Respiratory neural activity responses to chemical stimuli in newborn rats:reversible transition from normal to 'secondary' rhythm during asphyxia and its implication for 'respiratory like' activity of isolated medullary preparation

To clarify a possible origin of 'respiratory like' rhythmic activities obse rved in in vitro brainstem preparation, the phrenic (Phr) and cranial nerve (XII or IX) inspiratory activities were analyzed in halothane-anesthetized , vagotomized and artificially ventilated newborn (2-6 days after birth) an d young adult rats (30-50 days) during altered chemical stimuli and prolong ed asphyxia at 25 degreesC. The newborn rat showed regular rhythmic inspira tory discharges of short duration, and their responses to CO2 and hypoxia d id not differ from those seen in adult rats. In the newborn rat, the Phr an d cranial nerve inspiratory discharges increased first, then respiratory Fr equency decreased and finally ceased completely for similar to 1-2 min duri ng asphyxia. Thereafter, 'secondary' rhythmic inspiratory activity emerged at a slower rate with decremental inspiratory discharge profile, which pers isted for a period more than 40 min of asphyxia. A normal respiratory activ ity recovered after resumption of artificial ventilation. Though young adul t rats exhibited similar sequential changes in respiratory activity during asphyxia, the 'secondary' rhythmic activity persisted For a period of sever al min only. The pattern of 'secondary' respiratory activity corresponded w ell with that of rhythmic activities seen ill the isolated medullary block preparation of newborn rat. 'Respiratory like' activity found in isolated m edullary preparations of newborn animals may arise from a mechanism that ge nerates 'secondary' (or so called 'gasping' type) rhythmic inspiratory acti vity during prolonged asphyxia in in vivo preparations. (C) 2000 Elsevier S cience Ireland Ltd and the Japan Neuroscience Society. All rights reserved.