Lecture 14 + 15 Flashcards
Inotropic Receptors
Used in rapid chemical transmission
Post-synaptic ligand-gated ion channels
Metabotropic Receptors
Used in slow chemical transmission
Activation of membrane-expressed receptors
• Receptors couple with intracellular signaling proteins
• Often directly or indirectly modulate the activity of ion channels
The GABAa receptor is __________
The GABAA receptor is ionotropic
The GABAb receptors is __________
The GABAB receptors is metabotropic
• Attenuates Cav channel activity, thus reducing pre-synaptic Ca2+ influx and vesicle
exocytosis
Metabotropic synaptic processes tend to be ______ but _______ lasting than ionotropic ones
Metabotropic synaptic processes tend to be slower but longer lasting than ionotropic ones
G protein- coupled receptors (GPCRs)
Account for most of the SLOW synaptic processes in the nervous system
Structure of GPCRs
- Ligand-activated transmembrane proteins with 7 trans-membrane helices
- Associate with cytoplasmic G-proteins via cytoplasmic regions
- III-IVandV-VIloops
- C-terminus
Function of GPCRs
G proteins mediate intracellular signals
Humans have ~______ GPCRs
Humans have ~800 GPCRs
What Ligands activate GPCRs?
Amine NTs such as dopamine, serotonin, histamine,
norepinephrine, and nucleotides such as adenosine
• Bind in pockets between transmembrane helices
Neuropeptides
• Bind in outer regions of transmembrane helices
NTs such as ACh, GABA & L-glutamate
• Bind specialized external ligand-binding domains in GPCR N-termini
Other ligands include chemical odorants, light- activatable ligands (i.e. rhodopsin GPCRs bind retinal), cell-cell contact via extracellular proteins, and external proteolysis
GPCRs and pharmacology
GPCRs are a major pharmacological target for human disease
• Both in the nervous system and elsewhere in the body
Approx. 60% of currently prescribed drugs target GPCRs
Thus, GPCRs are important ALL OVER the body
Ligand activation of GPCRs causes…
G protiens to activate
G Protien Activation
Binding causes exchange of GDP for GTP on the α subunit
α-GTP acts as an intracellular second messenger
• Slow and long lasting
Gβγ complex moves short distances along the membrane, and directly interacts with membrane proteins to modulate their function (e.g. ion channels)
• Less slow and less long lasting
G Protien Structure
α,β & γ subunits
Promiscuous Coupling
The coupling between specific GPCRs and G proteins is somewhat variable
In the CNS, GPCR-G protein coupling tends to be _______
In the CNS, GPCR-G protein coupling tends to be more specific
Types of G Proteins:
Gs
↑Adenylyl cyclase –> ↑ cyclic AMP –> ↑ protein kinase A
Types of G Protiens:
Gq
↑ Phospholipase C –> inositol triphosphase –> Ca Diacylgylcerol –> protein kinase C
Types of G Proteins:
Gi
↓ Adenylyl cyclase –> ↓ cyclic AMP –> ↑ K channels opened –> inhibition
Types of G Proteins:
Go
↓ Ca channels shut –> ↓ transmitter release
Sympathetic Nervous System
“Fight or Flight”
Parasympathetic Nervous System
“Rest or Digest”
Parasympathetic Nervous System:
The Vagus Nerve - Slows Heartbeat
- Cholinergic neurons in the vagus nerve innervate pacemaker cells in the heart sinoatrial (SA) node
- ACh release at vagus-SA node cell synapses leads to a slowing down of rhythmic AP firing
- Thus cardiomyocyte APs and contraction also slow (i.e. decrease heart rate)
Mechanism for reduced AP frequency in SA node cells
involves activation of a transient K+ current
Ionophoresis of ACh, mimicking synaptic release at vagus nerve endings, creates a hyperpolarization that stops SA cell APs
• At lower physiological concentrations, ACh serves to slow the heart beat
V-clamp recording → ACh activates a K+ channel that lasts for > 1 second
• Considerably slower than ionotropic synaptic responses which lasts milliseconds
Activation of the K+ current occurs via ACh activation of____________________.
How does this happen?
Activation of the K+ current occurs via ACh activation of the M2 muscarinic GPCR
Activated Gi protein βγ subunit travels a short distance along the membrane and directly interacts with a G-protein activated inward rectifying K+ (GIRK) channel
David Clapham - Provided proof that the Gβγ subunit modulates the GIRK channel
Used Inside-Out Recording
- ACh in the pipette led to increased single channel activity
• Thus the membrane contains the necessary mechanisms for K+ channel activation (you don’t need stuff that’s in the cytoplasm) - Application of soluble in vitro expressed Gβγ to the bath mimicked the effect of “extracellular” ACh application (i.e. inside the pipette)
Soejima and Noma - showed that the GPCRs and their G proteins must be quite close to the GIRK channels
Used Cell-Attached patch method
ACh has to be in the pipette to increase the activity of recorded channels
• ACh in the bath had no effect on the channel in the patch
Indicates very close association between the GPCR, the G proteins and the GIRK channel
GIRK Channels
Also active at neuron-neuron synapses
Activated by NTs that target GPCRs coupled to Gi/Go proteins
• e.g.ACh,GABA,glutamate,dopamine,norepinephrine
GABAb receptors
GPCRs that couple with Go G proteins
Gβγ complex
The activated Gβγ proteins are in close proximity to the Cav2 channels
• i.e. membrane-delimited signalling
The Gβγ complex slows down Cav2 channel activation
The Gβγ complex is STUCK in the membrane
Gα proteins
activated Gα proteins are liberated into the cytoplasm
α-GTP
Acts as an intracellular second messenger
• Slow and long lasting,
• Amplified due to cascade activation of downstream signaling components
In cardiomyocytes, activation of β- adrenergic GPCRs…
- Causes a widening of the cardiac action potentials due to ↑Cav1 channel activity
- Increased Ca2+ influx leads to stronger contraction
- Voltage lamp recording of Cav1 current reveals stronger current
