ANALYSIS OF NOCIFENSIVE BEHAVIOR INDUCED IN RATS BY CO2-LASER PULSE STIMULATION

To characterize nocifensive behavior, a laser beam was applied to the hind footpad of nonanesthetized and unrestrained rats and the reaction pattern was analyzed. Fifty-four rats were divided into nine groups o f six animals, and each group was given one of nine combinations of la ser stimuli: intensity of 4, 8, or 12 W and duration of 10, 30, or 50 ms. A single pulse was applied to a 0.13 cm(2) area of right or left f ootpad and the trial was repeated 20 times with 3 min between trials. The behavior was videotaped and reviewed for a period of 2 min followi ng each stimulation. It seemed to consist of eight discrete responses, and each response was scored for whether it occurred and for its summ ed duration per trial. The component responses and the behavior as a w hole were characterized by their sensitivity in terms of the level of energy required to attain 50% of the maximum response, and their linea r or quadratic trends with increasing stimulus energy. The most sensit ive index of pain stimulation was the composite score, followed by foo t jumping, foot elevation, body movements, licking, and then foot move ments. As stimulus energy increased, rats exhibited a greater number o f different responses and a greater frequency of each component respon se. The results suggest that a pool of hierarchically organized respon ses in the nocifensive motor system are recruited partially or wholly by nociceptive stimuli of varying intensity.