Synaptic plasticity during cortical slow wave activity

When neurons in the brain become excited, they 'fire' electrical impulses, which they rapidly transmit to all their contacts. The points at which neurons contact each other are called synapses, and long-term changes in the strength of these synaptic connections are thought to provide the biological basis of the brain's ability to learn and remember. If synaptic strengths are continuously altered as we go about our daily lives, is there a point at which the flow of electricity through the brain will eventually become congested? One hypothesis is that sleep provides a critical period for retuning synaptic connections, to enable continued and efficient learning.

During sleep, the brain alternates between two different patterns of electrical activity - low-amplitude fast oscillations, associated with rapid eye movements and vivid dreaming, and slow wave oscillations of deep sleep. It is the slow wave oscillations that have been implicated in playing a role in retuning synapses in brain circuits, but the cellular mechanisms have yet to be resolved. The aim of this project to record directly the strength of synaptic connections in cortical circuits, and determine how they change during sleep-related slow wave oscillations. We will use thin slices of brain tissue, which preserve the intrinsic patterns of slow wave oscillations, while allowing unprecedented access to monitor synaptic events using electrical measurements and optical imaging techniques. These experiments will determine how the precise sequences of neuronal firing during slow wave oscillations are translated into increases versus decreases in synaptic strength, and thus provide insights into rules and mechanisms by which slow wave sleep recalibrates neuronal circuits.

Sleep has previously been viewed as an evolutionary adaptation to conserve energy during dark and cold nights, but we now know that the brain is highly active during sleep, suggesting it has a positive function. Nevertheless, the need to sleep is still commonly viewed as a nuisance in our busy schedules, and we take stimulants and sedatives to override our biological clocks. Disturbances in sleep are correlated with problems in physical and mental health, and resolving the function of sleep is a critical step to determining the pathway to healthy aging.

In slow wave sleep, cortical neurons display oscillations between hyperpolarised Down states and depolarised Up states. The rhythmic bouts of Up state activity have been proposed to provide the conditions for the downscaling synaptic weights built up during awake behaviour, and thus enable synaptic homeostasis in cortical networks. It has also been shown that sequences of neuronal firing observed during behaviour are replayed during slow wave sleep, supporting an alternative function of this sleep stage in memory consolidation and transfer. Our hypothesis is that synapses that participate directly in triggering postsynaptic spikes during Up state will be strengthened, while the majority that do not will be downscaled. To test this, we will record electrically-evoked synaptic events in layer 2/3 pyramidal neurons, in slices of medial entorhinal cortex that display spontaneous Up/Down states. Synaptic efficacy will be monitored in the Down state, and the effect of pairing protocols tested in the Up state. To resolve the dendritic calcium events underlying synaptic plasticity during Up states, we will load layer 2/3 pyramidal neurons with a high-affinity calcium indicator, and simultaneously image activated dendritic shafts and spines using a fast 3D scanning two-photon microscope. To determine whether isolated dendritic plateau potentials provide the salient trigger for synaptic strengthening, we will use the same microscopy system to image voltage changes in these thin dendritic structures. These experiments will provide insights into the functions of slow wave sleep in retuning cortical synapses.

There are four potential pathways to impact of the proposed research:

1) Improved understanding of the basic biological functions of sleep
The aim of this project is to determine the mechanisms of synaptic plasticity during sleep-related slow wave oscillations in cortical circuits. The outcome of these experiments, disseminated throughout the time course of the project, will offer valuable insights for researchers attempting to understand the mechanisms of synaptic plasticity and the functions of sleep, as well as informing models of neural processing in wider areas of neuroscience research.

2) Establishing a cross-disciplinary research platform
Our collaborative approach will provide immediate expose of physiology researchers to the current developments in optical techniques that can enhance biological and health science, whilst providing researchers in optical engineering with experience in biological experiments to inform equipment design and implementation. Moreover, the Research Co-I will be trained in electrophysiological techniques, which will complement his engineering background, and thus provide essential skills that can be applied to dissecting further basic biological questions.

3) Technology Transfer
The application of a newly developed remote focusing two-photon microscope to address the fundamental cellular functions of sleep, will highlight the potential of remote focusing as an enabling technology. Disseminating the results of the research will identify remote focusing microscopy as a valuable tool to deliver key insights into broader fields of biomedical research, thereby creating a wider market for the technology as a commercial instrument.

3) Public health
Sleep is increasingly sacrificed in the face demands from work and society. Through our public engagement, we hope to use our results to argue that sleep has a critical function for plasticity in the brain, and to ensure a public understanding that sleep is a free resource to promote human health.