Effects of chronic corticosterone administration in adolescent mice on endogenous cortical network activity and behaviour Konstantinos Armaos and Irini Skaliora Neurophysiology Lab, Biomedical Reseach Foundation of the Academy of Athens Background: Spontaneous network activity, in the form of recurring Up and Down states, is considered the default activity of the cerebral cortex, reflecting its endogenous connectivity and synaptic dynamics. This activity pattern is also preserved in brain slices ex vivo. The activity can be monitored with LFP recordings and consists of baseline activity called “down states”, interrupted by spontaneous bursts of high (and low) frequency oscillations called “Up states”. Upstates can be analyzed in order to quantify their properties, including their rate of occurrence, the duration, and the spectral power the different frequency bands. Chronic oral administration of corticosterone to rodents has been found to induce anxiety-like behaviour, anhedonia, immobility and stress-related memory deficits and has been proposed as a model of metabolic syndrome or depression. However, the effect of corticosterone administration to the network cortical function has not been addressed. Results: 4. Electrophysiology Upstate duration was longer for the control group (3) 1. The cort group had significantly reduced weight gain during the 2 weeks of the administration, even though food and liquid consumption was equal for both groups. than the cort. All the other metrics were similar between groups. 2. The adrenal glands of the cort group were smaller than control but with more surrounding adipose tissue. 3. EPM: The cort group covered less total distance in the EPM, spent less time in open and more time in closed arms.  Weight gain (gr) Food consumption (gr) Liquid consumption (ml) Duration (secs) Occurrence (events/min) Goal: To investigate the effect of chronic oral administration of corticosterone on the anatomy, physiology, behaviour and ex vivo electrophysiology of adolescent male mice.  Network Index Rectified Area (V*sec) Methods: Mice at the age of 28 days received corticosterone in drinking water for 2 weeks. Animal weight, food and liquid consumption (per cage) were checked daily. At the end of administration period, mice were submitted to Elevated Plus Maze. On the next day, mice were sacrificed, their brains were extracted immediately, put in artificial cerebrospinal fluid infused with carbogen and cut in 400μm slices. Spontaneous electrical activity was recorded from the brain slices with LFP recordings.  total distance (cm) time in open arms (sec) time in closed arms (sec) Conclusions: Even though corticosterone administration had obvious anatomical, physiological and behavioral detrimental effects, the network activity remained largely similar among groups. This result implies that spontaneous network activity in the form of Up and Down states is a robust phenomenon, resistant to corticosterone administration.