Sustained potassium currents in maturing CA1 hippocampal neurones

Actions of bis(7)-tacrine and tacrine on transient potassium current in rat DRG neurons and potassium current mediated by K(V)4.2 expressed in Xenopus oocyte

Bis(7)-tacrine [bis(7)-tetrahydroaminacrine] is a dimeric AChE inhibitor derived from tacrine with a potential to treat Alzheimer's disease. Actions of bis(7)-tacrine on ligand-gated ion channels and voltage-gated cation channels have been identified on neurons of both central and peripheral nervous systems. In the present study, the effect of bis(7)-tacrine was investigated on the K(V)4.2 encoded potassium currents expressed in Xenopus oocytes and the transient A-type potassium current (I(K(A))) on rat DRG neurons. Bis(7)-tacrine suppressed recombinant Kv4.2 potassium channels in a concentration-dependent manner, with IC(50) value of 0.53+/-0.13 muM. Tacrine also inhibited Kv4.2 channels, but with a much lower potency (IC(50) 74+/-15 muM).The possible mechanisms underlying the inhibition on potassium currents by bis(7)-tacrine/tacrine could be that inactivation of the transient potassium currents was accelerated and recovery of the native or Kv4.2 expressed potassium currents was suppressed by bis(7)-tacrine/tacrine.

Insulin increases excitability via a dose-dependent dual inhibition of voltage-activated K+ currents in differentiated N1E-115 neuroblastoma cells

A role in the control of excitability has been attributed to insulin via modulation of potassium (K(+)) currents. To investigate insulin modulatory effects on voltage-activated potassium currents in a neuronal cell line with origin in the sympathetic system, we performed whole-cell voltage-clamp recordings in differentiated N1E-115 neuroblastoma cells. Two main voltage-activated K(+) currents were identified: (a) a relatively fast inactivating current (I(fast) - time constant 50-300 ms); (b) a slow delayed rectifying K(+) current (I(slow) - time constant 1-4 s). The kinetics of inactivation of I(fast), rather than I(slow), showed clear voltage dependence. I(fast) and I(slow) exhibited different activation and inactivation dependence for voltage, and have different but nevertheless high sensitivities to tetraethylammonium, 4-aminopyridine and quinidine. In differentiated cells - rather than in non-differentiated cells - application of up to 300 nm insulin reduced I(slow) only (IC(50) = 6.7 nm), whereas at higher concentrations I(fast) was also affected (IC(50) = 7.7 microm). The insulin inhibitory effect is not due to a change in the activation or inactivation current-voltage profiles, and the time-dependent inactivation is also not altered; this is not likely to be a result of activation of the insulin-growth-factor-1 (IGF1) receptors, as application of IGF1 did not result in significant current alteration. Results suggest that the current sensitive to low concentrations of insulin is mediated by erg-like channels. Similar observations concerning the insulin inhibitory effect on slow voltage-activated K(+) currents were also made in isolated rat hippocampal pyramidal neurons, suggesting a widespread neuromodulator role of insulin on K(+) channels.

The development of synaptic plasticity induction rules and the requirement for postsynaptic spikes in rat hippocampal CA1 pyramidal neurones

Coincident pre- and postsynaptic activity induces synaptic plasticity at the Schaffer collateral synapse onto CA1 pyramidal neurones. The precise timing, frequency and number of coincident action potentials required to induce synaptic plasticity is currently unknown. In this study we show that the postsynaptic activity required for the induction of long-term potentiation (LTP) changes with development. In acute slices from adult rats, coincident pre- and postsynaptic theta burst stimulation (TBS) induced LTP and we show that multiple high-frequency postsynaptic spikes are required. In contrast, in acute slices from juvenile (P14) rats, TBS failed to induce LTP unless the excitatory postsynaptic potentials (EPSPs) were of sufficient magnitude to initiate action potentials. We also show that coincident individual pre- and postsynaptic action potentials are only capable of inducing LTP in the juvenile when given at a frequency greater than 5 Hz and that the timing of individual pre- and postsynaptic action potentials relative to one another is not important. Finally, we show that local tetrodotoxin (TTX) application to the soma blocked LTP in adults, but not juveniles. These data demonstrate that somatic spiking is more important for LTP induction in the adult as opposed to juvenile rats and we hypothesize that the basis for this is the ability of action potentials in the postsynaptic CA1 pyramidal neurone to back-propagate into the dendrites. Therefore, the pre- and postsynaptic activity patterns required to induce LTP mature as the hippocampus develops.

The sensitivity of sodium channels in immature and mature rat CA1 neurones to the local anaesthetics procaine and lidocaine

Sodium currents were recorded in CA1 hippocampal cells from new-born (P(4-10)) and older (P(>22)) rats, using whole-cell voltage clamp techniques. The effects of local anaesthetics (procaine and lidocaine) were studied in both cell populations. Parameters defining steady-state inactivation, removal of inactivation and the affinity of the anaesthetic molecules to the inactivated state were determined at both stages of maturation. Procaine and lidocaine induced a hyperpolarizing shift in steady-state inactivation curves, and slowed the rate of recovery from the inactivated state. Procaine disclosed differences between immature and older cells in what concerns block of the closed (resting) channels, drug affinity and binding to the inactivated state, i.e. the binding rate of procaine was found higher and the affinity lower in younger cells. The characteristics of procaine and lidocaine block on CA1 sodium currents differed in some particular aspects: magnitude of block on resting channels, shift in the voltage dependence and voltage sensitivity of steady-state inactivation, slow recovery from inactivation and use-dependent block.

Structure-Based Discovery of Potassium Channel Blockers from Natural Products

Potassium ion (K(+)) channels are attractive targets for rational drug design. Based upon a three-dimensional model of the eukaryotic K(+) channels, the docking virtual screening approach was employed to search the China Natural Products Database. Compounds were ranked according to the relative binding energy, favorable shape complementarity, and potential of forming hydrogen bonds with the K(+) channel. Four candidate compounds found by virtual screening were investigated by using the whole-cell voltage-clamp recording in rat dissociated hippocampal neurons. When applied extracellularly, compound 1 markedly depressed the delayed rectifier K(+) current (I(K)) and fast transient K(+) current (I(A)), whereas compounds 2, 3, and 4 exerted a more potent and selective inhibitory effect on I(K). Intracellular application of the four compounds had no effect on both the K(+) currents.

BmKK4, a novel toxin from the venom of Asian scorpion Buthus martensi Karsch, inhibits potassium currents in rat hippocampal neurons in vitro

A novel short-chain peptide BmKK4 was isolated from the venom of Asian scorpion Buthus martensi Karsch. It is composed of 30 amino acids including six cysteine residues, and shares less than 25% sequence identity with the known alpha-KTx toxins. The action of BmKK4 on voltage-dependent potassium currents was examined in acutely dissociated hippocampal neurons of rat. BmKK4 (10-100 microM) inhibited both the delayed rectifier and fast transient potassium current in concentration-dependent manners. The inhibition was reversible and voltage-independent. BmKK4 caused a depolarizing shift (about 10 mV) of the steady-state activation curve of the currents, without changing their steady-state inactivation behavior. The unique amino acid sequence and electrophysiological effects suggest that BmKK4 represent a new subfamily of potassium channel toxins.

Na+-activated K+ current contributes to postexcitatory hyperpolarization in neocortical intrinsically bursting neurons

The ionic mechanisms underlying the termination of action-potential (AP) bursts and postburst afterhyperpolarization (AHP) in intrinsically bursting (IB) neocortical neurons were investigated by performing intracellular recordings in thin slices of rat sensorimotor cortex. The blockade of Ca(2+)-activated K(+) currents enhanced postburst depolarizing afterpotentials, but had inconsistent and minor effects on the amplitude and duration of AHPs. On the contrary, experimental conditions resulting in reduction of voltage-dependent Na(+) entry into the cells caused a significant decrease of AHP amplitude. Slice perfusion with a modified artificial cerebrospinal fluid in which LiCl (40 mM) partially replaced NaCl had negligible effects on the properties of individual APs, whereas it consistently increased burst length and led to an approximately 30% reduction in the amplitude of AHPs following individual bursts or short trains of stimulus-induced APs. Experiments performed by partially replacing Na(+) ions with choline revealed a comparable reduction in AHP amplitude associated with an inhibition of bursting activity. Moreover, in voltage-clamp experiments carried out in both in situ and acutely isolated neurons, partial substitution of extracellular NaCl with LiCl significantly and reversibly reduced the amplitude of K(+) currents evoked by depolarizing stimuli above-threshold for Na(+)-current activation. The above effect of Na(+)-to-Li(+) substitution was not seen when voltage-gated Na(+) currents were blocked with TTX, indicating the presence of a specific K(+)-current component activated by voltage-dependent Na(+) (but not Li(+)) influx. The above findings suggest that a Na(+)-activated K(+) current recruited by the Na(+) entry secondary to burst discharge significantly contributes to AHP generation and the maintenance of rhythmic burst recurrence during sustained depolarizations in neocortical IB neurons.

Huperzine A inhibits the sustained potassium current in rat dissociated hippocampal neurons

The actions of huperzine A (HupA), a novel cholinesterase inhibitor, on the sustained potassium current were investigated in acutely dissociated hippocampal neurons of rat. HupA inhibited the current (IC(50) = 856 +/- 1 microM) with voltage-dependency. The effect was insensitive to 3 microM atropine. Tacrine (IC(50) = 43 +/- 3 microM) was 20 times more potent than HupA. HupA hyperpolarized the activation curve of the current by 16 mV, and markedly prolonged the decay time constant tau(2). HupA affected neither the steady-state inactivation of the current, nor its recovery from inactivation. The potential relevance of the inhibitory effect of HupA on the current to the treatment of Alzheimer's disease is discussed.

Sodium channel currents in maturing acutely isolated rat hippocampal CA1 neurones

Sodium channel currents were recorded in excised inside-out patches from immature (P(4-10)) and older (P(20-46)) rat CA1 neurones. Channel conductance was 16.6+/-0.013 pS (P(20-46)) and 19.0+/-0.031 pS (P(4-10)). Opening patterns varied with step voltage and with age. In some patches bursting was apparent at voltages positive to -30 mV. Non-bursting behaviour was more dominant in patches from younger animals. In older animals mean open time (m.o.t.) was best described by two exponentials especially in the older cells; in the immature, there were fewer cases with two exponentials. The time constant of inactivation (tau(h)) estimated in ensemble averages was best described by two exponentials (tau(hf) and tau(hs)) in most patches from older cells. tau(hf) decreased with depolarization; tau(hs) increased in the range -30 to 0 mV. The voltage dependence of tau(hf) in the older cells is identical to that of the single tau(h) found in the younger; the results indicate a dominance of tau(hf) in the younger. Patches from younger cells more often showed one apparent active channel; in such cases, m.o.t. was described by a single exponential. However, in two cases, channels showed bursting behaviour with one of these channels showing a shift between bursting and non-bursting modes. Our findings are consistent with a heterogeneous channel population and with changes in the population in the course of maturation.

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.