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5.4. Pain-state models using chemical stimuli

are nearby in nature to clinical pain.

332 Pain Relief - From Analgesics to Alternative Therapies

5.4.2. Acetic acid induced writhing test

5.4.3. Stimulation of hollow organs

inflatable balloon [75, 76].

direct inhibition of sensory neuron ASIC current [77].

6. Conclusion

5.4.1. Formalin test

The experimental models in which chemical stimulation is done with the administration of algogenic agents represent an irreversible, slow, and progressive form of stimulation, which

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 repre-

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].

In hollow organs such as rat colon, formalin is injected, and a complex biphasic type of 'true visceral pain' is exhibited in two phases. In first phase, contraction and stretching of body and in second phase, abdominal licking and nibbling behaviour is shown by the animal. Intravesical administration of capsaicin or turpentine produce bladder pain, glycerol produces abdominal constrictions and intrauterine injections of mustard oil show complex behaviour patterns in a number of models including rats. Another mean of stimulus for colorectal distension in rat is an

The ASIC family is a potential target for new analgesics. In rheumatoid arthritis and vascular ischemia, as well as in the routine perioperative settings, inflammation and ischemic pain conditions are a sign mark of acidic nociception which can be reduce by the NSAIDs and by

Work is currently in progress on a more selective and potent ASIC blocker and could potentially be an effective agent in the treatment of inflammatory and ischemic acute or chronic pain in the future [78]. Key regulators of membrane excitability are inflammatory mediators and key receptors (kinins, mPGEs), ion channels (TRPV1, NaV 1.7), and neurotrophins (NGF).

sents animal shows responses like licking, nibbling or shaking of injured paw [74].

Mohammad Saleem\* and Huma Naz

\*Address all correspondence to: saleem2978@hotmail.com

Faculty of Pharmaceutical Science, Government College University, Faisalabad, Pakistan
