High activity K+ channels in rat hippocampal neurones maintained in culture

A channel was identified in cell-attached recordings in rat hippocampal neu rones maintained in culture. This channel, which was highly active at the r esting membrane potential, was present in most (73 %) parches studied. The channel was characterized by long duration openings and a high open probabi lity (P-o, mean value 0.73 at -70 mV) at negative patch potentials with mil d voltage dependence over the range -40 to -120 mV. It showed inward rectif ication. There were up to five active channels in cell-attached recordings in experiments where the cells were bathed in sodium-containing Locke solut ion. The single channel conductances in cell-attached recordings with 140 o r 40 mM K+ in the patch pipette were 26 and 12 pS, respectively. The channe l was therefore selective for K+ over Na+. The channel was not permeable to Rb+ ions. The single channel conductance was 24 pS in excised inside-out p atches bathed in symmetrical K+ (140mM) solutions. Examination of the chann el kinetics revealed that both the open and closed time distributions could be fitted by the sum of three exponentials, there being no pronounced volt age sensitivity between -60 and -120 mV. The 26 pS K+ channel was insensiti ve to extracellular TEA, apamin, 4-AP and dequalinium. Neither was it sensi tive to intracellular Ca2+. Extracellular Ba2+ was effective in reversibly blocking the channel, the IC50 being 2.0 mM. There was no obvious effect of bath application of the K+ channel opener, lemakalim, or a cAMP analogue. This channel appears to contribute a significant proportion (at least 30 %) of the resting conductance in these neurones.