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H Korn

Publications and source records attributed to H Korn.

At least 109 records · Page 6Linked to original sources
PubMed1986

Variability of axonal arborizations hides simple rules of construction: a topological study from HRP intracellular injections.

Intracellular staining with horseradish peroxidase (HRP) allows the analysis of the extent and diversity of axonal field, as the Golgi techniques did for dendritic fields. In this study, we have used such HRP injections to investigate possible rules of construction that underlie the variability in the observed morphological patterns of axons. The Mauthner cell of the teleost provides a suitable material for such a work since it receives inhibitory inputs from two distinct classes of cells that can be identified physiologically prior to their intracellular staining: those that contribute to a recurrent collateral network and those that are part of the commissural vestibulovestibular pathway. The distribution of their terminal boutons over the M-cell surface was quantified and their axonal arborizations were analyzed topologically with the help of a centripetal method of terminal ordering. The results indicated that both types of interneurons have qualitatively a similar distribution of boutons over this target, encompassing the soma, initial portions of the main dendrites, and the cap-dendrites, and that furthermore, the terminals have a tendency to form clusters, the proportion of which is relatively constant regardless of the total number of boutons established by a given afferent cell. In respect to their projection on the M-cell, the two populations differ mainly in the number of established contacts, which averaged 44.1 +/- 28.2 (n = 10) and 14.4 +/- 11.3 (n = 43) for collateral and commissural interneurons, respectively. These differences reflect the variations in axonal arborizations. A topological analysis has revealed that branching occurs mainly as bifurcations, whereas in contrast three or four segments issuing from a branch point are only occasionally observed at the level of the last segment; for each cell, the distribution of boutons within orders is not random and rather follows an unimodal distribution, centered on the mean order (theta); the terminals of each subset, or class, of neurons have their own pattern of distribution within orders; and finally the mean order (theta) is linked to the number of terminal segments (nT) by the relation theta = 1.28 log2 nT. This parameter is of importance: it characterizes the axonal arborization and it allows one to predict the number of terminal segments of individual neurons. Similar analysis of camera lucida reconstruction of axonal arborizations of various cell types studied by other investigators suggests that this relation can be generalized.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

Instantaneous inward rectification in the Mauthner cell: a postsynaptic booster for excitatory inputs.

Single and double electrode voltage clamp techniques have been used to analyse the leakage conductance of the goldfish Mauthner cell. The results indicate that the input conductance of this neuron is maximal at the resting potential, having an average value of 4.68 microS. Approximately 50% of this conductance is voltage dependent and could be inactivated by large hyperpolarizing command pulses (30-50 mV magnitude) of 20-35 ms duration. The magnitude and apparent time constant of the inactivation are both functions of membrane potential, such that the relaxation of the inward current increased and occurred more rapidly for greater hyperpolarizations. The presence of instantaneous inward tail currents when the imposed voltage steps were terminated and membrane potential was returned to the resting level indicated that the conductance mechanism inactivated during hyperpolarization had generated a small outward current at the resting level. Thus, we propose that a major fraction of the Mauthner cell's input conductance is a voltage-dependent K+ conductance. Hyperpolarizing inactivation is a feature of an inward K+ rectifier in other cell types, and when the recording microelectrode was located in the Mauthner cell lateral dendrite, it was possible to demonstrate characteristics consistent with rectification, namely a low conductance for outward depolarizing currents and a high conductance for inward currents. The inward rectifier of the Mauthner cell is different from that of other neurons in that it is already maximally activated at the resting potential and therefore is a major determinant of that parameter. In addition, its activation and inactivation kinetics are quite fast.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

Evidence for ionic coupling between MDCK cells at non-confluent and confluent stages of culture.

The possibility of current flow between epithelial cells (MDCK) has been evaluated using intracellular electrophysiological techniques. We report here that a significant electrotonic coupling was found in this material at all ages of culture. This observation contrasts with previous reports that confluent MDCK are not ionically coupled and lack gap junctions. Alternative mechanisms for such coupling are considered, with emphasis on the role of tight junctions, should the absence of gap junctions be confirmed.

Animals↗
PubMed1986

Probabilistic determination of synaptic strength.

This work was carried on to analyze the presynaptic components of synaptic efficacy, which is designated by the term of synaptic strength. To assess the relations between synaptic strength and innervation density, the properties of unitary Cl(-)-dependent inhibitory postsynaptic potentials (IPSPs) evoked in the potentials (IPSPs) evoked in the goldfish Mauthner (M-) cell by single impulses in individual presynaptic cells, including quantal release parameters, were compared with the histological features of the same neurons, determined from their reconstructions after intracellular injection with horseradish peroxidase (HRP). Because the M-cell is a stereotyped target neuron, comparison of the synaptic strength from different experiments was accomplished by using as a quantitative measure of this parameter the mean unitary IPSP amplitude normalized with respect to the reversal potential for Cl- (i.e., the driving force). In 108 experiments at low stimulus frequency, the majority of the normalized responses (63%) were grouped in a rather narrow range, varying about fourfold, or from 1.5 to 6% of the driving force, with results from stained (n = 46) and unstained (n = 62) cells being the same. In contrast, for the same restricted set of responses, the number of presynaptic terminals (histological n) encompassed a larger range, varying from 3 to 52. Impulses in neurons with quite different complements of terminal boutons could evoke similarly sized, normalized IPSPs, and these two parameters were poorly correlated, with there being, at most, a tendency for the responses to increase with histological n for small values of the latter. Quantal fluctuations in IPSP amplitudes were analyzed according to a binomial model having three parameters, p, which is the probability of release, n, which is the number of releasing units and was previously shown to equal the number of presynaptic boutons or active sites, and q, or the quantal size. The normalized quantal size varied randomly, with a mean value of 0.51% (SD = 0.20) and was relatively independent of n. In contrast, the distribution of p, which ranged from 0.17 to 0.74 (mean = 0.40, SD = 0.155), was skewed to the right; this parameter tended to decrease as a function of increasing n. The normalized unitary inhibitory conductance (g'IPSP) underlying an IPSP is equal to the product of npg'q, where g'q is the normalized quantal conductance.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

[Development of sensitivity to glycine, GABA and beta-alanine in cultured spinal neurons].

Whole cell patch-clamp recording of cultured chick spinal neurons, presumed to be motoneurons, revealed currents elicited in these cells by GABA, glycine and beta-alanine, corresponding to the opening of chloride channels. During maturation, sensitivity to all three transmitters were first detected on the day 6 of culture, and appeared in most neurons by day 8. However, at all stages of development, a fraction of the cells were sensitive only to GABA; this observation supports the notion that the GABA and the glycine receptors are distinct. On the other hand, separate activation by glycine and beta-alanine was never observed, in agreement with the postulate that these amino acids bind to the same receptor.

Alanine↗
PubMed1985

Dorsolateral geniculate neurons in vitro: reduced postsynaptic excitability following repetitive activation of the optic tract.

Monosynaptic activation of dorsolateral geniculate neurons of the rat was studied in the in vitro slice preparation. (1) As previously described by others, monosynaptic excitatory postsynaptic potentials (EPSPs) trigger at membrane potential negative to -60 mV a low threshold spike (Lts). In addition, we report that the Lts amplitude is linked to that of its underlying EPSP. (2) EPSPs, EPSP-Lts sequences and nonsynaptic Lts are followed by a long period of refractoriness during which a test response is decreased in amplitude and the Lts component is cut off. These results are discussed in relation to the generation of spontaneous bursting activity of thalamic neurons during natural sleep, which could be accounted for by the properties of the Lts alone.

Animals↗
PubMed1985

Activity-dependent deformations of presynaptic grids at central synapses.

In the CNS, the exocytosis which accompanies transmitter releases occurs at the level of a presynaptic grid. Possible alterations in the grid as a function of this phenomenon were searched for at the synapses established by unmyelinated club endings on the Mauthner cell of teleosts. The number of vesicle openings generated by aldehyde fixation was diminished by cooling the preparation and enhanced after perfusion with a high-KCl Ringer solution. Morphometric analysis of the grid showed that under these conditions the mean distance between its constituent elements, the presynaptic dense projections, increased with the number of exocytotic events. Parallel changes were observed for the mean diameter of the spaces left free between these dense projections, suggesting that vesicle exocytosis produces a transient enlargement of the space where it takes place. These observations indicate that the presynaptic grid is more dynamically involved in the secretory process than previously conceived. It is therefore hypothesized that (i) the movement of the dense projections is a consequence of their interaction with the plasma membrane, and (ii) the distortion of the grid could underlie regulatory mechanisms by which the number of released vesicles is limited after each impulse. It is also proposed that the dense projections contribute to the stabilization of the plasma membrane, thereby preventing its randomization following intense release.

Animals↗
PubMed1985

Evidence that receptors mediating central synaptic potentials extend beyond the postsynaptic density.

Physiological recordings and computer simulations of unitary inhibitory postsynaptic potentials in the Mauthner cell of the goldfish central nervous system have been used to estimate the expected size of the postsynaptic receptor matrix at individual junctions. Simultaneous pre- and postsynaptic recordings were used to determine the kinetic parameters of the quantal responses under normal conditions and in the presence of strychnine, a competitive antagonist of glycine, which is the putative transmitter at these synapses. Calculations indicate that if the postsynaptic density, which has a radius of 0.1 micron, were to accommodate the population of channels estimated to be opened during a quantal response, the glycine binding site density in that region would be unrealistically high. Computer simulation of the quantal responses included transmitter diffusion, transmitter-receptor interactions, and channel activation under conditions including both normal and lowered binding site densities, the latter corresponding to the experimental data obtained with strychnine. The data indicate that the synaptic receptors involved in generating unitary responses are widely distributed to include regions located outside the junctional area, which directly faces the presynaptic release sites. We further suggest that the receptor matrix is surrounded by a restricted diffusional space; this geometrical organization may underlie the finding that response rise times are relatively independent of receptor binding site densities.

Action Potentials↗
PubMed1985

Distribution of glycine receptors at central synapses: an immunoelectron microscopy study.

The distribution of receptors for a neurotransmitter was investigated cytochemically for the first time in the central nervous system, at synapses established on cells of the ventral horn of the rat cervical spinal cord. Three monoclonal antibodies (mAb's) raised against glycine receptors were used. Immunofluorescent staining already showed discontinuous labeling at the surface of neurons, and immunoenzymatic electron microscopy further revealed that the antigenic determinants were confined to the postsynaptic membrane and concentrated at the level of the synaptic complex. More specifically, one mAb directed against the receptive subunit of the oligomeric receptor recognized an epitope on the extracellular side of the plasma membrane, whereas two other mAb's bound to the cytoplasmic face. Epitopes for the last two mAb's were more accurately localized with protein A-colloidal gold, using an intermediate rabbit anti-mouse immunoglobulin serum. (a) In addition to the presence of gold particles in areas facing the presynaptic active zone (visualized with ethanolic phosphotungstic acid), the labeling extended beyond this zone for approximately 50-60 nm, which corresponds to the width of one presynaptic dense projection. (b) The distances between the mid membrane and the gold particles were different for the two mAb's (with means of 21.7 +/- 8.5 nm and 29.8 +/- 10.4 nm, respectively). The data suggest that one of the recognized epitopes is close to the plasma membrane, whereas the second protrudes into the cytoplasm. Our results indicate that the receptor is a transmembrane protein which has a restricted spatial distribution on the postsynaptic neuronal surface.

Animals↗
PubMed1984

Transformation of binomial input by the postsynaptic membrane at a central synapse.

Although a binomial model gives an adequate description of the release properties of central afferent synapses, slight differences between experimental and predicted probability density functions are observed, with an excess number of responses recorded around the means. These discrepancies can be quantified by comparing experimental and theoretical entropies, which could relate to information transfer at these junctions. A model, based on the experimentally supported assumption that the postsynaptic membrane functions as a nonlinear processor, minimizes the differences between the distributions of the recorded and predicted potentials. According to this model, the nonlinearity is due to a localized interaction between the effects of simultaneously activated adjacent synapses.

Animals↗
PubMed1984

Voltage-dependent dye coupling at a rectifying electrotonic synapse of the crayfish.

At the crayfish giant motor synapse, the lateral giant axon (l.g.a.) and the giant motor fibre (g.m.f.) form an electrotonic junction which exhibits two states of ionic coupling (Furshpan & Potter, 1959a; Giaume & Korn, 1983). Junctional conductance is low at resting membrane potentials (i.e. with lateral axon more negative than the motor fibre) and high when the polarity of the voltage difference (delta V) across the synapse is reversed. For these two states of conductance, junctional permeability was investigated using the intercellular tracer Lucifer Yellow. The dye was ionophoretically injected into either the presynaptic (l.g.a.) or the post-synaptic (g.m.f.) cell. In the high conductance state (delta V greater than 0), fluorescence was detected in both neurones whether Lucifer Yellow had been injected pre- or post-synaptically. By contrast, at the resting junctional polarization (delta V less than 0) Lucifer Yellow spread from the giant axon to the g.m.f., but not from the g.m.f. to the giant axons. These data demonstrate that dye transfer at the giant motor synapse, like ionic coupling, is sensitive to junctional polarization and is more marked in the high conductance state. Possible explanations for the asymmetry observed in the low conductance state are discussed.

Animals↗
PubMed1984

Regulation of efficacy at central synapses.

The quantal nature of synaptic depression produced by high frequency stimulations has been analyzed at a central synapse for the first time. Simultaneous intracellular recordings were obtained from the Mauthner cell and adjacent identifiable inhibitory interneurons. The presynaptic cells were stimulated at frequencies from 2 to 33 Hz, and the corresponding release parameters were determined using a computational procedure described elsewhere (Korn, H., A. Triller, A. Mallet, and D. S. Faber (1981) Science 213: 898-901). As in our previous studies, these entities were correlated with histological features of the neurons following systematic horseradish peroxidase injections and reconstructions. Evidence was obtained that, in the range of physiological conditions used, the binomial parameter n (number of available quanta for release) remains constant; thus every synaptic bouton continues to function as an independent all-or-none releasing unit. The progressive reduction in amplitude of postsynaptic potentials can be attributed solely to a lower probability of release, as shown by the fall of the binomial parameter p. This evidence supports the concept that p is a critical variable for short-term modifications of synaptic efficacy and may provide insight for instances of synaptic plasticity underlying those behavioral changes which can be attributed to presynaptic loci. The present study also represents a necessary step toward linking mathematical variables of models for transmitter exocytosis with subcellular events.

Animals↗
PubMed1982

Ammonium sulfate induced uncouplings of crayfish septate axons with and without increased junctional resistance.

Cytoplasmic pH (pH1) of crayfish lateral giant axons was monitored using antimony microelectrodes placed near septate junctions and variations of internal pH was induced by short applications of ammonium sulfate in the perfusing bath of the preparation. This treatment produced a rapid cell alkalinization followed, after wash, by acidification rebound. Simultaneously, two successive phases of uncoupling of the septate junction occurred; they had the same time course as those of their associated pH1 movements. Calculation of the electronic coupling parameters indicated that, during alkalinization, coupling was accompanied by an increased axonal membrane conductance (the intimate origin of which was beyond the scope of this study) and resulted from a shunt of the gap junctions; the resistance proper of the latter was unaffected; thus involvement of Ca2+ was ruled out and uncoupling was only an indirect consequence of the electrotonic junction's network configuration. In contrast, and as expected from previous investigations, the junctional membrane resistance was increased during the second phase of cytoplasmic acidification. Evidence that uncoupling can be brought about by a non-junctional membrane increased permeability raises questions about some of the criteria commonly used during investigations of electrotonic transmission.

Ammonium Sulfate↗