2 TYPES OF BK CHANNELS IN IMMATURE RAT NEOCORTICAL PYRAMIDAL NEURONS

1. The properties of large conductance Ca2+-activated K+ channels (BK channels) were investigated in neocortical infragranular pyramidal neu rons by the use of inside-out patch recordings. Neurons were acutely i solated from slices of newborn to 28-day-old rats (P0-P28) by using mi nimal protease exposure followed by trituration with a vibrating glass probe. Two types of BK channels, slow-gating and fast-gating, were ob served in immature neurons (P0-P5), whereas only slow-gating BK were f ound in more mature neurons. Fast-gating BK channels differed in condu ctance, voltage dependence, and kinetics from the slow-gating ones. 2. The properties of fast-gating channels included a conductance of 145 +/- 12.9 (SE) pS; frequent openings with short mean open times that we re relatively voltage-independent, mean closed times that showed a vol tage-dependent increase, a voltage-dependent decrease in open probabil ity (P-o). The properties of slow-gating channels contrasted with thos e of the fast-gating ones, in that the former had a conductance of 181 +/- 3.9 pS, longer mean open times that showed a voltage-dependent in crease, mean closed times that showed a marked voltage-dependent decre ase, a volta,ae-dependent increase in P-o, and slight inward rectifica tion. The significance of these developmental variations in channel pr operties is discussed.