Figure 4. IS neurons express a fast transient outward current with similar kinetics to Kv4.

(a) Whole-cell currents in response to a family of voltage steps from −80 to 0 mV in 5 mV steps. (b) A-type current separated from other outward current components. The remaining step-evoked current following a pre-pulse (−30 mV, 200 ms) capable of inactivating A-type current was subtracted from total current. Voltage steps from −50 to +40 in 10 mV steps. Recordings were carried out in the presence of 5 mM TEA in order to block slowly activating K⁺ currents. (c) Voltage-dependence of steady-state activation and inactivation. (d) Activation (red) and inactivation (blue) time constants of dissected A-type current (top) and recovery of inactivation time constant (bottom). (e) The fast inactivating outward current found in these cells was sensitive to the A-type current blocker 4-AP (7 mM) and the Kv4-specific blocker phrixotoxin (PhTX, 5 µM). Top panels: total currents in control, drug application, and washout, as indicated. Lower panel: PhTX block of transient outward current fraction, separated as in (b). (f) Average of 537 aligned APs following ISIs lasting longer than 100 ms shows a small prespike dip or inflection, attributed to the activation of the transient K⁺ current. (g), (i) Isolated transient outward current with a single exponential fitted to the decay phase (τ = 13.05 ms). (ii) Example current from a cluster of transient K⁺ channels in a cell-attached patch (step from RP-30 mV to RP+50 mV at the time indicated by arrow, outward current plotted upwards), fitted with the same exponential time constant as in (i). (h) Dependence of patch current on the potential of a 500 ms prepulse before a step from RP-30 to RP+50 mV, showing that it inactivates over the range RP+10 mV to RP+40 mV.

DOI: http://dx.doi.org/10.7554/eLife.16475.009

Figure 4—figure supplement 1. Irregularity is not diminished by buffering intracellular calcium.

An IS cell recorded with a patch pipette containing normal intracellular solution and stimulated with a steady current stimulus of 150 pA (left) is then repatched with a pipette containing intracellular solution, to which 10 mM BAPTA, a fast calcium and high-affinity buffer has been added (right), and stimulated with the same current level. Control CV(ISI) = 0.22 (125 ISIs), BAPTA CV(ISI) = 0.44 (129 ISIs, excluding initial 700 ms transient responses).

DOI: http://dx.doi.org/10.7554/eLife.16475.011

Figure 4—figure supplement 2. Fast inactivating outward current is insensitive to TEA (2 mM).

Voltage steps to −10 mV from −80 mV holding potential (n = 6).

DOI: http://dx.doi.org/10.7554/eLife.16475.012

Figure 4—figure supplement 3. Gad2-GFP cortical interneurons from primary cultures display the two conductances required for spiking irregularity.

(a) Cells expressed a large, fast-inactivating outward current (n = 4). Steps from −75 mV to 0 mV, held at −80 mV (b) In some cases (n = 3), after measuring spike irregularity, cells were repatched with a Cs based solution and locally perfused with Cd, TEA and 4AP, to block K⁺ and Ca²⁺ currents. Slow ramp depolarization (from −80 to −10, 20 mV/s) revealed persistent sodium current activating approximately at −55 mV, as in the slice preparation.

DOI: http://dx.doi.org/10.7554/eLife.16475.013