Zinc modulates A-type potassium currents and neuronal excitability in snail neurons

Ionic mechanisms maintaining action potential conduction velocity at high firing frequencies in an unmyelinated axon

The descending contralateral movement detector (DCMD) is a high‐performance interneuron in locusts with an axon capable of transmitting action potentials (AP) at more than 500 Hz. We investigated biophysical mechanisms for fidelity of high‐frequency transmission in this axon. We measured conduction velocities (CVs) at room temperature during exposure to 10 mmol/L cadmium, a calcium current antagonist, and found significant reduction in CV with reduction at frequencies >200 Hz of ~10%. Higher temperatures induced greater CV reductions during exposure to cadmium across all frequencies of ~20–30%. Intracellular recordings during 15 min of exposure to cadmium or nickel, also a calcium current antagonist, revealed an increase in the magnitude of the afterhyperpolarization potential (AHP) and the time to recover to baseline after the AHP (Medians for Control: −19.8%; Nickel: 167.2%; Cadmium: 387.2%), that could be due to a T‐type calcium current. However, the removal of extracellular calcium did not mimic divalent cation exposure suggesting calcium currents are not the cause of the AHP increase. Computational modeling showed that the effects of the divalent cations could be modeled with a persistent sodium current which could be blocked by high concentrations of divalent cations. Persistent sodium current shortened the AHP duration in our models and increased CV for high‐frequency APs. We suggest that faithful, high‐frequency axonal conduction in the DCMD is enabled by a mechanism that shortens the AHP duration like a persistent or resurgent sodium current.

Inhibition of the activity of T lymphocyte Kv1.3 channels by extracellular zinc

The inhibitory effect of zinc on voltage-gated Kv1.3 channels in human T lymphocytes was investigated using the "whole-cell" patch-clamp technique. Application of 10 and 20 microM Zn caused a concentration-dependent shift of activation midpoint of the whole-cell currents from -19.65+/-1.03 mV (mean+/-SE) under control conditions to 9.84+/-0.66 mV upon application of 20 microM Zn. This effect was saturated at zinc concentrations higher than 20 microM. The activation rate was considerably slower, whereas the deactivation rate was not significantly affected by Zn. Inactivation midpoint was shifted from -53.06+/-0.44 mV under control conditions to -36.05+/-0.48 mV in the presence of 100 microM Zn. Inactivation rate was not significantly affected upon Zn treatment. Whole-cell potassium currents were reduced to about 70% of their control values with no clear concentration dependence in the zinc concentration range from 10 to 100 microM. When raising the zinc concentration to levels above 100 microM, a concentration-dependent inhibition of the whole-cell currents appeared additionally to the changes in channel gating. The channels were half-blocked at the zinc concentration of 346+/-40 microM and the Hill slope coefficient was 1.89+/-0.21. The inhibitory effect of zinc was not complete at micromolar concentrations and was saturated at concentrations higher than 1mM. This inhibitory effect was not accompanied by any further modification in the shift of the activation and inactivation midpoints nor by a slowing of the channel activation rate. The inhibitory effect of zinc was significantly diminished in the presence of 150 mM K(+) in the extracellular solution, whereas the zinc-induced shift of the activation threshold and slowing of the activation kinetics remained unchanged when raising extracellular potassium concentration. It is suggested that zinc acts on two independent binding sites on the channels. Binding to one site that is saturated at concentrations higher than 20 microM affects the channel gating. Binding to another site at concentrations higher than 100 microM inhibits the currents without affecting the channel gating.

Somatodendritic depolarization-activated potassium currents in rat neostriatal cholinergic interneurons are predominantly of the A type and attributable to coexpression of Kv4.2 and Kv4.1 subunits

Unlike other neostriatal neurons, cholinergic interneurons exhibit spontaneous, low-frequency, repetitive firing. To gain an understanding of the K+ channels regulating this behavior, acutely isolated adult rat cholinergic interneurons were studied using whole-cell voltage-clamp and single-cell reverse transcription-PCR techniques. Cholinergic interneurons were identified by the presence of choline acetyltransferase (ChAT) mRNA. Depolarization-activated potassium currents in cholinergic interneurons were dominated by a rapidly inactivating, K+-selective A current that became active at subthreshold potentials. Depolarizing prepulses inactivated this component of the current, leaving a delayed, rectifier-like current. Micromolar concentrations of Cd2+ dramatically shifted the voltage dependence of the A current without significantly affecting the delayed rectifier. The A-channel antagonist 4-aminopyridine (4-AP) produced a voltage-dependent block (IC50, approximately 1 mM) with a prominent crossover at millimolar concentrations. On the other hand, TEA preferentially blocked the sustained current component at concentrations <10 mM. Single-cell mRNA profiling of subunits known to give rise to rapidly inactivating K+ currents revealed the coexpression of Kv4.1, Kv4.2, and Kv1.4 mRNAs but low or undetectable levels of Kv4.3 and Kv3.4 mRNAs. Kv1.1, beta1, and beta2 subunit mRNAs, but not beta3, were also commonly detected. The inactivation recovery kinetics of the A-type current were found to match those of Kv4.2 and 4.1 channels and not those of Kv1.4 or Kv1. 1 and beta1 channels. Immunocytochemical analysis confirmed the presence of Kv4.2 but not Kv1.4 subunits in the somatodendritic membrane of ChAT-immunoreactive neurons. These results argue that the depolarization-activated somatodendritic K+ currents in cholinergic interneurons are dominated by Kv4.2- and Kv4. 1-containing channels. The properties of these channels are consistent with their playing a prominent role in governing the slow, repetitive discharge of interneurons seen in vivo.

Lamotrigine may limit pathological excitation in the hippocampus by modulating a transient potassium outward current

Actions of the new antiepileptic drug lamotrigine were characterised using whole cell patch clamp recordings from rat CA1 pyramidal cells in vitro. The results suggest that lamotrigine, besides its previously described effect on the fast sodium inward current and calcium currents, modulates the transient potassium outward current ID. This may be an effective mechanism to inhibit pathological excitation.