Glucocorticoids are good and bad for plasticity and learning

Circadian glucocorticoid peaks promote postsynaptic dendritic spine formation in the mouse cortex after motor skill learning, whereas troughs are required for stabilizing newly formed spines that are important for long-term memory retention. Conversely, chronic and excessive exposure to glucocorticoids eliminates learning-associated new spines and disrupts previously acquired memories. Together, these findings indicate that tightly regulated circadian glucocorticoid oscillations are important for learning-dependent synaptic formation and maintenance. 
Circadian glucocorticoid oscillations have dual roles in dendritic spine plasticity, controlling spine formation and elimination through distinct mechanisms important for motor learning.

Nature Neuroscience 

16, 

658–659 

(2013)