Block D Flashcards
what is GABA
amino acid that is the main inhibitory transmitter in the brain
what is GABA synthesised from
glutamate by (GAD)
where is GAD found
only in GABA-synthesising neurons in the brain
where is GABA found
exclusively in brain tissue
GABA metabolism steps
synthesis, synaptic removal, catabolism
GABA synthesis
glutamte to GABA by GAD in nerve terminals
GABA synaptic removal
GAT terminates action, sodium ion symporter (Na down GABA up)
GABA catabolism
within neuronal and non-neuronal tissue (mitochondria)
GABA to succinate by GABA-T
What is binding of GABA to the brain like
saturable and specific
GABA binding sites
GABA receptors
-binding is not sodium dependent
GABA uptake sites
-binding is sodium dependent
-neuronal and non-neuronal
-greatly outnumber GABA receptor sites
what are the two subtypes of GABA receptors
GABAa
-bicuculine sensitive
-baclofen insensitive
GABAb
-bicuculine insensitive
-baclofen sensitive
what type of receptors are GABAa
inotropic
what type of receptors are GABAb
metabotropic
structure of the GABAa receptor complex
-ligand gated chloride ion channel
-subunits are standard 4TM structure
-pentameric (5 subunits)
what are the GABAa receptor subunits
isoforms
GABA binding on GABAa binding sites
-two interfaces between alpha and beta subunits
-must bind GABA at both interfaces for activation
Benzodiazepine binding on GABAa binding sites
interfaces between alpha and gamma subunits
GABAa-rho receptors
-once known as GABAc
-similar structure to GABAa but a different pharmacology
-found in retinal bipolar cells
what kind of channel is GABAa-rho receptor
five subunit ligand-gated ion channel, composed solely of the three rho subunit varieties
what are GABAa-rho insensitive and sensitive to
insensitive
-bicucline, barbiturates, benzodiazepines, baclofen
sensitive
-CACA (agonoist), TPMPA (antagonist), picrotoxin (antagonist)
what kind of receptor is GABAb
metabotropic
what kind of dimer receptor is GABAb
hetrodimer of two 7TM receptors
-GABAb1 and GABAb2
-only b1 binds GABA to N-terminal
VFT region
-B2 binds postive allosteric modulators
what does GABAb couple to
Gi/Go
what is GABA in the CNS
inhibitory neurotransmitter, suppresses neuronal activation.
-presynaptic inhibition (axo-axonal inhibition)
-postsynaptic inhibition (recurrent inhibition)
what does GABA do in the spinal cord
reduces transmitter release from terminals of primary afferent fibres
why is GABA pharmacologically distinct from glycine
-bicuculline and picrotoxin block GABA effects
-strychnine blocks glycine effects
what kind of neurotransmitter is GABA in the CNS
inhibitory so surpasses neuronal activation
presynaptic inhibition
-axo-axonal inhibition
postsynaptic inhibition
-recurrent inhibition
GABA Presynaptic Inhibition
-reduces transmitter release from terminals of primary afferent fibres
-pharmacologically distinct from glycine
–>bicuclline and picrotoxin block GABA effects
–>styrchnine blocks glycine effects
-involves axo-axonal synaptic connections
axo-axonal synapses- presynaptic inhibition
-primary afferent fibre
-GABA neuron mediating presynaptic inhibition
-reduced release of neurotransmitter from terminal of primary afferent fibre
postsynaptic inhibition
-most GABA effects in brain
-increased chloride flux postsynpactically
examples of postsynaptic inhibition pathways
striatal (caudate putamen) -> substantia nigra
-inhibit firing of DA neurons
-direct and indirect (interneuron) effect
hippocampal and cerebral pyramidal cells
-external and recurrent inhibition
physiological roles of GABAb receptors
effects
-presynaptic decreased calcium ion fluxes
-postsynaptic increases potassium fluxes
implicated as target for management of
-pain, absence epilepsy, cocaine addition, asthma
benzodiazepine chemical structure
7 membered (2N) ring fused to an aromatic ring
4 main substituent groups
benzodiazepine mechanism of action
enhance presynaptic inhibition in spinal cord
-blocked by bicuculine
decreasing GABA prevents effects of BDZs
-effect requires normal levels of GABA
benzodiazepine binding
-BZD clinical efficacy correlates to binding affinity
-GABA does not displace BZDs
-BZDs do not displace GABA
location of BZD binding sites
-correlates to presence of GAD
-localised at GABAergic synapse
BZD binding site
a specific binding site on the alpha subunit of the GABAa receptor complex
-modulates the binding of GABA to its site
-modulates the opening of the chloride ion channel
what do classical BZDs acts as at the BZD site
postive allosteric modulators
-located at the interface of the alpha and gamma subunits
-enhance the inhibitory effects of GABA
do BZDs act as agonists
no, their binding evokes no response
what does BZDs enhancing the ability of activated GABA receptors to open mean
-increased frequency of opening
-no change in channel conductance
-no change in mean duration of opening
interaction of GABA and BZD
GABA facilitates BZD binding to its binding site on the receptor
BZDs facilitate GABA binding to its binding site on the receptor
what does the reciprocal relations between GABA and BZD result in
GABA shift
what happens to the dose-response curve if BZDs are absence
the curve is further rightwards
agonist has
affinity and positive efficacy (e=1)
inverse agonist has
affinity and negative efficacy (e=-1)
antagonist has
affinity and no efficacy (e=0)
partial agonist has
affinity and low positive efficacy (0<e<1)
what are endozepines
endogenous compounds with BZD like effects
diazepam binding inhibitor
- acyl-CoA-binding protein (10 kDa)
- binds medium- and long-chain acyl-CoA esters
- acts as an intracellular carrier of acyl-CoA esters
- displaces BZD and Z-drugs from GABAA complex
oleamide
- derived from the fatty acid oleic acid
- accumulate in thecerebrospinal fluidduringsleep deprivation
- induces sleep in animals
- interacts with multipleneurotransmittersystems (including cannabinoid and BDZ)
BZD receptor “agonists”
-positive allosteric modulators
-Enhance GABA-mediate inhibitory effects
->increase GABA affinity for its binding site
-> increase GABA-mediated ion channel opening frequency
-Behavioural effects
-> anxiolytic, anticonvulsant
-Examples
-> diazepam, clonazepam
Z-drugs
-Nonbenzodiazepine agonists of benzodiazepine receptor
-> chemically unrelated to BZD
-Zolpidem, zopiclone, zaleplon, and eszopiclone
-Clinical properties and therapeutic uses similar to BZD
what do all Z-drugs lack
characteristic 7 membered ring
BZD receptor “inverse agonists”
-negative allosteric modulators
-reduce GABA-mediate inhibitory effects
->decrease GABA affinity for binding site
->decrease GABA-mediated ion channel opening frequency
-reversed behavioural effects to BZDS
->anxiogenic, proconvulsant, panic attacks
-examples
->Beta-carbolines, FG-7142 (beta-carboline derivative
BZD receptor antagonists
Ro-15-1788
- imidazobenzodiazepine derivative
- flumazenil *
Blocks binding
- of agonists (e.g. benzodiazepines)
- of inverse agonists (e.g. β-carbolines)
Blocks behavioural effects
- of agonists (e.g. benzodiazepines) – >can be used to treat BZD overdose
- of inverse agonists (e.g. β-carbolines)
No intrinsic behavioural effects
BZD receptor “partial agonists”
Low efficacy
- cannot produce maximum response
Occupy a large proportion of receptors
- reduce response of full agonist
Partial BZD agonists
- restricted behavioural profile
- good anxiolytic effect (requires low receptor occupancy)
- poor sedative effect (requires high receptor occupancy)
- e.g. bretazenil and abecarnil
what affect to BZDs have on CNS
CNS depressants
-enhance GABAs actions leading to depressants
unwanted CNS effects of BZD-R agonists
Excessive CNS depression
- drowsiness, confusion, memory problems
Paradoxical CNS effects
- increased anxiety, aggressive behaviour
- more common in children, elderly and with shorter acting drugs
Habituation (dependence)
- associated with an unpleasant withdrawal syndrome
Tolerance
- loss of clinical effect on repeated administration
- greatest for hypnosis, less for anxiolysis and anticonvulsion
therapeutic uses of BZD-R agonists
To treat epilepsy
- bursts of electrical activity in the CNS causing seizures or fits
- BZD strengthen inhibitory inputs, increasing seizure threshold
To treat muscle spasm
- caused by injury, inflammation or nerve disorders
- due to inhibitory action of BZDs in spinal cord
To reduce anxiety
- due to inhibitory action of BZDs in limbic system
To sedate restless patients and promote sleep
To induce surgical anaesthesia
- intravenous administration of short-acting compound
BZD metabolism
Metabolised in liver
- by CYP3A4 and/or CYP2C19
- but not inducers of liver enzymes
Many primary metabolites are also BZD agonists
- needs to be factored into duration of drug effect
Many undergo glucuronidation
Excreted primarily via the kidneys
- and also in faeces
Very wide range of plasma half-lives
- from 1 hour to 5 days
Half-life influences clinical use of BZD
Long half-life (30 – 60 hours) - used as anticonvulsants
Intermediate half-life (10 – 20 hours) - used as anxiolytics
Short half-life (under 10 hours) - used as hypnotics, minimises daytime sedation (hangover effect)
- shorter for problems getting to sleep
- longer for problems staying asleep
Very short half-life (1 – 3 hours)
- used as IV anaesthetics
midazolam
half life= 2 hours
clinical use= intravenous anaesthetic
flunitrazepam
half-life= 20-30 hours
clinical use= anxiolytic
triazolam
half-life= 3-4 hours
clinical use=hypnotic
clonazepam
half-life= 20-60 hours
clinical use= anticonvulsant
flumazenil
Therapeutically only the BZD antagonists, i.e., flumazenil are used (in addition to agonists)
Pure antagonist of BZD and Z-drugs
Short half-life (approximately 1 hour)
- given intravenously, and often repeatedly to outlast BZD
Used in intensive care
- diagnosis of coma of unknown origin (to reveal BZD poisoning)
- treatment of BZD poisoning
Used in anaesthesia
- reversal of anaesthesia produced by a BZD
- reversal of sedation induced by a BZD used for short interventions
Used in treatment of hepatic encephalopathy
- where abnormal endogenous compounds are acting as BZD agonists
Z-drugs versus BXD
Z-drugs are largely used as hypnotics
Considered more effective than BZD
- short half-life of z-drugs minimises hangover effect
Considered safer than BZD
- particularly in elderly
- but similar side-effect profile
Produce less tolerance
Produce less habituation
- reduced withdrawal problems
where is glutamate widely distributed throughout
CNS
what kind of neurotransmitter is Glutamate
excitatory
metabotropic receptor structure
Dimeric G-protein coupled receptor
- each subunits has seven membrane-spanning region
- similar structure to muscarinic and adrenoceptors receptors
Eight identified subunit variants
- identified by molecular biology (gene sequencing)
- mGlu1 – mGlu8
Functional receptors are homodimers
- two of the same subunits
- form three physiologically and pharmacologically distinct groups
metabotropic receptor grouping
Group 1
- mGlu1 and mGlu5
Group 2
- mGlu2 and mGlu3
Group 3
- mGlu4, mGlu6, mGlu7 and Glu8
Group 1
coupling= Gq/11
effector mechanism= increase PLC
second messenger= increase IP3/DAG
Group 2
coupling= Gi/o
effector mechamism= decrease AC
second messenger= decrease cAMP
Group 3
coupling= Gi/o
effector mechanism= decrease AC
second messenger= decrease cAMP
group 1 cellular response
- located mainly on postsynaptic membranes
- activation increases Na+ and K + conductance’s (excitation)
- activation increases inhibitory postsynaptic potentials (inhibition)
- activation leads to modulation of voltage-dependent Ca2+ channels
Group 2 and 3 cellular response
- located mainly on presynaptic membranes
- activation has no direct effect on postsynaptic potentials
- activation leads to increased presynaptic inhibition
- activation leads to reduced activity of postsynaptic potentials (both excitatory and inhibitory)
metabotropic receptor functional roles
Modulation of NMDA receptor function
- group I increase NMDA receptor activity
- groups II/III decrease NMDA receptor activity (protect from excitotoxicity)
Synaptic plasticity
- participate in long-term potentiation and long-term depression (removed from synaptic membrane in response to agonist binding)
Control of hypothalamic-pituitary-adrenal axis
- regulation of cortisol levels and stress responses
Putative role in disease
- expression of mGlu1 may be involved in development of certain cancers
- evidence for mGlu2/3 agonists (eglumegad) in treatment of schizophrenia
- activation of mGlu4 receptors could treat Parkinson’s disease
inotropic glutamate receptors
NMDA, AMPA, kainate, delta
- integral receptor/cation channels
- each receptor composed of four subunits
Multiple subunits for each major class of receptor
- NMDA: GluN1, GluN2A, GluN2B, GluN2C, GluN2D, GluN3A, GluN3B- AMPA: GluA1, GluA2, Glu3, GluA4
- kainate: GluK1, GluK2, GluK3, GluK4, GluK5
- delta: GluD1, GluD2
