Characterization of four types of background potassium channels in rat cerebellar granule neurons

Cerebellar granule neurons express a standing outward (background) K+ current (I-K,I-SO) that regulates the resting membrane potential and cell excitability. As several tandem-pore (2P) K+ channel mRNAs are highly expressed in cerebellar granule cells, we studied whether, and which, 2P K+ channels contribute to I-K,I-SO. I-K,I-SO was highly sensitive to changes in extracellular pH and was partially inhibited by acetylcholine, as reported previously. In cell-attached patches from cultured cerebellar granule neurons, four types of K+ channels were found to be active when membrane potential was held at -50 mV or +50 mV in symmetrical 140 mm KCl. Based on single-channel conductances, gating kinetics and modulation by pharmacological agents and pH, three K+ channels could be considered as functional correlates of TASK-1, TASK-3 and TREK-2, which are members of the 2P K+ channel family. The fourth K+ channel (Type 4) has not been described previously and its molecular correlate is not yet known. Based on the measurement of channel current densities, the Type 2 (TASK-3) and the Type 4 K+ channels were determined to be the major sources of I-K,I-SO in cultured cerebellar granule neurons. The Type 1 (TASK-1) and Type 3 (TREK-2) activities were relatively low throughout cell growth in culture (1 - 10 days). Similar to TASK-I and TASK-3, the Type 4 K+ channel was highly sensitive to changes in extracellular pH, showing a 78% inhibition by changing the extracellular pH from 7.3 to 6.3. The results of this study show that three 2P K+ channels and an additional pH-sensing K+ channel (Type 4) comprise the I-K,I-SO in cultured cerebellar granule neurons. Our results also show that the high sensitivity of I-K,I-SO to extracellular pH comes from the high sensitivity of Type 2 (TASK-3) and Type 4 K+ channels.