The ontogeny of hypoxic neuroventilation in the brainstem of the developing American bullfrog, Rana catesbeiana

Abstract

Isolated brainstem models have been used twice to examine the hypoxic neuroventilatory response (HnVR) in developing bullfrogs with varying results, leading me to question if the results of these two previous studies were physiologically relevant. Based on in vivo and in vitro observations, I hypothesized that buccal neuroventilation exhibits no ventilatory response to hypoxia in any developmental stage and that lung neuroventilation exhibits a biphasic hypoxic response in late metamorphic tadpoles and a neuroventilatory depression in juvenile bullfrogs and early metamorphic tadpoles. I tested this by using brainstems isolated from intact early and late metamorphic tadpoles and juvenile bullfrogs exposed to 20% O2 saturation in artificial cerebrospinal fluid. Neuroventilatory patterns were recorded from the facial and hypoglossal nerves using suction electrodes and burst frequency, and duration for buccal and lung bursts were examined in all developmental stages. Results confirmed my hypothesis for buccal neuroventilation across development and lung neuroventilation in juveniles, but not my hypothesis about lung neuroventilation in tadpoles. Therefore, the HnVR of developing bullfrogs consists of constant maintenance of neuroventilatory activity in early metamorphic tadpoles, a trend towards a biphasic neuroventilatory response in late metamorphic tadpoles, and ventilatory depression that is mediated centrally in juvenile bullfrogs.