Chapter 8: Glutamate And GABA Flashcards
Glutamine—>
Glutamate
Enzymes: glutaminase
Glutamine storage and release
VGLUT (vesicular glutamate transporter)- found only in cells that use Glu as NT
Either VGLUT1 or 2 are found in most neurons
- VGLUT1- cortex and hippocampus
- VGLUT2- subcortical structures
VGLUT3- inner hair cells of ear
Glutamate inactivation
Excitatory amino acid transporters (EAATs)
EAAT1/2- expressed by astrocytes
- astrocytes play most important role in taking up Glu after release
EAAT3- found on postsynaptic cells
EAAT4- Purkinje cells in cerebellum
EAAT5- bipolar cells of retina
Glutamate—>
Glutamine
Enzyme: glutamine synthetase
Location: astrocyte
*helps in metabolism and removal of ammonia
Glutamate is most abundant […] NT in the brain
Glutamate is most abundant excitatory NT in the brain
- found in all neurons and glial cells
- glutametergic neurons segregate Glu they use for transmission vs metabolism
Location:
- Cortex
- Pyramidal neurons- project to striatum, thalamus, limbic system structures, and brain stem - Cerebellum
- Parallel fiber (GC’s) input to Purkinje neurons - Hippocampus
- DG, CA1, CA3
The functional roles of Glu are important in neurotransmission, plasticity, and disease
Fast excitatory neurotransmission
Learning and memory
- neuronal plasticity
Neurological disorders
- neurodegenerative disorders
- cells death (excitotoxicity; apoptosis)
Possible involvement of Glu in drug addiction, schizophrenia, and other psychopathology
Glu is an agonist of both […] and […] receptors
Glu is an agonist of both ionotropic and metabotropic receptors
iGlu receptors
NMDA- Na+ and Ca2+ flow through channel
- Ca2+ activates 2nd- messenger system
non-NMDA- Na+ and K+ flows through receptor channels
- AMPA - kainic acid (KA) - locomotor activity, coordination, and brain excitability
- excitatory response
- only has 4 subunits
mGlu receptors
Group I (mGlu1,5)- postsynaptic and excitatory responses via PIP2 system
Group II (mGlu2,3)- inhibit cAMP formation
Group III (mGlu4, 6 and 7)- inhibit cAMP formation
- L-AP4 is selective agonist
Non-NMDA Pharmacology
Agonists: AMPA and kainic acid
Antagonist: NBQX
Non-NMDA Structure
GluA1-A4; GluA5-A7; KA1, KA2
4 subunits; dimer of dimers
Non-NMDA Physiology
Most AMPA/KA-R’s increases gNa+/gK+
GluA2- lacking AMPA receptors are Ca2+ permeable
NMDA receptors are highly regulated
Ligand and voltage gated
Subunits:
GluN1 x2: Ser and Gly bind
GluN2 x2: Glu binds
Channels only open if:
1. Glu is released into NMDA receptors 2. Cell membrane is depolarized by stimulation of different excitatory receptor
NMDA Co-agonists
- Glutamate (NMDA)
- Glycine (D-serine)
NMDA receptor antagonists
- Competitive: AP5 (APV)
- Non-competitive: PCP, MK-801, and Namenda, ketamine
NMDA Mg2+ block
Depolarization removes Mg2+ block
Highly Ca2+ permeable
Voltage-dependence of NR is due to […] in the […]
Voltage-dependence of NR is due to Mg2+ in the pore of the channel
Hyperpolarized Vm
- Mg2+ blocks the channel
- Non-conductive
Depolarized Vm
- Mg2+ is expelled
- Highly permeable to Ca2+
NMDA receptors are critical in some forms of […]
NMDA receptors are critical in some forms of LTP
- Induction phase
- Expression phase
Induction Phase of LTP
- Depolarization
- NMDA receptor activation
- Ca2+ influx- activates protein kinases, including CAMKII
Expression phase of LTP
Many kinases phosphorylate several targets involved in AMPA receptor trafficking
- Receptor trafficking- NT receptors continuously moved into and out of cell membrane
- CAMKII can stay activated even after Ca2+ return to baseline
[…] enhance cognitive function
AMPA receptor modulators enhance cognitive function
- Nootropics
- Ampakines
Ampakines
- Positive allosteric modulators of AMPA receptors
- Prevent deactivation and/or desensitization
- Increase channel open time
The ampakine […] improves performance on the delayed match-to-sample task
The ampakine CX717 improves performance on the delayed match-to-sample task
The ampakine […] increases dendritic arborization and spine density in aged rats
The ampakine CX929 increases dendritic arborization and spine density in aged rats
Excitotoxicity hypothesis
Prolonged neuronal depolarization leads to neuronal damage and/ or cell death
Three forms of cell death from strong activation of NMDA receptors
Necrosis- mode of cell death characterized by lysis of cell due to osmotic swelling
Apoptosis (programmed cell death)- disruption of nucleus, DNA breakup, and cell death (cleared out by phagocytosis)
Programmed necrosis- provoked by excitotoxic treatment of cells (necroptosis)
High [Glu]
- cell death by necrosis- characterized by lysis due to osmotic swelling
- involves both NMDA and non-NMDA receptors
Lower [Glu]
Cell death by apoptosis (no lysis)
Amyotrophic Lateral Sclerosis (ALS)
Motor neuron disease caused by excitotoxicity
- Treatment: Riluzole- reduces Glu release
Excitotoxic brain damage: domoic acid
Toxin made up of several species of marine algae
NMDA receptors are involved in 2 forms of cell death
Apoptosis
Programmed necrosis
Apoptosis
- capsase- dependent
- cell shrinkage
- DNA cleavage
- Phagocytosis of debris
Programmed necrosis
- capable-independent
- GluN2B- dependent
- extrasynaptic location
[…] is a leading cause of death
- loss of blood supply to the brain
- ischemic (occlusion)
- hemorrhagic
- O2 deprivation
O2 Deprivation (hypoxia)
Ischemic core
- brain tissue that’s completely deprived of oxygen rapidly dies
- ischemia- interruption of blood flow to brain due to excitotoxic brain damage
Penumbra- area of partial deprivation surrounding the core
Loss of blood/ O2 initiated cell death
- Loss of ATP
- Pumps fail (Na+/K+ ATPase)
- loss of ionic gradient
- cell swelling/ rupture
- membrane depolarization - Increased neuronal firing
- excessive Glu release
- NMDA-R activation
- voltage-gated Ca2+ channel activation - Increased Ca2+ influx
- Ca2+- activated protease and lipases
- Free radical production - Cell death (programmed necrosis)
Treatment of stroke damage is largely unsuccessful
Restore blood supply
Block Ca2+ influx
Promote Glu clearance
*Severe side-effects (eg. Psychosis)- too much time elapses between stroke and treatment
Restore blood supply
- Thrombolytics (TPA)
- Anticoagulants (heparin, warfarin)
Block Ca2+ influx
- NMDA receptor blockers
- Glycine site
- PCP site - voltage-gated Ca2+ channels blockers
Promote Glu clearance
Increase EAAT2 expression or function
Drugs that block GABA synthesis
Allyglycine, thiosemicarbazide, 3-mercaptopropanoic acid
GABA is found in […]
Cerebral cortex, hippocampus, substantia nigra
- also cerebellum, striatum, globus pallidus, and olfactory bulbs
- can be found in local interneurons and projection neurons (carry info longer distances)
- only has job as NT and is manufactured by only GABAergic neurons
GABA is the most abundant […] NT in the brain
GABA is the most abundant inhibitory NT in the brain
CNS depressants
- Anxiolytics
- Anesthetics
- Sedatives
- Anticonvulsants
- Tiagabine (Gabitril)
- Vigabatrin (Sabril)
Tiagabine (Gabritril)
Selective inhibitory of GAT-1
- increases extracellular GABA levels
- increased GABA transmissions
Vigabratrin (Sabril)
Irreversible inhibitor of GABA-T
- prevents GABA metabolism and leads to buildup of GABA in brain
GABA precursor
Glutamate—> GABA
Enzyme: Glutamic acid decarboxylase (GAD)
GABA packaging enzyme
Vesicular GABA transporter (VGAT)
- same transporter used to load Gly into synaptic vesicles
- sometimes referred to as VIAAT (vesicular inhibitory amino acid transporter)
GABA is coreleased with […]
GABA is coreleased with Gly, ACh, DA, and Glu
- coexpression of multiple vesicular transporters must take place (ex VIAAT and VAChT)
*Inhibitory neurons in CNS may release GABA only, Gly only, or both
GABA transporters
GAT-1 and GAT-2- found in both neurons and astrocytes
GAT-3- astrocytes
GAT-1
Can be found in astrocytes and at nerve terminals of GABAergic neurons: important for GABA reuptake
GABA—> Glutamate
Enzyme: GABA amino-transferase (GABA-T)
Location: astrocytes and neurons
- final product: succinate
- byproduct: glutamate
- in astrocytes, further synthesized to glutamine (using glutamine synthetase) and transported back to nerve terminal via glutamine transporters)
GABAa ionotropic receptors are permeable to […] causing […]
GABAa ionotropic receptors are permeable to Cl- causing hyperpolarization
GABAa ionotropic receptor subunits
5 subunits
2 a, 2 B, 1 g
Dominant type: (a1)2(B2)2(g2)
Extrasynaptic receptors: (a4)2(Bx)2(d) OR (a6)2(Bx)2(d)
- d binds neurosteroids and anesthetics
- further away from release site on postsynaptic neuron
GABAa ionotropic receptor ligands
- muscimol (agonist)
- Bucucculine (comp antagonist)- convulsant
- Picrotoxin, PTZ (non- competitive antagonist)
GABA subunit binds on […] subunit
GABA subunit binds on B subunit
a5 subunit is expressed most in […]
a5 subunit is expressed most in hippocampus
Many CNS depressants are […] of GABAa receptors
Many CNS depressants are allosteric modulators of GABAa receptors
Positive allosteric modulators of GABAa
- Benzodiazepines
- Barbituates
- Ethanol, anesthetics, and neurosteroids
Benzodiazepines
Binding requires g2 subunit (a-g interface)
Increases potency of GABA
- Increases affinity of GABA for B-subunit
- increases probability channel will open
- increases Cl- influx
- increases hyperpolarization
- ex. Diazepam (Valium)- no effect on its own, but makes GABAergic response stronger
Behavioral effects of alpha subunits of benzodiazepines
a1- sedation
- enhancement of tonic cellular inhibition - zolpidem (Ambien)
a2/a3: anxiolytic
- alprazolam (Xanax): most selective anxiolytic substance)
a5- amnestic
- found in hippocampus
*low levels: reduce anxiety
High levels: sedation
Barbituates
Habit forming, lethal in overdose (addiction potential)
Binding site:
- on B-subunit distinct from GABA/ agonist site
- increases Cl- flux
- increases channel open probability
- increases duration of channel opening
- can function in absence of agonist
Ethanol
Enhance GABAa receptor activity
Anesthetics
Includes voltage-gated ion channels and ligand-gated ion channels
- propofol (Diprivan)
- a5- amnestic
Neurosteroids
- Steroid hormones synthesized in brain that act locally on GABAa receptor
- Binding site on d-subunit, converts GABA from partial agonist to full agonist (extrasynaptic receptor)
Ex. Allopregnanolone (Zulresso) used in post-partum depression, allotetrahydrodeoxycorticosterone, and androstanediol
Negative allosteric modulators
Picrotoxin, pentylenetetrazol (PTZ)
*convulsants
GABAb receptors are […]
GABAb receptors are metabotropic
Postsynaptic GABAb receptors
- open K+ channels
- inhibition of cAMP
Presynaptic GABAb receptors
- autoreceptors, heteroreceptors
- inhibition of Ca2+ channels
- inhibition of cAMP
GABAb agonist
Baclofen (Lioresal)
- muscle relaxant and anti spastic agent
GABAb antagonist
Competitive antagonist: Saclofen and 2- hydroxysaclofen
