Three types of depolarization-activated potassium currents in acutely isolated mouse vestibular neurons

The nature and electrophysiological properties of Ca(2+)-independent depolarization-activated potassium currents were investigated in vestibular primary neurons acutely isolated from postnatal mice using the whole cell configuration of the patch-clamp technique. Three types of currents were identified. The first current, sensitive to TEA (I(TEA)) and insensitive to 4-aminopyridine (4-AP), activated at -40 mV and exhibited slow activation (tau(ac), 38.4 +/- 7.8 ms at -30 mV, mean +/- SD). I(TEA) had a half activation potential [V(ac(1/2))] of -14.5 +/- 2.6 mV and was inactivated by up to 84.5 +/- 5.7% by 10-s conditioning prepulses with a half inactivation potential [V(inac(1/2))] of -62.4 +/- 0.2 mV. The second current, sensitive to 4-AP (maximum block around 0.5 mM) and to alpha-dendrotoxin (I(DTX)) appeared at -60 mV. Complete block of I(DTX) was achieved using either 20 nM alpha-DTX or 50 nM margatoxin. This current activated 10 times faster than I(TEA) (tau(ac), 3.5 +/- 0.8 ms at -50 mV) with V(ac(1/2)) of -51.2 +/- 0.6 mV, and inactivated only slightly compared with I(TEA) (maximum inactivation, 19.7 +/- 3.2%). The third current, also sensitive to 4-AP (maximum block at 2 mM), was selectively blocked by application of blood depressing substance (BDS-I; maximum block at 250 nM). The BDS-I-sensitive current (I(BDS-I)) activated around -60 mV. It displayed fast activation (tau(ac), 2.3 +/- 0.4 ms at -50 mV) and fast and complete voltage-dependent inactivation. I(BDS-I) had a V(ac(1/2)) of -31.3 +/- 0.4 mV and V(inac(1/2)) of -65.8 +/- 0.3 mV. It displayed faster time-dependent inactivation and recovery from inactivation than I(TEA). The three types of current were found in all the neurons investigated. Although I(TEA) was the major current, the proportion of I(DTX) and I(BDS-I) varied considerably between neurons. The ratio of the density of I(BDS-I) to that of I(DTX) ranged from 0.02 to 2.90 without correlation with the cell capacitances. In conclusion, vestibular primary neurons differ by the proportion rather than the type of the depolarization-activated potassium currents they express.     

Properties of voltage dependent potassium currents in acutely isolated rat oculomotor neurons

Voltage-dependent potassium currents in oculomotor neurons (OMNs) were studied with whole-cell patch clamp recordings. Fast inactivating outward currents (I(fast)) had half activation voltage (Vh) of -37.1 mV with slope factor (Vc) of 10.9 mV. I(fast) had half inactivation voltage (Vh) of -66.5 mV and Vc of 11.4 mV. I(fast) decayed with a time constant(tau) of 5.1 ms at +10 mV. I(fast) were sensitive to 4-aminopyridine, showing 50% inhibitory concentration (IC(50)) of 0.97 mM. Slowly inactivating outward currents (I(slow)) had two components. The low-concentration-TEA-sensitive currents had Vh of -3.7 mV with Vc of 9.7 mV in activation and had Vh of -54.7 mV with Vc of 23.8 mV in inactivation. The persistent currents had Vh of 7.4 mV and Vc of 11.8 mV in activation and Vh of -54.4 mV and Vc of 21.2 mV in inactivation. Decay of I(slow) (+10 mV) followed a double exponential time course (tau 215, 1165.6 ms). Low-concentration-TEA-sensitive currents were blocked completely by tetraethylammonium (TEA) of 3 mM with an IC(50) of 1.52 mM. Higher concentrations (3-20 mM) of TEA blocked the persistent currents, with an IC(50) of 6.9 mM.     

Transient voltage and calcium-dependent outward currents in hippocampal CA3 pyramidal neurons

Membrane currents activated by step changes in membrane potential were studied in hippocampal pyramidal neurons of region CA3 using the single microelectrode voltage-clamp technique. The transient outward current activated by depolarizing steps appeared to be composed of two transient currents that could be distinguished by differences in voltage sensitivity, time course, and pharmacological sensitivity. The more slowly decaying current was activated by voltage steps positive to -60 mV and declined exponentially with a time constant between 200 and 400 ms. This current inactivated as the holding potential was made more positive over the range of -75 to -45 mV and was 50% inactivated near -60 mV. The more slowly decaying transient current was selectively blocked by 0.5 mM 4-aminopyridine (4-AP) but not by 5-10 mM tetraethylammonium (TEA) or 2-5 mM Mn2+. The second transient current had a much faster time course than the 4-AP-sensitive current, having a duration of 5-20 ms. This very fast transient current was observed during potential steps positive to -45 mV. The fast transient current was inactivated when the holding potential was made positive to -45 mV. The amplitude of the fast transient current was greatly reduced by the application of 4 mM Mn2+ or Ca2+-free artificial cerebrospinal fluid (CSF). The fast transient current appeared to be unaffected by 0.5 mM 4-AP but was greatly reduced by 10 mM TEA. These results suggest that the transient outward current observed during depolarizing steps is composed of at least two distinct transient currents. The more slowly decaying current resembles the A-current originally described in molluscan neurons (9, 32, 42) in voltage sensitivity, time course, and pharmacological sensitivity. The faster transient current resembles a fast, Ca2+-dependent transient current previously observed in bull-frog sympathetic neurons (5, 27).     

Voltage-activated potassium outward currents in two types of spider mechanoreceptor neurons.

We studied the properties of voltage-activated outward currents in two types of spider cuticular mechanoreceptor neurons to learn if these currents contribute to the differences in their adaptation properties. Both types of neurons adapt rapidly to sustained stimuli, but type A neurons usually only fire one or two action potentials, whereas type B neurons can fire bursts lasting several hundred milliseconds. We found that both neurons had two outward current components, 1) a transient current that activated rapidly when stimulated from resting potential and inactivated with maintained stimuli and 2) a noninactivating outward current. The transient outward current could be blocked by 5 mM tetraethylammonium chloride, 5 mM 4-aminopyridine, or 100 microM quinidine, but these blockers also reduced the amplitude of the noninactivating outward current. Charybdotoxin or apamin did not have any effect on the outward currents, indicating that Ca2+-activated K+ currents were not present or not inhibited by these toxins. The only significant differences between type A and type B neurons were found in the half-maximal activation (V50) values of both currents. The transient current had a V50 value of 9. 6 mV in type A neurons and -13.1 mV in type B neurons, whereas the V50 values of noninactivating outward currents were -48.9 mV for type A neurons and -56.7 mV for type B neurons. We conclude that, although differences in the activation kinetics of the voltage-activated K+ currents could contribute to the difference in the adaptation behavior of type A and type B neurons, they are not major factors.     

17beta-estradiol inhibits outward potassium currents recorded in rat parabrachial nucleus cells in vitro

Evidence is increasingly accumulating in support of a role for the steroid hormone 17beta-estradiol to modify neuronal functions in the mammalian CNS, especially in autonomic centers. In addition to its well known slowly developing and long lasting actions (genomic), estrogen can also rapidly modulate cell signaling events by affecting membrane excitability (non-genomic). Little, however, is known regarding the mechanism(s) by which 17beta-estradiol produces its rapid effects on neuronal membrane excitability. As potassium channels play a crucial role in cell excitability, we hypothesized that 17beta-estradiol caused excitability by modulating potassium flux through the neuronal cell membrane. We tested this hypothesis by examining the effects of 17beta-estradiol on outward potassium currents recorded in cells from the parabrachial nucleus of rats, in vitro. Bath application of 17beta-estradiol (10-100 microM) reversibly reduced voltage-activated outward potassium currents in a concentration-dependent manner. This effect was mimicked by BSA-17beta-estradiol but not mimicked by 17alpha-estradiol and was significantly reduced by ICI 182,780, a selective estrogen receptor antagonist. The inhibitory effect of 17beta-estradiol was dependent on extracellular potassium concentration, with more profound effects observed at lower concentrations. The 17beta-estradiol-induced inhibition of the outward current was blocked by pretreatment with the potassium channel blockers tetraethylammonium and 4-aminopyridine. The time constants of deactivation of tail currents were decreased by 17beta-estradiol over a range of test potentials (-140 to -80 mV). Finally, the inhibitory effect of 17beta-estradiol on the outward potassium currents was blocked following pre-incubation of slices in lavendustin A, a tyrosine kinase inhibitor. Taken together, these results suggest that 17beta-estradiol acts rapidly at an extracellular membrane receptor to reduce tetraethylammonium- and 4-aminopyridine-sensitive outward potassium currents by accelerating the closure of potassium channels. This may be the ionic basis of 17beta-estradiol-induced enhancement of neuronal excitability.     

Nerve growth factor decreases potassium currents and alters repetitive firing in rat sympathetic neurons

The sympathetic nervous system is an essential regulator of the cardiovascular system and interactions with target tissue regulate sympathetic neuronal properties. The heart produces nerve growth factor (NGF), which promotes sympathetic noradrenergic innervation of cardiac tissue and affects sympathetic synaptic strength. Neurotrophins, including NGF, are important modulators of synaptic plasticity and membrane electrical properties. Here we show that acute application of NGF causes a change in the repetitive firing pattern of cultured sympathetic neurons of the rat superior cervical ganglion. Neurons fire fewer action potentials in NGF, but with increased frequency, demonstrating an NGF-dependent change from a tonic to a phasic firing pattern. Additionally, NGF decreases the spike time variance, making spikes more tightly time locked to stimulus onset. NGF causes a decrease in the amplitude of both calcium-dependent and -independent potassium currents, and inhibition of calcium-dependent potassium currents using CdCl(2) reproduces some, but not all, of the firing properties induced by NGF. This study suggests that NGF release from cardiac tissue may act to modulate the repetitive firing properties of sympathetic neurons to tune their output to meet the physiological needs of the organism.     

Proterties of large-conductance calciumactivated potassium channel in pyramidal neurons acutely isolated from hippocampal CA1 region of adult rat

<正> Properties of large-conductance Ca~(2+) -achvated K~+ (BK_(Ca)channel were studied in ex-cised patches of pyramidal neurons from adult rat hippocampal CAl region using inside-out single channel recording technique. Activity of BK_(Ca)channel was first oborved at[Ca] = l0~(-8)M with the membrane potential of + 20 mV, and the [Ca]_i at which the channel was half activated(P_0 = 0.5)was 2 x 10~(-6)M. Conductance of single BK_(Ca) channel was approx.245 pS with symmetrical 140 mM-K~+ on bo sides of the excisedmembrane,essentially independent of membrane potentials and [Ca]_i, tested. Two exponentials, with the time constants of 2.07 ma and 14.36ms at membrane potential of + 40 mV with 5×10~(-7) M-[Ca]_i,were requied to describe the observed open distrbution of BK_(Ca) channel, sug-gesting existence of two different open channel stathes with apparently normal conductance. BK_(Ca)channel occasionally entered an apparent thirdopen channel state with a single channel current amplitude about 45% the amplitude o     

Whole-cell voltage-clamp study of the fading of GABA-activated currents in acutely dissociated hippocampal neurons

The lability of the responses of mammalian central neurons to gamma-aminobutyric acid (GABA) was studied using neurons acutely dissociated from the CA1 region of the adult guinea pig hippocampus as a model system. GABA was applied to the neuronal somata by pressure ejection and the resulting current (IGABA) recorded under whole-cell voltage clamp. In initial experiments we examined several basic properties of cells in this preparation. Our data confirm that passive and active membrane properties are similar to those which characterize cells in other preparations. In addition, GABA-dependent conductance (gGABA), reversal potential (EGABA), and the interaction of GABA with pentobarbital and bicuculline all appeared to be normal. Dendritic GABA application could cause depolarizing GABA responses, and somatic GABA application caused hyperpolarizations due to chloride (Cl-) movements. Repetitive brief applications (5-15 ms) of GABA (10(-5) to 10(-3) M) at a frequency of 0.5 Hz led to fading of successive peaks of IGABA until, at a given holding potential, a steady state was reached in which IGABA no longer changed. Imposing voltage steps lasting seconds during a train of steady-state GABA responses led initially to increased IGABA that then diminished with maintenance of the step voltage. The rate of decrease of IGABA at each new holding potential was independent of the polarity of the step in holding potential but was highly dependent on the rate of GABA application. Application rates as low as 0.05 Hz led to fading of IGABA, even with activation of relatively small conductances (5-15 nS). Since IGABA evoked by somatic GABA application in these cells is carried by Cl-, the Cl- equilibrium potential (ECl) is equal to the reversal potential for IGABA, i.e., to EGABA. The fading of IGABA with changes in holding potential can be almost entirely accounted for by a shift in ECl resulting from transmembrane flux of Cl- through the GABA-activated conductance. Maneuvers that prevent changes in the intracellular concentration of Cl-ions, [Cl-]i, including holding the membrane potential at EGABA during repetitive GABA application or buffering [Cl-]i with high pipette [Cl-], prevent changes in EGABA. Desensitization of the GABA response (an actual decrease in gGABA) occurs in these neurons during prolonged application of GABA (greater than 1 s) but with a slower time course than changes in EGABA. Whole-cell voltage-clamp techniques applied to tissue-cultured spinal cord neurons indicated that rapid shifts in EGABA result from repetitive GABA application in these cells as well.(ABSTRACT TRUNCATED AT 250 WORDS)     

High- and low-threshold calcium currents in neurons acutely isolated from rat sensorimotor cortex

Neurons were isolated by papain treatment and trituration of the frontoparietal cortex of 14 to 28-day-old rats. Whole cell voltage clamp revealed a slowly inactivating high-threshold Ca²⁺ current, activated positive to −45 mV, and a transient low-threshold Ca²⁺ current, activated positive to −65 mV. The high-threshold current was more sensitive to block by Cd²⁺ and the low-threshold current was more sensitive to block by Ni²⁺. Replacement of Ca²⁺ by Ba²⁺ increased the high-threshold current and reduced the low-threshold current. The high-threshold current was enhanced by Bay K 8644 and reduced by nimodipine and ω-conotoxin. The low-threshold current was also reduced by nimodipine but was insensitive to Bay K 8644 and ω-conotoxin. The properties of the currents were consistent with different underlying Ca²⁺ channel types.