Increased central cholinergic drive contributes to the apneas of serotonin-deficient rat pups during active sleep.

Infant rat pups lacking CNS serotonin (5-hydroxytryptamine; 5-HT) have unstable breathing during prolonged periods of active sleep. Given that cholinergic neurons are drivers of active sleep and project to respiratory patterning regions in the brainstem, we hypothesized that 5-HT preserves respiratory stability in active sleep by dampening central cholinergic drive. We used whole-body plethysmography coupled with nuchal EMG to monitor the breathing pattern of 2-week-old TPH2+/+ and TPH2-/- pups in active sleep, before and after muscarinic blockade. Group 1 experiment: we injected methyl atropine (Ap-M), a CNS-impermeant form of atropine, followed ~30 min later by an injection of atropine sulfate (Ap-S), the CNS-permeant form (both 1mg/kg, 10 ul bolus i.v.); both injections occurred within an active sleep episode. We analyzed the effect of each drug on the co-efficient of variation of the respiratory period (CV-P) during active sleep. Group 2 experiment: rats cycled through several episodes of active and quiet sleep before administration of Ap-S (1 mg/kg, 200 ul i.p.), or vehicle. We assessed the effect of Ap-S on the apnea indices of both genotypes during quiet and active sleep. Group 1: Ap-S significantly reduced the CV-P of TPH2-/- pups (p=0.03), an effect not observed in TPH2+/+ pups or following Ap-M. Group 2: the apnea index of TPH2-/- pups was significantly reduced following Ap-S injection (p=0.04), while the apnea index of TPH2+/+ littermates was unaffected (p=0.58). These findings suggest that central 5-HT reduces apnea and stabilizes breathing by reducing cholinergic signaling through muscarinic receptors.