CO2 microdialysis in retrotrapezoid nucleus of the rat increases breathingin wakefulness but not in sleep

Central chemoreceptors are widespread within the brain stem. We suggest tha t their function at some sites may vary with the state of arousal. In this study, we tested the hypothesis that the function of chemoreceptors in the retrotrapezoid nucleus (RTN) varies with sleep and wakefulness. In unanesth etized rats, we produced focal acidification of the RTN by means of a micro dialysis probe (tip containing the semipermeable membrane = 1-mm length, 24 0-mu m diameter, and 45-nl volume). With the use of a dialysate equilibrate d with 25% CO2, the tissue pH change (measured in anesthetized animals) was 1) limited to within 550 mu m of the probe and, 2) at the probe tip, was e quivalent to that observed with end-tidal PCO2 of 63 Ton: This focal acidif ication of the RTN increased ventilation significantly by 24% above baselin e, on average, in 13 trials in seven rats only during wakefulness. The effe ct was entirely due to an increase in tidal volume. During sleep defined by behavioral criteria, ventilation was unaffected, on average, in 10 trials in seven rats. During sleep, the chemoreceptors in the RTN appear to be ina ctive, or, if active, the respiratory control system either is not respondi ng or is responding with very low gain. Because ventilation is increased du ring sleep with all central chemoreceptor sites stimulated via systemic CO2 application, other central chemoreceptor locations must have enhanced effe ctiveness.