Neuroscience - Neurotransmitter receptors Flashcards
What are the 2 main classes of neurotransmitter receptors?
- Ionotropic receptors
- G-protein coupled (metabotropic) receptors
What does transmitter binding cause in both ionotropic and G-protein coupled receptors?
Ionotropic - binding opens an ion channel
G-protein coupled - binding activates a G-protein which, in turn, activates an effector e.g. enzyme, ion channel
What are the 2 main types of ionotropic receptors?
- cys-loop receptors
- p-loop receptors
Cys-loop receptors are the receptors for what transmitters?
- Nicotinic ACH
- GABAa
- Glycine
- Serotonin 5-HT3 subclass
P-loop receptors are the receptors for what transmitter?
Glutamate (iGluR)
What are the main 2 types of G-protein coupled receptor?
Class A and C
Class A receptors are the receptors for what transmitters?
Muscarinic ACH Dopamine Serotonin 5-HT1-2,4-7 subclasses alpha and beta adrenergic Most neuropeptides
Class C receptors are the receptors for what transmitters?
Glutamate (mGluR)
GABAb
How was nACHR first isolated?
Isolated from the electric organ of electric fish
- dense arrays of nACHRs
What is the subunit organisation of the nACHR?
- Individual subunits designated alpha, beta, gamma, delta and epsilon
- Each individual subunit has 4 transmembrane (TM) domains and extracellular N- and C- termini
- 5 subunits assemble a pentameric structure
- TM2 from each subunit lines the pore of the ion channel
- 2 alpha subunits, which bind ligand, are required for a functional receptor
How many individual subunits are there in total? How many of each? (e.g how many alpha, beta etc)
17 subunits in total 10 alpha 4 beta 1 gamma 1 delta 1 e
Different combinations of subunits cause what?
Different pentameric receptors with different conductance properties
Give an example of how different subunits can cause different conductance?
Receptors composed of alpha and beta subunits more permeable to Ca2+
What structural feature of the nACHR facilitates cation permeability?
The pore is surrounded by rings of negatively-charged amino acids
Per receptor, there are always 2 subunits of what type?
alpha
Where does the ligand bind to on the nACHR?
- Binds to the N-terminal extracellular domain of alpha subunit and adjacent adjacent subunit (varies depending on receptor type)
What is the positive and negative face of the nACHR and what does each determine?
Positive - alpha-subunit side - determines ligand affinity
Negative - adjacent subunit - determines ligand selectivity
What distinguishes the alpha subunit from a non-alpha subunit?
2 neighbouring cysteine residues distinguish alpha from non-alpha subunits
What amino acids are important for binding on the negative face?
Leucine, methionine and tryptophan
What facilitates nACHR channel opening/closing
- Rotation of TM2 domains of each of the 5 receptor subunits closes or opens a ring of leucine and valine residues to regulate ion movement
What amino acids line the channel to facilitate ion permeability?
Serine and threonine
What are the 3 groups of ionotropic glutamate receptors (iGluR)?
1) NMDA
2) AMPA
3) Kainate
What family of ionotropic receptors are iGluRs part of?
pore-loop channels
(p-loop channels)
includes the voltage gated K+ and Na+ ion channels
What is the subunit organisation of the iGluR?
- Each subunit has 3 TM domains and one re-entrant hairpin loop
- Hairpin loop forms the pore of the receptor chennel
- Extracellular domains including N-terminus and TM2-TM3 loop are very large
- C-terminus is intracellular
- Receptor is a tetramer of 4 subunits
What is the overall structure of the iGluR?
From extracellular to transmembrane:
- Amino terminal domain
- Ligand binding domain
- Transmembrane domain
- LBD consists of 2 lobes formed from regions of N-terminus and extracellular loop between TM2 and TM3
- P-loop forms the ion channel
Binding of the agonist to the LBD causes what?
Induces a conformational change in LBD that pulls the ion channel open
What is the unique property of NMDA receptors vs non-NMDA receptors?
They are Ca2+ permeable, whereas non-NMDA receptors are permeable primarily to Na+ and K+
What are NMDAs 2 binding sites?
1) Mg2+ in the channel
2) Glycine acts as a cofactor required for channel opening
At resting potential, what blocks the channel of NMDA receptors?
Mg2+
What must occur to remove the Mg2+ black?
- Transmitter binding followed by membrane depolarisation
How many G-protein coupled receptor (GPCR) genes are there in the human genome?
Over 800
What is the largest class of GPCRs? How many genes
Class A
>600 genes
Includes most GPCRs for classical and peptide neurotansmitters
How many genes are there for Class C GPCRs and what are they receptors for?
20 genes
glutamate and GABAb
What is the structure of the beta-adrenergic receptor?
- Ligand binding protein is single polypeptide
- 7 TM domains interconnected by 3 extracellular loops and 3 intracellular loops
- N terminus extracellular
- C terminus intracellular
- G protein coupled to adenylate cyclase
- receptor binding leads to production of cAMP
How does the neurotransmitter bind in beta-adrenergic and muscarinic ACH receptors?
- Transmitter binds to a pocket below the membrane, withing the hydrophobic core
- Disulphide bond between e1 and e2 stabilses the binding pocket
What is the structure of G proteins?
Heterodimeric: composed of alpha, beta and gamma subunits
Describe the G protein binding cycle
Cycles between GDP and GTP bound states
1) alpha-beta-gamma G protein bound to GDP in an inactive state
2) GPCR interacts with this protein - reduced affinity for alpha subunit
3) GDP exhanged for GDP, Galpha dissociates from Gbeta-gamma
4) G protein has GTPase function, hydrolysis of GTP back to GDP
Ga-GTP and Gby are functional and can interact with effectors (adenylate cyclase)
What activates the Type 1 neuron specific adenylate cyclase?
Activated by Ca2+-calmodulin binding
What is the structure of type 1 adenylate cyclase?
- Active enzyme is a tandem repeat of 6 TM domain proteins
- HAs a cytoplasmic catalytic domain that converts ATP to cAMP
What is the primary target of cAMP?
Protein kinase A - serine/threonine kinase that phosphorylates many targets
What are the 2 pathways that can activate adenylate cyclase?
- Act through different G proteins: Gs and Gq
- Gs activates Gs-alpha
- Gq activates Ca2+-calmodulin
- Both lead to adenylate cyclase activation (AC behaves as a coincidence factor)
What is coincidence detection important for?
Associating events e.g in associative learning such as Pavlovian conditioning
In noradrenaline signalling, state the 1st messanger, receptor, G protein transducer, effector, 2nd messanger, mediator and cellular response
1st messanger - noradrenaline
Receptor - B-adrenergic GPCR
G protein transducer - Gs
Effector - Adenylate cyclase
2nd messanger - cAMP
Mediator - Protein kinase A
Cellular response - regulation of ion channels/enzymes, remoddeling of cytoskeleton, changes in gene expression
Where are pre-synaptic receptors found? What type can they be? What is their role?
- Located on pre-synaptic terminals
- May be ionotrpic or G-protein coupled
- Modulate synaptic activity by altering transmitter release
Give an example of how a neurotransmitter regulates the release of a seperate transmitter by pre-synaptic receptor activation?
- Release of dopamine from dopaminergic neurons
- Release modulated by ACH from a neighbouring neuron binding to presynaptic nACHR on terminals of dopaminergic neuron
- Activation of presynaptic nACHR leads to increased presynaptic Ca2+ which enhances dopamine exoxcytosis
What are the main 2 types of receptor regulation?
1) Desensitisation - reduces responsiveness of receptor to transmitter
2) Sensitisation - increases response of receptor to transmitter
What are the 2 types of desensitisation and what causes each?
1) Rapid (milliseconds to minutes)
- Due to conformational changed associated with phosphorylation of receptor subunits
2) Slow (minutes to hours)
- due to removal of receptors by endocytosis
- may be reversible or irreversible
