The sleep-wake cycle is probably the most accurate sign of vital functions. These inevitable interchangeable states together are the matrix of everything that happens in physiology, and their rhythms are regulated by homeostatic and circadian processes involving different neuronal structures and different neural substrates. The hypothalamic regulation of the sleep-wake cycle becomes related to the involvement of several neuropeptide-producing neurons involved in sleep and wake regulation. This article examines the hypothalamic regulation of the sleep-wake cycle, focusing on the hypocretin system and neurons that produce melanin-concentrating hormone (MCH) located in the lateral hypothalamic area (LHA).
Prepro-orexin protein is the precursor protein that produces excitatory neuropeptides or A and B (hypocretins 1 and 2). Orexin A (hypocretin 1), which has 33 amino acids and 3.5 kDa structure, is fully conserved among different mammals, reflecting its physiological interest. Orexin B (hypocretin 2) is a 28 amino acid peptide with 2.9 kDa with 46% identity to orexin A. The neurons in the LH spread over a wide area throughout the brain and spinal cord. Orexin stimulates target neurons through two types of expressed G-protein-coupled receptors. Orexin 1 receptor (OX1R) is predominantly expressed in the locus coeruleus (LC) and orexin 2 receptor (OX2R) is predominantly expressed in the arcuate nucleus (Arc), ventrolateral hypothalamus (VMH), LH and TMN. Both OX1R and OX2R are expressed in the raphe nucleus and ventral tegmental area (VTA).
Similar to other wakefulness-promoting neurons, orexin neurons are mainly fired during active wakefulness, when orexin levels are highest and are silenced consistent with the lowest orexin levels during NREM and REM sleep. Different neuronal pathways are described, including orexin and neurotransmitters that affect its activity. The neuropeptide Y (NPY) and agouti-associated peptide (AgRP) in Arc projects into orexin neurons. Also, serotoninergic neurons in the median / paramedian raphe nucleus and GABAergic neurons in the ventrolateral preoptic (VLPO) nucleus send axons to orexin neurons.
VLPO reduces NREM and REM sleep as its neurons are activated by the somnogens adenosine and prostaglandin D2, and VLPO damage. Orexinergic neurons are also targeted by neuronal projections from the stria terminalis (BST), supraventricular region, and the bed nucleus of the dorsomedial hypothalamus (DMH), and receive neuronal projections from the upper chiasmatic nucleus (main circadian clock). A direct neuronal pathway between SCN and orexinergic neurons has not been defined so far.
Since orexinergic neurons in LH are scarce and only morphology is difficult to distinguish from other neurons by the slice-way clamp technique, an electrophysiological method based on the expression of the enhanced green fluorescent protein (EGFP) under the control of the orexin promoter in transgenic mice is to identify substances that affect orexinergic neuron activity used. For example, this allows the effects of different neurotransmitters on orexin neurons to be hypothesized. These;
Glutamate receptor agonists AMPA and NMDA,
• depolarizing orexin neurons
• GABAA and GABAB receptor agonists muscimol and baclofen hyperpolarize these cells.
Serotonin and noradrenaline hyperpolarize all orexin neurons through two receptors linked to inhibitory Gi proteins (5HT1A and alpha 2A receptors, respectively) and then activate protein-bound inward rectifying potassium channels. Recent optogenetic methods have allowed verification that activation of serotoninergic neuron terminal inhibits orexin neurons directly (via the 5-HT1A receptor) or indirectly (through facilitation of GABAergic inhibitory inputs). Dopamine also hyperpolarizes orexin neurons, possibly through an indirect action via the alpha 2A receptor, and glycine inhibits the activity of orexin neurons directly or indirectly.
A complementary method for studying the function of orexinergic neurons is to look for the physiological consequences of their ablation. Hara et al. Produced transgenic mice in which orexin neurons were ablated and showed a phenotype similar to human narcolepsy. This also occurred in OX1r and OX2r knockout mice. In transgenic mice, gradual ablation of orexin neurons has been experimentally associated with a disruption of the usual sleep-wake cycle using a specific “time-controlled death” technique.
The anatomical proximity and genetic co-localization of orexin neurons that regulate the sleep-wake state have recently benefited from optogenetics. Using such an approach, Adamantidis and colleagues show that by increasing the activity of orexin neurons, the likelihood of transition from NREM or REM sleep to wakefulness is also increased. On the other hand, the results from the Zhang group using the same kind of approach show that acute inhibition of orexinergic neurons leads to induction of NREM sleep based on time of day. In order to overcome some of the challenges associated with studying neuronal networks deeper in the brain, several next-generation optogenetic tools are being developed with a major impact anticipated in the near future in the fields of chronobiology and sleep physiology.
Melanin Concentrating Hormone (MCH) And MCH Neurons
Melanin-condensing hormone is a 19 amino acid peptide that is predominant in specific neurons with the cell body located in the lateral hypothalamus and incerto-hypothalamic area of mammals. In addition to dormant neurons in the preoptic area, these groups of neurons are also active during sleep, especially REM sleep. MCH neurons project across the brain with an intense innervation of cholinergic and monoaminergic stimulation centers.
MCH lowers cAMP levels in the cell via MCH receptor 1 (MCHR1), a G-protein coupled receptor linked to Gq, Gi and Go subunits that are widely expressed in the brain, and cellular electrophysiological studies show that MCH has both presynaptic and postsynaptic potent inhibitory effects. It is evidence that MCHR1 is expressed in various areas of the brain, including those that are part of the physiological pathways within the sleep-wake control mechanisms (hippocampus, subiculum, basolateral amygdala, shell of the accumulator nucleus, ventromedial nucleus, arcuate nucleus, tuberomamillary nucleus, dorsolateral). It supports that pons MCH neurons, including dorsal raphe and locus coeruleus, should play an important role in sleep-wake physiology.
In addition, intracerebroventricular infusion of the MCH peptide facilitates REM and NREM sleep, while the knockout of MCH is associated with a more active state of wakefulness and a decrease in REM or NREM sleep. Optogenetically selectively activated MCH neurons generally prolong REM sleep time. Consistent results showed that MCH neurons are strongly activated during REM sleep and deactivated during NREM, suggesting that MCH neurons promote REM sleep. However, studies of timing-controlled ablation of MCH neurons revealed an increase in wakefulness and a decrease in NREM sleep, and show that MCH also plays a role in regulating NREM sleep.
MCH neurons appear to inhibit some awake center neurons via GABAergic-inhibiting synapses on the histaminergic neurons of the tuberomamillary nucleus. Recent studies have shown that acute activation of MCH neurons at the onset of REM sleep prolongs the duration of this sleep phase, but not the duration of NREM sleep. On the other hand, inhibition of MCH neurons reduces the frequency of theta rhythms from the hippocampus without affecting REM sleep time.
MCH neurons are stimulated by orexin, AMPA agonists, NMDA and cannabinoid type-1 receptor agonists and inhibit orexinergic and neighboring GABAergic neurons. However, it is clear that orexin can also inhibit MCH neurons via GABAa receptors. Dopamine is also a neuronal inhibitor of MCH via the alpha-2 receptor or D1- and D2-like receptors. Moreover, MCH neurons are inhibited by MCH itself and by GABA, noradrenaline, serotonin, acetylcholine, neuropeptide Y, and histamine. This mutual inhibitory interaction between orexin neurons in LH and MCH neurons is crucial for regulating the sleep-wake physiological cycle.
Writer: Ozlem Guvenc Agaoglu