TASK-like K+ channels mediate effects of 5-HT and extracellular pH in rat dorsal vagal neurones in vitro

摘要

Dorsal vagal neurones (DVN) receive serotonergic projections from the medullary raphé nuclei, suggesting that 5-HT modulates vagal activity. A previous study has shown that 5-HT excites DVN in part by inhibition of a K⁺ current via postsynaptic 5-HT2A receptors. As mRNA for the two-pore-domain K⁺ channels TASK-1 (KCNK3) and TASK-3 (KCNK9) has been found in DVN, we investigated the possibility that 5-HT exerts its effects via inhibition of these K⁺ channels using whole-cell patch-clamp techniques. In current clamp, 5-HT (20 μm) elicited a depolarization by 5.1 ± 1.5 mV and an increase in firing rate. In voltage clamp, 5-HT reduced the standing outward current (ISO) at −20 mV by 106 ± 17 pA, inhibiting a conductance (reversal, −95 ± 4 mV) which displayed Goldman-Hodgkin-Katz outward rectification, supportive of a TASK-like K⁺ current. Since TASK channels are modulated by extracellular pH (pHo), we next investigated the pH sensitivity of ISO in Hepes-buffered ACSF. At pHo 7.3, DVN exhibited an ISO of 147 ± 15 pA at −20 mV. Acidification to pHo 6.3 reduced ISO to 85 ± 13 pA, whereas raising pHo to 8.5 increased ISO to 216 ± 26 pA. At pHo 7.3, ISO was inhibited by BaCl2 (IC50 465 μm), but unaffected by ZnCl2 (100 μm). 5-HT (10 μm) reduced ISO by 114 ± 17 pA at pH_o 7.3, but at pH_o 6.3 the 5-HT-induced inhibition of I_SO was significantly smaller. The present data suggest that the excitatory effects of 5-HT on DVN are mediated in part by inhibition of a TASK-like, pH-sensitive K⁺ conductance. The pharmacological profile of this conductance excludes TASK-3 homomers, but rather implicates TASK-1-containing channels.