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D Swandulla

Publications and source records attributed to D Swandulla.

At least 37 records · Page 2Linked to original sources
PubMed1994

Modulation of fast excitatory synaptic transmission by cyclothiazide and GYKI 52466 in the rat hippocampus.

The effects of cyclothiazide, a drug which potentiates AMPA receptor-mediated responses and GYKI 52466, a non-competitive AMPA receptor antagonist, were studied on fast glutamatergic transmission in rat hippocampal slices. Cyclothiazide prolonged the decay of AMPA receptor-mediated EPSCs (AMPA-EPSCs) in a concentration-dependent manner. GYKI 52466 reduced the peak amplitude of AMPA-EPSCs and blocked the induction of LTP. When GYKI 52466 was applied in the presence of cyclothiazide it still reduced the peak amplitude of AMPA-EPSCs but was not able to reverse the cyclothiazide induced prolongation of AMPA-EPSC duration. These data suggest that GYKI 52466 and cyclothiazide probably mediate their effects on the AMPA receptor via different binding sites.

2-Amino-5-phosphonovalerate↗
PubMed1994

Calcium-activated non-selective channels in the nervous system.

In the decade, since the first description of calcium-activated non-selective (CAN) channels in cardiac myocytes, pancreatic acini and neuroblastoma, this type of channel has been shown to have a ubiquitous distribution across a variety of tissues. Recently, their role in the function of cells of the nervous system has become better delineated. Because CAN channels pass depolarizing current, respond to cytoplasmic Ca2+ activity and do not inactivate, they are capable of producing maintained depolarization of neurons. This property endows upon CAN channels an important role in both physiological functions and pathological processes of the nervous system.

Animals↗
PubMed1994

Molecular pathways for presynaptic calcium signaling.

The results presented in this article describe two distinct, Ca-regulated molecular pathways in presynaptic terminals and implicate these two pathways in differentially mediating neurotransmitter secretion and PTP. Our current view of the Ca-dependent triggering of secretion and PTP is shown in Fig. 9. According to this scheme, differential activation of these two pathways is achieved by a combination of diffusion-based dilution of Ca that enters the terminal through voltage-gated Ca channels and by coupling these pathways to Ca receptors with different affinities for Ca ions. A simple way to achieve these conditions is to position these two receptors at different distances from the Ca channels, as shown in Fig. 2. Given that Ca ions are involved in activating many different presynaptic processes (Fig. 1), we propose that closer scrutiny of the molecular physiology of nerve terminals will reveal a wide variety of Ca-activated pathways responsible for producing these diverse processes.

Amino Acid Sequence↗
PubMed1993

Molecular properties of the glutamate receptor mediating synaptic excitation in rat hypothalamic neurons.

AMPA-type glutamate receptors (GluRs) mediate synaptic excitation in networks of cultured rat hypothalamic neurons [18, 25]. Under voltage clamp the agonists quisqualate and AMPA induce current responses which consist of a maintained and/or transient component depending on the concentrations applied. The current-voltage relationship for both components is linear. The biphasic response patterns are due to receptor desensitization which is fast and does not require intracellular second messengers for its activation. Several GluR-subtype-encoding transcripts were found in these neurons using polymerase chain reaction (PCR) methods. While mRNAs encoding the GluR2 and 3 flip forms are expressed early, mRNAs encoding the GluR1, 2 and 3 flop forms and the GluR4 flip form appear only in cultures older than 3 weeks. By comparison to recombinant receptors, the properties of the native receptor can be accommodated by a heteromeric receptor containing GluR2 as one of the subunits.

Animals↗
PubMed1993

Characterization of neurochemical phenotypes in cultured hypothalamic neurons with immunohistochemistry and in situ hybridization.

The expression of neurochemical phenotypes was studied in long-term cultures of dissociated embryonic neurons from rat hypothalamus. With time in culture, these neurons establish a complex dendritic and axonal network, as indicated by staining with antibodies against microtubulin-associated protein (MAP2) and neurofilaments (SMI32 and SMI33) as well as GABA and glutamate decarboxylase mRNA immunoreactivity. Neurons expressing neuropeptide Y (NPY) mRNA and NPY peptide and opioid-like peptides as well as vasopressin were observed. Further, weakly acetylcholinesterase- and NADPH diaphorase (nitric-oxide synthase)-labelled neurons were present. In conclusion, the neurochemical phenotypes reported for hypothalamic neurons in vivo can be observed in these cultures. This indicates that the culture conditions allow morphological and molecular differentiation. These findings support the view that long-term hypothalamic cultures provide a valuable model for studying mechanisms of neurosecretion in hypothalamic networks.

Acetylcholinesterase↗
PubMed1993

Calcium buffering in bursting Helix pacemaker neurons.

Bursting pacemaker neurons of the snail Helix pomatia were voltage-clamped and Ca currents in response to depolarizing steps were recorded. Simultaneously, changes in intracellular Ca concentrations were measured using the fluorescent dye fura-2 and a highly sensitive digital camera. Ca influx through voltage-gated channels induced a spatially non-uniform increase in intracellular Ca. The Ca signals decayed with a time constant of about 5 s. By increasing the concentration of the indicator dye, its Ca-buffering capacity was enhanced and Ca transients in response to depolarization were diminished. Thereby, the endogenous Ca buffer capacity could be determined and was calculated to be about 480 buffered ions for every free Ca ion. The buffer capacity did not vary significantly with the amount of Ca influx within the range tested, suggesting that the buffer is not saturated at Ca concentrations of up to 1 microM.

Animals↗
PubMed1993

Inhibition of high voltage-activated calcium currents by L-glutamate receptor-mediated calcium influx.

The modulation of high voltage-activated (HVA) Ca2+ currents by L-glutamate and its agonists was investigated in cultured rat hypothalamic neurons. L-Glutamate and agonists selective for NMDA or non-NMDA receptors reversibly inhibited HVA Ca2+ currents. The putative presynaptic glutamate receptor agonist L-2-amino-4-phosphonobutyric acid and the selective metabotropic agonist trans-ACPD were ineffective. Inhibition was dependent on the presence of extracellular Ca2+ and blocked by internal perfusion of the cells with BAPTA. The calmodulin antagonists trifluoperazine and calmidazolium completely prevented the inhibition. Increases in the intracellular Ca2+ concentration due to Ca2+ influx through non-NMDA receptor channels were visualized using fura-2. These results indicate that not only NMDA but also non-NMDA receptor channels in these neurons are permeable for Ca2+ and that Ca2+ influx through these channels activates a calmodulin-dependent mechanism, which leads to HVA Ca2+ current inhibition.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1992

Differential effects of ketamine enantiomers on NMDA receptor currents in cultured neurons.

The effects of R- and S-ketamine on N-methyl-D-aspartate receptor-activated cation currents (NMDA receptor currents) of voltage-clamped cultured rat hippocampal neurons were investigated using the whole-cell patch-clamp technique. Both enantiomers exhibited a voltage- and use-dependent blockade of NMDA receptor currents, with the S-enantiomer being about twice as potent as the R-enantiomer. Calculated relative forward and backward rates suggest that conformational differences influence the dissociation from the binding site more than the association with it.

Animals↗
PubMed1992

Presynaptic calcium signals during neurotransmitter release: detection with fluorescent indicators and other calcium chelators.

Synthetic calcium buffers, including fluorescent calcium indicators, were microinjected into squid 'giant' presynaptic nerve terminals to investigate the calcium signal that triggers neurotransmitter secretion. Digital imaging methods, applied in conjunction with the fluorescent calcium indicator dye fura-2, reveal that transient rises in presynaptic calcium concentration are associated with action potentials. Transmitter release terminates within 1-2 ms after a train of action potentials, even though presynaptic calcium concentration remains at micromolar levels for many seconds longer. Microinjection of the calcium buffer, EGTA, into the presynaptic terminal has no effect on transmitter release evoked by single presynaptic action potentials. EGTA injection does, however, block the change in calcium concentration measured by fura-2. Therefore, the calcium signal measured by fura-2 is not responsible for triggering release. These results suggest that the rise in presynaptic calcium concentration that triggers release must be highly localized to escape detection with fura-2 imaging. Unlike EGTA, microinjection of BAPTA--a calcium buffer with an equilibrium affinity for calcium similar to that of EGTA--produces a potent, dose-dependent, and reversible block of action-potential evoked transmitter release. The superior ability of BAPTA to block transmitter release apparently is due to the more rapid calcium-binding kinetics of BAPTA compared to EGTA. Because EGTA should bind calcium within a few tens of microseconds under the conditions of our experiments, the inability of EGTA to block release indicates that transmitter release is triggered within a few tens of microseconds after the entry of calcium into the presynaptic terminal.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Ionic currents in cultured rat hypothalamic neurones.

1. Dissociated neurones from embryonic rat hypothalamus were grown for several weeks in culture where they formed complex networks. These synaptically coupled networks were capable of generating synchronized bursting activity. Voltage-activated membrane currents were studied in these neurones using a patch clamp in the whole-cell configuration. 2. Outward currents were carried by K+ ions and consisted of an inactivating and a non-inactivating component. These components were similar to the transient K+ current (IA) and the delayed rectifier current (IK) reported in neurones from the postnatal rat hypothalamus. Application of Zn2+ (1 mM) blocked the transient component completely while reducing the non-inactivating component by only approximately 20%. 3. Inward currents were carried by Na+ and Ca2+ ions. Rapidly activating transient Na+ currents were activated at approximately -25 mV. TTX entirely blocked these currents at low concentration (300 nM). Voltage sensitivity of the Na+ conductance was 5.8 mV per e-fold change with half-maximal activation occurring at -8 mV. Na+ current kinetics could be well described by the Hodgkin-Huxley model (m3h). 4. With depolarizing pulses from a holding potential of -80 mV two Ca2+ current components with different ranges of activation were identified. Low voltage-activated (LVA, T-type) Ca2+ currents were activated at approximately -50 mV. High voltage-activated (HVA; also called L- or N-type) Ca2+ currents were observed at membrane potentials more positive to approximately -30 mV. LVA Ca2+ currents were observed in hypothalamic neurones that had developed a network of dendritic processes in the course of several weeks in culture. Activation and inactivation time constants of LVA Ca2+ currents were 15-25 ms and 30-100 ms (-30 to -45 mV). In contrast to HVA Ca2+ currents, no LVA Ca2+ currents were seen in neuronal somata obtained from the network cultures by mechanical dissociation. This suggests that most of the LVA Ca2+ channels are located on the dendritic tree rather than on the soma membrane. 5. HVA Ca2+ currents were maximal between 0 and +10 mV (external [Ca2+] = 5 mM). The time-to-peak was in the range of 1.7-5.4 ms (+30 to -10 mV). Tail currents following repolarization decayed monoexponentially with a time constant of approximately 210 microseconds. During 500 ms depolarizations, 90% of the current inactivated. The time course of inactivation showed two time constants of approximately 40 and approximately 700 ms.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Burst activity and synaptic mechanisms in a hypothalamic network grown in culture.

In a cultured network of rat embryonic hypothalamic cells, synaptic interaction is through GABAA-receptors, that mediate inhibition by an increase in Cl- conductance, and AMPA-receptors, that mediate excitation by an increase in monovalent cationic conductance. Changes in the balance of inhibition and excitation towards a predominance of excitation lead to phasic synchronous activity of the cells. Synaptic interaction through these receptors is thus capable of modulating neurosecretion rapidly.

Action Potentials↗
PubMed1991

Do calcium channel classifications account for neuronal calcium channel diversity?

Calcium (Ca2+) ions are involved in the development and control of a variety of neuronal properties and functions such as channel expression, synaptic transmission and neurosecretion. The main pathway by which Ca2+ enters the intracellular space is through voltage-activated Ca2+ channels that can be classified according to their different biophysical and pharmacological properties. Identification and characterization of these channel types are prerequisites for understanding the mechanisms that underlie Ca2(+)-controlled processes. In this article we summarize the efforts made to identify neuronal Ca2+ channel types, and we attempt to evaluate how useful existing classifications are in assigning specific properties and functions to distinct channel types in neurons.

Animals↗
PubMed1991

Role of residual calcium in synaptic depression and posttetanic potentiation: fast and slow calcium signaling in nerve terminals.

Trains of action potentials evoked rises in presynaptic Ca2+ concentration ([Ca2+]i) at the squid giant synapse. These increases in [Ca2+]i were spatially nonuniform during the trains, but rapidly equilibrated after the trains and slowly declined over hundreds of seconds. The trains also elicited synaptic depression and augmentation, both of which developed during stimulation and declined within a few seconds afterward. Microinjection of the Ca2+ buffer EGTA into presynaptic terminals had no effect on transmitter release or synaptic depression. However, EGTA injection effectively blocked both the persistent Ca2+ signals and augmentation. These results suggest that transmitter release is triggered by a large, brief, and sharply localized rise in [Ca2+]i, while augmentation is produced by a smaller, slower, and more diffuse rise in [Ca2+]i.

Action Potentials↗
PubMed1990

Modulation of calcium-activated non-specific cation currents by cyclic AMP-dependent phosphorylation in neurones of Helix.

1. Currents through calcium-activated non-specific cation (CAN) channels were studied in the fast burster neurone of Helix aspersa and Helix pomatia. CAN currents were activated by reproducible intracellular injections of small quantities of Ca2+ utilizing a fast, quantitative pressure injection technique. 2. External application of forskolin (10-25 microM), an activator of adenylate cyclase, caused the endogenous bursting activity of the cells to be replaced by beating activity. These same concentrations of forskolin reduced CAN currents reversibly to about 50%. 3. External application of IBMX (3-isobutyl-1-methylxanthine, 100 microM), an inhibitor of phosphodiesterase, the enzyme which breaks down cyclic AMP, reduced CAN currents reversibly to about 40%. 4. External application of the membrane-permeable cyclic AMP analogues 8-bromo-cyclic AMP and dibutyryl-cyclic AMP (100 microM) caused almost complete block of the CAN current. A marked reduction in the CAN current was also observed following quantitative injections of cyclic AMP (internal concentrations up to 50 microM) directly into the cells from a second pressure injection pipette. 5. Similar results were obtained with quantitative injections of the catalytic subunit (C-subunit) of the cyclic AMP-dependent protein kinase (internal concentrations 10(-4) units of enzyme) directly into the cells from a second pressure injection pipette. 6. Injection of the non-hydrolysable GTP analogue, GTP-gamma-S (internal concentrations 100 microM), which stimulates G-proteins, produced a prolonged increase in CAN current amplitude by as much as 300%. 7. External application of serotonin (100-200 microM) caused a transition from bursting to beating activity of the neurones and mimicked cyclic AMP's effects on CAN currents. Two other neurotransmitters, dopamine and acetylcholine, were not significantly effective in reducing CAN currents. 8. Injection of a peptide inhibitor of cyclic AMP-dependent protein kinase suppressed serotonin's action on bursting and on CAN current. 9. Our results indicate that CAN currents in Helix burster neurones are modulated by cyclic AMP-dependent membrane phosphorylation. They suggest that the physiological transmitter that induces this second messenger action is serotonin. The dual control of CAN channels by two second messengers, namely Ca2+ and cyclic AMP, has important functional implications. While Ca2+ activates these channels which generate the pacemaker current in these neurones, cyclic AMP-dependent phosphorylation down-regulates them, thereby resulting in modulation of neuronal bursting activity.

1-Methyl-3-isobutylxanthine↗
PubMed1990

Development and properties of synaptic mechanisms in a network of rat hypothalamic neurons grown in culture.

1. Dissociated neurons from embryonic rat hypothalamus (E14-15) were cultured on a glial background monolayer for up to three months. Dendrites of cells 7-14 days in culture (DIC) intracellularly stained with the fluorescent dye Lucifer yellow were thin and smooth, and multiple growth cones could be observed. The length of the dendrites of older cells did not differ much, but dendrites were thicker and branched more profoundly, forming a complicated network. Growth cones were rare, but few spine-like protrusions could be observed. 2. Randomly occurring depolarizing potentials were recorded in 60% of the cells 7-14 DIC and in 90% of the cells 21 DIC. Activity became phasic when the gamma-aminobutyric acid (GABA) antagonists picrotoxin or bicuculline were applied. After 21 DIC the majority of the cells showed burst discharges, whereas only approximately 10% of the cells 7 DIC exhibited bursting. 3. With low [Cl] in the recording pipette, spontaneous activity consisted of hyperpolarizing and depolarizing potentials at -40-mV membrane potential. Some spontaneous activity persisted with Na channels blocked by tetrodotoxin (TTX, 0.3-1 microM), and when reducing the external [Ca]o from 5 to 0.3 mM. Picrotoxin blocked part of the activity, and the remaining activity was blocked by kynurenic acid. 4. Bursts of action potentials were superimposed on rhythmically occurring clusters of excitatory synaptic potentials (EPSPs), which had a steep rising phase and decayed within hundreds of milliseconds. Bursts of similar appearance could be triggered by brief (10 ms) depolarizing current injections, and a few cells had properties indicative for endogenous pacemakers. 5. From 7 DIC on, all cells responded to GABA and to the GABA agonist muscimol. Under voltage clamp, zero current potential depended on the Cl gradient across the membrane and corresponded to the zero current potential of picrotoxin-sensitive postsynaptic currents. 6. After 21 DIC all cells responded to glutamate and its agonist quisqualate. Under voltage clamp, nanomolar concentrations of quisqualate (100-500 nM) induced long-lasting inward currents, which did not decay substantially during prolonged drug application. Quisqualate concentrations greater than 1 microM induced a diphasic inward-current response consisting of an initial fast current transient followed by a maintained current component. With internal Cs replacing K and Na and external TTX (0.3 microM), both current components reversed sign at approximately 8 mV, as predicted by the Nernst equation for currents through channels that were permeable for monovalent cations. 7. Focal applications of GABA and muscimol elicited larger currents when applied near the soma than when applied to the dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1989

Quisqualate receptor-mediated rhythmic bursting of rat hypothalamic neurons in dissociated cell culture.

Dissociated hypothalamic neurons from embryonic rat brain exhibit a level of spontaneous synaptic activity after 21 days in culture. When GABA-mediated responses are blocked by picrotoxin or bicuculline (20 microM), the neurons burst rhythmically. Rhythmic burst activity is generated in most cells by postsynaptic excitatory currents (EPSCs) through non-specific cationic channels rather than by intrinsic pacemaker currents. We present evidence that EPSCs are mediated by an excitatory amino acid and a quisqualate receptor type.

Action Potentials↗
PubMed1989

A diacylglycerol analogue reduces neuronal calcium currents independently of protein kinase C activation.

Diacylglycerol analogues (for example 1,2-oleoylacetylglycerol, OAG) and phorbol esters are activators of protein kinase C, and have been widely used to study the function of this enzyme in both intact cells and cell-free preparations. Electrophysiological studies have shown that these activators can either depress or increase Ca2+ currents, or decrease K+ currents when applied outside the cell. It has been assumed that these effects are mediated by protein kinase C activation. Here we report that micromolar levels of OAG and phorbol esters depress Ca2+ currents in chick sensory neurons independently of their effect as activators of protein kinase C. The depression of the Ca2+ current is rapid and is unaffected by intracellular application of the protein kinase C inhibitors staurosporin, sphingosine and H-7. Furthermore, the activators were ineffective when applied intracellularly, indicating that their site of action is on the outside of the membrane.

Action Potentials↗