Acute FTY720 exposure increases neuronal excitability in a concentration- and time-dependent manner, but prolonged treatment blocks sensitization. (A) shows a representative recording wherein 4 APs were evoked under control conditions (left) whereas, after a 15-min exposure to 100 nM FTY720, the number of evoked APs were increased to 12. The dotted lines indicate the 0 mV level. (B) summarizes the effects of 100 nM FTY720 on sensory neurons. In one population, FTY720 significantly increased the number of APs after a 10-min exposure (n = 8), whereas another population appeared to be insensitive to FTY720 (n = 5). (C) A 30-min pretreatment with 5 μM SKI-II, a selective inhibitor of sphingosine kinases, blocked the capacity of 100 nM FTY720 to sensitize sensory neurons (n = 9 for the Cont-15 min and n = 7 for the 20-min time point, P = 0.16 Kruskal-Wallis ANOVA). (D) A 1-h pretreatment with 1 μM FTY720 blocks the ability of 100 nM SEW2871 and subsequent 1 μM S1P to sensitize sensory neurons (n = 6, P = 0.39 Friedman RM ANOVA on ranks). AP - action potential, Cont - control, S1P - sphingosine-1-phosphate.