The role of potassium conductances in determining input resistance was studied in 166 genioglossal (GG) motoneurons using sharp electrode recording in brain stem slices of the rats aged 5-7 days, 13-15 days, and 19-24 days postnatal (P). A high magnesium (Mg2+; 6 mM) perfusate was used to block calcium-mediated synaptic release while intracellular or extracellular cesium (Cs+) and/or extracellular tetraethylammonium (TEA) or barium (Ba2+) were used to block potassium conductances. In all cases, the addition of TEA to the high Mg2+ perfusate generated a larger increase in both input resistance (R(n)) and the first membrane time constant (τ(O)) than did high Mg2+ alone indicating a substantial nonsynaptic contribution to input resistance. With intracellular injection of Cs+, GG motoneurons with lower resistance (<40 MΩ), on the average, showed a larger percent increase in R(n) than cells with higher resistance (>40 MΩ). There was also a significant increase in the effect of internal Cs+ on R(n) and τ(O) with age. The largest percent increase (67%) in the τ(O) due to intracellular Cs+ occurred at P13-15, a developmental stage characterized by a large reduction in specific membrane resistance. Addition of external Cs+ blocked conductances (further increasing R(n) and τ(O)) beyond those blocked by the TEA perfusate. Substitution of external calcium with 2 mM barium chloride produced a significant increase in both R(n) and τ(O) at all ages studied. The addition of either intracellular Cs+ or extracellular Ba2+ created a depolarization shift of the membrane potential. The amount of injected current required to maintain the membrane potential was negatively correlated with the control R(n) of the cell at most ages. Thus low resistance cells had, on the average, more Cs+- and Ba2+- sensitive channels than their high resistance counterparts. There was also a disproportionately larger percent increase in τ(O) as compared with R(n) for both internal Cs+ and external Ba2+. Based on a model by Redman and colleagues, it might be suggested that the majority of these potassium conductances underlying membrane resistance are initially located in the distal dendrites but become more uniformly distributed over the motoneuron surface in the oldest animals.
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