The Mechanism Of Dexmetomidine And Isoliquiritigenin On GABAergic Neurons With Different Electrophysiological Characteristics Of VLPO Neurons And Their Effects On Sleep-Wake

BackgroundSleep and wake are very important to humans and the whole biological world.Humans spend eight hours every day to sleep.Sleep can allow the body to get a full rest,and wake can allow the body to eat,exercise,and work independently.Whether it is sleep or wake is extremely important for human beings.Sleep-wake is a complex process regulated by neural circuits composed of sleep nuclei and wake nuclei.When sleep begins,the sleep system suppresses the wake system.The wake system also projects axons into the sleep system and suppresses them.However,diseases such as insomnia or sleep disorders have plagued more and more people,and they can also cause many other diseases,such as obesity,forgetfulness,and emotional cognitive diseases.Thus,sleep-promoting drugs get more and more research,and it is particularly important to explore the mechanism of action and application of sleep-promoting drugs.The ventrolateral preoptic area（VLPO）is located in the rostral-caudal band of the hypothalamus.It is the core brain region that promotes sleep and plays an irreplaceable role in the sleep-wake process.It plays an important role in the initiation and maintenance of nonrapid eye movement sleep（NREM sleep）.There are two types of neurons or two types of cells in the VLPO brain region.There are two types of neurons:one that is inhibited by noradrenaline（NA）,called NA（-）neurons;another type of neuron that is excited by NA is called the NA（+）neuron.There are two cell types:one cell has a low-threshold spike（LTS）characteristic,called LTS cells;another type of cell that lacks an LTS is called non-LTS cells.From the physiological process analysis,it can be considered that LTS cells are equivalent to NA（-）neurons,and non-LTS cells are equivalent to NA（+）neurons.Dexmedetomidine（Dex）is anα₂ adrenergic receptor agonist and well-known for its unique sedative activities with rapid arousal.The unique sedative effects of Dex are widely used for sedation in intensive care units（ICU）,where patients can be waked up to communicate their needs.Sedation is a complex process involving multiple brain regions and circuits.Previous studies have suggested that Dex-induced sedation and sleep homeostasis may share a common mechanism.Genetic damage to galanergic neurons in the lateral preoptic area（LPO）significantly reduced the ability of Dex to induce sedation.However,it is not clear how Dex acts on the two types of neurons in VLPO.Isoliquiritigenin（ILTG）is obtained from caryophyllum and glycyrrhiza,mainly presented as yellow crystalline powder,and is a chalcone compound.ILTG has a hypnotic function by a pentobarbital-induced sleep test.The researchers also found that ILTG can act viaγ-aminobutyric acid A type（GABA_A）receptors in the dorsal raphe.In addition,studies have shown that ILTG may regulate GABAergic synaptic transmission.However,the effects of ILTG on cells in VLPO are unclear.ObjectivesTo explore the mechanism of dexmedetomidine and isogliquiritin on different types of VLPO neurons and their effects on sleep-wake.MethodsThis study used polysomnography recording to explore the effects of Dex on sleep-wake in mice.Immunofluorescence staining was used to observe the c-Fos expression of Dex in VLPO.In addition,whole-cell recording was used to explore the mechanism of Dex on two types of neurons in the VLPO.This study used whole-cell recording in brain slices to explore the mechanism of ILTG on two types of cells in the VLPO.ResultsThe result showed that Dex can increase NREM sleep and reduce wake at night.Dex increased the expression of c-Fos in the VLPO by about 4.14-fold.In addition,bath application of Dex increased the firing rate of both VLPO NA（-）and NA（+）neurons.Compared to the control group,there was no difference in the firing rate of both VLPO NA（-）and NA（+）neurons after Dex and RS79948 administration,anα2 receptor antagonist.No difference was detected regarding the resting membrane potential（RMP）amplitude of both VLPO NA（-）and NA（+）neurons after the application of Dex.Moreover,Dex significantly reduced the frequency of miniature inhibitory postsynaptic currents（m IPSCs）in both VLPO NA（-）and NA（+）neurons.Bath administration of ILTG increased the firing rate of VLPO LTS cells,reversed by flumazenil,a GABA_Abenzodiazepine site antagonist.However,the firing rate of VLPO non-LTS cells was inhibited by ILTG,also reversed by flumazenil.No differences were detected regarding RMP amplitude,spike threshold,afterhyperpolarization（AHP）amplitude,or action potential duration（APD50）after ILTG perfusion in VLPO LTS cells.RMP amplitude was more hyperpolarized and spike threshold was higher after ILTG application in VLPO non-LTS cells.In addition,ILTG significantly reduced the frequency of m IPSCs in VLPO LTS cells.ILTG significantly increased the amplitude of m IPSCs in VLPO non-LTS cells.ConclusionIn conclusion,Dex activates NA（-）and NA（+）neurons in the VLPO via presynapticα2receptors.This mechanism may explain the unique sedative properties with rapid arousal.ILTG suppresses presynaptic GABA release on VLPO LTS cells,thereby increasing their excitability.ILTG enhances postsynaptic GABA_A receptor function on VLPO non-LTS cells,thereby decreasing their excitability.These results suggest that ILTG may produce hypnotic effects by modulating the GABAergic synaptic transmission properties of these two cell types.