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Ruth Grahn

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The effects of two stressors, predator odor 2, 5-dihydro-2, 4, 5-trimethylthiazoline (TMT) or restraint, on object recognition memory were examined when the stressor was presented either in the subjective day or subjective night. The Novel Object Recognition (NOR) task, behavioral observations during odor exposure, and c-Fos immunohistochemistry were conducted using 48 male Sprague-Dawley rats. The lateral bed nucleus of the stria terminalis (BNST), basolateral amygdala, hippocampus, perirhinal cortex, suprachiasmatic nucleus (SCN), and paraventricular nucleus (PVN) were examined. Behavioral results illustrated that stressor had a significant impact on all stress-related behaviors measured, with circadian differences observed in locomotion and grooming behaviors. Results from the NOR task showed that memory function was significantly impaired in animals in the restraint condition during the subjective night and not the subjective day, while memory function was impaired in animals in the odor condition during the subjective day and not the subjective night. Neural analyses revealed significant differences in brain activation only in the PVN, such that higher activation occurred during the day than the night for both restraint and control animals, while higher activation occurred during the night than the day for odor-exposed animals. Such opposition between the activation patterns and memory functioning of restraint and odor groups could indicate neural mechanisms for how the stress response is modulated during different points of the circadian cycle to affect memory processing.



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