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

Magnesium ions block an N-methyl-D-aspartate receptor-mediated component of synaptic transmission in rat hippocampus

The involvement of N-methyl-D-aspartate (NMDA) receptors in synaptic transmission in the Schaffer collateral-commissural pathway of rat hippocampal slices has been examined in the presence and absence of Mg2+. Superfusion of slices with Mg2+ -free medium resulted in an increase in the amplitude of the population spike and the appearance of multiple population spikes. An NMDA antagonist, which had no effect on the synaptic response in the presence of Mg2+, completely prevented the appearance of multiple responses. If added to slices already bathed in Mg2+ -free medium, the antagonist eliminated the multiple responses and partly reduced the amplitude of the initial population spike. These data suggest that the extent to which NMDA receptors participate in synaptic transmission in rat hippocampus depends on the extracellular Mg2+ concentration.

Some properties of ionic channels activated by excitatory amino acids in hippocampal neurons

Properties of excitation induced by various excitatory amino acids were studied in thin slices of the guinea pig hippocampus in the presence of Mn2+, tetrodotoxin and tetraethylammonium chloride. Depolarizations induced by L-glutamate (Glu), quisqualate (Quis) and D-homocysteate (DH) were accompanied consistently by decreases in neuron input resistance. In the current-voltage function, increases in input resistance were never observed at any membrane potential. The amplitude of Glu, Quis and DH responses decreased during tonic outward currents and increased during tonic inward currents. Although neuron input resistance decreased with depolarizations induced by L-aspartate (Asp) as well, the magnitude of the resistance reduction was significantly smaller than that induced by Glu. Asp responses changed in amplitude as did Glu responses during tonic inward and outward currents. Depolarizations induced by N-Methyl-D-aspartate (NMDA) were accompanied by apparent increases in input resistance, and their amplitudes increased and decreased during tonic depolarization and hyperpolarization, respectively. Mn2+ was almost without effect at the concentration used (2.7 mM) on responses induced by Glu, DH or Asp. These results suggest that Glu, Quis and DH induce depolarizations in hippocampal neurons by activating only Quis receptors, and that Asp activates Quis receptors preferentially though it activates NMDA receptors as well.

The effects of potassium channel blocking agents on the responses of in vitro frog motoneurones to glutamate and other excitatory amino acids: an intracellular study

Intracellular recordings were obtained from motoneurones of the slice preparation of the in vitro frog spinal cord to assess the participation of K+ channels in the generation of amino acid-induced excitation. The amino acids were applied by fast superfusion and at low temperature to reduce their uptake processes. Intracellular Cs+, bath-applied tetraethylammonium or 4-aminopyridine prolonged the spike and blocked its afterhyperpolarizations. Tetraethylammonium and 4-aminopyridine enhanced the synaptic activation of motoneurones which could be abolished by including Mn2+ in the superfusion fluid. Mn2+ reduced the duration of action potentials already prolonged by 4-aminopyridine but appeared to cause plateaus to develop on tetraethylammonium-prolonged spikes. The hypothesis that amino acids depolarize motoneurones by closing K+ channels was tested by observing the effects of K+ channel blocking agents on amino acid responses. Neither tetraethylammonium nor 4-aminopyridine reduced the excitatory depolarizations evoked by glutamate, N-methyl-D-aspartate, quisqualate or DL-homocysteate. Intracellular Cs+ blocked responses to amino acids irrespective of their receptor preference; an action which has been interpreted as non-specific.

A role for taurine in the maintenance of homeostasis in the central nervous system during hyperexcitation?

Employing the brain dialysis technique, we demonstrate that the aspartate congener N-methyl-D,L-aspartic acid (NMA) stimulates the release of endogenous taurine in vivo in a Ca2+-dependent manner. Furthermore, exogenous taurine (1-10 mM) inhibits the NMA-induced Ca2+ influx into intracellular compartments. This suggests that the extracellular taurine concentration may control Ca2+ movement and thereby provide a homeostatic mechanism in situations of excessive excitation.

Blockade of N-methyl-D-aspartate receptors may protect against ischemic damage in the brain

In rats ischemia of the forebrain induced by a 30-minute occlusion of the carotid artery, followed by 120 minutes of arterial reperfusion, produced ischemic lesions of selectively vulnerable pyramidal cells in both hippocampi. Focal microinfusion into the dorsal hippocampus of 2-amino-7-phosphonoheptanoic acid, an antagonist of excitation at the N-methyl-D-aspartate-preferring receptor, before ischemia was induced protected against the development of ischemic damage. It is proposed that excitatory neurotransmission plays an important role in selective neuronal loss due to cerebral ischemia.

Multiple actions of a molluscan cardioexcitatory neuropeptide and related peptides on identified Helix neurones

The effects of the molluscan neuropeptide Phe-Met-Arg-Phe-NH2 (FMRF amide) and related peptides (Price & Greenberg, 1977) were tested on Helix aspersa neurones. Ionophoretic application of FMRFamide depolarized and excited some neurones, but hyperpolarized and inhibited others. In some neurones the sign of the response was dependent on the membrane potential. Two responses resulted from an increase in membrane conductance, a depolarizing response mediated mainly by an increase in Na+ ion permeability, and a hyperpolarizing response mediated by an increase in K+ ion permeability. In the C1 neurone a voltage-dependent response was observed, which only occurred when the neurone was depolarized from its resting level. This response was recorded as an inward current during voltage clamp and resulted from a decrease in K current(s), possibly Ca-activated K current. More than one response may occur in a single neurone. In the C1 neurone, the K-mediated hyperpolarization occurred as well as the voltage-dependent response, while the depolarization seen in the F2 neurone was a combination of an increase in Na conductance and an increase in K conductance.

Mg2+ dependence of membrane resistance increases evoked by NMDA in hippocampal neurones

The response of granule cells and CA1 pyramidal neurones to NMDA was studied in the presence and absence of Mg2+ using an in vitro slice preparation. In the absence of Mg2+ the depolarizing response of hippocampal neurones to NMDA is accompanied by a decrease in input resistance. In the presence of Mg2+ ions, however, the response to NMDA is always associated with an apparent increase in input resistance. These results indicate that the action of NMDA is by a classical mechanism of conductance increase and are in agreement with the suggestion that the apparent increase in input resistance associated with NMDA depolarizations is the result of voltage-dependent channel block by Mg2+ of the NMDA evoked current.

Mixed-agonist action of excitatory amino acids on mouse spinal cord neurones under voltage clamp

Neurones from the ventral half of mouse embryo spinal cord were grown in tissue culture and voltage clamped with two micro-electrodes. The current-voltage relation of responses evoked by brief pressure applications of excitatory amino acids was examined over a membrane potential range of -100 to +70 mV. Three types of current-voltage relation were observed. Responses to kainic and quisqualic acids were relatively linear within +/- 20 mV of the resting potential. N-methyl-D-aspartate (NMDA) and L-aspartic acid responses had a negative slope conductance at membrane potentials more negative than -30 mV. In contrast, over the same potential range the slope conductance of responses evoked by L-glutamic and L-homocysteic acids was close to zero. The membrane potential-chord conductance relation of the ionic mechanism activated by excitatory amino acids, derived using the driving force for ionic current, showed two types of behaviour. The conductance linked to NMDA receptors was highly voltage sensitive and increased on depolarization; a much weaker voltage sensitivity was observed for responses evoked by kainic and quisqualic acids. L-glutamic and L-homocysteic acid responses behaved as though due to simultaneous activation of both NMDA and either kainate or quisqualate receptors. In the presence of the NMDA receptor antagonist (+/-)-2-aminophosphonovaleric acid (2-APV) the response to L-glutamate became less voltage sensitive and resembled responses evoked by kainate or quisqualate. Simultaneous activation of both conductance mechanisms by mixtures of kainate and NMDA produced current-voltage and membrane potential-chord conductance relations similar to those of L-glutamate. The voltage sensitivity of the L-glutamate response was inversely related to the dose; for low doses of L-glutamate the slope conductance of responses recorded near the resting potential was close to zero. However, larger doses of L-glutamate evoked responses with a voltage sensitivity similar to that of kainate. We suggest that L-glutamate acts as a mixed agonist at both NMDA and non-NMDA receptors. This can explain the results of previous experiments that failed to demonstrate a membrane resistance change during L-glutamate-induced depolarizations.

The reversal potential of excitatory amino acid action on granule cells of the rat dentate gyrus

The responses of granule cells to glutamate, aspartate, N-methyl-D-aspartate (NMDA), quisqualate and kainate applied by ionophoresis on to their dendrites in the middle molecular layer of the dentate gyrus were studied with intracellular electrodes using an in vitro hippocampal slice preparation. On passive depolarization 75% of the granule cells displayed anomalous rectification, which persisted in the presence of TTX and TEA but was eliminated by Co2+ or the intracellular injection of Cs+. Short ionophoretic applications of all the excitatory amino acids evoked dose-dependent depolarizations that were highly localized: movement of the ionophoretic electrode by as little as 10 microns could substantially change the size of the response. The depolarizations evoked by glutamate, asparatate, quisqualate and kainate were unaffected by TTX and Co2+. The depolarization evoked by NMDA was unaffected by TTX but markedly reduced by Co2+. Following intracellular injection of Cs+, neurones could be depolarized to +30 mV and the depolarizations produced by glutamate, quisqualate, NMDA and kainate reversed. The reversal potentials (E) were Eglutamate: -5.6 +/- 0.4 mV; ENMDA: 1.8 +/- 1.9 mV; Equisqualate: -3.9 +/- 1.9 mV; Ekainate: -4.6 +/- 2.0 mV. The excitatory post-synaptic potential (e.p.s.p.) evoked by stimulation of the medial perforant path could also be reversed and Ee.p.s.p. was -5.5 +/- 1.1 mV. The 6 mV difference between ENMDA and the equilibrium potential for the other exogenously applied excitatory amino acids and the statistically significant difference between ENMDA and Ee.p.s.p. (P less than 0.005; d.f.: 7) is consistent with our earlier hypothesis that both the transmitter released by the medial perforant path and exogenously applied glutamate are unlikely to interact with NMDA receptors.

Voltage-dependent block by Mg2+ of NMDA responses in spinal cord neurones

Acidic amino acids are putative excitatory synaptic transmitters, the ionic mechanism of which is not well understood. Recent studies with selective agonists and antagonists suggest that neurones of the mammalian central nervous system possess several different receptors for acidic amino acids, which in turn are coupled to separate conductance mechanisms. N-methyl-D-aspartic acid (NMDA) is a selective agonist for one of these receptors. The excitatory action of amino acids acting at NMDA receptors is remarkably sensitive to the membrane potential and it has been suggested that the NMDA receptor is coupled to a voltage-sensitive conductance. Recently, patch-clamp experiments have shown the voltage-dependent block by Mg2+ of current flow through ion channels activated by L-glutamate. We now show using voltage-clamp experiments on mouse spinal cord neurones that the voltage-sensitivity of NMDA action is greatly reduced on the withdrawal of physiological concentrations (approximately 1 mM) of Mg2+ from the extracellular fluid. This provides further evidence that Mg2+ blocks inward current flow through ion channels linked to NMDA receptors.

A possible correlate of the postsynaptic condition for long-lasting potentiation in the guinea pig hippocampus in vitro

Picrotoxin and 2-amino-5-phosphonovalerate (APV) have previously been reported to influence the production of long-lasting potentiation in the hippocampus (facilitation and depression, respectively). We have examined how these drugs modify the postsynaptic responses to tetanic afferent activation used to elicit long-lasting potentiation. The experiments were performed in the CA1 area of transverse hippocampal slices maintained in vitro. A slow extracellular dendritic negativity-interpreted as a sign of a dendritic depolarizing process-was enhanced by picrotoxin and depressed by APV. This dendritic potential may represent a postsynaptic event involved in the production of long-lasting potentiation.

Lathyrus excitotoxin: mechanism of neuronal excitation by L-2-oxalylamino-3-amino- and L-3-oxalylamino-2-amino-propionic acid

Intracellular recordings were made in cultured neurones from foetal mouse spinal cord. The effects of applications of the neurotoxin, L-3-oxalylamino-2-amino-propionic acid (a constituent of the chickling pea, Lathyrus sativus) and its 2-oxalylamino isomer on membrane potential and conductance were examined in the presence of TTX and TEA and compared to those of other excitatory amino acids. Although both compounds produced membrane depolarization and an increase in input conductance, the 3-oxalylamino isomer (beta-ODAP) was approximately equal to 10 times more potent than the 2-oxalylamino isomer (alpha-ODAP). beta-ODAP caused a voltage-independent change in conductance, as compared to an apparent voltage-dependent decrease produced in the same neurons by L-aspartic acid (L-ASP). Although reversal potentials determined for beta-ODAP resembled those for alpha-ODAP and kainic acid, they were consistently and significantly lower than the reversal level for L-ASP. Although the receptor antagonist 2-amino-5-phosphonovaleric acid (APV) and the divalent cation Cd2+ did not alter the conductance increase evoked by beta-ODAP, they markedly depressed responses to L-ASP. Such differences suggest a mechanism of excitatory action for the neurotoxin, beta-ODAP, which does not involve a Ca2+-dependent mechanism and is quite different from that for L-ASP and N-methyl-D-aspartic acid, but similar to that of kainic and quisqualic acids.(ABSTRACT TRUNCATED AT 250 WORDS)