Synaptic Transmission in the CNS (Week 2 and 3--O'Dell)

Question 1

Fast excitatory synaptic transmission

Answer 1

Fast excitatory synaptic transmission is mediated by ligand-gated ion channels, which allow for rapid depolarization

NTs used: acetylcholine and glutamate

Question 2

Features of NMJ synapses

Answer 2

Synaptic inputs: one-to-one (each muscle fiber receives input from one motor neuron)

Excitatory or inhibitory: inputs are excitatory only

NTs and receptors: acetylcholine only NT and nicotinic ACh receptor only

Reliability of synapse: extremely reliable (high safety factor) because every AP in motor neuron leads to AP in postsynaptic muscle fiber

Question 3

Features of synapses in the CNS

Answer 3

Synaptic inputs: CNS neurons receive synaptic inputs from hundreds (or more) presynaptic cells

Excitatory or inhibitory: excitatory, inhibitory and modulatory

NTs and receptors: many different NTs act through many different postsynaptic receptors (even multiple receptors for a single NT!)

Reliability of synapse: some synapses have high safety factor but many are unreliable (some presynaptic APs only evoke NT release at the presyn terminal 10-20% of the time, even when NT is released doesn’t always cause postsyn AP, might need 100s of presyn axons at once to evoke postsyn AP)

Question 4

Why is location of stimulation (vesicle release) important?

Answer 4

Because synaptic potentials propagate passively

Synapse that contacts postsynaptic cell near cell body (trigger zone) will have much stronger influence on wehther or not postsynaptic cell reaches threshold

Question 5

Two distinct types of postsynaptic receptors in CNS

Answer 5

1) Ligand-gated ion channels: transmitter receptors that form ion channels through the plasma membrane
2) G protein-coupled receptors: act via heterotrimeric G proteins to modulate excitability in the postsynaptic cell through a variety of second messenger pathways

Question 6

What determines whether the postsynaptic cell fires an action potential?

Answer 6

Postsynaptic cell must summate opposing (or additive) synaptic potentials and integrate them

If summated synaptic potential reaching the action potential initiation site (trigger zone) near the cell body is above threshold, the postsynaptic cell will fire an AP

Question 7

Which properties of the membrane are important in determining how synaptic inputs influence firing in the postsynaptic cell?

Answer 7

Time constant: determines time course of synaptic potentials, so affects temporal summation (ability of synaptic potentials generated at diff points in time to summate with each other); long time constant means prolonged duration of potential so facilitates temporal summation and more likely to elicit AP

Length constant: determines how synaptic potentials will decay as a function of distance, so affects spatial summation (ability of synaptic potentials generated at different locations in a postsynaptic cellsto summate with each other); long length constant means synaptic potentials can propagate farther so facilitates spatial summation and more likely to elicit AP

Question 8

Where are inhibitory synapses usually found?

Answer 8

Inhibitory synapses often found on or near cell body of postsynaptic cell because this allows axosomatic inhibitory synapses to negate EPSPs arriving from synapses located on more distal regions of dendrites and prevent postsynaptic cell from firing

Question 9

Where are excitatory synapses usually found?

Answer 9

Excitatory synapses usually somewhere on dendritic tree of postsynaptic cell (axodendritic synapses onto dendritic spines)

Side note: dendritic spines are specialized biochemical compartments that may be involved in controlling processes involved in synaptic plasticity

Question 10

Two types of ACh receptors in the CNS

Answer 10

1) Nicotinic: activated by nicotine, ACh, some blocked by alpha-bungarotoxin; ligand-gated ion channels
2) Muscarinic: activated by muscarine and ACh; G protein-coupled receptor modulates intracellular signaling pathways

Question 11

Subunits of the nicotinic acetylcholine receptors in the CNS

Answer 11

5 subunits

Subunit composition is different and more variable than in those in NMJ

8 alpha subunits and 3 different beta subunits that combine in different ways to generate different types of nAchR

Subunit composition of neuronal nAchR has dramatic effects on properties of channels (high/low affinity, alpha bungarotoxin sensitivity, Ca2+ permeability)

alpha4beta2 subunit combinations are common

alpha7, alpha8, alpha9 subunits can form functional nAchR without combining with other subunits (homomeric channels)

Question 12

Different types of nAchR in CNS

Answer 12

Receptors made from alpha2-6 subunits (plus betas) have high affinity for nicotine and ACh but NOT blocked by alpha bungarotoxin

Receptors made from alpha7-9 have lower affinity for nicotine and ACh and are sensitive to alpha bungarotoxin

Receptors made from alpha2-4 (plus betas) are equally permeable to Ca2+ and Na+

Receptors made from alpha7 only (homomeric) are 20x more permeable to Ca2+ than Na+ (these are on presynaptic terminals and let Ca2+ in to enhance release of other NTs!)

Question 13

When Ca2+ enters the cell, what does it do?

Answer 13

Presynaptically: triggers release of NTs (glutamate, GABA, dopamine) to modulate synaptic transmission

Postsynaptically: turn on second messenger pathways (or is this presynaptically too?); allow Ca2+ to flow into the cell and adds to the Ca2+ that comes in just because of the presynaptic AP?

Question 14

How do we know that nicotinic receptors are important?

Answer 14

Smoking (nicotine) is very addictive

Cholinergic pathways are disrupted in Alzheimer’s disease where there is a loss of high-affinity nAch receptors (presumably alpha4beta2 type)

Question 15

Glutamate

Answer 15

The primary fast excitatory synaptic transmitter in the CNS

Receptors are all ligand-gated ion channels made of 4 subunits

3 main classes of receptors for glutamate are: AMPA, NMDA, kainate

Question 16

AMPA receptors

Answer 16

Ligand-gated ion channel for glutamate

Can be activated by AMPA

AMPA receptors are responsible for the bulk of fast excitatory synaptic transmission in the CNS

Made from 4 subunits using GluR1-4

Ion channel is permeable to both Na+ and K+ and usually has very low permeability to Ca2+ due to presence of GluR2 subunits

Question 17

What is different about an AMPA receptor that has no GluR2 subunit?

Answer 17

High permeability to Ca2+

Note: may have decreases in GluR2 expression in pathological states like after transient ischemia

Question 18

Delayed cell death induced by transient global ischemia

Answer 18

48 hours after ischemia, cells are fine but 7 days after ischemia, lots of cells are dead

Transient ischemia down regulates GluR2 expression –> make AMPA receptors that are permeable to Ca2+ –> cell death?

Question 19

NMDA receptors

Answer 19

Ligand-gated ion channels activated by NMDA

Require glycine (an AA) as a co-agonist

High permeability to Ca2+ ions

Blocked by extracellular Mg2+ in a voltage-dependent manner (when membrane potential is very negative)

Receptors need 3 signals in order to open: (1) glutamate binding, (2) postsynaptic depolarization (from other channels), and (3) glycine

Question 20

Kainate receptors

Answer 20

Ligand-gated ion channel for glutamate that is activated by kainic acid

Can be postsynaptic and generate slow excitatory postsynaptic potentials or presynaptic and regulate NT release

Sit out at periphery of synapse far away from where glutamate released so might not be stimulated unless lots of glutamate

Kainate receptors first make signal stronger by adding positive ions but then make signal weaker due to voltage gated inactivation of Ca2+ channels (presynaptically?)–negative feedback

Question 21

NTs that do fast inhibitory synaptic transmission

Answer 21

GABA

Glycine

Question 22

In normal adult neurons how does the resting membrane potential compare to the equilibrium potential for Cl-?

Answer 22

Equilibrium potential for Cl- is more negative than resting membrane potential

This means when GABA or glycine opens ion channels, Cl- flows in to hyperpolarize the membrane and cause inhibition

Question 23

What is the most common GABA-A receptor (60%)?

Answer 23

alpha1, beta2, gamma2 subunits

Question 24

GABA-A receptors with gamma vs. delta subunits

Answer 24

GABA-A receptors with gamma subunits localize to synaptic sites

GABA-A receptors with delta subunits (instead of gamma subunits) cannot localize to the synapse and are found 100’s of nm away! These are extrasynaptic receptors

Question 25

How do benzodiazepines work?

Answer 25

Benzodiazepines (used to treat anxiety) enhance GABA-mediated inhibitory synaptic transmission

Enhance phasic inhibitory synaptic transmission

In general, receptors containing gamma subunits are sensitive to benzodiazepines but those containing delta subunits are not

1) Bind to GABA-A receptor at allosteric site and increases affinity of receptor for GABA so receptor “holds on” to GABA for longer and thus prolongs duration of inhibitory synaptic currents
2) Enhance single channel conductance (so single channel will let more Cl- in)

Question 26

KCC2

Answer 26

K/Cl co-transporter that pumps Cl- out of the cell ensuring that Cl- equilibrium potential is more negative than resting membrane potential (so when GABA-A receptor channels open, Cl- ions flow in)

Most adult neurons express KCC2

Early in development, neurons lack KCC2 and instead express Na/K/Cl co-transporter that pumps Cl- into the cell

Question 27

Why is the Cl- equilibruim potential more positive than resting membrane potential early in development?

Answer 27

Because instead of K/Cl co-transporter KCC2, neurons express Na/K/Cl co-transporter NKCC1 which pumps Cl- into the cell

This means that when GABA-A receptors open, Cl- will flow out and membrane will depolarize

Question 28

How can a depolarizing GABA-A receptor-mediated synaptic potential be inhibitory?

Answer 28

Yes the opening of Cl- permeable channels causes membrane potential to depolarize initially, but once membrane potential reaches Cl- equilibrium potential, further depolarization will be prevented because any depolarizing influence trying to drive membrane more positive than Cl- equilibrium potential will increase driving force for Cl- to flow into the cell and oppose further depolarization

Question 29

In general, what does the subunit composition of a receptor determine?

Answer 29

Sensitivity to modulators (benzodiazepines and neurosteroids)

Cellular localization (synaptic or extrasynaptic)

Permeability to specific ions (Na+ vs. Ca2+)

Question 30

Two different types of GABA-A mediated inhibition

Answer 30

Phasic: transient presence of NT in synaptic cleft activates postsynaptic receptors and produces inhibitory synaptic current (IPSP) lasting 10’s of milliseconds; use GABA-A receptors with gamma subunits

Tonic: persistent activation of GABA receptors that produces a hyperpolarized “holding current” to prevent excitatory currents since large pulses of depolarizing current are needed to elicit AP firing; use GABA-A receptors with delta subunits (extrasynaptic)

Question 31

What happens when you use a GABA receptor blocker (SR95531)?

Answer 31

Decreased “holding current” which means less current needed to hold postsynaptic membrane potential at a particular potential

Enhanced excitability and increased AP firing

Same depolarizing current that was previously sub-threshold for AP firing can now elicit an AP

Easier to get an AP!

Question 32

How does alcohol enhance tonic inhibition?

Answer 32

Effects of alcohol on brain function during “social intoxication” are due to tonic inhibition

Alcohol enhances activity of delta subunit-containing GABA-A receptors

(no effect on phasic inhibition mediated by gamma subunit-containing GABA-A receptors)

Question 33

Why are GABA-A receptors with delta subunits well-suited for tonic inhibition?

Answer 33

Higher affinity for GABA

Less desensitization than gamma subunit-containing GABA-A receptors

Question 34

How does alcohol at high/lethal doses affect the brain?

Answer 34

Alters activity of AMPA, NMDA, K+ channels

Question 35

Depolarization-induced suppression of inhibition (DSI)

Answer 35

Have brief period of AP firing in postsynaptic cell, which then causes transient decrease in GABA-A receptor-mediated inhibitory synaptic transmission

Increases in postsynaptic Ca2+, decreases in GABA release, retrograde synaptic transmission

Usually, IPSC produced in postsynaptic neuron; but with DSI, the IPSC produced in postsynaptic neuron is strongly reduced after postsynaptic cell has been depolarized several times

Question 36

How exactly does DSI work?

Answer 36

1) Increase in postsynaptic Ca2+ (there are just voltage-gated Ca2+ channels at postsyn letting Ca2+ in…or maybe NMDA receptors??)
2) Endocannabinoids (anandamide or 2-arachidonylglycerol) are made from membrane lipids
3) Endocannabinoids travel backwards across synapse and bind to cannabinoid receptors (CB1 receptors) on presynaptic membrane of inhibitory interneurons
4) CB1 receptors are G protein coupled receptors and when activated stimulate intracellular signaling pathways that inhibit voltage-dependent Ca2+ channels in presynaptic terminal membrane
5) Decreased presynaptic Ca2+ means less GABA release so inhibition is suppressed!

Question 37

How does GABA produce inhibitory synaptic potentials through GABA-B receptors?

Answer 37

Produces slow, long lasting, phasic inhibitory postsynaptic potentials

Remember GABA-B receptors are G protein coupled receptors, so trigger activation of intracellular signaling pathways, where alpha and beta-gamma subunits are liberated and free to interact with K+ channel and that causes opening of K+ channel to let K+ out and hyperpolarize postsynaptic cell

Because beta-gamma subunits can remain active for relatively long periods of time after activation of the GABA-B receptor, the IPSP produced by activation of these K+ channels can persist for a long time (100’s of milliseconds)

Question 38

Phaclofen

Answer 38

GABA-B blocker

When Phaclofen is used it blocks slow component of IPSP while leaving initial fast IPSP (mediated by GABA-A receptors) intact

Question 39

Fast IPSP vs. slow IPSP

Answer 39

Two components of a prolonged IPSP: fast part and slow part

Fast IPSP: due to activation of ligand-gated ion channels for GABA (GABA-A receptors)

Slow IPSP: due to activation of G protein coupled receptors for GABA (GABA-B receptors)

Question 40

How do excitatory inputs cause IPSPs?

Answer 40

Excitatory neurons (glutamate) directly excite the postsynaptic cell but also send collaterals to inhibitory interneurons (GABA) –> GABA causes fast and slow IPSPs by activating postsynaptic GABA-A and GABA-B, respectively

Question 41

Barbituates vs. benzodiazepines in how they affect GABA receptor function

Answer 41

Benzodiazepines: increase binding affinity for GABA and also increase single channel conductance (= increase time channel stays open?)

Barbituates: increase time channel stays open

Question 42

How do presynaptic GABA-B receptors inhibit excitatory synaptic transmission?

Answer 42

GABA-B receptors found on presynaptic terminals of excitatory synapses (synapses that use glutamate as a NT)

GABA-B activated by GABA binding to it –> beta-gamma subunits liberated –> beta-gamma subunits inhibit voltage-gated Ca2+ channels in presynaptic membrane –> less Ca2+ influx during presynaptic AP –> less glutamate release –> inhibited excitatory synaptic transmission

Question 43

Synaptic plasticity

Answer 43

Synapses can undergo activity-dependent changes in strength

Strength of excitatory synaptic connections in the brain can persistently be up or downregulated by different patterns of synaptic activity

Question 44

What processes is synaptic plasticity important in?

Answer 44

Development

Learning and memory

Question 45

How does synaptic plasticity occur?

Answer 45

Multiple forms of synaptic plasticity from distinct cellular and molecular mechanisms

NMDA receptor-dependent long-term potentiation (LTP) is prominent at synapses in regions of brain important in learning and memory (hippocampus, amygdala, cortex)

Question 46

Induction of LTP

Answer 46

Brief periods of high-frequency synaptic activity induce long-lasting increase in strength of synaptic transmission (LTP)

Presynaptic cell at synapse must be given high-frequency stimulation (HFS) AND postsynaptic depolarization to get LTP (neither on its own is sufficient to get LTP!)

Question 47

What does it mean that LTP is synapse specific?

Answer 47

LTP is restricted to only the synapses that received HFS

Induction of LTP is not cell-wide phenomena (not all excitatory synapses onto postsynaptic cell are potentiated)

Question 48

Why does it make sense that NMDA receptors are responsible for LTP?

Answer 48

The conditions needed to induce LTP are exactly the conditions needed for activation of NMDA receptors!

1) Glutamate needed to activate receptor (need presynaptic depolarization to release glutamate)
2) Postsynaptic depolarization needed to relieve voltage-dependent Mg2+ ion block of NMDA receptor’s ion channel

Question 49

Signaling mechanism responsible for NMDA receptor-dependent LTP

Answer 49

Note: this is important during first hour after LTP induction!

1) Activation of postsynaptic NMDA receptors
2) Increase in postsynaptic Ca2+ levels due to influx via NMDA receptor ion channel
3) Activation of protein kinases such as CamKII and PKC
4) Phosphorylation of postsynaptic proteins that trigger insertion of additional AMPA type glutamate receptors into postsynaptic membrane
5) Increase in synaptic strength in LTP is due to increase in number of postsynaptic AMPA receptors

Question 50

Additional mechanisms that maintain LTP over long periods of time

Answer 50

Increases in Ca2+ and activation of protein kinases also activate signaling pathways that regulate gene expression

Adenylyl cyclase –> cAMP –> PKA –> translocates to nucleus and phosphorylates CREB –> transcription and translation –> upregulation of proteins that support maintenance of LTP over long periods of time (protein kinase PKMzeta)

Note: use protein synthesis inhibitor and disrupt later phases of LTP

Question 51

PKMzeta

Answer 51

Protein kinase that is persistently active and phosphorylates and enhances AMPA receptor insertion into synapses

Upregulated by induction of LTP and helps support maintence of LTP over long periods of time

Note: if inhibit PKMzeta, will get rid of LTP completely (shown in rats that memory from previous day is erased)

Question 52

Calcium/Calmodulin-Dependent Kinase II (CamKII)

Answer 52

Molecule that is inactive without Ca2+ because catalytic domain looped around and contacts regulatory domain

Once Ca2+ concentration increases, Ca2+ binds calmodulin and that binds regulatory domain to free up catalytic domain so CamKII is active and can phosphorylate substrates (to cause insertion of additional AMPA type glutamate receptors into postsynaptic membrane)

Can autophosphorylate, so even when Ca2+ is gone, CamKII is still active because it’s phosporylated/activated itself

Question 53

What is required for spatial learning?

Answer 53

Hippocampal LTP

Specifically, hippocampus itself, NMDA receptors, CamKII, etc

Question 54

What might protein synthesis cause regarding LTP?

Answer 54

Structural changes

Dendritic spine formation = new synapse formation

Enhance synaptic connectivity