Abstract

The traditional view of glial cells as mere supportive tissue has shifted, due to advances in technology and theoretical conceptualization, to include a diversity of other functions, such as regulation of complex behaviors. Astrocytes, the most abundant glial cells in the central nervous system (CNS), have been shown to modulate synaptic functions through gliotransmitter-mediated neurotransmitter reuptake, influencing neuronal signaling and behavioral functions. Contemporary studies further highlight astrocytes’ involvement in complex cognitive functions. For instance, inhibiting astrocytes in the hippocampus can lead to memory deficits, suggesting their integral role in memory processes. Moreover, astrocytic calcium activity and astrocyte–neuron metabolic coupling have been linked to changes in synaptic strength and learning. Microglia, another type of glial cell, also extend beyond their supportive roles, contributing to learning and memory processes, with microglial reductions impacting these functions in a developmentally dependent manner. Oligodendrocytes, traditionally thought to have limited roles postdevelopment, are now recognized for their activity-dependent modulation of myelination and plasticity, thus influencing behavioral responses. Recent advancements in technology and computational modeling have expanded our understanding of glial functions, particularly how astrocytes influence neuronal circuits and behaviors. This review underscores the importance of glial cells in CNS functions and the need for further research to unravel the complexities of neuron–glia interactions, the impact of these interactions on brain functions, and potential implications for neurological diseases.

Introduction

In 1846, Rudolf Virchow used the term “Nervenkitt,” or neuroglia, to describe nerve-glue–like connective substance where the nervous elements are embedded (Somjen, 1988). This conceptualization of glia as support tissue remained the prevalent view for over 100 years. Research studies in the past two decades have dramatically shifted this view providing growing evidence implicating glia in diverse metabolic and neurophysiological functions including regulation of neuronal cell numbers and migration, axon specification and growth, circuit-wide neuronal differentiation, synapse formation and pruning, synaptic communication and plasticity, ion homeostasis and neuro-glial-vascular coupling, and behavior (H. S. Lee et al., 2014;Perea et al., 2014b;Allen and Lyons, 2018). Drawing upon this knowledge, a broader role of glia in the central nervous system (CNS) has begun to emerge, pointing out these cells’ regulatory role in complex, higher-order brain functions and behaviors (Wyart and Prendergast, 2019).

Neuron–Astrocyte Interactions in Sensory Processing

Research has revealed that bidirectional interactions between neurons and glia are critical for the maturation of neural circuits (Kim et al., 2017;Allen and Lyons, 2018;Benfey et al., 2022). Early studies of visual cortex development using cats reared in the dark reported that mature appearance of the visual cortex (i.e., its circuitry and neuronal morphology) develops during a limited postnatal period under the influence of visual experience. Subsequent studies reported that compared with light-experienced animals, dark-reared animals show compromised glial cell proliferation (Gabbott et al., 1986) resulting in a reduced presence of the astroglial marker glial fibrillary acidic protein (GFAP;Stewart et al., 1986;Müller, 1990). These findings provided some of the first evidence suggesting that neuronal activity can influence astrocytic maturation in the visual cortex.

Additional studies of experience-dependent plasticity (EDP)-like changes in astrocytes have examined mouse somatosensory barrel cortex, which expresses a number of different EDP forms. For example, all-whisker deprivation in wild-type mice induced experience-dependent Hebbian depression accompanied by homeostatic upregulation in layer 2/3 barrel cortex that was not observed in mutant mice (IP3R2−/−) lacking the astrocyte-expressed IP3 receptor subtypes (Butcher et al., 2022). Using a whisker deflection protocol and novel calcium indicators for simultaneous two-photon calcium imaging of cortical astrocytes and neurons in awake mice, (Stobart et al., 2018) described fast astrocytic responses that were independent of IP3R2-mediated signaling. This led to conclusions that rapid astrocyte-mediated signals actively participate in synaptic modulation and neurovascular coupling (Stobart et al., 2018).

Indeed, astrocytes have been shown to be responsive to sensory stimuli in multiple different brain regions and are often activated on the timescales similar to those of neighboring neurons. In vivo studies in the visual cortex have shown that astrocytes, like neurons, respond to visual stimuli with distinct spatial receptive fields and tuning to visual features (Schummers et al., 2008). Using two-photon calcium imaging of astrocyte and neuronal responses to visual stimuli in the ferret primary visual cortex,Schummers et al. (2008) showed that astrocytes exhibited stimulus-locked responses to visual gratings with receptive fields similar to those of neighboring neurons (Schummers et al., 2008). Additionally, astrocytes exhibited clear orientation selectivity, with the same preferred orientation as nearby neurons. Finally, blocking astrocyte glutamate transporters affected visually evoked activity in nearby neurons, indicating that astrocytes influence synaptic responses to visual stimuli and hence neuronal stimulus processing. Together, these data were the first to demonstrate that astrocytes can modulate neuronal responses and play an active role in neuronal information representation.

To further probe how astrocyte activity in visual cortex affects neuronal processing,Perea et al. (2014b) optogenetically photostimulated astrocytes in the primary visual cortex while recording nearby neurons. PV+ interneurons exhibited an increase in firing rate in response to astrocyte stimulation, while SOM+ and excitatory neurons had more variable changes in firing rate. PV+ neurons also showed increased visual responses and decreased orientation selectivity, while SOM+ and excitatory neurons again showed both increased and decreased visual responses and corresponding changes in orientation selectivity. Together these studies indicate that astrocyte signaling in the visual cortex influences neuronal integration during sensory information processing, with effects dependent on neuronal subtype. Interestingly, later studies showed that the strength of astrocyte responses to visual stimuli appears to be dependent on brain state, in that astrocyte response amplitude and pupil size were highly correlated (Slezak et al., 2019). Depleting NE inputs to the cortex using the toxin DSP-4 resulted in reduced amplitude of visual responses but did not affect the temporal dynamics of astrocyte responses to visual stimuli, suggesting that NE modulates astrocyte stimulus encoding (Slezak et al., 2019).

In studies investigating the role of astrocytes in olfactory sensory processing and mapping the lines of communication between olfactory neurons and astrocytes, mice were exposed to novel odors. This produced widespread changes in astrocyte transcriptomes, highlighted by a complete shift in the patterns of DNA binding by the Sox9 transcription factor. This shift in Sox9 DNA binding led to the identification of Slc22a3 as an odor-experience-dependent target gene of Sox9 that is induced in astrocytes after exposure to novel odors. Expanding upon these results, the authors reported that astrocytic Slc22a3 is required for olfactory detection and that Slc22a3 functions in part by enabling serotonin to be transported into the nucleus, where it binds histones and serves as an activating epigenetic mark to regulate astrocytic gene expression (Sardar et al., 2023). Among the genes and pathways that are subject to epigenetic regulation by histone serotonylation in astrocytes are key enzymes in the GABA synthesis pathway. Further analysis of these changes revealed the key roles for GABA release from olfactory bulb astrocytes in mediating adaptive responses to sensory input (Sardar et al., 2023). Taken together, these findings suggest that increased neuronal activity in response to sensory experiences can induce EDP-like responses in astrocytes which in turn integrate neural signaling necessary for sensory information processing.

Neuron–Astrocyte Interaction in Learning, Goal-Directed Behaviors, and Social Experiences

In addition to their role in sensory processing, recent work has demonstrated a critical role for astrocytes in learning and other complex behaviors (Padmashri et al., 2015;Delepine et al., 2023). Mice with attenuated astrocyte calcium signaling in motor cortical area M1 show deficits in a motor learning task (Padmashri et al., 2015). Furthermore, astrocytes show hypertrophy in rats that learn motor skills (Anderson et al., 1994). Using a lever push task that requires coordinated neuronal ensemble activity in the primary motor cortex (Peters et al., 2014),Delepine et al. (2023) investigated the contributions of astrocytes to motor learning and the corresponding neuronal ensemble activity in the primary motor cortex. Using astrocyte-specific knockdown of glucagon-like peptide-1 (GLT1) in the motor cortex, they showed that astrocyte uptake of extracellular glutamate is important for developing a stereotyped movement trajectory in a lever push task, but not for learning the cue–lever push association. However, expressing Gq protein-designer receptors exclusively activated by designer drugs (Gq-DREADDs) selectively in primary motor cortex astrocytes to disrupt astrocyte calcium signaling, affected the associatively conditioned hit rate response and response time, in addition to movement stereotypy. These data suggest that astrocytes play a role in motor learning through multiple distinct mechanisms. To evaluate how astrocyte signaling during motor learning influences neuronal activity,Delepine et al. (2023) recorded neuronal calcium activity during the task. During both astrocyte GLT1 inhibition and astrocyte Gq protein activation, neuronal population encoding of movement trajectory was decreased, indicating that astrocyte signaling mediates neuronal ensemble formation during motor learning.

Astrocytes respond to nearby neuronal activity but may also themselves integrate behaviorally relevant information. Astrocytes mediate switches in cortical state by regulating extracellular glutamate (Poskanzer and Yuste, 2016), and recent work indicates that astrocytes respond directly to NE to modulate cortical state (Reitman et al., 2023). Studies in zebrafish have shown that NE–astrocyte signaling modulates behavior as well (Mu et al., 2019). Zebrafish were put in a virtual environment with visual cues to mimic swimming forward. When the visual cues stopped, and swim attempts repeatedly failed to generate perceived movement, the zebrafish eventually stopped swimming. NE neurons encoded the mismatch between motor output and visual input and signaled to radial astrocytes. Radial astrocytes then accumulated the NE failure signals and eventually triggered a change in strategy (Mu et al., 2019). These data suggest that, through regulation of behavioral state, astrocytes themselves play a critical role in integrating neuromodulatory signals to motor learning relevant behaviors.

Astrocyte responses to neuromodulators may be a critical mechanism underlying their integrative function. Indeed, astrocytes in the nucleus accumbens of the ventral striatum respond robustly to dopamine, and, remarkably, may activate a specific adenosine tri-phospahate (ATP) signaling pathway to depress presynaptic transmission at excitatory synapses (Corkrum et al., 2020). Recent work from the Sur lab has highlighted a critical role for NE signaling in reinforcement learning, such that high levels of NE release after a surprising outcome facilitates behavioral optimization (Breton-Provencher et al., 2022). Specifically, mice were trained in a go/no-go task to press a lever on go tones to receive a water reward and to refrain from pressing on no-go tones to avoid a mild air-puff punishment. When mice received the air-puff on false-alarm trials, they changed their behavior on the subsequent trial, so they performed more hits and fewer false alarms. This behavioral updating was mediated by NE signaling. When mice were presented with a surprising water reward, they changed their behavior on the next trial and performed more false alarms, suggesting that NE release modulates behavior in a context-specific manner (Breton-Provencher et al., 2022). The phasic NE activity following a surprising outcome lasts only milliseconds, while the effect of a trial outcome on the next trial requires that the signal persists for several seconds. Since astrocytes have been shown to respond strongly to NE to mediate cortical and behavioral state changes, we propose that astrocytes integrate and alter neuronal signals over longer timescales to facilitate behavioral optimization during reinforcement learning (Drummond et al., 2023). In line with this hypothesis, a recent review has suggested that astrocytes reconfigure neuronal circuits in a context-dependent manner (Murphy-Royal et al., 2023).

Further investigations of whether NE influences memory formation through its corresponding receptors expressed on astrocytes used a combination of virus-mediated, cell-specific knockdown approaches (Gao et al., 2016). These investigators have shown that hippocampal beta-2 noradrenergic receptors (β2Ars), specifically expressed on astrocytes, but not β1ARs, are required for hippocampal-mediated contextual fear memory consolidation—the process required to stabilize the initially fragile long-term memory (Gao et al., 2016). The β2Ars subtypes were also shown to be necessary for the learning-evoked release of lactate from astrocytes, which is necessary to support the neuronal molecular changes essential for long-term memory formation. In sum, from these studies, it emerged that critical mechanisms of long-term memory previously believed to directly act on neurons act instead on astrocytes and that astrocytic functions are necessary for the consolidation and long-term storage of memories (Alberini et al., 2018).

Furthermore, astrocytes have well-established roles in regulating hippocampal circuit function and its associated learning and memory processes (Adamsky et al., 2018;Huang et al., 2020). Learning experiences have been shown to activate ensembles of neurons called engrams (Goode et al., 2020;Josselyn and Tonegawa, 2020) and reactivation of these engrams can elicit memory recall (Tonegawa et al., 2015;Frankland et al., 2019;Luft et al., 2024). However, whether learning experiences activate ensembles of astrocytes that participate in memory recall is unknown. Using fear conditioning as an associative learning paradigm, a not-yet-published study by Williamson et al. showed that in a small subset of astrocytes, c-Fos is induced after fear conditioning. Conditional knock-out of c-Fos in astrocytes resulted in impaired learning and memory and circuit activity. In subsequent experiments, these authors created a suite of viral-based tools that enabled conditional tagging of c-Fos-expressing astrocytes after fear conditioning. Using these tools, a subset of astrocytes, termed learning-associated astrocytes (LAAs), was labeled. These LAAs were near engram neurons that exhibit elevated numbers of synapses. Using DREADD-based chemogenetics, it was found that reactivation of LAAs promotes memory recall in a novel context, suggesting that astrocytes themselves store memories. Additional experiments addressing the mechanisms of this phenomenon have found that knock-out of the astrocytes’ transcription factor NFIA abolishes the capacity of LAAs to stimulate recall (Williamson et al., 2024). Altogether, these results implicate LAAs as a new form of plasticity that is sufficient to provoke memory recall.

Expanding efforts to unravel the role of glia in complex behaviors suggest that neuron–glia interaction also contributes to social experiences. Environmental manipulation studies using social deprivation and environmental enrichment paradigms show widespread effects on glia, neuronal activity, brain function, and cognitive ability (Robinson et al., 2008;Ferle et al., 2020;Curley and Champagne, 2023). For glia, evidence suggest that environmental enrichment promotes myelination (Forbes et al., 2020) while social deprivation impairs oligodendrocyte differentiation and myelination in the prefrontal cortex and leads to aberrant social interactions and working memory (Makinodan et al., 2012). However, how astrocytes respond and adapt to social experiences is not well understood. In a recent study byCheng et al. (2023), juvenile social deprivation was shown to produce minor increases in calcium activity with no changes in astrocyte morphology. Subsequent experiments using single-cell RNA-Seq on the hippocampus revealed widespread molecular changes in astrocyte transcriptomes, highlighted by an increase in TRPA1 expression (Cheng et al., 2023). TRPA1 is a calcium channel that has previously been implicated in astrocyte roles in hippocampal circuit function (Shigetomi et al., 2011), though its role in astrocyte adaptation to social experience remains unknown. Pharmacological and genetic deletion of TRPA1 from astrocytes after social deprivation reversed cognitive deficits and hippocampal circuit impairment. Further studies revealed that hippocampal astrocytes in socially deprived mice release GABA via a TRPA1-dependent mechanism and that genetic inhibition of GABA production in astrocytes also restores cognitive function and hippocampal circuit activities after social deprivation. Together, the results of this study demonstrated that astrocyte function is tuned to social experience, where social deprivation provokes astrocytes to release GABA and thereby suppress hippocampal circuit function.

While the importance of astrocytes and neuron–glia interaction in complex behaviors is becoming increasingly evident, future studies are needed to evaluate the strong possibility that astrocytes act as integrators of stimulus and environmental information to facilitate complex learning processes, goal-directed behaviors, and social experiences.

The Active Cooperation between Astrocytes and Neurons in Long-Term Memory via Metabolic Coupling

Long-term memory formation, the ability to retain information over time about an experience, is a complex function that affects multiple behaviors and is an integral part of an individual's identity (Wilson and Ross, 2003). The understanding of the biological mechanisms underlying memory formation and processes has focused most extensively on neuronal mechanisms. However, neurons and glia have been documented to cooperate to mediate learning and memory.

Studies have provided evidence that the breakdown of astrocytic glycogen (in the adult brain glycogen is stored mainly in astrocytes), i.e., glycogenolysis, is an important mechanism involved in the formation of memories (O'Dowd et al., 1994;Gibbs et al., 2006;Alberini et al., 2018). Furthermore, astrocytic–neuronal metabolic coupling via the glycolysis product lactate was found to be critical in the hippocampus for the formation of episodic-like long-term memories in rats (Suzuki et al., 2011). Using an inhibitory avoidance learning task, it was observed that learning leads to an increase in the extracellular lactate levels in the rat hippocampus, hinting on the involvement of astrocytes in learning and memory. Further investigations showed that astrocytic glycogen breakdown in the hippocampus and lactate release from astrocytes are essential for long-term memory and for the maintenance of long-term potentiation of synaptic strength elicited in vivo but dispensable for short-term memory formation. Disrupting the expression of the astrocytic lactate monocarboxylate transporters 4 (MCT4) or MCT1 led to amnesia, which was rescued by ʟ-lactate but not equicaloric concentration of glucose. Disrupting the expression of the neuronal lactate transporter MCT2 also led to memory impairment, which however was not affected by the administration of either ʟ-lactate or glucose, suggesting that lactate import into neurons is necessary for long-term memory. Subsequent examination of molecular mechanisms of plasticity revealed that glycogenolysis and astrocytic lactate transporters are also critical for the induction of molecular changes accompanying memory consolidation, including the induction of phospho-CREB, ARC, and phospho-cofilin. Similar conclusions were reached byNewman et al. (2011) who provided evidence for a critical role of astrocyte–neuron lactate transport in a spatial working memory task.

Following up on these findings,Descalzi et al. (2019) investigated the nature of the function(s) supported by lactate in neurons. The Krebs cycle substrates pyruvate and ketone body B3HB were shown to be able to functionally replace lactate in rescuing memory impairment caused by inhibition of glycogenolysis or expression knockdown of glia MCTs 1 and 4 in the dorsal hippocampus of rats. In contrast, neither metabolite was able to rescue memory impairment produced by expression knockdown of the neuronal MCT2, suggesting that a critical role of astrocytic lactate is to provide energy for neuronal responses required for long-term memory (Descalzi et al., 2019). Given that de novo protein synthesis—one of the hallmark mechanisms required for long-term memory formation, is an energy-intensive cellular process (Buttgereit and Brand, 1995;Sutton and Schuman, 2006;Costa-Mattioli et al., 2009;D. O. Wang et al., 2009;Richter and Klann, 2009;Topisirovic and Sonenberg, 2011),Descalzi et al. (2019) subsequently asked whether lactate fuels energy required to support learning-induced de novo protein synthesis. They found that lactate or pyruvate is important for learning-induced de novo mRNA translation in both excitatory and inhibitory neurons and for the induced expression of neuronal ARC/ARG3.1. However, these findings did not exclude the possibility that astrocytic–neuronal lactate coupling plays additional critical roles in long-term memory formation, e.g., by regulating redox or cellular signaling through direct activation of the G-protein-coupled lactate receptor GPR81/HCAR1, prostaglandin modulation, cerebral vasoconstriction and vasodilation, or other mechanisms. Altogether, these studies suggested that cooperative molecular mechanisms, working across multiple cell types, i.e., astrocytes and neurons, might server as key biological substrates for complex behavioral responses such as memory formation.

In light of the role of neuron–glia interaction in memory formation, an open question was how the astrocytic and neuronal metabolism changes over development, influence memory processes when the consumption of energy and glucose in the brain is remarkably higher compared with adult. The extremely high energy requirement during development (Kuzawa et al., 2014) likely reflects the energetic costs associated with developmental structural changes, including cell proliferation, migration, differentiation, axonal growth, and refinement of connections (Agathocleous and Harris, 2009;Magistretti and Allaman, 2015;Agostini et al., 2016;Vaarmann et al., 2016) which all can influence learning and memory. Yet, whether and how the increased energy needs of the young brain reflect experience-dependent processes and astrocyte-mediated metabolic changes in response to learning and in memory formation in the juvenile brain was unknown.

Given that formation of episodic-like memories in adult animals requires glycogenolysis and the delivery of astrocytic-derived lactate into neurons in the hippocampus,Cruz et al. (2022) investigated whether similar mechanisms underlie learning and memory in the hippocampus of juvenile rats. Using an inhibitory avoidance learning paradigm, they showed that the juvenile rats, as compared with adult rats, have significantly higher mRNA levels of several glucose metabolism enzymes, higher levels of the monocarboxylate transporters MCT1 and MCT4, and higher levels of the glucose transporters endothelial-GLUT1 and GLUT3 proteins. Furthermore, relative to adults, long-term episodic memory formation in juvenile animals needed significantly higher rates of aerobic glycolysis and astrocytic–neuronal lactate coupling in the hippocampus. Only juvenile but not adult long-term memory formation recruited GLUT3 and neuronal 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (PFKFB3)—a powerful stimulator of glycolysis. In addition, the juvenile hippocampus was found to differentially engage the direct transport of glucose into neurons (Cruz et al., 2022). In sum, these results indicated that glucose metabolism regulation greatly shifts as the brain matures, and, while the adult hippocampus preferentially employed glycogenolysis and astrocyte–neuronal lactate-mediated coupling, the juvenile hippocampus selectively required higher astrocyte–neuronal lactate transport and the direct neuronal glucose transport relying upon glycolytic enzyme PFKFB3 in neurons (Cruz et al., 2022).

These studies provided important evidence addressing the long debated on how the brain regulates the use of glucose to fuel its functions, and particularly whether glucose is directly consumed by all brain cell types, as classically it has been thought, or whether the variety of brain cell types differentially regulate glucose metabolism pathways and the use of different energy substrates including glycogen and lactate (Chih and Roberts, 2003;Aubert et al., 2005;Dienel and Hertz, 2005;Dienel, 2012,2013;Pellerin and Magistretti, 2012;Alberini and Travaglia, 2017;Rich et al., 2019). Altogether these findings underscore the importance of further research about experience-induced metabolic regulation and investigating underlying mechanisms by which glia–neuron interactions regulate behavioral functions in health and in diseases.

Computational Modeling of Neuron–Astrocyte Interactions Predicts Astrocytic Calcium Involvement in Spike-Timing–Dependent Plasticity

In recent years, computational modeling of astrocytic activity has increasingly been used to study the various roles of astrocytes in brain functions (Manninen et al., 2018a,b,2023;Linne et al., 2022). Models of astrocytic function have mainly been of four types. Initially, single-astrocyte and astrocyte network models were developed, but lately, the focus has shifted to neuron–astrocyte interaction models, including single-synapse and network models (Fig. 1A). These models have helped us understand the combined aspects of various mechanisms underlying different brain functions and dysfunctions, test hypotheses easily, and guide experimental studies. Despite the development of almost 150 models for astrocytic calcium signaling by 2020, the advancement of astrocyte models still lags that of neuron models. Much work is needed to make these in silico models more biologically relevant by incorporating additional mechanisms beyond the typically modeled ones. Ideally, these models should incorporate the multiple mechanisms that regulate calcium transport through the plasma membrane, and the mechanisms through which cytosolic calcium levels are regulated intracellularly.

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Figure 1.

Computational astrocyte models developed for calcium signaling.A, 147 models for astrocytes developed by 2020 can be divided into four groups: 41 single-astrocyte models, 24 astrocyte network models, 49 neuron–astrocyte-synapse models, and 33 neuron–astrocyte network models.B, Percentage of models considering different intracellular calcium mechanisms in astrocytes.C, Percentage of models considering different calcium mechanisms on the plasma membrane of astrocytes. Data gathered from the work byManninen et al. (2018b,2023).

Among the intracellular calcium mechanisms, almost all the astrocyte models focus primarily on modeling astrocytic IP3-dependent calcium-induced calcium release (CICR) via IP₃ receptors from the endoplasmic reticulum (ER), Sarco/ER calcium-ATPase (SERCA), and leakage from the ER to the cytosol (Fig. 1B). Other intracellular calcium mechanisms in astrocytes, such as calcium buffers, calcium diffusion, ryanodine receptors (RyRs), intracellular diacylglycerol (DAG), and protein kinase C (PKC)-related pathways, and mitochondrial calcium mechanisms, including mitochondrial calcium uniporter (MCU) and sodium-calcium exchanger (NCX), are less frequently incorporated in astrocyte models (Fig. 1B). Each of these calcium mechanisms is included in similar proportions of models across the four classes (Fig. 1B).

Various mechanisms for transporting calcium through the astrocyte plasma membrane are considered in some models across the four types described, but they are included in fewer than 40% of the models, and significantly less frequently than the most commonly used intracellular calcium mechanisms (Fig. 1B,C). On the other hand, a greater variety of plasma membrane mechanisms than intracellular mechanisms are included in the models. The modeled plasma-membrane calcium mechanisms include efflux, influx, capacitive calcium entry (CCE), gap junctions, neuronal membrane potential (V_m)-dependent influx, plasma membrane calcium-ATPase (PMCA), ionotropic purinergic ATP receptor (P2XR), transient receptor potential vanilloid 4 channel (TRPV4), and voltage-gated calcium channel (VGCC). Various combinations of plasma membrane calcium mechanisms are used in the models, indicating that single-astrocyte and astrocyte network models often focus on similar mechanisms. The same applies to neuron–astrocyte synapse and network models (Fig. 1C).

Conclusion

Recent advances in technologies and computational approaches in cellular and circuit-level neuroscience have transformed the historical view of glial cells as a supportive matrix for the other cell types within the CNS. The past two decades of research describe glia involvement in a diversity of brain functions, from modulation of neurotransmitter release to synaptic remodeling to regulation of neural circuits, functions, and behavior (Knowles et al., 2022;Bataveljic et al., 2024;Bollinger et al., 2024). In particular, it appears that astrocytes significantly contribute synaptogenesis, synaptic transmission, neuronal excitability, and angiogenesis and vascularization (Allen, 2014;Khakh and Deneen, 2019).

Despite significant strides in understanding the role of astrocytes, several questions remain open. One of them concerns the mechanisms by which astrocytes modulate local neurotransmission and behaviors, given that they do not necessitate rapid transmission of electrical signals as a form of communication with other cells. Thus far, assessments of astrocyte activity have been limited to indirect measurements, such as the intracellular calcium levels and the diffusion of second messengers like inositol 1,2,5-triphosphate (IP3), ATP, and diacylglycerol (Paniccia et al., 2022). Although these indirect readouts have implicated astrocytes in sensory processes (Schummers et al., 2008;Butcher et al., 2022), social behaviors (Cheng et al., 2023), goal-directed behaviors (Delepine et al., 2023), and learning (Kol et al., 2020), the exact neurophysiological mechanisms underlying astrocytic modulation of these behaviors await further investigation.

An additional open question is whether other glial cells use similar or different neurobiological mechanisms to modulate neuronal functions. Recent studies have suggested that these diverse mechanisms of interaction between astrocytes, neurons, and other cell types depend on coordinated activity patterns that support the regulation of brain functions and behaviors (Suzuki et al., 2011;Ung et al., 2020). Further studies interrogating how glia interacts with diverse cellular players across the brain may yield novel insights into how alterations in glia–neuron interactions might have an important role in pathogenesis of brain diseases (Khakh and Sofroniew, 2015).

Computational modeling approaches hold promise for addressing complex questions regarding the mechanisms of astrocytic signaling (Manninen et al., 2018b,2023) in their interactions with neurons. Although there are currently >150 models of astrocytic calcium signaling, further refinement of the existing models and the development of new computational models will facilitate better understanding of the mechanisms by which astrocytes modulate synaptic functions, neural circuit activity, and different behaviors (De Pittà and Brunel, 2016;Manninen et al., 2020).

In summary, future research on the role of glia in the brain could help address challenges such as the following: (1) the complexity of the glia interactions with other cell subtypes, (2) the mechanisms that underlie glia–neuron interactions in brain functions and behavior, (3) the heterogeneity of glial cells and their functional roles throughout development, (4) further development of the existing technologies to image and manipulate glial cell interactions in vivo, (5) refinement of computational models of glia functional roles in the brain, and (6) the translation of basic neuroscience findings (seeFig. 2 for a summary schematic).

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Figure 2.

Summary figure that presents the contents included in this review paper.

In conclusion, ongoing research on glial cells and their interaction with neuronal cells in regulation of complex behavior is transforming previously accepted conceptions of their role in the brain. Continuing advances in experimental and computational approaches are poised to provide better understanding of the complexity of glial cells and their involvement in complex brain function in health and diseases.

Footnotes

• M.S.: National Institutes of Health (NIH) grants R01MH126351, R01MH133066, and R01NS130361; Multidisciplinary University Research Initiative Grant W911NF2110328; and The Picower Institute Innovation Fund. G.T.D.: NIH Fellowship F31MH129112. T.M.: Research Council of Finland (Decision No. 355256). C.M.A.: NIH Grants R01MH065635 and R01MH100822. B.D.: NIH Grant R21MH134002.

• The authors declare no competing financial interests.

• Correspondence should be addressed to Mauricio Rangel-Gomez atmauricio.rangel-gomez{at}nih.gov.

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