Does glutamate released by astrocytes cause focal epilepsy?

Novel astrocyte targets: new avenues for the therapeutic treatment of epilepsy

During the last 20 years, it has been well established that a finely tuned, continuous crosstalk between neurons and astrocytes not only critically modulates physiological brain functions but also underlies many neurological diseases. In particular, this novel way of interpreting brain activity is markedly influencing our current knowledge of epilepsy, prompting a re-evaluation of old findings and guiding novel experimentation. Here, we review recent studies that have unraveled novel and unique contributions of astrocytes to the generation and spread of convulsive and nonconvulsive seizures and epileptiform activity. The emerging scenario advocates an overall framework in which a dynamic and reciprocal interplay among astrocytic and neuronal ensembles is fundamental for a fuller understanding of epilepsy. In turn, this offers novel astrocytic targets for the development of those really novel chemical entities for the control of convulsive and nonconvulsive seizures that have been acknowledged as a key priority in the management of epilepsy.

New vistas on astroglia in convulsive and non-convulsive epilepsy highlight novel astrocytic targets for treatment

Our current knowledge of the role of astrocytes in health and disease states supports the view that many physiological brain functions and neurological diseases are finely tuned, and in certain cases fully determined, by the continuous cross-talk between astrocytes and neurons. This novel way of interpreting brain activity as a dynamic and reciprocal interplay between astrocytic and neuronal networks has also influenced our understanding of epilepsy, not only forcing a reinterpretation of old findings, but also being a catalyst for novel experimentation. In this review, we summarize some of the recent studies that highlight these novel distinct contributions of astrocytes to the expression of convulsive and non-convulsive epileptiform discharges and seizures. The emerging picture suggests a general framework based on bilateral signalling between astrocytes and neurons for a fuller understanding of epileptogenic and epileptic mechanisms in the brain network. Astrocytes potentially represent targets for the development of those novel chemical entities with improved efficacy for the treatment of convulsive and non-convulsive epilepsy that expert groups have recognized as one of the key priorities for the management of epilepsy.

The role of astroglia in the epileptic brain

Epilepsies comprise a family of multifactorial neurological disorders that affect at least 50 million people worldwide. Despite a long history of neurobiological and clinical studies the mechanisms that lead the brain network to a hyperexcitable state and to the intense, massive neuronal discharges reflecting a seizure episode are only partially defined. Most epilepsies of genetic origin are related to mutations in ionic channels that cause neuronal hyperexcitability. However, idiopathic epilepsies of unclear origin represent the majority of these brain disorders. A large body of evidence suggests that in the epileptic brain neurons are not the only players. Indeed, the glial cell astrocyte is known to be morphologically and functionally altered in different types of epilepsy. Although it is unclear whether these astrocyte dysfunctions can have a causative role in epileptogenesis, the hypothesis that astrocytes contribute to epileptiform activities recently received a considerable experimental support. Notably, currently used antiepileptic drugs, that act mainly on neuronal ion channels, are ineffective in a large group of patients. Clarifying astrocyte functions in the epileptic brain tissue could unveil astrocytes as novel therapeutic targets. In this review we present first a short overview on the role of astrocytes in the epileptic brain starting from the "historical" observations on their fundamental modulation of brain homeostasis, such as the control of water content, ionic equilibrium, and neurotransmitters concentrations. We then focus our review on most recent studies that hint at a distinct contribution of these cells in the generation of focal epileptiform activities.

Hippocampal tissue of patients with refractory temporal lobe epilepsy is associated with astrocyte activation, inflammation, and altered expression of channels and receptors

Temporal lobe epilepsy (TLE) is the most common form of focal epilepsy. Previous research has demonstrated several trends in human tissue that, undoubtedly, contribute to the development and progression of TLE. In this study we examined resected human hippocampus tissue for a variety of changes including gliosis that might contribute to the development and presentation of TLE. The study subjects consisted of six TLE patients and three sudden-death controls. Clinicopathological characteristics were evaluated by H&E staining. Immunohistological staining and Western blotting methods were used to analyze the samples. Neuronal hypertrophy was observed in resected epileptic tissue. Immunohistological staining demonstrated that activation of astrocytes was significantly increased in epileptic tissue as compared to corresponding regions of the control group. The Western blot data also showed increased CX43 and AQP4 in the hippocampus and downregulation of Kir4.1, α-syntrophin, and dystrophin, the key constituents of AQP4 multi-molecular complex. These tissues also demonstrated changes in inflammatory factors (COX-2, TGF-β, NF-κB) suggesting that these molecules may play an important role in TLE pathogenesis. In addition we detected increases in metabotropic glutamate receptor (mGluR) 2/3, mGluR5 and kainic acid receptor subunits KA1 (Grik4) and KA2 (Grik5) in patients' hippocampi. We noted increased expression of the α1c subunit comprising class C L-type Ca(2+) channels and calpain expression in these tissues, suggesting that these subunits might have an integral role in TLE pathogenesis. These changes found in the resected tissue suggest that they may contribute to TLE and that the kainic acid receptor (KAR) and deregulation of GluR2 receptor may play an important role in TLE development and disease course. This study identifies alterations in number of commonly studied molecular targets associated with astrogliosis, cellular hypertrophy, water homeostasis, inflammation, and modulation of excitatory neurotransmission in hippocampal tissues from TLE patients.

Astrocyte calcium signaling and epilepsy

Studies performed over the last decade, in both animal models and human epilepsy, support the view that a defective K(+) buffering due to an altered expression of K(+) and aquaporin channels in astrocytes represents a possible causative factor of the pathological neuronal hyperexcitability in the epileptic brain. More recent studies, however, reappraised the role of neurons in epileptogenesis and suggested that Ca(2+)-dependent gliotransmission directly contributes to the excessive neuronal synchronization that predisposes the brain network to seizures. Significant support for this view comes from the finding that astrocytes from hyperexcitable networks respond to neuronal signals with massive Ca(2+) elevations and generate a recurrent excitatory loop with neurons that has the potential to promote a focal seizure. The specific aim of this review is on the one hand, to provide an overview of the experimental findings that hinted at a direct role of Ca(2+)-dependent gliotransmission in the generation of seizure-like discharges in models of focal epilepsy; and on the other hand, to emphasize the importance of developing new experimental tools that could help us to understand the amazing complexity of neuron-astrocyte partnership in brain disorders.

An excitatory loop with astrocytes contributes to drive neurons to seizure threshold

Studies in rodent brain slices suggest that seizures in focal epilepsies are sustained and propagated by the reciprocal interaction between neurons and astroglial cells

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.

In focal epilepsy, seizures are generated by a localized, synchronous neuronal electrical discharge that may spread to large portions of the brain. Despite intense experimental research in this field, a key question relevant to the human epilepsy condition remains completely unanswered: what are the cellular events that lead to the onset of a seizure in the first place? In various in vitro models of seizures using rodent brain slices, we simultaneously recorded neuronal firing and Ca²⁺ signals both from neurons and from astrocytes, the principal population of glial cells in the brain. We found that activation of astrocytes by neuronal activity and signalling from astrocytes back to neurons contribute to the initiation of a focal seizure. This reciprocal excitatory loop between neurons and astrocytes represents a new mechanism in the pathophysiology of epilepsy that should be considered by those aiming to develop more effective therapies for epilepsies that are not controlled by currently available treatments.

Which clinical and experimental data link temporal lobe epilepsy with depression?

The association of temporal lobe epilepsy with depression and other neuropsychiatric disorders has been known since the early beginnings of neurology and psychiatry. However, only recently have in vivo and ex vivo techniques such as Positron Emission Tomography, Magnetic Resonance Imaging and Magnetic Resonance Spectroscopy in combination with refined animal models and behavioral tests made it possible to identify an emerging pattern of common pathophysiological mechanisms. We now have growing evidence that in both disorders altered interaction of serotonergic and noradrenergic neurons with glutamatergic systems is associated with abnormal neuronal circuits and hyperexcitability. Neuronal hyperexcitability can possibly evoke seizure activity as well as disturbed emotions. Moreover, decreased synaptic levels of neurotransmitters and high glucocorticoid levels influence intracellular signaling pathways such as cAMP, causing disturbances of brain-derived and other neurotrophic factors. These may be associated with hippocampal atrophy seen on Magnetic Resonance Imaging and memory impairment as well as altered fear processing and transient hypertrophy of the amygdala. Positron Emission Tomography studies additionally suggest hypometabolism of glucose in temporal and frontal lobes. Last, but not least, in temporal lobe epilepsy and depression astrocytes play a role that reaches far beyond their involvement in hippocampal sclerosis and ultimately, therapeutic regulation of glial-neuronal interactions may be a target for future research. All these mechanisms are strongly intertwined and probably bidirectional such that the structural and functional alterations from one disease increase the risk for developing the other. This review provides an integrative update of the most relevant experimental and clinical data on temporal lobe epilepsy and its association with depression.