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S Hestrin

Publications and source records attributed to S Hestrin.

At least 37 records · Page 2Linked to original sources
PubMed1990

Mechanisms generating the time course of dual component excitatory synaptic currents recorded in hippocampal slices.

We studied with the whole-cell recording techniques, the mechanisms underlying the time course of the slow N-methyl-D-aspartate (NMDA), and fast non-NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) in hippocampal slices. The rising phase of the NMDA receptor-mediated component of the EPSC as well as the decaying phase of the NMDA and non-NMDA component were highly temperature-sensitive, suggesting that neither of these processes is determined by free diffusion of transmitter. Moreover, glutamate uptake blockers enhanced the responses to exogenously applied glutamate, but had no effect on the decay of either the NMDA or non-NMDA components of the EPSCs. On the other hand, open channel blockers known to modify NMDA channel kinetics reduced the EPSC decay time. Thus, the present results support a model in which the rise time and decay of the NMDA component are determined primarily by slow channel kinetics and the decay of the non-NMDA component is due either to channel kinetics or to desensitization.

Animals↗
PubMed1990

Analysis of excitatory synaptic action in pyramidal cells using whole-cell recording from rat hippocampal slices.

1. The pharmacological and biophysical properties of excitatory synapses in the CA1 region of the hippocampus were studied using patch electrodes and whole-cell recording from thin slices. 2. Excitatory postsynaptic currents (EPSCs) had a fast component whose amplitude was voltage insensitive and a slow component whose amplitude was voltage dependent with a region of negative slope resistance in the range of -70 to -30 mV. 3. The voltage-dependent component was abolished by the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovalerate (APV; 50 microM), which had no effect on the fast component. Conversely, the fast voltage-insensitive component was abolished by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) which had no effect on the slow component. 4. In Ringer solution with no added Mg2+ the current-voltage relation of the NMDA component was linear over a much larger voltage range than in the presence of 1.3 mM-Mg2+. 5. The NMDA component of the EPSC could be switched off with a hyperpolarizing voltage step at the soma. The kinetics of this switch-off was used to estimate the speed of clamp control of the subsynaptic membrane as well as the electrotonic distance from the soma. The kinetic analysis of the EPSC was restricted to synapses which were judged to be under adequate voltage control. 6. For those synapses that were close to the soma the time constant for decay for the non-NMDA component, which was voltage insensitive, ranged from 4-8 ms. 7. The rise time for the NMDA component was 8-20 ms and the time constant for decay ranged from 60-150 ms. 8. During increased transmitter release with post-tetanic potentiation or application or phorbol esters, both components of the EPSC increased to a similar extent. 9. These experiments provide a detailed description of the dual receptor mechanism operating at hippocampal excitatory synapses. In addition, the experiments provide an electrophysiological method for estimating the electrotonic distance of synaptic inputs.

2-Amino-5-phosphonovalerate↗
PubMed1990

Properties of excitatory postsynaptic currents recorded in vitro from rat hippocampal interneurones.

1. We studied excitatory synaptic currents activated by stimulation of Schaffer collateral-commissural fibres and recorded from interneurones in the CA1 region of hippocampal slices using whole-cell techniques. 2. Interneurones were identified by their location outside the cell layer and their morphology as seen with differential interference contrast (DIC) microscopy and by filling with Lucifer Yellow (LY). 3. The excitatory postsynaptic current (EPSC) had a fast, voltage-insensitive component and a slow component which had a region of negative slope resistance between -70 and -40 mV. The slow voltage-dependent component was abolished by the N-methyl-D-aspartate (NMDA) receptor antagonist (DL-2-amino-5-phosphonovalerate (APV) 50 microM) which had little effect on the fast component. Conversely, the fast component was abolished by the non-NMDA receptor antagonist 6-cyano-7-nitoquinoxaline-2,3-dione (CNQX; 10 microM), which had no effect on the slow component. 4. The rise time of the fast component ranged from 1 to 3 ms and the decay time constant ranged from 3 to 15 ms. The rise time of the slow component ranged from 5 to 11 ms and the decay time constant ranged from 50 to 100 ms. 5. It is concluded that although the morphology of the excitatory synapses onto interneurones differs considerably from those onto pyramidal cells, their electrophysiological and pharmacological properties are very similar.

2-Amino-5-phosphonovalerate↗
PubMed1990

Activation kinetics of retinal cones and rods: response to intense flashes of light.

Cone photoreceptors are less sensitive to light and the duration of their photoresponse is shorter than that of rods. In salamander rods and cones, we identified 3 components in membrane currents activated by bright flashes of light: an early receptor current (ERC) resulting from charge displacement within visual pigments, a saturation photocurrent generated by the closure of the cGMP-sensitive channels, and a putative Na-Ca exchanger current. The time courses of both the ERC and the onset of the saturation photocurrent were similar in rods and cones. The putative Na-Ca exchanger current, on the other hand, is 4- to 8-fold faster in cones. The onset of the saturation photocurrent consisted of a delay followed by a fast relaxation with an exponential time course. In both photoreceptor types the delay and the time course of the fast relaxation are dependent on light intensity and reach a limiting value when about 1% of the photopigment is bleached. The limiting value of the delay, about 8 msec, and of the relaxation time constant, about 2 msec, are nearly identical in rods and cones. The near identity of these parameters implies that at least 2 kinetic steps in the activation response of rods and cones are quantitatively similar. These findings suggest that the functional differences between rods and cones may arise from disparities in the processes that restore the components of the phototransduction cascade to their dark level and not from differences in the activation processes.

Animals↗
PubMed1989

Tonic activation of NMDA receptors by ambient glutamate enhances excitability of neurons.

Voltage clamp recordings and noise analysis from pyramidal cells in hippocampal slices indicate that N-methyl-D-aspartate (NMDA) receptors are tonically active. On the basis of the known concentration of glutamate in the extracellular fluid, this tonic action is likely caused by the ambient glutamate level. NMDA receptors are voltage-sensitive, thus background activation of these receptors imparts a regenerative electrical property to pyramidal cells, which facilitates the coupling between dendritic excitatory synaptic input and somatic action potential discharge in these neurons.

2-Amino-5-phosphonovalerate↗
PubMed1987

Kinetics of activation of acetylcholine receptors in a mouse muscle cell line under a range of acetylcholine concentrations.

We studied, using the patch-clamp technique, the kinetics of single acetylcholine (ACh)-activated channels in a mouse muscle cell line. In the presence of high ACh concentrations we estimated the rate of channel isomerization into the open state (beta) from the dwell time between openings. Also, we obtained estimates for beta under low agonist concentrations by assuming a linear sequential model of channel activation and applying burst analysis. If the linear model is correct, then the two estimates of beta should agree since beta should be independent of ACh concentration. However, the estimates of beta obtained under low ACh concentrations were slower than those obtained independently under high ACh concentrations. The discrepancy in the estimates of beta suggests that the linear model is inadequate, but the discrepancy can be explained if open channels can close through two separate pathways. Two alternative kinetic models that can account for our data are discussed.

Acetylcholine↗
PubMed1987

Effects of cyclic GMP on the kinetics of the photocurrent in rods and in detached rod outer segments.

We investigated the effects of high concentrations of cytoplasmic cyclic GMP on the photocurrent kinetics and light sensitivity of the tiger salamander rod both in intact cells and in detached outer segments. Photoreceptors were internally perfused with cGMP by applying patch pipettes containing cGMP to the inner or outer segment. Large increases in the concentration of cGMP in the outer segment cytoplasm were achieved only when the patch pipette was applied directly to the outer segment. The dark-current amplitude increased with increasing cGMP concentrations up to approximately 1,400 pA. Internal perfusion with 5.0 mM cGMP introduced a delay of 1-3 s in the photocurrent. The magnitude of the delay was inversely proportional to the light intensity. In addition, the photocurrent time course was slowed down and the light sensitivity, measured 1 s after the flash, was decreased approximately 100-fold when compared with that of the intact cell. The observed effects of cGMP were compared with those predicted by a model that assumes that the initial photocurrent time course is determined by the kinetics of the light-activated phosphodiesterase (PDE) and the cGMP dependence of the light-sensitive channels. At high concentrations of cGMP, the experimental data were similar to those predicted by the model and based on the known biochemical properties of the light-activated PDE and cGMP-activated channels.

Ambystoma↗
PubMed1987

The properties and function of inward rectification in rod photoreceptors of the tiger salamander.

1. Rod photoreceptors were isolated from the retinae of tiger salamanders and voltage clamped using the whole-cell patch-clamp technique. 2. Hyperpolarizing the cell to potentials more negative than -50 mV evoked an inward current termed Ih. 3. Ih did not turn on immediately following a hyperpolarizing step but showed a marked delay. The activation time course of Ih could be described by the sum of two exponential components of opposite polarity. 4. The steady-state chord-conductance was half activated at -67 mV. 5. The reversal potential of Ih was close to -30 mV in normal standard salt solution. Increasing the external potassium concentration tenfold shifted the reversal potential by +17 mV. 6. The conductance-voltage relation and the kinetic parameters were not affected by changes in the external potassium concentration. 7. When fully activated, the zero-current conductance underlying Ih depended on the square root of the concentration of external potassium. 8. The permeability ratio PNa/PK depended on the external potassium concentration. It was 0.2 at an external potassium concentration of 2.0 mM and 0.3 at an external potassium concentration of 10.0 mM. The interaction of potassium with Ih suggests that Ih is a multi-ion pore. 9. It is concluded that Ih differs from the inward rectifier that is found in egg cells, frog muscle and heart muscle. 10. The kinetics and voltage sensitivity of Ih suggest that it does not play a role in the dark resting state or in the response to dim flashes of light. Its properties indicate that it may have a major role in the response to bright flashes.

Action Potentials↗
PubMed1987

Voltage-activated potassium channels in the plasma membrane of rod outer segments: a possible effect of enzymatic cell dissociation.

Using patch-clamp techniques, we recorded single-channel currents from the plasma membrane of the outer segment of isolated light-adapted rods. The channels are potassium-selective and their conductance is about 87 pS. The channels are activated by depolarization and are not sensitive to cytoplasmic calcium, they are exclusively found in rods isolated with the proteolytic enzyme papain, and are not detected in rods isolated by mechanical means. Thus, these channels do not exist in an activatable form in the outer segment plasma membrane under physiological conditions. The channels might be derived from a normally inaccessible structure, such as the disk membrane, or, alternatively, they might be a modified form of a channel that is not active in the intact rod.

Ambystoma↗
PubMed1985

The effects of a myasthenic serum on the acetylcholine receptors of C2 myotubes. II. Functional inactivation of the receptor.

We have investigated the effect of antibodies from a myasthenic serum on the physiological properties of acetylcholine receptors (AChRs) in myotubes of a mouse muscle cell line, C2. The antibodies in this serum blocked the binding of 125I-alpha-bungarotoxin to the myotubes to an extent of about 50%. The antibodies also inhibited the increase in 22Na influx caused by carbamylcholine (CARB). At a concentration of antibody that blocked about 50% of toxin binding, greater than 80% of the AChR-mediated 22Na influx was blocked. The apparent KD for CARB, estimated from the dose-response curve for 22Na influx, was unaffected. The effect of the antibodies was further examined by patch-clamp recording. In greater than 30% of the patches from antibody-treated cells, no channel activity in response to acetylcholine was seen; in contrast, every patch from control cells showed activity. The channels that were seen after antibody treatment were indistinguishable from those seen in normal cells, both in their single-channel conductance and in the kinetic constants used to describe channel opening and closing. We conclude that the antibodies in this serum inhibit the functional response of AChRs in C2 myotubes to acetylcholine and do so by inactivating individual receptors.

Animals↗
PubMed1983

Activation of acetylcholine receptors causes the partition of hydrophobic cations into postsynaptic membrane vesicles.

In the continued presence of cholinergic ligands, the acetylcholine receptor-channel complex (AChR) in postsynaptic membranes undergoes a sequence of conformational changes. On addition of the ligand, the receptor rapidly changes from a closed channel to an open channel conformation, then slowly changes to a nonconducting state termed desensitization. The lifetime of the open channel conformation and the rate of desensitization are both dependent on the magnitude of the membrane potential, suggesting that the ligand-induced conformational changes in AChR may involve the movement of electrical charges within the membrane. Measurements of charge redistribution in AChR-containing membranes following ligand binding have not been reported. Recently, measurements of changes in the membrane partition coefficient of hydrophobic ions have been used to detect electrostatic changes in both biological and model membranes. We report here that cholinergic ligands induce changes in the partition coefficient of the hydrophobic cation tetraphenylphosphonium (TPP) into AChR-enriched membranes. The extent and time course of these changes in TPP partition coefficient are accounted for in a kinetic model. We conclude that TPP movement is a monitor of a molecular event which may be associated with the slow component of AChR desensitization.

Animals↗
PubMed1981

The interaction of potassium with the activation of anomalous rectification in frog muscle membrane.

1. Inward rectification of frog muscle membrane was analysed with the Vaseline gap method. 2. Hyperpolarization, under voltage clamp, produced inward potassium currents, which had a component that activated with a time constant, tau K. 3. The activation time constant tau K of the inward potassium currents was voltage dependent. For a given external potassium concentration, the time constant was maximal for potentials near the potassium equilibrium potential, EK. 4. The potassium chord conductance gK, had a sigmoidal voltage dependency, increasing initially e-fold per 11.6 mV of hyperpolarization. 5. When the internal potassium concentration was fixed, raising external potassium induced a shift of the tau K-V and the gK-V relations in the positive direction along the voltage axis. That shift was comparable to the change in EK. 6. No shift of the tau K-V and the gK-V relations was observed when the internal potassium was reduced from 150 to 50 mM. 7. Changes of internal sodium concentration between 5 and 100 mM did not significantly effect the magnitude of inward rectification.

Animals↗