Waking and sleeping following water deprivation in the rat

Translational and post-translational dynamics in a model peptidergic system

The cell bodies of hypothalamic magnocellular neurones are densely packed in the hypothalamic supraoptic nucleus whereas their axons project to the anatomically discrete posterior pituitary gland. We have taken advantage of this unique anatomical structure to establish proteome and phosphoproteome dynamics in neuronal cell bodies and axonal terminals in response to physiological stimulation. We have found that proteome and phosphoproteome responses to neuronal stimulation are very different between somatic and axonal neuronal compartments, indicating the need of each cell domain to differentially adapt. In particular, changes in the phosphoproteome in the cell body are involved in the reorganisation of the cytoskeleton and in axonal terminals the regulation of synaptic and secretory processes. We have identified that prohormone precursors including vasopressin and oxytocin are phosphorylated in axonal terminals and are hyperphosphorylated following stimulation. By multi-omic integration of transcriptome and proteomic data we identify changes to proteins present in afferent inputs to this nucleus.

Water deprivation induces hypoactivity in rats independently of oxytocin receptor signaling at the central amygdala

INTRODUCTION

Vasopressin (AVP) and oxytocin (OXT) are neuropeptides produced by magnocellular neurons (MCNs) of the hypothalamus and secreted through neurohypophysis to defend mammals against dehydration. It was recently demonstrated that MCNs also project to limbic structures, modulating several behavioral responses.

METHODS AND RESULTS

We found that 24 h of water deprivation (WD) or salt loading (SL) did not change exploration or anxiety-like behaviors in the elevated plus maze (EPM) test. However, rats deprived of water for 48 h showed reduced exploration of open field and the closed arms of EPM, indicating hypoactivity during night time. We evaluated mRNA expression of glutamate decarboxylase 1 (Gad1), vesicular glutamate transporter 2 (Slc17a6), AVP (Avpr1a) and OXT (Oxtr) receptors in the lateral habenula (LHb), basolateral (BLA) and central (CeA) amygdala after 48 h of WD or SL. WD, but not SL, increased Oxtr mRNA expression in the CeA. Bilateral pharmacological inhibition of OXTR function in the CeA with the OXTR antagonist L-371,257 was performed to evaluate its possible role in regulating the EPM exploration or water intake induced by WD. The blockade of OXTR in the CeA did not reverse the hypoactivity response in the EPM, nor did it change water intake induced in 48-h water-deprived rats.

DISCUSSION

We found that WD modulates exploratory activity in rats, but this response is not mediated by oxytocin receptor signaling to the CeA, despite the upregulated Oxtr mRNA expression in that structure after WD for 48 h.

Use of Actigraphy for a Rat Behavioural Sleep Study

Previous studies of animal behavioural sleep is mainly divided into two study types, observation by video recording or counts by sensor, both of which require a complex environment and procedure. An actigraph unit is a commercially available product which can provide non-invasive monitoring human rest/activity cycles. The goal of this study was to evaluate whether actigraphy can be applied for analysing behavioural sleep in rats, since no reports have described utilization of the actigraphy unit for monitoring sleep of small animals. The actigraph unit was held on the chest of eight male rats by a loose elastic belt. The rats spent two days in a normal condition, followed by two days of sleep deprivation. Total counts measured by the actigraph could be clearly divided into two phases, sleep phase and awake phase, when the rats were kept in the normal cage. Next, the rats were moved into the sleep-deviation cage, and the total counts were significantly higher during daytime, indicating the successful induction of sleep deprivation. These results showed that the actigraphy unit monitored rest/activity cycles of rats, which will contribute to making sleep behaviour experiments easier.

REM Sleep and Endothermy: Potential Sites and Mechanism of a Reciprocal Interference

Numerous data show a reciprocal interaction between REM sleep and thermoregulation. During REM sleep, the function of thermoregulation appears to be impaired; from the other hand, the tonic activation of thermogenesis, such as during cold exposure, suppresses REM sleep occurrence. Recently, both the central neural network controlling REM sleep and the central neural network controlling thermoregulation have been progressively unraveled. Thermoregulation was shown to be controlled by a central “core” circuit, responsible for the maintenance of body temperature, modulated by a set of accessory areas. REM sleep was suggested to be controlled by a group of hypothalamic neurons overlooking at the REM sleep generating circuits within the brainstem. The two networks overlap in a few areas, and in this review, we will suggest that in such overlap may reside the explanation of the reciprocal interaction between REM sleep and thermoregulation. Considering the peculiar modulation of thermoregulation by REM sleep the result of their coincidental evolution, REM sleep may therefore be seen as a period of transient heterothermy.

Wake-sleep and cardiovascular regulatory changes in rats made obese by a high-fat diet

Obesity is known to be associated with alterations in wake-sleep (WS) architecture and cardiovascular parameters. This study was aimed at assessing the possible influence of diet-induced obesity (DIO) on sleep homeostasis and on the WS state-dependent levels of arterial pressure (AP) and heart rate in the rat. Two groups of age-matched Sprague-Dawley rats were fed either a high-fat hypercaloric diet, leading to DIO, or a normocaloric standard diet (lean controls) for 8 weeks. While under general anesthesia, animals were implanted with instrumentation for the recording of electroencephalogram, electromyogram, arterial pressure, and deep brain temperature. The experimental protocol consisted of 48h of baseline, 12h of gentle handling, enhancing wake and depressing sleep, and 36-h post-handling recovery. Compared to lean controls, DIO rats showed: i) the same amount of rapid-eye movement (REM) and non-REM (NREM) sleep in the rest period, although the latter was characterized by more fragmented episodes; ii) an increase in both REM sleep and NREM sleep in the activity period; iii) a comparable post-handling sleep homeostatic response, in terms of either the degree of Delta power increase during NREM sleep or the quantitative compensation of the REM sleep loss at the end of the 36-h recovery period; iv) significantly higher levels of AP, irrespectively of the different WS states and of the changes in their intensity throughout the experimental protocol. Overall, these changes may be the reflection of a modification in the activity of the hypothalamic areas where WS, autonomic, and metabolic regulations are known to interact.

Temporal Organization of the Sleep-Wake Cycle under Food Entrainment in the Rat

STUDY OBJECTIVES

To analyze the temporal organization of the sleep-wake cycle under food entrainment in the rat.

METHODS

Eighteen male Sprague-Dawley rats were chronically implanted for polysomnographic recording. During the baseline (BL) protocol, rats were recorded under a 12:12 light-dark (LD) schedule in individual isolation chambers with food and water ad libitum. Food entrainment was performed by means of a 4-h food restriction (FR) protocol starting at photic zeitgeber time 5. Eight animals underwent a 3-h phase advance of the FR protocol (A-FR). We compared the mean curves and acrophases of wakefulness, NREM sleep, and REM sleep under photic and food entrainment and after a phase advance in scheduled food delivery. We further evaluated the dynamics of REM sleep homeostasis and the NREM sleep EEG delta wave profile.

RESULTS

A prominent food-anticipatory arousal interval was observed after nine or more days of FR, characterized by increased wakefulness and suppression of REM sleep propensity and dampening of NREM sleep EEG delta activity. REM sleep exhibited a robust nocturnal phase preference under FR that was not explained by a nocturnal REM sleep rebound. The mean curve of sleep-wake states and NREM sleep EEG delta activity remained phase-locked to the timing of meals during the A-FR protocol.

CONCLUSIONS

Our results support the hypothesis that under food entrainment, the sleep-wake cycle is coupled to a food-entrainable oscillator (FEO). Our findings suggest an unexpected interaction between FEO output and NREM sleep EEG delta activity generators.

Waking and sleeping in the rat made obese through a high-fat hypercaloric diet

Sleep restriction leads to metabolism dysregulation and to weight gain, which is apparently the consequence of an excessive caloric intake. On the other hand, obesity is associated with excessive daytime sleepiness in humans and promotes sleep in different rodent models of obesity. Since no consistent data on the wake-sleep (WS) pattern in diet-induced obesity rats are available, in the present study the effects on the WS cycle of the prolonged delivery of a high-fat hypercaloric (HC) diet leading to obesity were studied in Sprague-Dawley rats. The main findings are that animals kept under a HC diet for either four or eight weeks showed an overall decrease of time spent in wakefulness (Wake) and a clear Wake fragmentation when compared to animals kept under a normocaloric diet. The development of obesity was also accompanied with the occurrence of a larger daily amount of REM sleep (REMS). However, the capacity of HC animals to respond to a "Continuous darkness" exposure condition (obtained by extending the Dark period of the Light-Dark cycle to the following Light period) with an increase of Sequential REMS was dampened. The results of the present study indicate that if, on one hand, sleep curtailment promotes an excess of energy accumulation; on the other hand an over-exceeding energy accumulation depresses Wake. Thus, processes underlying energy homeostasis possibly interact with those underlying WS behavior, in order to optimize energy storage.