Metabotropic and ionotropic glutamate receptors as potential targets for the treatment of alcohol use disorder

Abstract

Emerging evidence indicates that dysfunctional glutamate neurotransmission is critical in the initiation and development of alcohol and drug dependence. Alcohol consumption induced downregulation of glutamate transporter 1 (GLT-1) as reported in previous studies from our laboratory. Glutamate is the major excitatory neurotransmitter in the brain, which acts via interactions with several glutamate receptors. Alcohol consumption interferes with the glutamatergic signal transmission by altering the functions of these receptors. Among the glutamate receptors involved in alcohol-drinking behavior are the metabotropic receptors such as mGluR1/5, mGluR2/3, and mGluR7, as well as the ionotropic receptors, NMDA and AMPA. Preclinical studies using agonists and antagonists implicate these glutamatergic receptors in the development of alcohol use disorder (AUD). Therefore, the purpose of this review is to discuss the neurocircuitry involving glutamate transmission in animals exposed to alcohol and further outline the role of metabotropic and ionotropic receptors in the regulation of alcohol-drinking behavior. This review provides ample information about the potential therapeutic role of glutamatergic receptors for the treatment of AUD.

Keywords: AMPA; Alcohol; Glutamate; NMDA; mGluR1/5; mGluR2/3; mGluR7.

Figure 1. Neurocircuitry involved in AUD

The brain reward circuitry is comprised of five major brain regions – nucleus accumbens (NAc), prefrontal cortex (PFC), amygdala (AMG), hippocampus (HPC) and ventral tegmental area (VTA) – which are interconnected by the glutamatergic and dopaminergic excitatory pathways as well as the inhibitory GABAergic pathway. (A) Glutamatergic System – NAc receives glutamatergic inputs from PFC, AMG and HPC, while all three latter regions are interconnected by reciprocating glutamatergic projections. (B) Dopaminergic System – VTA relays dopaminergic projections to NAc, PFC, AMG and HPC. (C) GABAergic System – NAc sends GABAergic inputs to VTA.

Figure 2. Glutamatergic Neurotransmission

In the presynaptic neuron, glutaminase catalyzes the conversion of glutamine to glutamate, which is further loaded into the vesicles by vesicular glutamate transporters (VGLUTs). Following depolarization of the presynaptic terminal, the vesicle interacts with SNARE proteins on the synaptic membrane, consequently leading to the release of glutamate into the synapse. After being released from the presynaptic terminal, the glutamate in the synapse interacts with the post-synaptic mGluRs and iGluRs, initiating further cell signaling. Group 2 and Group 3 mGlu receptors on the presynaptic terminal inhibit the adenylyl cyclase activity and negatively regulate the glutamate release from the presynaptic terminal. The excess extracellular glutamate is taken up by several glial glutamate transporters such as GLT-1 (also known as excitatory amino acid transporter 2, EAAT2) and GLAST (also known as excitatory amino acid transporter 1, EAAT1). Inside the glial cell, the glutamine synthetase enzyme catalyzes the conversion of glutamate to glutamine, which is further transported to the presynaptic neuronal terminal and can be further used in the glutamate-glutamine cycle. Cystine-glutamate exchanger, (xCT) located on the glial cell, also plays a vital role in elevating the synaptic glutamate concentrations, using l-cystine for exchange.

Figure 3. Schematic representation of metabotropic glutamate receptors (mGluRs)

Glutamate activates the receptor by binding to the extracellular N-terminal domain. (A) Upon activation of group 1 mGluR, Gq proteins are stimulated, which further activates phospholipase C (PLC). The activation of PLC subsequently catalyzes the production of diacylglycerol (DAG) and inositol (1,4,5)-triphosphate (IP3). DAG activates protein kinase C (PKC), while IP3 increases the release of Ca²⁺ from intracellular stores. (B) Activation of group 2 mGluRs and group 3 mGluRs leads to stimulation of Gi/o proteins, which further inhibits adenylyl cyclase (AC) activity, eventually reducing the intracellular concentrations of cAMP.

Figure 4. Schematic diagram of ionotropic glutamate receptors (iGluRs) subunits

iGluRs contain a large extracellular amino-terminal (N) domain and an intracellular carboxy-terminal (C) domain. These receptors constitute four transmembrane domains (M1-M4), wherein the M2 domain forms a re-enterant loop. Two distinct extracellular loops containing S1 and S2 form the ligand-binding region in the receptor.

Figure 5

Schematic representation of orthosteric and allosteric binding sites of metabotropic glutamate receptors (mGluRs). mGluRs belong to Class C subtype of G-protein-coupled receptors (GPCRs). These receptors are characterized by a large extracellular N-terminal domain, termed as the Venus flytrap domain (VFD), which is exclusively used to bind orthosteric ligands (e.g. glutamate) (Conn et al., 2009; Gregory et al., 2011; May et al., 2007). These VFDs are involved in the dimerization of the mGluRs. The transmembrane region of mGluRs forms a pocket where the small-molecule modulators bind allosterically, with the potential to have more than one binding site. Thus, the allosteric modulators bind to a site, which is topographically different from the orthosteric ligand binding site, causing a change in receptor conformation further modifying the receptor activity in a positive or negative modulation of neutral direction. This modulation in receptor activity can be either affected by binding efficacy, binding affinity or varying degrees of both (Melancon et al., 2012a).