Acetic acid-induced writhing test
Intraperitoneal injection of acetic acid triggers writhing, most likely due to the release of TNF-α, interleukin 1β and interleukin 8 by macrophages and mast cells (Ribeiro et al. 2000). The mice were divided into groups of 6 for evaluation of the analgesic effect of Z. nummularia roots extract. One group was injected with normal saline (10 ml/kg) and used as negative control. Another group was injected with diclofenac (5 and 10 mg/kg). The last group was treated with Z. nummularia root extract at 50 and 100 mg/kg. The mice were injected with acetic acid (1 % intraperitoneal) and the subsequent abdominal constriction (writhing) was counted for 10 min after 5 min of acetic acid injection.