5.1.1. The tail-flick model using radiant heat/immersion of the tail in hot water

It is one of the most simplified procedures used in human subjects with radiant heat [55]. In fact, Hardy et al. finally used this method in rats [56]. After the exposure to thermal radiation of the tail of an animal it takes out the tail by a brief dynamic movement [57]. This separation of the tail from the heat source is termed as 'tail-flick latency'. In this method, a timer is started at the time of application of heat and the time taken for the rat to withdraw its tail from heat source is recorded. Withdrawal time is usually within 2–10 s. It is advisable to not to lengthen the exposure to radiant heat more than 20 s as the skin of the tail may be burnt. In order to control the intensity of the current passing through the filament, a rheostat is inserted in the apparatus which further controls the intensity of radiant heat. Some investigators have used cold as a substitute of hot stimuli; this test can be used on monkeys as well. The use of immersion of the tail is apparently a variant of the test described above [58].
