Glutamate Flashcards
Amino acid neurotransmitters
- Not required in diet
- Synthesized in most cells of the body
Two functional groups of Amino acid neurotransmitters
Excitatory amino acid NT
Inhibitory amino acid NT
Excitatory amino acid NT (4)
Glutamate, Aspartate, Cysteate, Homocysteate
Inhibitory amino acid NT (4)
γ-aminobutyric acid (GABA), Glycine, Taurine,
Alanine
Excitatory amino acid neurotransmitters Aspartate Released in a
Ca2+
-dependent manner
Aspartate May not be stored in
secretory vesicles
Aspartate May be directly released from
cell cytoplasm Not considered a ‘classic’ neurotransmitter
Aspartate Acts at
glutamatergic receptors
Aspartate Physiological functions
unclear
Excitatory amino acid neurotransmitters Glutamate
Most widely used excitatory neurotransmitter
Glutamate____of all neurons, ______of all synapses are
glutamatergic
90% of all neurons, 80-90% of all synapses are
glutamatergic
Glutamate mediates fast
excitatory neurotransmission
Glutamate Mediates fast excitatory neurotransmission
Sensory, motor coordination, emotion, cognition,
memory formation and retrieval
Glutamate Proteinogenic amino acid
Abundant throughout the cell
Concentrated in presynaptic compartments
Glutamate
synthesis from
glutamine
Glutamine —glutaminase —> glutamate
In the CNS the majority
of glutamate is
recycled from
glutamine by the
enzyme glutaminase
Glutamate
transporters
Vesicular glutamate transporter (VGLUT) can be
used to identify glutamatergic neurons
VGLUTs are structurally and functionally similar to
VMAT
- VGLUT1 and 2 are expressed
on
distinct glutamatergic
populations in the CNS.
VGLUT3 is expressed in (3 neurons
)
various neurons including
GABAergic, cholinergic, and
monoaminergic neurons
suggesting possible
modulatory functions.
VGLUT2 locations
Deep cerebellar nuclei, inferior colliculus, thalamus
VGLUT 1 locations
cerebellar cortex, hippocampus, cortex
Glutamate is
metabolized to
glutamine
Glutamine
synthetase is the
enzyme responsible
for
conversion of
glutamate to
glutamine
Glutamate transporters on the
cell membrane are termed
excitatory amino acid
transporters (EAATs)
Glutamate transporters on the
cell membrane are termed
excitatory amino acid
transporters (EAATs) Non-specific for both
glutamate and aspartate
how many families are in EAAT
5
- EAAT1 and 2 are expressed on
astrocytes
EAAT3 and 4 are expressed on
neurons
EAAT5 is expressed in the
retina
- EAAT expression
compartmentalizes
glutamate
recycling
Neurons comprise only
50% of the cells in the
CNS
Glia in the CNS
Astrocytes, Oligodendrocytes
Ependymal cells
Microglia
Astrocytes
define the brain side of the BBB
- Oligodendrocytes
myelinate axons in white
matte
Ependymal cells
generate and regulate CSF
Microglia
Immune surveillance and
development
Astrocyte functions * Define the
blood brain barrier -Regulate intake of nutrients and O2
Astrocyte functions -Regulate
blood flow in the brain
Astrocyte functions Form extensive
signalling networks -Coupled with electrical synapses – Gap junctions
Astrocyte functions Regulate synaptic
functions and
contribute to plasticity
Astrocytes and cognition
- Proposed to contribute to cognitive
processes - more astrocytes increased cognition
High levels of extracellular glutamate are
toxic to neurons
Genetic knockdown of EAAT 1 and 2
(astrocytic) result in
widespread increases in
glutamate levels esp. in the striatum
Knockdown of activity of EAAT3 (neuronal
type)
has much more limited effects
Astrocyte pathway of glutamate recycling is
the
dominant pathway
EAAT2 abnormalities are observed in
amyotrophic lateral sclerosis (ALS)
Glutamatergic synapses are wrapped by
astrocyte processes expressing EAAT1/2.
Glutamate uptake into astrocytes is
Is rapid,
high efficiency, and prevents spillover of
glutamate into adjacent synapses
Astrocytes are the principal site of
glutamate breakdown.
Glutamine is exported from
astrocytes and
taken up into neurons to be converted back
to glutamate
- MSG can be used experimentally
to
induce glutamatergic lesions
MSG is proposed as one of the
five basic
tastes (referred to as
umami)
MSG is proposed as one of the
five basic tastes (referred to as
umami)
Acts on glutamate receptors on
the tongue
MSG syndrome is a widely
reported reaction to
MSG
Glutamatergic neurons and systems - Pyramidal neurons of the cerebral cortex (4)
Projections to striatum, thalamus,
limbic, brainstem
Glutamatergic neurons and systems Corticospinal tracts
Voluntary motor control
Glutamatergic neurons and systems Parallel fibers of the cerebellum
Excitatory inputs to Purkinje cells
Glutamatergic neurons and systems area
Hippocampus
Glutamate receptors
The most important receptors are
ionotropic
Synaptic transmission elicits
excitatory
postsynaptic potentials (EPSP)
Ionotropic receptors (3)
AMPA receptors
Kainate receptors
NMDA receptors
Metabotropic receptors Group I
mGluR1, mGluR5
Gq → PLC, Ca2+
Metabotropic receptors Group II
mGluR2, mGluR3
Gi → ↓ cAMP
Metabotropic receptors Group III
mGluR4, mGluR6, mGluR7, mGluR8
Gi → ↓ cAMP
AMPA Receptors
Four types of subunits and form
(GluR1-4)
form heterotetramers (dimers of dimers)
AMPA Receptors Rapid kinetics Onset, offset, desensitization
occur within
milliseconds
AMPA Receptors Rapid kinetics Single channel conductance on
n picosecond
timescale (10
-12 s)
AMPA Receptors * Experimental antagonists (3)
NBQX, CNQX, DNQX
- Specific mutations in AMPAR
(GRIN2A gene)
associated with
58% decrease in Parkinson’s
risk if also a heavy coffee drinker
Kainate receptors Functionally similar to
AMPA receptors
Kainate receptors 5 subunits (termed
GluK1
-5
Kainate receptors Selective agonist is
kainat
Kainate receptors Somewhat slower than
AMPAR
Kainate receptors Limited role in
fast, excitatory
transmission
Kainate receptors Limited role in fast, excitatory
transmission
Can be expressed presynaptically at
GABAergic synapses
AMPA and kainate receptors have very
similar pharmacology
Agonists at kainate and AMPA receptors
cause
seizures
- Kainic acid is used as a model of
epilepsy
Kainic acid is used as a model of epilepsy Repeated administration causes
development of spontaneous seizures
AMPA and Kainate pharmacology Agonists (3)
Kainate/Kainic acid
AMPA
Domoic acid
Kainate/Kainic acid
Kainate > AMPAR
AMPA
AMPAR»_space;
Kainate
Domoic acid
Kainate > AMPAR
Domoic acid causes
Causes amnesiac
shellfish poisoning
in humans
AMPA and Kainate pharmacology Antagonists: (2)
NBQX
NS102
NBQX
AMPAR»_space;
kainate
NS102
Kainate»_space;
AMPAR
NMDA Receptors Widely distributed (5)
Cortex, hippocampus, basal
ganglia, septum, cerebellum
NMDA Receptors Always co-expressed with
either AMPA or kainate
receptors
NMDA Receptors Permeable to
Ca2+ as well as
Na+ and K+
NMDA Receptors Highly regulated
- 6 binding sites for endogenous
ligands and modulators
NMDA Receptors * Important in
learning and
memory processes by
modulating synaptic strength
NMDAR binding sites (6)
Glutamate site
Glycine/D-serine site
Polyamine binding site
Mg2+ binding site
Zn2+ binding site
H+ binding
NMDAR binding sites Glutamate site
obligatory agonist binding site
NMDAR binding sites Glycine/D-serine site
obligatory co-agonist binding site
NMDAR binding sites Polyamine binding site
Site of endogenous allosteric
modulation (positive)
NMDAR binding sites Mg2+ binding site
voltage dependent block of channel
opening
NMDAR binding sites Zn2+ binding site
negative allosteric modulation site
NMDAR binding sites H+ binding
pH sensitive negative modulation
NMDAR Gating resting state
resting state - Mg2+ occupies the
channel pore
NMDAR Gating Agonist binding alone is
Is insufficient
to allow ion flux.
NMDAR Gating The pore is ‘unblocked’ when a
depolarization is previously present – displacing Mg2+ in n voltage-dependent manner
NMDAR Gating NMDA receptors are only active
after
an initial depolarization
(through AMPA receptors).
NMDA is described as a
coincidence detector
opening
only under conditions of strong or
repeated stimulation.
NMDAR Pharmacology Agonists:
Endogenous
exogenous
Endogenous NMDAR * Agonists
Glutamate and glycine / D-serine (both obligatory)
Polyamines (e.g. spermine, spermidine) – allosteric modulators
Exogenous NMDAR agonist
NMDA (N-methyl-D-aspartate) – synthetic amino acid
NMDAR Pharmacology Antagonists Endogenous:
Zn2+ (allosteric), Mg2+
NMDAR Pharmacology Antagonists Exogenous
MK801 – widely used experimental antagonist – non-competitive
PCP and ketamine – dissociative anesthetics / recreational – non-competitive