Sleep is critical for hippocampus-dependent memory consolidation. However, the underlying mechanisms of synaptic plasticity are poorly understood. The central controversy is on whether long-term potentiation (LTP) takes a role during sleep and which would be its specific effect on memory. To address this question, we used immunohistochemistry to measure phosphorylation of Ca2+/calmodulin-dependent protein kinase II (pCaMKIIα) in the rat hippocampus immediately after specific sleep-wake states were interrupted. Control animals not exposed to novel objects during waking (WK) showed stable pCaMKIIα levels across the sleep-wake cycle, but animals exposed to novel objects showed a decrease during subsequent slow-wave sleep (SWS) followed by a rebound during rapid-eye-movement sleep (REM). The levels of pCaMKIIα during REM were proportional to cortical spindles near SWS/REM transitions. Based on these results, we modeled sleep-dependent LTP on a network of fully connected excitatory neurons fed with spikes recorded from the rat hippocampus across WK, SWS and REM. Sleep without LTP orderly rescaled synaptic weights to a narrow range of intermediate values. In contrast, LTP triggered near the SWS/REM transition led to marked swaps in synaptic weight ranking. To better understand the interaction between rescaling and restructuring during sleep, we implemented synaptic homeostasis and embossing in a detailed hippocampal-cortical model with both excitatory and inhibitory neurons. Synaptic homeostasis was implemented by weakening potentiation and strengthening depression, while synaptic embossing was simulated by evoking LTP on selected synapses. We observed that synaptic homeostasis facilitates controlled synaptic restructuring. The results imply a mechanism for a cognitive synergy between SWS and REM, and suggest that LTP at the SWS/REM transition critically influences the effect of sleep: Its lack determines synaptic homeostasis, its presence causes synaptic restructuring.

Dopamine release and phase-amplitude cross-frequency coupling (CFC) have independently been implicated in prefrontal cortex (PFC) functioning. To causally investigate whether dopamine release affects phase-amplitude comodulation between different frequencies in local field potentials (LFP) recorded from the medial PFC (mPFC) of behaving rats, we used RuBiDopa, a light-sensitive caged compound that releases the neurotransmitter dopamine when irradiated with visible light. LFP power did not change in any frequency band after the application of light-uncaged dopamine, but significantly strengthened phase-amplitude comodulation between delta and gamma oscillations. Saline did not exert significant changes, while injections of dopamine and RuBiDopa produced a slow increase in comodulation for several minutes after the injection. The results show that dopamine release in the medial PFC shifts phase-amplitude comodulation from theta-gamma to delta-gamma. Although being preliminary results due to the limitation of the low number of animals present in this study, our findings suggest that dopamine-mediated modification of the frequencies involved in comodulation could be a mechanism by which this neurotransmitter regulates functioning in mPFC.

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The role of sleep on memory consolidation is thought to involve experience-dependent changes in spindle oscillations and protein phosphorylation, but how these phenomena are related remains poorly understood. To gain insight into this relationship, we used electrophysiological recordings and quantitative phosphoproteomic analysis to assess spindle oscillations and phosphoprotein levels in the hippocampus (HP) and primary somatosensory cortex (S1) of adult male rats recorded across the sleep cycle. Animals were surgically implanted with multielectrode probes and after recovery were exposed or unexposed to novel objects (+ and – groups, respectively). HP and S1 samples were obtained after periods rich in either slow-wave sleep (SWS) or rapid-eye-movement sleep. Bottom-up shotgun mass spectrometry in a two-dimensional liquid chromatography-tandem mass spectrometry setup (MSE mode with label-free quantification) showed that the proteomes differed in the numbers of phosphoproteins identified by phosphoryl modification STY tags, with a total of 337 validated phosphoproteins identified in S1 and 198 in the HP. A comparison of the phosphoproteomic profiles of the treatments and regions (SWS+ versus SWS-, REM+ versus REM-, REM+ versus SWS+ and REM- versus SWS-), using clustering analysis of the significantly identified phosphoproteins, found that 51 phosphoproteins from S1 were sufficient to separate the four experimental conditions, while 37 phosphoproteins from the HP could only partially separate the groups. Fold change analysis identified 90 significantly modulated phosphoproteins related to synaptic function, actin-microtubule regulation, DNA-RNA binding, proteases-phosphatases-kinases and other regulatory functions, including CaMKII and MAPK. In both the HP and S1, nearly one third of the clustering-relevant phosphoproteins had levels significantly correlated with the abundance of spindle oscillations pooled across the transition from SWS to REM. In S1, phosphorylated Reelin was upregulated during REM compared to SWS, in proportion to the number of spindle oscillations during the transition from SWS to REM. In the HP, a voltage-gated calcium channel subunit (Cacna2d1) was down-regulated during REM+ compared to SWS+, in proportion to spindle counts. Since spindles facilitate calcium entry through the opening of voltage-dependent calcium channels, Cacna2d1 down-regulation may lead to a hippocampus-specific, REM-dependent downregulation of synaptic plasticity after exposure to novel objects. The results point to major experience-dependent differences between HP and S1 in phosphoproteomic regulation across the sleep cycle, with potential implications for memory corticalization.

The role of sleep on memory consolidation is thought to involve experience-dependent changes in spindle oscillations and protein phosphorylation, but how these phenomena are related remains poorly understood. To gain insight into this relationship, we used electrophysiological recordings and quantitative phosphoproteomic analysis to assess spindle oscillations and phosphoprotein levels in the hippocampus (HP) and primary somatosensory cortex (S1) of adult male rats recorded across the sleep cycle. Animals were surgically implanted with multielectrode probes and after recovery were exposed or unexposed to novel objects (+ and – groups, respectively). HP and S1 samples were obtained after periods rich in either slow-wave sleep (SWS) or rapid-eye-movement sleep. Bottom-up shotgun mass spectrometry in a two-dimensional liquid chromatography-tandem mass spectrometry setup (MSE mode with label-free quantification) showed that the proteomes differed in the numbers of phosphoproteins identified by phosphoryl modification STY tags, with a total of 337 validated phosphoproteins identified in S1 and 198 in the HP. A comparison of the phosphoproteomic profiles of the treatments and regions (SWS+ versus SWS-, REM+ versus REM-, REM+ versus SWS+ and REM- versus SWS-), using clustering analysis of the significantly identified phosphoproteins, found that 51 phosphoproteins from S1 were sufficient to separate the four experimental conditions, while 37 phosphoproteins from the HP could only partially separate the groups. Fold change analysis identified 90 significantly modulated phosphoproteins related to synaptic function, actin-microtubule regulation, DNA-RNA binding, proteases-phosphatases-kinases and other regulatory functions, including CaMKII and MAPK. In both the HP and S1, nearly one third of the clustering-relevant phosphoproteins had levels significantly correlated with the abundance of spindle oscillations pooled across the transition from SWS to REM. In S1, phosphorylated Reelin was upregulated during REM compared to SWS, in proportion to the number of spindle oscillations during the transition from SWS to REM. In the HP, a voltage-gated calcium channel subunit (Cacna2d1) was down-regulated during SWS+ compared to REM+, in proportion to spindle counts. Since spindles facilitate calcium entry through the opening of voltage-dependent calcium channels, Cacna2d1 down-regulation may lead to a hippocampus-specific, REM-dependent downregulation of synaptic plasticity after exposure to novel objects. The results point to major experience-dependent differences between HP and S1 in phosphoproteomic regulation across the sleep cycle, with potential implications for memory corticalization.