Serotonin inhibits naive T-cell production of cAMP. (A) Change in levels of cAMP in naive T cells in response to single or combined treatment as indicated with 5-HT (10 μM), the PDE-4 inhibitor, rolipram (30 μM), and the type 1 5-HT receptor inhibitor, S-Way 100135 (1 μM). Each data point is the mean (± SE) of 4 to 5 experiments (^*P < .01 compared with control; †P < .05 compared with other rolipram groups by ANOVA). (B) Dose-response relationship for inhibition of cAMP by 5-HT in T cells pretreated with rolipram. The smooth line is the fit to a Hill equation.