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C M Armstrong

Publications and source records attributed to C M Armstrong.

At least 19 recordsLinked to original sources
PubMed1992

Sodium action potentials in the dendrites of cerebellar Purkinje cells.

We report here that in cerebellar Purkinje cells from which the axon has been removed, positive voltage steps applied to the voltage-clamped soma produce spikes of active current. The spikes are inward, are all-or-none, have a duration of approximately 1 ms, and are reversibly eliminated by tetrodotoxin, a Na channel poison. From cell to cell, the amplitude of the spikes ranges from 4 to 20 nA. Spike latency decreases as the depolarizing step is made larger. These spikes clearly arise at a site where the voltage is not controlled, remote from the soma. From these facts we conclude that Purkinje cell dendrites contain a sufficient density of Na channels to generate action potentials. Activation by either parallel fiber or climbing fiber synapses produces similar spikes, suggesting that normal input elicits Na action potentials in the dendrites. These findings greatly alter current views of how dendrites in these cells respond to synaptic input.

Action Potentials↗
PubMed1992

Inhibitory synaptic currents in rat cerebellar Purkinje cells: modulation by postsynaptic depolarization.

1. Synaptic currents were recorded in voltage-clamped cerebellar Purkinje cells using the tight-seal whole-cell recording technique. Cells were dialysed with a CsCl solution and were held at -60 or -70 mV. Inhibitory interneurones (basket and stellate cells) were stimulated using an extracellular pipette positioned in the molecular layer. Blockers of excitatory glutamatergic synapses were included in the bath solution. 2. Evoked synaptic currents were observed after a latency of 3-4 ms. The time course of synaptic currents could in most cases be fitted to a biexponential curve, with a rise time constant, tau on, of 1-3 ms and a decay time constant, tau off, of 7-13 ms. These currents were blocked by bicuculline. 3. The mean amplitude of evoked synaptic currents increased in discrete steps when the voltage applied to the stimulating pipette was increased. At each level, very prominent fluctuations of the amplitude were observed among trials. 4. Complex synaptic currents corresponding to repetitive activity of the presynaptic interneurone were occasionally observed, particularly with high intensity presynaptic stimulation. This repetitive activity could lead to bursts of synaptic currents lasting for several seconds. 5. Following a depolarizing voltage train in the postsynaptic Purkinje cell, the amplitude of evoked synaptic currents was first inhibited, and then potentiated. The inhibition was accompanied by a small but consistent increase in tau off and by no alteration in tau on. When using small intensity presynaptic stimuli, it was found that the probability of failures was greatly enhanced. The inhibitory phase lasted for about 1 min before giving way to potentiation. The potentiation returned to the control with a time to half-decay of 12.9 +/- 0.9 min. 6. The present results give further evidence to a previously proposed hypothesis that the inhibition produced by Purkinje cell depolarization is mainly presynaptic. The longer lasting potentiation, on the other hand, has most probably a postsynaptic origin. Cerebellar Purkinje cells receive inhibitory GABAergic inputs from two classes of interneurones located in the molecular layer (reviewed in Palay & Chan-Palay, 1974; Ito, 1984). Basket cells are closest to the Purkinje cell layer and address their inhibitory signal primarily to the soma and to the main dendrites. Stellate cells are more externally located and have contacts with the more distal part of the dendritic arborization of Purkinje cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Calcium ion as a cofactor in Na channel gating.

Calcium ions in the external medium stabilize the resting state of voltage-dependent channels, including Na channels. This effect of calcium on channel gating is usually explained in terms of the surface charge hypothesis, which proposes that local adsorption of calcium ion to the outside of the membrane alters the intramembranous electric field, thus influencing channel behavior indirectly. Calcium ion has also been shown to block Na channels, most strongly at negative voltage. We have examined these two apparently separate effects of calcium, the gating effect and Ca block, and find the two are closely correlated. We propose that calcium (or a suitable substitute) is an essential cofactor in normal gating and that it produces gating and blocking effects by binding within the channel.

Animals↗
PubMed1991

Potassium channel block by internal calcium and strontium.

We show that intracellular Ca blocks current flow through open K channels in squid giant fiber lobe neurons. The block has similarities to internal Sr block of K channels in squid axons, which we have reexamined. Both ions must cross a high energy barrier to enter the blocking site from the inside, and block occurs only with millimolar concentrations and with strong depolarization. With Sr (axon) or Ca (neuron) inside, IK is normal in time course for voltages less than about +50 mV; but for large steps, above +90 mV, there is a rapid time-dependent block or "inactivation." From roughly +70 to +90 mV (depending on concentration) the current has a complex time course that may be related to K accumulation near the membrane's outer surface. Block can be deepened by either increasing the concentration or the voltage. Electrical distance measurements suggest that the blocking ion moves to a site deep in the channel, possibly near the outer end. Block by internal Ca can be prevented by putting 10 mM Rb in the external solution. Recovery from block after a strong depolarization occurs quickly at +30 mV, with a time course that is about the same as that of normal K channel activation at this voltage. 20 mM Mg in neurons had no discernible blocking effect. The experiments raise questions regarding the relation of block to normal channel gating. It is speculated that when the channel is normally closed, the "blocking" site is occupied by a Ca ion that comes from the external medium.

Animals↗
PubMed1991

Synaptic- and agonist-induced excitatory currents of Purkinje cells in rat cerebellar slices.

1. Postsynaptic currents originating from activation of the two major excitatory inputs to Purkinje cells were studied in thin slices of rat cerebellum, using the tight-seal whole-cell recording technique. Two types of excitatory postsynaptic currents were analysed: those evoked by stimulation of the granule cell-parallel fibre system (PF-EPSC) and those elicited by stimulation of the climbing fibres (CF-EPSC). 2. Both types of postsynaptic currents had a linear current-voltage relation, reversing at membrane potentials close to 0 mV. Their time course of activation was independent of the membrane potential. 3. For both types of postsynaptic currents, the time course of decay was well described by a single exponential function, with a time constant which increased as the membrane potential was made more positive. 4. Postsynaptic currents arising from stimulation of the climbing fibre generally had a slightly faster time course of onset and decay than those associated with stimulation of the granule cell-parallel fibre system. The average values of the 10-90% rise time were 1.8 +/- 0.4 ms (means +/- S.D., n = 7) for PF-EPSCs and 0.8 +/- 0.3 ms (n = 9) for CF-EPSCs. Time constants of decay, at a holding potential of -60 mV, had values of 8.3 +/- 1.6 ms (n = 7) and 6.4 +/- 1.1 ms (n = 9) for PF-EPSCs and CF-EPSCs respectively. 5. CF-EPSCs and PF-EPSCs had the characteristics described above in slices derived from animals aged 9-22 days old and 9-15 days old, respectively. The PF-EPSCs in animals older than 15 days had very slow time courses and positive apparent reversal potentials, suggesting that they originated from distal locations, not under accurate voltage control. 6. In order to assess the quality of the voltage clamp, responses to hyperpolarizing pulses from -70 mV were analysed. The capacitive currents could be fitted by the sum of two exponentials, and were interpreted with an equivalent electrical circuit comprising two main compartments (soma and proximal dendrites on one hand, distal dendrites on the other). Analysis of synaptic currents in terms of this model suggested that the recorded time course of decay was approximately correct. 7. CF-EPSCs as well as PF-EPSCs were insensitive to the NMDA receptor antagonist 3-3(2-carboxypiperazine-4-yl)propyl-1-phosphonate (CPP), but were blocked in a dose-dependent reversible manner by the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX).(ABSTRACT TRUNCATED AT 400 WORDS)

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1990

Do voltage-dependent K+ channels require Ca2+? A critical test employing a heterologous expression system.

Removal of Ca2+ from the solution bathing neurons is known in many cases to alter the gating properties of voltage-dependent K+ channels and to induce a large, nonselective "leak" conductance. We used a heterologous expression system to test whether the leak conductance observed in neurons is mediated by voltage-dependent K+ channels in an altered, debased conformation. Voltage-dependent K+ channels were expressed in an insect cell line infected with a recombinant baculovirus carrying the cDNA for Drosophila Shaker "A-type" K+ channels. These expressed channels respond to low Ca2+ identically to voltage-dependent K+ channels in native neuronal membranes; upon removal of external Ca2+, Shaker K+ currents disappear and are replaced by a steady, nonselective leak conductance. However, control cells devoid of Shaker channels were free of any voltage-dependent conductances and did not generate a leak when external Ca2+ was removed. These results show that Ca2+ is essential for proper function of voltage-dependent K+ channels and is required to stabilize the native conformations of these membrane proteins.

Animals↗
PubMed1990

Synaptic currents in cerebellar Purkinje cells.

Cerebellar Purkinje cells are known to receive strong excitatory input from two major pathways originating outside the cerebellum and inhibitory input from two types of neurons in the cerebellar cortex. The functions and synaptic strengths of these pathways are only partially known. We have used the patch-clamp technique applied to Purkinje cells in thin slices of rat cerebellum to measure directly the postsynaptic currents arising from the two major excitatory pathways and one of the inhibitory inputs. Inhibitory synaptic currents occur spontaneously with high frequency and are variable in amplitude, ranging, in our recording conditions with high internal Cl-, from less than 100 pA to more than 1 nA. These currents are blocked by the gamma-aminobutyrate type A antagonist bicuculline. One of the excitatory inputs is all or none. For threshold stimulation, the synaptic current is either full amplitude, when the presynaptic fiber is successfully stimulated, or completely absent. This synaptic current is often larger than 1 nA and is virtually eliminated by 2 microM 6-cyano-7-nitroquinoxaline-2,3-dione, a blocking agent thought to be specific for glutamate receptors that are not of the N-methyl-D-aspartate type. Its all-or-none character identifies it as arising from a climbing-fiber synapse. The other excitatory input produces a synaptic current that is smoothly graded as a function of stimulus intensity. This response we believe arises from the stimulation of mossy fibers or granule cells. The synaptic current associated with this input is also largely eliminated by 2 microM 6-cyano-7-nitroquinoxaline-2,3-dione.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1990

Modification of sodium channel gating by lanthanum. Some effects that cannot be explained by surface charge theory.

In clonal pituitary (GH3) cells we studied the changes in sodium channel gating caused by substitution of La3+ for Ca2+ ion. Gating of sodium channels was simplified by using intracellular papain to remove inactivation. To quantify La effects, we empirically fitted closing and the late phase of opening of the channels with single exponentials, determined the opening (a) and closing (b) rate, and plotted these rates as a function of Vm (membrane voltage). The midpoint of the fraction open-Vm curve was also determined. Changing from Ca to La shifted the curves for these three measures of Na channel gating along the voltage axis and changed their shape somewhat. Surface charge theory, in the form usually presented, predicts equal shifts of all three curves, with no change in shape. We found, however, that the shift for each of the measurements was different. 2 mM La, for example, shifted opening kinetics by +52 mV (i.e., 52 mV must be added to the depolarization to make activation in 2 mM La as fast as in 2 mM Ca), the fraction open voltage curve by +42.5 mV, and the closing rate curve by +28 mV. The shift was an almost linear function of log [La] for each of the measures. The main finding is that changing from 2 mM Ca to 10 microM La causes a positive shift of the opening rate and fraction open curves, but a negative shift of the closing rate curve. The opposite signs of the two effects cannot be explained in terms of surface charge theory. We briefly discuss some alternatives to this theory.

Adenoma↗
PubMed1989

Potassium currents in dissociated cells of the rat pineal gland.

The properties of K currents of pineal cells were studied using the whole-cell variant of the patch-clamp technique. The total K current could be separated in two distinct components: a fast, transient current (It) and a slow current (Is). The activation threshold of It was at -35 to -30 mV. On depolarization to +50 mV it reaches a peak in 2-3 ms and inactivates almost completely in 50 ms. Half steady state inactivation occurs at -45 mV. Inactivation of It is voltage-dependent and is well fitted by single exponentials with time constants between 17.2 ms at +50 mV and 27.2 ms at -10 mV. Inactivation is removed with time and the recovery period shortened by membrane hyperpolarization. The slow K current has a threshold at -20 to -15 mV. It reaches a maximum in about 30-40 ms and inactivates slightly, to about 80% of the peak value at the end of pulses lasting 200 ms. With 80 mM external K, tail currents recorded after short (1-2 ms) depolarizations were about 2.5 times faster than the tails recorded at the end of 50 ms pulses. The fast tails were removed by depolarizing prepulses but the slow tails remained unaltered. Thus, the fast and slow tails are probably a reflection of the closing of the transient and slow K channels. The transient K current of pineal cells has general characteristics similar to transient currents recorded in non-secretory cells, but also has particular kinetic properties.

Animals↗
PubMed1989

Calcium channel block by cadmium in chicken sensory neurons.

Cadmium block of calcium channels was studied in chicken dorsal root ganglion cells by a whole-cell patch clamp that provides high time resolution. Barium ion was the current carrier, and the channel type studied had a high threshold of activation and fast deactivation (type FD). Block of these channels by 20 microM external Cd2+ is voltage dependent. Cd2+ ions can be cleared from blocked channels by stepping the membrane voltage (Vm) to a negative value. Clearing the channels is progressively faster and more complete as Vm is made more negative. Once cleared of Cd2+, the channels conduct transiently on reopening but reequilibrate with Cd2+ and become blocked within a few milliseconds. Cd2+ equilibrates much more slowly with closed channels, but at a holding potential of -80 mV virtually all channels are blocked at equilibrium. Cd2+ does not slow closing of the channels, as would be expected if it were necessary for Cd2+ to leave the channels before closing occurred. Instead, the data show unambiguously that the channel gate can close when the channel is Cd2+ occupied.

Animals↗
PubMed1989

Sodium channel gating in clonal pituitary cells. The inactivation step is not voltage dependent.

We have determined the time course of Na channel inactivation in clonal pituitary (GH3) cells by comparing records before and after the enzymatic removal of inactivation. The cells were subjected to whole-cell patch clamp, with papain included in the internal medium. Inactivation was slowly removed over the course of 10 min, making it possible to obtain control records before the enzyme acted. Papain caused a large (4-100x) increase in current magnitude for small depolarizations (near -40 mV), and a much smaller increase for large ones (approximately 1.5x at +40 mV). For technical reasons it was sometimes convenient to study outward INa recorded with no Na+ outside. The instantaneous I-V (IIV) curve in this condition was nonlinear before papain, and more nearly linear afterwards. The gNa-V curve after papain, obtained by dividing the INa-V curve by the IIV curve, was left-shifted by at least 20 mV and steepened. A spontaneous 5-10 mV left shift occurred in the absence of papain. The rate of the inactivation step was found to vary only slightly from -100 mV to +60 mV, based on the following evidence. (a) Before papain, inactivation rate saturated with voltage and was constant from +20 to +60 mV. (b) We activated the channels with a brief pulse, and studied the time course of the current on changing the voltage to a second, usually more negative level (Na+ present internally and externally). The time course of inactivation at each voltage was obtained by comparing control traces with those after inactivation was removed. When the 5-10-mV spontaneous shift was taken into account, inactivation rate changed by less than 10% from -100 to +60 mV. The data are considered in terms of existing models of the Na channel.

Animals↗
PubMed1988

Potassium channel "inactivation" induced by soft-glass patch pipettes.

We have studied the potassium currents of rat pituitary pars intermedia cells kept in primary culture using whole-cell recording with patch pipettes. The potassium current recorded with hard-glass pipettes is mainly carried by voltage-dependent channels that show slow inactivation in the presence of 0.5 mM internal EGTA. Fast "inactivation" of the potassium current is seen with patch pipettes fabricated from soft glass (soda glass or potash lead glass), and is probably caused by block of the potassium channels by di- or multivalent cations released from the glass.

Animals↗
PubMed1988

Fast-deactivating calcium channels in chick sensory neurons.

Whole-cell Ca and Ba currents were studied in chick dorsal root ganglion (DRG) cells kept 6-10 in culture. Voltage steps with a 15-microseconds rise time were imposed on the membrane using an improved patch-clamp circuit. Changes in membrane current could be measured 30 microseconds after the initiation of the test pulse. Currents through Ca channels were recorded under conditions that eliminate Na and K currents. Tail currents, associated with Ca channel closing, decayed in two distinct phases that were very well fitted by the sum of two exponentials. The time constants tau f and tau s were near 160 microseconds and 1.5 ms at -80 mV, 20 degrees C. The tail current components, called FD and SD (fast-deactivating and slowly deactivating), are Ca channel currents. They were greatly reduced when Mg2+ replaced all other divalent cations in the bath. The SD component inactivated almost completely as the test pulse duration was increased to 100 ms. It was suppressed when the cell was held at membrane potentials positive to -50 mV and was blocked by 100-200 microM Ni2+. This behavior indicates that the SD component was due to the closing of the low-voltage-activated (LVA) Ca channels previously described in this preparation. The FD component was fully activated with 10-ms test pulses to +20 mV at 20 degrees C, and inactivated to approximately 30% during 500-ms test pulses. It was reduced in amplitude by holding at -40 mV, but was only slightly reduced by micromolar concentrations of Ni2+. Replacement of Ca2+ with Ba2+ increased the FD tail current amplitudes by a factor of approximately 1.5. The deactivation kinetics did not change (a) as channels inactivated during progressively longer pulses or (b) when the degree of activation was varied. Further, tau f was affected neither by changing the holding potential nor by varying the test pulse amplitude. Lowering the temperature from 20 to 10 degrees C decreased tau f by a factor of 2.5. In all cases, the FD component was very well fitted by a single exponential. There was no indication of an additional tail component of significant size. Our findings indicate that the FD component is due to closing of a single class of Ca channels that coexist with the LVA Ca channel type in chick DRG neurons.

Animals↗