Exogenous serotonin induces rapid phosphorylation of IκBα that is inhibited by a 5-HT₇ receptor–selective antagonist. Freshly isolated naive T cells were incubated with SB 269970 (5-HT₇ receptor antagonist) or SB 216641 (5-HT_1B receptor antagonist) for 1 hour, 37°C. Samples were then pulsed with exogenous 5-HT (10μM) for 5 minutes at 37°C and analyzed for phospho-IκBα (top). Blots were stripped and reprobed for β-actin to confirm equal loading in each lane (bottom). Densitometric analysis was performed showing phospho-IκBα (relative to β-actin).