Time stamp memory is modulated by the phase of the estrous cycle in wistar rats

Estrogen levels have been reported to influence cognitive performance measures. Notably, cognition is also subjected to the time stamp phenomenon, in which optimal performance occurs when the time of testing matches the time of training, highlighting the importance of carefully examining external cues in the synchronization of consciously controlled brain activity. The current study tested intact, sham-operated and ovariectomized Wistar rats to investigate the time stamp phenomenon at different phases of the estrous cycle to control for internal hormonally regulated time-dependent neuroendocrine activity that may influence learning outcome measures. Rats were trained and tested in an object recognition task, at matching Zeitgeber Time (ZT14- ZT14) or nonmatching times (ZT14-ZT18), during proestrus or estrus phases. Intact rats tested in the estrus phase significantly exhibited increased exploration of the new object, but only when training and testing times matched. Contrastingly, intact rats tested in proestrus exhibited increased exploration of the new object at both times tested. Moreover, the ovariectomized rats resulted in an abolished preference selectivity for new objects, even when the testing was performed at the same time as the training. Finally, the sham-operated rats tested in proestrus also exhibited an increased exploration of the new object at both times tested. Taken together, these results suggest that high estrogen levels during the proestrus phase in rats can facilitate object recognition.