Peeking into the sleeping brain: Using in vivo imaging in rodents to understand the relationship between sleep and cognition

Abstract

Sleep is well known to benefit cognitive function. In particular, sleep has been shown to enhance learning and memory in both humans and animals. While the underlying mechanisms are not fully understood, it has been suggested that brain activity during sleep modulates neuronal communication through synaptic plasticity. These insights were mostly gained using electrophysiology to monitor ongoing large scale and single cell activity. While these efforts were instrumental in the characterisation of important network and cellular activity during sleep, several aspects underlying cognition are beyond the reach of this technology. Neuronal circuit activity is dynamically regulated via the precise interaction of different neuronal and non-neuronal cell types and relies on subtle modifications of individual synapses. In contrast to established electrophysiological approaches, recent advances in imaging techniques, mainly applied in rodents, provide unprecedented access to these aspects of neuronal function in vivo. In this review, we describe various techniques currently available for in vivo brain imaging, from single synapse to large scale network activity. We discuss the advantages and limitations of these approaches in the context of sleep research and describe which particular aspects related to cognition lend themselves to this kind of investigation. Finally, we review the few studies that used in vivo imaging in rodents to investigate the sleeping brain and discuss how the results have already significantly contributed to a better understanding on the complex relation between sleep and plasticity across development and adulthood.

Keywords: Cognition; Electrophysiology; In vivo imaging; Plasticity; Rodents; Sleep.

Fig. 1

Schematic representation of key aspects relevant to sleep research that cannot be addressed using standard electrophysiological approaches.Middle: The different layers of the cortex are populated by different types of neurons (inhibitory = red, blue and light green; excitatory = light and dark grey, orange, and dark green). Non-neural cells are not represented here, but populate the entire cortex. Outside: Electrophysiology does not provide access to defined neuronal subpopulations on a single cell (left) or network level (top) and does not allow the observation of subcellular structure and function (right). However, these aspects of neuronal circuits are key when investigating sleep (bottom) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

Fig. 2

Summary of the main findings using in vivo imaging applied to sleep separated into functional categories. Details for each study are found in the text and Table 2.