Fig. 9. Buccal burst frequency data obtained during each series of experiments for brainstem preparations from pre-metamorphic tadpoles (grey bars) and adult bullfrogs (black bars). Data were obtained before and after application of (A) noradrenaline (NA; 5 µmol l^–1), (B) GABA_A/glycine antagonist mixture (bicuculline 1.25 µmol l^–1/strychnine 1.5 µmol l^–1), (C) the ₁-adrenoceptor agonist phenylephrine (Phe; 25 µmol l^–1), (D) the ₂-adrenoceptor agonist clonidine (Clo; 25 µmol l^–1), or (E) aCSF hypoxia. Note that application of the bicuculine/strychnine mixture abolished fictive buccal burst frequency in all preparations (only data from NA experiments are shown), and this could not be restored by any experimental treatment. Values are means ± s.e.m. ^*Values statistically different from baseline at P<0.05; ^**values statistically different from baseline at P<0.10.