lecture 16: GPCRs Flashcards
Several steps of synaptic neurotransmission
- depolarisation of the presynaptic neuron - influx of Na+
- influx of ca2+ via voltage dependent ca2+ channels
- vesicle mobilization, fusion, and release
- receptor activation
- removal of neurotransmitters from cleft
what are the key drug targets in neuropharmacology
receptors and transporters
what are the two most common receptors in the brain
- ligand gated ion channels (ionotropic receptors)
- G protein-coupled receptors (metabotropic)
ligand-gated ion channels and GPCRs time scale and examples
Ligand-gated ion channels:
time = milliseconds
examples = nicotinic, ACh receptor
GPCRs:
time = seconds
examples = muscarinic, ACh receptor
activation of GPCRs vs ion channels
ion channels:
- binding of agonist causes a confirmational change in receptor protein
- conformational change opens gate allowing ion flow
GPCRs:
- binding of agonist causes a conformational change in receptor protein
- more stuff happens
- a lot more complicated
GPCR structure
- 7 transmembrane proteins which folds up on itself to form 7 folds in the transmembrane
- when ligand binds to fold causes 3D change in structure which activates g protein, which is made up of 3 different subunits (a,b,y)
- alpha has three different subtypes which all cause different intracellular signalling
GPCR activation cycle
- resting state = not doing much
- bind an agonist = increase in signalling = change in 3D shape
- causes GTP to exchange GDP on alpha subunit
- GTP-bound alpha subunit breaks off from receptor complex
- GTP-bound alpha subunit interacts with a target protein to modify cell signalling
what does it mean by GPCR has constitutive activity
always have a base level of signalling
subunits of GPCRs
–> a receptor that attaches to a G protein
- called G proteins because during activation GDP is exchanged for GTP
- the GPCR protein chain crosses the cell membrane 7 times
- G protein has 3 subunits = a, B, Y
3 main subtypes of a subunit
ai, as, aq
- causes cellular effects by modifying (or preventing modification of intracellular proteins)
- examples include adrenergic receptors (bind adrenaline) and dopamine receptors
what does the activation of GPCR activate
- GPCR activation leads to activation (or deactivation) of enzymes adenylyl cyclase or phospholipase C which amplify signal
Adenyl cyclase signalling pathway
target enzymes = adenylyl cyclase
second messenger = cAMP
protein kinases = PKA
effectors =enzymes, transport proteins, etc.
–> cAMP is an intracellular signalling molecule/secondary messengers
- activates protein kinases by phosphorylation of enzymes/proteins (protein kinase A)
phospholipase C signalling pathway
target enzymes = phospholipase C
second messenger = IP3 or DAG
protein kinases = PKC
effectors = contractile proteins or ion channels
–> phospholipase C breaks down molecules in the cell membrane into DAG and IP3
- IP3
increases free Ca2+ from the endoplasmic reticulum (increases probability of depolarisation = stimulatory)
- DAG
activates protein kinase C
different alpha subunits
ai (Gi), as (Gs), aq (Gq)
as/Gs
–> activates adenylyl cyclase
- stimulatory
- dopamine type 1 receptor (D1) has a Gs subunit
- amplifies cAMP
- activates PKA = activates sodium ion channels = makes the membrane potential more positive = more likely for neuron to fire
ai/Gi
–> deactivates adenylyl cyclase
- inhibitory
- dopamine type 2 receptor has a Gi subunit
- decreases cAMP
- activates potassium ion channels (GIRK)
- closes calcium ion channels = inhibits neurons
what does GIRK do
pushes positive ions outside of the cell which hyperpolarizes the neuron
what does AMPA activate
- sodium ion channel = influx of Na+
- causes neuron to fire
what does activation of GABA cause
- chloride ion channel = causes an influx of cl-
- stops neurons from firing
- membrane potential becomes more negative = inhibition of action potential
D1 and D2 neurotransmission
–> Activation of D1 activates sodium (Na) ion channels
- influx of Na+ makes membrane threshold more positive = neuron fires
–> activation of D2 activates potassium ion channels (GIRK)
- GIRK pushes potassium out of the postsynaptic neuron making membrane potential more negative = neuron does not fire
Gq, aq
–> activates phospholipase C
- serotonin, muscarinic, alpha1 (noradrenaline)
muscarinic type 3 (M3) Gq-coupled GPCR
- M3 receptors are found on intestinal smooth muscle cells
- acetylcholine activates M3 receptors to activate PLC
–> produce IP3
–> releases calcium from sarcoplasmic reticulum
–> calcium binds to calmodulin
–> calmodulin activates myosin light chain kinase
–> phosphorylation of myosin light chains and muscle contraction
how does GPCR activation affect neurotransmission
- activate ion channels
–> depolarisation/hyperpolarisation - regulate receptor internalisation
- modulate presynaptic neurotransmitter release
common GPCRs
- cannabinoid
- adenosine
- serotonin
- dopamine
- alpha
- opioid
- muscarinic
example of a GPCR and its neurological effect
–> opioid receptors
- opioid receptors are found in the brainstem on neurons that transmit pain signals
- opioids activate Gi coupled GPCRs activating potassium channels (GIRK) and closing calcium channels
- neuron does not fire
- pain signal stops
- pain is not perceived
