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Dendrites are the post-synaptic sites of most excitatory and inhibitory synapses in the brain, making them the main location of cortical information processing and synaptic plasticity. Although current hypotheses suggest a central role for sleep in proper cognitive function and brain plasticity, virtually nothing is known about changes in dendritic activity across the sleep-wake cycle and how waking experience modifies this activity. <br/>To start addressing these questions, we developed a method that allows long-term recordings of EEGs/EMG combined with in vivo cortical calcium (Ca2+) activity in freely moving and sleeping rats. We measured Ca2+ activity from populations of dendrites of layer (L) 5 pyramidal neurons (n = 13 rats) that we compared with Ca2+ activity from populations of neurons in L2/3 (n = 11 rats). L5 and L2/3 neurons were labelled using bolus injection of OGB1-AM or GCaMP6 (1). Ca2+ signals were detected using a fiber-optic system (cannula diameter = 400µm), transmitting the changes in fluorescence to a photodiode. Ca2+ fluctuations could then be correlated with ongoing changes in brain oscillatory activity during 5 major brain states: active wake [AW], quiet wake [QW], NREM, REM and NREM-REM transition (or intermediate state, [IS]). <br/>Our Ca2+ recordings show large transients in L5 dendrites and L2/3 neurons that oscillate predominantly at frequencies < 1Hz (transient duration (sec.): L2/3 = 2.97 ± 0.54; L5 dend. = 2.81± 0.5) irrespective of brain states. Dendritic Ca2+ signals show a significant increase (as measured by Ca2+ signal power (< 1Hz) and Ca2+ transients amplitude changes) specifically during IS sleep, when spindle and theta activity prevails (2). Interestingly, this trend wasn’t seen in L2/3 neurons that display greatest Ca2+ activity during AW. Correlation analyses between dendritic Ca2+ transient amplitude and selected brain oscillations revealed a strong positive relationship during sleep (especially NREM) compared to waking states. Remarkably, this relation was reversed for L2/3 neurons that display negative correlations for the same frequency bands, especially during NREM and IS. This inverse correlation trend is strongest for higher frequencies range (i.e. gamma: 20-60 Hz) and during IS. <br/>In summary, we show that this technique is successful in monitoring fluctuations in ongoing dendritic Ca2+ activity during natural brain states and allows, in principle, to combine behavioral measurement with imaging from various brain regions (e.g. deep structures) in freely behaving animals. Using this method, we show that Ca2+ transients from populations of L2/3 neurons and L5 dendrites are deferentially regulated across the sleep/wake cycle, with dendritic activity being the highest during the IS sleep. Our correlation analysis suggests that specific sleep EEG activity during NREM and IS, but not REM, has opposite effect on Ca2+ activity in dendrites (positive correlation) and L2/3 neurons (negative correlation). These results support the idea that specific sleep oscillations affect differently Ca2+ activity fluctuations in different neuronal populations and/or cellular compartments. Behavioral manipulations (e.g. sleep deprivation [ongoing experiments], learning) will provide important information on the functional relevance of those observations and on the impact of waking experience on wake- and sleep-dependent dendritic activity changes. <br/>