Fig. 3. Effects of electrical stimulation of medullary Raphé neurons on fictive lung ventilation frequency. (A,C) Responses were measured in brainstem preparations from (A) pre-metamorphic tadpoles, (B) metamorphic tadpoles, and (C) adult bullfrogs under control conditions [artificial cerebrospinal fluid (aCSF) only; open diamonds] and in the presence of a 5-HT₃ receptor antagonist (tropisetron; closed circles). Note that the tropisetron concentration required to affect the frequency response (when present) decreases during development. (D,E) Stage-dependent lung burst frequency responsiveness to medullary Raphé stimulation expressed as the percentage change from baseline of the frequency response following 20 Hz stimulation (D), and as percentage change from baseline expressed as the percentage of the maximum lung burst frequency produced by each preparation (E). The black bars represent the control response (aCSF) and the grey bars represent the response in the presence of the 5-HT₃ antagonist (when necessary). Values are expressed as means ± s.e.m. Asterisks indicate values statistically different from baseline values (no stimulation) at *P<0.05 and *P<0.01; indicates a value significantly different from control at P<0.01.