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S Marom

Publications and source records attributed to S Marom.

16 recordsLinked to original sources
PubMed2001

Learning in networks of cortical neurons.

The results presented here demonstrate selective learning in a network of real cortical neurons. We focally stimulate the network at a low frequency (0.3-1 Hz) until a desired predefined response is observed 50 +/- 10 msec after a stimulus, at which point the stimulus is stopped for 5 min. Repeated cycles of this procedure ultimately lead to the desired response being directly elicited by the stimulus. By plotting the number of stimuli required to achieve the target response in each cycle, we are able to generate learning curves. Presumably, the repetitive stimulation is driving changes in the circuit, and we are selecting for changes consistent with the predefined desired response. To the best of our knowledge, this is the first time learning of arbitrarily chosen tasks, in networks composed of real cortical neurons, is demonstrated outside of the body.

Action Potentials↗
PubMed2001

Frequency tuning of input-output relation in a rat cortical neuron in-vitro.

The input-output relation of a single neuron stands at the basis of every biologically oriented description of the brain. This report shows that the input-output relation of cultured cortical neurons is non-linearly tuned by the input frequency. Increasing the rate of stimulation results in the appearance of ordered temporal firing patterns, which are qualitatively different for different input frequencies. The experimental results of this study lead to the conclusion that frequency tuning of neuronal input-output relation arises from activity-dependent rates at the molecular level underlying the mechanism of excitability itself.

Action Potentials↗
PubMed2001

Social phobia subtyping with the MMPI-2.

The present article examines the utility of the MMPI-2 for the subtyping of social phobia (SP). Cluster analysis was conducted on the MMPI-2 profiles of 109 patients with SP. Clusters were compared on demographic and clinical variables prior to treatment, as well as following completion of cognitive-behavioral group therapy (CBGT). Three distinct clusters emerged. The first is characterized by an absence of significant scale elevations and appears to be consistent with the reported "circumscribed" subtype of SP. It is associated with a significantly later age at onset of SP, a higher proportion of married individuals, and lower scores on pretreatment clinical variables. Significant elevations on Scales 2 (Depression) and 7 (Psychasthenia) and moderately high scores on pretreatment clinical variables characterize the second cluster. The third cluster is characterized by significantly high elevations on Scales 8, 7, and 2, and the highest scores on pretreatment clinical variables. Patients from all three groups improved significantly following a course of CBGT.

Adult↗
PubMed1999

A global defect in scaling relationship between electrical activity and availability of muscle sodium channels in hyperkalemic periodic paralysis.

Hyperkalemic periodic paralysis (HyperPP) is a hereditary disorder characterized by alternate episodic attacks of muscle weakness and muscle myotonia. The most common mutation associated with HyperPP is a T704M substitution in the skeletal-muscle sodium channel. This mutation increases sodium persistent currents, alters voltage dependence of activation and impairs slow inactivation. The present study shows experimental evidence in support of a potentially important global defect caused by the T704M mutation. While the effective rate of recovery from slow inactivation, in both normal and mutated channels, is related to the duration of past activity by a power law function, the scaling power of the mutated channel is significantly greater. This difference between the channels offers a clue for an explanation to the wide range of time scales, history dependence, and the mixed myotonic/paralysis effect, which mark the clinical picture of HyperPP.

Cell Line↗
PubMed1998

Interaction between duration of activity and time course of recovery from slow inactivation in mammalian brain Na+ channels.

NaII and NaIIA channels are the most abundant voltage-gated channels in neonatal and adult cortex, respectively. The relationships between activity and availability for activation of these channels were examined using the Xenopus expression system. The main point of this work is that the time constant (tau) of recovery from the unavailable (inactivated) pool is related to the duration (t) of previous activation by a power law: tau(t) = p . tD, with a scaling power D congruent to 0.8 and 0.5 for NaII and NaIIA, respectively, and p as a constant kinetic setpoint. These relationships extend from tens of milliseconds to several minutes and are intrinsic to the channel protein. Coexpression of beta1 auxiliary subunit, together with the alpha subunit of the NaIIA channel, modulates the constant kinetic setpoint but not the scaling power of the latter. The power law scaling between activity and availability is not a universal property of ion channels; unlike that of voltage-gated sodium channels, the rate of recovery from slow inactivation of the ShakerB channel is virtually insensitive to the duration of previous stimuli. It is suggested that the power law scaling described here can act as a molecular memory mechanism that preserves traces of previous activity, over a wide range of time scales, in the form of modulated reaction rates. This mechanism should be considered when theorizing about the dynamics of threshold and firing patterns of neurons.

Animals↗
PubMed1996

Effects of density and gating of delayed-rectifier potassium channels on resting membrane potential and its fluctuations.

The aim of this study is to evaluate directly, using a reduced experimental system, the nature of interactions between voltage-gated potassium channels and the resting membrane potential. Xenopus oocytes were injected with various concentrations of cRNA coding for a delayed-rectifier potassium channel Shaker-IR. The effects of the density and kinetics of the expressed channels on resting membrane potential is explored in isolated ("inside-out") patches. The channel density is given in terms of maximal conductance (Gmax), measured from the maximal slope of the I-V curve under voltage clamp conditions. The capacitance of the experimental setup is approximately 1 pF. At high channel densities (Gmax > 10 pA/mV) the mean membrane potential is stabilized at approximately -60 mV. This resting membrane potential is more than 35 mV positive to the reversal potential for potassium ions under the same experimental conditions. Analyses of voltage clamp experiments indicate that at high channel densities the mean membrane potential is determined by the rates of channel activation and deactivation, but is not affected by the rates involved in the process of slow (C-type) inactivation. In contrast, at lower channel densities membrane potential is very unstable, and its mean value and amplitude of fluctuations are strongly affected by the process of slow (C-type) inactivation.

Animals↗
PubMed1995

Rich dynamics in a simplified excitable system.

The study aims at exploring effects of microscopic channel fluctuations on macroscopic dynamics of excitable systems. Molecular biology techniques are used in order to construct a minimal excitable system that is built of cloned channels embedded in a small (approximately 1 micron 2) isolated patch of membrane. This simple synthetic "point" system exhibits dynamics in time scales that are several orders of magnitude longer then a single spike.

Animals↗
PubMed1995

Intracellular and extracellular amino acids that influence C-type inactivation and its modulation in a voltage-dependent potassium channel.

The rate of C-type inactivation of the cloned voltage-gated potassium channel, Kv1.3, measured in membrane patches from Xenopus oocytes, increases when the patch is detached from the cell; the structural basis for this on-cell/off-cell change was examined. First, four serine and threonine residues, that are putative sites for phosphorylation by protein kinases A and C, were mutated to alanines. Mutating any one of these residues, or two or three of them simultaneously, does not eliminate the change in C-type inactivation. However, the basal rate of C-type inactivation in the cell-attached patch is markedly slower in the triple phosphorylation site mutant. Second, a homologous potassium channel, Kv 1.6, does not exhibit the on-cell/off-cell change. When an extracellular histidine at position 401 of Kv1.3 is replaced with tyrosine, the residue at the equivalent position (430) in Kv1.6, the resulting Kv1.3 H401Y mutant channel does not undergo the on-cell/off-cell change. The results indicate that several potentially phosphorylatable intracellular amino acids influence the basal rate of C-type inactivation, but are not essential for the on-cell/off-cell change in inactivation kinetics. In contrast, an extracellular amino acid is critical for this on-cell/off-cell change.

Amino Acid Sequence↗
PubMed1994

State-dependent inactivation of the Kv3 potassium channel.

Inactivation of Kv3 (Kv1.3) delayed rectifier potassium channels was studied in the Xenopus oocyte expression system. These channels inactivate slowly during a long depolarizing pulse. In addition, inactivation accumulates in response to a series of short depolarizing pulses (cumulative inactivation), although no significant inactivation occurs within each short pulse. The extent of cumulative inactivation does not depend on the voltage during the depolarizing pulse, but it does vary in a biphasic manner as a function of the interpulse duration. Furthermore, the rate of cumulative inactivation is influenced by changing the rate of deactivation. These data are consistent with a model in which Kv3 channel inactivation is a state-dependent and voltage-independent process. Macroscopic and single channel experiments indicate that inactivation can occur from a closed (silent) state before channel opening. That is, channels need not open to inactivate. The transition that leads to the inactivated state from the silent state is, in fact, severalfold faster then the observed inactivation of current during long depolarizing pulses. Long pulse-induced inactivation appears to be slow, because its rate is limited by the probability that channels are in the open state, rather than in the silent state from which they can inactivate. External potassium and external calcium ions alter the rates of cumulative and long pulse-induced inactivation, suggesting that antagonistic potassium and calcium binding steps are involved in the normal gating of the channel.

Animals↗
PubMed1994

Modeling state-dependent inactivation of membrane currents.

Inactivation of many ion channels occurs through largely voltage-independent transitions to an inactivated state from the open state or from other states in the pathway leading to opening of the channel. Because this form of inactivation is state-dependent rather than voltage-dependent, it cannot be described by the standard Hodgkin-Huxley formalism used in virtually all modeling studies of neuronal behavior. Using two examples, cumulative inactivation of the Kv3 potassium channel and inactivation of the fast sodium channel, we extend the standard formalism for modeling macroscopic membrane currents to account for state-dependent inactivation. Our results provide an accurate description of cumulative inactivation of the Kv3 channel, new insight into inactivation of the sodium channel, and a general framework for modeling macroscopic currents when state-dependent processes are involved. In a model neuron, the macroscopic Kv3 current produces a novel short-term memory effect and firing delays similar to those seen in hippocampal neurons.

Animals↗
PubMed1993

Mechanism and modulation of inactivation of the Kv3 potassium channel.

The mechanism and modulation of inactivation of the rat brain voltage-gated potassium channel, Kv3, expressed in Xenopus oocytes, were examined. In the cell-attached patch recording mode, Kv3 macroscopic current amplitude remains constant from one pulse to the next when a series of short (15 ms) repetitive depolarizing pulses are given. However, when the membrane patch is detached from the cell, the current amplitude decreases substantially from one depolarizing pulse to the next, even though there is no significant inactivation-like behaviour within a single current trace. In contrast, the closely related Kv2 potassium channel does not show a similar modulation of inactivation on changing from the cell-attached to the detached-patch recording mode. The data indicate the existence of a prolonged non-conducting state which the Kv3 channel enters in response to a series of short depolarizing pulses; this state is identical to the C-type inactivation state into which channels are driven during individual long depolarizing pulses. The probability of finding the channel in this state differs between the cell-attached and detached-patch modes, indicating that it is modulated by some cytoplasmic factor.

Animals↗
PubMed1992

Immunological rejection of heart transplant: how lytic granules from cytotoxic T lymphocytes damage guinea pig ventricular myocytes.

We investigated the mechanism by which lytic granules extracted from cytotoxic T lymphocytes (CTL) damage guinea pig ventricular myocytes in order to determine whether their actions can be related to the overall immunological rejection of the transplanted heart. Granule-induced myocyte morphological changes and final destruction were preceded by shortening of action potential duration (APD) and reductions of the resting potential and the action potential amplitude. APD shortening was probably caused by a granule-induced increase in outward current (most likely non-specific). Ryanodine, which blocks Ca2+ release from the sarcoplasmic reticulum, did not interfere with the morphological and electrophysiological effects of lytic granules. Fura-2 imaging indicated that [Ca2+]i initially increased about 2-fold from 90.0 +/- 11.5 nM, while cell length decreased less than 5% from a mean value of 99.0 +/- 9.0 microns. A further increase in [Ca2+]i (greater than 10 fold) was associated with progressive contracture and destruction, suggesting that the structural damage inflicted by lytic granules is caused by [Ca2+]i overload. The results indicate that the cytocidal action of CTL-derived lytic granules may be involved in immunologically induced damage, even to the extent of rejection of the transplanted heart.

Action Potentials↗
PubMed1990

The mechanism of muscle injury in the crush syndrome: ischemic versus pressure-stretch myopathy.

Crush injuries are ubiquitous, common sequelae in victims of seismic, industrial and military catastrophes, and were considered to be mainly due to ischemia of the affected limbs. Our clinical experience suggests that early in the crush syndrome, interference with the circulation may occur but is rare. The predominant earliest lesion in the crush syndrome is postulated to be pressure-stretch myopathy, rather than ischemic myopathy. It is proposed that at the membrane level, stretch increases sarcoplasmic influx of Na, Cl, H2O and Ca down their electrochemical gradient. Energy-requiring cationic extrusion pumps work at maximal capacity, but are unable to cope with the increased load. This results in cell swelling and increase in cytosolic and mitochondrial calcium with activation of autolytic destructive processes and interference with cellular respiration. Extensive muscle swelling may cause late muscle tamponade and myoneural ischemic damage (compartmental syndrome). Thus, whereas prevalent theory suggests that the sarcolemmal cationic pump activity is attenuated in the crush syndrome due to early ischemia, we propose that the cationic extrusion pump is maximally activated as in the amphotericin B model. Because the cationic pump is maximally activated in the stretched muscle and in cells exposed to amphotericin, these models rapidly deplete their scarce ATP stores and are susceptible to hypoxia in the face of initially normal circulation.

Biological Transport, Active↗
PubMed1989

Brush-border membrane cation conducting channels from rat kidney proximal tubules.

This is a description and kinetic characterization of cation channels from rat kidney brush-border membrane vesicles and from apical membranes of proximal tubule cells in culture. Channel activity was demonstrated and characterized in both artificial phospholipid bilayers and in tissue culture. Intermediate conductance, approximately 50 pS, cation-selective channels were observed by both methods. Channels were characterized by a Na permeability (PNa)/K permeability (PK) of 1-5:1. Open-channel current-voltage curves were linear in symmetric 300 mM NaCl. In tissue culture the gating kinetics are described by two open-time constants and two closed-time constants. Channel activity was neither voltage nor Ca2+ dependent and the probability of being in the open state ranged from 0.6 to 0.95. In tissue culture experiments the channel demonstrated nonstationary gating activity. A second, 15-pS cation channel, seen in planar bilayers, demonstrated a higher selectivity for Na+ with a (PNa/PK ratio of greater than 10).

Animals↗
PubMed1987

Calcium current in growth balls from isolated Helix aspersa neuronal growth cones.

Growth cones were severed from their neurites in primary cultures of Helix aspersa neurons. Following isolation, growth cones rolled up into 5-10-micron-diameter spheres, which remained attached to a poly-L-lysine or lectin-coated glass coverslip. Whole-cell-configuration patch-clamp recordings from isolated growth cones revealed inward calcium currents upon block of outward currents with internally perfused CsCl. Up to 50 microM tetrodotoxin did not affect this current. In 20-micron-diameter spheres, a peak current of 1.2 nA was reached within 3 ms under voltage-clamp conditions for a 60-mV pulse from a holding potential of -50 mV. Channel density calculations averaged to approximately one channel per square micrometer. A two-phase inactivation was evident under voltage-clamp steps from -50 mV to +15 mV. The growth balls described can be internally perfused and voltage clamped to measure ionic currents involved in growth cone function.

Action Potentials↗