In Focus: Ionotropic Glutamate Receptors

The excitatory effects of glutamate (the major excitatory neurotransmitter) are mediated by the activation of ionotropic and metabotropic receptors. These receptors differ in their molecular, biochemical, physiological and pharmacological properties⁶.

The ionotropic glutamate receptors are divided into three main categories: AMPA, NMDA (N-methyl-D-aspartate) and Kainate receptors.

AMPA receptors consist of four closely related genes with a high sequence homology. Their related products, GluR1-GluR4 subunits combine in tetramers in different stoichiometries⁶ which determine their channel function. These receptors, like many other cellular proteins, undergo posttranslational modifications. Such modifications influence protein-protein interactions, trafficking, internalization and their accumulation in the Golgi. Also, post-translational modifications are also involved in regulating LTP driven incorporation of AMPA receptors into the post-synaptic membrane (extensively reviewed in reference 19).

AMPA receptors are responsible for the primary depolarization in glutamate mediated neurotransmission. in situ hybridization and immunocytochemistry studies show that AMPA receptors are mainly distributed in the brain. These receptors play important roles in synaptic plasticity, excitatory neurotransmission, LTP and LTD effects, and take part in learning and memory formation^14,15,18,26. The important functions AMPA receptors have acquired also imply that deregulation of these receptors leads to pathologies.

NMDA receptors are the second type of ionotropic glutamate-gated ion channels. They have a pivotal role in the regulation of synaptic function in the central nervous system (CNS). They are molecularly composed of three different subunits: NR1, NR2 and NR3 which can undergo alternative splicing. These subunits already form functional channels in the endoplasmic reticulum in a co-translational-dependent manner. Their physiological properties depend on the different subunits making up the functional channel⁹. NMDA receptors are highly permeable to Ca²⁺ for which its influx is essential for NMDA receptor-dependent synaptic events.

Synaptic NMDA receptors are localized to postsynaptic densities, where they are organized in a signaling complex along with adaptor proteins and signaling molecules. While having important contribution to synaptic plasticity such as LTP and LTD²³, NMDA receptors have been a major target for drug development due to their broad involvement in many neurological diseases^8,25. Overly active NMDA receptors could instigate some neurological disorders¹³ and neuropathic pain²⁷. Conversely, underactive NMDA receptors may underlie neurodevelopmental disorders such as schizophrenia³.

GluR5, GluR6, GluR7, KA1, and KA2 kainate receptors were cloned in the 1990s. Later on it was shown that these receptors have various splice variants although their exact cellular functions are not quite clear, and some of their mRNAs undergo editing, two modifications responsible for increasing the structural complexity. Primary structure analysis shows that kainate receptors share 40% sequence homology to AMPA receptors and 20% to NMDA receptors¹⁰. These receptors form tetrameric structures where each monomer has its own ligand binding site. The GluR5-7 subgroup can form homomeric functional structures when heterologously expressed^4,20,21, whereas KA1 and KA2 require the coexpression of other subunits in order to form kainate-gated functional channels⁵. As kainate and AMPA receptors are difficult to differentiate, the development of an AMPA specific antagonist enabled proper characterization (spatial and functional) of these two receptors16. The kainate receptors are widely distributed throughout the nervous system and their activity has been found in regions of the hippocampus²², and in the CA1 hippocampal interneurons², in the cerebellum¹ as well as the spinal cord¹². In addition, it is also clear that kainate receptors are present at presynaptic membranes where they can control the inhibitory or excitatory release of neurotransmitters¹¹. The application of specific agonists/antagonist made it clear that kainate receptors are evolving to become therapeutic targets as they are clearly involved in epilepsy, neurodegeneration, pain, migraine and may be implicated in some psychiatric disorders like bipolar disease and autism^7,17,24.

Alomone Labs is pleased to offer specific antibodies targeted to ionotropic glutamate receptors, most against extracellular epitopes enabling detection in live cells and on the membrane surface. These antibodies can be used in various applications, namely western blot, immunoprecipitation, immunohistochemistry and immunocytochemistry. In addition, some antibodies are offered conjugated to a fluorescent probe, ideal for colocalization studies and enabling fast and easy detection of the receptor.

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