Microinjection of adenosine into the hypothalamic ventrolateral preoptic area enhances wakefulness via the A1 receptor in rats

Functions and mechanisms of adenosine and its receptors in sleep regulation

Sleep is a natural and recurring state of life. Long-term insomnia can lead to physical and mental fatigue, inattention, memory loss, anxiety, depression and other symptoms, imposing immense public health and economic burden worldwide. The sleep and awakening regulation system is composed of many nerve nuclei and neurotransmitters in the brain, and it forms a neural network that interacts and restricts each other to regulate the occurrence and maintenance of sleep-wake. Adenosine (AD) is a neurotransmitter in the central nervous system and a driver of sleep. Meanwhile, the functions and mechanisms underlying sleep-promoting effects of adenosine and its receptors are still not entirely clear. However, in recent years, the increasing evidence indicated that adenosine can promote sleep through inhibiting arousal system and activating sleep-promoting system. At the same time, astrocyte-derived adenosine in modulating sleep homeostasis and sleep loss-induced related cognitive and memory deficits plays an important role. This review, therefore, summarizes the current research on the functions and possible mechanisms of adenosine and its receptors in the regulation of sleep and homeostatic control of sleep. Understanding these aspects will provide us better ideas on clinical problems such as insomnia, hypersomnia and other sleep disorders.

Astrocyte-derived adenosine excites sleep-promoting neurons in the ventrolateral preoptic nucleus: Astrocyte-neuron interactions in the regulation of sleep

Although ATP and/or adenosine derived from astrocytes are known to regulate sleep, the precise mechanisms underlying the somnogenic effects of ATP and adenosine remain unclear. We selectively expressed channelrhodopsin-2 (ChR2), a light-sensitive ion channel, in astrocytes within the ventrolateral preoptic nucleus (VLPO), which is an essential brain nucleus involved in sleep promotion. We then examined the effects of photostimulation of astrocytic ChR2 on neuronal excitability using whole-cell patch-clamp recordings in two functionally distinct types of VLPO neurons: sleep-promoting GABAergic projection neurons and non-sleep-promoting local GABAergic neurons. Optogenetic stimulation of VLPO astrocytes demonstrated opposite outcomes in the two types of VLPO neurons. It led to the inhibition of non-sleep-promoting neurons and excitation of sleep-promoting neurons. These responses were attenuated by blocking of either adenosine A₁ receptors or tissue-nonspecific alkaline phosphatase (TNAP). In contrast, exogenous adenosine decreased the excitability of both VLPO neuron populations. Moreover, TNAP was expressed in galanin-negative VLPO neurons, but not in galanin-positive sleep-promoting projection neurons. Taken together, these results suggest that astrocyte-derived ATP is converted into adenosine by TNAP in non-sleep-promoting neurons. In turn, adenosine decreases the excitability of local GABAergic neurons, thereby increasing the excitability of sleep-promoting GABAergic projection neurons. We propose a novel mechanism involving astrocyte-neuron interactions in sleep regulation, wherein endogenous adenosine derived from astrocytes excites sleep-promoting VLPO neurons, and thus decreases neuronal excitability in arousal-related areas of the brain.

Flavonoid-Like Components of Peanut Stem and Leaf Extract Promote Sleep by Decreasing Neuronal Excitability

SCOPE

Peanut stem and leaf (PSL), a traditional Chinese medicine, is widely used as a dietary supplement to improve sleep quality; however, the underlying mechanism is unclear. Here, the study aims to determine whether active compounds in PSL extract exert their effects by mediating neuronal excitability.

METHODS AND RESULTS

Aqueous PSL extract (500 mg kg^-1 BW) increases the duration of total sleep (TS), slow wave sleep (SWS) and rapid eye movement sleep (REMS) in BALB/c mice after 7 and 14 continuous days of intragastric administration. Two PSL extract components with flavonoid-like structures: 4',7-di-O-methylnaringenin (DMN, 61 µg kg^-1 BW) and 2'-O-methylisoliquiritigenin (MIL, 12 µg kg^-1 BW), show similar effects on sleep in BALB/c mice. Moreover, incubation with DMN (50 µM) and MIL (50 µM) acutely reduces voltage-gated sodium and potassium currents and suppresses the firing of evoked action potential in mouse cortical neurons, indicating the inhibition on neuronal excitability. Meanwhile, RNA-seq analysis predicts the potential regulation of voltage-gated channels, which is according with the molecular docking simulation that both MIL and DMN can bind to voltage gated sodium channels 1.2 (Na_v 1.2).

CONCLUSIONS

DMN and MIL are the active ingredients of PSL that improve sleep quality, suggesting that PSL promotes sleep by regulating the excitability of neurons.

Review: Neuropathology findings in autonomic brain regions in SUDEP and future research directions

Autonomic dysfunction is implicated from clinical, neuroimaging and experimental studies in sudden and unexpected death in epilepsy (SUDEP). Neuropathological analysis in SUDEP series enable exploration of acquired, seizure-related cellular adaptations in autonomic and brainstem autonomic centres of relevance to dysfunction in the peri-ictal period. Alterations in SUDEP compared to control groups have been identified in the ventrolateral medulla, amygdala, hippocampus and central autonomic regions. These involve neuropeptidergic, serotonergic and adenosine systems, as well as specific regional astroglial and microglial populations, as potential neuronal modulators, orchestrating autonomic dysfunction. Future research studies need to extend to clinically and genetically characterized epilepsies, to explore if common or distinct pathways of autonomic dysfunction mediate SUDEP. The ultimate objective of SUDEP research is the identification of disease biomarkers for at risk patients, to improve post-mortem recognition and disease categorisation, but ultimately, for exposing potential treatment targets of pharmacologically modifiable and reversible cellular alterations.

Estradiol Influences Adenosinergic Signaling and NREM Sleep Need in Adult Female Rats

Gonadal steroids and gender are risk factors for sleep disruptions and insomnia in women. However, the relationship between ovarian steroids and sleep is poorly understood. In rodent models, estradiol (E2) suppresses sleep in females suggesting that E2 may reduce homeostatic sleep need. The current study investigates whether E2 decreases sleep need and the potential mechanisms that govern E2 suppression of sleep. Our previous findings suggest that the median preoptic nucleus (MnPO) is a key nexus for E2 action on sleep. Using behavioral, neurochemical and pharmacological approaches, we tested whether (1) E2 influenced the sleep homeostat and (2) E2 influenced adenosine signaling in the MnPO of adult female rats. In both unrestricted baseline sleep and recovery sleep from 6-hour sleep deprivation, E2 significantly reduced non-rapid eye movement sleep (NREM)-delta power, NREM-Slow Wave Activity (NREM-SWA, 0.5-4.0Hz), and NREM-delta energy suggesting that E2 decreases homeostatic sleep need. However, coordinate with E2-induced changes in physiological markers of homeostatic sleep was a marked increase in MnPO extracellular adenosine (a molecular marker of homeostatic sleep need) during unrestricted and recovery sleep in E2-treated but not oil control animals. While these results seemed contradictory, systemically administered E2 blocked the ability of CGS-21680 (adenosine A2A receptor agonist) microinjected into the MnPO to increase NREM sleep suggesting that E2 may block adenosine signaling. Together, these findings provide evidence that E2 may attenuate the local effects of the A2A receptors in the MnPO which in turn may underlie estrogenic suppression of sleep behavior as well as changes in homeostatic sleep need.

Astrocytes in the Ventrolateral Preoptic Area Promote Sleep

Although ventrolateral preoptic (VLPO) nucleus is regarded as a center for sleep promotion, the exact mechanisms underlying the sleep regulation are unknown. Here, we used optogenetic tools to identify the key roles of VLPO astrocytes in sleep promotion. Optogenetic stimulation of VLPO astrocytes increased sleep duration in the active phase in naturally sleep-waking adult male rats (n = 6); it also increased the extracellular ATP concentration (n = 3) and c-Fos expression (n = 3-4) in neurons within the VLPO. In vivo microdialysis analyses revealed an increase in the activity of VLPO astrocytes and ATP levels during sleep states (n = 4). Moreover, metabolic inhibition of VLPO astrocytes reduced ATP levels (n = 4) and diminished sleep duration (n = 4). We further show that tissue-nonspecific alkaline phosphatase (TNAP), an ATP-degrading enzyme, plays a key role in mediating the somnogenic effects of ATP released from astrocytes (n = 5). An appropriate sample size for all experiments was based on statistical power calculations. Our results, taken together, indicate that astrocyte-derived ATP may be hydrolyzed into adenosine by TNAP, which may in turn act on VLPO neurons to promote sleep.SIGNIFICANCE STATEMENT Glia have recently been at the forefront of neuroscience research. Emerging evidence illustrates that astrocytes, the most abundant glial cell type, are the functional determinants for fates of neurons and other glial cells in the central nervous system. In this study, we newly identified the pivotal role of hypothalamic ventrolateral preoptic (VLPO) astrocytes in the sleep regulation, and provide novel insights into the mechanisms underlying the astrocyte-mediated sleep regulation.

Adenosine kinase and adenosine receptors A1 R and A2A R in temporal lobe epilepsy and hippocampal sclerosis and association with risk factors for SUDEP

OBJECTIVE

The "adenosine hypothesis of SUDEP" (sudden unexpected death in epilepsy) predicts that a seizure-induced adenosine surge combined with impaired metabolic clearance can foster lethal apnea or cardiac arrest. Changes in adenosine receptor density and adenosine kinase (ADK) occur in surgical epilepsy patients. Our aim was to correlate the distribution of ADK and adenosine A_2A and A₁ receptors (A_2A R and A₁ R) in surgical tissue from patients with temporal lobe epilepsy and hippocampal sclerosis (TLE/HS) with SUDEP risk factors.

METHODS

In 75 cases, patients were stratified into high-risk (n = 16), medium-risk (n = 11) and low-risk (n = 48) categories according to the frequency of generalized seizures before surgery. Using whole-slide scanning Definiens image analysis we quantified the labeling index (LI) for ADK, A_2A R, and A₁ R in seven regions of interest: temporal cortex, temporal lobe white matter, CA1, CA4, dentate gyrus, subiculum, and amygdala and relative to glial and neuronal densities with glial fibrillary acidic protein (GFAP) and neuronal nuclear antigen (NeuN).

RESULTS

A₁ R showed predominant neuronal, A_2A R astroglial, and ADK nuclear labeling in all regions but with significant variation. Compared with the low-risk group, the high-risk group had significantly lower A_2A R LI in the temporal cortex. In HS cases with severe neuronal cell loss and gliosis predominantly in the CA1 and CA4 regions, significantly higher A₁ R was present in the amygdala in high-risk than in low-risk cases. There was no significant difference in neuronal loss or gliosis between the risk groups or differences for ADK labeling.

SIGNIFICANCE

Reduced cortical A_2A R suggests glial dysfunction and impaired adenosine modulation in response to seizures in patients at higher risk for SUDEP. Increased neuronal A₁ R in the high-risk group could contribute to periictal amygdala dysfunction in SUDEP.

Inhibition of adenosine A1 receptors abolished the nutritional ketosis-evoked delay in the onset of isoflurane-induced anesthesia in Wistar Albino Glaxo Rijswijk rats

Background

It has been demonstrated that administration of exogenous ketone supplement ketone salt (KS) and ketone ester (KE) increased blood ketone level and delayed the onset of isoflurane-induced anesthesia in different rodent models, such as Wistar Albino Glaxo Rijswijk (WAG/Rij) rats. The modulatory effect of adenosinergic system may have a role in the ketone supplementation-evoked effects on isoflurane-generated anesthesia. Thus, we investigated whether adenosine receptor antagonists can modulate the effect of exogenous ketone supplements on the onset of akinesia induced by isoflurane.

Methods

To investigate the effect of exogenous ketone supplements on anesthetic induction we used ketone supplement KE, KS, KEKS (1:1 mix of KE and KS), KSMCT and KEMCT (1:1 mix of KS and KE with medium chain triglyceride/MCT oil, respectively) in WAG/Rij rats. Animals were fed with standard diet (SD), which was supplemented by oral gavage of different ketone supplements (2.5 g/kg/day) for 1 week. After 7 days, isoflurane (3%) was administered for 5 min and the time until onset of isoflurane-induced anesthesia (time until immobility; light phase of anesthesia: loss of consciousness without movement) was measured. Changes in levels of blood β-hydroxybutyrate (βHB), blood glucose and body weight of animals were also recorded. To investigate the putative effects of adenosine receptors on ketone supplements-evoked influence on isoflurane-induced anesthesia we used a specific adenosine A1 receptor antagonist DPCPX (intraperitoneally/i.p. 0.2 mg/kg) and a selective adenosine A2A receptor antagonist SCH 58261 (i.p. 0.5 mg/kg) alone as well as in combination with KEKS.

Results

Significant increases were demonstrated in both blood βHB levels and the number of seconds required before isoflurane-induced anesthesia (immobility) after the final treatment by all exogenous ketone supplements. Moreover, this effect of exogenous ketone supplements positively correlated with blood βHB levels. It was also demonstrated that DPCPX completely abolished the effect of KEKS on isoflurane-induced anesthesia (time until immobility), but not SCH 58261.

Conclusions

These findings strengthen our previous suggestion that exogenous ketone supplements may modulate the isoflurane-induced onset of anesthesia (immobility), likely through A1Rs.

Preemptive Caffeine Administration Blocks the Increase in Postoperative Pain Caused by Previous Sleep Loss in the Rat: A Potential Role for Preoptic Adenosine A2A Receptors in Sleep-Pain Interactions

Sleep and pain are reciprocally related, but the precise mechanisms underlying this relationship are poorly understood. This study used a rat model of surgical pain to examine the effect of previous sleep loss on postoperative pain and tested the hypothesis that preoptic adenosinergic mechanisms regulate sleep-pain interactions. Relative to ad libitum sleep, 6 hours of total sleep deprivation prior to a surgical incision significantly enhanced postoperative mechanical hypersensitivity in the affected paw and prolonged the time to recovery from surgery. There were no sex-specific differences in these measures. There were also no changes in adrenocorticotropic hormone and corticosterone levels after sleep deprivation, suggesting that this effect was not mediated by the stress associated with the sleep perturbation. Systemic administration of the nonselective adenosine receptor antagonist caffeine at the onset of sleep deprivation prevented the sleep deprivation-induced increase in postoperative hypersensitivity. Microinjection of the adenosine A2A receptor antagonist ZM 241385 into the median preoptic nucleus (MnPO) blocked the increase in surgical pain levels and duration caused by prior sleep deprivation and eliminated the thermal hyperalgesia induced by sleep deprivation in a group of nonoperated (i.e., without surgical incision) rats. These data show that even a brief sleep disturbance prior to surgery worsens postoperative pain and are consistent with our hypothesis that adenosine A2A receptors in the MnPO contribute to regulate these sleep-pain interactions.

GABAergic ventrolateral pre‑optic nucleus neurons are involved in the mediation of the anesthetic hypnosis induced by propofol

Intravenous anesthetics have been used clinically to induce unconsciousness for seventeen decades, however the mechanism of anesthetic-induced unconsciousness remains to be fully elucidated. It has previously been demonstrated that anesthetics exert sedative effects by acting on endogenous sleep-arousal circuits. However, few studies focus on the ventrolateral pre-optic (VLPO) to locus coeruleus (LC) sleep-arousal pathway. The present study aimed to investigate if VLPO is involved in unconsciousness induced by propofol. The present study additionally investigated if the inhibitory effect of propofol on LC neurons was mediated by activating VLPO neurons. Microinjection, target lesion and extracellular single-unit recordings were used to study the role of the VLPO-LC pathway in propofol anesthesia. The results demonstrated that GABAA agonist (THIP) or GABAA antagonist (gabazine) microinjections into VLPO altered the time of loss of righting reflex and the time of recovery of righting reflex. Furthermore, propofol suppressed the spontaneous firing activity of LC noradrenergic neurons. There was no significant difference observed in firing activity between VLPO sham lesion and VLPO lesion rats. The findings indicate that VLPO neurons are important in propofol-induced unconsciousness, however are unlikely to contribute to the inhibitory effect of propofol on LC spontaneous firing activity.

Distinctive recruitment of endogenous sleep-promoting neurons by volatile anesthetics and a nonimmobilizer

BACKGROUND

Numerous studies demonstrate that anesthetic-induced unconsciousness is accompanied by activation of hypothalamic sleep-promoting neurons, which occurs through both pre- and postsynaptic mechanisms. However, the correlation between drug exposure, neuronal activation, and onset of hypnosis remains incompletely understood. Moreover, the degree to which anesthetics activate both endogenous populations of γ-aminobutyric acid (GABA)ergic sleep-promoting neurons within the ventrolateral preoptic (VLPO) and median preoptic nuclei remains unknown.

METHODS

Mice were exposed to oxygen, hypnotic doses of isoflurane or halothane, or 1,2-dichlorohexafluorocyclobutane (F6), a nonimmobilizer. Hypothalamic brain slices prepared from anesthetic-naive mice were also exposed to oxygen, volatile anesthetics, or F6 ex vivo, both in the presence and absence of tetrodotoxin. Double-label immunohistochemistry was performed to quantify the number of c-Fos-immunoreactive nuclei in the GABAergic subpopulation of neurons in the VLPO and the median preoptic areas to test the hypothesis that volatile anesthetics, but not nonimmobilizers, activate sleep-promoting neurons in both nuclei.

RESULTS

In vivo exposure to isoflurane and halothane doubled the fraction of active, c-Fos-expressing GABAergic neurons in the VLPO, whereas F6 failed to affect VLPO c-Fos expression. Both in the presence and absence of tetrodotoxin, isoflurane dose-dependently increased c-Fos expression in GABAergic neurons ex vivo, whereas F6 failed to alter expression. In GABAergic neurons of the median preoptic area, c-Fos expression increased with isoflurane and F6, but not with halothane exposure.

CONCLUSIONS

Anesthetic unconsciousness is not accompanied by global activation of all putative sleep-promoting neurons. However, within the VLPO hypnotic doses of volatile anesthetics, but not nonimmobilizers, activate putative sleep-promoting neurons, correlating with the appearance of the hypnotic state.

Sedative and hypnotic activity of N(6)-(3-methoxyl-4-hydroxybenzyl) adenine riboside (B2), an adenosine analog

N(6)-(3-methoxyl-4-hydroxybenzyl) adenine riboside (B2) is an N(6)-substitued adenosine analog. Previous studies have shown that B2 binds to the adenosine A1 and A2A receptors with moderate affinity and produces protective effects on serum deprivation-induced cell damage. However, central nervous system effects of B2 have not been studied. We aimed to investigate the sedative and hypnotic effects and the mechanism of action of B2 in mice. Our behavioral studies showed that oral administration of B2 decreased spontaneous locomotor activity and potentiated the hypnotic effect of pentobarbital in mice. Sleep architecture analyses revealed that B2 decreased wakefulness and increased non-rapid eye movement (NREM) sleep in both normal mice and mice with caffeine-induced insomnia. Using immunohistochemistry, we showed that B2 increased c-Fos expression, a cellular marker for neuronal activity, in the ventrolateral preoptic (VLPO) area, a sleep center in the anterior hypothalamus. Altogether, these results indicate that oral administration of B2 produces sedative and hypnotic effects. Furthermore, the activation of VLPO neurons may be involved in the central depressant effects of B2.