Effects of Sleep-Wake State on the Genioglossus Versus Diaphragm Muscle Responses to CO2 in Rats

The effects of sleep on the ventilatory responses to hypercapnia have been well described in animals and humans. In contrast, there is little information for genioglossus (GG) responses to a range of CO2 stimuli across all sleep-wake states. Given the notion that sleep, especially rapid-eye-movement (REM) sleep, may cause greater suppression of muscles with both respiratory and non-respiratory functions, this study tests the hypothesis that GG activity will be differentially affected by sleep-wake states with major suppression in REM sleep despite excitation by CO2. Seven rats were chronically implanted with electroencephalogram, neck, GG and diaphragm electrodes and responses to 0, 1, 3, 5, 7 and 9% CO2 were recorded. Diaphragm activity and respiratory rate increased with CO2 (P<0.001) across sleep-wake states with significant increases at 3-5% CO2 compared to 0% CO2 controls (P<0.05). Phasic GG activity also increased in hypercapnia but required higher CO2's (7-9%) for significant activation (P<0.05). Further studies in fifteen urethane-anesthetized rats with the vagi intact (n=6) and cut (n=9) showed that intact vagi delayed GG recruitment with hypercapnia but did not affect diaphragm responses. In the naturally sleeping rats, we also showed that GG activity was significantly reduced in non-REM and REM sleep (P<0.04) and was almost abolished in REM even with stimulation by 9% CO2 (decrease=80.4% vs. wakefulness). Such major suppression of GG activity in REM, even with significant respiratory stimulation, may explain why obstructive apneas are more common in REM sleep.