Recent Insights on Supraspinal Astrocytes in Chronic Pain

Abstract

Chronic pain is a debilitating, life-altering condition that affects a significant portion of the global population, impacting approximately one fifth of people in Europe and one third of people worldwide. Clinical and experimental efforts are increasingly converging to deepen our comprehension of the molecular, cellular and circuit-level mechanisms underlying persistent pain. While most studies have traditionally focused on alterations of nociceptive pathways in neurons, growing evidence highlights the critical role of astrocytes in modulating these pathways and contributing to the development of the central sensitization that characterizes chronic pain. Moreover, astrocytes are also implicated in pain-associated maladaptive behaviours and cognitive impairments. In this context, we review the latest findings on astrocyte involvement in chronic pain and its related mood and cognitive comorbidities.

Graphical Abstract

Created in BioRender. Chiavegato, A. (2025). https://BioRender.com/42hblyi

Introduction

The International Association for the Study of Pain (IASP) defines pain as an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage [1, 2]. Organisms have evolved this mechanism to ensure self-protection against potential dangerous situations. Indeed, pain confers a biological advantage by serving as a warning signal of an injury or medical condition, thus promoting the avoidance of further injuries and encouraging healing and recovery.

In the physiological pathway of nociception, a noxious stimulus is transduced into an electrical signal by peripheral nociceptors, primary afferents whose cell bodies are in the dorsal root ganglia and whose axons contact secondary order neurons in the spinal cord. The nociceptive signal follows then the ascending path across brainstem and is conveyed to the thalamus through the direct spinothalamic tract or the indirect reticulothalamic tract. From the thalamus, the signal is relayed to a number of higher brain centers that are key players in the modulation of sensory, emotional and cognitive dimensions of pain, such as somatosensory cortex (SSCx), anterior cingulate cortex (ACC), insular cortex, prefrontal cortex, amygdala and hippocampus (HIP). From these regions, modulatory signals are conveyed into a descending path encompassing brainstem regions such as the periaqueductal gray (PAG) and rostroventral medulla (RVM), acting as a natural pain relief system that modulates the transmission of pain signals.

Under certain conditions, the sensation of pain persists beyond the normal recovery period of an injury or illness, becoming pathological and chronic and developing into a disease itself. Constant or recurrent pain can also be linked to persistent inflammation such as in diabetes or arthritis, or arise from a disease or its treatment, such as in the case of cancer and chemotherapy. The official definition of IASP describes chronic pain as a pain that persists or recurs for more than 3 months [3]. This debilitating condition, highly diffuse worldwide [4–6], is characterized by spontaneous pain (perceived in the absence of external stimuli), allodynia (pain in the presence of non-painful stimuli) and hyperalgesia (enhanced sensitivity to painful stimuli). Among the different forms of chronic pain, the most prevalent is neuropathic pain, which is caused by damage to or dysfunction of the somatosensory nervous system [7].

The development of neuropathic pain involves the central sensitization of nociceptive circuitry through functional and anatomical remodelling of neural connections, leading to dysfunctional circuit activation also in the absence of a painful stimulus. One fundamental mechanism that underlies the transition from acute to chronic pain in the central nervous system is thus synaptic plasticity. In acute pain, synaptic changes such as increased neurotransmitter release and receptor sensitivity temporarily enhance pain perception, serving as an adaptive response to injury, but these mechanisms become maladaptive during pain chronicization [8], with maladaptive neuroplasticity potentially raising at different levels in pain circuit. Neuropathic pain is accompanied by different factors, such as neuroinflammation, changes in long-term potentiation (LTP), altered excitatory/inhibitory balance and reduced synaptic flexibility, which all contribute to persistent hyperexcitability and exaggerated responses to stimuli even in the absence of ongoing tissue damage [8–12]. Concurrent upregulation of proteins involved in the modulation of synaptic plasticity and LTP, e.g. NMDA receptors and neurotrophic factors such as BDNF, leads to enhanced excitatory transmission and increased neuronal excitability [13, 14]. Alongside this increase in excitatory transmission, chronic pain is also characterized by reduced GABAergic inhibition, which has been attributed to decreased gamma-aminobutyric acid (GABA) synthesis, impaired receptor function or altered interneuron activity [11, 15]. The loss of GABAergic tone not only amplifies pain signals but also contributes to the spread of pain beyond the original injury site. Over time, the central nervous system undergoes a shift in which plastic changes that are usually reversible become more stable and resistant to modulation. This loss of synaptic adaptability may contribute to establish pain circuits and impair the ability to return to a physiological state [8].

Neuroglial Cells in Chronic Pain

Within the central nervous system (CNS), glial cells play fundamental roles in maintaining brain homeostasis and modulating brain function. We here focus on astrocytes, acknowledged partners of neurons in different brain circuits underpinning physiological processes, as well as in the development of neuropathological conditions [16–22]. Many studies have highlighted the role of astrocytes and microglia in the regulation of chronic pain within the spinal cord [23, 24]. Both cell types undergo molecular and morphological changes during the transition from acute to chronic pain, releasing cytokines and other molecules that promote the establishment of a pro-inflammatory milieu and ultimately affect neuronal activity. The analysis of the timecourse of glial cells recruitment reveals that microglia activation peaks and resolves much earlier than that of astrocytes during chronic pain development, revealing a closer temporal association between astrocyte activation and chronic pain induction and upkeep. For a comprehensive synopsis of the literature on the inflammatory pathways triggered in spinal astrocytes and microglia during chronicization of pain, we refer the readers to a number of previous publications [25–33]. While much of the research has focused on the spinal cord, fewer studies have explored how astrocytes in the brain respond or contribute to chronic pain. Nevertheless, emerging evidence suggests that these cells exhibit region-specific changes in various brain areas in different models of neuropathic pain. The aim of this review is to gather the most recent evidence on the ability of astrocytes to modulate chronic pain in higher brain regions, ranging from the thalamus to somatosensory cortex, up to anterior cingulate cortex and amygdala. In addition, we will add some examples of recent literature revealing how astrocyte modulation can influence mood disorders or cognitive outputs associated to chronic pain. In the graphical abstract, we summarize the main topics and evidence we will present in this review. Before delving into this, however, we will briefly describe key aspects of astrocyte excitability for a better understanding of the mechanisms and implications discussed throughout the review.

Astrocyte Excitability

Cell excitability can be defined as the intrinsic ability of a cell to generate rapid changes in specific physiological parameters in response to external - or internal - stimuli. This ability relies on the presence of tightly controlled electrical and/or chemical gradients across cellular membranes, combined with the expression of ion channels and/or transporters that can be switched between OFF and ON permeability states to allow ion fluxes. While the term excitability commonly recalls electrically excitable neurons, voltage-gated channels and action potential firing, another universal form of cell excitability is enclosed in intracellular Ca²⁺ changes, which are often triggered by metabotropic (G-protein coupled) receptor activation and Ca²⁺ release from intracellular stores. Astrocytes are electrically non-excitable cells that rely on tightly regulated gradients of various ions, such as Ca²⁺, Na²⁺, Cl⁻ and H⁺, to sustain cell homeostasis [34–37]. While multiple ions with their associated signaling pathways contribute to astrocyte function [38], the majority of scientific research has historically focused on astrocyte excitability mediated by dynamic intracellular Ca²⁺ signaling. Accordingly, most studies cited in this review employed Ca²⁺ dynamics as a readout for astrocyte activity during physiological or pathological processes, as well as modulatory tools to promote or inhibit astrocyte intracellular Ca²⁺ signal. To aid in the comprehension of the experimental work presented throughout this review, we here provide a brief introduction to these topics.

Astrocytes express metabotropic receptors for many neurotransmitters and neuromodulators, such as glutamate, GABA, adenosine triphosphate (ATP), acetylcholine, norepinephrine, dopamine, endocannabinoids [17, 39–48]. These receptors are characterized by high affinity, slow desensitization and multi-step signal amplification, associated to intracellular pathways using Ca²⁺ or cAMP as second messengers. Following metabotropic agonist binding to Gq receptors, the activation of phospholipase C (PLC) leads to inositol triphosphate (InsP₃) production and Ca²⁺ release along electrochemical gradient through InsP₃ receptors at the endoplasmic reticulum membrane [48]. Intracellular Ca²⁺ signals in astrocytes are highly variegate in term of both spatial and temporal features, especially at the level of fine processes contacting synapses [49], and their analysis along astrocyte soma and subcellular compartments is a current challenge in neuroglial research. Notably, increases in the intracellular concentration of Ca²⁺ in astrocytes can lead to a regulated release of gliotransmitters (including glutamate, D-serine, ATP, GABA etc.), which have been shown to affect synaptic transmission, as well as to modulate circuits and behaviour [50, 51]. Monitoring Ca²⁺ signals in astrocytes is thus crucial to decode their involvement in brain circuit regulation.

Similarly crucial is the ability to modulate Ca²⁺ signals in astrocytes. Optogenetic and chemogenetic approaches have been developed to allow targetable expression of non-native ionic channels and metabotropic receptors in different cells, with the goal of controlling cell activation or inhibition. However, while neurons are easier to tune through spatially and temporally controlled application of depolarizing or hyperpolarizing stimuli, astrocyte manipulation is more complex. For example, optogenetic activation of ion fluxes across astrocyte membrane cannot mimic the variegate spatial and temporal dynamics of intracellular Ca²⁺ signals accompanying release from internal stores. Chemogenetic activation of DREADDs (Designer Receptors Exclusively Activated by Designer Drugs), conversely, is closer to physiological astrocyte activation since it triggers the intracellular cascade initiated by G-protein receptors. However, while Gq DREADDs are consistently reported to induce Ca²⁺ activity in stimulated astrocytes, it is more difficult to interpret data obtained with Gi DREADDs. Although these latter receptors are classically coupled to cAMP (cyclic adenosine monophosphate) pathway, they have been reported in different studies to result in Ca²⁺ increases [52] or in negative Ca²⁺ modulation [53]. Despite these limitations in determining the exact direction of astrocyte modulation, evidence of circuit and behavioural effects following astrocyte manipulation supports the proof of concept that astrocytes can play a significant role in regulating brain physiology and pathology.

Finally, it is noteworthy to clarify here the expression “astrocyte activation”, which is commonly used to define distinct and sometimes unrelated events. On one hand, the term activation can refer to the development of reactive astrogliosis, a state characterized by alterations in astrocyte gene expression, including up-regulation of glial fibrillary acidic protein (GFAP), cytokines and inflammatory mediators, as well as changes in their morphology and complexity, potentially resulting in atrophy or hypertrophy (for appropriate nomenclature, please refer to the consensus article [54]). On the other hand, activation can also refer to the initiation of intracellular signals in astrocytes, such as Ca²⁺ increases, commonly elicited by chemical, chemogenetic or optogenetic stimulation. Finally, we can also describe activation of Gi DREADDs in astrocytes, regardless of their ultimate stimulatory or inhibitory effect on Ca²⁺ signaling. We therefore encourage readers to adopt a critical perspective when interpreting the term “astrocyte activation”, as its meaning can vary substantially depending on the experimental context.

Thalamus

The thalamus is the first supraspinal region to receive the nociceptive stimuli from the spinal cord and it relays them to the somatosensory cortex for further processing [55]. Neuroimaging study in patients suffering from chronic low back pain have showed an increased expression of translocator protein (TSPO), a marker of activated astrocytes and microglia, particularly in the thalamus and in the somatosensory cortex, highlighting the possible involvement of neuroglial cells in human chronic pain [56].

A role for the thalamic astrocytes in modulating neuropathic pain associated with nerve damage in diabetes mellitus has been recently described [57]. In a rat model of diabetic neuropathic pain, characterized by mechanical hindpaw allodynia, the authors observed various alterations in thalamic astrocytes. Specifically, astrocytes in the paraventricular thalamus (PVT) from diabetic rats exhibited increased expression of the astrogliosis marker GFAP, along with significant enlargement of soma size, increased number of branches and enhanced structural complexity. Chemogenetic activation of the Gq signaling pathway in PVT astrocytes of control rats was sufficient to induce morphological changes similar to those described in diabetic rats, along with mechanical hindpaw allodynia. Conversely, chemogenetic activation of the Gi signaling pathway in PVT astrocytes reversed the astrogliosis-related alterations and, most importantly, exerted an analgesic effect in diabetic rats with mechanical allodynia. Overall, these findings suggest a critical role for PVT astrocytes in mediating diabetic neuropathic pain. The authors also investigated the activity marker c-fos in PVT neurons and found a significant decrease of c-fos expression in rats with diabetic neuropathic pain. Remarkably, activation of Gi signaling in PVT astrocytes– which reduced allodynia– increased the number of c-fos positive GABAergic neurons in diabetic neuropathic pain rats. Consistently, chemogenetic activation of PVT GABAergic neurons relieved mechanical allodynia in diabetic rats, while their inhibition induced mechanical allodynia in control rats. Collectively, these results highlight the role of the PVT in diabetic neuropathic pain, with astrocytic Gq and GABAergic neuron activation exerting opposing effects on pain processing. In addition, these data suggest that astrocyte activation during the development of mechanical allodynia may promote neuropathic pain by inhibiting GABAergic neurons.

Amygdala

The thalamus, besides relaying information to cortical areas, is also directly and indirectly connected to the amygdala [58], a region primarily involved in the emotional perception of pain but also in pain modulation [59, 60]. Beyond investigating the role of thalamic astrocytes in diabetic neuropathic pain rats, the group of Chang-Xi reported that astrocytes from the basolateral amygdala (BLA) also play a crucial role in the same model [61]. Similarly to what reported for the thalamus, GFAP expression was markedly increased in the BLA of diabetic rats and chemogenetic activation of Gi or Gq signaling in BLA astrocytes produced an analgesic effect in diabetic rats and an algesic effect in control rats, respectively. This dual effect was associated with bidirectional changes in BLA expression levels of inflammatory cytokines such as TNF-α, IL-1β, CXCL1 and MCP-1, which were reduced after astrocytic Gi activation in diabetic rats and increased after astrocytic Gq activation in control rats. The authors also explored the mechanism of action of koumine, a novel alkaloid analgesic that does not induce tolerance or dependence, and could therefore be safer than conventional analgesics such as gabapentin. Intragastric administration of koumine, which exerted an analgesic effect in diabetic neuropathic pain rats, was found to attenuate GFAP expression in BLA astrocytes and, consistently, to suppress the inflammatory response in the BLA. Collectively, these results highlight the involvement of BLA astrocytes in the pathophysiology of diabetic neuropathic pain.

Hypothalamus

Paraventricular hypothalamus (PVH) is a supraspinal region centrally involved in perceiving and decoding somatic and visceral pain [62], as well as a potential center of sensitization in chronic pancreatitis due to its functional connection with the pancreas and its known role in visceral pain during pancreatic cancer [63]. Chronic upper abdominal pain is a common symptom in patients with chronic pancreatic inflammation. A recent study by Luo and colleagues investigated the role of hypothalamic astrocytes in this type of visceral pain. In a chronic pancreatitis model induced by intraperitoneal injection of caerulein, the authors confirmed persistent abdominal mechanical allodynia lasting up to 28 days. Interestingly, fiber photometry revealed astrocyte Ca²⁺ hyperactivity in response to abdominal stimulation in caerulein-injected mice, whereas sham mice showed no such activation. Elevated spontaneous Ca²⁺ activity was also observed in injected mice, compared to control mice. To investigate the role of astrocyte Ca²⁺ hyperactivity in abdominal allodynia, the authors used the Gi-coupled kappa-opioid receptor (KORD), activated by the ligand Salvinorin B. In brain slices from mice expressing KORD in PVH astrocytes, treatment with Salvinorin B reduced both basal Ca²⁺ levels and spontaneous Ca²⁺ elevations. Although in vivo suppression of astrocyte Ca²⁺ activity was not directly assessed, KORD activation successfully reduced abdominal mechanical allodynia in mice with chronic pancreatitis. In these mice, the authors also observed hyperactivity of PVH glutamatergic neurons and down-regulation of the astrocyte glutamate transporter GLT-1 (the rodent homolog of human EAAT2). Notably, activation of Gi signaling in astrocytes via KORD was sufficient to reverse both alterations, suggesting that the reduction of astrocyte Ca²⁺ signaling in mice with chronic pancreatitis exerts an analgesic effect by restoring GLT-1 expression levels and normalizing extracellular glutamate levels and neuronal excitability. Supporting this hypothesis, systemic administration of the GLT-1 agonist LDN-212,320 also ameliorated PVH glutamatergic neuron hyperactivity and abdominal pain in caerulein-injected mice. Although the precise mechanisms underlying astrocyte Ca²⁺ hyperactivity and GLT-1 dysfunction remain unexplored, these results highlight the crucial role of astrocytic GLT-1 in the PVN during chronic pancreatitis-associated allodynia.

Primary Somatosensory Cortex

The primary somatosensory cortex (SSCx) receives and processes afferent thalamocortical (TC) inputs from the ventral posterolateral (VPL) nucleus of the thalamus. It serves as the principal region where peripheral signals are decoded and interpreted as noxious stimuli [64]. Due to the importance of this region in pain perception, several studies have investigated the involvement of the somatosensory cortex in the development of chronic pain. As already mentioned, neuroglial activation has been reported in the somatosensory cortex from patients with chronic low back pain [56].

Other studies conducted in animal models have further elucidated the role of astrocytes from the SSCx in chronic pain. Kim and colleagues, using partial sciatic nerve ligation (PSL) to model chronic pain in mice, demonstrated a transient increase in dendritic spine turnover in the SSCx during the first week following PSL. Notably, this narrow time window coincides with the onset of mechanical allodynia [10]. In a successive work, the same group revealed that astrocytes in the somatosensory cortex exhibit an increase in the frequency of basal intracellular Ca²⁺ transients, temporally correlated with both increased dendritic spine formation and allodynia development. Astrocyte Ca²⁺ signals were shown to be sufficient and necessary to enhance spine formation in somatosensory cortical dendrites, revealing the central astrocyte contribution to maladaptive synaptic remodelling. Indeed, photolysis of Ca²⁺ caged compounds to generate Ca²⁺ elevations in SSCx astrocytes increased dendritic spine turnover in control mice, while buffering astrocyte Ca²⁺ signals through sustained BAPTA-AM infusion in PSL-injured mice prevented spine turnover changes. The increase in astrocyte Ca²⁺ activity and the consequent synapse remodelling underlying the pain sensation was due to the upregulation of the metabotropic glutamate receptor-5 (mGluR5) receptors within the first week following the PSL. Notably, pharmacological blockade of mGluR5 during this period significantly suppressed mechanical allodynia for at least one month– as did BAPTA-mediated astrocyte Ca²⁺ buffering or the use of transgenic mice devoid of IP₃ receptors in astrocytes (IP3RKO), characterized by reduced Ca²⁺ increases [65]. Years later, the same group further confirmed the central role of astrocytic mGluR5 for the development of chronic pain. After PSL, mGluR5 upregulation occurs in astrocytes in both the somatosensory cortex and the spinal cord. Danjo and colleagues, using a Cre-inducible mouse line, were able to knock out mGluR5 specifically in cortical astrocytes without affecting spinal astrocytes. In line with previous findings obtained with pharmacological mGluR5 inhibition, astrocyte-specific mGluR5 knockout animals did not develop allodynia following PSL. Moreover, deletion of mGluR5 reduced spontaneous and evoked Ca²⁺ activity of astrocytes in SSCx, reversed astrocyte-dependent synaptogenesis and prevented neuronal activity enhancement induced by PSL. These findings are particularly fascinating, since targeting a single astrocytic receptor in the SSCx is sufficient to completely prevent the development of neuropathic pain [66].

Following a similar line of research, Takeda and collegues investigated the effect of manipulating astrocytes after allodynia was already established. The rationale underlying this work resides in the hypothesis that while early astrocyte activation can induce maladaptive connections, a successive astrocyte reactivation may revert them by exploiting the same mechanism, i.e. by triggering spine turnover. Activation of astrocytes two weeks after PSL was combined with the transient blockade of noxious afferent input (using TTX or lidocaine therapy). Increasing astrocytic Ca²⁺ activity with low intensity transcranial direct current stimulation (tDCS) or via DREADDs approach significantly relieved allodynia-like behaviours in mice, with effects lasting up to two months. Mechanistically, astrocyte activation via Gq-coupled DREADDs was sufficient to significantly and specifically reduce the PSL-induced maladaptive synapses by increasing spine turnover [67]. Similar observations were made in another study using a chemogenetic approach to manipulate astrocytic Ca²⁺ activity. Gq-coupled stimulation of astrocytes in the somatosensory cortex of rats suppressed acid-induced thermal hyperalgesia, while a Gi-coupled approach blocked acupuncture-induced analgesia (electrical stimulation using acupuncture needle) [68].

Taken together, these studies suggest that astrocytes, and more specifically the intracellular Ca²⁺ activity of astrocytes in the SSCx are involved in the establishment of the maladaptive circuits underlying pain sensation in the neuropathic models. Interestingly, a second activation of astrocytes appears to contribute to the restoration of physiological conditions acting on spine turnover.

Anterior Cingulate Cortex

The anterior cingulate cortex (ACC) is a limbic region involved in numerous emotional and cognitive processes. It also serves as the major hub for affective pain perception and associated behavioural responses, such as aversion, anxiety and memory formation [69, 70]. Cumulative evidence from animal models demonstrate that chronic pain and anxiety-like behaviours are associated with ACC neuronal hyperactivity [71, 72], while ACC neuronal response in the human brain during peripheral nociception correlates with the unpleasantness of pain [73, 74]. In this context, a critical role is highlighted for ACC astrocytes in the modulation of the emotional response to pain and in the induction of anxiety-like or aversive behaviours.

ACC hyperactivation is described by Wei and colleagues in a rat model of chronic inflammatory pain, where astrocyte activation - characterized by gliosis and neuroinflammation, with concurrent up-regulation of GFAP, TNFα, Il-6 and IL-1β - contributes to the comorbidity of chronic pain with anxiety. The authors demonstrated that interfering with astrocytic Ca²⁺ activity by a Gi-coupled DREADDs approach was sufficient to alleviate both allodynia and anxiety behaviour induced by Complete Freund’s adjuvant (CFA) injection in the hind paw of rats [75]. Similarly, in a mouse neuropathic pain model induced by nerve injury, ACC astrocytes displayed the ability to modulate anxiety-like behaviour, with their Gi-coupled chemogenetic activation reducing neuronal c-fos expression and ameliorating anxiety-like behaviours tested via open-field and elevated plus maze tests [76]. In this work, however, the possible effects of astrocyte modulation on allodynia were not assessed.

Other studies suggest an active role of astrocytes of the ACC in negative emotions underlying maladaptive behaviours under chronic pain condition. Interestingly, ACC astrocytes contribute to pain-related aversive behaviour in rats [77]. In particular, in a visceral chronic pain model, β2-adrenergic receptors (β2AR) expressed by astrocytes are activated by noradrenaline from locus ceruleus (LC) projections on ACC and are responsible of pain-evoked aversive behaviour. Indeed, while optogenetic activation of LC projections to ACC astrocytes promotes the consolidation and memory of pain-aversive learning, specific knockdown of astrocytic β2AR by miRNAi impairs animal performance in the conditioned place aversion test. In a different study on escape/avoidance behaviour in rats with induced allodynia, local injection of L-alpha-aminoadipate, a cytotoxin relatively specific for astrocytes, completely reverses the escape/avoidance behaviour in rats without affecting the CFA-induced mechanical allodynia [78], supporting distinct roles of ACC astrocytes in affective pain processing.

In a mouse model of neuropathic pain, Shen and collegues describe an abnormal Ca²⁺ activity of ACC astrocytes, caused by the up-regulation of astrocytic mGluR5 and associated with elevated extracellular glutamate concentration, enhanced synaptic transmission and allodynia. Selective attenuation of Gq signaling pathway through iβARK [79] or targeted RNA silencing of mGluR5 in astrocytes were each sufficient to restore normal astrocytic Ca²⁺ activity and significantly reduce both synaptic hypexcitability and mechanical allodynia. Additionally, Gq- coupled chemogenetic activation of ACC astrocytes exacerbated neuropathic pain compared to controls, although it was insufficient to induce pain by itself, suggesting a pain-facilitating role of these cells [80]. Interestingly, as already described in a previous section, comparable results on mGluR5 re-expression were observed in the somatosensory cortex. These findings suggest that mGluR5 plays a crucial role in modulating the transition from acute to chronic pain across different brain regions involved in pain processing, highlighting the mGluR5 pathway as a promising therapeutic target for reducing allodynia.

Notably, rodent models of sciatic nerve ligation or chronic inflammatory pain often show an imbalance between glutamate and GABA, with a shift toward elevated glutamate levels [71, 75, 81]. Yamashita and colleagues suggest a model in which the excess of glutamate induces Ca²⁺ dependent translocation of the GABA transporter 3 (GAT3) to the astrocyte membrane. Interestingly, optogenetic activation of ACC astrocytes led to increased wakefulness and reduced NREM sleep, mimicking conditions experienced by patients suffering sleep disorder in neuropathic pain [71].

Hippocampus

The hippocampus plays a pivotal role in spatial memory and emotional regulation and it has been implicated in the development of mood disorders following nerve injury [82–84], as well as in comorbid cognitive impairments [85]. Notably, patients suffering from trigeminal neuralgia exhibit reduced hippocampal volume [86], and studies in rodent models of persistent pain have revealed multiple hippocampal abnormalities, including impaired function and altered neurogenesis [84, 87]. The research field on the contribution of hippocampus to pain modulation is relatively new. A recent work, for example, highlighted the direct participation of ventral hippocampus to the modulation of pain behaviours, through ensembles of nociceptive neurons showing inhibitory responses to noxious stimuli and encoding pain modality. Activation of the projectons of these neuronal ensembles to basolateral amygdala and infralimbic cortex is sufficient to mitigate pain behaviour [88].

To our knowledge, no study has yet addressed the possible role of astrocytes in hippocampal pathways contributing to chronic pain. We here report some examples of the involvement of astrocyte from ventral hippocampus in behavioral and mood alterations associated to neuropathic pain. The study of Fiore and Austin, for instance, disclosed an remarkable correlation between hippocampal neuroglia activation and exploratory behaviour of rats. The authors exposed rats to sciatic nerve chronic constriction injury (CCI) and observed different behavioural phenotypes. While all rats developed mechanical allodynia and impaired motor activity, only a subset of animals exhibited long-lasting deficits in exploratory behaviour. Interestingly, only this subgroup of rats exhibited astrocytic atrophy, with decreased GFAP expression, reduced IBA1 immunoreactivity, and elevated levels of pro-inflammatory mediators such as IL-1β and MCP-1, particularly in the ventral hippocampus [89]. While this work is more descriptive and correlative, another recent study used direct, functional activation of astrocytes from dorsal hippocampus via Gq DREADDs in a spared nerve injury (SNI) model. This paradigm induces neuropathic pain and cognitive dysfunction, associated with decreased levels of IL-33, a cytokine involved in synaptic plasticity and protection from oxidative stress. Astrocyte chemogenetic activation led to the recovery of IL-33 levels, reduced oxidative stress, restored synaptic connectivity and finally improved cognitive function [90]. In a model of clinical trigeminal neuropathic pain, chronic constriction of the infraorbital nerve (CION) induced allodynia and anxiety/depression-like behaviour, as well as gliosis in the CA1 region of the ventral hippocampus. Furthermore, CION triggered microglia activation, which in turn release IL-17 A to recruit astrocytes and induce a Ca²⁺-dependent ATP release from these cells. Notably, inhibiting astrocytic intracellular Ca²⁺ elevations, blocking connexin 43 hemichannels or preventing ATP degradation via the microglial CD39 enzyme were each sufficient to rescue the anxiety/depression-like behaviour. The authors ultimately propose a model in which anxiety/depression-like behaviour following CION depends on microglial activation inducing ATP release by astrocytes. This ATP is then degraded by microglial CD39 into adenosine, which increases the excitability of CA1 pyramidal neurons [91]. These findings support a deep connection between dysfunctional hippocampal glial communication and mood alterations in chronic pain.

Conclusions and Future Directions

In conclusion, a substantial body of evidence highlights the central role of supraspinal astrocytes in the modulation of chronic pain across multiple brain regions involved in nociceptive processing. Several studies have shown that astrocytes are active players in different processes underlying chronic pain such as neuroinflammation, synaptic remodeling and regulation of excitatory/inhibitory balance. In particular, astrocytic intracellular Ca²⁺ dynamics and mGluR5-mediated signaling appear to be crucial for the transition from acute to chronic pain. Notably, astrocytes are not only involved in the establishment of allodynia, but they seem to play a critical role also in modulating the emotional and cognitive dimensions of chronic pain. While additional studies are necessary to fully recognize the role of astrocytes in brain regions involved in pain processing, the findings so far obtained by the scientific community encourage further research to pave the way for innovative therapeutic strategies targeting astrocyte-specific mechanisms, potentially offering more selective and long-lasting relief for neuropathic pain and its associated neuropsychiatric comorbidities.

Author Contributions

A.D.S. wrote the initial draft of the manuscript and prepared the graphical abstract. M.G.G. expanded the sections on “Thalamus”, “Amygdala” and “Hypothalamus”. A.C. expanded the section on “Anterior Cingulate Cortex”. M.Z. expanded the Introduction and the section on “Hippocampus”. All authors reviewed the manuscript.

Funding

Open access funding provided by Consiglio Nazionale Delle Ricerche (CNR) within the CRUI-CARE Agreement. This work was supported by Fondazione Telethon (Grant number GMR23T1234 to MZ).

Data Availability

No datasets were generated or analysed during the current study.

Declarations

Competing Interests

The authors declare no competing interests.