Formalin assay parameters differ in confirming the antinociceptive mechanism of domperidone in mice

Domperidone, a prokinetic drug with minimal extrapyramidal side-effects was investigated for its antinociceptive response in mice using formalin assay procedure. Two parameters namely the pain score and the time spent by the animal in licking/biting the formalin injected paw were considered. Domperidone (1, 2.5 or 5 mg/kg; ip) injected 15 min prior to formalin effectively reduced the pain score bringing it to zero at the 15th minute and was also effective till 30 min but to a lesser degree. This effect of domperidone (2.5 mg/kg) was significantly attenuated in naloxone pretreated mice indicating a partial role for opioid pathways. In the other parameter i.e. time spent in licking/biting, domperidone in all the doses employed failed to modify significantly the same by the animal in the early phase. In contrast, a dose related inhibition of the time spent was recorded in the late phase. Besides, a trend towards the enhancement of the inhibitory effect of domperidone (2.5 mg/kg) in the late phase was noticed in naloxone pretreated mice. Possibly, the peripheral analgesic mechanisms may play a role in this response since the late phase was considered akin to inflammation. The results confirm the antinociceptive effect of domperidone and suggest that caution be exercised while selecting the parameters when formalin assay is employed.