In the present study, we show that hypersensitivity to noxious thermal stimulation can be seen clearly in developing rats. Rats, at postnatal days 3, 6, 9, 12, 15, 21 and 90 were tested for reflex responsiveness to noxious heat, using tail withdrawal from hot water as the assay. Thermal nociceptive thresholds are considerably lowered, relative to adults, up to postnatal day 12. Thresholds were 39, 37.5, 40.8, 43.3, 46.5, 45.2 and 47.2 degrees C for the respective age groups. Enhanced sensitivity 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 the decrease in the slope of the temperature-response curve (system gain). Spinal transections at postnatal days 13, 17, 20, 60, or 100 did not produce a change in nociceptive thresholds in any of the age groups. In contrast, sensitivity to noxious stimulation (system gain) was enhanced 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.

Nociceptive and antinociceptive systems change dramatically during the first 10 days of life in rats. The present studies sought to determine whether exposure to water stress can produce analgesia during this period, and when in the ontogeny changes in magnitude and duration of such stress-induced analgesia occur. It was found that exposure to water reliably induced analgesia as early as 3 days postnatally. This analgesia increased in magnitude and duration at the age of 17-20 days, when supra-spinal descending inhibition develops, and depended on the temperature of the water and duration of exposure.