Maturational changes in the thermal nociceptive responses of developing rats

Some find developmental differences in rodent thermal nociceptive responses and others do not. To address these inconsistencies, the escape latencies of immature (5-to 25-day-old) and adult (3-to 4-month-old) albino rats were recorded following tail exposure to different intensities of radiant heat (650-W halogen lamp placed 10-30 mm from the tail) or conductive heat 35-50 degrees C water). Developmental differences in tail flick latencies were not observed in immature rats when the lamp was closest to the tail (although adult latencies were longer than 5-and 15-day-old responses) When radiant heat intensity was reduced, 5-day-old rats had shorter escape latencies than 15-, 25-, and 90-day-old animals. Age differences persisted in the latencies of immature animals even when the test aperture was varied to compensate for maturational changes in tail width (whereas adult responses no longer differed from those of 5-and 15-day-old rats). Developmental differences were eliminated when the tail skin was blackened so as to normalize the absorption of radiant heat across age. Similar age-and intensity-dependent differences were observed in rats exposed to conductive heat: Five-and 10-day-old pups had shorter escape responses than older rats when tails were immersed in intermediate (40 or 45 degrees C) but not lower (35 degree C) or higher (50 degrees C) temperature water. Blackening the tails did not change conductive heat escape latencies. No sex differences were found at any age or stimulus intensity with either type of heat. Higher intensities of thermal stimuli applied to the tail are required to elicit escape responses in older rats compared to younger ones, but the use of relatively intense thermal test stimuli can mask age-dependent differences in nociception. Some of the inconsistent results reported previously about maturational changes in thermal nociception may be due to intensity differences in the noxious test stimuli used. Maturational differences in the radiant absorption properties of the tail seem to account for most of the age-related changes in rodent responses to radiant heat, but the mechanism(s) which subserve developmental differences in conductive heat nociception need to be elucidated.