Imaging Dendrites Across Wake and Sleep: Fiberoptic Measurements of Calcium Activity in Freely Behaving Animals

It is now clear that the sleeping cortex is not at rest and that the patterns of activity that manifest in the different phases of sleep are at least as strong as during wake (e.g. REM sleep). The need for a better understanding of neuronal activity during sleep is of outmost importance in the context of recent findings suggesting that one of the functions of sleep is to promote brain plasticity. But direct evidence that sleep actually promotes network remodeling - supported by synaptic plasticity - is lacking. Dendritic spines are the postsynaptic sites of most excitatory synapses in the brain and changes in spine morphology and dynamism serve as good indicators of synaptic plasticity. Thus a more direct assessment of neuronal activity, and specifically synaptic activity, is the increase in dendritic [Ca2+]i as it remains confined within the spine (i.e. synapse). Dendritic Ca2+ entry during sleep could provide the long timescales needed to mobilize the machinery hypothesized as being responsible for the consolidation of brain plasticity induced during the state of wakefulness. Our hypothesis is that experience-dependent plasticity processes increase neuronal activity through enhancement of [Ca2+]i during sleep. To test this hypothesis, we will focus on dendritic calcium activation during sleep following enriched environment (EE), which has been shown to enhance synaptic plasticity in individual branches of the dendritic arbor. To achieve this, we will combine in vivo assessment of calcium activity using fiberoptical imaging technique in different cortical neuronal populations (supragranulare pyramidal neurons [layer 2/3], apical tuft dendrite of L5 neurons, and interneurons) with EEG/EMG polysomnographic recordings in freely behaving rats. As dendrites are the main site of plasticity-induced changes, we expect our results to provide new insight into the role of sleep in brain activity in general and plasticity in particular.