5-HT induces rapid phosphorylation of ERK1/2 that is inhibited by a 5-HT₇ receptor–selective antagonist. Freshly isolated naive T cells were incubated with exogenous 5-HT (10 μM) at 37°C. Samples were lysed, immunoblotted, and probed for (A) phospho-ERK1/2 (top). Blots were stripped and reprobed for total ERK1/2 to confirm equal loading in each lane (bottom). Densitometric analysis was performed showing a maximal increase in phospho-ERK1/2 at 5 minutes following stimulation with exogenous 5-HT (relative to total ERK). (B) 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 5-HT (10 μM) for 5 minutes at 37°C) and analyzed for phospho-ERK1/2 (top) and total ERK1/2 (bottom). Densitometric analysis was performed, showing phospho-ERK p42 (relative to total ERK).