LIFESCI 2C03 Chapter 6: LS2CC3_CH6 notes

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Glutamate
most important for normal brain function
glutamate is a nonessential amino acid and doesn't cross blood-brain barrier
therefore must be synthesized in neurons from local precursors
1. precursor glutamine taken up into presynaptic terminals by SAT2
2. metabolized to glutamate by the glutaminase
2ii. glucose metabolized by neurons can also synthesize glu
3. glu packaged into vesicles by VGLUT (at least 3 di&erent VGLUT)
4. once released, glu taken up form the cleft by Na dependent co-transporters (5),
EAATs
some E9AATs present in glial cells -> glu transported into glial cells via EAATs
converted into glutamine by glutamine synthetase -> transported out of the
gial cells by SN1 and transported into nerve terminals via SAT2
this sequence is called the glutamate-glutamine cycle (cooperation of glial
cells and presynaptic terminals)
ionotropic glutamate receptors
AMPA receptors, NMDA receptors and kainate receptors -> glutamate-gated
cation channels allow passage of Na and K; therefore, all these receptors
produce excitatory postsynaptic responses
most central synapses -> AMPA and NMDA receptors
antagonist that block the two receptors revealed that EPSCs by NMDA
receptor is slower and last longer than AMPA receptors
EPSCs by AMPA receptors is larger than other ionotropic glu receptors,
thereby AMPA are the primary mediators of excitatory transmission in the
brain.
kainate receptors - found on presynaptic terminal serve as a feedback
mechanism to regulate glu release; when found on postsynaptic cells, kainate
generate EPSCs that rise quickly but decay more slowly than AMPA
NMDA unique physiologically
- pore of the channels allows the entry of calcium -> calcium acts as a 2nd
messenger to cause intracellular signalling cascade ; at hyperpolarized
membrane, mg blocks the pore and vice versa
- mg confers the voltage dependence therefore NMDA requires the coincident
presence of both glu and postsynaptic depolarization to open NMDA is the
reason to long term synaptic plasticity
- also, gating of NMDA need co-agnosit (glycine)
AMPA - composed of 4 di&erent subunits (GLU1-4) contains di&erent domains
(i.e. extracellular ligand binding domain and transmembrane domain etc)
subunits organized into tetrameric structure (extracellular structure is
asymmetrical, clamshell shape- where glu binds, clamshell shut -> gate
helices move and open the pore)
- transmembrane domain has helices form channel pore and a gate (closed
when glu not bound)
NMDA - GluN2 subunits bind glu, GluN1 and 3 bind glycine. typically
composed of two GLluN1 and two GluN3 (GLUN3 sometimes replaces two
GLuN2)
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