Age-related changes in the response to thermal noxious heat and reduction of C-fibers by neonatal treatment with capsaicin.

Developmental changes of the response to nociceptive heat were examined in mice treated with capsaicin (50 mg/kg) on postnatal days 2-15.Tests of hot-plate (55 C) and irradiation by infrared (IR test) were carried out after 10 days of capsaicin administration up to 120 days at intervals of 10 or 20 days. The time until forepaw (hot-plate) and hindpaw (IR test) withdrawal was assessed as the response latencies to suprathreshold and thermal threshold, respectively. Moreover, the numbers of unmyelinated C-fibers and myelinated fibers in the L4 dorsal roots of the animals treated on postnatal days 2 and 5 were counted on electron micrograph montages. Despite the marked reduction of C-fibers (60% mean decrease) in the 4 dorsal roots of the animals treated on postnatal day 2, thresholds were normal compared with those of controls. However, the animals treated with capsaicin on postnatal day 5 showed an apparent delay of thermal threshold latency only in the IR test, although the mean reduction of C-fibers was v...