Transient potassium currents in avian sensory neurons

The effects of lead on transient outward currents of acutely dissociated rat dorsal root ganglia

The effects of Pb2+ on transient outward currents (TOCs) were investigated on rat dorsal root ganglia (DRG) neurons at postnatal days of 15 approximately 21, using the conventional whole-cell patch-clamp technique. In media-sized (35 approximately 40 microm) neurons and in the presence of 50 mM TEA, TOCs that preliminarly included an A-current (IA) and a D-current (ID), were clearly present and dominant. Application of Pb2+ lengthened the initial delay of TOCs and increased the onset-peak time in a concentration-dependent manner. The amplitudes of initial outward current peak were reduced with increasing Pb2+ concentrations. The inhibitory effects of Pb2+ on TOCs were reversible with 80 approximately 90% of current reversed in 2 approximately 10 min at 1 approximately 400 microM Pb2+. For the normalized activation curves fitted by a single Boltzmann equation under each condition, there was a shift to more depolarized voltages with increasing concentrations of Pb2+. The V1/2 and the slope factor (k) increased from 12.76+/-1.49 mV and 15.31+/-1.66 mV (n=10) under control condition to 39.91+/-5.44 mV (n=10, P<0.01) and 21.39+/-3.13 mV (n=10, P<0.05) at 400 microM Pb2+, respectively, indicating that Pb2+ decreased the activation of TOCs. For the normalized steady-state inactivation curves, the V1/2 and the k increased from -92.31+/-2.72 and 8.59+/-1.36 mV (n=10) to -55.65+/-3.67 (n=10, P<0.01) and 23.02+/-2.98 mV (n=10, P<0.01) at 400 microM Pb2+, respectively. The curves were shifted to more depolarized voltages by Pb2+, indicating that channels were less likely to be inactivated at higher concentrations of Pb2+ at any given potential. The fast (tf) and slow (ts) decay time-constants were both significantly increased by increasing concentrations of Pb2+ (n=10, P<0.05), indicating that Pb2+ increased the decay time-course of TOCs. These effects were concentration-dependent and partly reversible following washing. Ca2+ modulated the TOCs gating and might share same binding site with Pb2+, for which Ca2+ had very low affinity. In summary, the results demonstrated that Pb2+ was a dose- and voltage-dependent, and reversible blocker of TOCs in rat DRG neurons. After Pb2+ application, normal sensory physiology of DRG neurons was affected, and these neurons might display aberrant firing properties that resulted in abnormal sensations. This variation caused by Pb2+ could underlie the toxical modulation of sensory input to the central nervous system.

Voltage-gated currents distinguish parvocellular from magnocellular neurones in the rat hypothalamic paraventricular nucleus

1. Magnocellular and parvocellular neurones of the hypothalamic paraventricular nucleus (PVN) differentially regulate pituitary hormone secretion and autonomic output. Previous experiments have suggested that magnocellular, or type I neurones, and parvocellular, or type II neurones, of the PVN express different electrophysiological properties. Whole-cell patch-clamp recordings were performed in hypothalamic slices to identify the voltage-gated currents responsible for the electrophysiological differences between type I and type II PVN neurones. 2. Type I neurones, which display transient outward rectification and lack a low-threshold spike (LTS), generated a large A-type K+ current (IA) (mean +/- s.e. m.: 1127.5 +/- 126.4 pA; range: 250-3600 pA; voltage steps to -25 mV) but expressed little or no T-type Ca2+ current (IT). Type II neurones, which lack transient outward rectification but often display an LTS, expressed a smaller IA (360.1 +/- 56.3 pA; range: 40-1100 pA; voltage steps to -25 mV), and 75 % of the type II neurones generated an IT (-402.5 +/- 166.9 pA; range: -90 to -2200 pA; at peak). 3. The voltage dependence of IA was shifted to more negative values in type I neurones compared to type II neurones. Thus, the activation threshold (-53.5 +/- 0.9 and -46.1 +/- 2.6 mV), the half-activation potential (-25 +/- 1.9 and -17.9 +/- 2.0 mV), the half-inactivation potential (-80.4 +/- 9.3 and -67.2 +/- 3.0 mV), and the potential at which the current became fully inactivated (-57.4 +/- 2.1 and -49.8 +/- 1.5 mV) were more negative in type I neurones than in type II neurones, respectively. 4. IT in type II neurones activated at a threshold of -59.2 +/- 1.2 mV, peaked at -32. 6 +/- 1.7 mV, was half-inactivated at -66.9 +/- 2.2 mV, and was fully inactivated at -52.2 +/- 2.2 mV. 5. Both cell types expressed a delayed rectifier current with similar voltage dependence, although it was smaller in type I neurones (389.7 +/- 39.3 pA) than in type II neurones (586.4 +/- 76.0 pA). 6. In type I neurones IA was reduced by 41.1 +/- 7.0 % and the action potential delay caused by the transient outward rectification was reduced by 46.2 +/- 10.3 % in 5 mM 4-aminopyridine. In type II neurones IT was reduced by 66.8 +/- 10.9 % and the LTS was reduced by 76.7 +/- 7.8 % in 100 microM nickel chloride, but neither IT nor LTS was sensitive to 50 microM cadmium chloride. 7. Thus, differences in the electrophysiological properties between type I, putative magnocellular neurones and type II, putative parvocellular neurones of the PVN can be attributed to the differential expression of voltage-gated K+ and Ca2+ currents. This diversity of ion channel expression is likely to have profound effects on the response properties of these neurosecretory and non-neurosecretory neurones.

Single voltage-gated K+ channels and their functions in small dorsal root ganglion neurones of rat

1. Single voltage-activated K+ channels were investigated by means of the patch-clamp technique in small dorsal root ganglion (DRG) neurones in 150 microns thin slices of new-born rat DRG. It was found that K+ conductance in small DRG neurones is formed by one type of fast inactivating A-channel and four types of delayed rectifier K+ channels, which could be separated on the basis of their single-channel conductance, kinetics and sensitivity to external tetraethylammonium (TEA). 2. Potassium A-channels were observed at relatively moderate density. They were weakly sensitive to TEA and activated between -70 and +20 mV. The conductance of A-channels was about 40 pS for inward currents in symmetrical high-K+ solutions with external 5 mM TEA added to suppress other types of K+ channels. The time constant of channel inactivation (tau in) was 18.8 ms at -70 mV and 6 ms at potentials positive to -20 mV. 3. A fast delayed rectifier (DRF) channel with a conductance of 55 pS in symmetrical high-K+ solutions was the most frequent type of K+ channel. The channel activated in a broad potential range between -50 and +60 mV and demonstrated a fast deactivation within 1-3 ms after potential return to -80 mV in high-Ko+ solution. The tau in value was 90-150 ms at positive membrane potentials. The single-channel current amplitudes were blocked to 55% by 1 mM TEA. 4. Three further types of delayed rectifier K+ channels were called DR1-, DR2- and DR3- channels. Their single-channel conductances for inward currents in symmetrical high-K+ solutions were distributed between 30 and 44 pS. The channels activated in almost the same voltage range between -60 and -10 mV. Deactivation of the channels at -80 mV lasted tens of milliseconds. The channels were separated on the basis of their sensitivities to TEA. DR1-channel currents were reduced to 50% in the presence of 1 mM TEA, DR2-channel currents were reduced to about 50% by 5 mM TEA, whereas the amplitudes of currents through DR3-channels were almost unaffected by 5 mM TEA. 5. Addition of external 1 and 5 mM TEA to whole cells under current-clamp condition depolarized the cell membrane, lowered the threshold for action potential firing, prolonged action potential duration and reduced the amplitude of after-hyperpolarization. 6. It is concluded that potassium A-, DRF-, DR1-, DR2- and DR3-channels play multiple roles in the excitability of DRG neurones. Possible influences of these channels on the shape of the action potential, its firing threshold and the resting membrane potential of small DRG neurones are discussed.

Identification of single transiently opening ("A-type") K channels in guinea-pig colonic myocytes

Two K+ channel populations were identified in depolarized cell-attached membrane patches of myocytes freshly dispersed from the circular smooth muscle of guinea-pig proximal colon. First, a large-conductance (150 pS) Ca(2+)-activated K+ channel which was non-inactivating and sensitive to blockade by tetraethylammonium (TEA, 0.5-5 mM); and second, a smaller conductance K+ channel which opened and closed within 100 ms, was insensitive to TEA (0.5-5 mM), but was blocked by 5 mM 4-aminopyridine (4-AP) or maintained depolarization, and which had a unitary conductance of 12-13 pS. The averaged time course of these smaller conductance K+ channels closely resembled the time course of the 4-AP-sensitive, Ca(2+)-insensitive transient outward K+ current recorded in the whole-cell recording mode.

Permeation in ionic channels: a statistical rate theory approach

A novel way to model permeation through ionic channels is formulated. Our method does not require that equilibrium exists in the channel or at the channel interfaces. In addition, the potential profile does not need to be specified and the assumption of constant field across the membrane does not need to be made. Our formulation relies on statistical rate theory for its development and uses a form of the electrochemical potential which assumes that the ions are in solution. We show that the conductance and the degree of nonlinearity are dependent on the relative equilibrium exchange rates in the channel and at the interfaces. Nonlinear current-voltage plots can be obtained in symmetric solutions as well as a nonunity exponent for the Ussing flux ratio. Due to the dependence of the partition coefficient on solubility, it is highly unlikely that the intracellular and extracellular partition coefficients are the same. A manifestation of unequal partition coefficients is a current reversal at a membrane voltage that is different from the Nernst potential of the current-carrying ionic species.

Properties of transient K+ currents and underlying single K+ channels in rat olfactory receptor neurons

The transient potassium current, IK(t), of enzymatically dissociated rat olfactory receptor neurons was studied using patch-clamp techniques. Upon depolarization from negative holding potentials, IK(t) activated rapidly and then inactivated with a time course described by the sum of two exponential components with time constants of 22.4 and 143 ms. Single-channel analysis revealed a further small component with a time constant of several seconds. Steady-state inactivation was complete at -20 mV and completely removed at -80 mV (midpoint -45 mV). Activation was significant at -40 mV and appeared to reach a maximum conductance at +40 mV (midpoint -13 mV). Deactivation was described by the sum of two voltage-dependent exponential components. Recovery from inactivation was extraordinarily slow (50 s at -100 mV) and the underlying processes appeared complex. IK(t) was reduced by 4-aminopyridine and tetraethylammonium applied externally. Increasing the external K+ concentration ([K+]o) from 5 to 25 mM partially removed IK(t) inactivation, usually without affecting activation kinetics. The elevated [K+]o also hyperpolarized the steady-state inactivation curve by 9 mV and significantly depolarized the voltage dependence of activation. Single transient K+ channels, with conductances of 17 and 26 pS, were observed in excised patches and often appeared to be localized into large clusters. These channels were similar to IK(t) in their kinetic, pharmacological, and voltage-dependent properties and their inactivation was also subject to modulation by [K+]o. The properties of IK(t) imply a role in action potential repolarization and suggest it may also be important in modulating spike parameters during neuronal burst firing. A simple method is also presented to correct for errors in the measurement of whole-cell resistance (Ro) that can result when patch-clamping very small cells. The analysis revealed a mean corrected Ro of 26 G omega for these cells.

A five-conductance model of the action potential in the rat sympathetic neurone

The origin of the action potential in neurones has yet to be answered satisfactorily for most cells. We present here a five-conductance model of the somatic membrane of the mature and intact sympathetic neurone studied in situ in the isolated rat superior cervical ganglion under two-electrode voltage-clamp conditions. The neural membrane hosts five separate types of voltage-dependent ionic conductances, which have been isolated at 37 degrees C by using simple manipulations such as conditioning-test protocols and external ionic pharmacological treatments. The total current could be separated into two distinct inward components: (1) the sodium current, INa, and (2) the calcium current, ICa; and three outward components: (1) the delayed rectifier, IKV, (2) the transient IA, and (3) the calcium-dependent IKCa. Each current has been kinetically characterized in the framework of the Hodgkin-Huxley scheme used for the squid giant axon. Continuous mathematical functions are now available for the activation and inactivation (where present) gating mechanisms of each current which, together with the maximum conductance values measured in the experiments, allow for a satisfactory reconstruction of the individual current tracings over a wide range of membrane voltage. The results obtained are integrated in a full mathematical model which, by describing the electrical behaviour of the neurone under current-clamp conditions, leads to a quantitative understanding of the physiological firing pattern. While, as expected, the fast inward current carried by Na+ contributes to the depolarizing phase of the action potential, the spike falling phase is more complex than previous explanations. IKCa, with a minor contribution from IKV, repolarizes the neurone only under conditions of low cell internal negativity. Their role becomes less pronounced in the voltage range negative to -60 mV, where membrane repolarization allows IA to deinactivate. In the spike arising from these voltage levels the membrane repolarization is mainly sustained by IA, which proves to be the only current sufficiently fast and large enough to recharge the membrane capacitor at the speed observed during activity. Different modes of firing coexist in the same neurone and the switching from one to another is fast and governed by the membrane potential level, which makes the selection between the different voltage-dependent channel systems. The neurone thus seems to be prepared to operate within a wide voltage range; the results presented indicate the basic factors underlying the different discrete behaviours.

Potassium currents in chick sensory neurons change with development

We used the whole-cell configuration of the giga-seal voltage-clamp to study voltage-gated potassium currents in sensory neurons dissociated from dorsal root ganglia from embryonic and hatched chicks. Neurons from 8-, 10-, 14-, and 18-day-old embryos (E8, E10, E14, E18) and 1- to 5-day-old chicks were studied under conditions which inhibited inward currents and calcium-activated currents (tetrodotoxin, no added calcium, intracellular EGTA). At all ages, potassium currents were activated by depolarizations to potentials positive to -40 mV. At a given age the amount of inactivation of outward current during 50- to 100-ms steps varied from cell to cell; some cells showed no inactivation while in others the outward current declined to about half of the peak current. On average, the amount of inactivation was fairly stable at E8, E10, E18, and in hatched chicks but showed a transient increase at E14. In contrast, currents elicited by 50-ms test steps following 2-s conditioning steps showed an age dependent change. In E8 neurons, shifting the conditioning voltage from -100 to -90 mV had little or no effect on the current at the end of the test step while earlier outward current was reduced. In cells from older embryos or hatched chicks, similar conditioning voltages caused reductions of both early and late currents during the test step. The relative amount of late current inactivated by this protocol increased as the age of the chicks increased. In addition, the amount of variation in the inactivation properties was larger in cells from older embryos and hatched birds. The changes in outward current occur during a period in which new neurons are formed and existing neurons mature and establish function.