The gliotransmitter glutamate in different brain regions modulates neuronal excitability and synaptic transmission through a variety of mechanisms. Among the hallmarks of astrocytic glutamate release ...are the slow depolarizing inward currents (SICs) in neurons mediated by N‐methyl‐d‐aspartate receptor activation. Different stimuli that evoke Ca2+ elevations in astrocytes induce neuronal SICs suggesting a Ca2+‐dependent exocytotic glutamate release mechanism of SIC generation. To gain new insights into this mechanism, we investigated the relationship between astrocytic Ca2+ elevations and neuronal SICs in mouse hippocampal slice preparations. Here we provide evidence that SICs, occurring either spontaneously or following a hypotonic challenge, are unchanged in the virtual absence of Ca2+ signal changes at astrocytic soma and processes, including spatially restricted Ca2+ microdomains. SICs are also unchanged in the presence of Bafilomycin A1 that after prolonged slice incubation depletes glutamate from astrocytic vesicles. We also found that hemichannels and TREK family channels‐that recent studies proposed to mediate astrocytic glutamate release ‐ are not involved in SIC generation. SICs are reduced by the volume‐sensitive anion channel antagonists diisothiocyanatostilbene‐2,2′‐disulfonic acid (DIDS), quinine and fluoxetine, suggesting a possible contribution of these channels in SIC generation. Direct measurements of astrocytic glutamate release further confirm that hypotonicity‐evoked gliotransmission is impaired following DIDS, quinine and fluoxetine while the exocytotic release of glutamate—that is proposed to mediate synaptic transmission modulation by astrocytes—remains unaffected. In conclusion, our data provide evidence that the release of glutamate generating SICs occurs independently on exocytotic Ca2+‐dependent glutamate release mechanism.
Main Points
Astrocyte‐to‐neuron signaling can occur independently of astrocytic Ca2+ elevations.
Hypotonicity‐evoked gliotransmission can be impaired without affecting physiological exocytotic gliotransmission.
Astrocytes play crucial roles in brain homeostasis and are emerging as regulatory elements of neuronal and synaptic physiology by responding to neurotransmitters with Ca2+ elevations and releasing ...gliotransmitters that activate neuronal receptors. Aging involves neuronal and astrocytic alterations, being considered risk factor for neurodegenerative diseases. Most evidence of the astrocyte–neuron signaling is derived from studies with young animals; however, the features of astrocyte–neuron signaling in adult and aging brain remain largely unknown. We have investigated the existence and properties of astrocyte–neuron signaling in physiologically and pathologically aging mouse hippocampal and cortical slices at different lifetime points (0.5 to 20 month‐old animals). We found that astrocytes preserved their ability to express spontaneous and neurotransmitter‐dependent intracellular Ca2+ signals from juvenile to aging brains. Likewise, resting levels of gliotransmission, assessed by neuronal NMDAR activation by glutamate released from astrocytes, were largely preserved with similar properties in all tested age groups, but DHPG‐induced gliotransmission was reduced in aged mice. In contrast, gliotransmission was enhanced in the APP/PS1 mouse model of Alzheimer's disease, indicating a dysregulation of astrocyte–neuron signaling in pathological conditions. Disruption of the astrocytic IP3R2 mediated‐signaling, which is required for neurotransmitter‐induced astrocyte Ca2+ signals and gliotransmission, boosted the progression of amyloid plaque deposits and synaptic plasticity impairments in APP/PS1 mice at early stages of the disease. Therefore, astrocyte–neuron interaction is a fundamental signaling, largely conserved in the adult and aging brain of healthy animals, but it is altered in Alzheimer's disease, suggesting that dysfunctions of astrocyte Ca2+ physiology may contribute to this neurodegenerative disease. GLIA 2017 GLIA 2017;65:569–580
Main Points
Fundamental Neuron–Astrocyte signaling pathway is largely conserved over the life span.
Dysfunction of astrocyte IP3R2‐dependent calcium signaling is associated with early progression of Alzheimer's disease.
Long-term potentiation (LTP) of synaptic transmission represents the cellular basis of learning and memory. Astrocytes have been shown to regulate synaptic transmission and plasticity. However, their ...involvement in specific physiological processes that induce LTP in vivo remains unknown. Here we show that in vivo cholinergic activity evoked by sensory stimulation or electrical stimulation of the septal nucleus increases Ca²⁺ in hippocampal astrocytes and induces LTP of CA3-CA1 synapses, which requires cholinergic muscarinic (mAChR) and metabotropic glutamate receptor (mGluR) activation. Stimulation of cholinergic pathways in hippocampal slices evokes astrocyte Ca²⁺ elevations, postsynaptic depolarizations of CA1 pyramidal neurons, and LTP of transmitter release at single CA3-CA1 synapses. Like in vivo, these effects are mediated by mAChRs, and this cholinergic-induced LTP (c-LTP) also involves mGluR activation. Astrocyte Ca²⁺ elevations and LTP are absent in IP₃R2 knock-out mice. Downregulating astrocyte Ca²⁺ signal by loading astrocytes with BAPTA or GDPβS also prevents LTP, which is restored by simultaneous astrocyte Ca²⁺ uncaging and postsynaptic depolarization. Therefore, cholinergic-induced LTP requires astrocyte Ca²⁺ elevations, which stimulate astrocyte glutamate release that activates mGluRs. The cholinergic-induced LTP results from the temporal coincidence of the postsynaptic activity and the astrocyte Ca²⁺ signal simultaneously evoked by cholinergic activity. Therefore, the astrocyte Ca²⁺ signal is necessary for cholinergic-induced synaptic plasticity, indicating that astrocytes are directly involved in brain storage information.
Endocannabinoids (eCBs) play key roles in brain function, acting as modulatory signals in synaptic transmission and plasticity. They are recognized as retrograde messengers that mediate long-term ...synaptic depression (LTD), but their ability to induce long-term potentiation (LTP) is poorly known. We show that eCBs induce the long-term enhancement of transmitter release at single hippocampal synapses through stimulation of astrocytes when coincident with postsynaptic activity. This LTP requires the coordinated activity of the 3 elements of the tripartite synapse: 1) eCB-evoked astrocyte calcium signal that stimulates glutamate release; 2) postsynaptic nitric oxide production; and 3) activation of protein kinase C and presynaptic group I metabotropic glutamate receptors, whose location at presynaptic sites was confirmed by immunoelectron microscopy. Hence, while eCBs act as retrograde signals to depress homoneuronal synapses, they serve as lateral messengers to induce LTP in distant heteroneuronal synapses through stimulation of astrocytes. Therefore, eCBs can trigger LTP through stimulation of astrocyte-neuron signaling, revealing novel cellular mechanisms of eCB effects on synaptic plasticity.
The glial cells astrocytes have long been recognized as important neuron-supporting elements in brain development, homeostasis, and metabolism. After the discovery that the reciprocal communication ...between astrocytes and neurons is a fundamental mechanism in the modulation of neuronal synaptic communication, over the last two decades astrocytes became a hot topic in neuroscience research. Crucial to their functional interactions with neurons are the cytosolic Ca
2+
elevations that mediate gliotransmission. Large attention has been posed to the so-called Ca
2+
microdomains, dynamic Ca
2+
changes spatially restricted to fine astrocytic processes including perisynaptic astrocytic processes (PAPs). With presynaptic terminals and postsynaptic neuronal membranes, PAPs compose the tripartite synapse. The distinct spatial-temporal features and functional roles of astrocyte microdomain Ca
2+
activity remain poorly defined. However, thanks to the development of genetically encoded Ca
2+
indicators (GECIs), advanced microscopy techniques, and innovative analytical approaches, Ca
2+
transients in astrocyte microdomains were recently studied in unprecedented detail. These events have been observed to occur much more frequently (∼50–100-fold) and dynamically than somatic Ca
2+
elevations with mechanisms that likely involve both IP
3
-dependent and -independent pathways. Further progress aimed to clarify the complex, dynamic machinery responsible for astrocytic Ca
2+
activity at microdomains is a crucial step in our understanding of the astrocyte role in brain function and may also reveal astrocytes as novel therapeutic targets for different brain diseases. Here, we review the most recent studies that improve our mechanistic understanding of the essential features of astrocyte Ca
2+
microdomains.
The signaling diversity of GABAergic interneurons to post-synaptic neurons is crucial to generate the functional heterogeneity that characterizes brain circuits. Whether this diversity applies to ...other brain cells, such as the glial cells astrocytes, remains unexplored. Using optogenetics and two-photon functional imaging in the adult mouse neocortex, we here reveal that parvalbumin- and somatostatin-expressing interneurons, two key interneuron classes in the brain, differentially signal to astrocytes inducing weak and robust GABA
receptor-mediated Ca
elevations, respectively. Furthermore, the astrocyte response depresses upon parvalbumin interneuron repetitive stimulations and potentiates upon somatostatin interneuron repetitive stimulations, revealing a distinguished astrocyte plasticity. Remarkably, the potentiated response crucially depends on the neuropeptide somatostatin, released by somatostatin interneurons, which activates somatostatin receptors at astrocytic processes. Our study unveils, in the living brain, a hitherto unidentified signaling specificity between interneuron subtypes and astrocytes opening a new perspective into the role of astrocytes as non-neuronal components of inhibitory circuits.
Seizures in focal epilepsies are sustained by a highly synchronous neuronal discharge that arises at restricted brain sites and subsequently spreads to large portions of the brain. Despite intense ...experimental research in this field, the earlier cellular events that initiate and sustain a focal seizure are still not well defined. Their identification is central to understand the pathophysiology of focal epilepsies and to develop new pharmacological therapies for drug-resistant forms of epilepsy. The prominent involvement of astrocytes in ictogenesis was recently proposed. We test here whether a cooperation between astrocytes and neurons is a prerequisite to support ictal (seizure-like) and interictal epileptiform events. Simultaneous patch-clamp recording and Ca2+ imaging techniques were performed in a new in vitro model of focal seizures induced by local applications of N-methyl-D-aspartic acid (NMDA) in rat entorhinal cortex slices. We found that a Ca2+ elevation in astrocytes correlates with both the initial development and the maintenance of a focal, seizure-like discharge. A delayed astrocyte activation during ictal discharges was also observed in other models (including the whole in vitro isolated guinea pig brain) in which the site of generation of seizure activity cannot be precisely monitored. In contrast, interictal discharges were not associated with Ca2+ changes in astrocytes. Selective inhibition or stimulation of astrocyte Ca2+ signalling blocked or enhanced, respectively, ictal discharges, but did not affect interictal discharge generation. Our data reveal that neurons engage astrocytes in a recurrent excitatory loop (possibly involving gliotransmission) that promotes seizure ignition and sustains the ictal discharge. This neuron-astrocyte interaction may represent a novel target to develop effective therapeutic strategies to control seizures.
The plasticity of glutamatergic transmission in the ventral tegmental area (VTA) represents a fundamental mechanism in the modulation of dopamine neuron burst firing and phasic dopamine release at ...target regions. These processes encode basic behavioral responses, including locomotor activity, learning and motivated behaviors. Here we describe a hitherto unidentified mechanism of long-term synaptic plasticity in mouse VTA. We found that the burst firing in individual dopamine neurons induces a long-lasting potentiation of excitatory synapses on adjacent dopamine neurons that crucially depends on Ca
elevations in astrocytes, mediated by endocannabinoid CB1 and dopamine D2 receptors co-localized at the same astrocytic process, and activation of pre-synaptic metabotropic glutamate receptors. Consistent with these findings, selective in vivo activation of astrocytes increases the burst firing of dopamine neurons in the VTA and induces locomotor hyperactivity. Astrocytes play, therefore, a key role in the modulation of VTA dopamine neuron functional activity.
Abstract The tight spatial and temporal coupling between neuronal activity and blood flow ensures that active brain regions receive an adequate supply of oxygen and energetic metabolites. There ...clearly is still an enormous amount of experimental and theoretical work to be done to unravel the precise mechanism of neurovascular coupling, but over the last decade significant advances have been made. The most recent studies confirm the original finding that the activation of Ca2+ elevations in astrocyte endfeet is an essential step but also reveal new levels of complexity in the astrocyte control of neurovascular coupling. The recent evidence for a link between Ca2+ signalling in astrocytes and local metabolic states of the brain tissue has broad implications for the interpretation of data from functional brain imaging studies. Unraveling the full molecular mechanism of the astrocyte control of cerebral blood flow represents a formidable challenge in neurobiological research in the years to come that might also create opportunities for the development of new therapeutic strategies for cerebrovascular diseases such as ischemic stroke, hypertension and migraine as well as neurodegenerative diseases as Alzheimer's disease.
The prefrontal cortex (PFC) is implicated in processing of the affective state of others through non-verbal communication. This social cognitive function is thought to rely on an intact cortical ...neuronal excitatory and inhibitory balance. Here combining in vivo electrophysiology with a behavioral task for affective state discrimination in mice, we show a differential activation of medial PFC (mPFC) neurons during social exploration that depends on the affective state of the conspecific. Optogenetic manipulations revealed a double dissociation between the role of interneurons in social cognition. Specifically, inhibition of mPFC somatostatin (SOM
), but not of parvalbumin (PV
) interneurons, abolishes affective state discrimination. Accordingly, synchronized activation of mPFC SOM
interneurons selectively induces social discrimination. As visualized by in vivo single-cell microendoscopic Ca
imaging, an increased synchronous activity of mPFC SOM
interneurons, guiding inhibition of pyramidal neurons, is associated with affective state discrimination. Our findings provide new insights into the neurobiological mechanisms of affective state discrimination.
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