To investigate possible ionic current mechanisms underlying ischemic arrhythmias, we studied single Na+ channel currents in rat and rabbit cardiac myocytes treated with the ischemic metabolite lysophosphatidylcholine (LPC) using the cell-attached and excised inside-out patch-clamp technique at 22 degrees C. LPC has been reported previously to reduce open probability and to induce sustained open channel activity at depolarized potentials. We now report two new observations for Na+ currents in LPC-treated patches: 1) The activation-voltage relation of the peak of the ensemble currents is shifted in the negative (hyperpolarizing) direction by approximately 20 mV compared with control currents. This effect was observed in all patches for depolarizations from a holding potential of -150 mV to different test potentials. 2) In some LPC-treated patches, Na+ channels exhibited sustained bursting activity at potentials as negative as -150 mV, giving a nondecaying inward current. This bursting activity was accompanied by double and triple simultaneous openings and closings, suggesting tight cooperativity in channel gating. These LPC-modified channels were identified as Na+ channels, because their unitary conductance was the same as Na+ channels in control solutions, because the single channel current-voltage relation was extrapolated to reverse at the Na+ Nernst potential, and because the current was blocked by the local anesthetic QX-222. This novel depolarizing current may play a role in the electrophysiological abnormalities in ischemia, including abnormal automaticity and reentrant arrhythmias, and could be a target for antiarrhythmic drugs.

It is widely believed that, in cortical pyramidal cells, action potentials (APs) initiate in the distal portion of axon initial segment (AIS) because that is where Na ⁺ channel density is highest. To investigate the relationship between the density of Na ⁺ channels and the spatiotemporal pattern of AP initiation, we simultaneously recorded Na ⁺ flux and action currents along the proximal axonal length. We found that functional Na ⁺ channel density is approximately four times lower in the AP trigger zone than in the middle of the AIS, where it is highest. Computational analysis of AP initiation revealed a paradoxical mismatch between the AP threshold and Na ⁺ channel density, which could be explained by the lopsided capacitive load imposed on the proximal end of the AIS by the somatodendritic compartment. Favorable conditions for AP initiation are therefore achieved in the distal AIS portion, close to the edge of myelin, where the current source–load ratio is highest. Our findings suggest that cable properties play a central role in determining where the AP starts, such that small plastic changes in the local AIS Na ⁺ channel density could have a large influence on neuronal excitability as a whole.

Vesicular Zn²⁺regulates postsynaptic neuronal excitability upon its corelease with glutamate. We previously demonstrated that synaptic Zn²⁺acts via a distinct metabotropic zinc-sensing receptor (mZnR) in neurons to trigger Ca²⁺responses in the hippocampus. Here, we show that physiological activation of mZnR signaling induces enhanced K⁺/Cl⁻cotransporter 2 (KCC2) activity and surface expression. As KCC2 is the major Cl⁻outward transporter in neurons, Zn²⁺also triggers a pronounced hyperpolarizing shift in the GABA_Areversal potential. Mossy fiber stimulation-dependent upregulation of KCC2 activity is eliminated in slices from Zn²⁺transporter 3-deficient animals, which lack synaptic Zn²⁺. Importantly, activity-dependent ZnR signaling and subsequent enhancement of KCC2 activity are also absent in slices from mice lacking the G-protein-coupled receptor GPR39, identifying this protein as the functional neuronal mZnR. Our work elucidates a fundamentally important role for synaptically released Zn²⁺acting as a neurotransmitter signal via activation of a mZnR to increase Cl⁻transport, thereby enhancing inhibitory tone in postsynaptic cells.

1. In whole cell recordings from layer V neurons in slices of mouse somatosensory neocortex, tetrodotoxin (TTX)-sensitive persistent Na+ current (INaP) was studied by blocking K+ currents with intracellular Cs+ and Ca2+ currents with extracellular Cd2+. During slow voltage ramps, INaP began to activate at around -60 mV, and attained a peak at around -25 mV. The peak amplitude of INaP varied widely from cell to cell (range 60-3,160 pA; median 308 pA, n = 77). At potentials more positive than -35 mV, INaP in all cells was superimposed on a large, TTX-resistant outward current. 2. In hybrid clamp experiments, INaP was significantly reduced by a preceding high-frequency train of spikes. 3. INaP underwent pronounced slow inactivation, which was revealed by systematically varying the ramp speed between 233 and 2.33 mV/s, or varying the duration of a depolarizing prepulse between 0.1 and 10 s. 4. Onset of slow inactivation at +20 mV was monoexponential with tau = 2.06 s (n = 17 cells). Recovery from slow inactivation was voltage dependent. It followed a monoexponential time course with tau = 2.31 s (n = 6) at -70 mV and tau = 1.10 s (n = 4) at -90 mV. These values are not significantly different than values previously reported for slow inactivation of fast-inactivating INa. 5. Slow inactivation of neocortical INaP will influence all neuronal functions in which this current plays a role, including spike threshold determination, synaptic integration, and active propagation in dendrites. The kinetics of slow inactivation suggest that it may be a factor not only during extremely intense spiking, but also during periods of "spontaneous" activity.