VLPO sleep-promoting neurons integrate and modulate local energy supply – Metabosleep

To decipher the neurobiological mechanisms involved in the effects of glucose on sleep neurons, we use a variety of approaches ranging from ex vivo electrophysiology in rodents brain slices to monitoring of vigilance states in freely moving animals. Ex vivo techniques consist of patch-clamp recordings of sleep promoting neurons in the hypothalamus. To identify the molecular mechanisms involved in the glucose-induced effects, we use ex vivo pharmacological approaches and molecular biology. The ultimate goal is to combine changes in vigilance states identified in vivo (quantity wake / sleep) to changes in cellular pathways find out ex vivo. In addition, we validated some of our assumptions in animals whose vigilance states have been quantified by means of polygraphic recordings, coupling electroencephalogram (EEG) and the electromyogram (EMG).

Our results show that neurons responsible for the onset of sleep in the ventolateral preoptic nucleus (VLPO) are selectively excited by an increase in glucose concentration in mice. Indeed, only the neurons presenting the properties of the sleep promoting neurons are sensitive to changes in the glucose concentration. This important discovery is reinforced by our analysis of the underlying molecular mechanisms, revealing that these neurons can be considered as glucose «sensors«. In order to demonstrate the physiological relevance of our discovery, we show in vivo in mice that glucose perfusion in the VLPO induces an increase in the amount of sleep and a decrease in the sleep onset latency. Altogether these results suggest that glucose-induced excitation of sleep-promoting VLPO neurons should therefore be involved in the drowsiness that one feels after a high-sugar meal. This novel mechanism regulating the activity of VLPO neurons reinforces the fundamental and intimate link between sleep and metabolism.

The consequences of sleepiness in terms of incidents / accidents are increasingly recognized and the implications in public health and in quality of life can not be denied. However, the cellular mechanisms responsible for sleep induction and those related to sleepiness in normal condition of sleep and in situation of sleep deprivation remain unknown. Our results show that sleep promoting neurons would also be metabolic sensors and metabolic regulators. But the disturbance of vigilance states could affect the excitability of these cells. The demonstration of a direct effect of glucose on neurons responsible for sleep onset is important and could improve on-going education of the general public on the links between sleep, metabolism and health and thus would significantly contribute to increase public awareness and facilitate prevention of sleep related diseases. On the other hand, knowledge of the mechanisms responsible for the onset and maintenance of sleep could help develop a sleeping pill and / or physiological stimulant molecules that specifically act on the activity of sleep promoting neurons. Indeed, these types of substances may have more selective action with fewer side effects compared with sleeping pills and / or stimulant molecules currently available.

- Varin C. et coll. Glucose Induces Slow Wave Sleep by Exciting the Sleep-Promoting Neurons in the Ventrolateral Preoptic Nucleus: A New Link between Sleep and Metabolism. J Neurosci. 2015 Jul 8 ;35(27):9900-11.
This mechanism would allow the brain to regulate the states of arousal and sleep according to the brain metabolic fuel. Thus, glucose-induced excitation of VLPO sleep promoting neurons may be involved in in the drowsiness that one feels after a high-sugar meal.

- Scharbarg E. et coll. Astrocyte-derived adenosine is central to the hypogenic effect of glucose. Scientific Report . 2016 Jan 12 ;6:19107.
The increase in extracellular glucose concentration would strengthen a well-established mechanism in which the progressive accumulation of adenosine during arousal gradually induces sleep pressure.

- Dubourget R et coll. Multiparametric characterization of neuronal subpopulations in the ventrolateral preoptic nucleus. Brain Struct Funct. 2016 Jul 8. [Epub ahead of print]
The exhaustive characterization of VLPO neuronal subtypes is a crucial step toward a better understanding of the neuronal network within the VLPO and thereby sleep physiology.

So much people are chronically suffering from sleep disorders, hindering daily functioning and adversely affecting health and longevity. One of the proposed functions of sleep is to replenish energy stores in the brain that have been depleted during wakefulness. However, the direct influence of metabolic state on the activity of neurons responsible for the induction and maintenance of sleep remains to be established and is at the very essence of this project. The goal of this current project is to demonstrate that sleep-promoting neurons from the VLPO are able to sense and regulate local energy status. This regulation would have a significant impact on the activity of these cells and consequently on the regulation of vigilance states. Preliminary results obtained from mouse brain slices indicate that VLPO sleep-promoting neurons are selectively excited by an increase in extracellular glucose concentration within physiological range. The first objective of this project is to identify the molecular mechanisms involved in this process (task 1). We will pay particular attention to highlight the role of ATP-sensitive potassium channels (KATP) known to couple cell metabolism to electrical activity. Glucose sensing by sleep-promoting neurons represents an unusual physiological pathway through which energy supply may affect sleep. Secondly, we will determine whether intracellular energy stores, represented by intracellular ATP could modulate the activity of VLPO neurons and if the energy status of these cells correlates with vigilance states of animals (task 2). This finding would demonstrate that metabolic status of sleep-promoting neurons is tightly correlated with their excitability and thus with the behavioural state of the animal. Finally, we will demonstrate that VLPO sleep-promoting neurons are able to regulate local metabolic supply (task 3). According to our preliminary data, activation of VLPO sleep-promoting neurons would locally dilate blood vessels supplying the VLPO that in turn would lead to an increase in energy availability. Our hypothesis is that sleep-promoting neurons are able to release nitric oxide (NO) that dilates arterioles irrigating the VLPO. This mechanism could constitute an auto-excitatory loop in the VLPO that would be important to provide an energy supply necessary to maintain neuronal activity and consequently to maintain sleep. Here we will identify and characterize the cellular and molecular mechanisms involved in all these phenomena by using different electrophysiological recordings techniques, on acute mouse brain slices, associated with RT-PCR, immunohistochemistry, imaging and amperometry. In addition, we will attempt to validate some of these assumptions in animals whose vigilance states will have been quantified by means of polygraphic recordings. All these results would allow to establish for the first time a direct link between brain metabolism and the excitability of neurons responsible for the onset of sleep. VLPO sleep-promoting neurons could then be considered as metabolic-sensors and local metabolic regulators.