Dysregulation of Ambient Glutamate and Glutamate Receptors in Epilepsy: An Astrocytic Perspective

Abstract

Given the important functions that glutamate serves in excitatory neurotransmission, understanding the regulation of glutamate in physiological and pathological states is critical to devising novel therapies to treat epilepsy. Exclusive expression of pyruvate carboxylase and glutamine synthetase in astrocytes positions astrocytes as essential regulators of glutamate in the central nervous system (CNS). Additionally, astrocytes can significantly alter the volume of the extracellular space (ECS) in the CNS due to their expression of the bi-directional water channel, aquaporin-4, which are enriched at perivascular endfeet. Rapid ECS shrinkage has been observed following epileptiform activity and can inherently concentrate ions and neurotransmitters including glutamate. This review highlights our emerging knowledge on the various potential contributions of astrocytes to epilepsy, particularly supporting the notion that astrocytes may be involved in seizure initiation via failure of homeostatic responses that lead to increased ambient glutamate. We also review the mechanisms whereby ambient glutamate can influence neuronal excitability, including via generation of the glutamate receptor subunit GluN2B-mediated slow inward currents, as well as indirectly affect neuronal excitability via actions on metabotropic glutamate receptors that can potentiate GluN2B currents and influence neuronal glutamate release probabilities. Additionally, we discuss evidence for upregulation of System ${\text{x}}_{\text{c}}^{-}$ , a cystine/glutamate antiporter expressed on astrocytes, in epileptic tissue and changes in expression patterns of glutamate receptors.

Keywords: NMDAR; System xc-; astroglia; epilepsy; glutamate homeostasis; metabotrophic glutamate receptor.

Figure 1

*Vesicular glutamate release during action potentials is the primary source of synaptic glutamate. **SXC, primarily expressed on astrocytes, is a major source of ambient, extrasynaptic glutamate. Ambient glutamate concentration around the synapse, after EAAT activity, follows a gradient with the lowest level in the synaptic cleft to the highest in the extrasynaptic compartment. (1) Astrocyte homeostatic responses to increased activity from hyperexcitable neurons. (1a) Increased vesicular glutamate release from hyperexcitable neurons leads to increased astrocytic EAAT activity. (1b) Elevated neuronal activity also causes release of K⁺, in attempts to maintain homeostatic neuronal resting membrane potential. Next, astrocytic buffering of extracellular K⁺ through elevated Kir4.1 activity, which is accompanied by increased H20 uptake through aquaporin-4, ultimately results in activity-induced astrocytic swelling and reduction in ECS. (1c) Astrocytic swelling leads to activation of VRAC and release of glutamate and other gliotransmitters into the ECS. (2) Pathophysiological effects of increased activity and changes in expression of neuronal extrasynaptic glutamate receptors. (2a) Activation of N2B-containing NMDARs leads to the generation of slow, depolarizing currents. (2b) Elevated expression and activity of group 1 mGluRs in epilepsy has been linked to increased NMDAR-mediated currents via a mechanism involving Ca²⁺-calmodulin dependent tyrosine phosphorylation of NMDAR subunits NR2A/B. (2c) Presynaptic group 2 mGluRs have been shown to inhibit glutamate and GABA release. Tissue from epileptic patients and animal models have revealed decreased mGluR_2/3 expression, which can contribute to a pro-epileptic brain state. (3) Changes in astrocytic glutamatergic protein expression in epilepsy. (3a) SXC expression has been found to be elevated in human epileptic tissue, as well as various epilepsy animal models. SXC activity leads to the release of glutamate from astrocytes. (3b) Animal models of epilepsy have revealed that persistent upregulation of astrocytic mGluR₅ was a reliable indicator of epileptogenesis. mGluR₅ activation leads to altered GLAST/GLT-1 expression and induces NR2B-dependent NMDAR mediated neuronal currents. (3c) Upregulation of mGluR₃ has been reported in epilepsy animal models and experimental activation of group 2 mGluRs in cultured astrocytes was shown to upregulate GLAST/GLT-1 expression, suggesting that a balance of group 1 and group 2 mGluRs on astrocytes is important in maintaining homeostatic extracellular glutamate.