Developmental changes in central O2 chemoreflex in Rana catesbeiana: the role of noradrenergic modulation

The in vitro brainstem preparation from Rana catesbeiana shows a functional central O(2) chemoreflex. Acute brainstem exposure to hypoxic superfusate elicits lung burst frequency responses that change over the course of development. Based on studies suggesting that brainstem noradrenergic neurons are involved in this reflex, we tested the following two hypotheses in vitro: (1) activation of adrenoceptors is necessary for the expression of the fictive lung ventilation response to hypoxia, and (2) changes in fast, Cl(-)-dependent neurotransmission (GABA/glycine) contribute to developmental changes in noradrenergic modulation. Experiments were performed on preparations from pre-metamorphics tadpoles (TK stages V-XIII) and adult bullfrogs. Acute exposure to hypoxic superfusate (98% N(2), 2% CO(2)) increased fictive lung ventilation frequency in the pre-metamorphic group, whereas a decrease was observed in adults. Buccal burst frequency was unchanged by hypoxia. Noradrenaline (NA; 5 micromol l(-1)) bath application mimicked both fictive breathing responses and application of the alpha(1)-antagonist prazosine (0.5 micromol l(-1)) blocked the lung burst response to hypoxia in both groups. Blocking GABA(A)/glycine receptors with a bicuculine/strychnine mixture (1.25 micromol l(-1)/1.5 micromol l(-1), respectively) or activation of GABA(B) pre-synaptic autoreceptors with baclofen (0.5 micromol l(-1)) prevented the lung burst response to hypoxia and to the alpha(1)-agonist phenylephrine (25 micromol l(-1)) in both stage groups. We conclude that NA modulation contributes to the central O(2) chemoreflex in bullfrog, which acts via GABA/glycine pathways. These data suggest that maturation of GABA/glycine neurotransmission contributes to the developmental changes in this chemoreflex.