High level of corticosterone (CORT) is known as stress hormone, which is released from adrenal cortex upon stress stimulus. CORT reaches the brain through the blood-brain barrier. Hippocampus is affected by CORT, because it has a high expression of glucocorticoid receptor (GR). Hippocampus is a center for learning and memory. Recently, we observed that high concentration of CORT induces rapid increase of the spine density within 1 hour in hippocampal slices (Komatsuzaki Y et al., 2012, PLoS one). Following the circadian rhythm, CORT level changes in both the blood and the brain. In control with no stress, CORT concentration is around 30nM in the awake state and around 3nM in the sleep state (Hojo Y et al., 2011, Front Neuroendocrine Science). We observed that such a low level of CORT can modulate the spine density in the hippocampus. The rat spine density was higher in the awake state than in the sleep state. Supplementation of metyrapon decreased the spine density due to decrease of CORT level. The density and head-size of spine was analyzed by "Spiso-3D", the automated mathematical software (Mukai et al., 2011, Cerebral Cortex).
The current results suggest the molecular base of difference in memory performance between in the awake and in the sleep states.