DEVELOPMENT OF THERMAL NOCICEPTION IN RATS

In the present study, we show that hypersensitivity to noxious thermal stimulation can be seen clearly in developing rats. Rats, at postnata l days 3, 6, 9, 12, 15, 21 and 90 were tested for reflex responsivenes s to noxious heat, using tail withdrawal from hot water as the assay. Thermal nociceptive thresholds are considerably lowered, relative to a dults, up to postnatal day 12. Thresholds were 39, 37.5, 40.8, 43.3, 4 6.5, 45.2 and 47.2 degrees C for the respective age groups. Enhanced s ensitivity to suprathreshold noxious stimuli is seen in neonates up to postnatal day 15 (but not on day 9). Starting on day 21, sensitivity to noxious stimuli decreases with increasing age, as can be seen by th e decrease in the slope of the temperature-response curve (system gain ). Spinal transections at postnatal days 13, 17, 20, 60, or 100 did no t produce a change in nociceptive thresholds in any of the age groups. In contrast, sensitivity to noxious stimulation (system gain) was enh anced by spinalization in rats 20 days of age or older. Based on these results we suggest that threshold elevation with increasing age most probably reflects changes in local spinal properties, while changes in responsiveness to suprathreshold noxious stimuli involves maturation of both spinal and descending supraspinal structures.