Habituation to sham testing procedures modifies tail-flick latencies: effects on nociception rather than vasomotor tone.

Habituation of rats to the testing environment and procedures reduces flexor withdrawal latencies to those of spinalised animals. We have now recorded surface temperatures at 3 sites on the tail and tail-flick latencies simultaneously in experimentally naive (novice) rats and in habituated rats. At usual ambient temperatures (20 +/- 1 degree C), tail temperatures fluctuated in accordance with the predictions of an on-off controller. There was an inverse correlation between the tail-flick latency and the temperature at the site of noxious stimulation. A similar correlation was found when the pre-stimulus temperature of the tip of the tail was held at temperatures ranging from 21 degrees C to 35 degrees C. Habituated animals exhibited a similar linear regression slope factor but lower latencies than novice animals at each temperature. We conclude (1) that tail-flick latency is determined partly by the pre-stimulus temperature at the site of noxious thermal stimulation, and (2) that the effects of habituation on tail-flick latency are more likely to be explained by differences in nociception than in regional vasomotor tone.