Central chemoreceptors are widespread within the brainstem. This conclusion is based on in vitro evidence of neurons from many locations that are responsive to pH changes and on in vivo evidence of respiratory responses to focal acidification at these same locations. Why are there so many chemoreceptor sites? As one possibility we hypothesize that function at different sites varies with arousal state. In unanesthetized rats, we produce focal acidification at single sites with microdialysis (probe tip = 1 mm×240 μm diameter) using artificial cerebrospinal fluid equilibrated with 25% CO₂. Tissue acidosis, measured at the region of the retrotrapezoid nucleus (RTN) in unanesthetized rats, is approximately equivalent to that observed with end-tidal PCO₂ = 7–8 mmHg above the eupneic value. Focal acidification of the retrotrapezoid nucleus (RTN) increased ventilation by 24% only in wakefulness via an increase in tidal volume [1]. In the medullary raphé, the excitatory effect of such focal acidification was observed only in sleep (defined by EEG and EMG criteria); ventilation and frequency increased by 15–20% in NREM sleep and frequency by 15% in REM sleep. There was no effect in wakefulness [2]. In our most recent study, focal acidification of nucleus tractus solitarius (NTS) increased ventilation by up to 20% in both wakefulness and NREM sleep due to an effect on both tidal volume and frequency. The increase in ventilation is greater with focal acidification in the caudal part of NTS at the level of the area postrema. Acidification of more rostral parts of the NTS produced variable results.

The medullary raphé contributes to chemoreception in sleep, the RTN in wakefulness, and the cNTS in both sleep and wakefulness. Central chemoreceptors at these three different locations do appear to vary in effectiveness with arousal state. The response at each site is only a fraction of the response attributable to all sites being stimulated at the same intensity. Hypocapnia resulting from single site stimulation can lower the contribution of other sites minimizing the apparent effectiveness of single site stimulation.