Effect of crocin on the morphine-induced antinociception in the formalin test in rats.

In this study, the effects of intraperitoneal (i.p.) injection of crocin in the absence and presence of subcutaneous (s.c.) injections of morphine and naloxone were investigated on the formalin test in rats. The formalin test was induced by intra-plantar (i.pl.) injection of formalin (50 microL, 1%), and the time spent licking and biting of the injected paw was measured for 1 h. Formalin induced a marked biphasic (first phase: 0-5 min and second phase: 15-45 min) pain response. Morphine (1 mg/kg, s.c.) significantly (p < 0.05) suppressed both phases of pain. Naloxone (2 mg/kg, s.c.) alone did not change the intensity of pain, but pretreatment with naloxone (2 mg/kg) significantly (p < 0.05) prevented morphine (1 mg/kg)-induced antinociception. Crocin at doses of 50, 100 and 200 mg/kg significantly (p < 0.05) attenuated pain. Crocin (100 mg/kg, i.p.) significantly (p < 0.05) increased the morphine (1 mg/kg, s.c.)-induced antinociception. Naloxone (2 mg/kg) did not reverse the suppressive effect of crocin (100 mg/kg) on pain. Crocin at a dose of 400 mg/kg significantly (p < 0.05) suppressed locomotor activities. These findings indicate that morphine through a naloxone-sensitive mechanism produced analgesia. Crocin produced a dose-dependent antinociceptive effect. In addition, crocin increased morphine-induced antinociception, but naloxone did not change the antinociceptive effect of crocin.