5.4.1. Formalin test

The formalin test in rats, a chronic pain model is used to assess the centrally active analgesic agents. In this test, excessive licking and biting of the paw is recorded as response after the administration of formalin (37% solution of formaldehyde) into the front paw. Both paws resting on the floor indicated the analgesic response or protection of the test drug. The response as painful behaviour can be evaluated on a four-level scale related to posture: 0 represents normal texture; 1 represents the injected paw not supporting the animal but leftovers on the ground; 2 represents animal raised up the injected paw visibly; and 3 represents animal shows responses like licking, nibbling or shaking of injured paw [74].

#### 5.4.2. Acetic acid induced writhing test

In this method, pain is indicated as a characteristic behaviour of contraction of abdominal muscles and stretching of hind paws along with twisting of dorso-abdominal muscles, and motor in co-ordination in rats or mice (called writhing) after the administration of allogenic agents like phenyl quinone or acetic acid into the peritoneal cavity which irritate the serous membrane; therefore, this test is called 'writhing test'. These writhings are counted as per unit of time [74].
