Salubrinal, an endoplasmic reticulum stress blocker, modulates sleep homeostasis and activation of sleep- and wake-regulatory neurons.

Endoplasmic reticulum (ER) stress has been associated with the regulation of sleep and wake. We have previously shown that i.c.v. administration of a specific ER stress modulator, Salubrinal (SALUB), which inhibits global protein translation by blocking the dephosphorylation of eukaryotic initiation factor 2α (p-eIF2α), increased non-rapid eye movement (NREM) sleep. Here we report on the relationship between ER stress response and sleep homeostasis by measuring the amount and intensity of homeostatic recovery sleep in response to the i.c.v. administration of SALUB in adult freely behaving rats. We have also tested the hypothesis that SALUB induces sleep by activating sleep-promoting neurons and inhibiting wake-promoting neurons in the basal forebrain (BF) and hypothalamus by quantifying the effects of SALUB treatment on c-Fos expression in those neuronal groups. The present study found that i.c.v. administration of SALUB significantly modified the homeostatic sleep response. SALUB administered during sleep deprivation increased sleep intensity, indicated by slow-wave activity (SWA), during recovery sleep, whereas its administration during recovery sleep increased the amount of recovery sleep. We also found that SALUB induced c-Fos activation of GABAergic neurons in the sleep-promoting rostral median preoptic nucleus while simultaneously reducing c-Fos activation of wake-promoting lateral hypothalamic orexin-expressing neurons and magnocellular BF cholinergic neurons. The current findings suggest that ER stress pathway plays a role in the homeostatic control of NREM sleep in response to sleep deprivation and provides a mechanistic explanation for the sleep modulation by molecules signaling the need for brain protein synthesis.

Sustained sleep fragmentation results in delayed changes in hippocampal-dependent cognitive function associated with reduced dentate gyrus neurogenesis.

Sleep fragmentation (SF) is prevalent in human sleep-related disorders. In rats, sustained SF has a potent suppressive effect on adult hippocampal dentate gyrus (DG) neurogenesis. Adult-generated DG neurons progressively mature over several weeks, and participate in certain hippocampal-dependent cognitive functions. We predicted that suppression of neurogenesis by sustained SF would affect hippocampal-dependent cognitive functions in the time window when new neurons would reach functional maturity. Sprague-Dawley rats were surgically-prepared with electroencephalogram (EEG) and electromyogram (EMG) electrodes for sleep state detection. We induced sleep-dependent SF for 12 days, and compared SF animals to yoked sleep fragmentation controls (SFC), treadmill controls (TC) and cage controls (CC). Rats were injected with bromodeoxyuridine on treatment days 4 and 5. Rats were returned to home cages for 14 days. Cognitive performance was assessed in a Barnes maze with 5 days at a constant escape position followed by 2 days at a rotated position. After Barnes maze testing rats were perfused and DG sections were immunolabeled for BrdU and neuronal nuclear antigen (NeuN), a marker of mature neurons.SF reduced BrdU-labeled cell counts by 32% compared to SFC and TC groups. SF reduced sleep epoch duration, but amounts of rapid eye movement (REM) sleep did not differ between SF and SFC rats, and non-rapid eye movement (NREM) was reduced only transiently. In the Barnes maze, SF rats exhibited a progressive decrease in escape time, but were slower than controls. SF animals used different search strategies. The use of a random, non-spatial search strategy was significantly elevated in SF compared to the SFC, TC and CC groups. The use of random search strategies was negatively correlated with NREM sleep bout length during SF. Sustained sleep fragmentation reduced DG neurogenesis and induced use of a non-spatial search strategy, which could be seen 2 weeks after terminating the SF treatment. The reduction in neurogenesis induced by sleep fragmentation is likely to underlie the delayed changes in cognitive function.

Administration of the protein synthesis inhibitor, anisomycin, has distinct sleep-promoting effects in lateral preoptic and perifornical hypothalamic sites in rats.

Although a robust relationship between sleep and increased brain protein synthesis is well-documented, there have been few reports of the effects of local application of a protein synthesis inhibitor (PSI) on sleep. In this study, we compared the effects of local microdialytic administration of the protein synthesis inhibitor, anisomycin (ANI) into the lateral preoptic area (LPOA), a sleep promoting area vs. the perifornical/lateral hypothalamus (PF/LH), a wake and rapid eye movement (REM) sleep-promoting area. ANI administered to the LPOA at night resulted in an increase in stage 2 of rat non-REM sleep, whereas ANI delivered into the PF/LH during the daytime increased REM sleep. ANI microdialysis into hippocampus did not affect sleep or waking. These differential effects of local protein synthesis inhibition on sleep support a hypothesis that mechanisms controlling protein synthesis are critically involved in the regulation of both NREM sleep and REM sleep.

Sleep-promoting functions of the hypothalamic median preoptic nucleus: inhibition of arousal systems.

Recent work supports the hypotheses developed by von Economo and Nauta and elaborated by Sallanon et al. that the POA contains a sleep-promoting output that opposes wake-promoting neuronal groups in the PH. The POA gives rise to descending pathways that terminate within wake-promoting populations in pLH, PH and midbrain. Current evidence suggests that this output originates in POA sleep-active GABAergic neurons. This output also seems to convey the signals of homeostatic drive. Disynaptic projections from the SCN to both MnPN and VLPO were recently identified. These may regulate the circadian control of sleep propensity. The hypothesis that the descending projections from POA sleep-active neurons to sites of arousal-related neurons originates in GABAergic neurons must be confirmed. Also to be further clarified is the anatomical distribution of putative sleep-active GABAergic neurons within the POA. Segregated groups have been found in the MnPN and VLPO, but unit recording studies of sleep-active neurons, lesion studies and local neurochemical application studies all indicate that sleep-active neurons may be found diffusely in the POA and adjacent areas. The MnPN has been shown previously to be involved in water balance and blood pressure regulation and to be responsive to hyperthermia. Our studies suggest that this nucleus also contains sleep-active, putative sleep-promoting neurons. However, interactions between sleep control and physiological variables must be considered. In particular, the details of neuronal basis of the coupling of warm-sensitive neurons in MnPN to the POA hypnogenic output has not been explored. It is also worth noting that both the VLPO and MnPN lie close to the ventricular and subarachnoid surface and are punctuated by radial arterioles. The possibility that the sleep-regulatory functions of these sites is coupled to physiological signals conveyed through epithelial cells has been suggested for the actions of PGD2 but has yet to be explored in detail for other putative hypnogens.

Effects of lateral preoptic area application of orexin-A on sleep-wakefulness.

Deficiency of orexin, a newly discovered hypothalamic peptide, is thought to lead to abnormal sleepiness and cataplexy in both human narcolepsy and animal models of the disease. As the POA contains extensive orexin terminals and is established as a sleep/arousal regulatory site, we evaluated a hypothesis that this site is a target for the arousal-inducing effects of orexin. Orexin-A was microinjected into lateral preoptic area (IPOA) and the effects on sleep-wakefulness and brain temperature were studied. Compared to saline vehicle control, orexin-A induced an increase in wakefulness for 70 min and suppressed all sleep stages, especially SWS2 and REM for 80 and 90 min, respectively. Brain temperature was not differentially affected by orexin-A compared to saline control. The orexin-induced arousal and REM suppression are consistent with the orexin-deficiency model of narcolepsy. Our results suggest that the IPOA orexin terminal field or adjacent structures may be a locus of arousal regulation by this peptide and a substrate of sleep-wake regulatory deficits in narcolepsy.