Alzheimer’s Disease (AD) is the most prevalent form of dementia, posing a significant challenge and having a profound impact on the health and quality of life of the elderly population. In recent years, China has witnessed an increase in the incidence, prevalence, and mortality rates of AD, imposing a considerable economic burden on patients’ families, society, and the healthcare system (Li et al., 2022; Ren et al., 2022; Ji et al., 2024). Despite advancements in diagnostic techniques and a deepening understanding of its pathology, the lack of a clearly defined underlying mechanism has hindered the development of effective treatments to slow the progression of AD. Pathologically, AD is featured by substantial atrophy and death of brain cells. The primary cause of this degeneration is the accumulation of extracellular amyloid-β protein (Aβ) plaques, coupled with intraneuronal neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau proteins (Braak and Braak, 1991; Winblad et al., 2016). The presence of Aβ plaques and NFTs disrupts neuronal communication, resulting in synaptic loss and ultimately causing widespread neuronal death (Selkoe, 2002; Mattson, 2004). Further pathological features include gliosis, neuroinflammation, and significant synaptic changes (Itagaki et al., 1989; Calsolaro and Edison, 2016; Tönnies and Trushina, 2017).

Orexin, also referred to as hypocretin, is a neuropeptide synthesized in the hypothalamus and consists of two isoforms: orexin-A and orexin-B (de Lecea et al., 1998; Sakurai et al., 1998). Orexinergic neurons are primarily located in the lateral hypothalamus (LH) and extensively project their fibers to different regions of the brain (Vraka et al., 2023). The orexinergic system is essential in regulating sleep-wake cycles, feeding behavior, energy homeostasis, as well as reward processing (Chemelli et al., 1999; Edwards et al., 1999; Volkoff et al., 1999; Willie et al., 2001; Carter et al., 2009; Tsujino and Sakurai, 2009; Leinninger et al., 2011). Extensive research has demonstrated that the orexinergic system exerts an influence on the pathophysiology of AD, particularly in relation to the generation and accumulation of Aβ. Kang et al. discovered that injecting orexin into the ventricles of APP/PS1 mice elevates the levels of Aβ in the brain ISF, while infusion of a dual orexin receptor antagonist (DORA) effectively decreases Aβ levels (Kang et al., 2009). Similarly, studies have indicated that knocking out the orexin gene in APP/PS1 mice results in a reduction of Aβ deposition in the brain (Roh et al., 2014). Human studies have revealed that patients with moderate to severe AD present higher levels of orexin in their CSF compared to the control individuals (Liguori et al., 2014). However, there remains some disagreement regarding whether the orexinergic system actually promotes the progression of AD. On one hand, focal overexpression of orexin in the hippocampus of APP/PS1/OR^−/− mice did not alter the levels of Aβ accumulation (Roh et al., 2014). Additionally, postmortem analysis of hypothalamic tissue from patients with AD displayed a 40% reduction in orexin-immunoreactive neurons, accompanied by a slight decrease in the ventricular cerebrospinal fluid (CSF) levels of orexin-A. (Fronczek et al., 2012).

On the other hand, some physiological functions of orexin are inconsistent with a role in the development of AD. Orexin plays a crucial role in hippocampus-dependent social memory consolidation, and the introduction of exogenous orexin can ameliorate this deficit by modulating synaptic plasticity in the hippocampal region (Yang et al., 2013). By regulating the activity of various neurotransmitter systems, including cholinergic and dopaminergic pathways, orexin can enhance learning processes, facilitate the retrieval of memories, as well as support the consolidation of acquired information (Telegdy and Adamik, 2002; Palotai et al., 2014; Piantadosi et al., 2015). The administration of orexin-A via the nasal cavity or directly into the hippocampus has been shown to alleviate memory deficits in orexin/ataxin-3 transgenic mice (Yang et al., 2013; Mavanji et al., 2017). The impairment of memory caused by orexin deficiency has been confirmed in humans. For instance, patients with narcolepsy frequently exhibit memory and cognitive decline that is associated with the disease (Henry et al., 1993; Naumann et al., 2006; Witt et al., 2018). Other studies have demonstrated that orexin exerts neuroprotective effects. Specifically, orexin-A inhibits neuroinflammation by decreasing astrocyte proliferation, microglial activation, as well as the expression of chemokines and cytokines (Becquet et al., 2019). Orexin receptors hinder excessive autophagy through the MAPK/ERK/mTOR signaling pathway and confer neuroprotective benefits in AD via heterodimerization with GPR103 (Davies et al., 2015; Xu et al., 2021).

In summary, considering the diverse physiological functions of orexin, it is unreasonable to regard it as a direct causal factor in AD. The function of orexin in bolstering cognitive abilities, particularly memory and learning, fundamentally contradicts the underlying pathological mechanisms of AD. Although some studies suggest that orexin influences Aβ dynamics, its overall beneficial effects indicate a protective, rather than pathogenic, role in AD. Therefore, we hypothesize that the increased orexin levels observed in the brains of AD patients may represent a compensatory neuroprotective mechanism, rather than a causative factor. The accumulation of Aβ stimulates a compensatory increase in orexin, which, in turn, enhances a range of neuroprotective mechanisms, thereby mitigating the memory and cognitive impairments related to Aβ deposition. To certify this hypothesis, the pathological characteristics of Aβ accumulation in the hippocampus of AD patients were simulated in this study by injecting Aβ_1-42 oligomers into the bilateral hippocampal dentate gyrus. Subsequently, we observed the impact of various concentrations of Aβ_1-42 oligomers on neuroinflammation in the hippocampus and the functionality of the orexinergic system in the brain. Additionally, the intervention effects of dexamethasone (Dex) on these impacts were evaluated.

Neuroinflammation is widely recognized as a central mechanism underlying the accumulation of amyloid-β protein (Aβ)-induced pathology in Alzheimer’s disease (AD). Research has shown that Aβ can stimulate microglia, causing them to shift towards the pro-inflammatory M1 phenotype and triggering the secretion of various pro-inflammatory factors, such as IL-6 and TNF-α, thereby initiating neuroinflammatory responses (Heneka et al., 2015; Tang and Le, 2016). This inflammatory response disrupts neuronal signal transmission, decreases synaptic plasticity, and enhances oxidative stress reactions, ultimately exacerbating neuronal damage (Block et al., 2007; Cai et al., 2019). Synaptic plasticity is a fundamental process that underlies learning and memory. The persistent release of pro-inflammatory factors can disrupt neuronal signaling pathways, causing synaptic loss and ultimately contributing to significant cognitive impairment in AD (De Strooper and Karran, 2016; Ransohoff, 2016). In this study, we successfully induced a significant upregulation of pro-inflammatory factors by injecting Aβ_1-42 oligomers into the dentate gyrus of the mouse hippocampus, effectively simulating the neuroinflammatory pathological features of AD.

Previous studies have suggested that elevated orexin levels may lead to increased Aβ levels. For instance, Kang et al. found that injecting orexin into APP transgenic mice resulted in higher Aβ deposition, whereas administering a dual orexin receptor antagonist (DORA) reduced Aβ levels (Kang et al., 2009). Similarly, Roh et al. observed reduced Aβ deposition following the knockout of the orexin gene in APP/PS1 transgenic mice (Roh et al., 2014). These studies, along with our research, demonstrate a positive correlation between orexin and Aβ, although the causal relationships differ. It is important to note that the orexin system plays a key role in promoting and maintaining wakefulness (Arrigoni et al., 2010), facilitating voluntary activity (Kiwaki et al., 2004), and supporting energy metabolism (Tsujino and Sakurai, 2009). On one hand, during wakefulness, metabolic products such as adenosine and Aβ inevitably accumulate in the brain (Huang et al., 2011; Spira et al., 2014). On the other hand, there is literature showing that Aβ-degrading enzymes, such as neprilysin (NEP), exhibit increased activity during sleep when orexinergic neurons are inhibited, thereby enhancing Aβ degradation (Ogawa et al., 2005). However, previous studies seem to have overlooked the impact of metabolic product accumulation during wakefulness and the corresponding decrease in Aβ-degrading enzyme activity.

Recent research has demonstrated that the orexinergic system enhances cognitive functions. Specifically, orexin-A promotes the transformation of microglia into the anti-inflammatory M2 phenotype (Duffy et al., 2015), which exhibits anti-inflammatory and neuroprotective properties, improving cognitive deficits (Saijo and Glass, 2011; Wang et al., 2020). The role of the orexinergic system in regulating the body’s energy metabolism is well-established (Tsujino and Sakurai, 2009). From the survival perspective, it also fundamentally aids humans and animals in adapting to their environment and maintaining survival capabilities. For example, when animals are hungry or hypoglycemic, the excitability of orexinergic neurons increases (Risold et al., 1999), resulting in increased secretion of orexin-A (Ouedraogo et al., 2003). The enhanced functionality of the orexinergic system fosters and maintains wakefulness, augments autonomous activity, and boosts muscle energy consumption. Furthermore, it also enhances alertness and cognitive functions (Kiwaki et al., 2004; Yang et al., 2013). Collectively, these effects promote feeding behavior to fulfill energy requirements. This adaptive mechanism underscores the crucial role of the orexinergic system in ensuring survival by modulating physiological and behavioral responses according to the environment.

Neuroinflammation induced by the accumulation of Aβ results in cognitive impairments in both humans and animals, evidently compromising their adaptability and survival capabilities. To counteract the cognitive toxicity induced by Aβ, the body inevitably initiates compensatory responses. Our study demonstrates that intracerebral injection of Aβ led to a concentration-dependent increase in both orexin-A levels in the hippocampus, as well as an enhancement in the excitability of orexinergic neurons in the lateral hypothalamus (LH). This suggests that the accumulation of Aβ not only triggers neuroinflammation but also simultaneously augments the functionality of the orexinergic system.

Dexamethasone (Dex) is a glucocorticoid anti-inflammatory agent capable of penetrating the blood-brain barrier (BBB) (Vohra et al., 2021). In our study, intervention with Dex significantly mitigated Aβ-induced neuroinflammation, leading to decreased levels of orexin-A in the hippocampus and reduced excitability of orexinergic neurons in the LH. These findings further affirm that the elevation in orexin-A levels is a consequence of Aβ-induced neuroinflammation, and they bolster the hypothesis that the orexinergic system serves as a neuroprotective mechanism against Aβ-induced cognitive toxicity.

While our study provides valuable insights into the compensatory role of the orexinergic system in AD, there are several limitations that should be considered. First, although we focused on male mice to reduce variability caused by hormonal fluctuations, this choice limits our ability to fully assess potential sex-based differences in the impact of Aβ and the orexinergic system. Future studies should include both male and female animals to explore whether sex-based differences influence the compensatory mechanisms observed in AD. Furthermore, while our study utilized c-Fos as a marker of neuronal excitability, it is worth noting that more advanced techniques, such as electrophysiological recordings or calcium imaging, could offer a more comprehensive understanding of the dynamic activity of orexinergic neurons. This limitation could be addressed in future studies.

Besides, caution is needed when applying these findings to humans due to the inability of animal models to fully replicate the complexity of human AD, particularly in terms of genetics and the multifactorial nature of the disease. Further research is required to clarify the molecular mechanisms and signaling pathways through which the orexinergic system regulates AD pathology. Additionally, it is crucial to consider the implications of dosage, treatment duration, and long-term effects when exploring the orexinergic system as a therapeutic target. These considerations will help guide future research directions and their potential clinical applications.

The latest studies have explored various anti-inflammatory treatment strategies for AD. Both lupeol, a natural compound, and NB-02, a botanical therapeutic drug, have demonstrated neuroprotective effects by regulating neuroinflammation (Lee et al., 2021; Choe, 2024). Additionally, the activation of the receptor TREM2 on microglia has been proven to reduce neuroinflammation and improve cognitive outcomes in AD, while monoclonal antibodies targeting TREM2 further support its therapeutic potential (Fassler et al., 2021). Methylprednisolone and low-dose aspirin have also shown promise in regulating neuroinflammation and mitigating cognitive decline (Vallés et al., 2020; Sun et al., 2023). Furthermore, GLP-1 receptor agonists have been found to reduce neuroinflammation and amyloid precursor protein (APP) deposition, while improving memory and synaptic function (Nowell et al., 2022). In summary, these studies underscore the potential of targeting neuroinflammation as a therapeutic strategy for AD, and the findings of our study are in alignment with this growing body of work.

Neuroinflammation induced by Aβ is accompanied by an augmentation in the functionality of the orexinergic system, featured by an increase in the excitability of orexinergic neurons and an elevated secretion of orexin-A. The enhanced functionality of the orexinergic system acts as a compensatory response to counteract Aβ-induced neuroinflammation, ultimately aiming to improve cognition. This study contributes to our understanding of the pathophysiological mechanisms underlying AD.