic cytoplasm. The results support the proposal that amino acid neurotoxicity is a consequence of an intracellular Ca2+ overload brought about by excessive Ca2+ influx.

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.