Low Concentrations of the Organophosphate VX Affect Spontaneous and Evoked Transmitter Release from Hippocampal Neurons: Toxicological Relevance of Cholinesterase-Independent Actions

The N-methyl-d-aspartate (NMDA) receptor has been implicated in the formation of synaptic connections. To investigate the role of the ε2 (NR2B) NMDA receptor subunit, which is prominently expressed during early development, we used neurons from mice lacking this subunit. Although ε2−/− mice die soon after birth, we examined whether NMDA receptor targeting to the postsynaptic membrane was dependent on the ε2 subunit by rescuing hippocampal neurons from these mice and studying them in autaptic cultures. In voltage-clamp recordings, excitatory postsynaptic currents (EPSCs) from ε2−/− neurons expressed an NMDA receptor–mediated EPSC that was apparent as soon as synaptic activity developed. However, compared with wild-type neurons, NMDA receptor–mediated EPSC deactivation kinetics were much faster and were less sensitive to glycine, but were blocked by Mg2+ or AP5. Whole cell currents from ε2−/− neurons were also more sensitive to block by low concentrations of Zn2+ and much less sensitive to the ε2-specific antagonist ifenprodil than wild-type currents. The rapid NMDA receptor–mediated EPSC deactivation kinetics and the pharmacological profile from ε2−/−neurons are consistent with the expression of ζ1/ε1 diheteromeric receptors in excitatory hippocampal neurons from mice lacking the ε2 subunit. Thus ε1 can substitute for the ε2 subunit at synapses and ε2 is not required for targeting of NMDA receptors to the postsynaptic membrane.

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Clumping and oscillations in evoked transmitter release at the frog neuromuscular junction.

The Journal of Physiology ◽

10.1113/jphysiol.1978.sp012321 ◽

1978 ◽

Vol 278 (1) ◽

pp. 513-523 ◽

Cited By ~ 26

Author(s):

H Meiri ◽

R Rahamimoff

Keyword(s):

Neuromuscular Junction ◽

Transmitter Release ◽

Frog Neuromuscular Junction ◽

Evoked Transmitter Release

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Dynamics of Excitatory Transmitter Release: Analysis of Synaptic Responses in CA3 Hippocampal Neurons After Repetitive Stimulation of Afferent Fibers

Journal of Neurophysiology ◽

10.1152/jn.1998.79.4.1977 ◽

1998 ◽

Vol 79 (4) ◽

pp. 1977-1988 ◽

Cited By ~ 15

Author(s):

Marco Canepari ◽

Enrico Cherubini

Keyword(s):

Transmitter Release ◽

Hippocampal Neurons ◽

Repetitive Stimulation ◽

Patch Clamp Technique ◽

Afferent Fibers ◽

Presynaptic Mechanisms ◽

Release Analysis ◽

Excitatory Transmitter ◽

Stimulation Of ◽

Synaptic Responses

Canepari, Marco and Enrico Cherubini. Dynamics of excitatory transmitter release: analysis of synaptic responses in CA3 hippocampal neurons after repetitive stimulation of afferent fibers. J. Neurophysiol. 79: 1977–1988, 1998. The patch-clamp technique (whole cell configuration) was used to record excitatory postsynaptic currents (EPSCs) evoked by repetitive stimulation (4 pulses at 50-ms intervals) of afferent fibers in the stratum lucidum-radiatum. Different synaptic behaviors (EPSC patterns) were classified in terms of facilitation or depression of the mean amplitude of the second, third, and fourth EPSC with respect to the previous one. A large variety of EPSC patterns was observed by stimulating different afferent fibers. Experiments with the mGluR2/mGluR3 agonist 2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) (1 μM), a compound that reduces release at mossy but not at associative commissural fibers and therefore allows to identify the origin of synaptic responses, showed that particular EPSC patterns could not be associated to the activation of a specific type of synaptic input. To investigate the role of the probability of release in the dynamics of synaptic activity, the extracellular calcium concentration was varied from 0.8 to 4 mM in several experiments. EPSC patterns dominated by depression, characteristics of high release probability conditions, could be observed in the majority of the cases in the presence of higher calcium concentrations. A quantitative model for dynamics of transmitter release has been developed. Experimental results were compared with data computed with the model taking into account the probability of release and the time course of reavailability. This work indicates that short-term changes of presynaptic conditions occurring during a train of action potentials can account for the high variability of EPSC responses. The model that is proposed also suggests a general method of experimental data analysis to investigate the possible presynaptic mechanisms underlying long-lasting changes in synaptic efficacy.

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Calcium From Internal Stores Triggers GABA Release From Retinal Amacrine Cells

Journal of Neurophysiology ◽

10.1152/jn.00604.2005 ◽

2005 ◽

Vol 94 (6) ◽

pp. 4196-4208 ◽

Cited By ~ 24

Author(s):

Ajithkumar Warrier ◽

Salvador Borges ◽

David Dalcino ◽

Cameron Walters ◽

Martin Wilson

Keyword(s):

Transmitter Release ◽

Metabotropic Glutamate Receptor ◽

Amacrine Cell ◽

Ryanodine Receptors ◽

Amacrine Cells ◽

Gaba Release ◽

Voltage Gated ◽

Metabotropic Glutamate ◽

Inositol 1,4,5 Trisphosphate ◽

Evoked Transmitter Release

The Ca2+ that promotes transmitter release is generally thought to enter presynaptic terminals through voltage-gated Ca2+channels. Using electrophysiology and Ca2+ imaging, we show that, in amacrine cell dendrites, at least some of the Ca2+ that triggers transmitter release comes from endoplasmic reticulum Ca2+ stores. We show that both inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs) are present in these dendrites and both participate in the elevation of cytoplasmic [Ca2+] during the brief depolarization of a dendrite. Only the Ca2+ released through IP3Rs, however, seems to promote the release of transmitter. Antagonists for the IP3R reduced transmitter release, whereas RyR blockers had no effect. Application of an agonist for metabotropic glutamate receptor, known to liberate Ca2+ from internal stores, enhanced both spontaneous and evoked transmitter release.

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The Neuromuscular Junction Revisited: Ca2+ Channels and Transmitter Release in Cholinergic Neurones in Xenopus Nerve and Muscle Cell Culture

Journal of Experimental Biology ◽

10.1242/jeb.153.1.129 ◽

1990 ◽

Vol 153 (1) ◽

pp. 129-140 ◽

Cited By ~ 1

Author(s):

T. P. FENG ◽

ZHENG-SHAN DAI

Keyword(s):

Neuromuscular Junction ◽

Transmitter Release ◽

Single Channel ◽

Calcium Currents ◽

Patch Clamps ◽

Whole Cell ◽

Voltage Gated ◽

Cholinergic Neurones ◽

Ca2 Channels ◽

Evoked Transmitter Release

Although the entry of calcium ions into the presynaptic nerve terminals through voltage-gated Ca2+ channels is now universally recognized as playing an essential role in evoked transmitter release at the neuromuscular junction (NMJ), and indeed in chemical synapses generally, we have as yet very little direct knowledge of the Ca2+ channels of the presynaptic terminals. In this work, making use of cocultured nerve and muscle cells from Xenopus embryos, we studied the NMJ formed between the soma of identified cholinergic neurones and myoball, which allowed the use of patch-clamps on both the pre- and postsynaptic components. Both whole-cell and single-channel recordings of Ca2+ channels in the presynaptic cell were made. We found only one type of voltage-gated Ca2+ channel with highvoltage activation and slow inactivation characteristics, allowing its classification either as the L or the N type. The channels were susceptible to block by metenkephalin but not to block by nifedipine or to enhancement by Bay K 8644. This combination of pharmacological properties favours their classification as the N type. Preliminary observations on the correlation between calcium currents and transmitter release disclosed a strikingly rapid run-down of the evoked release with unchanged calcium currents and spontaneous release during whole-cell recording, indicating a specific wash-out effect on some link between calcium entry and evoked transmitter release.

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Gadolinium Reduces AMPA Receptor Desensitization and Deactivation in Hippocampal Neurons

Journal of Neurophysiology ◽

10.1152/jn.2001.86.1.173 ◽

2001 ◽

Vol 86 (1) ◽

pp. 173-182 ◽

Cited By ~ 3

Author(s):

Saobo Lei ◽

John F. MacDonald

Keyword(s):

Steady State ◽

Ampa Receptor ◽

Hippocampal Neurons ◽

Time Course ◽

Single Channel ◽

Trivalent Cation ◽

Receptor Desensitization ◽

Whole Cell ◽

Low Concentrations ◽

Cultured Hippocampal Neurons

The actions of the trivalent cation Gd3+ on whole cell AMPA receptor-mediated currents were studied in isolated hippocampal neurons, in nucleated or outside-out patches taken from cultured hippocampal neurons, and on miniature excitatory postsynaptic currents (mEPSCs) recorded in cultured hippocampal neurons. Glutamate, AMPA, or kainate was employed to activate AMPA receptors. Applications of relatively low concentrations of Gd3+ (0.1–10 μM) substantially enhanced steady-state whole cell glutamate and kainate-evoked currents without altering peak currents, suggesting that desensitization was reduced. However, higher concentrations (>30 μM) depressed steady-state currents, indicating an underlying inhibition of channel activity. Lower concentrations of Gd3+also increased the potency of peak glutamate-evoked currents without altering that of steady-state currents. An ultrafast perfusion system and nucleated patches were then used to better resolve peak glutamate-evoked currents. Low concentrations of Gd3+ reduced peak currents, enhanced steady-state currents, and slowed the onset of desensitization, providing further evidence that this cation reduces desensitization. In the presence of cyclothiazide, a compound that blocks desensitization, a low concentration Gd3+ inhibited both peak and steady-state currents, indicating that Gd3+ both reduces desensitization and inhibits these currents. Gd3+ reduced the probability of channel opening at the peak of the currents but did not alter the single channel conductance calculated using nonstationary variance analysis. Recovery from desensitization was enhanced, and glutamate-evoked current activation and deactivation were slowed by Gd3+. The Gd3+-induced reduction in desensitization did not require the presence of the GluR2 subunit as this effect was seen in hippocampal neurons from GluR2 null-mutant mice. Gd3+ reduced the time course of decay of mEPSCs perhaps as a consequence of its slowing of AMPA receptor deactivation although an increase in the frequency of mEPSCs also suggested enhanced presynaptic release of transmitter. These results demonstrate that Gd3+ potently reduces AMPA receptor desensitization and mimics a number of the properties of the positive modulators of AMPA receptor desensitization such as cyclothiazide.

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