A Sensitive Period for the Development of Motor Function in Rats: A Microgravity Study

Abstract Spaceflight studies offer a unique opportunity to examine the impact of gravity on developing motor skills. Previously, we reported that young rats experiencing microgravity in low Earth orbit (LEO) beginning on postnatal day (P)14 showed impaired swimming, walking, and surface righting after returning to 1 g, with immature motor skills persisting until adulthood. Here, we report on post-flight surface righting and swimming of rats experiencing spaceflight from P7 or P8. Litters with dams were flown aboard a space shuttle Space Transportation System (STS) 9-day (NIH-R3, STS-72) or 16-day mission (Neurolab, STS-90). Flight rats from both missions showed significantly fewer mature, age-appropriate righting tactics after landing compared to ground controls. Flight rats also had a steeper body angle while floating in the water before swimming, started swimming sooner, and swam faster. The effects on surface righting persisted for the duration of behavior tests (6 days [9-day mission] or 23 days [16-day mission]), after landing. Differences in pre-swimming behavior resolved by return day (R)2, and differences in swimming speed and posture resolved by R10. These data suggest that exposure to microgravity at a young age prevents the normal development of surface righting and that the normal development of swimming can recover if animals return from LEO by P16 or P24. These findings lend additional support to the existence of a critical period of development for motor function. However, studies are needed with improved housing during spaceflight to ensure that maternal offspring behavior is not disrupted, as was observed during the Neurolab mission.