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H D Lux

Publications and source records attributed to H D Lux.

At least 19 recordsLinked to original sources
PubMed1992

Evans blue reduces macroscopic desensitization of non-NMDA receptor mediated currents and prolongs excitatory postsynaptic currents in cultured rat thalamic neurons.

Fast application of L-glutamate, AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) or kainate to cultured rat thalamic neurons revealed properties of non-NMDA (N-methyl-D-aspartate) receptors similar to those described in hippocampal neurons. The kinetics of non-NMDA receptor-mediated currents were altered by the addition of the dye Evans Blue (EB). Macroscopic desensitization was reduced and activation and deactivation kinetics were slowed. Delayed addition of EB, after desensitization of non-NMDA receptors, resulted in reactivation of desensitized receptors. Thus, both ion channel gating and entry into the desensitized state were affected. Evans blue also slowed the activation and the decay of glutamatergic miniature EPSCs (excitatory postsynaptic currents), demonstrating that receptor kinetics determine the time course of the synaptic response.

Animals↗
PubMed1992

Glutamate selectively increases the high-threshold Ca2+ channel current in sensory and hippocampal neurons.

Previous studies resulted in conflicting conclusions that glutamate application either decreases or increases the activity of Ca2+ channels in hippocampal neurons. We studied whole-cell Ca2+ currents (ICa) in chick dorsal root ganglion neurons and rat hippocampal cells. For both cell types glutamate (1-30 microM) increased high-threshold Ca2+ current. It was independent of the charge carriers, Ca2+ or Ba2+. Low-threshold Ca2+ channel current and the fast sodium current were not changed with glutamate application. The effect developed within 1-2 min and then further facilitated after washout of the agonist. A second application of glutamate produced no additional increase in ICa. No changes in the time-course of whole-cell currents were observed, suggesting that glutamate recruits 'sleepy' Ca2+ channels. Whatever its mechanism, overlasting increase of ICa by glutamate may be important in neuronal plasticity.

Animals↗
PubMed1992

The selective action of quinacrine on high-threshold calcium channels in rat hippocampal cells.

1. The whole-cell patch-clamp technique has been used to examine Ca channel currents carried by Ba (IBa) in rat hippocampal neurones. 2. Quinacrine selectivity decreased the high-threshold current activated by membrane depolarization from a holding potential of -70 mV. Neither the low-threshold Ca channel current nor the fast tetrodotoxin (TTX)-sensitive sodium current were affected by quinacrine. 3. Bath application of quinacrine caused a dose-dependent reduction of the peak amplitude of IBa. This effect was fast, voltage-independent, reversible and had a Kd of 30 +/- 5 microM. 4. The quinacrine-induced block did not change the time-course and the voltage dependence of IBa activation and deactivation. The inhibition revealed no use-dependence, ruling out an open channel block by quinacrine. 5. p-Bromophenacyl bromide had no effect on IBa suggesting the lack of involvement of phospholipase A2 in the action of quinacrine. In addition, the quinacrine-induced block was not related to the calmodulin pathway and internal quinacrine did not affect the peak amplitude of IBa. 6. The effect of quinacrine on the amplitude of IBa was dependent of the external pH, and suggested that only the single-protonated form of the drug can bind to the channel receptor with a Kd of 3 microM. Quinacrine and other substituted acridines can thus be useful for pharmacological and structure-activity studies of Ca channels.

Animals↗
PubMed1992

Pharmacological characterization of calcium currents and synaptic transmission between thalamic neurons in vitro.

We recorded from pairs of cultured, synaptically connected thalamic neurons. Evoked excitatory postsynaptic currents (EPSCs) reversed at +17 mV and were blocked reversibly by 1 mM kynurenic acid, a glutamate receptor antagonist. NMDA and non-NMDA receptors mediated excitatory post-synaptic responses, as shown by selective block of EPSC components with 50 microM (+/-)-2-amino-5-phosphonopentanoic acid and 10 microM 6,7-dinitroquinoxaline-2,3-dione, respectively. Inhibitory postsynaptic responses were evoked less frequently and were blocked by the GABAA receptor antagonist (-)-bicuculline methochloride. The pharmacological profiles of whole-cell calcium currents and evoked EPSCs were compared. With 50 microM cadmium chloride (Cd), whole-cell low voltage-activated (LVA) calcium currents were reduced in amplitude and high voltage-activated (HVA) calcium currents and excitatory synaptic transmission were completely blocked. This suggests that the residual calcium influx through LVA channels into the presynaptic terminal does not suffice to trigger transmitter release. A saturating concentration of omega-conotoxin GVIA (omega-CgTx) (2.5 microM) blocked one-third of whole-cell HVA calcium currents and evoked EPSCs. The dihydropyridine nifedipine (50 microM) reversibly reduced whole-cell HVA calcium currents in a voltage-dependent manner but not excitatory synaptic transmission. Cd and omega-CgTx did not alter amplitude distributions of miniature EPSCs, demonstrating that the inhibition of synaptic transmission was due to block of presynaptic calcium channels. We conclude that excitatory glutamatergic transmission in thalamic neurons in vitro was mediated mainly by HVA calcium currents, which were insensitive to omega-CgTx and nifedipine.

Animals↗
PubMed1991

Calmodulin antagonists and protein phosphatase inhibitor okadaic acid fasten the 'run-up' of high-voltage activated calcium current in rat hippocampal neurones.

Voltage-activated Ca2+ channel currents were recorded from cultured rat hippocampal neurones using the whole-cell clamp technique with Ba2+ as a charge carrier. After breaking into the cell the amplitude of low-voltage activated Ca2+ channel current increased to a new steady value within 1 min whereas several minutes were required for a full development of the high-voltage activated current (IHVA). Pretreatment of cells with calmodulin antagonists (trifluoperazine or W-13) or protein phosphatase inhibitor, okadaic acid, fastened the development of IHVA. Trifluoperazine (6-40 microM) also increased IHVA when applied after breaking into the cell in standard external solution. Incubation of cells in the presence of permeable precursor of Ca2+ chelator, BAPTA, was without effect. The effects of all inhibitors studied allow to suggest that IHVA in intact cells is largely masked due to activity of calmodulin-activated protein phosphatase.

Animals↗
PubMed1991

Cytoplasmic alkalinization increases high-threshold calcium current in chick dorsal root ganglion neurones.

Changes of calcium currents with intracellular pH (pHi) were investigated in chick dorsal root ganglion (DRG) neurones. High-threshold calcium currents decreased after extracellular application of a permeable weak acid, sodium acetate (CH3COONa), and increased when applying a permeable weak base, ammonium chloride (NH4Cl), whereas both compounds were ineffective against the low-threshold calcium current. These weak electrolytes, employed to change pHi, did not alter the kinetic and steady-state parameters of activation and inactivation of the calcium current. Extracellular application of concanavalin A (Con A) and wheat germ agglutinin to elevate pHi increased the high-threshold calcium current. Their effect developed within 2-5 min and was independent of lectin concentration varied from 0.1 to 1 mg/ml. The lectin effects were greatly diminished if Na/H exchange was blocked by amiloride or suppressed by low external sodium. Succinilated Con A and Con A in the presence of D-mannose were less effective. Calcium currents were recorded simultaneously with the pHi, monitored with a proton-sensitive microelectrode. It was found that 50% inhibition of the calcium current occurred at pHi of 6.5. Histidine-specific reagents--diethylpyrocarbonate, Rose Bengal and Methylene Blue--prevented the modulation of the calcium conductance by CH3COONa and NH4Cl. Extracellular baclofen and theophylline or intracellular phorbol esters, staurosporine, calmodulin antagonists R24571 and W-13, and neomycine failed to prevent the modulation of the calcium current by weak electrolytes. These observations are consistent with an interaction between intracellular protons and calcium channels.

Animals↗
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

Distribution of Ca2+ and Na+ conductances during neuronal differentiation of chick DRG precursor cells.

The distribution of Ca2+ and Na+ conductances on neuronal precursor cells was investigated during differentiation. Ionic conductances on the soma or on the growth cone were isolated by superfusing all other parts of the cells with sucrose. Conductances on the neuritic shaft were detected as additional conductances after removing sucrose from the neuritis shaft. Neuronal precursor cells were isolated from chick dorsal root ganglia by selectively killing differentiated neurons. Cultured precursor cells differentiated into morphological and functional mature neurons. Functionally undifferentiated precursor cells (during the first 10 hr in culture) expressed only low-voltage-activated (LVA) Ca2+ currents. High-voltage-activated (HVA) Ca2+ and Na+ currents appeared delayed after more than 10 hr in culture. Voltage-dependent conductances, if expressed by a cell, were present on all parts of the surface membrane at all stages of differentiation. LVA Ca2+ conductances were well represented on the growth cone as well as on the soma in functionally undifferentiated precursor cells. During differentiation of precursor cells, LVA Ca2+ and HVA Ca2+ as well as Na+ conductances were expressed on the somatic membrane, on the neuritic shaft, and on the growth cone. These results demonstrate the expression of Ca2+ channels on growth cones during differentiation.

Animals↗
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↗
PubMed1990

Neurons sensitive to pH in slices of the rat ventral medulla oblongata.

The effects of extracellular pH changes on neurons in slices of the rat ventral medulla oblongata were investigated by extracellular recording. Changes in discharge rate were correlated with pH changes in the tissue next to the recorded cell, as measured by H(+)-selective microelectrodes. pH was altered by varying the bicarbonate concentration ([HCO3-]) in the superfusion solution. In 136 out of 316 neurons, the number of spontaneous or electrically evoked discharges per unit time increased with decreasing pH and decreased with increasing pH. Changes of only 0.01-0.04 pH unit were effective in these pH-sensitive neurons. The response was transient; the discharge rate returned to the control value within a few minutes. The pH sensitivity persisted in the presence of 0.5 microM atropine, 20 microM bicuculline and after replacing Ca2+ by Mg2+ in the superfusion solution to reduce synaptic transmission. The response to the same pH decrease was stronger when increasing PCO2 than when reducing [HCO3-]0. The pH-induced response significantly increased during hypoxia. The results show that in the ventral medulla oblongata neurons exist that transiently respond to small decreases and increases of pH. The pH sensitivity is an intrinsic property of these neurons; it is not due to a synaptic mechanism but is modulated by PCO2 and PO2.

Animals↗
PubMed1990

Temperature sensitivity of Ca currents in chick sensory neurones.

We have investigated the effects of temperature on the Ca currents of chick sensory neurones. Raising the temperature from 17 to 37 degrees C, caused low-threshold (LVA, T) and high-threshold (HVA, L and N) Ca currents to show a marked amplitude increase and a drastic acceleration of their activation-inactivation gatings. Compared to HVA channels, the LVA type showed a weaker temperature sensitivity. Its average Q10 values were closer to those of other voltage-operated ion channels: 1.7 (permeability), 1.9 (activation) and 2.2 (inactivation). Alternatively, the activation kinetics and peak permeability of HVA Ca channels showed maximal Q10 values of about 5 and 2.8, respectively. HVA channel deactivation was less sensitive to temperature (Q10 1.8). Inactivation of these channels was slow and monoexponential between 17 and 22 degrees C, but faster and double exponential above 30 degrees C, uncovering a fast temperature-sensitive decaying phase. The size and rate of decay of this component decreased with increasing membrane depolarizations and persisted at holding potentials positive to -80 mV, suggesting the involvement of temperature-sensitive Ca-mediated processes in the mechanism of HVA channel inactivation. Our data are consistent with the view that heating from 17 to 37 degrees C causes both an increased probability of Ca channels to open and a drastic acceleration of their activation-inactivation kinetics.

Animals↗
PubMed1990

Na+ currents through low-voltage-activated Ca2+ channels of chick sensory neurones: block by external Ca2+ and Mg2+.

1. Whole-cell currents through low-voltage-activated (LVA) Ca2+ channels carried by monovalent cations were studied in chick dorsal root ganglion (DRG) cells. 2. With 120 mM [Na+] on both sides of the membrane and [Ca2+]o less than or equal to 100 microM, the currents reversed at 0 mV. Their half-times of activation and inactivation were strictly voltage-dependent and decreased to near-constant values of 0.6-0.85 and 40 ms, respectively, at positive membrane potentials. The longer activation times were observed with [Ca2+]o greater than or equal to 50 microM. 3. The selectivity of the Ca2+ channel for monovalent ions with reference to internal Na+ was evaluated from the reversal potential. The Li+ and Na+ permeabilities were similar. The permeability ratios of K+ and Rb+ were 0.45, and 0.33 for Cs+. 4. Micromolar increases in [Ca2+]o produced small voltage shifts of half-times of activation (less than or equal to +3 mV at 10 microM and +10 mV at 500 microM), but strongly depressed the Na+ current. The Ca2(+)-induced block of Na+ current satisfied a 1:1 stoichiometry with an apparent KD of 1.8 microM at -20 mV. The block was, however, relieved with more positive and negative potentials, with KDs of 55 and 8.5 microM at +90 and -110 mV, respectively. 5. Relaxation time constants of block and unblock of Na+ currents through the LVA Ca2+ channel were measured on step changes to and from membrane potentials at which pronounced Ca2(+)-induced block occurred. 6. At -20 mV, the time constants of block decreased with micromolar increase in [Ca2+]o in line with a blocking rate coefficient of 1.9 x 10(8) M-1 s-1, but settled to values of 0.18 ms at [Ca2+]o beyond 50 microM. The Na+ currents were unblocked with time constant (tau u) of around 0.25 ms at strongly positive and negative membrane potentials at 22 degrees C. 7. Tau u failed to show any obvious dependence on [Ca2+]o up to the millimolar range. This finding contradicts suggestions that removal of the block occurs in a [Ca2+]o-dependent manner as a result of an increased probability of Ca2+ ion mobilization by repulsive forces with increased Ca2+ occupation of the channel. 8. The time course of unblock of Na+ currents was strongly temperature-dependent showing a Q10 of 2.5 for tau u. 9. The voltage dependence of the Na+ current block by Ca2+ ions is best accounted for by a single, centrally located Ca2+ binding site.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1979

Stimulus induced and seizure related changes in extracellular potassium concentration in cat thalamus (VPL).

Extracellular potassium activity (ak) and field potentials (fp) were measured in the nucleus ventro-postero-lateralis (VPL) thalami in order to assess the extent of thalamic participation in cortical seizure activity. Small increases (up to 0.7 mmole/l) or decreases (up to 0.2 mmole/l) in ak were induced by electrical stimulation of the contralateral forepaw. These changes in ak were spatially more limited than the simultaneously recorded fp. Similar observations were made during weak electrical stimulation of the somatosensory cortex and during interictal spikes in a cortical penicillin focus. Large and widespread increases in ak to levels of 11.6 mmoles/l and slow negative fps of 8 mV accompanied seizure generation either in a cortical penicillin focus or during intense repetitive electrical stimulation of the cortical surface. Subsequent to such increases ak fell to subnormal levels. The amplitudes and durations of such undershoots were correlated with the amplitudes of the preceding increases in ak. Sometimes thalamic seizures ceases before cortical epileptic episodes. This resulted in a decrease of cortical EEG amplitudes. After ablation of the sensorimotor cortex seizures in forepaw-VPL could be induced by stimulation of the somatosensory cortex. These results further support the conclusion that specific thalamic nuclei participate in seizure generation and may serve as a subcortical route of seizure spread.

Afferent Pathways↗
PubMed1979

Decrease of inhibitory driving force in crayfish stretch reception: a mechanism of the convulsant action of penicillin.

The effect of penicillin on the evoked IPSP was investigated in the isolated crayfish stretch receptor. The IPSP driving force (IPSP reversal potential minus membrane potential) was reduced in a dose-dependent fashion but, when necessary correction was made for the decrease in resting membrane conductance, the synaptic conductance was only slightly reduced. The possibility that a penicillin-induced intracellular acidification was responsible for the decrease in IPSP driving force is considered.

Animals↗
PubMed1978

Ionic changes during experimentally induced seizure activity.

Changes in intra- and extracellular ionic activity and their relation to generation and termination of seizure phenomena can be studied with the help of ion-selective microelectrodes. Transient changes in extracellular potassium activity (aK) of the cortex regularly accompany paroxysmal activity induced by electrical stimulation and pentylenetetrazol injections or occur within active penicillin and aluminum foci. A rise of aK from baseline levels of about 3 mmoles/l up to ceiling levels of 8--12 mmoles/l, followed by subnormal K activity, is typically found during seizure discharge. Extracellular K accumulation during seizures facilitates the spread into extrafocal regions. Ceiling levels of extracellular aK are characterized by pronounced K reabsorption which is probably a limiting mechanism for the rise in extracellular aK. It may be a consequence of a simultaneous rise in intracellular Na activity that an electrogenic Na--K exchange process is involved in the termination of ictal activity. Seizures are also accompanied by significant reductions in extracellular Ca2+ activity (aCa) to as low as 0.7 mmoles/l (resting aCa 1.25 mmoles/l). There is no critical level of lowered aCa at which a seizure ultimately results. However, unlike changes in aK reductions in aCa can precede ictal activity. Thus, a fall of aCa occurs before the onset of paroxysmal periods during cyclical spike driving in a penicillin focus and before seizures induced by pentylenetetrazol. Ca2+-dependent mechanisms may contribute to seizure generation. In addition to changes in aK and aCa, intracellular chloride activity (aCl) can increase during seizure activity, as a result of an impaired chloride extrusion mechanism, which would lead to a reduced efficacy of inhibitory synaptic transmission and, therefore, to facilitation of seizure generation.

Animals↗