Fast excitatory synaptic transmission
Fast excitatory synaptic transmission is mediated by ligand-gated ion channels, which allow for rapid depolarization
NTs used: acetylcholine and glutamate
Features of NMJ synapses
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
Features of synapses in the CNS
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)
Why is location of stimulation (vesicle release) important?
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
Two distinct types of postsynaptic receptors in CNS
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
What determines whether the postsynaptic cell fires an action potential?
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
Which properties of the membrane are important in determining how synaptic inputs influence firing in the postsynaptic cell?
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
Where are inhibitory synapses usually found?
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
Where are excitatory synapses usually found?
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
Two types of ACh receptors in the CNS
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
Subunits of the nicotinic acetylcholine receptors in the CNS
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)
Different types of nAchR in CNS
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!)
When Ca2+ enters the cell, what does it do?
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?
How do we know that nicotinic receptors are important?
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)
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
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
What is different about an AMPA receptor that has no GluR2 subunit?
High permeability to Ca2+
Note: may have decreases in GluR2 expression in pathological states like after transient ischemia
Delayed cell death induced by transient global ischemia
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?
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
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
NTs that do fast inhibitory synaptic transmission
In normal adult neurons how does the resting membrane potential compare to the equilibrium potential for Cl-?
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
What is the most common GABA-A receptor (60%)?
alpha1, beta2, gamma2 subunits
GABA-A receptors with gamma vs. delta subunits
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
How do benzodiazepines work?
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)
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
Why is the Cl- equilibruim potential more positive than resting membrane potential early in development?
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
How can a depolarizing GABA-A receptor-mediated synaptic potential be inhibitory?
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
In general, what does the subunit composition of a receptor determine?
Sensitivity to modulators (benzodiazepines and neurosteroids)
Cellular localization (synaptic or extrasynaptic)
Permeability to specific ions (Na+ vs. Ca2+)
Two different types of GABA-A mediated inhibition
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)
What happens when you use a GABA receptor blocker (SR95531)?
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!
How does alcohol enhance tonic inhibition?
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)
Why are GABA-A receptors with delta subunits well-suited for tonic inhibition?
Higher affinity for GABA
Less desensitization than gamma subunit-containing GABA-A receptors
How does alcohol at high/lethal doses affect the brain?
Alters activity of AMPA, NMDA, K+ channels
Depolarization-induced suppression of inhibition (DSI)
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
How exactly does DSI work?
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!
How does GABA produce inhibitory synaptic potentials through GABA-B receptors?
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)
When Phaclofen is used it blocks slow component of IPSP while leaving initial fast IPSP (mediated by GABA-A receptors) intact
Fast IPSP vs. slow IPSP
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)
How do excitatory inputs cause IPSPs?
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
Barbituates vs. benzodiazepines in how they affect GABA receptor function
Benzodiazepines: increase binding affinity for GABA and also increase single channel conductance (= increase time channel stays open?)
Barbituates: increase time channel stays open
How do presynaptic GABA-B receptors inhibit excitatory synaptic transmission?
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
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
What processes is synaptic plasticity important in?
Learning and memory
How does synaptic plasticity occur?
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)
Induction of LTP
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!)
What does it mean that LTP is synapse specific?
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)
Why does it make sense that NMDA receptors are responsible for LTP?
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
Signaling mechanism responsible for NMDA receptor-dependent LTP
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
Additional mechanisms that maintain LTP over long periods of time
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
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)
Calcium/Calmodulin-Dependent Kinase II (CamKII)
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
What is required for spatial learning?
Specifically, hippocampus itself, NMDA receptors, CamKII, etc
What might protein synthesis cause regarding LTP?
Dendritic spine formation = new synapse formation
Enhance synaptic connectivity