REM sleep promotes bidirectional plasticity in developing visual cortex in vivo.

Sleep is required for the full expression of plasticity during the visual critical period (CP). However, the precise role of rapid-eye-movement (REM) sleep in this process is undetermined. Previous studies in rodents indicate that REM sleep weakens cortical circuits following MD, but this has been explored in only one class of cortical neuron (layer 5 apical dendrites). We investigated the role of REM sleep in ocular dominance plasticity (ODP) in layer 2/3 neurons using 2-photon calcium imaging in awake CP mice. In contrast to findings in layer 5 neurons, we find that REM sleep promotes changes consistent with synaptic strengthening and weakening. This supports recent suggestions that the effects of sleep on plasticity are highly dependent upon the type of circuit and preceding waking experience.

Anatomical correlates of rapid eye movement sleep-dependent plasticity in the developing cortex.

Rapid eye movement (REM) sleep is expressed at its highest levels during early life when the brain is rapidly developing. This suggests that REM sleep may play important roles in brain maturation and developmental plasticity. We investigated this possibility by examining the role of REM sleep in the regulation of plasticity-related proteins known to govern synaptic plasticity in vitro and in vivo. We combined immunohistochemistry with a classic model of experience-dependent plasticity in the developing brain known to be consolidated during sleep. We found that after the developing visual cortex is triggered to remodel, it is reactivated during REM sleep (as measured by FOS+ and ARC+ cells). This is accompanied by expression of several proteins implicated in synaptic long-term potentiation (PSD95 and phosphorylated (p), mTOR, cofilin, and CREB) across the different cortical layers. These changes did not occur in animals deprived of REM sleep, but were preserved in control animals that were instead awakened in non- (N) REM sleep. Collectively, these findings support a role for REM sleep in developmental brain plasticity.

Differential origin of the activation of dorsal and ventral dentate gyrus granule cells during paradoxical (REM) sleep in the rat.

We recently demonstrated that granule cells located in the dorsal dentate gyrus (dDG) are activated by neurons located in the lateral supramammillary nucleus (SumL) during paradoxical sleep (PS) hypersomnia. To determine whether these neurons are glutamatergic and/or GABAergic, we combined FOS immunostaining with in situ hybridization of vesicular glutamate transporter 2 (vGLUT2, a marker of glutamatergic neurons) or that of the vesicular GABA transporter (vGAT, a marker of GABAergic neurons) mRNA in rats displaying PS hypersomnia (PSR). We found that 84 and 76 % of the FOS+ SumL neurons in PSR rats expressed vGLUT2 and vGAT mRNA, respectively. Then, we examined vGLUT2 and FOS immunostaining in the dorsal and ventral DG of PSR rats with a neurochemical lesion of the Sum. In PSR-lesioned animals but not in sham animals, nearly all vGLUT2+ fibers and FOS+ neurons disappeared in the dDG, but not in the ventral DG (vDG). To identify the pathway (s) responsible (s) for the activation of the vDG during PS hypersomnia, we combined Fluorogold (FG) injection in the vDG of PSR rats with FOS staining. We found a large number of neurons FOS-FG+, specifically in the medial entorhinal cortex (ENTm). Altogether, our results suggest that SumL neurons with a unique dual glutamatergic and GABAergic phenotype are responsible for the activation of the dDG during PS hypersomnia, while vDG granule neurons are activated by ENTm cortical neurons. These results suggest differential mechanisms and functions for the activation of the dDG and the vDG granule cells during PS.

The supramammillary nucleus and the claustrum activate the cortex during REM sleep.

Evidence in humans suggests that limbic cortices are more active during rapid eye movement (REM or paradoxical) sleep than during waking, a phenomenon fitting with the presence of vivid dreaming during this state. In that context, it seemed essential to determine which populations of cortical neurons are activated during REM sleep. Our aim in the present study is to fill this gap by combining gene expression analysis, functional neuroanatomy, and neurochemical lesions in rats. We find in rats that, during REM sleep hypersomnia compared to control and REM sleep deprivation, the dentate gyrus, claustrum, cortical amygdaloid nucleus, and medial entorhinal and retrosplenial cortices are the only cortical structures containing neurons with an increased expression of Bdnf, FOS, and ARC, known markers of activation and/or synaptic plasticity. Further, the dentate gyrus is the only cortical structure containing more FOS-labeled neurons during REM sleep hypersomnia than during waking. Combining FOS staining, retrograde labeling, and neurochemical lesion, we then provide evidence that FOS overexpression occurring in the cortex during REM sleep hypersomnia is due to projections from the supramammillary nucleus and the claustrum. Our results strongly suggest that only a subset of cortical and hippocampal neurons are activated and display plasticity during REM sleep by means of ascending projections from the claustrum and the supramammillary nucleus. Our results pave the way for future studies to identify the function of REM sleep with regard to dreaming and emotional memory processing.

Rapid eye movement sleep promotes cortical plasticity in the developing brain.

Rapid eye movement sleep is maximal during early life, but its function in the developing brain is unknown. We investigated the role of rapid eye movement sleep in a canonical model of developmental plasticity in vivo (ocular dominance plasticity in the cat) induced by monocular deprivation. Preventing rapid eye movement sleep after monocular deprivation reduced ocular dominance plasticity and inhibited activation of a kinase critical for this plasticity (extracellular signal-regulated kinase). Chronic single-neuron recording in freely behaving cats further revealed that cortical activity during rapid eye movement sleep resembled activity present during monocular deprivation. This corresponded to times of maximal extracellular signal-regulated kinase activation. These findings indicate that rapid eye movement sleep promotes molecular and network adaptations that consolidate waking experience in the developing brain.

Extracellular signal-regulated kinase (ERK) activity during sleep consolidates cortical plasticity in vivo.

Ocular dominance plasticity (ODP) in the cat primary visual cortex (V1) is induced during waking by monocular deprivation (MD) and consolidated during subsequent sleep. The mechanisms underlying this process are incompletely understood. Extracellular signal-regulated kinase (ERK) is activated in V1 during sleep after MD, but it is unknown whether ERK activation during sleep is necessary for ODP consolidation. We investigated the role of ERK in sleep-dependent ODP consolidation by inhibiting the ERK-activating enzyme MEK in V1 (via U0126) during post-MD sleep. ODP consolidation was then measured with extracellular microelectrode recordings. Western blot analysis was used to confirm the efficacy of U0126 and to examine proteins downstream of ERK. U0126 abolished ODP consolidation and reduced both phosphorylation of eukaryotic initiation factor 4E (eIF4E) and levels of the synaptic marker PSD-95. Furthermore, interfering with ERK-mediated translation by inhibiting MAP kinase-interacting kinase 1 (Mnk1) with CGP57380 mimicked the effects of U0126. These results demonstrate that ODP consolidation requires sleep-dependent activation of the ERK-Mnk1 pathway.

Tuberal hypothalamic neurons secreting the satiety molecule Nesfatin-1 are critically involved in paradoxical (REM) sleep homeostasis.

The recently discovered Nesfatin-1 plays a role in appetite regulation as a satiety factor through hypothalamic leptin-independent mechanisms. Nesfatin-1 is co-expressed with Melanin-Concentrating Hormone (MCH) in neurons from the tuberal hypothalamic area (THA) which are recruited during sleep states, especially paradoxical sleep (PS). To help decipher the contribution of this contingent of THA neurons to sleep regulatory mechanisms, we thus investigated in rats whether the co-factor Nesfatin-1 is also endowed with sleep-modulating properties. Here, we found that the disruption of the brain Nesfatin-1 signaling achieved by icv administration of Nesfatin-1 antiserum or antisense against the nucleobindin2 (NUCB2) prohormone suppressed PS with little, if any alteration of slow wave sleep (SWS). Further, the infusion of Nesfatin-1 antiserum after a selective PS deprivation, designed for elevating PS needs, severely prevented the ensuing expected PS recovery. Strengthening these pharmacological data, we finally demonstrated by using c-Fos as an index of neuronal activation that the recruitment of Nesfatin-1-immunoreactive neurons within THA is positively correlated to PS but not to SWS amounts experienced by rats prior to sacrifice. In conclusion, this work supports a functional contribution of the Nesfatin-1 signaling, operated by THA neurons, to PS regulatory mechanisms. We propose that these neurons, likely releasing MCH as a synergistic factor, constitute an appropriate lever by which the hypothalamus may integrate endogenous signals to adapt the ultradian rhythm and maintenance of PS in a manner dictated by homeostatic needs. This could be done through the inhibition of downstream targets comprised primarily of the local hypothalamic wake-active orexin- and histamine-containing neurons.