Glutamatergic Neurotransmission

what type of NT is glutamate?

• small clear core molecule NT

• primary excitatory NT

____ cells have the capacity to make glu

all

the glu/gln cycle is the only NT system that __________

recycles

what cells are a glu sink (limit excitation)?

astrocytes

describe process of going from glucose in a glutamatergic neuron to an astrocyte

1. glucose

2. pyruvate (via glycolysis)

3. alpha-keto glutarate (via TCA cycle)

4. glutamate

5. vessicle

6. synapse

7. glu transporter on astrocyte

how is glu recycled?

• once in an astrocyte, glu is turned to gln via glutamine synthetase

• gln is released into synapse and transported back into glutamatergic neuron

• glutamatergic neuron turns gln back into glutamate

why is glu converted to gln?

gln doesn’t activate anything when it is released, which allows it to return to the glutamatergic neuron and be recycled into glu

• prevents a prolonged signal

• maintains glu levels

EAAT

Excitatory Amino Acid Transporter (for glu)

function of EAAT

terminates neurotransmission by “eating” glu out of synapse - brings glu into cell

EAAT is dependent on:

Na and K gradients (formed by Na/K ATPase)

where is the concentration of glutamate the lowest?

synapse

where is the concentration gradient of glutamate the highest?

in a vesicle

list locations of glu from least to most concentrated

1. synapse

2. cytoplasm

3. vesicle

as glutamate is moved from a synapse into a vesicle, it is moving _______ its concentration gradient

against

the VGlut transport system uses ___________ across the synaptic vesicle to concentrate glu into the vesicle

H+ gradient

packaging of glu into vesicles requies…

functional proton ATPase in synaptic vesicle membrane

VGlut

vesicular glu transporter

• concentrate glu into vesicle

describe pH inside vesicle

low (lots of H+)

ionotropic glu receptors are coupled to:

ion channels

are post-synaptic glu receptors excitatory or inhibitory?

excitatory

why are post-synaptic glu receptors excitatory?

they’re permeable to positively charged ions

list 3 typesof ionotropic receptors

1. AMPA

2. Kainate

3. NMDA

AMPA receptors are the _______ of the glutamatergic system

workhorse

AMPA receptors consist of ______ subunits

4

each AMPA subunit consists of ________ primary regions in the membrane

3

3 primary regions in the membrane of AMPA receptors include:

1. transmembrane domain

2. ligand binding domain

3. N-terminal domain

4 potential subunits for AMPA receptors

GluR1-GluR4

how many potential subunits for AMPA receptors?

4

ligand of AMPA receptors

glu

effect of having different combinations of subunits in a receptor

change properties

most AMPA subunits are only permeable to:

Na+

if an AMPA receptor lacks GluR2, then it is permeable to ________ in addition to Na (RARE)

Ca2+

NMDA receptors only work if…

something depolarizes the membrane first, causing Mg2+ to move out of the way (AMPA will do this)

how many subunits does NMDA need to be functional?

4

NMDA receptors provide the opportunity for _________

synaptic plasticity

NMDA receptors are obligatory __________ assemblies

heterotetrameric

NMDA receptors are usually composed of:

• 2 glycine binding GluN1 subunits

• 2 glutamate binding GluN2A-D (occasionally GluN3)

NMDA receptors are always more permeable to which ion?

Ca2+

NMDA receptors are more permeable to Ca2+ than ___

Na+

which receptor type is voltage-gated?

NMDA (Mg2+)

what feature makes NMDA receptors voltage-gated?

blockage of pore with magnesium

what moves the Mg2+ out of the pore of an NMDA receptor?

depolarization

how many transmembrane domains in an NMDA receptor?

3

what helps depolarize the membrane causing the Mg2+ plug to be removed from NMDA receptors?

AMPA receptor activation

kainate receptors

• heterotetrameric assemblies which are similar to AMPA and NMDA receptors

• not as prevalent

• primarily permeable to Na+

what are the 3 activities of kainate receptors?

1. critical for the reception of excitatory synaptic signaling

2. regulation of pre-synaptic glutamate and GABA release (volume control)

3. development of dendrites

how many subunits in kainate receptors?

4

kainate receptors are primarily permeable to which ion?

Na+

What G-proteins are associated with Group 1 of metabotropic glutamate receptors?

Gq and or Gs

What G-protein is associated with Group 2 of metabotropic glutamate receptors?

Gi/o

how are metabotropic glutamate receptors differentiated?

by the G-proteins they couple to, their function, and localization of expression

how many transmembrane domains in each subunit of mGluR?

7 (14 total in receptor)

how many subunits in mGluRs?

2

unlike other receptors, NMDA receptors require ___________

co-agonists

• Glycine/D-serine

describe localization of mGluRs

• postsynaptic

• presynaptic

• autoreceptors (excitatory or inhibitory)

autoreceptor

control release of their own NT

function of presynaptic mGluRs

monotiro and modify amount of Glu in the synapse (by modulating Ca2+ levels)

function of system xc-

• sets extracellular Glu concentration in some regions of the brain

• important in anti-oxidant repsonse of the cells (cystine → glutathione → reduces ROS)

• expressed highly in astrocytes

system xc- pumps Glu _____ its concentration gradient

down

system xc- pumps cystine _______ its concentration gradient

against

how deos system xc- regulate the oxidant response of cells?

transports cystine into the cell where it is transformed into glutathione, an antioxidant

glutathione

protects cells from oxidative stress (results from high metabolic activity in the brain)

• produced from cystine

what is the hallmark of glutamatergic neurotransmission?

synaptic plasticity

after a brief tetanus (strong stimulation), we see an increase in the _______ response

post-synaptic

two types of synaptic response in glutamatergic neurotransmission

1. post-tetanic potentiation (pre-synaptic and calcium mediated)

2. long term potentiation (post-synaptic changes)

post-synaptic potentiation is mediated by:

calcium

is post-tetanic potentiation pre or post-synaptic?

pre-synaptic

effect of post-tetanic potentiation

increases glu release

describe process of post-tetanic potentiation

• calcium increases

• calmodulin (CaM) increases

• CaM recruits more vesicles to dock at the plasma membrane

• more Glu is released

3 phases of long-term potentiation

1. induction

2. expression (early LTP)

3. stabilization (late LTP)

early stages of long-term potentiation are primarily an ___________ event

AMPA receptor trafficking

describe early stages of LTP

• increased AMPA signal → release of Mg2+ from pore of NMDA receptors

• Ca2+ enters through NMDA receptors

• activates CaMKinase IIa

• CaMKinase IIa phosphorylates AMPA receptors in storage vesicles

• increased trafficking of vesicles to the plasma membrane

• more AMPA receptors receive the signal

• leads to a larger post-synaptic response in the short term

function of CaMKinase IIa

phosphorylates AMPA receptors in storage vesicles (early stage LTP)

effect of phosphorylation of AMPA receptors in storage vesicles (early LTP)

increased trafficking of vesicles to the plasma membrane

late stage of long term potentiation requires:

protein synthesis

late stage of long term potentiation is due to:

gene expression

describe late stage of long term potentiation

• Increases in calcium post-synaptically also activates calmodulin.

• Calmodulin increases adenylyl cyclase activity leading to more cAMP

• cAMP activates kinases that phosphorylate a transcription factor (cAMP Response Element Binding protein-1 CREB-1

• Genes with CREB binding domains will be stimulated to produce BDNF and other neurotrophic factors that lead to the production of new dendritic spikes near by.

what does calmodulin do in late stage of LTP?

activates adenylyl cyclase, leading to an increase in cAMP

role of cAMP in late stage LTP

activates kinases that phosphorylate transcription factor CREB

role of CREB in late stage LTP

activates genes, stimulating production of BDNF and other neurotrophic factors that lead to the production of new dendritic spines

how does CREB activate genes?

recruits transcription machinery to a gene to turn on production of mRNA

CREB only functions when…

phosphorylated

what part of CREB interacts with DNA?

flared part/pore

does long term depression rely on NMDA receptors?

NO

what triggers long term depression (LTD)?

calcium

describe long term depression (LTD)

• triggered by calcium

• Often through activation of mGluR receptors

• This leads to a lower, but longer lasting rise in intracellular calcium that activate phosphatases (not enough to activate calmodulin)

• This removes phosphorylation on AMPA receptors

• Diminishes post-synaptic response

• Diminished NMDA receptor activation, leading to less CREB activation, less BDNF production and synapse loss

effect of LTD

lower but longer lasting rise in intracellular calcium that activates phosphatases

does LTD activate calmodulin?

NO

does LTD increase or decrease post-synaptic response?

decrease

what happens to synapses in LTD?

synapses are loss (less BDNF is produced)

how is glutamatergic neurotransmission involved in ischemic stroke?

• malfunction of cytC (ETC) in mitochondria leads to oxidative stress

• less ATP is produced

• loss/reduced function of Na/K ATPase

• membrane is depolarized

• lots of Glu is released

• excess Glu acts stimulates NMDA and inhibits system xc-

• produces more free radicals and less glutathione to regulate them

excitotoxicity

excess Glu leads to high levels of free radicals, creating oxidative damage and cell death

• additionally, excess Glu causes system xc- to operate in reverse, inhibiting glutathione production