Properties of single potassium channels in hypothalamic neurons

In this work the cell-attached and the excised membrane patch configurations have been used to determine properties of voltage-dependent K⁺ channels in cultured rat hypothalamic neurons. With inside-out patches and 140 mM K⁺ in the bath and 5 mM K⁺ in the pipette step depolarizations, in excess of 20 mV, elicited channel activity with at least two independent current levels. The larger current level was studied presently and current-voltage plots showed the conductance of the channel to be 48 pS; measurements of the changes in reversal potential with 140 mM or 5 mM K⁺ in the bathing solution indicated the channel to be selective for K⁺. The 48 pS channel was blocked when the bathing solution contained 140 mM Cs⁺. A concentration of 10 mM TEA applied to the outside of the patch in the outside-out configuration caused a loss of channel activity whereas 4-AP had no obvious effect on channel conductance or kinetics. Changes in the bath concentration of Ca²⁺ had no apparent effect to alter the frequency or duration of channel openings for inside-out patches. In some instances obvious changes in channel kinetics were observed during the course of an experiment; these included long periods where no channel opening events occurred and on a few other occasions progressive increases in mean open time. In cell-attached patch experiments with no TTX in the bathing medium to block sodium channels, action potentials could be recorded through capacitive coupling between the cell and the pipette. In such cases the heights of spontaneous K⁺ channel currents were very similar to those associated with channel openings during the repolarization phase of the spike. The properties of the K⁺ channel studied here are consistent with those associated with the delayed rectifier K⁺ conductance (I_K) and would serve to control electrical excitability in hypothalamic neurons.