Deactivation of calcium currents in the soma of spinal ganglion neurons after elimination of the depolarizing shift in the membrane potential

Kinetic and voltage-dependent characteristics of deactivation of calcium inward currents with the removal of membrane depolarization were studied in the somatic membrane of rat dorsal root ganglion neurons by intracellular dialysis technique. The "tail" of low-threshold calcium current could be described reliably by one exponent with time constant tau 1-1.2-1.8 ms at repolarization to --90 mV. The "tail" of the high-threshold calcium current represented a sum of several exponents; the time constant of the main component tau h was in the range of 250-380 microseconds. tau 1 and tau h remained practically unchanged for repolarization potentials in the subthreshold region; however, they increased if it was in the range of potentials at which the corresponding component of the calcium current started to activate. A dependence of tau 1 and tau h on the duration of depolarizing shift was observed. The results obtained are discussed in the framework of a three-level kinetic model of calcium channels.     

Different Types of Potassium Outward Current in Relay Neurons Acutely Isolated from the Rat Lateral Geniculate Nucleus

Different classes of potassium (K+) outward current activated by depolarization were characterized in relay neurons acutely isolated from the rat lateral geniculate nucleus (LGN), using the whole-cell version of the patch-clamp technique. A fast-transient current (IA), activated at around - 70 mV, declined rapidly with a voltage-dependent time constant (tau=6 ms at + 45 mV), was 50% steady-state inactivated at - 70 mV, and rapidly recovered from inactivation with a monoexponential time course (tau=21 ms). IA was blocked by 4-aminopyridine (4-AP, 2 - 8 mM) and was relatively insensitive to tetraethylammonium (TEA, 2 - 10 mM). After elimination of IA by a conditioning prepulse (30 ms to - 50 mV), a slow-transient K+ current could be studied in isolation, and was separated into three components, IKm, IKs and a calcium (Ca2+)-dependent current, IK[Ca]. The slow-transient current was not consistently affected by 4-AP (up to 8 mM), while TEA (2 - 10 mM) predominantly blocked IKs and IK[Ca]. The component IKm persisted in a solution containing TEA and 4-AP, activated at around - 55 mV, declined monoexponentially during maintained depolarization (tau=98 ms at + 45 mV), was 50% inactivated at - 39 mV, and recovered with tau=128 ms from inactivation. IKs activated at a similar threshold, but declined much slower with tau=2662 ms at + 45 mV. Steady-state inactivation of IKs was half-maximal at - 49 mV, and recovery from inactivation occurred relatively fast with tau=116 ms. From these data and additional current-clamp recordings it is concluded that the K+ currents, due to their wide range of kinetics and dependence on membrane voltage or internal Ca2+ concentration, are capable of cooperatively controlling the firing threshold and of shaping the different states of electrophysiological behaviour in LGN relay cells.     

Separation of the sodium and potassium channels in the surface membrane of mollusk nerve cells]

Characteristics of transmembrane ionic currents under controlled changes in ionic composition of extra-and intracellular medium were studied by means of intracellular dialysis and voltage clamp in isolated neurons from the molluscs Helix pomatia and Limnea stagnalis. The outward potassium currents were eliminated by replacement of intracellular potassium by Tris and the pure inward current could be measured. Replacement of the Ringer solution by NA-free or Ca-free solutions in the extracellular medium made it possible to separate the inward current into additive components, one of which is carried by sodium ions, and the other, by calcium ions. The sodium and calcium inward currents are shown to have different kinetics and potential dependence: taumNa = 1+/-0.5 ms, taumCa = = 3+/-1 ms, tauhNa = 8+/-2 ms, tauhCa = 115+/-10 ms when Vm = 0, GNa = 0.5 when Vm==-21+/-2 mV, GCa = 0.5 when Vm=-8+/-2 mV. Both currents were not altered by tetrodoxin (TTX), however calcium current is specifically blocked by externally applied calcium ions (2 X 10(-3) M), verapamil, D = 600 as well as by fluoride while introduced inside a cell. These data prove the existence of separate systems of sodium and calcium ion-conducting channels in the somatic membrane.     

Three types of voltage-dependent calcium current in cultured rat hippocampal neurons

Voltage-dependent calcium (Ca2+) currents in cultured rat hippocampal neurons were studied with the whole-cell recording mode of the patch-clamp technique. On the basis of the voltage-dependence of activation, kinetics of inactivation and pharmacology, 3 types of Ca2+ currents were distinguished. The low-threshold Ca2+ current (Il) was activated at -60 mV, and completely inactivated during a 100-ms depolarization to -40 mV (time constant: tau = 16 +/- 1 ms). The high-threshold currents (Ih), which were activated at -20 mV, could be separated into two types. The high-threshold, fast inactivating current (Ih,f) decayed quickly during a maintained depolarization (tau = 33 +/- 3 ms at 0 mV), whereas the high-threshold, slowly inactivating current (Ih,s) decayed with a much slower time constant (tau = 505 +/- 42 ms at 0 mV). The inactivations of Ih,f and Ih,s exhibited different time- and voltage-dependencies. Nickel ions (Ni2+, 25 microM) markedly suppressed Il, but little affected Ih. Cadmium ions (Cd2+, 10 microM) almost completely suppressed Ih, but left a small amount of Il. Lanthanum ions (La3+, 10 microM) almost completely suppressed both Il and Ih. Ih,s was sensitive to block by the dihydropyridine antagonist nicardipine (10 microM).     

2 calcium currents in the somatic membrane of neurons of Helix pomatia

Two different calcium currents were revealed in the somatic membrane of Helix pomatia neurons. In addition to the main current described in literature, depolarizing the membrane from the holding potential level (-120 divided by -100 mV) an additional calcium current was observed. It was activated at depolarizations to -80 divided by -40 mV. Contrary to the main calcium current it did not deteriorate during intracellular perfusion by solutions containing fluoride. Time-dependence of this current could be described in the framework of the Hodgkin-Huxley model with time constants for activation and inactivation equal to tau m = 6-8 ms and tau h = 300-600 ms, respectively. The amplitude of this current increased with increase of extracellular Ca2+ concentration and decreased after addition of Co2+, Ni2+, Cd2+, nifedipine and verapamil. Dissociation constants of these substances with corresponding channels determined for the maximum of current-voltage relationship were 2 (Ca2+), 3 (Co2+), 0.06 (nifedipine) and 0.2 mmol/l (verapamil). Properties of the fluoride-insensitive calcium current and data obtained for other calcium channels are compared. Its possible functional role is also discussed.     

Sodium conductance in cultured myenteric AH-type neurons from guinea-pig small intestine

Whole-cell patch clamp methods were used to investigate sodium conductance in after-hyperpolarization-type (AH) enteric neurons in culture after dissociation from the myenteric plexus of guinea-pig small intestine. Inward current carried by Na+ (I(Na)) was identified and its current-voltage characteristics were compared with those for inward Ca2+ current (I(Ca)). The I(Na) current was a rapidly inactivating current relative to I(Ca). Application of tetrodotoxin (TTX) blocked I(Na) with an EC50 of 10.7 nM. Activation curves for I(Na) showed a rapid decrease in time to peak for test potentials from holding potentials of -80 mV to between -40 and -10 mV. Voltage-dependence of steady-state inactivation curves for I(Na) was fit to the Boltzmann equation with potential for half-inactivation (V(1/2)) = -55.6 mV and slope factor (k) = 6.4 mV. Steady-state inactivation for I(Ca) fit the Boltzmann equation with a V(1/2) = -38.9 mV and k= 14.4 mV. Kinetics for inactivation of I(Na) were voltage dependent at potentials between -70 and -30 mV and accelerated and became less voltage-dependent at more positive potentials. The time constant (tau) for inactivation at -70 mV was tau = 161 +/- 23 ms and decreased to tau = 2.3 +/- 0.2 ms at -30 mV. Rapid acceleration of inactivation occurred between -50 and -40 mV. This was also the range where activation began. Recovery from inactivation with the membrane potential clamped at -100 or -80 mV was rapid and fit by a single exponential with tau = 7.3 +/- 1.1 ms for -100 mV and 21.5 +/- 5.1 ms for -80 mV. The results suggest that AH-type enteric neurons have only one type of Na+ channel that behaves like the "classical" voltage-gated tetrodotoxin-sensitive fast channel. The findings support the hypothesis that I(Na) current is an important factor in determination of excitability and firing behavior in AH neurons. I(Na) and I(Ca) together determine the properties of the rising phase of the spike and thereby contribute to global determinants of excitability as the neurons are exposed to multiple depolarizing and hyperpolarizing stimuli from synaptic inputs and mediators released from enteroparacrine cells.     

Passive electrical membrane properties of rat neostriatal neurons in an in vitro slice preparation

The passive electrical membrane properties of rat neostriatal neurons were studied in in vitro slice preparations. The data are only from neurons having stable resting membrane potentials of more than 50 mV and able to generate action potentials of amplitudes greater than 70 mV evoked by local or intracellular stimulation. All neurons measured for current-voltage relationship (n = 52) showed non-linearity of the input resistance in the hyperpolarizing direction. The mean input resistance at the resting membrane potential was 16.6 M omega. Depolarizing postsynaptic potentials evoked by local stimulation were decreased both in their amplitude and half-decay time by inward current injections exceeding more than 1 nA due to the strong membrane rectification at these levels of hyperpolarization. The mean membrane time constant (tau 0) was 5.3 ms, as measured from the semilogarithmic plots of transmembrane potential shift produced by small hyperpolarizing current pulses. In some neurons, the equalizing term (tau 1) could be determined as well and had a mean value of 1.0 ms. Measurement of (tau 0) using the strength-latency relation showed a similar value (5.0 ms) to that measured from the voltage transients. Intracellular labeling of the recorded neurons with horseradish peroxidase suggested that the recordings were obtained from medium spiny neurons.     

Inactivation of potassium outward current depending on the extracellular calcium ions

Inactivation of the potassium outward current depending on the extracellular calcium ions was studied in voltage clamp experiments on nonidentified intracellularly perfused neurons of the snail Helix pomatia. The decay of this current can be approximated by two exponents with time constants of 50-70 ms and 220-300 ms, respectively. The steady-state inactivation depended on the intracellular concentration of K ions. With a decrease of the latter to 20 mmol/l the current was inactivated completely. The inactivation degree was independent of the level of depolarizing shifts of the membrane potential and reduced with a rise of the extracellular K ions concentration. Addition of 5-10 mmol/l K+ to K+-free extracellular solution induced a slow-down of the fast component of the decay (tau = 167 ms) and acceleration of deinactivation. The possible mechanism of inactivation of the investigated current is discussed.     

Action of omega-conotoxin on calcium currents of the rat pituitary GH3 cell line]

Whole-cell modification of the patch clamp method was used to examine the action of omega-CgTX on calcium currents in GH3 pituitary cells. Two quite distinct components of inward calcium currents were observed in the presence of 15 mmol/l of calcium in the external solution. One was activated from the holding potential -80 mV by testing pulses more positive than -50 mV. The shift of the holding potential to -40 mV resulted in the stationary inactivation of this low voltage activated current component. It was found that omega-CgTX activated both low-threshold and high-threshold calcium currents at the first moment of application, but low-threshold current component increased more significantly. Full effect was developed for less than 30 s. Then time decay of currents was comparable with that of the "wash-out" process. Incubation of cells in the growth medium that contained 5 mumol/l omega-CgTX during 2 hour induced an increase in density of both types of calcium currents, then it fell after 2 hours of incubation in the same medium.     

Outward currents in isolated ventral cochlear nucleus neurons

Neurons of the ventral cochlear nucleus (VCN) perform diverse information processing tasks on incoming activity from the auditory nerve. We have investigated the cellular basis for functional diversity in VCN cells by characterizing the outward membrane conductances of acutely isolated cells using whole-cell, tight-seal, current- and voltage-clamp techniques. The electrical responses of isolated cells fall into two broad categories. Type 1 cells respond to small depolarizations with a regular train of action potentials. Under voltage clamp, these cells exhibit a noninactivating outward current for voltage steps positive to -35 mV. Analysis of tail currents reveals two exponentially decaying components with slightly different voltage dependence. These currents reverse at -73 mV, near the potassium equilibrium potential of -84 mV, and are blocked by tetraethylammonium (TEA). The major outward current in Type I cells thus appears to be mediated by potassium channels. In contrast to Type I cells, Type II cells respond to small depolarizations with only one to three short-latency action potentials and exhibit strong rectification around -70 mV. Under voltage clamp, these cells exhibit a noninactivating outward current with a threshold near -70 mV. Analysis of tail currents reveals two components with different voltage sensitivity and kinetics. A low-threshold current with slow kinetics is partly activated at rest. This current reverses at -77 mV and is blocked by 4-aminopyridine (4-AP) but is only partly affected by TEA. The other component is a high-threshold current activated by steps positive to -35 mV. This current is blocked by TEA, but not by 4-AP. A simple model based on the voltage dependence and kinetics of the slow low-threshold outward current in Type II cells was developed. The model produces current- and voltage-clamp responses that resemble those recorded experimentally. Our results indicate that the two major classes of acoustic response properties of VCN neurons are in part attributable to the types of outward (potassium) conductances present in these cells. The low-threshold conductance in the Type II (bushy) cells probably plays a role in the preservation of information about the acoustic stimulus phase from the auditory nerve to central auditory nuclei involved in low-frequency sound localization.     

Desensitization kinetics of a chloride acetylcholine response in Aplysia

The kinetics of desensitization of acetylcholine-evoked Cl conductance increased response of Aplysia RC neurons of the abdominal ganglion were studied under voltage-clamp conditions for comparison with results of similar studies on acetylcholine Na and K responses. The response evoked by acetylcholine on RC neurons was an outward current at resting potential (about -45 mV) that reversed at about -65 mV and was blocked by D-tubocurarine and strychnine but not hexamethonium and was not activated by arecoline. From the current-voltage relation this response can be ascribed to a pure conductance increase to Cl. The apparent KD was 40.6 microM. Upon prolonged exposure to acetylcholine the response peaked within 200-400 ms, and then decayed to a plateau current in the continued presence of the agonist. The peak and plateau currents reversed at the same potential, indicating that there had not been significant redistribution of Cl. The current decay in every cell was best fit by a double exponential function plus a constant, and the average time constants were tau fast = 1.8 +/- 0.2 s and tau slow = 16.2 +/- 1.0 s. Both components were slowed by cooling. While tau fast did not change with dose, tau slow increased with dose. Both components accelerated with hyperpolarization and upon application of trifluoperazine (2 microM). These results are consistent with the interpretation that desensitization of the acetylcholine Cl response is composed of two independent processes. This conclusion is the same as that derived from studies of the acetylcholine Na and K responses, and is in general consistent with desensitization being a property of a common acetylcholine receptor, and independent of the ionic selectivity of the associated channel. There are, however, significant differences in voltage, temperature and trifluoperazine dependence of the two components of the three ionic responses which may reflect influence of the different ion channels and/or transduction mechanisms.     

The cortically evoked secondary depolarization affects the integrative properties of thalamic reticular neurons

Corticothalamic terminals on thalamic reticular (RE) neurons account for most synapses from afferent pathways onto this nucleus and these inputs are more powerful than those from axon collaterals of thalamocortical neurons. Given the supremacy of cortical inputs, we analysed here the characteristics and possible mechanisms underlying a secondary component of the cortically elicited depolarization in RE neurons, recorded in cats under barbiturate anesthesia. Electrical stimulation of corticothalamic axons in the internal capsule evoked fixed and short-latency excitatory postsynaptic potentials (EPSPs) that, by increasing stimulation intensity and at hyperpolarized levels (< -70 mV), developed into low-threshold spikes and spindle oscillations. The threshold for spindle oscillations was 60% higher than that required for evoking minimal EPSPs. The evoked EPSPs included a secondary depolarizing component, which appeared approximately 5 ms after the peak of the initial component and was voltage dependent, i.e. most prominent between -70 mV and -85 mV, while being greatly reduced or absent at more hyperpolarized levels. The secondary depolarizing component was sensitive to QX-314 in the recording micropipette. We suggest that the secondary component of cortically evoked EPSPs in RE neurons is due to the dendritic activation of T-currents, with a probable contribution of the persistent Na+ current. This late component affected the integrative properties of RE neurons, including their spiking output and temporal summation of incoming cortical inputs.     

Intracellular fluoride alters the kinetic properties of calcium currents facilitating the investigation of synaptic events in hippocampal neurons

We have attempted to suppress voltage-dependent conductances in hippocampal neurons by introducing various intracellular agents. Voltage-clamp studies were carried out using acutely dissociated hippocampal neurons from adult guinea pigs. Synaptic events were examined using intracellular recordings in the slice preparation. Sodium conductance was suppressed when the quaternary lidocaine derivative QX 314 was introduced intracellularly. Potassium conductances were blocked by intracellular cesium or Tris. We also found that the anion fluoride could affect calcium conductance by an intracellular action. When anions other than fluoride were used for intracellular recordings, the voltage-dependent calcium current inactivated slowly and showed persistent activation at membrane potentials between -40 and -10 mV. In contrast, when fluoride was present intracellularly, the inactivation kinetics of the calcium current were accelerated and the persistent component of the current was largely suppressed. Intracellular recordings in the hippocampal slice showed that when electrodes contained cesium, QX 314, and fluoride, the spiking and nonlinear responses of the neuronal membrane to depolarization were blocked. In these conditions the time course and voltage-dependence of EPSPs could be examined in detail without complications due to voltage-dependent currents of the postsynaptic cell.     

Changes in the ionic mechanisms of the electro-excitability of the somatic membrane of rat sensory neurons during ontogenesis. Density ratios of inward currents

Correlations between densities of different types of inward currents in the somatic membrane of dorsal root ganglion neurons were studied in three age groups of rats (5-9 days, 45 days and 90 days postnatally). A linear dependence between the densities of high-threshold calcium and slow sodium currents was found. No correlation was observed between the densities of different inward currents in neurons with low-threshold calcium inward current. An inverse dependence was observed between the densities of transmembrane currents in cells having only two types of channels ("fast" sodium and high-threshold calcium ones). Neurons exhibiting slow TTX-resistant sodium and/or low-threshold calcium channels did not show inverse dependence between the densities of "fast" sodium and high-threshold calcium currents.     

Active and Passive Membrane Properties of Spinal Cord Neurons that Are Rhythmically Active during Swimming in Xenopus Embryos

Cellular properties have been examined in ventrally located Xenopus spinal cord neurons that are rhythmically active during fictive swimming and presumed to be motoneurons. Resting potentials and input resistances of such neurons are - 75 +/- 2 mV (mean +/- standard error) and 118 +/- 17 M ohm respectively. Most cells fire a single impulse, 0.5 to 2.0 ms in duration and 48.5 +/- 1.8 mV in amplitude, in response to a depolarizing current step. A minority fire several spikes of diminishing amplitude to more strongly depolarizing current. Cells held above spike, threshold fire on rebound from brief hyperpolarizing pulses. Spikes are blocked by 0.1 to 1.0 microM tetrodotoxin (TTX) and are therefore Na+-dependent. Current/voltage (I/V) plots to injected current are approximately linear near the resting potential but become non-linear at more depolarized levels. Cells recorded in TTX with CsCI-filled microelectrodes show a linearized I/V plot at depolarized membrane potentials suggesting the normal presence of a voltage-dependent K+ conductance activated at relatively depolarized levels. Most cells recorded in this way but without TTX fire long trains of spikes of near constant amplitude, pointing to a role of the K+ conductance in limiting firing in normal cells. Spike blockage with TTX reveals, in some cells, a transient depolarizing Cd2+-sensitive and therefore presumably Ca2+-dependent potential that increases in amplitude with depolarization. Cells in TTX, Cd2+, and strychnine, and recorded with CsCI-filled microelectrodes to block active conductances respond to hyperpolarizing current steps with a two component exponential response. The cell time constant (tau0) obtained from the longer of these by exponential peeling is relatively long (mean 15.7 ms). These findings contribute to an increased understanding of the cellular properties involved in spinal rhythm generation in this simple vertebrate.     

Changes in the ionic mechanisms of the electro-excitability of the somatic membrane of rat sensory neurons during ontogenesis. Distribution of ion channels of the inward current

Distribution of different types of ionic channels carrying inward currents was studied in the somatic membrane of rat dorsal root ganglion neurons within three age groups: 5-9 days, 45 days and 90 postnatally. The number of neurons whose membrane contained simultaneously four types of inward current channels ("fast" tetrodotoxin-sensitive and "slow" tetrodotoxin-resistant sodium low- and high-threshold calcium one's) was progressively reduced in successive groups. The first group contained 14.5%, the second 5% and the third group 1% of such neurons. These changes were due to disappearance of "slow" sodium and low-threshold calcium channels from the membrane; the number of neurons whose somatic membrane contained only two types of inward current channels ("fast" sodium and high-threshold calcium) has increased, respectively.     

Functional diversity of recombinant human AMPA type glutamate receptors: possible implications for selective vulnerability of motor neurons.

Lower motor neurons are known to be susceptible to glutamate-mediated cell damage via overstimulation of AMPA type glutamate receptors (GluR). The molecular basis of an important hypothesis in investigating amyotrophic lateral sclerosis (ALS) is glutamate-excitotoxicity. The aim of this study was to define desensitization and deactivation kinetics of recombinant human GluR1 and GluR2 receptor channels and their splice variants by means of patch-clamp experiments employing ultrafast solution exchange techniques. By this approach, the desensitization time constants of homooligomeric channels could be measured as tau(Des)=2.95+/-0.22 ms (n=10) for GluR1flip, tau(Des)=3.17+/-0.19 ms (n=10) for GluR1flop, tau(Des)=9.86+/-0.79 ms (n=10) for GluR2flip, and tau(Des)=1.87+/-0.26 ms (n=10) for GluR2flop, respectively. In the case of GluR1flip/flop and GluR2flop, a nondesensitising steady state current of less than 1% of peak current amplitude was observed, while GluR2flip channel currents showed a marked steady state component of about 10% of the maximum current. No significant differences were detected comparing the deactivation time course of GluR1 and GluR2 splice variants. These results suggest that the human GluR subtypes tested comprise no fundamental difference to their rodent analogous. Therefore, we describe a preparation that will be useful for further investigation of motor neuron physiological properties and a methodological approach allowing to study functional recombinant human GluR channels under reliable conditions.     

Kinetic parameters of calcium currents in maturing acutely isolated CA1 cells

Calcium currents were recorded in CA1 hippocampal cells from immature (P(4-10)) and older (P(22-55)) rats, using whole-cell voltage clamp techniques. Parameters defining the voltage-dependence of activation (tau(m)) and inactivation (tau(h)), steady-state inactivation and activation were determined at both stages of maturation. Current density increased with maturation. A transient low voltage activated (l.v.a.) current was found in P(4-10) cells, but not in the older cells. At voltages less negative than -30 mV, current inactivation was best described by two exponentials (tau(hf), tau(hs)); the ratio of the amplitudes of the two components changed with maturation, with a dominance of the faster component (tau(hf)) in the younger cells. The voltage dependence of tau(hf) followed a simple dependence model, decreased with increasing depolarization, in all cells at both stages of maturation. In P(4-10) cells, tau(hs) was voltage insensitive (range -25 to +30 mV); in P(22-55) cells, the voltage dependence of tau(hs) was found to be complex. Two current components were identified from the voltage dependence of the conductance in both groups. The first, more hyperpolarized component, the l.v.a. current found in P(4-10) cells; this was absent in the older cells, in which we found a component with a different voltage dependence. The voltage dependence of the conductance of the second, more depolarized component did not differ in younger and older cells. In the course of maturation, the steady-state inactivation of the second component underwent a hyperpolarizing shift and a decrease in voltage sensitivity.     

BK Channel Regulation of Afterpotentials and Burst Firing in Cerebellar Purkinje Neurons

BK calcium-activated potassium channels have complex kinetics because they are activated by both voltage and cytoplasmic calcium. The timing of BK activation and deactivation during action potentials determines their functional role in regulating firing patterns but is difficult to predict a priori. We used action potential clamp to characterize the kinetics of voltage-dependent calcium current and BK current during action potentials in Purkinje neurons from mice of both sexes, using acutely dissociated neurons that enabled rapid voltage clamp at 37°C. With both depolarizing voltage steps and action potential waveforms, BK current was entirely dependent on calcium entry through voltage-dependent calcium channels. With voltage steps, BK current greatly outweighed the triggering calcium current, with only a brief, small net inward calcium current before Ca-activated BK current dominated the total Ca-dependent current. During action potential waveforms, although BK current activated with only a short (∼100 μs) delay after calcium current, the two currents were largely separated, with calcium current flowing during the falling phase of the action potential and most BK current flowing over several milliseconds after repolarization. Step depolarizations activated both an iberiotoxin-sensitive BK component with rapid activation and deactivation kinetics and a slower-gating iberiotoxin-resistant component. During action potential firing, however, almost all BK current came from the faster-gating iberiotoxin-sensitive channels, even during bursts of action potentials. Inhibiting BK current had little effect on action potential width or a fast afterhyperpolarization but converted a medium afterhyperpolarization to an afterdepolarization and could convert tonic firing of single action potentials to burst firing.SIGNIFICANCE STATEMENT BK calcium-activated potassium channels are widely expressed in central neurons. Altered function of BK channels is associated with epilepsy and other neuronal disorders, including cerebellar ataxia. The functional role of BK in regulating neuronal firing patterns is highly dependent on the context of other channels and varies widely among different types of neurons. Most commonly, BK channels are activated during action potentials and help produce a fast afterhyperpolarization. We find that in Purkinje neurons BK current flows primarily after the fast afterhyperpolarization and helps to prevent a later afterdepolarization from producing rapid burst firing, enabling typical regular tonic firing.     

Effect of ethanol on subthreshold currents of Aplysia pacemaker neurons

The subthreshold currents in bursting pacemaker neurons of the Aplysia abdominal ganglion were individually studied with the voltage clamp technique for sensitivity to 4% ethanol. The most prevalent effect of ethanol on unclamped bursting neurons was a hyperpolarization. This was shown to be due to a decrease of a voltage independent inward leakage current. Direct measurement of the Na-dependent slow inward current showed that this current was eliminated by 4% ethanol. Direct measurement of the Ca-dependent slow inward current showed that this current was substantially reduced by 4% ethanol. Injection of EGTA into cell bodies did not eliminate the ethanol-induced block of the slow inward calcium current. Thus, ethanol cannot be reducing the Ca-dependent slow inward current solely by an increase of internal calcium concentration. The effect of ethanol on voltage dependent outward current was measured by blockage of all inward current. The peak outward current was increased by ethanol. The rate of inactivation of this outward current was also increased. Calcium activated potassium current (IK(Ca)) is particularly complicated in its response to ethanol because it is dependent on both Ca and voltage for its activation. The level of IK(Ca) elicited in response to constant Ca injection was increased by ethanol treatment. The level of this current as activated by voltage clamp pulses was either increased or decreased depending on the neuron type. Ca2+ activated potassium conductance increased e-fold for a 26 mV depolarization in membrane holding potential. Ethanol decreased this voltage dependence to e-fold for a 55 mV change in potential. This result was interpreted to mean that ethanol shifted an effective Ca2+ binding site of these channels from about halfway through the membrane field to one quarter of the way across. The same theoretical approach allowed the further conclusion that ethanol caused an increased internal free calcium concentration probably by decreasing calcium binding by intracellular buffers.