Hippocampal-dependent Memory Retention is Unaffected by a T21 Light–dark Cycle in Female Fischer Brown Norway Rats

The present study investigated circadian rhythms and hippocampal-dependent memory in female Fischer brown Norway rats. As in past reports with female Long Evans rats, we hypothesized that six days of exposure to an unentrainable T21 light-dark cycle that occurred during distributed Morris water task training would not impair the retention of the acquired memory. As expected, the rats did not have impaired retention of the place memory acquired in the distributed Morris water task. We next, quantified circadian entrainment two months after exposure to the T21 cycle. As expected, measures of circadian rhythms suggested that the rats exposed to the T21 had since entrained to a 12:12 light-dark cycle. To confirm that the T21 paradigm induces free-running circadian rhythms in female Fischer brown Norway rats, the control animals were subjected to the same T21 paradigm and circadian rhythms were continuously measured. Signs of free-running circadian rhythms were present during T21 exposure such as delayed phase of entrainment, elongated period, and decreased activity during the dark phase of the cycle. To our knowledge, this is one of the first characterization of activity rhythms in Fischer brown Norway rats.