DIFFERENT TYPES OF POTASSIUM OUTWARD CURRENT IN RELAY NEURONS ACUTELY ISOLATED FROM THE RAT LATERAL GENICULATE-NUCLEUS

Different classes of potassium (K+) outward current activated by depolarization were characterized in relay neurons acutely isolated from the rat lateral geniculate nucleus (LGN), using the whole-cell version of the patch-clamp technique. A fast-transient current (I(A)), activated at around - 70 mV, declined rapidly with a voltage-dependent time constant (tau = 6 ms at + 45 mV), was 50% steady-state inactivated at - 70 mV, and rapidly recovered from inactivation with a monoexponential time course (tau = 21 ms). I(A) was blocked by 4-aminopyridine (4-AP, 2 - 8 mM) and was relatively insensitive to tetraethylammonium (TEA, 2 - 10 mM). After elimination of I(A) by a conditioning prepulse (30 ms to -50 mV), a slow-transient K+ current could be studied in isolation, and was separated into three components, I(Km), I(Ks) and a calcium (Ca2+)-dependent current, I(K[Ca]). The slow-transient current was not consistently affected by 4-AP (up to 8 mM), while TEA (2- 10 mM) predominantly blocked I(Ks) and I(K[Ca]). The component I(Km) persisted in a solution containing TEA and 4-AP, activated at around - 55 mV, declined monoexponentially during maintained depolarization (tau = 98 ms at + 45 mV), was 50% inactivated at - 39 mV, and recovered with tau = 128 ms from inactivation. I(Ks) activated at a similar threshold, but declined much slower with tau = 2662 ms at + 45 mV. Steady-state inactivation of I(Ks) was half-maximal at - 49 mV, and recovery from inactivation occurred relatively fast with tau = 116 ms. From these data and additional current-clamp recordings it is concluded that the K+ currents, due to their wide range of kinetics and dependence on membrane voltage or internal Ca2+ concentration, are capable of cooperatively controlling the firing threshold and of shaping the different states of electrophysiological behaviour in LGN relay cells.