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M L Mayer

Publications and source records attributed to M L Mayer.

At least 73 records · Page 4Linked to original sources
PubMed1989

Glial uptake of excitatory amino acids influences neuronal survival in cultures of mouse hippocampus.

Several reports have described apparently normal survival and development of hippocampal and spinal cord culture preparations grown in Ham's F-12 medium, which contains 100 microM each of L-glutamate and L-aspartate. As at this concentration these amino acids are neurotoxic, some adaptive mechanism must occur to allow neuronal survival. We have investigated the mechanism underlying such adaptation. Dissociated cultures of mouse hippocampal neurons were grown in either Eagle's minimum essential medium or Ham's F-12 medium, supplemented with 5% horse serum. Analysis of neuronal density in cultures stained for neuron-specific enolase showed that although large numbers of neurons were present in mature cultures grown in either medium, neuronal survival in cultures grown continuously in F-12 was reduced to 41% compared to controls grown in Eagle's minimum essential medium. Physiological studies showed that those neurons which survived in F-12 did not lose their sensitivity to excitatory amino acids. In addition, the acute application of fresh, serum-free F-12 to 10-14-day-old cultures grown in either minimum essential medium or F-12 was highly neurotoxic. Three lines of evidence suggest that glial uptake of amino acids, and reduction of the extracellular concentration of glutamate and aspartate below neurotoxic levels, rather than receptor desensitization underlies the adaptive mechanism allowing neuronal survival. First, application of fresh F-12 produced large depolarizations, and profound neuronal swelling in cultures grown in F-12; however, after several hours swelling reversed suggesting a slow onset of the adaptive process. Second, pressure application of conditioned F-12 obtained from sister cultures also elicited depolarizations in neurons grown in F-12, but the amplitude of the underlying inward current was 25-30% of that produced by fresh F-12, suggesting a loss of potency of F-12 exposed to prolonged contact with hippocampal cultures. Third, measurement by high performance liquid chromatography showed reduction of aspartate concentrations to around 10% of those present in fresh F-12, within 24 h after exposing glial cell cultures to fresh F-12. It is concluded that cellular uptake mechanisms for amino acids have a strong impact on excitotoxicity in vitro, and most likely play an important role in protecting neurons from the potentially damaging action of high concentrations of excitatory transmitters in vivo. In addition our experiments help to explain the mechanisms permitting neuronal survival in cultures grown in Ham's F-12 medium, which when applied acutely to mature cultures is strikingly neurotoxic.

Action Potentials↗
PubMed1989

Open channel block of NMDA receptor responses evoked by tricyclic antidepressants.

The action of desipramine (DMI) and promazine on the response of mouse hippocampal neurons to the excitatory amino acid N-methyl-D-aspartic acid (NMDA) was investigated using whole-cell and single-channel recording. DMI at 20-50 microM was a potent, selective antagonist of responses to NMDA but not kainate or quisqualate. At -60 mV, the Kd for DMI block of responses to NMDA was 10 microM. The potency of DMI as an NMDA antagonist was highly voltage-dependent and behaved as though the Kd increased e-fold per 36 mV depolarization, reflecting an increase in the dissociation rate constant. Prior block of NMDA receptors with Mg2+ prevented binding of DMI, suggesting an action in the open channel. Single-channel analysis showed a decrease in the open time and burst length distributions, consistent with binding of DMI to open channels. We suggest that the action of DMI on NMDA receptor channels is similar to that of MK-801 and does not reflect binding to other domains, such as the regulatory sites for Zn2+ and glycine.

Animals↗
PubMed1989

Concanavalin A selectively reduces desensitization of mammalian neuronal quisqualate receptors.

A fast perfusion system was used to apply excitatory amino acids to embryonic hippocampal neurons grown in dissociated culture and voltage clamped in the whole-cell recording configuration. Responses to quisqualic acid and DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA; a potent quisqualate-like agonist) showed rapid desensitization: at 100 microM the peak inward current declined to a plateau response on average 0.2 times the peak response (mean time constant, 30 ms). Responses to L-aspartic acid and N-methyl-D-aspartic acid also showed desensitization: at 100 microM, when recorded in Mg-free solution with 0.3 microM glycine, the peak inward current declined to a plateau value 0.5 times the peak, but with a time constant of desensitization (average, 248 ms) one order of magnitude slower than desensitization of responses to quisqualate. Responses to kainate and domoate (agonists at kainic acid receptors) did not show appreciable desensitization. Responses to L-glutamate and 5-Br-willardine (a potent non-NMDA receptor agonist), recorded in glycine-free solution with 1 mM Mg to suppress N-methyl-D-aspartic acid receptor activity, showed similar rapid desensitization to AMPA and quisqualate, but occurred with less depression of the peak current. The lectin concanavalin A (Con A) reduced desensitization at quisqualate receptors, with no effect on responses to kainate or N-methyl-D-aspartic acid. The effect of Con A developed slowly (average time constant at 2.5 microM, 250 s) but at steady state Con A increased the plateau current evoked by 100 microM quisqualate to 13 times control. Succinyl-Con A produced only a small reduction of desensitization to quisqualate, approximately 10% of that produced by native Con A. Con A did not change the decay time constant of fast excitatory synaptic currents evoked by stimulation of presynaptic neurons, although the peak synaptic current decreased after treatment with lectin. Con A was also without effect on the block of responses to kainate produced by coapplication of quisqualate.

Animals↗
PubMed1989

Modulation of excitatory amino acid receptors by group IIB metal cations in cultured mouse hippocampal neurones.

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

Amino Acids↗
PubMed1989

The action of zinc on synaptic transmission and neuronal excitability in cultures of mouse hippocampus.

1. The whole-cell configuration of the patch clamp method was used to record from hippocampal neurones in cell culture. Synaptic responses were evoked by loose patch stimulation of adjacent presynaptic neurones in low-density cultures. Agonists and antagonists were applied rapidly, using an array of flow pipes each of diameter 250 microns, positioned within 100 microns of the postsynaptic neurone. 2. Bath application of 50 microM-zinc produced prolonged periods of synaptic barrage and action potential discharge. Flow pipe application of 50 microM-zinc, in glycine-free solution with 1 mM-Mg2+, produced on average a 75% reduction of IPSP amplitude, but increased the average EPSP amplitude to 171% of control. However, after block of gamma-aminobutyric acid (GABA) receptors with bicuculline, zinc had no effect on EPSP amplitude, suggesting that potentiation recorded in control solutions reflects block of polysynaptic IPSPs. 3. Consistent with the block of IPSPs postsynaptic responses to flow pipe applications of GABA were blocked by zinc, with fast-on, fast-off kinetics. The equilibrium dissociation constant (Kd) for zinc block of GABA responses, estimated from fit of a single binding site adsorption isotherm, was 11 microM and sufficient to explain the degree of reduction of IPSPs by 50 microM-zinc. Zinc antagonism of responses to GABA was essentially independent of membrane potential over the range -60 to +60 mV. 4. With bicuculline methiodide and glycine added to a magnesium-free extracellular solution, to allow the study of synaptic responses mediated by N-methyl-D-aspartic acid (NMDA) receptors, zinc reduced the amplitude of EPSPs to 50% of control, and decreased the decay time constant of the EPSP, suggesting that zinc blocks synaptic activation of NMDA receptors. 5. Under conditions where synaptic transmission was completely blocked with postsynaptic receptor antagonists (1-3 mM-kynurenic acid and 10-20 microM-bicuculline methiodide) 50 microM-zinc decreased the amplitude of the spike after-hyperpolarization (AHP), but did not produce large changes in action potential amplitude or half-width. Under these conditions 50 microM-zinc also decreased the current threshold required to trigger action potential discharge, and blocked accommodation so that repetitive firing replaced single action potential responses to prolonged current pulses.

Action Potentials↗
PubMed1988

A modulatory action of divalent cations on transient outward current in cultured rat sensory neurones.

1. The effects of some divalent cations on the A-current (IA) in cultured rat dorsal root ganglion cells (DRGs) were studied using whole-cell patch recording. 2. IA was not affected by omission of calcium from the external medium; however it was significantly depressed by manganese (10 mM) applied by pressure ejection. This depressant effect of manganese resulted from a depolarizing shift of the activation curve by 17 mV, associated with only a slight reduction of the maximum conductance. At 10 mM manganese also caused a depolarizing shift of the steady-state inactivation curve by 34 mV. Divalent cations other than manganese also gave positive shifts of the steady-state activation and inactivation curves for IA but were of different potency; the sequence was: Cd2+ greater than Mn2+ = Co2+ greater than Mg2+. 3. A dose-response curve for the depolarizing shift of the activation and inactivation curves of IA, as a function of manganese concentration, could be fitted by a single binding site model with an apparent dissociation constant of approximately 17 mM. The depolarizing shift of the inactivation curve was on average twice as large as that of the activation curve. 4. In contrast to its effect on IA, manganese (10 mM) did not cause any appreciable change in the voltage dependence of the activation curve for the delayed rectifier K+ current. 5. A low concentration of manganese (1 mM) increased the amplitude of IA recorded at pre-pulse potentials ranging from -50 to -70 mV. This augmentation of IA resulted from a positive shift of the inactivation curve by 6 mV without an appreciable shift of the activation curve; as a result a population of A-channels is released from inactivation over pre-pulse potentials from -50 to -70 mV. 6. These results show that divalent cations can evoke a depolarizing shift of both the activation and inactivation gates controlling IA; this causes either depression or augmentation of IA, depending on the species and concentration of the divalent cation, and also on the pre- pulse potential used to de-inactivate IA. This modulatory effect of divalent cations on the gating of IA appears to reflect binding to a specific, saturable site, either the A-channel protein itself, or phospholipids electrically close to the gating apparatus.

Action Potentials↗
PubMed1988

Sites of antagonist action on N-methyl-D-aspartic acid receptors studied using fluctuation analysis and a rapid perfusion technique.

1. Mouse hippocampal neurons in dissociated culture were grown at low density on previously plated hippocampal glial cell cultures and voltage clamped using the tight seal whole-cell patch-clamp technique. Flow pipes were used to rapidly exchange the extracellular solution, and to apply N-methyl-D-aspartic acid (NMDA) and some NMDA antagonists. Fluctuation analysis was used to estimate changes in the behavior of NMDA-activated ion channels during application of antagonists. In the presence of NMDA control spectra were well fit by single Lorentzian functions consistent with mean open times of 5-6 ms. 2. Two antagonists thought to act at the NMDA receptor agonist recognition site, 2-amino-5-phosphonovaleric acid (AP5) and kynurenic acid, did not produce changes in the mean open time or single channel conductance, consistent with their action as competitive antagonists. Onset of antagonism and recovery from the action of both AP5 and kynurenic acid was rapid and complete within 1 s. However, raising the extra-cellular glycine concentration, from 1 microM to 1 mM, reduced the potency of 100 microM kynurenic acid as an NMDA antagonist, suggesting that kynurenate has an additional action as a competitive antagonist at the glycine modulatory site on NMDA receptor channels. 3. In the presence of 150 microM magnesium NMDA spectra recorded at -60 mV were fit by double Lorentzian functions, consistent with single-channel events consisting of bursts of openings lasting 3.3 ms in duration, interrupted by blocking and unblocking events of average duration 0.18 ms. The onset and recovery from magnesium antagonism was rapid, and complete within 1 s, but was highly voltage dependent and at +40 mV magnesium (150 microM) failed to produce NMDA antagonism. These results are consistent with a voltage-dependent channel block of NMDA receptor channels produced by binding of magnesium to a site within the ion channel. 4. Zinc (30 microM) was a potent NMDA antagonist at both -60 and +40 mV, and at either potential appeared to reduce the mean open time of NMDA-activated ion channels from about 5 ms to approximately 3 ms. Over the frequency range measured, 1-1,000 Hz, NMDA spectra were well fit by single Lorentzians during zinc antagonism, in contrast to results obtained with magnesium. The mean single channel conductance also decreased in the presence of zinc to approximately 75% of control. Onset of antagonism and recovery from the action of zinc was rapid and complete within 1 s.(ABSTRACT TRUNCATED AT 400 WORDS)

2-Amino-5-phosphonovalerate↗
PubMed1988

Modulation of excitatory synaptic transmission by glycine and zinc in cultures of mouse hippocampal neurons.

The monosynaptic EPSP between cultured hippocampal neurons is mediated by activation of 2 classes of excitatory amino acid receptors. Kainate or quisqualate receptors generate a fast conventional EPSP, while NMDA receptors mediate a slow, voltage-sensitive EPSP. Recently, 2 substances have been shown to modulate the activity of the NMDA receptor-channel complex: glycine increases the probability of channel opening, while zinc acts as a noncompetitive antagonist. Since these substances are present in the CNS and thus may function as neuromodulators, we have examined their role in excitatory synaptic transmission in hippocampal cultures using the whole-cell-patch-recording technique. The slow, NMDA-receptor-mediated EPSP was strikingly dependent on the presence of a conditioning substance that gradually accumulated in the extracellular fluid during a 30 min incubation in physiological saline. Washout of the conditioned medium eliminated the slow EPSP, and perfusion with physiological saline containing 1 microM glycine restored the slow EPSP to control levels. Furthermore, conditioned medium collected from astroglial-only cultures also potentiated the response to NMDA. Zinc (20-50 microM) overcame the potentiation of the response by glycine and resulted in a reversible block of the slow EPSP, providing the first evidence for a direct action of zinc on excitatory synaptic transmission. Our results show that the expression of the slow EPSP may be subject to regulation by several endogenous substances: positive modulation by glycine (or a glycine-like substance), which can be released from astroglial cells, and negative modulation by physiological levels of zinc.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Pathogenesis of post-herpetic neuralgia.

A model of post-herpetic neuralgia is put forward based on the physiological effects of herpes simplex type-1 virus infection of rat dorsal root ganglion neurons in tissue culture. This virus causes these normally electrically silent neurons to produce spontaneous action potentials. Furthermore, pairs of neurons tend to have synchronised discharges. This coupling is not synaptically mediated, but due to presumed electrical junctions. It is proposed that post-herpetic neuralgia is caused by abnormal impulses arising in dorsal root ganglion neurons as a direct result of viral infection.

Action Potentials↗
PubMed1987

Permeation and block of N-methyl-D-aspartic acid receptor channels by divalent cations in mouse cultured central neurones.

1. Spinal cord and hippocampal neurones in cell culture were voltage clamped using the tight-seal, whole-cell recording technique. The concentration of sodium and a series of divalent cations in the extracellular media was varied to study permeation through excitatory amino acid receptor channels activated by the selective agonists N-methyl-D-aspartic acid (NMDA), kainic acid and quisqualic acid. 2. On raising the extracellular calcium concentration, with [Na+]o held constant at 105 mM, the reversal potential of responses to NMDA shifted in the depolarizing direction. This shift was adequately described by the extended constant-field equation over the range 0.3-50 mM-calcium. Using ionic activity coefficients we calculate a value of PCa/PNa = 10.6. Under the same experimental conditions the reversal potential of responses to kainic and quisqualic acids was much less affected by raising the calcium concentration, such that PCa/PNa = 0.15. A depolarizing shift of the NMDA reversal potential was also recorded during application of 20 mM-barium, strontium or manganese, suggesting permeation of these ions. The permeability sequence was Ca2+ greater than Ba2+ greater than Sr2+ much greater than Mn2+. No depolarizing shift of the NMDA reversal potential occurred during application of 20 mM-cobalt, magnesium or nickel. 3. In experiments in which the extracellular Na+ concentration was varied the extended constant-field equation was adequate in predicting shifts of the NMDA reversal potential recorded on varying [Na+]o over the range 50-150 mM, but failed to accurately predict the reversal potential of responses to NMDA with 10 mM-[Ca2+]o and only 10 or 20 mM-[Na+]o. These results imply an apparent increase in PCa/PNa on lowering [Na+]o and may result from interaction of permeant ions within the channel. 4. Barium and to a lesser extent calcium, but not strontium (all 20 mM), reduced the slope conductance of responses to NMDA recorded within +/- 15 mV of the reversal potential; over this limited range of membrane potential the current-voltage relationship remained linear in the presence of each of these ions. In contrast manganese produced a strong, voltage-dependent block of responses to NMDA, similar to that produced by magnesium, such that even close to the reversal potential the NMDA current-voltage relationship was highly non-linear. Thus manganese both permeates and blocks the NMDA receptor channel. 5. Raising the extracellular calcium concentration, from 0.1 to 5 mM, had two effects on the conductance mechanism activated by NMDA.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

Agonist- and voltage-gated calcium entry in cultured mouse spinal cord neurons under voltage clamp measured using arsenazo III.

Spinal cord neurons is dissociated cell culture were loaded with the calcium indicator arsenazo III using the whole-cell patch-clamp recording technique. Under voltage-clamp, depolarizing voltage steps evoked transient increases in absorbance at 660 nm, with no change at 570 nm, the isosbestic wavelength for calcium-arsenazo III complexes. The optical response occurred with a threshold depolarization to -30 mV, peaked at +10 mV, and decreased with further depolarization, consistent with an elevation of cytoplasmic free calcium resulting from Ca2+ flux through voltage-dependent calcium channels. Inward current responses to the excitatory amino acids N-methyl-D-aspartic acid (NMDA) and L-glutamate were also accompanied by calcium transients; these were dose-dependent, varied with the driving force for inward current, and were blocked by extracellular Mg2+ in a voltage-dependent manner, suggesting Ca2+ flux through NMDA-receptor channels. Responses to kainate, quisqualate, and GABA were not accompanied by comparable calcium transients. [Ca2+]i transients evoked by depolarizing voltage steps were of maximal amplitude at the start of recording and declined with time, reflecting rundown of voltage-dependent calcium channels. In contrast, [Ca2+]i transients evoked by NMDA gradually increased in amplitude during periods of whole-cell recording lasting 1-2 hr. Procedures resulting in loading of the neuron with Ca2+ accelerated the increase in amplitude of [Ca2+]i transients evoked by NMDA, but slowed the decay of [Ca2+]i transients evoked by voltage steps. Our results provide evidence for 2 independent sources of transmembrane Ca2+ flux in vertebrate neurons, through voltage-gated calcium channels and through NMDA-receptor channels. The Ca2+ flux gated by NMDA-receptor-specific agonists may play a role in synaptic plasticity, in regulating excitability, and in the excitotoxic response to excitatory amino acids.

Amino Acids↗
PubMed1986

Selective block of inward but not outward rectification in rat sensory neurones infected with herpes simplex virus.

Neurones from the dorsal root ganglia of neonatal rat pups were grown in tissue culture and voltage clamped using patch electrodes for whole cell recording. The electrodes were filled with a KMeSO4 solution containing 1.1 mM-EGTA. 15-24 h prior to recording cultures were infected with one of three strains of herpes simplex virus type 1 previously characterized on the basis of a cell fusion assay in cultures of the MRC-5 and BHK-21 cell lines. Infection of sensory neurones with either of the two non-syncitial strains of virus caused a nearly complete loss of the hyperpolarization-activated inward rectifying conductance normally present in these neurones, but did not appear to affect depolarization-activated, calcium-dependent outward currents or voltage-dependent delayed outward rectification. The loss of inward rectification appeared to result from inactivation of the underlying conductance mechanism, rather than from a shift in the gating properties of the conductance mechanism to unphysiological values. In contrast to these results inward rectification was present in sensory neurones infected with another, syncitial, strain of herpes simplex virus. However, quantitative analysis suggested a reduced availability compared to that recorded in uninfected neurones.

Action Potentials↗
PubMed1986

Changes in excitability induced by herpes simplex viruses in rat dorsal root ganglion neurons.

The physiological properties of rat sensory neurons infected with herpes simplex type 1 viruses and maintained in cell culture were studied using intracellular recording techniques. Two syncytial (cell fusing) and two nonsyncytial strains of virus were examined; individual strains of virus had different effects on neuronal excitability. The nonsyncytial viruses caused a loss of tetrodotoxin-sensitive low-threshold action potentials and blocked hyperpolarization-activated inward rectification, but did not alter the resting membrane potential, depolarization-activated outward rectification, or render the cells leaky. These effects develop progressively over the period 5-15 hr postinfection. One syncytial strain of virus induced spontaneous electrical activity that appeared to be the result of discrete electrical coupling between sensory neuron processes; as a result, action potential discharge is synchronized in coupled neurons. A second syncytial strain of virus rendered neurons inexcitable; however, in these experiments the input resistance fell to low values, possibly as a result of extensive coupling between sensory neurons. Viral replication in sensory neurons was demonstrable with indirect immunofluorescence using an antibody to herpes simplex viruses and correlated with the onset of virally induced changes in excitability. Virally triggered changes in excitability were blocked by the specific herpes virus antimetabolite acyclovir, suggesting that viral adsorption and penetration are by themselves insufficient to evoke changes in excitability. These results suggest that herpes viruses have selective effects on the excitable mechanisms in sensory neurons that are not simply the result of a general loss of membrane conductances or the disruption of transmembrane ion gradients.

Action Potentials↗
PubMed1986

High concentrations of N-acetylaspartylglutamate (NAAG) selectively activate NMDA receptors on mouse spinal cord neurons in cell culture.

We examined the membrane action of the endogenous dipeptide and putative neurotransmitter N-acetylaspartylglutamate (NAAG) on the excitatory amino acid receptors of cultured mouse spinal cord neurons using electrophysiological methods. Responses to NAAG (1 microM-5 mM) were compared to those elicited by N-methyl-D-aspartate (1 microM-1 mM) and L-glutamate (0.5-500 microM). Under voltage clamp, concentration-response curves of agonist-evoked currents demonstrated that NAAG was much less potent than either L-glutamate or N-methyl-D-aspartate (NMDA), so that inward currents could be evoked only at NAAG concentrations above 300 microM. Analysis of the dipeptide by high-pressure liquid chromatography showed no evidence of contamination by excitatory amino acids, suggesting that NAAG has an intrinsic, although weak, neuroexcitatory action on spinal neurons. Previous studies have shown that activation of NMDA receptors produces a voltage-dependent response. The current-voltage relationship of responses evoked by NAAG was also voltage-dependent. The peptide-activated conductance decreased with hyperpolarization in the presence of extracellular Mg2+, such that little inward current could be evoked at a membrane potential of -80 mV. In addition, responses to NAAG were completely antagonized by 250 microM DL-2-amino-5-phosphonovaleric acid, a specific NMDA-receptor antagonist. Application of NAAG in Mg2+-free medium resulted in an inward current with a large increase in membrane current noise. The spectral density function of this current noise could be fitted with a single Lorentzian with a decay time constant near 5 msec and a calculated single-channel conductance of 50-60 pS.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1985

Spontaneous electrical activity induced by herpes virus infection in rat sensory neuron cultures.

Dissociated cultures of rat dorsal root ganglion neurons were infected with a syncytial strain of herpes simplex virus type 1. Over 90% of neurons in infected cultures were spontaneously active and fired action potentials which, on membrane potential hyperpolarization, were replaced by depolarizing events similar to excitatory postsynaptic potentials. Amplitude analysis of these events produced populations described by the sum of several unitary events with Gaussian rather than binomial or Poisson distributions. Such spontaneous activity was blocked by tetrodotoxin but not by low calcium high magnesium solutions containing cadmium. Simultaneous recording from pairs of spontaneously active neurons revealed excitatory connexions between cells. It is suggested that virus-induced fusion of nerve cell processes induces electrical coupling between sensory neurons, and that the resulting electrical network supports spontaneous activity.

Animals↗
PubMed1985

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.

Action Potentials↗