Neurotransmitters and Receptors Flashcards
What is the history of glutamate?
- Role as a neurotransmitter first suggest by Hayashi 1954 who injected glutamate into dogs and monkeys resulting in seizures
- Found to depolarise neurons in cat spinal cord in 1950s
- J.C watkins did a series of experiments in the 1970s that elucidated the ionotropic subtypes, showed the Mg2+ block in NMDA, and established L-glutamate as main neurotransmitter of CNS
Classes of glutamate receptors
- AMPA
- Kainate
- NMDA
^ all ionotropic - mGluRs
^ metabotropic
EPSP
An electrical change (depolarisation) in the membrane of a postsynaptic neuron caused by the binding of an excitatory neurotransmitter from a presynaptic cell to a postsynaptic receptor, makes it more likely for a postsynaptic neuron to generate an action potential.
IPSP
An electrical change (hyperpolarisation) in the membrane of a postsynaptic neuron caused by the binding of an inhibitory neurotransmitter from a presynaptic cell to a postsynaptic receptor, makes it more difficult for a postsynaptic neuron to generate an action potential.
AP1 subunit
glutaminergic subunit that facilitates calcium permeability
Current dynamic of NMDA
Slow rise and slow decay
Current dynamic of AMPA
Rapid rise and rapid decay
Have varying pore size, depending on # glutamate occupying subunits (more glutamate, bigger pore, bigger EPSP)
Current dynamic of Kainate
Rapid rise and slow decay
Slow decay partially due to nearby AMPA receptor activation
NMDA antagonist
APV/AP5
AMPA antagonist
CQNX, GYKI (AMPA specific)
Kainate antagonist
CQNX
Shared features of AMPA and kainate
Agonists: glutamate, aspartate, AMPA, quisqualate, kainate
Antagonist: CQNX
Distribution of AMPA
AMPA is the most widely expressed glutamate receptor in the brain and is concentrated in:
- CA1
- thalamus
- TE
- cortex
Kainate distribution
Concentrated in
- CA3
- dentate gyrus
- cortex
Subunit arrangement of glutamate receptors
Different subunit arrangements present in a variety of compositions to give rise to receptors with widely different properties
These compositions can change over time, influenced by changes in condition
GluA2
- AMPA without GluA2 or the unedited GluA2(Q) are permeable to calcium
- AMPA with edited GluA2(R) are NOT permeable to calcium (majority)
Catecholamines
chemicals that have an amine group with either one (monoamine) or two (indolamine) rings
Peripheral action of serotonin
Serotonin is produced by enterochromaffin cells in the intestinal wall and platelets in the serum
= 90% of serotonin
Acts to regulate vascular tone
Central action of serotonin
Synthesised across brain, but primarily in the raphe nuclei
Acts as a neurotransmitter
Synthesis of serotonin
Tryptophan
- tryptophan hydroxylase
5-hydroxytryptophan
- dopa decarboxylase
Serotonin
- MAO
Marker used to identify serotonergic neurons
Tryptophan hydroxylase isoform 2
Diversity of serotonin receptors
The primordial serotonin receptor is over 750 million years old, and has given rise to the 7 key receptors that provide wide diversity
In addition to the core receptors there are transporter proteins and auto/heteroreceptors
5HT1
Gai coupled GPCR located throughout the CNS
5HT2
Gq coupled GPCR that acts to increase phospholipase C, and is located throughout the cortex
5HT3
Cation channel expressed in cortical interneurons, the hypothalamus and the CTZ
Required the 3A subunit for expression, and can be permeable to calcium depending on subunits expressed
5HT4
Gs coupled GPCR located
in striatum/olfactory system/HPC
5HT5/6/7
Gs coupled GPCRs located
5: whole brain
6: striatum/cortex
7: hypothalamus/cortex
SERT
serotonin transport protein composed of 12 transmembrane domains
- transport is voltage dependent on Cl- and Na+ gradients
- targeted by SSRIs, TCAs, etc
Serotonin autoreceptor
5HTD
serotonin heteroreceptor
alpha-2 heteroreceptor
MAOa
Monoamine oxidase A
Degrades NA, 5HT, and DA but has highest affinity for 5HT
MAOb
Monoamine oxidase B
degrades DA and tyramine
Desensitisation of serotonin receptors
Desensitisation occurs rapidly through PKC-mediated phosphorylation, which is a calcium dependent process
Reversed via phosphatase action following ligand dissociation
Homeostatic responses to serotonergic activation
Desensitisation (rapid)
Internalisation (minutes to hours)
Downregulation (hours to days)
The monoamine hypothesis of depression
Depression is due to a deficiency of noradrenaline and serotonin
- some believe different forms are depression are more related to 5HT deficiency than NE or vice versa
- most antidepressants act on NE/5HT however clinical effects take far longer than pharmacological effects
Evidence against monoamine hypothesis of depression
Individuals can have high serotonin and depression, or low serotonin and depression
Antidepressants take ~5 weeks to exert clinical effects, yet their pharmacological effects occur rapidly
Development of anti-depressants
Relatively accidental
- first three were histamine based and created as an aesthetic
- TCAs developed as anti-psychotics
- SSRIs developed due to TCA toxicity
Antidepressants
- TCAs (nonselective inhibitors of monoamine uptake)
- MAOIs (selective MAO inhibitors)
- SSRIs (inhibit 5HT reuptake)
- SNRIs (selective noradrenaline uptake inhibitors)
Placebo effect of antidepressants
A significant portion of anti-depressant effects for mild/moderate depression are due to placebo
Potentially due to effect of participating in a study including
- reduced isolation
- feeling important
- taking initiative
MDMA
Reverses action of serotonin transporter, causing flood of serotonin at synapse (also affects DA/NA)
Structurally similar to mescaline and amphetamine
Neurotoxicity caused by primary metabolite MDA
GABA synthesis
conversion of glutamate to GABA vu glutamic acid decarboxylase (GAD65/GAD67)
GABA breakdown
reuptake into synapse/glial cells via GAT
or
transamination via GABA transaminase (GABA-T)
Percentage of GABA in brain synapses
30%
What is the history of GABA?
1800s: It was known as a metabolite of plants and micro-organisms
1900s: isolated as an amino acid in the mouse brain through paper chromatorgraphy
1950: Roberts and Frankel discover GABA in the human brain
GABA mechanism of action
Binds to result in Cl- influx, which causes an IPSP
GABAa structure
pentameric structure composed of three different distinct subtypes (a,b,y)
the combination determines the pharmacological characteristics of the receptor
Most common composition of GABAa subunits
2a 2b 1y
Where does GABA bind at GABAa receptors?
Between the alpha and beta subunits
Where do BZDs bind at GABAa receptors
between the alpha and gamma subunits
Distribution of GABAa
widespread through cortex and hippocampus
GABAa antagonists
bicuculline (competitive)
picrotoxin (irreversible)
GABAa agonists
muscimol and isoguvacine
BZDs
Benzodiazepenes are positive allosteric modulators than bind between the alpha-gamma subunits of GABAa receptors and act to increase Cl- conductance
Flumazenil
Antagonist at BZD site, thus acts to decrease Cl- conductance, and has an anxiogenic effect
Overall effect of high GABA agonists
anxiolytic, sedative, hypnotic, muscle relaxant, anti-convulsant, amnesiac, dependency
Overall effect of slight GABA agonists
anxiolysis
Overall effect of slight GABA inverse agonists
anxiogenic
Overall effect of GABA inverse agonists
anxiogenic and pro-convulsant
EPM
An anxiety assay that uses rodent’s innate fear of open spaces and heights to model anxiolysis
- time spent in open arms is used as an indication of anxiolysis
- can only really model acute PTSD
Light-dark test
An anxiety assay based on the conflict in rodents of innate light aversion and spontaneous exploratory activity
Anaesthetic drugs that potentiate GABA response
Propofol and Etomidate
Mutation that prevents anaesthetic drugs from potentiating GABA responses
S270W, a single mutation in the alpha subunit of GABAa
Link between GABA and Alzheimers
In AD there is abnormal communication between GABAergic neurons and principal neurons
This occurs at the same time as gliosis, toxic metabolite production, and plasticity impairement
GABAb structure
A GPCR composed of 2 subunits
- B1a/B1b (ligand binding)
AND
- B2
Effect of GABAb activation
Ligand binding to the B1 subunit results in
- inhibition of adenylate cyclase
- pre-synaptic inhibition through reducing calcium influx
- post-synaptic inhibition by increasing K+ permeability
GABAb distribution
Present throughout the cortex, in both pre and post synaptic terminals
GABAb antagonist
saclofen
GABAb agonist
baclofen
GABAc structure
Mono-pentameric structures composed of three receptor subunits p1, p2, and p3
Location of r1 GABAc
retina
Location of r2 GABAc
CNS
GABAc receptors are considered a variant of GABAa despite only 30-38% AA specificity, but do not respond to…
bicuculline, baclofen, and BZDs
Conditions associated with GABA
- mental health
- major depression
- autism
- huntington’s
- epilepsy
- parkinsons
What are the three dopaminergic pathways in the brain?
Mesolimbic: projections from the VTA to the NA and cortex
Nigrostriatal: projections from the SNpc to the dorsal striatum
Tuberoinfundibular: projections within the hypothalamus to control prolactin secretion
D1 receptor family mechanism
D1 & D5
- increased cAMP production
- K+ channel inhibition
- Ca2+ channel activation/inhibition
- enhancement of NMDA receptor response
D2 receptor family mechanism
D2, D3, D4
- decrease cAMP production
- K+ channel activation = decreased post-synaptic excitability
- increased presynaptic inhibition via Ca2+ channels
Dopamine receptors in striatum
D1, D2, D3, D4, D5 (all of them)
Dopamine receptors in the cortex
D1, D2, D4
Dopamine receptors in the hippocampus
D2, D3, D4, D5
Dopamine receptors in the nucleus accumbens
D3
Lifecycle of acetylcholine
Choline + Acetyl CoA
via choline acetyltransferase
= Acetylcholine + Coenzyme A
Acetylcholine
via Acetylcholine esterase
= Choline (recycled) + acetate (degraded)
nAChR
composed of 5 subunits, each with four transmembrane domains and act to facilitate the passage of Na+, K+, and on occasion Ca2+.
Homomeric nAChR
The homomeric a7 receptor is critically implicated in AD.
These receptor have low affinity for nicotine, can be blocked by a-bungarotoxin, are permeable to Ca2+, and undergo rapid desensitisation.
Heteromeric nAChR
Heteromeric a4b2 receptors have high affinity for nicotine, are not blocked by a-bungarotoxin, and are moderately permeable to Ca2+.
mAChR
muscarinic acetylcholine receptors are GPCRs and can be classified into 5 subtypes (M1-5) based on agonist/antagonist selectivity
What are 3 main ACh pathways in the brain?
- Septohippocampal pathway: modulates hippocampal neurons and the formation of new memories
- Striatal interneurons: modulates the reinforcing properties of dopamine, and contributes to selection of behaviours
- Nucleus basalis: radiate throughout the cortex, and are implicated in AD
