Tail flick test

The tail flick test is a test of the pain response in animals, similar to the hot plate test. It is used in basic pain research and to measure the effectiveness of analgesics, by observing the reaction to heat. It was first described by D'Amour and Smith in 1941.

Procedure
A light beam is focused on the animal's tail and a timer starts. When the animal flicks its tail, the timer stops and the recorded time is a measure of the pain threshold.

Instruments
Instruments have been designed for use with this testing method, including the conduction dolorimeter. The conduction dolorimeter has a resistance wire that has a constant heat flow. For the tail flick test, the wire is attached to the tail of the organism, and the wire applies heat to the tail. The researcher then records the time of the tail flick.

Applications
Researchers testing the effectiveness of drugs on the pain threshold often use the tail flick test to measure the extent to which the drug being tested has reduced the amount of pain felt by the model organism.

Rats are a common model organism for these tests. The rats are usually given analgesics which are responsible for weakening a rat's response to pain. Under these weakened responses to pain, with effectiveness peaking about 30 minutes after ingestion, researchers test the effectiveness of the drugs by exposing the rat's tail to constant heat and measuring how long it takes for the rat's tail to flick, signaling its response to the pain. It was found in experimental tests of the tail flick testing method that the temperature of the skin of the tail plays a major role in the critical temperature, the temperature at which the tail flicks in response to pain. Researchers found that if the tail has been exposed to a cooler temperatures before the test, the critical temperature decreases.

By using the tail flick test, researchers have found that genetics plays a role in pain sensation and the effectiveness of analgesics. A mouse of one genetic line may be more or less tolerant of pain than a mouse of another genetic line. Also, a mouse of one genetic line may experience a higher or lower effectiveness of an analgesic than a mouse of another genetic line. Using this test, researchers can also identify the genes that play a role in pain sensation. For example, the Calca gene is primarily responsible for the variability in thermal (heat) nociception. The Sprawling mutation resulted in a moderate sensory neuropathy but the mutation did not affect nociceptive modality or motor function in the mice. The mice with the Sprawling mutation were unable to sense the pain, but their other sensory functions were unaffected.

Limitations
The tail flick test is one of the many tests used to measure the sensitivity to heat-induced pain in pain research of live species. The reflex response to heat-induced pain is, for the most part, a good indicator of the pain sensitivity of an organism and the reduction of pain sensitivity by analgesics. However, there are a few significant limitations of heat tests such as the tail flick tests. First of all, there is still much more research needed in the field of pain research using murine subjects; therefore, the validity of translating the observed pain responses from these animals to humans cannot be certain. Also, researchers have found that skin temperature can significantly affect the results of the tail flick test and it's important for one to consider this effect when performing the test. Lastly, Dr. Pissiketti et al. found that many thermal tests do not distinguish between opioid agonists and mixed agonist-antagonists and consequently created a tail flick test for mice using cold water in place of heat in order to make that distinction.