Exam 3 Synaptic Transmission David Fraiser

What are inotropic ion channels?

Ligand-gated ion channels

Which are faster, ionotropic or metabotropic receptors?

Ionotropic has rapid onset effects, metabotropic has slow onset effects

What type of relationship do ionotropic receptors have with action and response?

1:1 relationship between action and response

What type of relationship do metabotropic receptors have with action and response?

1:1 Amplification of response by G-proteins and second messengers

What are the scope of effects of ionotropic receptors?

Effects are limited by the type of ion channel that is part of the receptor protein

What are the scope of effects of metabotropic receptors?

Diverse possible effects from a single neurotransmitter due to multitude of second messenger mediated signaling pathways

What concentration of neurotransmitter is used to cause effects in ionotropic channels?

MicroMolar ranges

What concentration of neurotransmitter is used to cause effects in metabotropic channels?

NanoMolar ranges

Where are ionotropic receptors located relative to the site of neurotransmitter release?

Often located near the site of neurotransmitter release

Where are metabotropic receptors located relative to the site of neurotransmitter release?

Often located at some distance from the site of neurotransmitter release

What are the 3 types of metabotropic receptors?

G-Protein coupled, Enzyme Linked, Cytoplasmic receptors

What are G-Protein coupled receptors?

-Ligand binfing of the G protein receptor leads to activation of GTP bidning protein

How common are G-protein coupled receptors?

Lots of receptors, lots of pathways

What are enzyme linked receptors?

Receptor tyrosine kinases, serine/threonine-specific kinases, guanylate cyclase receptors

What triggers kinase cascade (Enzyme linked receptors)?

Ligand binding initiates kinase cascade

What do cytoplasmic receptors do? (basic)

Often target gene transcription

What is an agonist?

A compound that elicits the same biological effect(s) as the endogenous (naturally-occurring) ligand when it binds to a receptor

What is an antagonist?

A compound that reduces or eliminates the effect of an agonist when bound to a receptor

What is a competitive antagonist?

Binds to the same site as an agonist (orthosteric binding) but does not activate the receptor. Reduces or prevents activation of the channel by an agonist.

What is a noncompetive anatgonsit?

Binds to receptor at a different sidte from agonist, but prevents or reduces activation. known as a NAM (Negative allosteric modulator)

What is a reversible antagonist?

Binds non covalently to receptor

What is an irreversible antagonist?

competing

WHat are the 3 main characteristic of ionotropic recptors?

-Selective ligand binding site

-Selective pores opening

-Equilibrium potential is determined by the electrochemical gradient for permeant ions

What are the 4 main familie sof ionotropic receptors?

-Pentameric (Cys-loop)

-Glumtamine

-Terimeric

-TRP

What are the pentameric receptors?

ACh,5-HY#,GABAa,Glycie,ZAC

What are the strcuture of Penatmeric recptor?

-Share common structure with extracellular loop with 2 cysteine

-M2 segments form the pore

How many subunits and membrane spanning segments do Pentameric receptors have?

5 separate subunits, 4 membrane spanning segments M1-M4

How many and where does acetylcholine bind to on nAChR receptors?

2  molecules of ACh, 2 a subunits

What are the 2 types of subunit combinations for neuronal nAChR subunits

What are the 5 main charcteristic of neuronal AChRs

1) different subunit comp

2) Varrying affinity for Nic and Ach

3) Can pass more caclium than muscle AChRs

4) located presynaptically in CNS

5) suscetpiable to desensitization

Binding of a ligand to a receptor is ______?

Probabilistic

What factors affect ligand binding?

Binding frequency depends on concentration of the ligand, and the amount of time a ligand spends bound to a receptor varies by ligand

What type of ACh receptors does ACh bind, what about nicotine?

• ACh=all receptors

• Nicotine= Only ionotropic Ach receptors

What is Kd?

The dissociation constant

-Indicates concentration at which 50% of binding sites are occupied

How is Kd calculated?

Ratio of the unbinding rate (K off) divided by binding rate (K on)

What is the relationship between Kd and affinity?

Inversely related

-Therefore a ligand with high affinity has low Kd

-Ligand with low binding affinity for a receptor has a high Kd

What is different between 5-HT3 receptors and other 5-HT receptors

Usually 5-HT receptors are metabotropic, 5-HT# is the exception

Where are 5-HT3 receptors found?

Found in GI tract and brainstem

What are the characteristics of 5-HT3 receptors?

-Nonselective monovalent cation channel

-5 types of subunits A-E (A has binding site_

What type of receptors are GABA, and how many types of GABA are there?

GABA-B are metabotropic

-GABA-A are in pentameric recptor familt with nAChRs; ligand gated chloride channels. Equilibrium arounf -70mv

-Can have a,b,y subunits

How does GABA a affec the cell?

GABA A causes a influx of CL- ions casuing the membrane to become more negative?

What does the GABA diagram look like?

Shunting inhibtion*

What are glycine receptors?

Pentameric receptors, permeant to chloride

What are the differences between glycine and GABA receptors?

• Glycine found more in the brainstem and spinal chord (compared to GABA)

• Faster channel opening and closing (compared to GABA)

What does Zinc to Gaba and Glutamte receptors?

Zinc can antagonize or modulate other receptors (mostly GABA and Glutamate)

What are Zinc-Activated Channel receptors?

-Proton receptors (?)

-Nonselective monovalent cation channels

-Present in some mammals but not others

What is the most common NT?

50 trillion synapses

What are the ionotropic glutamate receptors characteristic?

• 4 Subunits

• 3 Transmembrane domains

• Large extracellular loop

What are the characterstics of the loops?

-2 domains that form Pacman

-1st loop (amino terminal domain) sometimes bind antagonists

-Second loop domain (ligand binding domain) binds… glutamate and any antagonists

What is the structure of the inotropic glutamate receptor structure?

What type of iontropic glutamate receptors are there?

• NDMA

• NON-NMDA

  • AMPA

  • Kainate

AMPA and Kainate have ___ conductance

Low

NMDA receptors have ____ conductance

High

AMPA and Kainate have ____ gating speed

Fast

ndma has ____ gating speed

Slow

What ions are permeable to AMPA and Kainate receptors?

NA+ K+ and Ca2+

What ions are permeable in NMDA receptors?

Na+, K+, Ca 2+

What effect does glycine have on NMDARs?

Glycine is a co-agonist

What effect does Mg2+ have on NMDA and non-NMDA recptors

NMDA- Yes Mg2+ block at rest

Non-NMDA- No Mg2+ block

WHat is the structure of NMDA receptors?

2 GluN1 and 2CluN2 subunits

-Glutamate binds to GLUN2 subunits

-Also requires binding of glycine or D-sEine

How does Mg2+ block NMDA glutumate receptors?

What is the diagram for Mg2+ block of NMDA glutumate receptors?

What type of response do AMPA and Kainate receptors have?

Fast and Excitatory

What are the characteristic of AMPA receptors

GLuA1-A4 Subunit types

GluaA2 is the one that blocks calcium, so ones lacking GLUA1 will be permebale to calcium as well as sodium and potassium

-No magnesium block, no glycine needed

What are the characteriscs of Kainate receptors?

-Fast and excitatory

-Less common the AMPA and nmda

-Found in hippocampus, cerebellum, amygdala, retina and spinal cord

-Gluk1-5 subunits

-A little slower than AMPA (but not as slow as NMDA

What us the first signal for metbaotropic receptors? 

Extracellular binding of the NT to the receptor

What is the second signal of metabotropic recprors?

The second messenger inside the cell

What was the first identified 2nd messenger? (Metabotropic receptors)

Cyclic AMP

What does Experiemnt 1 of secondary messengers show (hint-adds Epi)

What does Experiment 2 of delivery of second messengers show? (Hint-Membrane)

What are the different classsifcations i.e Primary effector, secondary messanger etc.

-Epi=External signal (first messanger_

• G-Protein= Transducer

• Adenyl Cyclase=Primary effector

What is the first messanger?

Molecule outside cell (ligand)

What is a transducer?

Ex. G protein

• connects receptor to effector

• Translates signal for outside into an inside signal

• Transfers signal from receptor to effector by activating it

What is the primary effector?

• Usually an enzyme  (i..e adentlyl cyclase, phopholipase)

• Produces second messenger

• Effector=Enzyme

What is the second messanger?

• Small molecule inside the cell 

• Amplifies and spreads the signal

What is the secondary effector?

• the protein that responds to the second messenger (i.e PKA)

• Alter cellular activity-enzyme activation, gene expression, secretion

What is the common secondary structure of metabotropic receptors?

• 7 transmembrane domains

• 3 extra, 3 intraceullar loops

What happens before the Ligand binds the receptor (metabotropic)  (1/3)

• G protein is attach to the intter surface of the membrane

• Alpha sunuits has GDP bound-meaning its inactive

• B and y subunits are bond to Ga

• No signal= receptor and G-Protein are both “off”

What happens when the Ligand binds the receptor (metabotropic)  (2/3)

• receptor changes shape

• Receptor changes Ga subunit

• Subunit release GDP and binds GTP

• Ga-GTP seperates from Gby

What happens after GTP binds to Ga (metabotropic)  (3/3)

• activated subunit binds to enzyme (effector)

• Enzyme becomes activated and catalyzes second messenger

• Gby can modulate ion channels or other effectors

How does the G-Protein complex inactivate

• Ga subunit hyrolyzes GTP to GDP

• Turns off and allows rejoining Gby, rejoining trimeric state

How does effector interaction affect GTP hydrolysis?

Can speed up hydrolysis

How does regulatory protein effect G-protein signaling?

Regulatory protein accelerates GTPase activity

What to Somatostatin and Acetylcholine do in ocntext of G-prtoein subunits

• Somatostatin (SOM) acts via αO1, β3, γ3

• Acetylcholine (ACh) acts via αO2, β1, γ4

• Combinations used by 2 metabotopic receptor patwhays to jhibit calcium channels

what do Oligonucleotides do?

• Degrade mRNA

• Can use to knock down specific subunits and study which ones are required for particular receptor

what is the direct pathway? (GCPRs)

What is the cAMP pathway?

What is the Phosphinositol pathway?

What is the arachidonic acid pathway?

Which G-Protein dimer is involved in moldulating GIRK?

Beta-Gamma dimer activate GIRK channels but Alpha-GTP dimer do not.

What is paired pulse facilitation? “Weak snyapse”

• First AP occurs and voltage gated calcium channels are triggered

• Some channels open and release NT

• Second AP occurs fast enough to use previous calcium activation

• More NT is released than first AP causing stronger EPSP

What is paired pulse depression? “Strong synapse”

• First AP occurs, and all voltage gated calcium channels are activated

• Most vesicles are release from presynaptic terminal

• Second AP occurs and there is less available Neurotransmitter

What is tetanic potentiation?

• Think Titanic , big and crashing down

• More calcium channels are open but less NT available over time

What is a low pass filter? 

Strong synapse, tetanic depression

• repsonds will to single or few stimulus but losses strneght over time

What is a high pass filter?

• weak synapse, tetanic facilitation

• Responds will to many action potentials building up calcium precense allowing for more vesicle release

What are the 3 hypothesis about how proteins contribute to short term faciliation?

Buffer saturation

Calcium channel modulation

Synaptotagmin 7

What is the buffer saturation hypothesis?

Paired-pulse facilitation

• calcium buffers that soak up calcium are saturated after the first ap, making subsequent EPSP stronger with more calcium and vesicle release

What is the caclium modulation hypothesis for paired pulse facilitation?

Residual Ca2+ interacts with hining proteins which enhance opening of voltage ca2+

• When second AP arrives, channels open more easily or stay open longer