Breathing across sleep/wake statesNCT increases minute ventilation during room air breathing. We determined whether NCT alters ‘normal’ breathing across sleep/wake states by measuring ventilation with whole-body plethysmography at each sleep/wake state during room air breathing. NCT rats significantly increased minute ventilation compared to controls (Fig. 5A) during wakefulness (mean increase = 57%, P= 0.015), non-REM sleep (mean increase = 56%, P= 0.021), and REM sleep (mean increase = 41%, P= 0.021), an effect not dependant on the prevailing sleep/wake state (P= 0.874, two-way ANOVA-RM). The higher ventilation of NCT rats was mainly due to larger tidal volumes (mean increases of 40%, 28%, and 30%; P= 0.010, P= 0.037, and P= 0.022; for wakefulness, non-REM, and REM sleep respectively; Fig. 5B), again not dependant on the prevailing state (P= 0.92, two-way ANOVA-RM). Tidal volume measured during transitions between states was not significantly different in NCT rats compared to controls (P= 0.45, Table 1). Tidal volume measurements are corrected with the equation provided by Drorbaugh & Fenn (1955) which uses humidity, barometric pressure, and body and chamber temperatures (Table 1). Note that chamber temperature was higher in NCT rats (mean increase = 1.8°C, P= 0.044) compared to controls, whereas the other parameters were not affect by NCT. When tidal volumes were not normalized according to body weight, volume changes due to NCT were lower (mean increases of 27%, 18% and 20%; P= 0.039, P= 0.083 and P= 0.049; for wakefulness, non-REM and REM sleep, respectively), again not dependant on the prevailing state (P= 0.91, two-way ANOVA-RM). Although NCT did not change significantly body weight (mean body weights in controls 597 ± 29 g and in NCT 556 ± 29 g, P= 0.33), it slightly decreased it in some animals which contributed to further increase tidal volume in NCT rats. Respiratory frequency was not different between control and NCT rats during wakefulness (P= 0.20) and REM sleep (P= 0.31), but it was significantly higher during non-REM sleep (increase of 18%, P= 0.039, Fig. 5C).
NCT decreases the effects of hypercapnia on minute ventilation. In control rats, hypercapnia increased minute ventilation by 98%, 103% and 91% during wakefulness, non-REM sleep and REM sleep, respectively (P= 0.037, P= 0.026 and P= 0.045, respectively, Fig. 5A) with the increases in ventilation being similar across sleep/wake states (P= 0.74, two-way ANOVA-RM). The enhancements of minute ventilation observed in control rats were due to significant increases in tidal volumes at all sleep/wake states (Fig. 5B), with these stimulating effects also not depending upon the prevailing sleep/wake state (P= 0.54), i.e. they occurred across all sleep/wake states. Respiratory frequency also contributed to the significant enhancement of minute ventilation during non-REM sleep, but not during wakefulness or REM sleep (Fig. 5C). In NCT rats, however, the CO2-mediated enhancements of minute ventilation were reduced compared to control rats (Fig. 5A). Indeed, comparison between control and NCT rats showed that NCT blunted the stimulating effect of hypercapnia on minute ventilation at each sleep/wake state (P= 0.037, P= 0.025 and P= 0.024 for wakefulness, non-REM and REM sleep, respectively, two-way ANOVA-RM), with these effects being similar across sleep/wake states (P= 0.96, three-way ANOVA, treatments × states × hypercapnia). In fact, NCT per se was such a powerful activator of ventilation by itself that subsequent addition of hypercapnia produces a lesser increase in ventilation, and little change in tidal volume compared to controls. Indeed the level of tidal volume produced by NCT per se during room air breathing was equivalent to that produced in control rats by hypercapnia (Fig. 5B). The stimulatory effects of hypercapnia on tidal volume were significantly reduced in NCT rats compared to control rats during non-REM and REM sleep (P= 0.030 and P= 0.011, respectively, two-way ANOVA-RM, Fig. 5B) with these effects being similar across sleep/wake states (P= 0.85, three-way ANOVA, treatments × states × hypercapnia). The increases in respiratory frequency in response to hypercapnia were similar between control and NCT rats at each sleep/wake state (P= 0.85, P= 0.77 and P= 0.09, for wakefulness, non-REM and REM sleep, respectively, two-way ANOVA-RM, Fig. 5C).