Muscarinic blockade stabilizes the breathing pattern of serotonin‐deficient rat pups during active sleep
Authors: Davis Marina L. , Magnusson Jennifer L. , Cummings Kevin J. 
Published: June 2021

Infant rats deficient in tryptophan hydroxylase 2 (TPH2−/−) and hence lacking central serotonin (5‐hydroxytryptamine; 5‐HT) display abnormally destabilized breathing during active sleep; their breathing pattern is punctuated with periods of apnea and tachypnea. Given that acetylcholine is a key driver of active sleep, as well as the fact that muscarinic receptors are abundantly expressed within key respiratory patterning regions, we hypothesized that increased central muscarinic drive contributes to the unstable breathing pattern of infant TPH2−/− rats during active sleep. To test this hypothesis, we conducted two separate experiments using whole‐body plethysmography coupled with nuchal EMG to monitor the breathing pattern of 2‐week‐old TPH2−/− and WT pups during active sleep. Experiment 1: following ~1 hr settling period to allow for the appearance of active sleep, we injected atropine methyl nitrate (Ap‐M), a CNS‐impermeant form of atropine, followed ~30 min later by an injection of atropine sulfate (Ap‐S), a CNS‐permeant form (both 1mg/kg, 10 ul bolus i.v.) (n= 8 and 10 TPH2−/− for Ap‐M and Ap‐S, respectively; n=8 WT for both drugs). Each drug was injected during a period of active sleep. For most animals, arousal did not occur and the effect of the drugs on the co‐efficient of variation in the respiratory period (CV‐P%) could be determined within one episode of active sleep. If arousal occurred, CV‐P% prior to injection was compared to CV‐P% in the next episode of active sleep. Experiment 2: the breathing pattern of TPH2−/− pups was monitored over 2 hrs before and after the administration of Ap‐S (1 mg/kg, 200 ul bolus i.p; n=17) or vehicle (saline; n=16). Tachypnea and apnea indices (# events/hr) were calculated for the active sleep episodes before and after Ap‐S. Administration of Ap‐M increased heart rate (~30–50 beats/min) and decreased heart rate variability for the duration of the experiment. In WT pups, CV‐P% tended to increase after i.v. administration of Ap‐M, but was not significantly changed following Ap‐S. In contrast, the CV‐P% of TPH2−/− pups was significantly increased after Ap‐M (p<0.01) and significantly reduced following Ap‐S (p=0.01). Ap‐S significantly reduced the tachypnea index of TPH2−/− pups relative to TPH2−/− pups given vehicle alone (p=0.04), but had no significant effect on apnea index. The reduced CV‐P% of TPH2−/− pups after Ap‐S relative to values after Ap‐M suggest that muscarinic blockade in the CNS by Ap‐S reverses the destabilizing effect of 5‐HT deficiency on the respiratory pattern. This occurs in spite of a destabilizing peripheral effect of Ap‐M on the breathing pattern. These data also suggest that during active sleep, 5‐HT signaling helps stabilize the breathing pattern of infant rats mainly by inhibiting muscarinic drive to patterning neurons promoting tachypnea.Support or Funding InformationFunding for this research was provided by an American Heart Association Scientist Development Grant (14SDG18560022; PI: KJC) and a Ruth L. Kirschstein National Research Service Award (NRSA) Individual Predoctoral Fellowship (F31) to JLM.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.