Potassium currents in acutely isolated human hippocampal dentate granule cells

1. Properties of voltage- and Ca2+-dependent K+ currents were investigated in thirty-four dentate granule cells acutely isolated from the resected hippocampus of eleven patients with therapy-refractory temporal lobe epilepsy (TLE). 2. When intracellular Ca2+ was strongly buffered with 11.5 mM EGTA-1 mM Ca2+ in the recording pipette, K+ currents (I-K) with a slow activation and biexponential time-dependent decay could be elicited, which showed a threshold for activation around -30 mV. 3. A contribution of Ca2+-dependent K+ currents became apparent with intracellular solution containing 1 mM BAPTA-0.1 mM Ca2+. Superfusion of low-Ca2+ extracellular solution blocked 43% of outward currents in this recording configuration. Outward current components could also be blocked by substituting 5 mM Ba2+ for extracellular Ca2+ (78%), or by application of 100 mu M Cd2+ (25%). 4. The Ca2+-dependent K+ currents could be pharmacologically subdivided into two components. One component was sensitive to 500 mu M tetraethylammmonium (TEA; 41%) and 10 nM charybdotoxin (CTX; 47.2%). The blocking effects of 10 nM CTX and 500 mu M TEA were not additive, suggesting that both agents block the same conductance. A second, smaller outward current component was blocked by 50 nM apamin (13%). 5. A transient A-type K+ current could be observed in six neurones and showed a fast monoexponential time-dependent inactivation with a steady-state voltage dependence that was distinct from that of I-K. The A-type current was blocked by 4-aminopyridine (4-AP) but not by TEA or low-Ca2+ solution. 6. We conclude that outward currents in human hippocampal dentate granule cells can be separated into at least four types by their kinetic and pharmacological properties. These include at least one voltage-dependent current similar to those observed in mammalian hippocampal neurones, and two Ca2+-dependent K+ currents that most probably correspond to SK- and BK-type currents. A classical A-type current could be detected in some patients with Ammon's horn sclerosis (AHS) but not in patients with lesion-associated TLE.