Circadian pacemaker in “shift worker” rats: Effect of a sustained daily cognitive task on circadian gene expression and corticosterone

Previous studies have demonstrated that daily performance of sustained attention tasks (SATs) for a water reward during the light phase causes nocturnal rats to entrain to a diurnal activity pattern. In addition, attentional performance of rats during the light phase was significantly lowered when compared to performance during the dark phase. We hypothesized that the shift to a diurnal activity pattern observed in animals trained during the light phase was due to desynchronization or dampening of the SCN and peripheral oscillators. To test this hypothesis, we trained rats at times ZT4 and ZT16, and handled a group of non-performing animals randomly as a control. ZT4 and ZT16 trained animals were water deprived prior to SAT performance and were given 20 minutes of water after completing the task; food was provided ad libitum. Wheel running actogram data replicated previous observations that ZT4 trained animals were significantly diurnal. Preliminary corticosterone data suggests that day/night differences are attenuated in ZT4 trained animals as compared to ZT16 trained animals, suggesting that peripheral oscillators outside of the brain are dampened in this animal model of “shift work”. To observe effects on SCN and non-SCN oscillators in the brain, tissue was collected from ZT4 and ZT16 trained animals and control animals at times ZT2 and ZT14. Per2 expression was chosen as a measure of circadian oscillation and levels were measured via radioactive in situ hybridization. Results of the in situ hybridization will be presented at the conference.