Fig. 1.

Single unit extracellular recordings of CO₂-stimulated and unstimulated raphé serotonin-synthesizing (5-HT) neurons in situ. Recordings from a single CO₂-stimulated raphé neuron show a firing frequency of 0.26 Hz during exposure to 5% CO₂ (A), a 92% increase in firing frequency (to 0.51 Hz) with exposure to 9% CO₂ (B), and recovery with return to baseline conditions (C). Recordings from a single unstimulated raphé neuron show firing frequencies of 0.70 Hz during exposure to 5% CO₂ (D), 0.67 Hz during exposure to 9% CO₂ (E), and 0.72 Hz during recovery normocapnia (F). Firing frequency and regularity analysis indicated these to be putative 5-HT neurons.