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H Korn

Publications and source records attributed to H Korn.

At least 55 records · Page 3Linked to original sources
PubMed1994

Excitatory synaptic connections onto rat hippocampal inhibitory cells may involve a single transmitter release site.

1. Whole-cell tight-seal records of excitatory postsynaptic currents (EPSCs) were made from inhibitory cells in the CA3 region of thin hippocampal slices. We tested the hypothesis that excitatory synaptic connections made on inhibitory cells involve few transmitter release sites. 2. EPSCs impinging on inhibitory cells had a time to peak of 0.4-3.8 ms and an amplitude of 8-90 pA at a holding potential of -60 mV. They were suppressed by the excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and DL-2-amino-5-phosphonovaleric acid (APV). 3. Addition of tetrodotoxin (TTX) and Co2+ to the external solution reduced the frequency of EPSCs from 0.90 to 0.25 s-1 (n = 24 cells). In the majority of cells EPSC amplitude distributions were not significantly changed. 4. Increasing Ca2+ and reducing Mg2+ in the external solution, in order to enhance the probability of transmitter release, did not change EPSC amplitude distributions. In contrast, amplitude histograms for IPSCs recorded from pyramidal cells were shifted to higher mean values in this solution. 5. EPSCs were elicited in inhibitory cells by electrical stimulation via a glass pipette placed near to pyramidal cells in stratum pyramidale. EPSCs elicited by weak stimuli had similar amplitude distributions to excitatory synaptic events recorded in the presence of TTX and Co2+. 6. These findings suggest excitatory synaptic connections made with CA3 inhibitory cells involve few or possibly just one transmitter release site.

2-Amino-5-phosphonovalerate↗
PubMed1994

Glycinergic inhibitory synaptic currents and related receptor channels in the zebrafish brain.

To extend our study of the inhibitory synaptic network we have developed an isolated whole-brain preparation of the 52-h-old zebrafish (Brachydanio rerio) in which the structural and functional integrity of the brain is preserved. We report the characterization of quantal inhibitory events and the correlation of their properties with those of the underlying activated channels. During whole-cell recordings of the Mauthner cells, applications of 10(-6) M tetrodotoxin greatly reduced the frequency and amplitude of the spontaneously occurring synaptic events, which were dominated by Cl--dependent inhibitory postsynaptic currents (IPSCs). Lowering Ca2+ and adding Mg2+ to tetrodotoxin-containing solutions resulted in a further decrease in amplitude of the recorded synaptic currents, the remaining ones being considered as miniature IPSCs (mIPSCs). Applications of 0.5-1 microM strychnine in the presence of tetrodotoxin eliminated > 90% of the inhibitory currents in the preparation. The amplitude histograms of these mIPSCs exhibited two initial equally spaced peaks, followed by a skewed distribution for higher values. The first two components were well fitted by the sum of two Gaussian curves, giving a mean quantal amplitude of 35.7 pA (at a holding potential of -50 mV) and a coefficient of variation of 0.25 for the first peak. Outside-out recordings showed at least two classes of glycine receptor channels, one having multiple conductance levels with a main state of 81-86 pS and another displaying only one opening level of 41-43 pS. These two mean conductance states had similar mean open times, of 0.6-1 and 4.5-6 ms respectively. In addition, three mean closed times were observed for the 41-43 pS level. The shortest group (0.6-1 ms) was considered as representing gaps within bursts. Burst analysis revealed three mean burst durations, of 0.6, 4 and 35 ms. Comparisons of the amplitude of the first class of mIPSCs and of the open channel conductances indicated that one quantum opens 14-22 channels. Moreover, the correspondence between the mean decay time of mIPSCs and the mean open time or medium burst duration (4-5 ms) suggests that glycine-activated channels open only once in response to a single exocytosis. The pre- and postsynaptic origins of mIPSCs amplitude fluctuations are discussed in the context of multivesicular release versus the hypothesis of postsynaptic receptor saturation.

Animals↗
PubMed1994

Different patterns of calcium signaling triggered through two components of the B lymphocyte antigen receptor.

The engagement of the B cell antigen receptor is the first step of antigenic stimulation of B lymphocytes. This step is followed by a series of biochemical events, including the activation of protein-tyrosine kinases, phosphoinositide turnover, and multiple patterns of calcium mobilization, which lead to the regulation of gene transcription and cellular responses. The B cell antigen receptor complex is composed of membrane immunoglobulins (as antigen recognition subunits) and associated chains (Ig-alpha and Ig-beta) that couple the receptor to cytoplasmic protein kinases. To investigate independently the relative signaling capacity of Ig-alpha and Ig-beta, chimeric proteins containing their cytoplasmic domains were expressed in a B cell line. We found that Ig-alpha and Ig-beta activate two distinct intracellular signaling pathways. The engagement of Ig-alpha chimeras induces a complete release of calcium from intracellular stores, followed by transmembrane calcium influx and late cell activation signals, detected by lymphokine secretion. In contrast, Ig-beta chimeras do not induce lymphokine secretion or calcium influx, but induce short oscillatory release of calcium, dependent on the activity of the Ca-ATPase pump of the endoplasmic reticulum. These results provide a structural basis for the diversity of B cell responses.

B-Lymphocytes↗
PubMed1994

Origin and function of spiral fibers projecting to the goldfish Mauthner cell.

Two neuron types contact the Mauthner cell (M cell) in the axon cap, a specialized region of high electrical resistance surrounding the initial segment of the M cell axon. One type produces a mixed electrical and chemical inhibition of the M cell. The second sends axons into the central core of the axon cap, where they spiral around the initial segment making both conventional synapses and gap junction contacts. The origin and synaptic effects of these spiral fibers have not been studied previously. When goldfish M cells were filled with Lucifer yellow, presynaptic spiral fibers were seen in the axon cap. These fibers could be traced back through the medial longitudinal fasciculus to their somata, near the contralateral fifth nerve motor nucleus. The same somata were labeled by horseradish peroxidase injected extracellularly into the axon cap. Recordings were made in the axon cap and the M cell after stimulation of hindbrain areas near the spiral fiber somata and axons. Extracellularly, a negative potential was observed close to the termination of the spiral fibers and termed the spiral fiber potential (SFP). Intracellularly, a graded, short latency depolarization of the M cell corresponded to the SFP and could cause the M cell to spike. This depolarization did not shunt the membrane, indicating that it may be produced through gap junctions. Intracellular responses to hindbrain stimulation also had a chloride-dependent, second component that shunted the membrane during paired-pulse testing. This inhibitory second component was probably evoked by cells other than the spiral fiber cells themselves.

Animals↗
PubMed1994

Automatic detection of spontaneous synaptic responses in central neurons.

A fully automatized software package for detection and measurements of randomly occurring synaptic transients embedded in background noise is described. It is based on waveform recognition protocols, allows analysis of long data segments, and provides quantitative information about event amplitudes and kinetics. Simulated postsynaptic recordings have been used to assess its performance over a wide range of conditions mimicking those seen in physiological experiments.

Animals↗
PubMed1994

Synchronous bursting in a subset of interneurons inhibitory to the goldfish Mauthner cell: synaptic mediation and plasticity.

1. Presynaptic activity in the inhibitory network impinging on the Mauthner (M-) cell was investigated in the goldfish medulla in vivo using extra- and intracellular recordings. The inhibitory presynaptic volley elicited by stimulation of the contralateral vestibular nerve consisted of multiple successive peaks at high frequency (up to 1,000 Hz). Less pronounced multicomponent responses were recorded after antidromic activation of the M-cell. Such high-frequency "oscillatory" field potentials also occurred spontaneously. 2. In intracellular recordings, a subset of inhibitory interneurons showed evoked and spontaneous burst discharge. Burst action potentials were correlated with the peaks in the extracellular volley, suggesting that repetitive firing of these cells is synchronized. Nonbursting cells, on the other hand, fired single action potentials in response to vestibular stimuli and were not activated via the M-cell collateral network. 3. Bursting cells were determined morphologically to be part of the feedback inhibitory circuit. Their responses to stimulation of the contralateral vestibular nerve thus suggest the existence of a crossed excitatory pathway to these interneurons. 4. Vestibular-evoked excitatory postsynaptic potentials (EPSPs) in bursting interneurons had a short latency of 0.781 +/- 0.08 ms (mean +/- SD, n = 18) but reached threshold at 2.25 +/- 1 ms (n = 21). These characteristics are suggestive of a chemically mediated EPSP. Indeed, the evoked synchronous repetitive activity of these cells was prevented by superfusion with excitatory amino-acid receptor antagonists. 5. Bursting neurons showed several characteristics that differentiate them from nonbursting cells, including brief action potentials, plateau responses, and intense spontaneous subthreshold activity. 6. With extracellular recordings, tetanization of contralateral vestibular primary afferents evoked a long-lasting potentiation of oscillatory population responses in 11 of 27 cases. Furthermore in three experiments, the frequency of occurrence of spontaneous bursts was enhanced and a similar facilitation was detected at the intracellular level. 7. We conclude that a subset of interneurons in this inhibitory network is capable of repetitive discharges and that evoked as well as spontaneous firing in this population is synchronized. Although electrical coupling between interneurons may mediate synchronization and intrinsic membrane properties may promote burst activity, our data suggest strongly that repetitive firing requires chemically mediated transmission. Furthermore they indicate that the mechanisms underlying evoked as well as spontaneous bursting in this population show activity-dependent plasticity.

Animals↗
PubMed1994

Colocalization of somatostatin with GABA or glutamate in distinct afferent terminals presynaptic to the Mauthner cell.

The presence of somatostatin in afferent fibers impinging on the goldfish Mauthner (M-) cell was determined using immunohistochemical methods, combined with confocal and electron microscopy, and the relationship of this peptide with inhibitory and excitatory terminals was studied. Somatostatin-reactive boutons were present only on the distal part of the M-cell's lateral dendrite. Somatostatin immunoreactivity was observed in typical large myelinated club endings (LMCEs) corresponding to mixed (electrical and chemical) eighth nerve primary afferent fibers. The axoplasm of these fibers contained dense-core vesicles (DCVs) dispersed among round vesicles. We have made a novel finding that the excitatory transmitter glutamate is present in LMCEs. Colocalization of this amino acid with somatostatin was detected in 75% of these endings using postembedding staining with gold particles of various sizes. The other structures labeled by somatostatin antibody were found to be small vesicle boutons (SVBs), which establish symmetrical synapses and contain a population of pleiomorphic vesicles with DCVs scattered among them. Double labeling with antibodies against glutamic acid decarboxylase and GABA allowed the definition of three types of biochemically characterized terminals: [somatostatin-GABA], [GABA], and [somatostatin]. However, the occurrence of DCVs in SVBs stained for GABA alone suggests that neuropeptides other than somatostatin may also coexist with GABA in this class of boutons. The coexistence of somatostatin with both inhibitory and excitatory neurotransmitters acting on the same region of a postsynaptic cell is discussed in relation to the role postulated for this peptide in synaptic plasticity.

Afferent Pathways↗
PubMed1994

Defective transmembrane calcium influx demonstrated in a primary immunodeficiency by video-imaging.

A rise of intracellular calcium concentration triggered by the engagement of various membrane receptors is a key event in the control of cell growth. This increase involves both a release of calcium from intracellular stores and the opening of a transmembrane calcium conductive pathway. Using video imaging to measure intracellular calcium concentration in individual fura-2-loaded cells, we detected a defect in calcium influx in lymphocytes and fibroblasts collected from patients affected by a rare and new form of primary immunodeficiency. In these cells, pharmacological agents such as thapsigargin or ionomycin, and the physiological activator bradykinin, only induced transient increases in cytoplasmic calcium level, due to the emptying of internal stores, while in control cells, this initial step is followed by an additional and sustained transmembrane calcium influx. The fact that calcium influx is absent in patient's fibroblasts indicates that the related deficiency, which is clinically associated with a lack of proliferation of T lymphocytes, also affects cells of the non-hematopoietic lineages. This study emphasizes the adequacy of single cell imaging for determining whether some forms of pathologies are associated with a disregulation of ionic fluxes, and for identifying them accurately.

B-Lymphocytes↗
PubMed1993

Heterogeneous distribution of glycinergic and GABAergic afferents on an identified central neuron.

Immunocytochemical methods were used on serial sections to study the glycine- and gamma-amino butyric acid (GABA)ergic innervations of the teleost Mauthner (M) cell. We found different distributions for the boutons containing the two amino acids. Endings filled with GABA predominate on the distal portion of the lateral dendrite (LD) while glycine-positive profiles are more abundant on the soma and within the axon cap (AC), a specialized neuropil surrounding the M-cell initial segment. A few endings containing both transmitters are present on the soma and on the small dendrites issuing ventrally from it. At this level some glutamic acid decarboxylase (GAD)-containing boutons face glycine receptor-93 kD-associated protein, an observation suggesting that the associated glycine functions as a neurotransmitter. Elsewhere on the M-cell, where glycine and GABA are not colocalized, GAD-positive profiles were never observed in front of postsynaptic differentiations with 93 kD labelling. GABA was detected in the small vesicle boutons (SVBs), most of them, following the classification of Tuttle et al., J. Comp. Neurol. 265:254-274, 1987, belonging to the A-type, while glycine was found in the unmyelinated club endings in the AC, and in C- and B-type SVBs, outside this region. All terminals established symmetrical synapses and were filled with a population of pleiomorphic vesicles. Boutons with GABA also contained numerous dense-core vesicles suggesting the presence of an associated peptide(s). A quantitative study of the transmitter content based on the number of the gold particles revealed a variable intensity of the labelling over certain profiles. For GABA, it was maximum at the tip of the LD and it decreased proximally. In contrast, the staining density was constant for glycine along all parts of the cell, except for the ventral dendrite (VD) where it decreased progressively. Taken together, these data suggest that the amino acid content varies, depending upon the location of the synapses on their target neuron.

Afferent Pathways↗
PubMed1993

Pattern of potassium channel expression in proliferating B lymphocytes depends upon the mode of activation.

Ionic channel expression is highly regulated during mitogenesis. But it is not clear whether these regulations only follow intrinsic programs during the course of the cell cycle or if they also depend upon the external factors used to promote cell activation. B lymphocytes express two classes of potassium channels and can be stimulated to enter the cell cycle by distinct pathways. Thus, we have analyzed, with the patch-clamp technique, if the expression of channels varies when the cells are activated by different signals that lead to cell proliferation. We found that stimulation through Ag receptors increases the expression of calcium- and voltage-activated potassium channels, whereas a bacterial mitogen, LPS, only enhances the expression of the latter. Moreover, channel expression can still be modified in proliferating cells because stimulation of LPS-activated cells through Ag receptors induces rapid expression of calcium-activated channels. The use of inhibitors of mRNA synthesis revealed that this process depends upon gene transcription. Thus, differential induction of the expression of potassium channels is not only linked to the entry into the cell cycle but depends also on pathways of stimulation.

Animals↗
PubMed1993

Cross-linking of IgG receptors inhibits membrane immunoglobulin-stimulated calcium influx in B lymphocytes.

By cross-linking membrane immunoglobulins (mIg), the antigenic stimulation of B lymphocytes induces an increase in intracellular free calcium levels ([Ca2+]i) because of a combination of release from intracellular stores and transmembrane influx. It has been suggested that both events are linked, as in a number of other cases of receptor-induced increase in [Ca2+]i. Conversely, in B lymphocytes, type II receptors for the Fc fragment of IgG (Fc gamma RII) inhibit mIg-mediated signaling. Thus, we have investigated at the level of single cells if these receptors could act on specific phases of mIg Ca2+ signaling. Lipopolysaccharide-activated murine B splenocytes and B lymphoma cells transfected with intact or truncated Fc gamma RII-cDNA were used to determine the domains of Fc gamma RII implicated in the inhibition of the Ca2+ signal. [Ca2+]i was measured in single fura-2-loaded cells by microfluorometry. The phases of release from intracellular stores and of transmembrane influx were discriminated by using manganese, which quenches fura-2, in the external medium as a tracer for bivalent cation entry. The role of membrane potential was studied by recording [Ca2+]i in cells voltage-clamped using the perforated patch-clamp method. Cross-linking of mIgM or mIgG with F(ab')2 fragments of anti-Ig antibodies induced a sustained rise in [Ca2+]i due to an extremely fast and transitory release of Ca2+ from intracellular stores and a long lasting transmembrane Ca2+ influx. The phase of influx, but not that of release, was inhibited by membrane depolarization. The increase in [Ca2+]i occurred after a delay inversely related to the dose of ligand. Co-cross-linking mIgs and Fc gamma RII with intact anti-Ig antibodies only triggered transitory release of Ca2+ from intracellular stores but no Ca2+ influx, even when the cell was voltage-clamped at negative membrane potentials. These transitory Ca2+ rises had similar amplitudes and delays to those induced by cross-linking mIgs alone. Thus, our data show that Fc gamma RII does not mediate an overall inhibition of mIg signaling but specifically affects transmembrane Ca2+ influx without affecting the release of Ca2+ from intracellular stores. Furthermore, this inhibition is not mediated by cell depolarization. Thus, Fc gamma RII represents a tool to dissociate physiologically the phases of release and transmembrane influx of Ca2+ triggered through antigen receptors.

Animals↗
PubMed1993

Synaptic noise and multiquantal release at dendritic synapses.

1. The quantal nature of inhibitory synaptic noise recorded intracellularly from the lateral dendrite of the goldfish Mauthner cell was studied, using new detection and measurement procedures that eliminate operator intervention. In addition, we employed an analytical algorithm, not previously applied to this problem, which treats composite amplitude distributions as mixtures of gaussians of unknown separations and variances. 2. As in the soma of this neuron, the dendritic inhibitory noise is quantal, with the exception that in the dendrite multiple equally spaced classes may persist in the presence of tetrodotoxin (TTX), an observation that may be correlated with the finding that the inhibitory afferents at this level often contain more than one release site. The validity of the analysis was confirmed by superfusing with saline containing low calcium and high magnesium, which reduces composite histograms that are gaussian mixtures to a single class, equal in amplitude to that of the first component detected in the control. 3. These results suggest that spontaneous exocytotic events may be synchronized at adjacent active zones within single terminals and that lowering the probability of release by reducing calcium may then be a more effective method for isolating single miniature events than is TTX.

Animals↗
PubMed1992

Dopamine enhances both electrotonic coupling and chemical excitatory postsynaptic potentials at mixed synapses.

The transmitter dopamine reduces electrotonic coupling between retinal horizontal cells and increases their sensitivity to glutamate. Since in other systems single afferents establish mixed electrotonic and chemical excitatory synapses with their targets, dopamine might be expected there to depress one component of excitation while enhancing the other. This hypothesis was tested by applying dopamine locally in the vicinity of the lateral dendrite of the goldfish Mauthner cell (M cell) and monitoring the composite electrotonic and chemical excitatory postsynaptic potentials and currents evoked by ipsilateral eighth nerve stimulation. Dopamine produces persistent enhancements of both components of the postsynaptic response while it also increases input conductance. All these dopamine actions are prevented by superfusing the brain with saline containing the dopamine D1 receptor antagonist SCH-23390. Postsynaptic injections of the cAMP-dependent protein kinase inhibitor (Walsh inhibitor, or PKI5-24) block the dopamine-induced changes in synaptic transmission, implicating a cAMP-dependent mechanism. Furthermore, there is a dopaminergic innervation of the M cell, as demonstrated immunohistochemically with antibodies against dopamine and the rate-limiting enzyme in its synthetic pathway, tyrosine hydroxylase. Varicose immunoreactive fibers lie in the vicinity of the distal part of the lateral dendrite between the large myelinated club endings that establish the mixed synapses. As determined with electron microscopy, the dopaminergic fibers contain small vesicles, and they do not have synaptic contacts with either the afferents or the M cell, remaining instead in the synaptic bed. Taken together, these results suggest that dopamine released at a distance from these terminals increases the gain of this primary sensory input to the M cell, most likely through a phosphorylation mechanism.

Animals↗
PubMed1992

Intrinsic quantal variability due to stochastic properties of receptor-transmitter interactions.

Synaptic events at the neuromuscular junction are integer multiples of a quantum, the postsynaptic response to transmitter released from one presynaptic vesicle. At central synapses where quanta are small, it has been suggested they are invariant due to occupation of all postsynaptic receptors, a concept neglecting inherent fluctuations in channel behavior. If this did occur, the quantal release model would not apply there and could not be used to localize sites of synaptic modification. Monte Carlo simulations of quanta include transmitter diffusion and interactions with postsynaptic receptors that are treated probabilistically. These models suggest that when there are few postsynaptic channels available at a synapse, their stochastic behavior produces significant intrinsic variance in response amplitude and kinetics, and saturation does not occur. These results were confirmed by analysis of inhibitory quanta in embryonic and adult Mauthner cells involving a small and large number of channels, respectively. The findings apply to excitatory synapses as well.

Animals↗
PubMed1992

Differential regulation of voltage- and calcium-activated potassium channels in human B lymphocytes.

The expression and characteristics of K+ channels of human B lymphocytes were studied by using single and whole-cell patch-clamp recordings. They were gated by depolarization (voltage-gated potassium current, IKv, 11-20 pS) and by an increase in intracellular Ca2+ concentration (calcium-activated potassium current, IKCa, 26 pS), respectively. The level of expression of these channels was correlated with the activational status of the cell. Both conductances are blocked by tetraethylammonium, verapamil, and charybdotoxin, and are insensitive to apamin; 4-aminopyridine blocks IK, preferentially. We used a protein kinase C activator (PMA) or antibodies to membrane Ig (anti-mu) to activate resting splenocytes in culture. Although IKv was recorded in the majority of the resting lymphocytic population, less than 20% of the activated cells expressed this conductance. However, in this subset the magnitude of IKv was 20-fold larger than in resting cells. On the other hand, IKCa was detected in nearly one half of the resting cells, whereas all activated cells expressed this current. The magnitude of IKCa was, on average, 30 times larger in activated than in nonactivated cells. These results probably reflect that during the course of activation 1) the number of voltage-dependent K+ channels per cell decreases and increases in a small subset and 2) the number of Ca(2+)-dependent K+ channels per cell increases in all cells. We suggest that the expression of functional Ca(2+)- and voltage-activated K+ channels are under the control of different regulatory signals.

B-Lymphocytes↗
PubMed1992

Long-term potentiation of inhibitory circuits and synapses in the central nervous system.

Glycinergic inhibition evoked disynaptically in the teleost Mauthner cell by stimulation of the contralateral eighth nerve exhibits long-term potentiation following classical tetanization of that pathway. This enhancement occurs at the synapses between primary afferents onto second-order interneurons and the connections between these inhibitory cells and the Mauthner neuron. The evidence for modifications of glycinergic transmission is that the slope of the relation between the presynaptic volley and the synaptic conductance can be greater after the tetanus. This increase in gain is still manifest after pharmacological block of potentiation at the excitatory synapse with glutamate antagonists. Inhibitory long-term potentiation is induced by tetani weaker than those required for enhancement of the monosynaptic excitation of the other (ipsilateral) Mauthner cell. Thus, in vivo learning can alter the balance between excitation and inhibition within a network by modifying one or both of them.

Animals↗
PubMed1992

Calcium influx through nicotinic receptor in rat central neurons: its relevance to cellular regulation.

The Ca2+ permeability of a nicotinic acetylcholine receptor (nAChR) in the rat CNS was determined using both current and fluorescence measurements on medial habenula neurons. The elementary slope conductance of the nAChR channel was 11 pS in pure external Ca2+ (100 mM) and 42 pS in standard solution. Ca2+ influx through nAChRs resulted in the rise of cytosolic Ca2+ concentration ([Ca2+]i) to the micromolar range. This increase was maximal under voltage conditions (below -50 mV) in which Ca2+ influx through voltage-activated channels was minimal. Ca2+ influx through nAChRs directly activated a Ca(2+)-dependent Cl- conductance. In addition, it caused a decrease in the GABAA response that outlasted the rise in [Ca2+]i. These results underscore the physiological significance of Ca2+ influx through nAChR channel in the CNS.

Animals↗