Purinergic receptors in the central nervous system: mechanisms of synaptic modulation, neuroplasticity, and neuroinflammation

The discovery of purinergic receptors has significantly advanced our understanding of how nucleotides and nucleosides regulate a variety of physiological processes, particularly in the CNS. These receptors are classified into two major families: P1 receptors, which are selective for adenosine, and P2 receptors, which primarily respond to ATP and ADP (Figure 1). While ATP is well-known for its role in cellular energy metabolism, it also acts as a crucial extracellular signaling molecule in the CNS. Astrocytes and neurons are the primary sources of ATP release into the surrounding environment [16, 17]. Early studies faced challenges due to ATP’s rapid degradation into adenosine, which masked the distinct roles of ATP and adenosine in signaling. Later research clarified that P1 receptors, inhibited by methylxanthines, mediate adenosine’s effects, while ATP activates P2 receptors, establishing two distinct signaling pathways [1, 18, 19].

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Figure 1. 

Classification of purinergic receptors in the CNS. CNS: central nervous system

Purinergic signaling plays a fundamental role in cellular communication across various species, underscoring its evolutionary conservation. Homologs of P2X receptors have been identified in organisms like Dictyostelium and Schistosoma, and ATP signaling is even present in plants, highlighting its importance in maintaining cellular functions and enabling adaptive responses across biological systems [20, 21]. In the healthy brain, purinergic signaling coordinates processes between both neural and non-neural cells, promoting homeostasis and facilitating adaptive responses to physiological and environmental changes. Glial cells are central to orchestrating these processes, interacting with neurons and regulating synaptic activity, metabolism, and immune responses [7, 8, 22].

Purinergic receptors exhibit remarkable diversity, with specific subtypes performing distinct cellular functions (Figure 1). The P1 receptor family includes four subtypes (A1, A2A, A2B, and A3), each with varying affinities for adenosine, influencing pathways such as modulation of adenylate cyclase activity [1, 23, 24]. The P2 receptor family consists of P2X receptors, which are ligand-gated ion channels, and P2Y receptors, which are G protein-coupled receptors (GPCRs) [25]. The P2X receptor family consists of seven subtypes (P2X1–7), which can form both homo- and heteromeric channels [26, 27]. Notably, in addition to heteromeric channels, the P2X7 receptor forms homotrimeric channels that mediate cellular responses such as proliferation, apoptosis, and inflammation. P2X receptors facilitate ion flux (Na⁺, K⁺, Ca²⁺), resulting in increased intracellular calcium levels and membrane depolarization [28, 29]. In contrast, the eight subtypes of P2Y receptors (P2Y1, 2, 4, 6, and 11–14) initiate downstream signaling via GPCR pathways, regulating targets like phospholipase C (PLC) and adenylate cyclase [30, 31].

The activity of purinergic receptors is precisely regulated by ecto-nucleotidases, such as nucleoside triphosphate diphosphohydrolase-1 (CD39) and ecto-5’-nucleotidase (CD73), which hydrolyze ATP and ADP into adenosine, thus activating P1 receptors and maintaining the balance between ATP and adenosine signaling [32–34]. Adenosine’s effects are further fine-tuned by enzymatic degradation and uptake through equilibrative nucleoside transporters (ENT1–4) [35, 36], ensuring that purinergic signaling maintains cellular homeostasis and facilitates appropriate cellular responses.

Purinergic receptors are crucial for neuroplasticity, influencing neurotransmission, structural plasticity, and behavioral adaptation (Figure 2) [5, 24]. Glial cells play key roles in these processes due to their expression of purinergic receptors, extending the influence of purinergic signaling beyond neurons [37]. P2 receptors, particularly P2X receptors localized to postsynaptic sites, mediate rapid excitatory signaling in response to ATP release [38–40]. P1 receptors, such as A1 and A2A, modulate neurotransmitter release through feedback inhibition at presynaptic sites, fine-tuning synaptic activity, especially during prolonged neuronal firing, and contributing to synaptic plasticity [41, 42]. Astrocytes actively participate by releasing ATP and adenosine in response to neuronal activity, influencing synaptic function and maintaining extracellular homeostasis [43, 44].

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Figure 2. 

Functions of purinergic receptors in CNS. CNS: central nervous system; MS: multiple sclerosis

Purinergic signaling also regulates structural changes in neurons. P2X7 receptors are involved in dendritic remodeling and synapse formation, processes critical for learning and memory [45, 46]. P2Y receptors regulate the cytoskeleton and promote axonal growth, facilitating synapse formation and neuronal connections [47, 48]. Glial cells support structural plasticity by releasing ATP in response to neuronal activity, promoting synaptogenesis and neuronal growth [49, 50]. Microglia utilizes purinergic signaling to mediate neuroinflammatory responses that can impact synaptic plasticity [51, 52]. These glial-mediated effects emphasize the crucial role of glial cells in shaping neuronal networks during development, learning, and injury repair [50, 53].

Purinergic signaling is also implicated in regulating behavioral processes such as attention and memory [54]. Glial cells, particularly astrocytes, contribute to behavioral plasticity by modulating synaptic transmission through purine release, directly affecting neuronal excitability and cognitive functions [55].

Purines produced in the CNS regulate the infiltration, trafficking, and activation of peripheral immune cells under both physiological and pathological conditions. ATP and its metabolites, including ADP and adenosine, act as key extracellular signaling molecules through P2X and P2Y purinergic receptors expressed on glial and immune cells [1, 56]. Under normal conditions, purinergic signaling maintains CNS homeostasis by modulating microglial surveillance and synaptic activity [7]. However, in neuroinflammatory states such as TBI and neurodegenerative diseases, excessive ATP release from damaged cells acts as a damage-associated molecular pattern (DAMP), activating microglia and infiltrating immune cells via P2X7 receptors [57]. Purines also influence leukocyte adhesion and transmigration across the blood-brain barrier (BBB) by modulating endothelial activation. Adenosine, through A2A receptors, can suppress inflammation by inhibiting leukocyte adhesion and promoting the anti-inflammatory M2 microglial phenotype [58]. Conversely, ATP-P2X7 receptor signaling amplifies inflammation by promoting pro-inflammatory cytokine release, such as interleukin-1β (IL-1β) and tumor necrosis factor-alpha (TNF-α), further enhancing leukocyte recruitment [57].

Experimental models have provided critical insights into the multifaceted roles of purinergic signaling in CNS pathology. In experimental autoimmune encephalomyelitis (EAE), a widely used model for MS, the blockade of P2X7 receptors has been shown to attenuate neuroinflammation by reducing T cell activation and subsequent CNS infiltration [59, 60]. T cell activation in EAE is a highly coordinated process initiated by antigen-presenting cells (APCs), including dendritic cells, microglia, and macrophages, which process and present myelin-derived peptides via major histocompatibility complex (MHC) molecules to naïve CD4⁺ T cells [61]. This interaction, in conjunction with co-stimulatory signaling (e.g., CD28/B7) and cytokine-mediated polarization, drives the differentiation of naïve T cells into pathogenic effector subsets, particularly T helper 1 (Th1) and Th17 cells [62, 63]. ATP, acting through P2X7 receptors on both APCs and T cells, plays a pivotal role in modulating this activation process. P2X7 receptor activation promotes the assembly of the NOD-like receptor protein 3 (NLRP3) inflammasome, leading to the release of pro-inflammatory cytokines such as IL-1β and interleukin-18 (IL-18), which further amplify T cell expansion and effector function [64]. Moreover, P2X7 receptor signaling enhances the metabolic reprogramming of T cells, favoring glycolysis-driven proliferation, a key requirement for sustaining the inflammatory response. The blockade of P2X7 receptors disrupts this inflammatory cascade by impairing inflammasome activation, attenuating IL-1β release, and limiting the expansion of pathogenic Th1 and Th17 cells [65]. Consequently, the reduction in pro-inflammatory T cell infiltration across the BBB alleviates CNS inflammation and tissue damage, highlighting P2X7 as a potential therapeutic target for autoimmune neuroinflammatory disorders [66].

Similarly, in spinal cord injury (SCI), ATP release drives microglial activation and secondary tissue damage [67, 68]. In TBI, purinergic receptor activity exhibits both protective and detrimental effects depending on receptor subtype and activation timing, underscoring the complex and context-dependent nature of purinergic signaling in neuroinflammation [69].

Short- and long-term roles of purinergic signaling in the CNS mediated by astrocytes and microglia

Purinergic signaling plays a central role in regulating both short-term and long-term cellular functions in the CNS, supporting a diverse range of physiological and pathological processes. Short-term effects, ranging from milliseconds to seconds, involve rapid synaptic transmission and acute cellular responses, while long-term effects, spanning minutes to hours or even days, include sustained cellular changes such as gene expression modulation, neuroinflammation, and prolonged adaptations in synaptic plasticity and cellular proliferation [1, 5, 70]. Initially, purinergic signaling was primarily studied in the context of short-term events such as neurotransmission, neuromodulation, secretion, chemoattraction, and acute inflammation. These processes are primarily mediated by ATP, which engages P2X ionotropic and P2Y metabotropic receptors to trigger rapid cellular responses necessary for maintaining homeostasis under acute conditions [24, 25, 71]. However, emerging evidence highlights that purinergic signaling also has long-term (trophic) roles, such as cell proliferation, differentiation, migration, and apoptosis [72, 73]. These trophic functions are critical for developmental processes, tissue repair, and the progression of pathological states like cancer and atherosclerosis (Table 1).

Astrocytes and microglia are key mediators of purinergic signaling in both short-term and long-term contexts [80]. Astrocytes release ATP in response to neuronal activity and various physiological stimuli. In short-term contexts, ATP activates P2 receptors on neighboring neurons and glial cells, modulating synaptic transmission and enhancing synaptic plasticity. These processes are integral to neuromodulation, a crucial aspect of dynamic neuronal communication [75, 78]. Astrocytic purinergic signaling is also essential for long-term functions such as regulating cell migration, angiogenesis, and tissue repair. ATP released by astrocytes is metabolized to adenosine, acting on trophic receptors to influence cell proliferation and neurogenesis, supporting developmental and reparative processes [78, 81, 82].

Microglia are similarly regulated by purinergic signaling. In short-term responses, ATP acts as a “find-me” signal, guiding microglial migration to injury or infection sites through activation of P2Y12 receptors [75, 83]. This chemotactic response initiates inflammatory processes and facilitates the clearance of cellular debris, essential for resolving acute damage. Beyond these immediate actions, microglia are involved in long-term functions, including synaptic remodeling and pruning, crucial for maintaining synaptic integrity and plasticity [22, 51, 84]. These processes are regulated by ATP and adenosine signaling, with dysregulation contributing to neurodegenerative diseases. Chronic activation of P2X7 receptors on microglia, for example, has been associated with sustained inflammatory responses and neurotoxicity, highlighting the dual nature of purinergic signaling in microglial physiology [52, 76, 85, 86].

By integrating both short-term and long-term functions, purinergic signaling orchestrates a complex network of cellular processes in the CNS, with astrocytes and microglia playing critical roles. Astrocytes primarily contribute to neuromodulation and structural support, while microglia specialize in immune surveillance and synaptic remodeling [74, 80, 87]. Together, these glial cells modulate the dynamic and adaptive responses of the CNS to physiological demands and pathological challenges.

Astrocytic ATP release and purinergic signaling: key mechanisms in synaptic plasticity and intercellular communication

ATP serves as a critical mediator of synaptic function, particularly in excitatory neurotransmission. Ionotropic P2XRs mediate rapid synaptic responses, with hippocampal slice studies demonstrating their involvement in regulating synaptic plasticity. However, their role in long-term potentiation (LTP) and synaptic depression is debated, with some evidence suggesting P2XRs facilitate LTP, while others suggest they induce synaptic depression [38, 39, 142, 161–163].

Astrocytes contribute to synaptic regulation by releasing ATP, which can occur via exocytosis, modulating neuronal excitability and synaptic activity [44, 55, 145, 164]. The interaction between ATP-mediated P2 receptor activation and adenosine receptors (A1Rs and A2ARs) creates a dynamic balance that modulates synaptic transmission [165, 166]. A2ARs typically enhance NMDA receptor activity and glutamate release, promoting synaptic plasticity, while A1Rs inhibit synaptic transmission, offering neuroprotective effects [167–169]. This interplay is vital for maintaining functional neuronal networks and may influence neurodegenerative processes.

In addition to synaptic regulation, astrocytes use purinergic signaling to coordinate calcium (Ca²⁺)-mediated intercellular communication [170]. ATP acts as a critical extracellular messenger activating purinergic receptors like P2Y and P2X, inducing intracellular Ca²⁺ increases in astrocytes [38, 145]. These Ca²⁺ waves propagate through astrocyte networks, influencing synaptic modulation and metabolic support for neurons [171].

ATP signaling also plays a key role in neurovascular coupling, where astrocytes regulate blood flow in response to neuronal activity, and in maintaining redox homeostasis [152]. During injury, astrocytes undergo reactive changes influenced by ATP and adenosine signaling [172]. Adenosine, produced from ATP breakdown by ecto-nucleotidases on astrocyte surfaces, acts on A1 and A2A receptors, modulating astrocytic responses. A1Rs suppress excessive excitation, while A2ARs promote synaptic facilitation and inflammation [166, 169, 173].

ATP and adenosine mediated crosstalk between neurons, astrocytes, and microglia

Neuron-astrocyte communication is essential for brain homeostasis, with both neurons and astrocytes capable of releasing ATP and responding to its extracellular concentrations [4]. Synaptically released neurotransmitters trigger Ca²⁺ changes in astrocytes, leading to the release of gliotransmitters like ATP, which modulate synaptic activity [174]. This bidirectional communication enables astrocytes to regulate synaptic environments by clearing neurotransmitters, maintaining ion balance, and modulating synaptic strength [175]. ATP conversion to adenosine further refines synaptic transmission through P1 receptors (A1Rs and A2ARs) [144, 176].

The fine-tuning of synaptic activity by this purinergic system is vital for regulating excitatory and inhibitory synapses, which is essential for maintaining neural network activity [177, 178]. Dysregulation of purinergic signaling has been implicated in various pathological conditions, including neurodegenerative diseases [6].

Purinergic signaling is also essential for microglial function, particularly in neuroinflammatory processes. ATP acts as danger signal activating microglia via P2X and P2Y receptors, initiating immune responses [75, 179]. P2X4 receptors mediate pro-inflammatory responses, contributing to conditions like neuropathic pain, epilepsy, and stroke, while P2Y12 receptors guide microglial migration to injury sites, modulating their interaction with neurons and promoting neuroprotective processes [75, 180, 181].

Adenosine, through A2ARs, counterbalances ATP-mediated microglial activation, reducing inflammation by inhibiting cytokine release and regulating potassium currents [157, 182]. This balance ensures that microglia respond appropriately to injury without exacerbating inflammation, highlighting the purinergic system’s role in neuroprotection.

Astrocytes and microglia interact closely during both physiological and pathological conditions, with ATP playing a crucial role in this crosstalk [37, 80]. In response to brain injury, damaged cells release ATP, activating purinergic receptors on both astrocytes and microglia. Microglial ATP release can activate P2Y1 receptors on astrocytes, triggering additional ATP release and amplifying the response [75, 183, 184]. This interaction influences synaptic activity, with astrocyte-derived ATP modulating microglial motility and activation, often via P2Y12Rs.

The bidirectional communication between astrocytes and microglia, mediated by purinergic signaling, is critical for brain development, homeostasis, and injury responses [7]. The balance between ATP and adenosine signaling through P2 and P1 receptors shapes glial activity, impacting processes such as synaptic pruning, and neuroprotection [6].

Purinergic signaling in neuroinflammation: glial regulation and immune dynamics

Purinergic signaling orchestrates an extensive range of immune and inflammatory responses. In the CNS, glial cells serve as pivotal regulators of these processes, shaping the neuroinflammatory landscape (Figure 3). Dysregulation of glial-mediated purinergic signaling plays a central role in the progression of neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and MS [3, 6, 76]. Deciphering the intricate mechanisms underlying this signaling network is essential for advancing therapeutic strategies targeting neuroinflammatory and immune-related disorders.

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Figure 3. 

Purinergic signaling in neuroinflammation: mechanisms and glial cell interactions. CNS: central nervous system; NAD⁺: nicotinamide adenine dinucleotide; CD39: nucleoside triphosphate diphosphohydrolase-1; CD73: ecto-5’-nucleotidase; AD: Alzheimer’s disease; PD: Parkinson’s disease; MS: multiple sclerosis; TNF-α: tumor necrosis factor-alpha; IL-1β: interleukin-1β

In the CNS, ATP released from damaged or necrotic cells acts as a potent “danger signal”, triggering immune responses via purinergic receptor activation on glial cells (Figure 3) [185, 186]. Among these receptors, P2X7 stands out for its pivotal role in assembling inflammasomes—multiprotein complexes that mediate the production and release of pro-inflammatory cytokines, including TNF-α and IL-1β [2, 179, 187]. These cytokines amplify immune responses, creating a feed-forward loop of inflammation that exacerbates neurodegenerative disease pathology [9, 188, 189].

The extracellular environment of the CNS is meticulously regulated by ectoenzymes such as CD39 and CD73, which sequentially hydrolyze ATP into adenosine (Figure 3). This enzymatic conversion shifts the signaling milieu from a pro-inflammatory state dominated by ATP to an anti-inflammatory state mediated by adenosine [33, 190–192]. Acting through P1 receptors expressed on glial cells, adenosine suppresses immune activation, fosters tissue repair, and preserves CNS homeostasis [160, 193, 194]. This dynamic interplay highlights the dual and context-dependent roles of ATP and adenosine in neuroinflammation.

Astrocytes play dual roles during neuroinflammatory events, acting as both supportive and detrimental players in the pathology of neurodegenerative diseases. In conditions such as amyotrophic lateral sclerosis (ALS), astrocytes release ATP, which activates P2X7 receptors on neighboring microglia [195, 196]. This activation initiates a cascade of inflammatory responses that can exacerbate neuronal damage. While P2X7 receptor activation may promote neuroinflammation, it also serves a regulatory role in modulating the inflammatory response, underscoring its complex involvement in the neuroinflammatory process [26]. In vitro studies using astrocyte cell cultures have demonstrated that P2X7 receptor activation leads to the release of pro-inflammatory cytokines, further highlighting the importance of these receptors in the neuroinflammatory milieu.

This interaction amplifies the release of inflammatory cytokines, creating a positive feedback loop that intensifies neuroinflammation [37, 85]. Concurrently, microglia respond to purinergic signals by mounting immune responses. While this activation can be protective during acute phases, prolonged microglial activation contributes to chronic inflammation and tissue damage, which are hallmarks of neurodegenerative diseases [3, 197, 198].

Nicotinamide adenine dinucleotide (NAD⁺) emerges as a critical modulator of purinergic signaling, primarily through its enhancement of P2X7 receptor activation. Elevated NAD⁺ levels potentiate ATP-driven inflammatory signaling, positioning NAD⁺ metabolism as a promising therapeutic target for modulating immune responses and neuroinflammation in the CNS [199–201].

Targeting purinergic signaling offers a compelling approach to managing neuroinflammation and related neurodegenerative disorders. Key strategies include inhibiting ATP release, blocking P2 receptor activation, and modulating ectoenzymes like CD39 and CD73 to regulate glial activation, mitigate inflammation, and slow disease progression. A comprehensive understanding of its role in glial immune regulation is essential for advancing effective therapeutic interventions.

P2X receptors in microglia

P2X receptors, particularly P2X4 and P2X7, are expressed in microglia and play crucial roles in injury, inflammation, and pain responses [180, 231, 232].

P2X4 receptors play a pivotal role in regulating inflammatory responses and are closely associated with neuropathic pain. Activation by ATP induces calcium influx through the receptor, which triggers the p38 MAPK pathway and leads to the release of BDNF [181, 207, 233–235]. BDNF enhances dorsal horn neuron excitability, contributing to pain sensitivity, especially following nerve or SCI. Additionally, P2X4 receptors modulate microglial activity, amplifying pro-inflammatory signaling and cytokine release, which exacerbates neuroinflammation and promotes further neuronal damage. This dual role in pain modulation and glial activation underscores the importance of P2X4 in chronic pain and neuroinflammatory conditions [236, 237].

P2X7 receptors are minimally expressed in homeostatic microglia, with quantitative analyses indicating low surface expression and transcript levels under normal physiological conditions [238, 239]. However, these receptors are highly expressed in meningeal macrophages, highlighting cell-type-specific expression patterns within the CNS [239]. In the context of inflammation and neurodegeneration, microglial P2X7R expression may become upregulated, suggesting a role in pathological states [238]. Notably, P2X7R has been implicated in synaptic pathology and behavioral deficits. In a mouse model of Rett syndrome, pharmacological inhibition of P2X7R ameliorated dendritic spine abnormalities and improved social behavior, further supporting its involvement in neurodevelopmental disorders [238].

P2X7R plays a crucial role in neuroinflammatory signaling. Activated by high extracellular ATP concentrations (> 100 μM), P2X7R induces cation permeability, particularly for Ca²⁺, and activates diacylglycerol lipase (DAGL)-mediated inflammatory pathways [240–243]. These receptors are essential for the release of pro-inflammatory cytokines, emphasizing their significance in neuroinflammation.

Electrophysiological studies have provided no evidence supporting the formation of a large pore by P2X7R. The receptor’s single-channel conductance (15–20 pS) remains consistent even during prolonged ATP exposure, indicating that pore dilation is not an intrinsic property of P2X7R. Instead, the observed increase in membrane permeability to large molecules during sustained P2X7R activation may involve alternative mechanisms, such as the recruitment of pannexin-1 channels, the involvement of other ion channels and transporters, or specific intrinsic properties of the P2X7R channel [244–246]. High-resolution single-channel recordings further distinguish the unitary conductance of P2X7R (approximately 15 pS) from other surface channels in whole-cell recordings of rodent astrocytes.

P2Y receptors in microglia

The P2Y12 receptor is a crucial marker of microglial homeostasis. Under physiological conditions, its activation by ATP or ADP enhances microglial motility and structural responsiveness, promoting lamellipodia formation, which is essential for chemotaxis. This enables microglia to migrate toward ATP, signaling cellular distress or injury [115, 228, 247–249]. Impaired migration in P2Y12-deficient microglia underscores its critical role in chemotaxis [250]. P2Y12 signaling engages pathways such as phosphatidylinositol 3-kinase (PI3K)-Akt and PLC, which are integral to ATP/ADP-driven chemotaxis [251, 252]. Additionally, P2Y12 activation supports cellular protrusions by promoting integrin-β1 accumulation at microglial terminals [253]. While pharmacological inhibition or genetic deletion of P2Y12 disrupts injury-induced chemotaxis, basal activity remains unaffected [190, 191]. Interestingly, state-dependent dynamics reveal reduced microglial motility during wakefulness, mediated by β2-adrenergic receptors rather than P2Y12 [254].

Beyond chemotaxis, P2Y12 contributes to pro-inflammatory signaling, including nuclear factor kappa B (NF-κB) activation via potassium efflux and THIK-1 channels, which affects inflammasome activation [255, 256]. It also influences synaptic plasticity in the visual cortex by promoting microglial process arborization and maintaining neuron-microglia structural connections. Notably, ischemic events reduce microglial-neuron interactions upon P2Y12 inhibition [115, 257, 258]. Both P2Y12 and P2Y13 receptors mediate microglial process motility. In their surveying state, microglia extend and retract processes to monitor the CNS environment. Upon neuronal injury, ATP or ADP release activates P2Y12, driving chemotactic responses through PI3K and PLC signaling cascades, calcium influx, and cytoskeletal rearrangements [259–262]. These mechanisms are crucial in neurodegenerative conditions like MS, where microglia migrate to engulf apoptotic neurons and modulate inflammation [121, 263]. The absence of P2Y13 impairs microglial morphology and ramification, highlighting its chemotactic role [259, 264]. P2Y13 also influences hippocampal neurogenesis and reduces pro-inflammatory cytokine expression in neuropathic pain models [265, 266]. Despite these diverse roles, basal microglial surveillance persists in the absence of P2Y12 and P2Y13, suggesting that normal ATP/ADP levels may not fully activate these receptors [259, 267].

P2Y6 receptors are also vital in regulating microglial functions. P2Y6R initiates phagocytosis of apoptotic cells and neuronal debris. This process involves PLC activation, InsP3 production, and calcium release, enhancing debris clearance [268–270]. Excessive ATP release during pathological conditions, such as epilepsy, may desensitize microglia to ATP signals, impairing phagocytosis [268–270]. Furthermore, P2Y6R shifts microglial behavior from migration to phagocytosis based on environmental cues, demonstrating a complementary role to P2Y12 in microglial dynamics [179, 271–273]. Other P2Y receptors, such as P2Y1 and P2Y4, also contribute to microglial activities. P2Y1 activation by ADP supports migration, while P2Y4 facilitates pinocytosis and migration [274, 275].

A1 and A3 adenosine receptors in microglia

A1R is highly expressed in microglia and suppresses ATP-induced morphological changes without regulating cytoskeletal alterations like A2ARs. A1R activation inhibits calcium influx, reducing ATP-induced cell rounding and membrane ruffling [159, 166, 276, 277]. In TBI and EAE models, A1R activation limits microglial activation and neuroinflammation, as evidenced by comparisons between wild-type and A1R knockout mice [278–280].

A3R interacts with P2Y12R to mediate microglial process extension and migration in response to ATP and adenosine. A3R activation also mitigates neuropathic pain by suppressing microglial activation in the spinal cord dorsal horn [75, 281]. ATP released by microglia is degraded into adenosine via ectoenzymes like CD39 and CD73, establishing a feedback loop that recruits microglia to injury sites [282, 283].

In CD39-deficient mice, microglial migration toward ATP is impaired, while phagocytic activity increases independently of ATP stimulation [283, 284]. Combined CD39 and CD73 deficiency reduces microglial network complexity, which can be restored by elevated adenosine levels [283, 285].

In PD models, CD73-derived A2AR signaling modulates microglial morphology and activation. CD73 deficiency attenuates pro-inflammatory responses to LPS while promoting microglial process extension in injury and PD models [75, 286–288]. Understanding A1R and A3R roles offers promising therapeutic opportunities for managing neuroinflammatory processes in neurological disorders.

A2A receptors in microglia and neuroinflammation

Microglia are central to maintaining CNS homeostasis, dynamically monitoring their microenvironment through process extension and retraction [289, 290]. These movements are driven by chemoattractants like ATP and ADP, which activate P2Y12R on microglia [75, 83]. In response to inflammatory stimuli, microglia transition from a ramified, resting state to an amoeboid, activated form. This transition is characterized by process retraction, increased phagocytosis, and the release of bioactive molecules, which play a key role in neuroinflammatory processes [291, 292]. A2AR regulates this transition. Inflammatory conditions upregulate A2AR expression while reducing P2Y12R levels, promoting reactive microglial states [51, 291, 293]. Adenosine, derived from ATP, activates A2AR pathways, modulating microglial behavior and increasing their sensitivity to inflammatory signals [287, 294].

A2ARs have diverse roles in CNS, including enhancing glutamate release, which exacerbates excitotoxicity, and promoting microglial activation [295, 296]. A2AR antagonists have shown neuroprotective effects in preclinical models of neurodegenerative diseases [297, 298]. In PD, they are particularly effective when combined with dopamine D2 receptor (D2R) agonists, enhancing weakened D2R signaling in the striatum caused by dopaminergic neuron loss and reducing neuroinflammation in the extrapyramidal system [288, 299–303].

The activation of A2ARs exacerbates neuroinflammation in various contexts. In perinatal brain injury models, A2AR activation increases pro-inflammatory markers such as IL-1β, IL-6, and TNF-α, effects mitigated by A2AR antagonists [75, 159]. These antagonists also reverse glucocorticoid-induced microglial hyper-ramification and associated anxiety-like behaviors, while reducing cytokine levels following LPS exposure [75].

The role of glial purinergic receptors in epilepsy: mechanisms of neuroinflammation and therapeutic opportunities

In epilepsy, glial purinergic receptors play pivotal roles in modulating neuronal excitability and neuroinflammation—two interconnected processes critical to seizure initiation and propagation [226]. The P2X7R, predominantly expressed on microglia, serves as a key driver of neuroinflammatory responses. Its activation induces the release of pro-inflammatory cytokines such as IL-1β, TNF-α, and IL-6 via inflammasome activation, exacerbating neuronal excitability and contributing to increased seizure frequency and severity [304–306]. Furthermore, P2X7R activation facilitates ATP release from glial cells, amplifying purinergic signaling and sustaining a pro-inflammatory microenvironment that worsens seizure pathology [240, 307]. Targeted antagonism of P2X7R has shown significant promise in preclinical models by reducing neuroinflammation, dampening seizure severity, and protecting against neuronal loss.

The P2Y1R also plays a critical role in epilepsy pathophysiology. P2Y1R activation regulates astrocytic calcium signaling, which influences glutamate release into the synaptic cleft. Excessive glutamate release, driven by P2Y1R, can lead to excitotoxicity, a process that damages or kills neurons and contributes to epilepsy progression [15, 211, 308]. Therapeutic strategies targeting P2Y1R have demonstrated potential in mitigating excitotoxic damage by modulating glutamate dynamics [211, 309], thereby providing neuroprotection and enhancing seizure control.

Similarly, the P2X4R, primarily localized to microglia, is implicated in neuroinflammatory processes associated with epilepsy. Activation of P2X4R triggers the release of BDNF [235, 310], which exacerbates hyperexcitability in neuronal circuits. Targeting P2X4R may attenuate neuroinflammation and reduce seizure susceptibility [10, 311], offering a dual approach to improving seizure control and limiting neuronal damage.

These findings underscore the critical roles of glial purinergic receptors in epilepsy. Their involvement in maintaining the delicate balance between neuroinflammation and excitability not only drives disease progression but also highlights their potential as therapeutic targets. Advances in purinergic receptor modulation offer promising avenues for developing novel, glial-focused interventions aimed at reducing seizure burden and preserving neuronal integrity in epilepsy.

Glial purinergic receptors in MS: mediators of neuroinflammation and therapeutic targets

In MS, glial purinergic receptors, including P2X7R, P2Y1R, P2Y12R, and P2Y2R, play crucial roles in modulating neuroinflammatory responses and driving disease progression [85, 179, 312]. These receptors regulate the activation of glial cells, particularly microglia and astrocytes, which are central to the inflammatory processes underlying demyelination and neurodegeneration [129, 227].

The P2X7R, predominantly expressed on microglia, serves as a key mediator of pro-inflammatory signaling in MS [59, 85]. Activation of P2X7R triggers the release of cytokines such as IL-1β, TNF-α, and IL-6 via inflammasome pathways, exacerbating the inflammatory cascade. This heightened inflammatory response contributes to myelin sheath degradation and axonal damage, hallmark features of MS pathology [85, 313, 314]. Moreover, sustained P2X7R activation promotes ATP release, creating a feedback loop that perpetuates glial activation and inflammation. Preclinical studies suggest that P2X7R inhibition can significantly reduce pro-inflammatory cytokine production, attenuate microglial activation, and mitigate neuroinflammation, thereby offering neuroprotection and reducing myelin loss [59, 202].

MS is primarily driven by the infiltration and activation of autoreactive lymphocytes, particularly CD4⁺ T cells of the Th1 and Th17 subsets, which secrete pro-inflammatory cytokines such as interferon-gamma (IFN-γ) and IL-17, contributing to CNS tissue damage and immune tolerance breakdown [315, 316]. Purinergic signaling plays a pivotal role in regulating T cell activity during MS pathogenesis, where ATP released from damaged and activated immune cells acts as a DAMP by binding to P2X and P2Y receptors on CD4⁺ T cells, modulating their activation, differentiation, and cytokine secretion [76]. Activation of P2X7R on Th1 and Th17 cells further enhances IL-1β and IL-18 release, amplifying neuroinflammation and lesion progression [57]. Beyond direct T cell modulation, purinergic signaling influences immune cell infiltration and BBB integrity in MS, with adenosine signaling through A2A receptors inhibiting T cell adhesion and reducing cytokine production, while ATP-P2X7 signaling promotes BBB disruption and T cell trafficking into the CNS [58]. EAE, a widely used MS model, has demonstrated that pharmacological blockade of P2X7R reduces both CD4⁺ T cell infiltration and disease severity, highlighting the therapeutic potential of targeting purinergic pathways in MS [32].

The P2Y12R, predominantly expressed on resting microglia, plays a dual role in MS pathophysiology. Under physiological conditions, P2Y12R facilitates microglial surveillance [317]; however, during MS, its activation promotes microglial migration and aggregation at injury sites. This response amplifies inflammatory signaling and contributes to tissue damage [318].

Similarly, the P2Y2R is implicated in astrocytic activation and immune cell recruitment, further aggravating the inflammatory environment in MS. P2Y2R activation induces the release of chemokines and adhesion molecules, facilitating leukocyte infiltration into the CNS and exacerbating demyelination [318–320]. Targeting P2Y2R may help reduce astrocytic-driven inflammation and immune cell infiltration, thereby slowing disease progression.

The P2Y1R is another critical player in MS pathology, primarily through its role in modulating astrocytic calcium signaling. P2Y1R activation influences the release of pro-inflammatory mediators, exacerbating neuronal excitotoxicity [15, 184, 211]. Inhibiting P2Y1R activity has the potential to attenuate astrocyte-mediated neuroinflammation and protect neurons from further damage.

These findings underscore the complex roles of glial purinergic receptors in MS. Their involvement in regulating glial activation, neuroinflammation, and tissue damage highlights their potential as therapeutic targets. Advances in receptor-specific modulation offer promising strategies to mitigate inflammation, slow disease progression, and preserve neurological function in MS patients.

Role of P2X7 receptor-mediated neuroinflammation in AD pathogenesis and therapeutic targeting

In AD, purinergic signaling, particularly through the P2X7R, plays a critical role in the neuroinflammatory processes that contribute significantly to neuronal death and cognitive decline. P2X7R, predominantly expressed on microglia, is activated by extracellular ATP, which is released during cellular stress or injury [66, 202, 321, 322]. Upon activation, P2X7R triggers a cascade of pro-inflammatory cytokine release, primarily through inflammasome-mediated pathways. This cascade exacerbates neuroinflammation, a key pathological feature of AD, and accelerates disease progression by amplifying the inflammatory microenvironment surrounding neurons [189, 240].

Chronic activation of P2X7R contributes to a self-perpetuating cycle of neuroinflammation, characterized by persistent microglial activation. This, in turn, leads to further cytokine production, oxidative stress, and the recruitment of peripheral immune cells into the brain [3, 52, 197, 209]. These processes drive synaptic dysfunction, a critical early event in AD pathology, and neuronal loss, ultimately resulting in the cognitive deficits characteristic of the disease. The neuroinflammatory response triggered by P2X7R activation also contributes to the formation of amyloid plaques and tau tangles, the hallmark proteinopathies of AD. The inflammation surrounding these plaques exacerbates their toxic effects on neurons [3, 52], further accelerating disease progression.

Recent studies have highlighted the potential of targeting P2X7R as a therapeutic strategy to mitigate neuroinflammation and slow the progression of AD. Inhibition of P2X7R has shown promise in preclinical models by reducing the release of pro-inflammatory cytokines, dampening microglial activation, and protecting synaptic integrity [307, 323, 324]. Modulating this receptor may help prevent the amplification of neuroinflammatory pathways that contribute to neuronal damage. Additionally, P2X7R antagonism could help preserve cognitive function by reducing synaptic loss and protecting neurons from excitotoxicity induced by excessive glutamate release.

Purinergic receptors in PD: mechanisms of neuroinflammation and therapeutic implications

In PD, purinergic receptors such as P2X7R, P2X4R, P2Y1R, and P2Y12R play essential roles in the neuroinflammatory processes that drive dopaminergic neuron degeneration, a hallmark of the disease [24, 179, 266]. Activation of P2X7R and P2X4R, both predominantly expressed on microglia, triggers the release of pro-inflammatory cytokines, including IL-1β, TNF-α, and IL-6. These cytokines perpetuate a chronic neuroinflammatory environment that exacerbates neuronal damage, especially in dopaminergic pathways [325]. This microglial activation is a critical contributor to the progressive loss of dopaminergic neurons, leading to motor dysfunction and other PD-related symptoms [52, 326]. Additionally, P2Y1R activation is implicated in the exacerbation of glutamate-induced excitotoxicity, a process that significantly damages neurons and contributes to the ongoing neurodegeneration in PD [309, 327].

P2Y12R, another key receptor expressed on microglia, modulates glial cell responses and amplifies the inflammatory cascade, thereby promoting sustained neuroinflammation and neuronal loss [83, 227]. This receptor’s role in regulating microglial activation and migration further highlights its contribution to the chronic inflammatory environment that characterizes PD pathophysiology.

Given the central roles of these purinergic receptors in promoting neuroinflammation and neuronal damage, targeting them presents a multifaceted approach for mitigating disease progression. Antagonism of P2X7R has shown promise in preclinical models by reducing microglial activation and pro-inflammatory cytokine release, potentially preserving dopaminergic neurons and improving motor function. Similarly, modulating P2Y1R and P2Y12R activity could help limit excitotoxicity and attenuate neurodegeneration, further contributing to the protection of dopaminergic neurons [224, 328, 329]. These therapeutic strategies hold significant potential for slowing the progression of PD and improving patient outcomes by addressing the underlying neuroinflammatory mechanisms that drive disease pathogenesis.

Purinergic receptors in ALS: modulation of P2X7R and P2Y1R as therapeutic strategies to mitigate neuroinflammation and excitotoxicity

In ALS, purinergic signaling, particularly through the P2X7R, plays a pivotal role in driving microglial activation and neuroinflammation, both of which are key contributors to the progressive degeneration of motor neurons [66, 240, 330]. The activation of P2X7R on microglia leads to the release of a variety of pro-inflammatory cytokines, which further amplify the inflammatory milieu in the CNS. This enhanced neuroinflammatory environment exacerbates motor neuron damage, accelerating disease progression [331]. Additionally, P2Y1R contributes to excitotoxicity by modulating glutamate signaling, thereby intensifying neuronal injury [332].

Recent preclinical studies have underscored the potential therapeutic benefits of targeting P2X7R, with antagonists showing promise in inhibiting microglial activation, reducing neuroinflammation [333, 334], and slowing the rate of motor neuron degeneration [335]. Inhibition of P2Y1R, on the other hand, could help reduce glutamate-mediated excitotoxicity, offering further protection against neuronal loss [184, 309]. Together, these strategies provide a multifaceted approach to managing ALS, highlighting the potential of purinergic receptor modulation as a means to delay disease onset, slow progression, and ultimately preserve motor neuron function. These findings suggest that targeting glial purinergic receptors could become a valuable therapeutic strategy in the fight against ALS.

Purinergic receptors in TBI/SCI: pathophysiological roles and therapeutic potential

Following TBI/SCI, purinergic signaling plays a crucial role in the initiation and propagation of neuroinflammatory cascades that exacerbate secondary neuronal damage and tissue degeneration [52, 336–338]. In these contexts, the activation of purinergic receptors, particularly P2X7R, P2X4R, and P2Y2R, is central to the neuroinflammatory response that not only contributes to the immediate tissue injury but also impedes the long-term recovery process [56, 339, 340].

Activation of the P2X7R triggers the release of a wide array of pro-inflammatory cytokines from microglia and astrocytes. This cytokine release amplifies the neuroinflammatory milieu, further exacerbating neuronal injury and impairing the ability of the CNS to effectively repair itself [179, 208]. The inflammatory environment that emerges from P2X7R activation also plays a significant role in the disruption of the BBB and the recruitment of peripheral immune cells [59, 240, 341], thereby prolonging inflammation and increasing the extent of secondary damage.

P2X4R and P2Y2R also contribute to glial cell activation and modulate neuroinflammatory responses. P2X4R is involved in the regulation of microglial activity, where its activation leads to the release of additional pro-inflammatory mediators, such as reactive oxygen species (ROS) and cytokines [75, 232, 342]. P2Y2R, on the other hand, has been implicated in the regulation of astrocytic response to injury and modulating the release of glutamate [150, 343], further intensifying excitotoxicity and exacerbating neuronal death. These purinergic receptors collectively hinder the resolution of inflammation and complicate tissue repair processes, contributing to the progression of both brain and spinal cord injuries.

Targeting these glial purinergic receptors offers a promising strategy to mitigate the harmful effects of neuroinflammation and excitotoxicity following TBI/SCI. In particular, antagonism of the P2X7 receptor has shown potential in preclinical models by reducing microglial activation, cytokine production, and neuronal damage, thereby limiting the extent of secondary injury [240, 313, 344]. Additionally, modulating P2X4R and P2Y2R activity could help in fine-tuning the inflammatory response [14, 342, 345], facilitating glial cell reactivity and promoting tissue repair mechanisms.

Therapeutic approaches that focus on modulating purinergic signaling pathways hold the potential to enhance functional recovery by reducing neuroinflammation, promoting neuroprotection, and supporting tissue repair. By targeting purinergic receptors, it may be possible to mitigate the deleterious effects of inflammation while simultaneously enhancing the endogenous repair mechanisms that are critical for long-term recovery and functional outcomes following traumatic injury to the brain or spinal cord. These strategies, still under investigation, may offer novel therapeutic avenues for improving both the short- and long-term prognosis of TBI/SCI patients.

P2X receptor dysregulation in Rett syndrome and autism spectrum disorder

Emerging evidence increasingly implicates P2X receptors, particularly P2X7 and P2X4, in the regulation of neuroinflammatory processes associated with Rett syndrome and autism spectrum disorder (ASD) [346–349]. P2X receptors mediating Ca²⁺ and Na⁺ influx to regulate neuroimmune signaling, including NLRP3 inflammasome activation, cytokine release, and cell survival pathways [350, 351].

In Rett syndrome, mutations in the methyl-CpG-binding protein 2 (MECP2) gene impair synaptic plasticity and drive chronic neuroinflammation, a key pathological feature of the disorder [352]. P2X7 receptor activation in astrocytes and microglia has been implicated in excessive pro-inflammatory cytokine release, contributing to synaptic dysfunction and neuronal injury. Astrocytic P2X7 activation disrupts synaptic homeostasis by promoting excessive gliotransmitter release (e.g., glutamate and ATP), heightening excitotoxicity and neuronal stress [66, 85]. Concurrently, microglial P2X7 activation amplifies ATP-driven calcium influx, oxidative stress, and a self-sustaining inflammatory feedback loop. This chronic microglial reactivity, characterized by elevated pro-inflammatory cytokines and ROS, exacerbates neurodegeneration, synaptic instability, and phenotypic severity in Rett syndrome [179, 238]. Moreover, P2X7-mediated calcium dysregulation can disrupt neurodevelopmental processes, including dendritic growth and synapse formation, further compounding the disorder’s neurological deficits [238, 353].

Similarly, ASD has been increasingly linked to altered purinergic signaling through the dysregulation of P2X7 and P2X4 receptors [347, 354]. Elevated extracellular ATP levels and overactivation of microglial P2X7 receptors have been observed in ASD models, correlating with increased expression of pro-inflammatory cytokines such as IL-6 and IL-1β. This neuroinflammatory state can impair synaptic pruning and neurogenesis, critical processes during early brain development [355]. Microglial cells exhibit exaggerated activation due to P2X7 stimulation, contributing to aberrant microglial engulfment of synaptic elements and a failure to appropriately regulate neuronal connectivity [86, 304]. Astrocytes, similarly, become reactive in ASD models, leading to excessive ATP release and disruption of normal neuronal-glial signaling pathways [356].

P2X4 receptor involvement in microglial motility and synaptic plasticity has also been implicated in ASD, with aberrant activity contributing to altered neuronal circuit formation and behavioral abnormalities, such as deficits in social interaction and sensory processing [310]. Notably, microglial P2X4 activation has been associated with impaired phagocytic clearance of apoptotic cells and debris, further compromising the neurodevelopmental environment [357].

Therapeutically, targeting the P2X7 receptor has emerged as a promising strategy for mitigating neuroinflammation and its pathological consequences in both Rett syndrome and ASD. Pharmacological inhibitors of P2X7, such as Brilliant Blue G and selective small-molecule antagonists, have shown potential in reducing microglial activation and pro-inflammatory cytokine release in preclinical models [358]. Additionally, genetic approaches, such as conditional knockout models, have demonstrated reduced neuroinflammation and improved synaptic function [222], highlighting the receptor’s role as a viable therapeutic target. Targeting astrocyte activation through P2X7 inhibition may also restore synaptic homeostasis and limit glutamate excitotoxicity in both disorders [359].