Phosphodiesterase 5 (PDE5) inhibitors are often used in combination with club drugs such as 3,4-methylenedioxymethamphetamine (MDMA or ecstasy). We investigated the consequences of such combination in the serotonergic system of the rat. Oral administration of sildenafil citrate (1.5 or 8 mg/kg) increased brain cGMP levels and protected in a dose-dependent manner against 5-hydroxytryptamine...