Endogenous excitatory drive to the respiratory system in rapid eye movement sleep in cats

1. A putative endogenous excitatory drive to the respiratory system in rapi d eye movement (REM) sleep may explain many characteristics of breathing in that state, e.g. its irregularity and variable ventilatory responses to ch emical stimuli. This drive is hypothetical, and determinations of its exist ence and character are complicated by control of the respiratory system by the oscillator and its feedback mechanisms. In the present study, endogenou s drive was studied during apnoea caused by mechanical hyperventilation. We reasoned that if there was a REM-dependent drive to the respiratory system , then respiratory activity should emerge out of the background apnoea as a manifestation of the drive. 2. Diaphragmatic muscle or medullary respiratory neuronal activity was stud ied in five intact, unanaesthetized adult cats who were either mechanically hyperventilated or breathed spontaneously in more than 100 REM sleep perio ds. 3. Diaphragmatic activity emerged out of a background apnoea caused by mech anical hyperventilation an average of 34 s after the onset of REM sleep. Em ergent activity occurred in 60% of 10s epochs in REM sleep and the amount o f activity per unit time averaged approximately 40% of cupnoeic activity. T he activity occurred in episodes and was poorly related to pontogeniculo-oc cipital waves. At low CO2 levels, this activity was non-rhythmic. At higher CO2 levels (less than 0.5% below eupnoeic end-tidal percentage CO2 levels in non-REM (NREM) (NREM), activity became rhythmic. 4. Medullary respiratory neurons were recorded in one of the five animals. Nineteen of twenty-seven medullary respiratory neurons were excited in REM sleep during apnoea. Excited neurons included inspiratory, expiratory and p hase-spanning neurons. Excitation began about 43 s after the onset of REM s leep. Activity increased from an average of 6 impulses s(-1) in NREM sleep to 15.5 impulses s(-1) in REM sleep, Neuronal activity was non-rhythmic at low CO2 levels and became rhythmic when levels were less than 0.5% below eu pnoeic end tidal levels in NREM sleep. The level of CO2 at which rhythmic n euronal activity developed corresponded to eupnoeic end-tidal CO2 levels in REM sleep. 5. These results demonstrate an endogenous excitatory drive to the respirat ory system in REM sleep and account for rapid and irregular breathing and t he lower set-point to CO2 in that. state.