L10 & 11 - Metabotropic Receptors Flashcards
Two types of synaptic transimission
1) Ionotropic - leads to EPSP (Na+ inflow) or IPSP (Cl- inflow
2) Metabotropic - GPCR, slower than Ionotropic receptors, plays a modulator role - alters ion-channel permeability
GPCR has _ TM segments
7 TM alpha helical segments
- aka “serpentine” receptors
- They have no intrinsic activity
G protein is a _ protein, with _ subunits
Trimeric protein, alpha, beta, gamma
Which parts of the G protein have covalently attached lipid tails to the cell membrane?
Alpha and Gamma
GPCR binding process
1) GPCR is bound weakly to G-protein. Upon receptor activation via ligand binding, the affinity for receptor to protein is high
2) Alpha subunit which is bound to GDP (inactive) swaps it for GTP (abundant in cytosol).
3) GTP binding leads to conformational change - dissociationg of a from b, g subunits OR sometimes they also stay together and no change is seen.
4) Leads to TWO effectors. Where Alpha activates target protein, relaying signals to other targets of signalling cascade and Beta-gamma complex does something else to another effector target.
5) GTP hydrolyses to GDP - often accelerated by RGS (Regulator of G protein Signalling)
6) GDP and alpha subunit joins back to b, g compelx
Upon prolonged stimulation of GPCR, what occurs?
Receptor kinase activates cytosolic components of GPCR, phosphorylating it. Next, arrestin protein binds to it - preventing GPCR binding to G-protein. Arrestin act as adpator proteins, recruiting the phosphorylated receptor to clathrin coated pits leading to endocytosis via degradation using lysosomes.
There are _ different a subunits. There are _ different B subunits. There are __ different gamma subunits. ____ possible G-proteins. How many different effector possibilities can you get?
20, 5, 12
1200
20 alpha + 60 (5x12) beta-gamma different effector possibilites - alpha acts as one effectors and beta-gamma acts as another.
3 Ways Metabotropic Receptors Regulate Ion channels
- Coupling by G-protein directly to an ion channel (E.g. muscarinic receptors)
- Coupling by G-protein to a second messenger system, where the 2nd messenger (eg cAMP, cGMP) directly regulates an ion channel (E.g. cAMP gated ion channels)
- Coupling by G-protein to a second messenger system, leading to ion channel phosphorylation (e.g. activating PKA which phosphorylates ion channels)
_ types of muscarinic receptors
5
Gαs, Gαi, Gαq, Gαt
What do they stimulate or inhibit? Qαq in detail?
- Gαs stimulates Adenylyl Cyclase -> cAMP -> Protein Kinases -> Phosphorylation of Target Protein
- Gαi inhibits Adenylyl Cyclase (sensitive to pertussis toxin)
- Gαq stimulates Phospholipase C -> cleaves PIP2 into DAG and IP3 -> both activate PKC -> Phosphorylation of Target protein -> Cellular response
a) DAG binds to PKC
b) IP3 binds to Ca2+ ion channel allows Ca2+ flow into cytosol from ER -> Ca2+ binds to PKC
• Gαt stimulates cGMP Phosphodiesterase, reducing cGMP levels
Why are there 1200 different G-proteins?
Due to Signalsomes: preformed
scaffoldings in which GPCR, G-protein, effector and regulatory molecules are locked into position
How are GPCR receptor signals switched off?
Detachment of ligand -> hydrolysis of GTP -> removal of 2nd messengers (may be labile (changed), removed, destroyed) -> activation of phosphotases (undo the work of whatever kinases were activated, so that protein activity, channel conductance etc all return to their pre-stimulus level)
*Turning off signals is more complex
PKA - does not only phosphorylate ion channels but also?
GPCR kinase - phosphorylating GPCR so arrestin will bind to it
Peptide NT are transported along _____ to ____ via ____
Transported along microtubules to axon terminals via vesicles.
*ALL peptide NT are metabotropic
List 4 peptide NT
- Substance P (pain transmission)
- Neuropeptide Y (blood pressure, appetite stimulation)
- Angiotensin II (thirst and salt appetite)
- Endorphins (pleasure, reward, inhibition of pain)
Are peptide NT recycled?
They are not recycled at the nerve terminal. The fused vesicular membrane is endocytocised and transported back to the cell body. There is no re-up take of peptide NT - they undergo proteolytic cleavage after release.
Peptide NT - larger or smaller than A.acid? Light or dark stain under electron microscopy?
Larger than A.acid NT, dark-stained
Autoreceptors – where is it found and how does it work?
-Presynaptic receptor
e.g. Dapamagernic terminal but also found in Ach, glutamatergic and etc.
Form of negative feedback that reduces amount of dopamine released
Dopamine binds to autoreceptor -> Inhibits VACC (Voltage Activated Ca2+ Channel) -> decreases amount of Ca2+ flowing into the terminal -> Inhibit release of NT
Non-synaptic Receptors
An example of neuromodulatory action, but located outside of the synapse altogether.
Neuromodulators
They alter the excitability of the post-synaptic neuron, but don’t regulate the membrane voltage directly
Another name for vesicles containing peptide transmitters? Size? Dense or not under electron microscope? What else do they contain?
LDCV - Large dense core vesicles
- 200-400 nm in diameter
- dense under the electron microscope
- they contain proteins which act as a scaffolding
- they contain peptidases which may act after release
Some metabotropic synapses have different morphology
- More spread out vs. ionotropic receptors which are clustered tightly with the PSD
- PSDs are less dense
- Some receptors lie outside the PSD
T/F: Kinases phosphorylate GPCR without a ligand bound to it
False – they only phosphorylate ligand bound GPCRs
Tachyphylaxis
Acute rapid decrease in response to a drug after its administration. This concept is present in GPCRs when GPCRs are internalized and removed after prolonged stimulation and hence drug won’t be as effective anymore.
Is it the receptor or transmitter that determines the nature of transmission?
The receptor – you can have transmitters that activate both ionotropic and metabotropic receptors
Metabotropic transmitter – GABA A or GABA B?
GABA B
2 different responses from dopamine
Initiating movement or feeling pleasure
2 different areas of the brain Ach stimulates
Pons – sleep and wakefulness
Forebrain – Learning, memory, attention
Which area of the brain is Serotonin found?
Raphe nuclei (in medulla, pons and midbrain) *Appetite, libido, sleep, mood, pain
Which area of the brain is NA system found in?
Locus coreleus (pons) *Mood, vigilance, pain
The major neuromodulatory systems – dopamine, ach, serotonin, NA. Most is found in brainstem but one is found in the basal forebrain?
One type of Ach
What does the ascending reticular activation system consist of? What does it control?
1) Serotoninergic raphe nuclei (of which there are 9)
2) Cholinergic system of the pons
3) Noradrenergic locus coeruleus
This system controls sleep wake cycles (including REM), arousal and vigilance
The Cholinergic Neuromodulatory System in the Basal Forebrain – What structures are present? Where do these structures innervate?
It consists of several contiguous groups (nuclei) of magnocellular cholinergic neurons. From rostral to caudal, these are
1) Septum – innervates hippocampus aka Septo Hippocampus system (traverse via a bundle of axons aka fornix to innervate it)
2) Diagonal Band (of Broca) – innervates Olfactory cortex
3) Basal Nucleus (of Meynert) aka Meynert Band – innervates entire neocortex (from frontal, occipital to parietal cortex)
Septum and hippocampus used to be neighbours but grew apart with the development of the cerebral cortex. Fornix traces evolutionary separation between the two structures.
Where are the only cells responsive to NGF (Nerve Growth Factor) found? This are is also the most severely and early affected area in Alzheimer’s Disease
In the cholinergic neuromodulatory system
One early symptom of Alzheimer’s disease?
Loss of smell
Most important signalling cells of hippocampus
CA3, CA2, CA1 pyramidal neurons – have Ach muscarinic R
-Other neurons are usually inhibitory interneurons
What types of neurons and receptors are present on the hippocampus?
• Pyramidal neurons (glutamatergic) - m1 receptors (excitatory) • Interneurons (GABAergic) - m2 receptors (inhibitory) • Autoreceptors - m4 (inhibit release)
M1-5, which ones are excitatory and which ones are inhibitory?
M1, 3, 5 – excitatory, activates Phospholipase C -> IP3 -> Ca2+ -> Excitation
M2, 4 – inhibitory, inhibit Adenylyl Cyclase -> Decrease cAMP -> decrease excitability (post-synaptic) -> decrease release (pre-synaptic)
Activation of M1ACh receptor in hippocampus – what is its effect on M type K+ channels? Effect on Na+ channels?
- Slight depol
- Inhibits M type K+ channels (Beta-gamma) which allows pyramidal neuron to discharge at high frequency, maximising effect of hippocampus
- Activates pacemaker sodium channels (increases oscillatory behaviour of membrane potential) -> rhythmic action potential discharges
90% of cholinergic release sites in the neocortex and hippocampus are not ____ but Ach is released from ______ into the ___ Cholinergic axons form many branches in the target tissue, and each branch has varicosities spaced about __ microns apart – ranging from _ to _ release sites
Synaptic, varicosities, ECF, 2.5, 200 K, 500 K
What happens when hippocampus loses cholinergic input?
Hippocampal pyramidal neurons become hypo-excitable – impairing memory function (especially spatial and anterograde memory e.g. formation of new long term memories)
What happens to Cholinergic Neurons after Fornix axotomy? What happens to cholinergic synapses in hippocampus? i.e. cutting the connections between forebrain cholinergic neurons and their target cells in the hippocampus
- Neurons shrink
- Synthesis of Ach decreases -> synthesis of proteins such as choline acetyltransferase and choline transporters also decrease
- Neurons lose their cholinergic phenotype, neuronal phenotype, and even undergoing apoptosis
- IN HIPPOCAMPUS: Cholinergic synapses retract and die
Describe the two-way signal flow of the CA1 Pyramidal neurons in the Hippocampus
Hippocamus depends on cholinergic neurons to function
Cholinergic neurons have NGF receptors TrkA and p75, and need NGF from pyramidal neurons to make Ach.
