Ionotropic receptors

Ligand-gated ion channels - mediators of fast excitatory or inhibitory neurotransmission

For revision on ionotropic receptors, go to BB329 webnotes
Website: Ion Channel Network: http://www.pain.med.umn.edu/csn/

Neuronal excitability is dependent on voltage- and ligand-gated ion channels (see Lodish 4th ed. pp. 939-951). The activity of these ion channels can be regulated through phosphorylation - thus through activation of second messenger pathways and second messenger dependent protein kinases.

Ligand-gated ion channels - neurotransmitter receptors

The first receptors were cloned by functional expression. To clone ligand-gated ion channels, it was possible to detect their expression by electrophysiological measurements of ion activity following ligand exposure. The functional cloning method most commonly used, (and still used to study ion channel function) was to express mRNA transcribed from cDNA libraries derived from cells which express these receptors in Xenopus oocytes (see Siegel, E (2001) Microinjection into Xenopus oocytes. Encyclopedia of Life Sciences).

Xenopus oocytes

The first receptor to be cloned was the nicotinic cholinergic receptor (nAChR) from the Torpedo electric organ. This receptor is the major receptor mediating cholinergic neurotransmission at the neuromuscular junction.

Because the Torpedo electric organ contains such a high concentration of nAChRs, it was possible to isolate the protein, and determine its subunit composition and other biochemical properties - and relate these to those of the cloned receptors. Combined with mutagenesis of the cloned receptor genes, it has been possible to determine structural/functional relationships. This remains the best characterised receptor of its type.

nAChR - heteropentameric structure

Several genes have been cloned now which encode various subunits of ligand-gated ion channels, and these have been divided into families according to sequence and functional criteria.

Ionotropic receptor families

  1. Nicotinic receptor family
  2. Glutamate receptor family (Na+/K+ and Ca2+ channels)
Glutamate receptors
Glutamate is the major excitatory neurotransmitter in the central nervous system. Excessive glutamate neurotransmission underlies ischaemia and excitotoxicity and neurodegeneration. Biophysical analysis of membrane patches taken from neuronal cell bodies indicate that different glutamate-sensitive channel types co-exist in many cells.

Three receptor types have been identified

  1. AMPA (a-amino-3-hydroxy-5-methyl-4-isoxazolepropionate) - sensitive receptors
  2. kainate (neurotoxin) -sensitive receptors
  3. NMDA (N-methyl-D-aspartate) -sensitive receptors
Molecular cloning of glutamate receptor subunits indicate they differ in structure to those belonging to the nAChR family Mutagenesis studies have indicated that a specific amino acid residue in region 2 can determine receptor ion permeability
Non-NMDA receptors
Properties
  1. AMPA-sensitive receptors
  2. kainate-sensitive receptors
Subunits encoded by different genes Subunits encoded by GluR1-7 are modified Editing of the Q/R site in region 2 is physiologically very important because it determines channel properties. Heterozygous transgenic mice with uneditable Q/R site develop early-onset epilepsy followed by premature death
Various combinations of different spliced and edited subunits alter the current-voltage relation (rectification) and Ca2+ permeability of non-NMDA glutamate receptors To learn more about glutamate receptor channels and the role of RNA editing please consult:
Sprengel, R, Higuchi, M, Monyer, H & Seeburg, P (1999) Glutamate receptor channels: a possible link between RNA editing in the brain and epilepsy. Advances in Neurology 79, 525-534.
Simpson, L & Emeson, RB (1996) RNA editing. Annual Review Neuroscience 19, 27-52.


NMDA receptors
Mediate Ca2+ influx

Properties Two subunit isoforms properties of heteromeric channels are dependent on which of 4 NR2 subunits are present NMDA receptor activity depends on neuronal activity (have to remove Mg2+ block) Importance of receptor heterogeneity Lecture notes last updated 24/1/2002
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