Nociception assay

The formalin assay is the most popular chemical assay of nociception. It entails the injection of a dilute solution of formalin into the surface of the rodent's hindpaw, followed by the scoring of stereotypical behaviors such as flinching, licking, and biting of the affected hindpaw. The behaviors last for approximately 1 hour, with the early or acute stage (directly after injection) reflecting direct activation of nociceptors and the late or tonic phase (15 to 20 minutes after the injection) reflecting inflammation. One major advantage of the formalin assay over other models of inflammatory pain is the limited duration (approximately 1 hour) of the response. Additionally, as described before, this assay produces a response in two discrete stages, allowing researchers to model both acute and tonic pain using a single noxious chemical. ==Writhing==

In the writhing test, the peripheral nociceptive activity of a test compound is determined by the number of abdominal writhes induced by the intraperitoneal injection of acetic acid. ==Von Frey==

The Von Frey assay, introduced by Maximilian von Frey and modified by Weinstein, uses Von Frey hair or fibers, which are small pieces of nylon rod, approximately 50 mm in length, and of varying diameters, to test a rodent's sensitivity to a mechanical stimulus. Alternatively, automated von Frey systems have recently been discovered that gradually increase the force of a single probe so that a researcher can observe when withdrawal responses occur. ==Thermal assays==

Sensitivity to acute thermal stimulation is the most common test used in live species pain research. Another important factor to consider is the level of restraint used; rodents held too tightly may exhibit greater tail flick latencies due to heightened stress levels. ==Hargreaves==

The Hargreaves assay uses a high-intensity beam of light directed at the hindpaw rather than the tail to induce pain; an investigator then measures the time it takes for the animal to withdraw its hindpaw. The main advantage of this test over the tail flick assay is that it allows independent assessment of treatment effects on both sides of the body. ==Applications==

One of the most common applications of nociception assays is to test the effectiveness of new pain medications and drugs of the like. One can then perform comparative tests to measure the differences in the effects of the drug on varying populations, such as men versus women, or young versus old. These tests can also identify certain harmful diseases or abnormalities in subjects if they display atypical nociception test responses. Additionally, nociception tests can be used to test the heritability of nociception itself. One can also use nociception assays to assess the physiology of the "pain" pathways. The role capsaicin receptors play in the pain pathways has been measured by comparing results from nociception assays in mice with and without the receptor. In addition, they are useful in other tests to make sure control subjects have normal nociception responses. ==See also==