The circadian system: plasticity at many levels.

Over the years it has become crystal clear that a variety of processes encode time-of-day information, ranging from 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 physiology and behavior are properly synchronized to a changing environment. The purpose of this review is to put forward examples (as opposed to generate a comprehensive revision of all the available literature) in which the circadian system displays a remarkable degree of plasticity, from cell autonomous to circuit-based levels. In the literature, the term circadian plasticity has been used to refer to different concepts. The obvious one, more literally, refers to any change that follows a circadian (circa=around, diem=day) pattern, i.e. a daily change of a given parameter. The discovery of daily remodeling of neuronal structures will be referred herein as structural circadian plasticity, and represents an additional and novel phenomenon modified daily. Finally, any plasticity that has to do with a circadian parameter would represent a type of circadian plasticity; as an example, adjustments that allow organisms to adapt their daily behavior to the annual changes in photoperiod is a form of circadian plasticity at a higher organizational level, which is an emergent property of the whole circadian system. Throughout this work we will revisit these types of changes by reviewing recent literature delving around circadian control of clock outputs, from the most immediate ones within pacemaker neurons to the circadian modulation of rest-activity cycles.

Imaging odor coding and synaptic plasticity in the mammalian brain with a genetically-encoded probe.

We have used the genetically-encoded fluorescent exocytosis indicator synaptopHluorin (spH), expressed selectively in mouse olfactory receptor neurons, to image odor representations at the input to the olfactory bulb. The olfactory bulb is a powerful system for in vivo fluorescence imaging because its inputs are segregated into receptor-specific functional units (glomeruli) that are optically accessible and receive massively convergent input from sensory neurons. In a line of transgenic mice expressing spH under the control of a receptor neuron-specific promoter (OMP), odorant-evoked patterns of receptor neuron input to approximately 10% of the olfactory bulb can be imaged with excellent spatial resolution and sensitivity during single brief odorant presentations. Odor representations are similar across mice and can be imaged repeatedly in the same animal for months. In olfactory bulb slices from OP-spH mice, shock-evoked spH signals are rapid and linear reporters of transmitter release, although control for changes in extracellular pH is critical for proper interpretation of the spH signals. These features have allowed us to characterize the functional organization and mechanisms of presynaptic modulation of transmitter release at the first olfactory synapse. The capacity for long-term chronic imaging permits the direct visualization of the function regeneration and remapping of input to the olfactory bulb after lesions of the nasal epithelium.