Circadian plasticity: from structure to behavior.

Over the years it has become clear that the biological clock acts at different levels, ranging from the control of gene expression, protein stability, or subcellular localization of key proteins, to the fine tuning of network properties and modulation of input signals, ultimately ensuring that the organism will be best synchronized to a changing environment at the physiological and behavioral levels. The purpose of this chapter is to discuss the circadian control of clock outputs, spanning the most immediate ones within pacemaker neurons (i.e., membrane excitability, release of neurotransmitters, structural changes) to the circadian modulation of different behaviors (locomotor activity, learning and memory, social interaction), with a focus on the examples that shed light on the surprising degree of plasticity that characterizes the underlying circuits.