The ITFR, impulsive tail flick reflex by short duration nociceptive stimuli.

A new method for evoking the tail flick reflex is introduced, using short duration or "impulsive" nociceptive stimuli, which allow synchronization and recording of electrophysiological responses. Ten adult rats were studied, by means of thermal (CO(2) laser infrared pulse with 30 ms duration, 7.5 or 10W), electric (a 25 ms train of five 0.2 ms pulses, with 5 or 10 mA intensity) or mechanical (pin pressed with 5 g force) stimuli. Both electromyographic and strain gauge mechanical responses were recorded from the tail. All three types of stimulation gave rise to three components, named early, late and ultralate, respectively occurring in the range of 19-97 ms, 190-519 ms, 1,523-2,765 ms. Conduction velocities of the underlying afferent fibres were calculated by moving the stimulation site. The early component could be linked to Aδ afferents, while late and ultralate components were due to unmyelinated C afferents. Experiments with Fentanyl (20 μg/kg) showed that only the C linked components were depressed, with the ultralate component the most affected, possibly because supraspinally originated. Tail flick reflex evoked by impulsive stimuli is believed to be an important electrophysiological complement to behavioural procedures, useful in identifying the site of action of analgesics and other drugs upon the spinal and supraspinal centres involved in nociception.