Chlorpheniramine, a first-generation antihistamine, is widely used for allergic reactions. Previous studies showed the interaction between antidepressant activity and nitric oxide and cyclic guanosine monophosphate (NO/cGMP) pathway. Thus, we aimed to assess the possible involvement of NO/cGMP pathway in this effect using forced swim test (FST) in male mice. To evaluate the locomotor activity and immobility time, we performed open field test (OFT) and FST on each mouse. Chlorpheniramine was administered intraperitoneally (i.p.) (0.1, 0.3, 1, 10 mg/kg) 30 minutes before FST. To assess the involvement of NO/cGMP pathway, a non-selective nitric oxide synthase (NOS) inhibitor, L-NAME (10mg/kg, i.p.), a selective inducible NOS (iNOS) inhibitor, aminoguanidine (50 mg/kg, i.p.), a selective neural NOS (nNOS) inhibitor, 7-nitroindazole (7-NI, 30 mg/kg, i.p.), a NO precursor, L-arginine (750 mg/kg, i.p.) and a selective phosphodiesterase-5 (PDE-5) inhibitor, sildenafil (5 mg/kg, i.p.) was co-administered with chlorpheniramine. Chlorpheniramine significantly decreased the immobility time at doses of 1mg/kg (P0.05). Moreover, pretreatment with L-arginine (P<0.01) and sildenafil (P<0.001) significantly reduced the anti-immobility effect of chlorpheniramine. These treatments did not alter the locomotor activity of mice in OFT. Our results revealed that the antidepressant-like effect of chlorpheniramine is mediated through inhibition of NO/cGMP pathway
