Activity of AM1241 at the formalin test in naïve rats and in rats treated with nitroglycerin. In the vehicle groups (DMSO and Control, CT), the injection of formalin resulted in a highly reliable, typical, biphasic pattern of flinches/shakes of the injected paw, being characterized by an initial acute phase of nociception within the first 5 min, followed by a prolonged tonic response from 15 to 60 min after formalin injection. DMSO administration induced a significant increase in the nocifensive behaviour of animals only in phase I, when compared with the CT group. No difference was reported between CT and DMSO groups in phase II. NTG administration, alone or with DMSO, significantly increased the total number of flinches/shakes in phase II of formalin test, when compared either to CT group or to DMSO group. In naive rats, the lower dose of AM1241 (2 mg/kg) significantly inhibited nocifensive behaviour induced by formalin injection during phase I, while the higher dose (4 mg/Kg) inhibited the nocifensive behaviour in both phases of the test when compared with DMSO group. In rats treated with nitroglycerin (NTG), both doses of AM1241 significantly reduced the total number of flinches/shakes in phase I of test, when compared to NTG + DMSO group, and phase II of the test when compared to both NTG and NTG + DMSO groups. Data are expressed as mean ± SEM. ANOVA analysis followed by Tukey’s Multiple Comparison test, *p < 0.05 vs CT; °p < 0.05 vs NTG; #p < 0.05 NTG + DMSO; §p < 0.05 vs DMSO.