DEVELOPMENT OF VOLTAGE-ACTIVATED POTASSIUM CURRENTS IN CULTURED CEREBELLAR GRANULE NEURONS UNDER DIFFERENT GROWTH-CONDITIONS

1. The functional expression of two potassium currents in cultured cer ebellar granule cells was investigated with the whole cell patch-clamp technique in relation to development and growth condition, Cells were grown in medium containing different concentrations of potassium: 25 mM (K25) and 40 mM (K40), together referred to as ''high K+''; 10 mM ( K10) or ''low K+''; and K10 with 100 mu M N-methyl-D-aspartate (KNMDA) . All conditions are known to influence maturation and survival of gra nule cells in culture. 2. At 2 days in vitro (DIV) the membrane capaci tance, taken as index of membrane surface area, was the same for cells grown in each growth condition. At 7-9 DIV it had increased in each c ondition, but to a substantially larger extent in cells grown in KNMDA , K25, and K40 than in cells grown in K10, During development the inpu t resistance only decreased in cells grown in KNMDA and high K+. 3. A delayed potassium current (I-K) and a fast transient potassium current (I-A) could both be recorded at 2 DIV in each growth condition, altho ugh a few neurons only expressed the I-K. The I-K was partially suppre ssed by tetraethylammonium (5 mM), whereas I-A was predominantly sensi tive to 4-aminopyridine (5 mM). 4. Normalized for cell capacitance, th e specific I-A conductance hardly changed during development in cells grown in high K+ and KNMDA. Cells in K10, however, displayed an I-A wi th totally different properties in 23 of 24 cells; the specific I-A co nductance in these cells was considerably smaller at 7-9 DIV, suggesti ng a deletion of these channels during development. 5. At 2 DIV, I-A i nactivated monoexponentially with a time constant that did not signifi cantly differ between growth conditions, The time constant was slightl y voltage dependent. At 7-9 DIV the voltage dependence became more pro minent, in particular at voltages more positive than + 10 mV. In many cells grown in K10 in which a transient current still could be detecte d, the voltage dependence was even stronger, with the fastest inactiva tion at similar to 0 mV. 6. The voltage of half-maximal steady-state i nactivation of I-A was -90.8 +/- 0.3 (SE) mV at 2 DIV for cells in eac h growth condition. It shifted to more depolarized levels at 7-9 DIV, dependent on growth condition: -80 mV in K10 and KNMDA, -77 mV in K25, and -72 mV in K40. At 2 DIV the slope factor of the Boltzmann functio n describing the voltage-dependent removal of inactivation was 8.2 +/- 0.9 mV, which did not change at 7-9 DIV, when it was 8.9, 8.4, and 8. 0 mV in K10, K25, and K40, respectively. 7. The activation of the I-K was voltage dependent and half-maximal at similar to 0 mV in all growt h conditions and independent of time in culture. During development th e specific I-K conductance increased in the cells grown in the K10 con dition but not in cells grown in the other conditions. Because I-A was greatly reduced in this condition, then seems to be a reciprocal modu lation of these two currents in neurons grown in K10. 8. These finding s suggest that the functional expression of I-A and I-K in cerebellar granule cells in culture can be regulated by activity-dependent proces ses during a critical period of development, Because the high K+ and K NMDA conditions could mimic the early afferentiation of granule cells, similar mechanisms may play a role in the voltage gated potassium cha nnel expression in cerebellar granule cells in situ.