Sevoflurane activates hippocampal CA3 kainate receptors (Gluk2) to induce hyperactivity during induction and recovery in a mouse model

Ventral pallidum GABAergic and glutamatergic neurons modulate arousal during sevoflurane general anaesthesia in male mice

BACKGROUND AND PURPOSE

The induction and emergence of general anaesthesia involve an altered process of states of consciousness, yet the central nervous system mechanisms remain inadequately understood. The ventral pallidum (VP) within the basal ganglia is crucial in sleep-wake modulation. However, its involvement in general anaesthesia and the underlying neuronal mechanisms are not well elucidated.

EXPERIMENTAL APPROACH

In vivo electrophysiological recordings were conducted to examine changes in the activity of different types of VP neurons before and after sevoflurane exposure. Fibre photometry, combined with electroencephalogram and electromyography recordings, was employed to analyse neuronal activity during both the induction and recovery phases of sevoflurane anaesthesia. Chemogenetics was implemented to investigate the impact of modulated neuronal activity on anaesthesia induction and emergence, whereas optogenetics was used for real time activation of neurons at different depths of anaesthesia.

KEY RESULTS

Sevoflurane exposure reduced the firing activity of both VP GABAergic (VPGABA) and VP glutamatergic (VPglu) neurons, without affecting cholinergic neurons. VPGABA and VPglu neuronal activity decreased during sevoflurane anaesthesia induction and increased during emergence. Manipulation of VPGABA neurons bidirectionally influenced the duration of induction and emergence. Inhibiting VPglu neurons accelerated induction. Real time activation of VPGABA neurons triggered cortical activation and behavioural emergence during steady-state sevoflurane anaesthesia and reduced the burst suppression ratio during deep anaesthesia.

CONCLUSION AND IMPLICATIONS

These findings highlight the role of VPGABA and VPglu neurons in modulating transitions between anaesthesia stages, providing valuable insights into the neuronal mechanisms underlying sevoflurane-induced anaesthesia.

GLT-1 downregulation in hippocampal astrocytes induced by type 2 diabetes contributes to postoperative cognitive dysfunction in adult mice

AIMS

Type 2 diabetes mellitus (T2DM) is related to an increased risk of postoperative cognitive dysfunction (POCD), which may be caused by neuronal hyperexcitability. Astrocyte glutamate transporter 1 (GLT-1) plays a crucial role in regulating neuron excitability. We investigated if T2DM would magnify the increased neuronal excitability induced by anesthesia/surgery (A/S) and lead to POCD in young adult mice, and if so, determined whether these effects were associated with GLT-1 expression.

METHODS

T2DM model was induced by high fat diet (HFD) and injecting STZ. Then, we evaluated the spatial learning and memory of T2DM mice after A/S with the novel object recognition test (NORT) and object location test (OLT). Western blotting and immunofluorescence were used to analyze the expression levels of GLT-1 and neuronal excitability. Oxidative stress reaction and neuronal apoptosis were detected with SOD2 expression, MMP level, and Tunel staining. Hippocampal functional synaptic plasticity was assessed with long-term potentiation (LTP). In the intervention study, we overexpressed hippocampal astrocyte GLT-1 in GFAP-Cre mice. Besides, AAV-Camkllα-hM4Di-mCherry was injected to inhibit neuronal hyperexcitability in CA1 region.

RESULTS

Our study found T2DM but not A/S reduced GLT-1 expression in hippocampal astrocytes. Interestingly, GLT-1 deficiency alone couldn't lead to cognitive decline, but the downregulation of GLT-1 in T2DM mice obviously enhanced increased hippocampal glutamatergic neuron excitability induced by A/S. The hyperexcitability caused neuronal apoptosis and cognitive impairment. Overexpression of GLT-1 rescued postoperative cognitive dysfunction, glutamatergic neuron hyperexcitability, oxidative stress reaction, and apoptosis in hippocampus. Moreover, chemogenetic inhibition of hippocampal glutamatergic neurons reduced oxidative stress and apoptosis and alleviated postoperative cognitive dysfunction.

CONCLUSIONS

These findings suggest that the adult mice with type 2 diabetes are at an increased risk of developing POCD, perhaps due to the downregulation of GLT-1 in hippocampal astrocytes, which enhances increased glutamatergic neuron excitability induced by A/S and leads to oxidative stress reaction, and neuronal apoptosis.

Selective vulnerability of hippocampal CA1 and CA3 pyramidal cells: What are possible pathomechanisms and should more attention be paid to the CA3 region in future studies?

Transient ischemia and reperfusion selectively damage neurons in brain, with hippocampal pyramidal cells being particularly vulnerable. Even within hippocampus, heterogeneous susceptibility is evident, with higher vulnerability of CA1 versus CA3 neurons described for several decades. Therefore, numerous studies have focused exclusively on CA1. Pediatric cardiac surgery is increasingly focusing on studies of hippocampal structures, and a negative impact of cardiopulmonary bypass on the hippocampus cannot be denied. Recent studies show a shift in selective vulnerability from neurons of CA1 to CA3. This review shows that cell damage is increased in CA3, sometimes stronger than in CA1, depending on several factors (method, species, age, observation period). Despite a highly variable pattern, several markers illustrate greater damage to CA3 neurons than previously assumed. Nevertheless, the underlying cellular mechanisms have not been fully deciphered to date. The complexity is reflected in possible pathomechanisms discussed here, with numerous factors (NMDA, kainate and AMPA receptors, intrinsic oxidative stress potential and various radicals, AKT isoforms, differences in vascular architecture, ratio of pro- and anti-apoptotic Bcl-2 factors, vulnerability of interneurons, mitochondrial dysregulation) contributing to either enhanced CA1 or CA3 vulnerability. Furthermore, differences in expressed genome, proteome, metabolome, and transcriptome in CA1 and CA3 appear to influence differential behavior after damaging stimuli, thus metabolomics-, transcriptomics-, and proteomics-based analyses represent a viable option to identify pathways of selective vulnerability in hippocampal neurons. These results emphasize that future studies should focus on the CA3 field in addition to CA1, especially with regard to improving therapeutic strategies after ischemic/hypoxic brain injury.

Egr2 contributes to age-dependent vulnerability to sevoflurane-induced cognitive deficits in mice

Sevoflurane inhalation is prone to initiate cognitive deficits in infants. The early growth response-2 (Egr-2) gene is DNA-binding transcription factor, involving in cognitive function. In this study we explored the molecular mechanisms underlying the vulnerability to cognitive deficits after sevoflurane administration. Six-day-old (young) and 6-week-old (early adult) mice received anesthesia with 3% sevoflurane for 2 h daily for 3 days. We showed that multiple exposures of sevoflurane induced significant learning ability impairment in young but not early adult mice, assessed in Morris water maze test on postnatal days 65. The integrated differential expression analysis revealed distinct transcription responses of Egr family members in the hippocampus of the young and early adult mice after sevoflurane administration. Particularly, Egr2 was significantly upregulated after sevoflurane exposure only in young mice. Microinjection of Egr2 shRNA recombinant adeno-associated virus into the dentate gyrus alleviated sevoflurane-induced cognitive deficits, and abolished sevoflurane-induced dendritic spins loss and BDNF downregulation in young mice. On the contrary, microinjection of the Egr2 overexpression virus in the dentate gyrus aggravated learning ability impairment induced by sevoflurane in young mice but not early adult mice. Furthermore, we revealed that sevoflurane markedly upregulated the nuclear factors of activated T-cells NFATC1 and NFATC2 in young mice, which were involved in Egr2 regulation. In conclusion, Egr2 serves as a critical factor for age-dependent vulnerability to sevoflurane-induced cognitive deficits.

Sevoflurane Exposure in the Developing Brain Induces Hyperactivity, Anxiety-Free, and Enhancement of Memory Consolidation in Mice

Background

Sevoflurane exposure at brain developmental stages has been reported to induce neurotoxicity and, subsequently, results in learning deficits at the juvenile age. In this study, we aimed to investigate the effects of prior early-age sevoflurane exposure on locomotor activity, anxiety, CA1-dependent learning, and spatial memory, as well as synapse changes in mice.

Methods

Totally, 3% sevoflurane was given to neonatal mice at postnatal day 7 for 4 h. These sevoflurane-treated mice were later subjected to open field and Morris water maze tests at their adult age (postnatal days 60–90) to assess their motor activity and spatial learning ability, respectively. The brain slices of sevoflurane-treated and control mice were examined for dendritic spine density and long-term potentiation (LTP) features following behavior tests (postnatal day 60). Protein levels of N-methyl-D-aspartate (NMDA) receptor subtypes and PSD95 in brain lysate were measured by using immunoblotting at the same age (postnatal day 60).

Results

Prior early-age sevoflurane exposure increased the overall moving distance, prolonged the central-area lingering time, and increased the central-area entries of adult mice. Sevoflurane-treated mice spent more time in the target quadrant during the probe test. An increase of the spine density of pyramidal neurons in the CA1 region was observed in sevoflurane-treated mice. NMDA receptor GluN2A subunit, but not the GluN2B or PSD95, was increased in the brain lysate of sevoflurane-treated mice compared with that of control mice. LTP in the hippocampus did not significantly differ between sevoflurane-treated and control mice.

Conclusion

Exposure to sevoflurane for mice during an early brain developmental stage (P7) induces later-on hyperactivity, anxiety-free, and enhancement of memory retention. These observations shed light on future investigations on the underlying mechanisms of sevoflurane’s effect on neuronal development.

Exposure to Sevoflurane, But Not Ketamine, During Early-life Brain Development has Long-Lasting Effects on GABAA Receptor Mediated Inhibitory Neurotransmission

Understanding the different mechanisms associated with different anesthetic targeted receptors is critical towards identifying accurate long-term outcome measures as a result of early-life anesthetic exposure. We examined changes in GABA_A receptor mediated neurotransmission by a predominately GABA_A receptor targeted anesthetic, sevoflurane or a predominately NMDA receptor targeted anesthetic, ketamine. Postnatal day 7 male mice were exposed to sevoflurane or ketamine and examined as adults for changes in inhibitory neurotransmission and its associated change in induced seizure activity. Paired pulse stimulation experiment showed that early-life sevoflurane treated mice had significantly less hippocampal CA1 inhibition later in life. There was significantly increased CA1 excitatory output in the sevoflurane treated group compared to the no sevoflurane treated group after the GABA agonist muscimol. Similar to our previously established data for early-life sevoflurane, here we established early-life ketamine administration resulted in neurodevelopmental behavioral changes later in life. However, muscimol did not produce a significant difference on the excitatory CA1 output between early-life ketamine group and saline group. While sevoflurane treated mice showed significantly higher induced seizure intensities and shorter latency periods to reach seizure intensity stage 5 (Racine score) compared with no sevoflurane treated mice, this phenomenon was not observed in the ketamine vs. saline treated groups. Early-life sevoflurane, but not ketamine, exposure reduced GABAergic inhibition and enhanced seizure activity later in life. The results indicate that early-life exposure to different anesthetics lead to distinct long-term effects and their unique pathways require mechanistic studies to understand induced long-lasting changes in the brain.

Isoflurane Suppresses Hippocampal High-frequency Ripples by Differentially Modulating Pyramidal Neurons and Interneurons in Mice

BACKGROUND

Isoflurane can induce anterograde amnesia. Hippocampal ripples are high-frequency oscillatory events occurring in the local field potentials of cornu ammonis 1 involved in memory processes. The authors hypothesized that isoflurane suppresses hippocampal ripples at a subanesthetic concentration by modulating the excitability of cornu ammonis 1 neurons.

METHODS

The potencies of isoflurane for memory impairment and anesthesia were measured in mice. Hippocampal ripples were measured by placing recording electrodes in the cornu ammonis 1. Effects of isoflurane on the excitability of hippocampal pyramidal neurons and interneurons were measured. A simulation model of ripples based on the firing frequency of hippocampal cornu ammonis 1 neurons was used to validate the effects of isoflurane on neuronal excitability in vitro and on ripples in vivo.

RESULTS

Isoflurane at 0.5%, which did not induce loss of righting reflex, impaired hippocampus-dependent fear memory by 97.4 ± 3.1% (mean ± SD; n = 14; P < 0.001). Isoflurane at 0.5% reduced ripple amplitude (38 ± 13 vs. 42 ± 13 μV; n = 9; P = 0.003), rate (462 ± 66 vs. 538 ± 81 spikes/min; n = 9; P = 0.002) and duration (36 ± 5 vs. 48 ± 9 ms; n = 9; P < 0.001) and increased the interarrival time (78 ± 7 vs. 69 ± 6 ms; n = 9; P < 0.001) and frequency (148.2 ± 3.9 vs. 145.0 ± 2.9 Hz; n = 9; P = 0.001). Isoflurane at the same concentration depressed action potential frequency in fast-spiking interneurons while slightly enhancing action potential frequency in cornu ammonis 1 pyramidal neurons. The simulated effects of isoflurane on hippocampal ripples were comparable to recordings in vivo.

CONCLUSIONS

The authors' results suggest that a subanesthetic concentration of isoflurane can suppress hippocampal ripples by differentially modulating the excitability of pyramidal neurons and interneurons, which may contribute to its amnestic action.

EDITOR’S PERSPECTIVE

Transcriptome analysis of sevoflurane exposure effects at the different brain regions

Backgrounds

Sevoflurane is a most frequently used volatile anesthetics, but its molecular mechanisms of action remain unclear. We hypothesized that specific genes play regulatory roles in brain exposed to sevoflurane. Thus, we aimed to evaluate the effects of sevoflurane inhalation and identify potential regulatory genes by RNA-seq analysis.

Methods

Eight-week old mice were exposed to sevoflurane. RNA from medial prefrontal cortex, striatum, hypothalamus, and hippocampus were analysed using RNA-seq. Differently expressed genes were extracted and their gene ontology terms were analysed using Metascape. These our anesthetized mouse data and the transcriptome array data of the cerebral cortex of sleeping mice were compared. Finally, the activities of transcription factors were evaluated using a weighted parametric gene set analysis (wPGSA). JASPAR was used to confirm the existence of binding motifs in the upstream sequences of the differently expressed genes.

Results

The gene ontology term enrichment analysis result suggests that sevoflurane inhalation upregulated angiogenesis and downregulated neural differentiation in each region of brain. The comparison with the brains of sleeping mice showed that the gene expression changes were specific to anesthetized mice. Focusing on individual genes, sevoflurane induced Klf4 upregulation in all sampled parts of brain. wPGSA supported the function of KLF4 as a transcription factor, and KLF4-binding motifs were present in many regulatory regions of the differentially expressed genes.

Conclusions

Klf4 was upregulated by sevoflurane inhalation in the mouse brain. The roles of KLF4 might be key to elucidating the mechanisms of sevoflurane induced functional modification in the brain.

The Critical Roles of the SUMO-Specific Protease SENP3 in Human Diseases and Clinical Implications

Post-translational modification by SUMO (small ubiquitin-like modifier) proteins has been shown to regulate a variety of functions of proteins, including protein stability, chromatin organization, transcription, DNA repair, subcellular localization, protein–protein interactions, and protein homeostasis. SENP (sentrin/SUMO-specific protease) regulates precursor processing and deconjugation of SUMO to control cellular mechanisms. SENP3, which is one of the SENP family members, deconjugates target proteins to alter protein modification. The effect of modification via SUMO and SENP3 is crucial to maintain the balance of SUMOylation and guarantee normal protein function and cellular activities. SENP3 acts as an oxidative stress-responsive molecule under physiological conditions. Under pathological conditions, if the SUMOylation process of proteins is affected by variations in SENP3 levels, it will cause a cellular reaction and ultimately lead to abnormal cellular activities and the occurrence and development of human diseases, including cardiovascular diseases, neurological diseases, and various cancers. In this review, we summarized the most recent advances concerning the critical roles of SENP3 in normal physiological and pathological conditions as well as the potential clinical implications in various diseases. Targeting SENP3 alone or in combination with current therapies might provide powerful targeted therapeutic strategies for the treatment of these diseases.

Sevoflurane depresses neurons in the medial parabrachial nucleus by potentiating postsynaptic GABAA receptors and background potassium channels

Despite persistent clinical use for over 170 years, the neuronal mechanisms by which general anesthetics produce hypnosis remain unclear. Previous studies suggest that anesthetics exert hypnotic effects by acting on endogenous arousal circuits. Recently, it has been shown that the medial parabrachial nucleus (MPB) is a novel wake-promoting component in the dorsolateral pons. However, it is not known whether and how the MPB contributes to anesthetic-induced hypnosis. Here, we investigated the action of sevoflurane, a widely used volatile anesthetic agent that best represents the drug class of halogenated ethers, on MPB neurons in mice. Using in vivo fiber photometry, we found that the population activities of MPB neurons were inhibited during sevoflurane-induced loss of consciousness. Using in vitro whole-cell patch-clamp recordings, we revealed that sevoflurane suppressed the firing rate of MPB neurons in concentration-dependent and reversible manners. At a concentration equal to MAC of hypnosis, sevoflurane potentiated synaptic GABA_A receptors (GABA_A-Rs), and the inhibitory effect of sevoflurane on the firing rate of MPB neurons was completely abolished by picrotoxin, which is a selective GABA_A-R antagonist. At a concentration equivalent to MAC of immobility, sevoflurane directly hyperpolarized MPB neurons and induced a significant decrease in membrane input resistance by increasing a basal potassium conductance. Moreover, pharmacological blockade of GABA_A-Rs in the MPB prolongs induction and shortens emergence under sevoflurane inhalation at MAC of hypnosis. These results indicate that sevoflurane inhibits MPB neurons through postsynaptic GABA_A-Rs and background potassium channels, which contributes to sevoflurane-induced hypnosis.

Sevoflurane induces neuronal activation and behavioral hyperactivity in young mice

Sevoflurane, a commonly used anesthetic, may cause agitation in patients. However, the mechanism underlying this clinical observation remains largely unknown. We thus assessed the effects of sevoflurane on neuronal activation and behaviors in mice. Ten-day-old mice received 2% sevoflurane, 1% isoflurane, or 6% desflurane for 10 minutes. The behavioral activities were recorded and evaluated at one minute after the loss of righting reflex in the mice, which was about two minutes after the anesthetic administration. The neuronal activation was evaluated by c-Fos expression and calcium imaging at one minute after the anesthetic administration. Propofol, which reduces neuronal activation, was used to determine the cause-and-effect of sevoflurane. We found that sevoflurane caused an increase in neuronal activation in primary somatosensory cortex of young mice and behavioral hyperactivity in the mice at one minute after the loss of righting reflex. Desflurane did not induce behavioral hyperactivity and isoflurane only caused behavioral hyperactivity with borderline significance. Finally, propofol attenuated the sevoflurane-induced increase in neuronal activation and behavioral hyperactivity in young mice. These results demonstrate an unexpected sevoflurane-induced increase in neuronal activation and behavioral hyperactivity in young mice. These findings suggest the potential mechanisms underlying the sevoflurane-induced agitation and will promote future studies to further determine whether anesthetics can induce behavioral hyperactivity via increasing neuronal activation.

The General Anesthetic Isoflurane Bilaterally Modulates Neuronal Excitability

Volatile anesthetics induce hyperactivity during induction while producing anesthesia at higher concentrations. They also bidirectionally modulate many neuronal functions. However, the neuronal mechanism is unclear. The effects of isoflurane on sodium channel currents were analyzed in acute mouse brain slices, including sodium leak (NALCN) currents and voltage-gated sodium channels (Nav) currents. Isoflurane at sub-anesthetic concentrations increased the spontaneous firing rate of CA3 pyramidal neurons, whereas anesthetic concentrations of isoflurane decreased the firing rate. Isoflurane at sub-anesthetic concentrations enhanced NALCN conductance but minimally inhibited Nav currents. Isoflurane at anesthetic concentrations depressed Nav currents and action potential amplitudes. Isoflurane at sub-anesthetic concentrations depolarized resting membrane potential (RMP) of neurons, whereas hyperpolarized the RMP at anesthetic concentrations. Isoflurane at low concentrations induced hyperactivity in vivo, which was diminished in NALCN knockdown mice. In conclusion, enhancement of NALCN by isoflurane contributes to its bidirectional modulation of neuronal excitability and the hyperactivity during induction.

The Effects of General Anesthetics on Synaptic Transmission

General anesthetics are a class of drugs that target the central nervous system and are widely used for various medical procedures. General anesthetics produce many behavioral changes required for clinical intervention, including amnesia, hypnosis, analgesia, and immobility; while they may also induce side effects like respiration and cardiovascular depressions. Understanding the mechanism of general anesthesia is essential for the development of selective general anesthetics which can preserve wanted pharmacological actions and exclude the side effects and underlying neural toxicities. However, the exact mechanism of how general anesthetics work is still elusive. Various molecular targets have been identified as specific targets for general anesthetics. Among these molecular targets, ion channels are the most principal category, including ligand-gated ionotropic receptors like γ-aminobutyric acid, glutamate and acetylcholine receptors, voltage-gated ion channels like voltage-gated sodium channel, calcium channel and potassium channels, and some second massager coupled channels. For neural functions of the central nervous system, synaptic transmission is the main procedure for which information is transmitted between neurons through brain regions, and intact synaptic function is fundamentally important for almost all the nervous functions, including consciousness, memory, and cognition. Therefore, it is important to understand the effects of general anesthetics on synaptic transmission via modulations of specific ion channels and relevant molecular targets, which can lead to the development of safer general anesthetics with selective actions. The present review will summarize the effects of various general anesthetics on synaptic transmissions and plasticity.

Dexmedetomidine suppresses sevoflurane anesthesia-induced neuroinflammation through activation of the PI3K/Akt/mTOR pathway

Background

Sevoflurane, an inhalational general anesthetic, has become one of the most widely used inhalational anesthetics in surgery. However, previous studies have found that sevoflurane anesthesia can trigger an inflammatory response, resulting in secondary damage. Dexmedetomidine (DEX), a highly-selective α adrenergic receptor agonist, is widely used as an anesthetic adjuvant in the clinic. In this study we investigated whether DEX was able to suppress sevoflurane-induced neuroinflammation.

Methods

The aim was to determine the mechanism of action of the suppressive effect of DEX using a rat model. Rats were randomly divided into a control group (n = 10), low-dose sevoflurane group (L-Sev; n = 10), high-dose sevoflurane group (H-Sev; n = 10), vehicle group (n = 10), DEX group (n = 10) and DEX + LY294002 (a specific inhibitor of PI3K) group (n = 10). The rats in vehicle, DEX and DEX + LY294002 groups were in the presence of high-dose sevoflurane exposure. Western blotting was used to measure the expression of proinflammatory cytokines (IL-6, IL-8, TNF-α) and the activity level of the phosphatidylinositol 3-hydroxy kinase/protein kinase B/mammalian target of rapamycin (PI3K/Akt/mTOR) pathway.

Results

We found that sevoflurane anesthesia induced an increase in the levels of pro-inflammatory cytokines, while decreasing activation of the PI3K/Akt/mTOR pathway in both the cortex and hippocampus of rats. Treatment with DEX reduced pro-inflammatory cytokine levels and prevented inactivation of the PI3K/Akt/mTOR pathway. Moreover, LY294002, an inhibitor of the PI3K/Akt/mTOR pathway, reduced the anti-inflammatory activity of DEX.

Conclusions

These data suggest that the PI3K/Akt/mTOR pathway contributes to sevoflurane-induced neuroinflammation and that activation of PI3K/Akt/mTOR signaling by DEX could help reduce the neuroinflammatory effects of sevoflurane.

Sevoflurane increases locomotion activity in mice

Clinical observations show emergence of agitation and hyperactivity during the anesthesia induction and/or recovery period post-anesthesia. However, an animal model to illustrate this clinical phenomenon has not yet been established. We therefore set out to investigate whether sevoflurane, a commonly used anesthetic, could alter locomotion in mice during the anesthesia induction and recovery period post-anesthesia. The activity of the mice was recorded 5 minutes before, during (for 30 minutes), and 40 minutes after the administration of the anesthetic sevoflurane [1-, 1.5- and 2-fold minimum alveolar concentration] at 37⁰ C. The total walking distance and velocity of movement were measured and quantified as the indexes of locomotion. We found that the anesthetic sevoflurane increased the locomotion of the mice during the induction period of the anesthesia. During the recovery phase after anesthesia, the mice exhibited increased locomotion for a short period of time (about 5 minutes) and then displayed a sharp decrease in mobility for up to 60 minutes following the end of anesthesia administration. The anesthetic sevoflurane did not significantly alter the food intake and body weight of the mice. Furthermore, we found that Alzheimer’s disease transgenic mice exhibited a greater degree of sevoflurane-induced hyperactivity than the wild-type mice did. Our results showed that inhalation of the anesthetic sevoflurane induced an acute hyperactivity in mice, particularly among Alzheimer’s disease transgenic mice. These findings from the pilot studies have established an animal model to promote further studies into postoperative emergence agitation, hyperactivity and the underlying mechanisms into these conditions.