1) Wright State University, Boonshoft School of Medicine, Department of Pharmacology and Toxicology, Dayton, OH, USA
2) Institute of Experimental Pharmacology and Toxicology, Slovak Academy of Sciences, Bratislava, Slovakia

Experiments were performed in C57BL/6J male mice to determine 1) light/dark effects of acute and chronic shaker stress on open field behavioral patterns and 2) light/dark effects of chronic stress on plasma corticosterone and oxytocin. Shaker stress was applied acutely (15 min) or chronically (3 or 7 days). Mice were tested in the open field in the light or dark phase of the circadian cycle. For the endocrine study, mice were exposed to 3 days of intermittent shaker stress and sacrificed after the last stress event (09:00 or 19:00 h). Acute or chronic shaker stress had no significant effects on intensity of motor activity and rearing of mice tested under either light condition. Mice tested in the dark phase had higher motor activity and exhibited lower anxiety-like behavior as expressed by central zone activities and had higher emotionality as expressed by increased defecation. Chronic stress increased corticosterone with a greater absolute increase in the dark period. However, the percentage stress-induced increase was not different between the day and night periods. The oxytocin response to stress was observed only during the light phase with no change seen at dark phase. These results show that there is a marked difference in the light/dark pituitary stress response with no alteration in stress induced behavioral changes. They also suggest that there are circadian interactions in the endocrine stress axis that are without consequences for open field behavior.