Voltage-activated potassium outward currents in two types of spider mechanoreceptor neurons

We studied the properties of voltage-activated outward currents in two type s of spider cuticular mechanoreceptor neurons to learn if these currents co ntribute to the differences in their adaptation properties. Both types of n eurons adapt rapidly to sustained stimuli, but type A neurons usually only fire one or two action potentials, whereas type B neurons can fire bursts l asting several hundred milliseconds. We found that both neurons had two out ward current components, I) a transient current that activated rapidly when stimulated from resting potential and inactivated with maintained stimuli and 2) a noninactivating outward current. The transient outward current cou ld be blocked by 5 mM tetraethylammonium chloride, 5 mM 4-aminopyridine, or 100 mu M quinidine, but these blockers also reduced the amplitude of the n oninactivating outward current. Charybdotoxin or apamin did not have any ef fect on the outward currents, indicating that Ca2+-activated K+ currents we re not present or not inhibited by these toxins. The only significant diffe rences between type A and type B neurons were found in the half-maximal act ivation (V-50) values of both currents. The transient current had a (V-50), value of 9.6 mV in type A neurons and -13.1 mV in type B neurons, whereas the V-50 values of noninactivating outward currents were -48.9 mV for type A neurons and -56.7 mV for type B neurons. We,lie conclude that, although d ifferences in the activation kinetics of the voltage-activated K+ currents could contribute to the difference in the adaptation behavior of type A and type B neurons, they are not major factors.