Biomedical subjects

J Streit

Publications and source records attributed to J Streit.

16 recordsLinked to original sources

PubMed1998

Pattern generation by two coupled time-discrete neural networks with synaptic depression.

Numerous animal behaviors, such as locomotion in vertebrates, are produced by rhythmic contractions that alternate between two muscle groups. The neuronal networks generating such alternate rhythmic activity are generally thought to rely on pacemaker cells or well-designed circuits consisting of inhibitory and excitatory neurons. However, experiments in organotypic cultures of embryonic rat spinal cord have shown that neuronal networks with purely excitatory and random connections may oscillate due to their synaptic depression, even without pacemaker cells. In this theoretical study, we investigate what happens if two such networks are symmetrically coupled by a small number of excitatory connections. We discuss a time-discrete mean-field model describing the average activity and the average synaptic depression of the two networks. Depending on the parameter values of the depression, the oscillations will be in phase, antiphase, quasiperiodic, or phase trapped. We put forward the hypothesis that pattern generators may rely on activity-dependent tuning of synaptic depression.

Action Potentials↗

PubMed1997

Synaptic plasticity in dissociated hippocampal cultures: pre- and postsynaptic contributions.

The distinction between pre- or postsynaptic expression of synaptic plasticity is difficult to make, unless the postsynaptic receptors can be investigated in isolation. We have studied single synaptic contacts in dissociated cultures of rat hippocampus. The reaction of postsynaptic receptor assemblies to the induction of synaptic plasticity was measured and compared with changes in the rate of spontaneous miniature excitatory postsynaptic currents (mEPSCs), which can reflect changes in the transmitter release mechanism. The response of a receptor assembly to locally applied exogenous glutamate was measured before and after synchronized application of glutamate and a train of postsynaptic depolarizations ('pairing'). Pairing induced a variety of changes: (i) the majority of the receptor assemblies showed no change in their response to glutamate before and after pairing; (ii) the postsynaptic current due to exogenous glutamate showed a rapid increase in five out of 26 cases. This was not due to changes in the single channel conductance; (iii) the rate of mEPSCs increased, if it had previously been below 25 Hz; (iv) the rate of mEPSCs decreased, if it had previously been above 25 Hz. Effects 2 and 3 were blocked by antagonists of NMDA receptors. These findings provide direct evidence for an increase of the number of glutamate receptors at a subset of the investigated postsynaptic sites during synaptic potentiation.

Animals↗

PubMed1996

Mechanisms of pattern generation in co-cultures of embryonic spinal cord and skeletal muscle.

Spontaneous output patterns of embryonic spinal cord slices in vitro were investigated in order to study the formation of pattern-generating networks. Patterns of spontaneous contractions of skeletal muscle fibers were recorded in co-cultures of embryonic rat spinal cord, dorsal root ganglia and skeletal muscle. A part of these contractions was shown to be driven by spinal circuits. These neuron-driven activity patterns changed from random to rhythmic when the inhibitory synapses in the spinal cord were blocked by strychnine, bicuculline or both. Rhythmic patterns consisted of bursts of activity (tetanic contractions) followed by periods of relaxation. The transition from random to rhythmic patterns occurred during a period of heavily increased rate of activity. Presynaptic inhibition was not involved critically in the generation of rhythmic patterns. Such patterns were, however, modulated through muscarinic and alpha-adrenergic receptors. Neither NMDA nor glutamate nor its uptake blocker dihydrokainate induced rhythmic patterns of contraction, although NMDA in the presence of low magnesium increased moderately the rate of random activity. In order to study the size of pattern-generating networks, parts of the spinal cord slices were sectioned during rhythmic activity. Tangential cuts at the lateral or dorsal side of the slices reduced either the rate or the duration of the bursts or both. Sagittal cuts suppressed the activity almost totally. These findings suggest that the pattern generators in the slices consist of excitatory networks covering the entire slice, and that these networks reverberate following spontaneous activity of some distributed elements.

Animals↗

PubMed1996

Transmitter concentration profiles in the synaptic cleft: an analytical model of release and diffusion.

A three-dimensional model for release and diffusion of glutamate in the synaptic cleft was developed and solved analytically. The model consists of a source function describing transmitter release from the vesicle and a diffusion function describing the spread of transmitter in the cleft. Concentration profiles of transmitter at the postsynaptic side were calculated for different transmitter concentrations in a vesicle, release scenarios, and diffusion coefficients. From the concentration profiles the receptor occupancy could be determined using alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor kinetics. It turned out that saturation of receptors and sufficiently fast currents could only be obtained if the diffusion coefficient was one order of magnitude lower than generally assumed, and if the postsynaptic receptors formed clusters with a diameter of roughly 100 nm directly opposite the release sites. Under these circumstances the gradient of the transmitter concentration at the postsynaptic membrane outside the receptor clusters was steep, with minimal cross-talk among neighboring receptor clusters. These findings suggest that for each release site a corresponding receptor aggregate exists, subdividing an individual synapse into independent functional subunits without the need for specific lateral diffusion barriers.

Animals↗

PubMed1995

Analysis of synaptic transmission at single identified boutons on rat spinal neurons in culture.

The spatial organization of receptor channels has a major influence on the speed and possible plasticity of synaptic signal transmission. We have studies glutamatergic synapses on neurons in organotypic cultures of rat spinal cord. In order to avoid the problems related to the analysis of currents of unknown origin within a neuron, we chose to examine the functional properties of single identified synapses. Iontophoretic mapping of the cell surface revealed hot spots of high glutamate sensitivity coincident with presynaptic boutons stained with the dye FM 1-43. Local application of KCl to these sites caused bursts of synaptic release. Hot spots typically consisted of 330 receptors with an average single-channel conductance of 8.3 pS. Evoked synaptic currents involved only about 40-50 receptors and nevertheless showed characteristics of saturation. This suggests that glutamate receptor clusters at sites of presynaptic terminals are organized into well separated subclusters opposite release sites.

Animals↗

PubMed1994

Action potential propagation through embryonic dorsal root ganglion cells in culture. I. Influence of the cell morphology on propagation properties.

1. In this and the companion paper the reliability of action potential (AP) propagation through dorsal root ganglion (DRG) cells was investigated. Experimental data were collected from DRG cells of embryonic rat slice cultures of the spinal cord. A field stimulation electrode was used to elicit an AP in the axon. The propagated AP or, in case of conduction block, its electronic residue (ER), was measured intracellularly in the soma of the DRG cell. 2. The morphological and electrophysiological data combined with published data from voltage-clamp studies were taken to implement a compartmental computer model, which allows a precise description of the propagating AP and the channel kinetics at any point along the axon. 3. The safety factor for conduction was found to be low. Thus failures of AP invasion of the DRG cell soma could occur at sites of impedance mismatch when a hyperpolarizing current was applied, a second stimulus felt into the relative refractory period of the first, or when the axon was repetitively stimulated. 4. The ERs of the failed APs had discrete amplitude levels, suggesting that the failures were always caused at the same site along the axon. These sites of low safety factor were found to be the branch point in the unipolar DRG cell and the entrance of the stem piece into the soma in both cell types, the bipolar as well as the unipolar. 5. A systematic comparison of bipolar and unipolar DRG cells showed that the AP conduction through the latter is more reliable. For large cell bodies, the unipolar configuration is needed for save conduction. 6. Conduction through unipolar DRG cells is faster than through bipolar cells because the electrical load of the soma is masked by the high-resistive stem piece. The length of this stem piece is correlated inversely to the delay caused at the branch point, as the electrical load of the soma is more efficiently masked by a long stem piece.

Action Potentials↗

PubMed1994

Action potential propagation through embryonic dorsal root ganglion cells in culture. II. Decrease of conduction reliability during repetitive stimulation.

1. The reliability of the propagation of action potentials (AP) through dorsal root ganglion (DRG) cells in embryonic slice cultures was investigated during repetitive stimulation at 1-20 Hz. Membrane potentials of DRG cells were recorded intracellularly while the axons were stimulated by an extracellular electrode. 2. In analogy to the double-pulse experiments reported previously, either one or two types of propagation failures were recorded during repetitive stimulation, depending on the cell morphology. In contrast to the double-pulse experiments, the failures appeared at longer interpulse intervals and usually only after several tens of stimuli with reliable propagation. 3. In the period with reliable propagation before the failures, a decrease in the conduction velocity and in the amplitude of the afterhyperpolarization (AHP), an increase in the total membrane conductance, and the disappearance of the action potential "shoulder" were observed. 4. The reliability of conduction during repetitive stimulation was improved by lowering the extracellular calcium concentration or by replacing the extracellular calcium by strontium. The reliability of conduction decreased by the application of cadmium, a calcium channel blocker, 4-amino pyridine, a fast potassium channel blocker, or apamin or muscarine, the blockers of calcium-dependent potassium channels. The reliability of conduction was not effected by blocking the sodium potassium pump with ouabain or by replacing extracellular sodium with lithium. 5. In the period with reliable propagation cadmium, apamin, and muscarine reduced the amplitude of the AHP. The shoulder of the action potential was more pronounced and not sensitive to repetitive stimulation when extracellular calcium was replaced by strontium. It disappeared when cadmium was applied. 6. In DRG somata changes of the intracellular Ca2+ concentration were monitored by measuring the fluorescence of the Ca2+ indicator Fluo-3 with a laser-scanning confocal microscope. During repetitive stimulation, an accumulation of intracellular calcium occurred that recovered very slowly (tens of seconds) after the AP trains. 7. Computer model simulations performed in analogy to the experimental protocols produced conduction failures during repetitive stimulation only when the calcium currents during the APs were reduced. 8. From these findings it is concluded that conduction failures during repetitive stimulation are dependent on an accumulation of intracellular calcium leading to an inactivation of calcium currents, combined with small contributions of an accumulation of extracellular potassium and a summation of slow potassium conductances.

Acetylcholine↗

PubMed1993

Regular oscillations of synaptic activity in spinal networks in vitro.

1. Spontaneous synaptic potentials were recorded in motoneurons grown in organotypic slice cultures of embryonic rat spinal cord. In 71 of 85 cells these potentials appeared without obvious temporal structure (random patterns); in the remaining 14 cells they appeared in bursts (rhythmic patterns). 2. Random activity patterns could be converted into rhythmic patterns by treating the cultures with strychnine, bicuculline, or both. The excitatory amino acid N-methyl-D-aspartate (NMDA) transiently increased the rate of spontaneous synaptic activity without inducing rhythmic patterns. The NMDA antagonist 7-chloro-kynurenate reduced the burst rate while leaving the burst length unchanged in rhythmic patterns. In random patterns it reduced the rate of spontaneous synaptic activity by 68%. 3. Histograms of interevent times of the random patterns were best fitted by the sum of two expontentials, suggesting that the random type of activity could not be described simply as a Poisson process but involved at least one additional mechanism. 4. Rhythmic patterns consisted of bursts of activity with a mean burst length of 2.2 s that were separated by interburst intervals with a mean length of 6.6 s. Within the bursts autocorrelograms revealed regular oscillations with a mean period of 226 ms in 6 of 11 experiments with rhythmic patterns. The period showed little variation between individual experiments (202-288 ms). In random patterns no oscillations were detected. 5. Within the spontaneous bursts the excitatory postsynaptic potentials (EPSPs) progressively declined in amplitude. A corresponding depression of EPSPs was observed when trains of electrical stimuli were applied at 5 Hz to the dorsal horns of the spinal cord slices.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Miniature excitatory postsynaptic potentials in embryonic motoneurons grown in slice cultures of spinal cord, dorsal root ganglia and skeletal muscle.

Miniature excitatory postsynaptic potentials (mEPSPs) were recorded in motoneurons grown in organotypic cocultures of embryonic rat spinal cord, dorsal root ganglia and muscle in the presence of TTX. The motoneurons were electrically compact with a mean electrotonic length of 0.6. Spontaneous EPSPs were found in most of these motoneurons. With TTX the large EPSPs disappeared, whereas in more than half of the experiments mEPSPs persisted with a range in size of 1 to 4 mV (mean: 2.1 mV), probably originating from the spontaneous release of single vesicles. The net inward charge transfer at the soma ranged from 0.12 to 0.34 pC. The mEPSPs were heterogeneous in size even within pools of potentials that were homogeneous in shape. They had similar shapes and amplitudes as the smallest spontaneous unitary EPSPs mediated by presynaptic impulses, suggesting that for the smallest afferents not more than one vesicle was released per afferent impulse. Both the miniature and the TTX-sensitive EPSPs were readily blocked by the glutamate antagonist DNQX.

Animals↗

PubMed1992

Depression of postsynaptic potentials by high-frequency stimulation in embryonic motoneurons grown in spinal cord slice cultures.

1. In embryonic cocultures of spinal cord, dorsal root ganglia, and muscle, excitatory postsynaptic potentials (EPSPs) were recorded in motoneurons during focal electrical stimulation of the dorsal root ganglia or the spinal cord. 2. EPSPs were depressed in amplitude at high-frequency stimulation relative to a control frequency of 0.5 Hz by 47 and 75% at 5 and 10 Hz, respectively. This was true for composite EPSPs and unitary EPSPs. 3. The depression showed a wide range of variability between individual experiments. The degree of depression at 5 Hz was negatively correlated to the rate of spontaneous excitatory input the motoneurons received. There was no correlation to the soma size, the average amplitude of the EPSPs, the rheobase, or the input resistance of the motoneurons. 4. An increase in latency of EPSPs was observed concomitant with or preceding the synaptic depression in most experiments. Total transmission failures, which were absent at low-frequency stimulation, appeared during depression. 5. Large incremental steps in amplitude could be seen during depression, suggesting that several release sites were switched off and on together. 6. Decreasing the extracellular calcium concentration from 5 to 1 mM led to a decrease in the frequency sensitivity of the synaptic efficacy and to a decrease of the EPSP amplitude and latency. 7. Measurements of the antidromic conduction of action potentials evoked in the axons and recorded in the somata of dorsal root ganglion cells revealed an increase in latency and the appearance of conduction failures at stimulation frequencies of 1-10 Hz. The frequency modulation of conduction was decreased in 1 mM compared with 5 mM external calcium. 8. Together these findings suggest that conduction failures in the presynaptic axons contribute to the synaptic depression of EPSPs in embryonic motoneurons.

Action Potentials↗

PubMed1990

Calcium current inactivation during nerve-growth-factor-induced differentiation of PC12 cells.

The inactivation of calcium currents during nerve growth factor (NGF)-induced differentiation of rat pheochromocytoma (PC12) cells was investigated. Whole cell calcium and barium currents were recorded in PC12 cells using the patch-clamp method. A shift of the steady-state inactivation curve towards more negative potentials, as well as an increase in the strength of time-dependent inactivation, was observed in differentiating PC12 cells (+NGF) compared to undifferentiated cells (-NGF). The fraction of current inactivated after 200 ms normalized to the peak current amplitude significantly increased from 0.27 +/- 0.01 (n = 39) to 0.39 +/- 0.01 (n = 109) following NGF treatment. The increase in the strength of inactivation preceded the increase in the peak calcium current amplitude observed in PC12 cells during NGF treatment. In differentiating cells, regional differences in the strength of inactivation paralleled differences in current density. Barium currents recorded from growth cones, where the current density was high, showed 30% more inactivation than soma currents. The strength of inactivation of calcium currents in individual cells was not correlated to the effectiveness of nifedipine, nor was the effect of nifedipine on calcium currents altered during differentiation. These results are discussed, suggesting a redistribution of calcium channels during the differentiation of PC12 cells.

Adrenal Gland Neoplasms↗

PubMed1989

A modified roller tube technique for organotypic cocultures of embryonic rat spinal cord, sensory ganglia and skeletal muscle.

The roller tube technique as initially described in the literature in 1981, was modified in several aspects for the coexplantation of embryonic rat spinal cord with attached dorsal root ganglia and skeletal muscle from newborn rats. The high metabolic activity of this coculture system required a particular culturing protocol to stabilize pH and osmotic pressure. The appropriate adjustment of the partial pressure of carbon dioxide gas in the incubator proved to be essential for the control of the pH within narrow limits (7.3 +/- 0.1). The adjustment of the osmotic pressure of the medium (290-300 mOsm) improved the growth of the cultures considerably. Roller drum speed was set to 120 revolutions per hour for enhanced flattening of the culture. A simple rating system was used to evaluate neuronal and non-neuronal outgrowth under different modifications of the culture system. Furthermore, morphological and electrophysiological criteria were defined for evaluating individual neurons. The technique described insures the growth of long-term organotypic cocultures of spinal cord, sensory ganglia and skeletal muscle.

Animals↗

PubMed1989

Simulation of action potential propagation in terminal arborizations of cultured sensory ganglia.

In order to study possible integrative properties of axon terminations, impulse propagation in arbitrarily complex terminal arborizations is simulated using a general purpose network analysis program. Detailed, anatomically based models are constructed from HRP-filled arborizations of sensory ganglia impinging upon motoneurons grown in an organotypic rat spinal cord culture. On assuming an excitable membrane the action potential propagates into all the ramifications. If some branches are assumed to be inexcitable, the electrotonically propagated potential may not depolarize the synaptic endings sufficiently to release transmitter. Under such conditions only part of the morphologically found synapses are expected to release transmitter following stimulation of sensory neurons.

Action Potentials↗

PubMed1989

Distribution of calcium currents in sprouting PC12 cells.

Whole-cell calcium and barium currents were recorded from PC12 cell bodies and growth cones during nerve growth factor (NGF)-induced neurite outgrowth. Depolarizing voltage steps were applied to activate the currents, and pharmacological agents were used to separate them from other ionic currents. In recordings from growth cones still attached to neurites, current flow from central parts of the cell could be distinguished from the growth cone currents. On the other hand, currents from neuritic shafts and growth cones contributed to whole-cell currents recorded in the soma. Such currents were isolated by alternatively comparing recordings of whole-cell calcium currents with recordings in which calcium currents of defined parts of the cell were suppressed by exposing these regions to laminar streams of solutions with low calcium. The boundary between such streams and the bath solution was shown to be sharp using a calcium-selective microelectrode. The current deficits recorded when the growth cones were exposed to solutions with low calcium (growth cone currents) were within 10-50% of the total cell currents and similar to the currents recorded when the whole cell except the growth cone was exposed to low calcium. The current densities in the growth cones during initial sprouting were 5.4 times higher than those in the somata. Growth cone currents showed more inactivation than currents originating from the soma during pulses of 200 msec. In most experiments no calcium currents could be resolved in the neuritic shaft during initial neurite growth (1-10 d of NGF application), indicating low current density. In proximal segments of the neurites, however, a somatofugal decrease of the current density was observed. It is concluded from these results, that in outgrowing neurites of PC12 cells high densities of calcium channels are maintained in the growth cones, whereas in the neuritic shaft calcium channel density is initially low and later increases during consolidation of the neurites.

Animals↗

PubMed1987

Voltage dependent calcium currents in PC12 growth cones and cells during NGF-induced cell growth.

The role of calcium currents in the regulation of neurite outgrowth is still rather speculative. As a contribution to this field, macroscopic voltage dependent calcium currents were investigated in relation to the nerve growth factor (NGF)-induced outgrowth of neurites in PC 12 cells. Calcium currents were recorded in isolated growth cones of PC 12 cells using the whole cell patch clamp method. The currents were activated at high voltages and only slightly inactivated with time. The currents were identical to those found in the cell soma of PC 12 cells and similar to the classical high-voltage-activated calcium current found in many neuronal cells. The peak current density in the growth cones was in the same range as in the cell somata. The calcium currents of the cell somata were not modified during the early phase of NGF application, despite the occurrence of NGF-induced soma growth and outgrowth of neurites. The current density at this time was therefore lower in NGF-treated cells than in untreated cells. In a later phase, maximal current amplitudes of NGF-treated cells were higher than in untreated cells indicating an increase in current density to values similar to that found in the untreated cells. In addition, the calcium current inactivation was found to be more pronounced in the NGF-treated cells by that time. The results are discussed with regard to a possible role of calcium currents in the regulation of NGF-induced neurite outgrowth in these cells.

Adrenal Gland Neoplasms↗

PubMed1987

Effects of hypoxia and glycolytic inhibition on electrical properties of sheep cardiac Purkinje fibres.

Active and passive electrical properties of sheep cardiac Purkinje fibres were determined using conventional microelectrode techniques. In addition, oxygen tension was monitored immediately adjacent to the fibres with oxygen sensing microelectrodes. Oxygen was withdrawn for 130 min in a glucose-free solution (PO2 less than 15 mmHg), glycogen breakdown inhibited by 2-4-deoxyglucose and the following effects observed: (i) a decrease in action potential duration by as much as 80%; (ii) a decrease in maximal rate of rise of the action potential (Vmax) by 52%; (iii) an increase in internal longitudinal resistance (ri) by 176%; and (iv) a decrease in conduction velocity (theta) by 47%. The magnitude of the effects depended on PO2 (range: 4 to 46 mmHg). In particular, ri was steeply dependent upon the PO2 of the medium (glycogenolysis also inhibited by 2-4-deoxyglucose): ri increased to 470% above control when PO2 was reduced to 4 mmHg versus only a 47% increase in ri at PO2 = 11 mmHg. These effects were largely reversible after re-oxygenation and washout of 2-4-deoxyglucose although the time course of recovery of ri lagged behind that of action potential duration. Similar to previous studies, practically no effect of hypoxia was observed on the electrical properties of Purkinje fibres when glycogenolysis was not inhibited. The measured decrease of theta correlated relatively well with that predicted from linear cable theory using the changes of (i) Vmax and (ii) ri. No significant changes in membrane capacitance and only minor changes in action potential amplitude developed during combined hypoxia and inhibition of glycolysis.

Action Potentials↗