Kinetic theory of plasma sheaths surrounding electron-emitting surfaces.

A one-dimensional kinetic theory of sheaths surrounding planar, electron-emitting surfaces is presented which accounts for plasma electrons lost to the surface and the temperature of the emitted electrons. It is shown that ratio of plasma electron temperature to emitted electron temperature significantly affects the sheath potential when the plasma electron temperature is within an order of magnitude of the emitted electron temperature. The sheath potential goes to zero as the plasma electron temperature equals the emitted electron temperature, which can occur in the afterglow of an rf plasma and some low-temperature plasma sources. These results were validated by particle in cell simulations. The theory was tested by making measurements of the sheath surrounding a thermionically emitting cathode in the afterglow of an rf plasma. The measured sheath potential shrunk to zero as the plasma electron temperature cooled to the emitted electron temperature, as predicted by the theory.

Experimental studies of the Bohm criterion in a two-ion-species plasma using laser-induced fluorescence.

The Bohm criterion is studied experimentally in the case of a two ion species plasma. Measurements are carried out in Ar and Ar+He plasmas (PA(r I) approximately 0.1 mtorr, 0< or =P(He)/P(Ar)< or =25, and 0< or =n(+)(He)/n(e)< or =0.5, T(e)< or =2 eV) created in an unmagnetized dc hot filament discharge confined by surface multidipole magnetic fields. Laser-induced fluorescence (LIF) measurements of Ar II ion velocity distribution functions (ivdfs) within the presheath up to the sheath edge show that the ions reach the sheath edge traveling faster than their individual Bohm speed by more than 75%, approaching a speed equal to the ion sound speed of the system.

First experimental measurements of the plasma potential throughout the presheath and sheath at a boundary in a weakly collisional plasma.

Experimental data obtained with emissive probes and Langmuir probes show that the plasma potential profile in the presheath scales as -ephi /T(e)= sqrt[(x(0) -x)/lambda ], consistent with ion flux conservation, and that the sheath consists of a transition region and an electron-free collisionless sheath with thicknesses scaling as lambda( 1/5)lambda (4/5 )(D) and lambda(D )(ephi/ T(e))(3 /4), respectively, where lambda is the ion-neutral collision length. Results support Rieman's presheath and transitional region model [Phys. Plasmas 4, 4158 (1997)]]. The potential drop over the presheath and transition sheath region were the order of T(e) /e and 2T(e )/3e, respectively, increasing with increasing pressure.

Nitric oxide modulates synaptic glutamate release during anoxia.

The ability of nitric oxide to enhance vesicular glutamate release during anoxia was examined in the present study. Whole-cell patch clamp recordings were obtained from CA1 pyramidal neurons in rat hippocampal slices perfused in media containing tetrodotoxin. These cells exhibit spontaneous inward currents previously identified as glutamatergic miniature excitatory postsynaptic currents (mEPSCs). The frequency of these mEPSCs increases during exposure to anoxia. The anoxia-induced increase in frequency is reduced when experiments are performed in the presence of the competitive nitric oxide (NO)-synthase inhibitors N(G)-nitro-L-arginine methyl ester and N(G)-nitro-L-arginine, as well as reduced hemoglobin. Arginine reversed the suppression by the NO-synthase inhibitors. The N-methyl-D-aspartate (NMDA) receptor antagonists 3-(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid and MK-801 also suppressed the anoxia-induced increase in mEPSC frequency. These data indicate that NMDA receptor-activated NO production may enhance vesicular synaptic glutamate release, which would in turn contribute to excitotoxicity during hypometabolic states.

Adenosine A1 antagonism increases specific synaptic forms of glutamate release during anoxia, revealing a unique source of excitation.

The role of the adenosine A1 receptor in the modulation of anoxia-induced synaptic glutamate release was examined in CA1 pyramidal neurons by whole-cell voltage-clamp recording in the rat hippocampal slice preparation. Anoxia leads to an increased action potential-independent synaptic glutamate release in the form of a higher frequency of miniature excitatory postsynaptic currents (mEPSCs). This increase is not significantly affected when slices are preincubated in the adenosine A1 receptor antagonist, 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX). A second population of spontaneous inward currents, however, occurs in DPCPX-treated slices during a well-defined period following the onset of anoxia. Their suppression by glutamate antagonists, tetrodotoxin, or by the cutting of the Schaffer collateral pathway indicates that they represent action potential-dependent, glutamatergic excitatory postsynaptic currents (ap-EPSCs) originating from CA3 pyramidal neurons. CA3 neurons were examined in current-clamp whole-cell patch mode to determine the origin of this increased orthodromic excitation. After the onset of anoxia, CA3 cells initially exhibit a small depolarization or hyperpolarization associated with a decrease in input resistance. This is followed by transient depolarization (the depolarizing "nub"), which is associated with an increase in input resistance. The nub evoked single as well as bursts of action potentials in CA3 neurons. The occurrence of these CA3 nub-elicited action potentials coincides with that of ap-EPSCs recorded in the CA1 cells. Recording with cesium- rather than standard potassium-containing electrodes results in the suppression of the nub and its associated increase in input resistance. In conclusion we have shown that adenosine tone plays an important role in suppressing anoxia-induced spontaneous ap-EPSCs but not action potential-independent mEPSCs in CA1 neurons. These EPSCs originate from a depolarization in CA3 pyramidal neurons, which is associated with an increase in resistance. This previously undescribed phenomenon likely results from a decrease in the conductance of an unidentified potassium channel.

Mechanism of early anoxia-induced suppression of the GABAA-mediated inhibitory postsynaptic current.

1. We investigated the mechanism of hypoxia-induced depression of gamma-aminobutyric acid-A (GABAA)-mediated inhibitory postsynaptic currents (IPSCs) in CA1 neurons of hippocampal slices from 21- to 28-day-old rats. Cells were examined by whole-cell patch-clamp recording and hypoxia was induced by switching perfusion of the slice from oxygenated artificial cerebral spinal fluid (ACSF) to ACSF saturated with 95% N2-5% CO2. 2. Synaptic responses evoked by stimulation of the Schaffer collateral-commissural projection at a fixed holding potential (VH = -60 mV) during anoxia revealed that the IPSC appeared more sensitive than the excitatory postsynaptic current to anoxia-induced depression. All subsequent studies examined the GABAA-mediated IPSC synaptic responses in isolation by direct stimulation of GABA interneurons in the stratum radiatum in the presence of extracellular 3-(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (CPP) (20 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (50 microM) to block glutamatergic currents and intracellular QX-314 (lidocaine N-ethyl bromide, 1 mM) to block GABAB-mediated currents. When studied in this manner (VH = -60 mV) the GABAA-mediated IPSC appeared to change from an outward to inward current after exposure to anoxia. 3. The current-voltage relationship of GABAA-mediated IPSCs revealed that these changes resulted from a positive shift in the IPSC reversal potential without a significant change in the conductance. Thus under patch clamp apparent IPSC inhibition may result from a decrease in the extracellular concentration of chloride ions. Similar findings were observed with micropipettes that contained high intracellular chloride concentrations. 4. Miniature spontaneous IPSCs were examined in the presence of tetrodotoxin (1 microM) with micropipettes containing high intracellular chloride concentrations. The miniature IPSCs (mIPSCs) appeared as spontaneous transient inward currents. Consistent with an anoxia-induced decrease in extracellular chloride, the mean amplitude of the mIPSCs increased after the onset of anoxia. A significant decrease in rise and decay time was also noted during anoxia. The frequency of the mIPSCs also increased by approximately 300%. 5. The resting input resistance of the cells was examined by measuring the current resulting from a 20-mV hyperpolarizing pulse. A significant reduction in resistance was observed 2 min after the onset of anoxia. This still occurred, although to a lesser degree, in the presence of glutamatergic blockers (20 microM CPP plus 50 microM CNQX). In the presence of both GABAergic (picrotoxin, 100 microM) and glutamatergic blockers no significant reduction in resting input resistance was apparent after 2 min of anoxia.(ABSTRACT TRUNCATED AT 400 WORDS)

Arachidonic acid participates in the anoxia-induced increase in mEPSC frequency in CA1 neurons of the rat hippocampus.

Patch clamp in the whole cell configuration was used to examine the effects of a variety of agents that influence arachidonic acid metabolism on vesicular glutamate release in CA1 neurons of rat hippocampal slices. As previously demonstrated, anoxia induced a significant increase in the frequency of spontaneous glutamate-mediated miniature excitatory postsynaptic currents (mEPSCs) during the first 5 min following anoxia. This increase in frequency was almost completely abolished if slices were preincubated in artificial cerebral spinal fluid (ACSF) containing the phospholipase C/A2 inhibitor, bromophenacyl-bromide (BPB; 20 microM) or the cyclooxygenase inhibitors, indomethacin (20 microM) and piroxicam (10 microM). This observation may be important to our understanding of the neuroprotective action of these agents. These data suggest that arachidonic acid (AA) and its cyclooxygenase products or by-products (oxygen free radicals) contribute to vesicular glutamate release during the early phase of anoxia.

Adenosine antagonists prevent hypoxia-induced depression of excitatory but not inhibitory synaptic currents.

Hypoxia induces depression of excitatory postsynaptic currents (EPSCs) and inhibitory postsynaptic currents (IPSCs) in CA1 neurons of the hippocampus. The effect of antagonists that act at the A1 adenosine receptor on hypoxia-induced depression of EPSCs and IPSCs were examined in hippocampal slices with the patch clamp technique (whole-cell configuration). The A1 receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) (200 nM) and 8-phenyltheophilline (8-PT) (10 microM) significantly prevented depression of EPSCs by hypoxia but failed to protect IPSCs. This result suggests that the hypoxia-induced depression of the EPSC involves the activation of adenosine receptors (possibly of the A1 subtype), whereas depression of the IPSC results from a different mechanism.

Early anoxia-induced vesicular glutamate release results from mobilization of calcium from intracellular stores.

1. The cause of the increased frequency of glutamatergic miniature excitatory postsynaptic currents (mEPSCs) resulting from anoxia was investigated in CA1 neurons of the in vitro rat hippocampal slice. These neurons were examined by whole-cell patch-clamp recording, and hypoxia was induced by switching the perfusion of the slice from oxygenated artificial cerebral spinal fluid (ACSF) to ACSF saturated with 95% N2-5% O2. Except where noted, experiments were carried out in ACSF containing 1 microM tetrodotoxin (TTX). 2. Although anoxia resulted in a significant increase in the frequency of mEPSCs, the amplitude, rise time, and half-decay time of the mEPSCs were unchanged. This increase in frequency indicates that there is a change in presynaptic neurotransmitter release mechanisms, probably an increase in calcium concentration, soon after the onset of anoxia. The unchanged kinetics and amplitude of the mEPSCs indicate that anoxic-induced synaptic changes are not a result of changes in the postsynaptic glutamate receptor. 3. When hippocampal slices were exposed to anoxic conditions in ACSF with calcium excluded, an increase in mEPSC frequency equal to that in normal ACSF was observed. When 0.2 mM of CdCl2 was added to the zero-calcium ACSF, anoxia still resulted in increases in mEPSC frequency equal to those of normal ACSF. It is therefore concluded that the anoxia-induced increase in mEPSC frequency does not result from an increase in a transmembrane calcium influx. The zero-calcium plus 0.2 mM CdCl2 ACSF solution completely abolished orthodromically elicited synaptic potential (in the absence of TTX), indicating that calcium currents that mediate normal orthodromic transmitter release were completely abolished in the latter experiments.(ABSTRACT TRUNCATED AT 250 WORDS)

Site of synaptic depression during hypoxia: a patch-clamp analysis.

1. The effect of hypoxia on synaptic physiology was investigated in hippocampal slices from 16- to 23-day-old rats. CA1 pyramidal cells were examined by whole cell patch-clamp recording, and hypoxia was induced by switching perfusion of the slice from oxygenated artificial cerebrospinal fluid (ACSF) to ACSF saturated with 95% N2-5%CO2. Synaptic responses were assessed by stimulating the Schaffer collateral-commissural projection with an electrode in the stratum radiatum every 20 s. 2. Within 100-200 s of the onset of hypoxia, the orthrodromically elicited synaptic response of the CA1 cells was largely inhibited. In addition, a slow inward current was observed after the onset of hypoxia. A transient outward current, preceding the inward current, was observed in only 2 of 17 cells examined. The slow inward current culminated in an irreversible rapid inward current at approximately 140 s after hypoxia. This rapid inward current occurred simultaneously with spreading depression as measured by field potentials. Tetrodotoxin (TTX) had no effect on the onset of this current, whereas kynurenic acid significantly delayed its occurrence. 3. Before the onset of hypoxia, spontaneous transient inward currents were apparent. The frequency of these events increased by three- to fourfold after hypoxia. The transient inward currents persisted in slices incubated in TTX, but were almost completely inhibited in slices incubated with the mixed N-methyl-D-aspartate (NMDA)/non-NMDA antagonist kynurenic acid. This identified the spontaneous events that were increased in frequency by hypoxia as glutamatergic miniature excitatory postsynaptic currents (mEPSCs). 4. The mean amplitude of the mEPSCs was not affected by hypoxia at a time at which the orthodromically elicited synaptic response was almost completely inhibited by hypoxia. In addition, the response of the postsynaptic cell to pressure ejection of glutamate was not inhibited under conditions of nearly complete blocked the synaptic response. Thus, by two measures, the postsynaptic response was not affected by hypoxia, indicating that the site of hypoxia-induced synaptic failure was at the presynaptic terminal. 5. The orthodromically elicited synaptic response consisted of an EPSC followed closely by an inhibitory postsynaptic current (IPSC). The IPSC portion of the elicited postsynaptic response was more sensitive to inhibition by hypoxia than was the EPSC. In some cells the EPSC exhibited a monophasic decline in amplitude during hypoxia. However, in a majority of cells, an initial decline in the amplitude of the EPSC was followed by a transient increase and subsequent depression.(ABSTRACT TRUNCATED AT 400 WORDS)

Cognitive dysfunction in a patient with long-term occupational exposure to ethylene oxide. Role of ethylene oxide as a causal factor.

This case illustrates a comprehensive approach to assessing causality in a woman with apparent cognitive dysfunction, as measured by neuropsychological testing, and a 10-year history of occupational exposure to ethylene oxide. The analysis included a multidisciplinary examination of the patient, which took place several years after the termination of her exposure. In addition, all of the patient's prior medical and psychiatric records were reviewed, as were the records of her employer to ascertain her exposure history. Our evaluation revealed a pattern of neuropsychological findings not consistent with nervous system damage secondary to an organic effect of ethylene oxide. A more likely causal hypothesis is adopted: the patient's apparent cognitive dysfunction had a psychiatric etiology. This case also illustrates the potential impact of a patient's involvement in legal proceedings related to claims of neurocognitive dysfunction.

Tetrahydroaminoacridine block of N-methyl-D-aspartate-activated cation channels in cultured hippocampal neurons.

The action of tetrahydroaminoacridine (THA), a centrally active cholinesterase inhibitor that may provide symptomatic benefit in Alzheimer's disease, was studied on responses to the excitatory amino acid N-methyl-D-aspartate (NMDA) in cultured hippocampal neurons, using whole-cell voltage-clamp and single-channel recording techniques. THA produced a concentration-dependent block of NMDA-evoked inward current responses (IC50, 190 microM at -60 mV), without affecting responses to quisqualate or kainate. THA block of NMDA responses was voltage dependent and was nearly completely relieved at positive holding potentials. Analysis of the voltage dependency indicated that the THA binding site senses 56% of the transmembrane electrostatic field. In single-channel recordings from outside-out membrane patches, THA appeared to reduce the frequency and duration of NMDA-evoked single-channel currents, without affecting the single-channel amplitude. The effects of THA on NMDA responses occur at concentrations 1-2 orders of magnitude greater than the therapeutic serum concentrations and, therefore, blockade of NMDA receptor-mediated responses is unlikely to contribute to the putative therapeutic action of THA. However, because NMDA receptors may play a critical role in cognitive and memory function, THA has the potential to produce undesirable central nervous system side effects at high doses.

Cycloleucine blocks NMDA responses in cultured hippocampal neurones under voltage clamp: antagonism at the strychnine-insensitive glycine receptor.

1. Radioligand binding studies have demonstrated that the neutral amino acid cycloleucine may act as a competitive antagonist at the glycine modulatory site on the N-methyl-D-aspartate (NMDA) receptor complex. In the present study, we examined the effects of cycloleucine on NMDA-evoked inward current responses in dissociated hippocampal neuronal cultures using the whole cell voltage-clamp technique. 2. In the presence of 1 microM glycine, cycloleucine caused a reversible, dose-dependent inhibition of NMDA responses with an IC50 of 24 microM. An increase in glycine to 100 microM resulted in a shift to the right of the cycloleucine concentration-effect curve (IC50, 1.4 mM). However, with cycloleucine concentrations less than or equal to 100 microM, a fraction of the block could not be overcome by glycine even at concentrations as high as 1 mM. 3. The cycloleucine block was unaffected by shifts in the holding potential (-60 to +60 mV), and there was no effect of cycloleucine on the reversal potential of the NMDA-evoked current. 4. Cycloleucine failed to effect kainic acid- and quisqualic acid-evoked currents at concentrations which inhibited NMDA responses. 5. We conclude that cycloleucine is a potent and selective antagonist of NMDA-receptor mediated responses. Although this effect occurs in part via competitive antagonism at the glycine modulatory site, the cycloleucine block cannot be completely reversed by glycine indicating an interaction with an additional site on the receptor-channel complex.