Ugo Faraguna

Molecular and electrophysiological evidence for net synaptic potentiation in wake and depression in sleep

Plastic changes occurring during wakefulness aid in the acquisition and consolidation of memories. For some memories, further consolidation requires sleep, but whether plastic processes during wakefulness and sleep differ is unclear. We show that, in rat cortex and hippocampus, GluR1-containing AMPA receptor (AMPAR) levels are high during wakefulness and low during sleep, and changes in the phosphorylation states of AMPARs, CamKII and GSK3β are consistent with synaptic potentiation during wakefulness and depression during sleep. Furthermore, slope and amplitude of cortical evoked responses increase after wakefulness, decrease after sleep and correlate with changes in slow-wave activity, a marker of sleep pressure. Changes in molecular and electrophysiological indicators of synaptic strength are largely independent of the time of day. Finally, cortical long-term potentiation can be easily induced after sleep, but not after wakefulness. Thus, wakefulness appears to be associated with net synaptic potentiation, whereas sleep may favor global synaptic depression, thereby preserving an overall balance of synaptic strength.

Cortical Firing and Sleep Homeostasis

The need to sleep grows with the duration of wakefulness and dissipates with time spent asleep, a process called sleep homeostasis. What are the consequences of staying awake on brain cells, and why is sleep needed? Surprisingly, we do not know whether the firing of cortical neurons is affected by how long an animal has been awake or asleep. Here, we found that after sustained wakefulness cortical neurons fire at higher frequencies in all behavioral states. During early NREM sleep after sustained wakefulness, periods of population activity (ON) are short, frequent, and associated with synchronous firing, while periods of neuronal silence are long and frequent. After sustained sleep, firing rates and synchrony decrease, while the duration of ON periods increases. Changes in firing patterns in NREM sleep correlate with changes in slow-wave activity, a marker of sleep homeostasis. Thus, the systematic increase of firing during wakefulness is counterbalanced by staying asleep.

A causal role for brain-derived neurotrophic factor in the homeostatic regulation of sleep

Slow-wave activity (SWA), the EEG power between 0.5 and 4 Hz during non-rapid eye movement (NREM) sleep, is one of the best characterized markers of sleep need, because it increases as a function of preceding waking duration and decreases during sleep, but the underlying mechanisms remain unknown. We hypothesized that SWA is high at sleep onset because it reflects the occurrence, during the previous waking period, of widespread synaptic potentiation in cortical and subcortical areas. Consistent with this hypothesis, we recently showed that the more rats explore, the stronger is the cortical expression of BDNF during wakefulness, and the larger is the increase in SWA during the subsequent sleep period. There is compelling evidence that BDNF plays a causal role in synaptic potentiation, and exogenous application of BDNF in vivo is sufficient to induce long-term increases in synaptic strength. We therefore performed cortical unilateral microinjections of BDNF in awake rats and measured SWA during the subsequent sleep period. SWA during NREM sleep was higher in the injected hemisphere relative to the contralateral one. The effect was reversible within 2 h, and did not occur during wakefulness or rapid eye movement sleep. Asymmetries in NREM SWA did not occur after vehicle injections. Furthermore, microinjections, during wakefulness, of a polyclonal anti-BDNF antibody or K252a, an inhibitor of BDNF TrkB receptors, led to a local SWA decrease during the following sleep period. These effects were also reversible and specific for NREM sleep. These results show a causal link between BDNF expression during wakefulness and subsequent sleep regulation.Slow wave activity (SWA), the EEG power between 0.5 - 4 Hz during NREM sleep, is one of the best characterized markers of sleep need, as it increases as a function of preceding waking duration and decreases during sleep, but the underlying mechanisms remain unknown. We hypothesized that SWA is high at sleep onset because it reflects the occurrence, during the previous waking period, of widespread synaptic potentiation in cortical and subcortical areas. Consistent with this hypothesis, we recently showed that the more rats explore, the stronger is the cortical expression of BDNF during wakefulness, and the larger is the increase in SWA during the subsequent sleep period. There is compelling evidence that BDNF plays a causal role in synaptic potentiation, and exogenous application of BDNF in vivo is sufficient to induce long-term increases in synaptic strength. We therefore performed cortical unilateral microinjections of BDNF in awake rats and measured SWA during the subsequent sleep period. SWA during NREM sleep was higher in the injected hemisphere relative to the contralateral one. The effect was reversible within 2 hours, and did not occur during wakefulness or REM sleep. Asymmetries in NREM SWA did not occur after vehicle injections. Furthermore, microinjections, during wakefulness, of a polyclonal anti-BDNF antibody or K252a, an inhibitor of BDNF TrkB receptors, led to a local SWA decrease during the following sleep period. These effects were also reversible and specific for NREM sleep. These results show a causal link between BDNF expression during wakefulness and subsequent sleep regulation.

Sleep and waking modulate spine turnover in the adolescent mouse cortex

Cortical development involves synaptic formation and elimination. Although synaptogenesis predominates in the early stages and pruning in the later stages, the two processes are thought to happen concurrently. In adults, synaptic strength is modulated by behavioral state, and we asked whether synaptic remodeling may be affected by sleep and waking states. Using two-photon microscopy in adolescent mice, we found that waking results in a net increase in cortical spines, whereas sleep is associated with net spine loss.

Direct evidence for wake-related increases and sleep-related decreases in synaptic strength in rodent cortex.

Despite evidence that waking is associated with net synaptic potentiation and sleep with depression, direct proof for changes in synaptic currents is lacking in large brain areas such as the cerebral cortex. By recording miniature EPSCs (mEPSCs) from frontal cortex slices of mice and rats that had been awake or asleep, we found that the frequency and amplitude of mEPSCs increased after waking and decreased after sleep, independent of time of day. Recovery sleep after deprivation also decreased mEPSCs, suggesting that sleep favors synaptic homeostasis. Since stronger synapses require more energy, space, and supplies, a generalized renormalization of synapses may be an important function of sleep.

Changes in brain gene expression after long-term sleep deprivation

Long‐term sleep deprivation in rats produces dramatic physiological changes including increase in energy expenditure, decrease in body weight, and death after 2–3 weeks. Despite several studies, the sleep deprivation syndrome remains largely unexplained. Here, to elucidate how prolonged sleep loss affects brain cells we used microarrays and screened the expression of > 26 000 transcripts in the cerebral cortex. Rats were sleep deprived using the disk‐over‐water method for 1 week. Seventy‐five transcripts showed increased expression in these animals relative to controls that had been spontaneously awake or sleep deprived for a few hours. Most of them were induced as a result of chronic sleep loss and not non‐specific effects of the disk stimulation. They include transcripts coding for several immunoglobulins, stress response proteins (macrophage inhibitor factor‐related protein 14, heat‐shock protein 27, α‐B‐crystallin), minoxidil sulfotransferase, globins and cortistatin. Twenty‐eight transcripts decreased their expression in long‐term sleep‐deprived rats. Sixteen of them were specifically decreased as a result of chronic sleep loss, including those coding for type I procollagen and dihydrolipoamide acetyltransferase. We also compared sleeping rats to short‐term and long‐term sleep‐deprived rats, and found that acute and chronic sleep loss led to some differences at the molecular level. Several plasticity‐related genes were strongly induced after acute sleep deprivation only, and several glial genes were down‐regulated in both sleep deprivation conditions, but to a different extent. These findings suggest that sustained sleep loss may trigger a generalized inflammatory and stress response in the brain.

Triggering Slow Waves During NREM Sleep in the Rat by Intracortical Electrical Stimulation: Effects of Sleep/Wake History and Background Activity

In humans, non-rapid eye movement (NREM) sleep slow waves occur not only spontaneously but can also be induced by transcranial magnetic stimulation. Here we investigated whether slow waves can also be induced by intracortical electrical stimulation during sleep in rats. Intracortical local field potential (LFP) recordings were obtained from several cortical locations while the frontal or the parietal area was stimulated intracortically with brief (0.1 ms) electrical pulses. Recordings were performed in early sleep (1st 2-3 h after light onset) and late sleep (6-8 h after light onset). The stimuli reliably triggered LFP potentials that were visually indistinguishable from naturally occurring slow waves. The induced slow waves shared the following features with spontaneous slow waves: they were followed by spindling activity in the same frequency range ( approximately 15 Hz) as spontaneously occurring sleep spindles; they propagated through the neocortex from the area of the stimulation; and compared with late sleep, waves triggered during early sleep were larger, had steeper slopes and fewer multipeaks. Peristimulus background spontaneous activity had a profound influence on the amplitude of the induced slow waves: they were virtually absent if the stimulus was delivered immediately after the spontaneous slow wave. These results show that in the rat a volley of electrical activity that is sufficiently strong to excite and recruit a large cortical neuronal population is capable of inducing slow waves during natural sleep.

Intraneural stimulation elicits discrimination of textural features by artificial fingertip in intact and amputee humans

Restoration of touch after hand amputation is a desirable feature of ideal prostheses. Here, we show that texture discrimination can be artificially provided in human subjects by implementing a neuromorphic real-time mechano-neuro-transduction (MNT), which emulates to some extent the firing dynamics of SA1 cutaneous afferents. The MNT process was used to modulate the temporal pattern of electrical spikes delivered to the human median nerve via percutaneous microstimulation in four intact subjects and via implanted intrafascicular stimulation in one transradial amputee. Both approaches allowed the subjects to reliably discriminate spatial coarseness of surfaces as confirmed also by a hybrid neural model of the median nerve. Moreover, MNT-evoked EEG activity showed physiologically plausible responses that were superimposable in time and topography to the ones elicited by a natural mechanical tactile stimulation. These findings can open up novel opportunities for sensory restoration in the next generation of neuro-prosthetic hands. DOI: http://dx.doi.org/10.7554/eLife.09148.001

Sleep Patterns and Homeostatic Mechanisms in Adolescent Mice

Sleep changes were studied in mice (n = 59) from early adolescence to adulthood (postnatal days P19–111). REM sleep declined steeply in early adolescence, while total sleep remained constant and NREM sleep increased slightly. Four hours of sleep deprivation starting at light onset were performed from ages P26 through adulthood (>P60). Following this acute sleep deprivation all mice slept longer and with more consolidated sleep bouts, while NREM slow wave activity (SWA) showed high interindividual variability in the younger groups, and increased consistently only after P42. Three parameters together explained up to 67% of the variance in SWA rebound in frontal cortex, including weight-adjusted age and increase in alpha power during sleep deprivation, both of which positively correlated with the SWA response. The third, and strongest predictor was the SWA decline during the light phase in baseline: mice with high peak SWA at light onset, resulting in a large SWA decline, were more likely to show no SWA rebound after sleep deprivation, a result that was also confirmed in parietal cortex. During baseline, however, SWA showed the same homeostatic changes in adolescents and adults, declining in the course of sleep and increasing across periods of spontaneous wake. Thus, we hypothesize that, in young adolescent mice, a ceiling effect and not the immaturity of the cellular mechanisms underlying sleep homeostasis may prevent the SWA rebound when wake is extended beyond its physiological duration.

Unilateral Cortical Spreading Depression Affects Sleep Need and Induces Molecular and Electrophysiological Signs of Synaptic Potentiation In Vivo

Cortical spreading depression (CSD) is an electrophysiological phenomenon first described by Leao in 1944 as a suppression of spontaneous electroencephalographic activity, traveling across the cerebral cortex. In vitro studies suggest that CSD may induce synaptic potentiation. One recent study also found that CSD is followed by a non-rapid eye movement (NREM) sleep duration increase, suggesting an increased need for sleep. Recent experiments in animals and humans show that the occurrence of synaptic potentiation increases subsequent sleep need as measured by larger slow wave activity (SWA) during NREM sleep, prompting the question whether CSD can affect NREM SWA. Here, we find that, in freely moving rats, local CSD induction increases corticocortical evoked responses and strongly induces brain derived neurotrophic factor (BDNF) in the affected cortical hemisphere but not in the contralateral one, consistent with synaptic potentiation in vivo. Moreover, for several hours after CSD, large slow waves occur in the affected hemisphere during rapid eye movement sleep and quiet waking but disappear during active exploration. Finally, we find that CSD increases NREM sleep duration and SWA, the latter specifically in the affected hemisphere. These effects are consistent with an increase in synaptic strength triggered by CSD, although nonphysiological phenomena associated with CSD may also play a role.