Slide 1

Sleep function and synaptic homeostasis Guilio Tononi, Chiara Cirelli Seminar presentation Kristjan-Julius Laak March 2014 Slide 2 Todays presentation The synaptic homeostasis hypothesis The main claims in detail Why does it matter? Discussion about the hypothesis Slide 3 The synaptic homeostasis hypothesis Sleep is the price we pay for the placticity of our brain {Slow wave} sleep plays {the main} role in the regulation of synaptic weight in the brain. nREM The amount of influence the firing of one neuron has on another. Slide 4 The hypothesis in brief 1.Wakefulness is associated with synaptic potentiation 2.Synaptic potentiation is tied to the homeostatic regulation of slow wave activity (SWA) 3.Slow wave activity is associated with synaptic downscaling 4.Downscaling is tied to the beneficial effects of sleep on neural function and performance enhancement in signal transmission between neurons Slide 5 Two-process model Sleep is regulated by both the circadian component C (24h) and a homeostatic process S Borbely & Achermann, 1999 Slide 6 Homeostatic regulation derived from the SWA in the EEG of NREM sleep total amount of synaptic weight in cerebral cortex Slide 7 Schematic diagram of the synaptic homeostasis 1. Synaptic potentiation2. SWA 3. Downscaling 4. Benefits Slide 8 The main claims in detail 1.Wakefulness is associated with synaptic potentiation 2.Synaptic potentiation is tied slow wave homeostasis 3.SWA is associated with synaptic downscaling 4.Downscaling is necessary for efficiency Slide 9 1. Wakefulness is associated with synaptic potentiation Plastic changes occur much of waking life The changes result more often in long-term potentiation (LTP) than in long-term depression Thus resulting in a net potentiation of synaptic strength Slide 10 Evidence Animals in enriched environment -> an increase in synaptic density -> LTP-like molecular changes Slide 11 Evidence Local increases in synaptic density Slide 12 Evidence Induction of LTP genes during spontaneous wakefulness VS. during sleep the genes are almost abolished Slide 13 Evidence Human brain metabolism increases throughout the wakefulness Slide 14 Mechanisms Evolutionary: potentiation should happend during wakefulness High-activity subset of neurons may give rise to LTP-related gene expression and thus LTP Specific effect of noradrenaline, which is high during waking period VS. absent during sleep (important for LTP) Slide 15 The main claims in detail 1.Wakefulness is associated with synaptic potentiation 2.Synaptic potentiation is tied slow wave homeostasis 3.SWA is associated with synaptic downscaling 4.Downscaling is necessary for efficiency Slide 16 2. Synaptic potentiation is tied slow wave homeostasis The higher the amount of synaptic potentiation in cortical circuits during wakefulness, the higher the increase in SWA during subsequent sleep. Or... Duration of wakefulness Expression of markers of synaptic potentiation SWA Slide 17 Evidence For the same duration of wakefulness, more exploring the environment- > SWA increased Slide 18 Evidence Lesioned noradrenergic system (less LTP) -> less SWA It is not wakefulness as such, but the induction of LTP-related molecules normally associated with wakefulness, that is responsible for driving the homeostatic increase in SWA Slide 19 Evidence Local effect of learning -> increase in local SWA in that specific region Slide 20 Evidence Developmental studies: SWA changes during the lifespan follows cortical synaptic density (in adolescence) Slide 21 Mechanisms Modeling studies show that stronger cortico- cortico connections result in stronger activation of sodium dependent potassium current, which leads to a longer down-phase -> large amplitude. Slide 22 The main claims in detail 1.Wakefulness is associated with synaptic potentiation 2.Synaptic potentiation is tied slow wave homeostasis 3.SWA is associated with synaptic downscaling 4.Downscaling is necessary for efficiency Slide 23 3. SWA is associated with synaptic downscaling The slow waves promote generalized depression/downscaling of synaptic strength Slide 24 Evidence The fact that EEG SWA decreased exponentially during sleep is an evidence to synaptic downscaling Slide 25 Evidence If SWA is suppressed in the first part of sleep, it increases greatly in the second part Slide 26 Evidence Brain metabolism absolute levels decrease during sleep Slide 27 Evidence Upregulation of molecules implicated in synaptic depression Sleep is not just unfavorable for synaptic potentiation, but conductive to synaptic downscaling Slide 28 Mechanisms Low frequency oscillations are ideally suited to induce depotentiation i.e. depression in stimulation paradigms Low frequency oscillations during sleep may promote depression through changes in calcium dynamics Temporal pairing of up-phase spiking and down- phase may indicate to synapses that the input was not effective in driving postsynaptic activity The process is self-limiting Slide 29 The main claims in detail 1.Wakefulness is associated with synaptic potentiation 2.Synaptic potentiation is tied slow wave homeostasis 3.SWA is associated with synaptic downscaling 4.Downscaling is necessary for efficiency Slide 30 The functional advantages of synaptic downscaling during sleep 1.Energy savings 2.Space savings 3.Benefits for learning and memory Slide 31 1. Energy savings The higher the synaptic weight -> the higher the energy budget 40% postsynaptic repolarization process 40% action potentials Slide 32 2. Space savings Synaptic strengthening is accompanied by increased size of terminal parts, and synapses may even grow in number. Slide 33 3. Benefits for learning and memory Downscaling promotes synaptic competition. E.g. development: stronger synapses survive. Signal-to-noise ratio grows when we sleep (red synapses, 5 vs 150 etc) Slide 34 3. Benefits for learning and memory Acquiring new information is not possible if there is no space. Sleep would not only be the price we have to pay for plasticity the previous day, but also an investment to allow the organism learn afresh the next day. Slide 35 Further questions need to be addressed What are the complex relationships between the local regulation of sleep as mediated by synaptic homeostasis, and the global regulation of sleep as mediated by hypothalamic and brainstem centers? How does the hypothesis apply to brain structures other than the cerebral cortex, e.g. hippocampus? Other species? (4h vs 20h) What is the role of REM sleep? (only part of the story) What is the role of sleep spindles? What happens if synaptic downscaling is incomplete? Do other mechanisms intervene to reduce neuronal excitability and thereby metabolic needs?