Astrocytes, the most abundant glial cells of the mammalian brain, respond to neurotransmitters through Gq-coupled G protein-coupled receptors (GPCRs) that mobilize intracellular Ca2+^{2+}, yet the physiological purpose of this prominent signaling pathway remains unsettled. Agulhon, Fiacco and McCarthy (Science, 2010) showed that neither selectively augmenting nor obliterating astrocytic Gq GPCR Ca2+^{2+} signaling affects excitatory synaptic transmission or short- and long-term plasticity at hippocampal Schaffer-collateral–CA1 synapses, concluding that, at least in the hippocampus, the mechanisms of gliotransmission (astrocyte-to-neuron chemical signaling) need to be reconsidered. Sixteen years of follow-up work has fragmented rather than settled the question: new cell-specific tools have produced in vivo evidence that astrocytic Gq/Ca2+^{2+} signaling modulates cortical network oscillations, late-phase long-term potentiation, memory consolidation, sleep-related adenosine dynamics, and behavior, while the identification of a specialized VGLUT1-expressing astrocyte subpopulation and of release pathways driven by ryanodine receptors (Ca2+^{2+}-sensitive release channels of the endoplasmic reticulum) has partially answered how gliotransmitters could be mobilized. The open problem is to establish a causal, cell-specific, mechanism-resolved account of what astrocytic Gq GPCR Ca2+^{2+} signaling physiologically does in the intact brain — which effector pathways operate under which conditions, regions, and behavioral states — that addresses the divergence between the negative acute-slice data and the positive in vivo data.

Background

Astrocytes are star-shaped glial cells that tile the central nervous system, ensheathing synapses and maintaining the chemical environment of the extracellular space. Unlike neurons, they are not electrically excitable; instead, they respond to neurotransmitters with rises in intracellular Ca2+^{2+}, largely via Gq-coupled GPCRs (G protein-coupled receptors whose Gq alpha subunit activates phospholipase C, producing inositol 1,4,5-trisphosphate, IP3, which opens IP3 receptors on the endoplasmic reticulum and releases stored Ca2+^{2+}; the type 2 IP3 receptor, IP3R2, is the predominant astrocytic isoform). In the 1990s–2000s, a large body of work proposed the 'tripartite synapse' model, in which astrocytes sense synaptic transmitter release and feed back by releasing their own signaling molecules — so-called gliotransmitters such as glutamate, ATP/adenosine, and D-serine — in a Ca2+^{2+}-dependent manner, thereby modulating synaptic transmission and plasticity (long-term potentiation, LTP, and long-term depression, LTD, the persistent activity-dependent changes in synaptic strength that underlie learning; short-term plasticity comprises transient use-dependent changes in release probability).

This picture was challenged by a series of studies from the McCarthy laboratory using mouse lines with astrocyte-specific genetic manipulations. Fiacco et al. (Neuron, 2007) expressed an exogenous Gq GPCR (the mast-cell receptor MrgA1) selectively in astrocytes so that astrocytic Ca2+^{2+} could be raised by an inert ligand, and found no effect on excitatory transmission at Schaffer-collateral–CA1 synapses (the hippocampal pathway from CA3 pyramidal cells to CA1). Agulhon, Fiacco and McCarthy (Science, 2010) completed the bidirectional test: neither selectively augmenting astrocytic Gq GPCR Ca2+^{2+} signaling nor abolishing it (in mice lacking IP3R2, and hence astrocytic Ca2+^{2+} elevations) altered spontaneous or evoked excitatory synaptic transmission, short-term plasticity, or LTP/LTD at these synapses. They concluded that astrocytic Gq GPCR Ca2+^{2+} elevations do not drive gliotransmitter release modulating excitatory, action-potential-mediated transmission in this preparation and that 'at least in the hippocampus, the mechanisms of gliotransmission need to be reconsidered,' leaving open the broader question of the fundamental physiological role of neuron-to-astrocyte Gq GPCR Ca2+^{2+} signaling. The identification questions — how gliotransmitters are mobilized, if at all, and which cellular sources account for previously reported gliotransmission effects — were raised in the subsequent debate literature (Savtchouk and Volterra, 2018; Fiacco and McCarthy, 2018).

The subsequent literature split into two camps. Sceptics (Fiacco and McCarthy, 2018) argued that gliotransmission is a pharmacological artifact of non-physiological stimulation. Proponents (Volterra and colleagues) argued that the negative studies rest on over-simple assumptions — that deleting IP3R2 removes all Ca2+^{2+} sources (ignoring Ca2+^{2+}-induced Ca2+^{2+} release mediated by ryanodine receptors — Ca2+^{2+}-sensitive channels of the endoplasmic reticulum distinct from IP3 receptors — and Ca2+^{2+} entry routes), and that a foreign Gq GPCR fails to reproduce physiological receptor-specific signaling — while an artifact of the widely used dnSNARE mouse line (in which a dominant-negative SNARE protein meant to block astrocytic vesicular release was found by Fujita et al., 2014, to be expressed in neurons) had contaminated part of the positive literature. Beginning around 2019, a new generation of cell-specific tools (CalEx and iβARK to attenuate astrocytic Ca2+^{2+} or Gq signaling in vivo; optogenetic and chemogenetic Gq actuators such as Optoα1AR and hM3Dq; genetically encoded fluorescent neurotransmitter sensors such as the glutamate sensor GluSnFR) produced in vivo demonstrations that astrocytic Gq/Ca2+^{2+} signaling does modulate circuit function in specific contexts — cortical gamma oscillations, late-phase LTP via astrocyte-derived brain-derived neurotrophic factor (BDNF, a secreted trophic factor that supports synaptic plasticity), remote memory consolidation, sleep/wake-dependent adenosine dynamics, and specific behaviors — and single-cell transcriptomics revealed molecularly heterogeneous astrocyte subpopulations, one of which possesses bona fide vesicular glutamate transporter 1 (VGLUT1, the vesicular carrier that loads glutamate into synaptic vesicles). No unified physiological account exists: 2024–2025 field reviews still frame the function of astrocyte Ca2+^{2+} signaling as an open, actively contested question.

Problem Statement

What is the physiological role of neuron-to-astrocyte Gq GPCR-mediated Ca2+^{2+} signaling in the intact central nervous system? Specifically, determine (i) under which physiological conditions — brain region, behavioral or developmental state, and signaling modality — astrocytic Gq-driven Ca2+^{2+} elevations causally modulate neuronal circuit function (synaptic transmission, plasticity, network oscillations, or behavior), and through which molecular effector pathways (vesicular or channel-mediated gliotransmitter release, or Ca2+^{2+}-dependent trophic/metabolic outputs such as BDNF or adenosine); (ii) by what molecular mechanisms gliotransmitters are mobilized from astrocytes under physiological conditions, given the demonstrated heterogeneity of astrocyte subpopulations and Ca2+^{2+} sources (IP3R2, ryanodine receptors, Ca2+^{2+} entry); and (iii) which cellular sources (astrocytic versus neuronal) account for the effects previously attributed to gliotransmission in the literature; and (iv) how the divergence between the negative acute-slice results at hippocampal Schaffer-collateral–CA1 synapses (Agulhon et al. 2010) and the positive in vivo demonstrations of astrocyte Ca2+^{2+}-dependent circuit modulation is to be reconciled. A resolution demonstrating that one body of evidence reflects artifacts or non-gliotransmission mechanisms (as argued for the dnSNARE and D-serine literatures) is a legitimate scientific outcome and is not excluded.

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