Brief circadian rhythm disruption does not impair hippocampal dependent memory when rats are over-trained and given more re-entrainment days

Abstract

Circadian rhythm disruption can impair cognition in both humans and rodents. Brief exposure to circadian rhythm disruption that occurs after Morris water task training impairs retention of the place memory. This is quite unexpected as circadian rhythms were entrained both during training and memory testing. In the present study, we investigated this phenomenon, but with a few procedural elements implemented that would likely improve Morris water task retention. These procedural elements consisted of more Morris water task training, an easier final light-dark cycle to entrain to, and more re-entrainment days following circadian rhythm disruption. To determine that a retention deficit in the Morris water task was not a performance issue, rats were also trained in a simple stimulus-response task, and a one-trial hippocampal dependent Y-maze task. As expected, the procedural changes resulted in no Morris water task retention deficit despite experiencing circadian rhythm disruption in between training and retention testing. The rats also did not display impaired retention of the stimulus-response task and were able to acquire a one-trial hippocampal dependent task in the Y-maze post-circadian rhythm disruption. These data add to the body of literature suggesting that circadian rhythm disruption induced memory impairments is a complex phenomenon that is yet to be understood fully.