Does circadian rhythm disruption during their early development have lasting effects on cognition of the elder rats?

This study aimed to investigate the changes in learning and memory after chronic circadian disruption followed by a long period of circadian recovery.

Eleven adult male spontaneously hypertensive rats were randomly divided into control group, 12-h light/12-h dark circadian disruption group (12L/12D) and 6-h light/6-h dark circadian disruption group (6L/6D). Rats in control group remained under the original 12-h light/12-h dark cycle throughout the experiment; rats in the 12L/12D group were exposed to 12-h light/12-h dark cycle with light-dark order changed every 3 days; rats in the 6L/6D group were exposed to 6-h light/6-h dark cycle. The disruption period continued for 18 weeks. Then after 8 weeks and 32 weeks of circadian re-entrainment, all animals were tested by Morris Water Maze (MWM), respectively, followed by an MRI examination.

Rats in the 12L/12D group demonstrated longer escape latency and swim distance in the MWM test than rats in the other two groups (P < 0.05). The MRI results showed volume% reduction and concentration% decrease of brain regions in the two circadian disruptive groups, while the changes were more significant and comprehensive in the 12L/12D group (P < 0.05).

Circadian disruption during early life accelerates cognition decline in later life in rats. Frequent light-dark order shift is more harmful.