CNS and neurotransmitters

Question 1

What are features of transmission at the NMJ?

Answer 1

each muscle fibre receives only one synaptic input
• each AP leads to calcium influx into the presynaptic terminal, and the release
of 100s of vesicles
• the endplate potential recruits voltage-gated Na+ channels buried in synaptic
folds, and reliably triggers a muscle action potential
• transmission is terminated by the breakdown of acetylcholine

Question 2

What is the diversity of neuronal subtypes in the CNS?

Answer 2

differing morphology,
ability to sustain spiking at over 100 Hz
or propensity to high-frequency fire bursts of spikes;

depends on the pattern of expression of voltage-gated ion channels
and their mechanisms of communication.

Question 3

What are electrical synapses?

Answer 3

formed between two hemichannels - gap junctions that are aggregates of intercellular channels which allow direct cell-cell transfer of ions

occur between the dendrites of neurons of the same subtype
effective between neurons of similar size and therefore impedence
e.g 2 bipolar cells
does occur in CNS

enable graded and bidrectional communicatoin useful for synchronising local clusters of neurons
cannot be used for long distance communication

Question 4

What are chemical synapses?

Answer 4

large variety of chemical transmitter used in the CNS
neurons often release more than one substance
half are glutamatergic (excitatory amino acid)
quater use gamma-aminobutyric acid (GABA) (inhibitory amino acid)

Question 5

How can chemical synapses be distinguished?

Answer 5

ultrastructural organisation
Glu synapses - Gray’s Type 1 synapses
have spherical vesicles, thicker postsynpatic densitiy (asymmetrical)
found on dendritic spines and shafts

GABA synapses - Gray’s Type 2 sysnapses
flattened or elongated vesicles
pre/postsynaptic densities of similar width
occur primarily on dendrite shafts, neuronal cell bodies and axon initial segment.

Question 6

How is glutamate synthesised?

Answer 6

gutamate is synthesised from glutamine by glutaminase

Question 7

How is glutamate in vesicles?

Answer 7

glutamate is concentrated in vesicles by vesicle glutamate transporters (vGluTs)

Question 8

What does glutamate release trigger?

Answer 8

can activate a range of ligand-gated ion channels (iGluR); categorised based on the binding/efficacy of
different exogenous ligands: AMPA, kainate and NMDA receptors

Question 9

how is fast glutamatergic synaptic transmission terminated?

Answer 9

by the diffusion of glutamate out of the synaptic cleft
subsequently removed from the extracellular fluid via excitatory amino acid transporters (EAATs) expressed in presynaptic terminals, postsynaptic neurons and astrocytes.

Question 10

How do astrocytes remove Glu?

Answer 10

predominant reuptake/recycling pathway
convert glutamate to glutamine via glutamine synthetase
release glutamine into the extracellular space where it is taken up by neurons

Question 11

Why is glutamate excitatory?

Answer 11

Fast glutamatergic transmission is mediated by iGluR
• iGluR are permeable to cations, and thus a synaptic glutamate release evokes
an excitatory postsynaptic potential (EPSP)

Question 12

What determines postsynaptic spiking in the CNS?

Answer 12

Many central neurons receive thousands of convergent weak synaptic inputs –
it is the integration of these inputs that determines postsynaptic spiking.

As each EPSP lasts for tens of milliseconds or more, this integration can occur over both space and time.

Question 13

How does release at a given synapse change?

Answer 13

release at a given synapse changes over time in a way that reflects the immediate history of presynaptic
activity – shows short-term plasticity

The degree of facilitation/depression depends on the frequency of presynaptic
action potentials.
• Facilitation is thought to be due to residual Ca2+ in the presynaptic terminal
which increases the probability of vesicle release following a successive action
potential.
• Depression is thought to be due to the refractory state of the release site
following vesicle fusion, and continues until a new vesicle can be primed for
release.
facilitation is likely to occur at synapses that show a low initial probability of
release (effect of residual Ca2+ dominates), while depression occurs at
synapses that show a high initial probability of release (effect of vesicle
depletion dominates).
Even for synapses that show paired-pulse facilitation, long high-frequency
trains of action potentials will induce subsequent depression.
• Synaptic short-term dynamics vary across synapses, even for the same axon
targeting different post-synaptic neurons
• Short-term plasticity is a feature of all synapses (including GABAergic synapses and the NMJ). It is not obvious at the NMJ as the basal EPP
exceeds action potential threshold by a safety margin – despite short-term
plasticity, action potentials in the presynaptic motor neuron reliably evoke
muscle action potentials even at high spike frequencies.

Question 14

How is GABA synthesised?

Answer 14

GABA is synthesised from glutamate by glutamate decarboxylase (GAD), and
concentrated in vesicles via vesicle GABA transporters (vGATs).

Question 15

What does GABA trigger?

Answer 15

GABA release can activate a range of ligand-gated ion channels
GABAa receptors;
blocked by picrotoxinand bicuculline

G protein-coupled receptors
GABAb receptors;
agonist: baclofen;
antagonists: phaclofen and saclofen).

Question 16

How is fast GABAergic synaptic transmission terminated?

Answer 16

Synaptic transmission is terminated by the diffusion of GABA out of the synaptic cleft,
GABA is subsequently removed from the extracellular fluid via GABA transporters (GATs),
recycled in a similar way to glutamate.

Question 17

What does fast GABAergic inhibition depend on?

Answer 17

the activation of GABAa receptors,

Question 18

what is the structure of GABAa receptors?

Answer 18

pentameric assemblies GABAa receptor subunits
there are at least 14 different receptor subunits expressed in the brain,
giving a large number of possible subunit combinations
synaptic GABAa receptors predominantly have a
conformation of 2α(1-3), 2β(1-3) and a γ(1-3) subunit

Question 19

Why is GABA inhibitory?

Answer 19

GABAa receptors are permeable to Cl- and HCO3
permeability ratio 3:1
activation tends to lead to a net influx of anions, and membrane hyperpolarisation

Question 20

what are benzodiazephines?

Answer 20

e.g diazepam and midazolam
allosteric modulators that bind to the surface of the alpha and beta subunits of GABAa receptors
this increases the frequency of single channel openings and prolongs synaptic inhibition
they are used clinically as anxiolytics, hypnotics, anticonvulsants and myorelaxants.
highlights importance of synaptic inhibition in controlling excitability in the CNS

Question 21

What are the key differences between central synapses and the NMJ?

Answer 21

Central synapses may release on average only a single vesicle for each
presynaptic action potential, which evoke weak and variable EPSPs.
• Central synapses can be inhibitory, as well excitatory.
• Short-term facilitation/depression affects synaptic communication at excitatory
and inhibitory synapses.
• Non-cholinergic transmission (including glutamatergic and GABAergic) is
largely terminated by diffusion and reuptake.

Question 22

What is neuromodulation?

Answer 22

the paracrine/spillover effects of neurotransmitters,

which simultaneously modulate synaptic function and/or cellular excitability across volumes of tissue

and exert slow modulatory effects largely via activation of G protein-coupled receptors

Question 23

What is synaptic plasticity?

Answer 23

activity-dependent changes in synaptic strength that outlast the direct effects of neurotransmitter release.

short-term plasticity, in the form of facilitation and depression of synaptic release.

Certain patterns of synaptic activity can also induce long lasting changes in synaptic strength, which can last for hours to years, and these forms of plasticity are thought to be important for behavioural learning
and memory.

Question 24

What are the the four main transmitter systems used for long-range and diffuse modulation of brain state?

Answer 24

Acetylcholine
Serotonin
Dopamine
Noradrenaline

Question 25

Where is ACh active in the brain?

Answer 25

cortical-projecting cholinergic neurons are found in the basal
forebrain,
while brain stem cholinergic neurons are located in the dorsolateral pontine tegmental area

Question 26

Where is serotonin active in the brain?

Answer 26

serotonergic neurons are located in the raphe nuclei

Question 27

Where is dopamine active in the brain?

Answer 27

there four major dopaminergic pathways:
1) nigrostriatal pathway from substantia nigra to dorsal striatum in basal ganglia,
2) mesolimbic pathway from ventral tegmental area to ventral striatum, hippocampus & amygdala,
3) mesocortical pathway from ventral tegmental area to frontal cortex,
4) tuberoinfindibular pathway from arcuate nucleus in
hypothalamus to pituitary gland, which can regulate the release of hormones, including prolaction.

Question 28

where is noradrenaline active in the brain?

Answer 28

the major source : neurons located in the locus coeruleus

Question 29

How is neuromodulation mediated?

Answer 29

via metabotropic receptors activate intracellular transduction pathways via G-protein coupled receptors

Question 30

what is the structure of G protein-coupled receptors?

Answer 30

each have 7 transmembrane domains, and often appear to exist as dimers.
GPCRs couple to GTP-binding heterotrimeric G-proteins that consist of Gα, Gβ, and Gγ subunits
with the identities of the G proteins determining which downstream pathway is modulated following receptor activation.

Question 31

How are heterotrimeric G proteins classified?

Answer 31

According to the type of alpha subunit they contain
3 canonical classes
Gαs (s for ‘stimulatory’) – activates plasma membrane adenylyl cyclases,
increasing the cytosolic second messenger cyclic AMP (cAMP).

Gαi (i for ‘inhibitory’) - inhibits most adenylyl cyclases, allowing cellular cAMP
to fall.

Gαq – activates phospholipase Cβ (PLCβ), a lipase that cleaves the signaling
phosphoinositide lipid (PI(4,5)P2) of the plasma membrane, generating
several second messengers including inositol trisphosphate (IP3) that
releases Ca2+ from intracellular stores and diacylglycerol that activates
phosphorylation by protein kinase C.

Question 32

What is the evidence that neuromodulation occurs via paracrine transmission?

Answer 32

Only a small percentage (5–40%) of the varicosities of monoaminergic and
cholinergic axons form synapses – the swellings along the axon that contain clusters of vesicles are not opposed to postsynaptic specialisations, and the GPCRs are not clustered around the release sites

Question 33

What are the key features of paracrine transmission via GPCRs

Answer 33

Axonal release of transmitter along a chain of varicosities will affect a large
volume of tissue (diffuse signal).

Due to diffusion in the extracellular space, the concentration of
neurotransmitter experienced the receptors will be in the order of a few
micromolar.

GPCR have sufficient sensitivity to be activated by these low neurotransmitter
concentrations, and can amplify the signal via the intracellular signalling
cascades.

GPCR can have divergent targets, including ion channels and gene
expression.

The response of each cell type to neurotransmitter release will depend on the
pattern of GPCR expression.

This type of signalling is relatively slow, but sufficient to mediate
behaviourally-relevant changes in arousal, mood, etc. It is analogous to the
mechanisms of transmission employed by the post-ganglionic neurons (final
output) in the autonomic nervous system.

Question 34

Is the identity of neurotransmitter important in fast synaptic transmission?

Answer 34

No
flies use glutamate instead of acetylcholine at the NMJ
the message is conveyed via the activation of a specific set of synapses

Question 35

Is the identity of neurotransmitter important in paracrine transmission?

Answer 35

yes
different signals have to be converyed by the chemical identity of the neurotransmitter
explains evolution and persistence of neurotransmitter diversity

Question 36

how can spillover of fast transmitters cause neuromodulation?

Answer 36

The synaptic release of glutamate and GABA also leads to spillover into the extracellular space

which can activate their metabotropic receptors (mGluR and GABAB, respectively)
(also has the potential to activate some extrasynaptic NMDAR and GABAaR).

This allows the modulation of neuronal processing, depending on thecurrent levels of activity in the circuit – more local activity leads to more spillover.

Question 37

What are the targets of neuromodualtion?

Answer 37

3 principal targets:

Presynaptic release –
presynaptic GPCRs can modulate ion channels, and
thus terminal excitability and action potential-induced Ca2+ influx.
Neuromodulation tends to decrease evoked release.

Postsynaptic response –
postsynaptic GPCRs can modulate ligand-gated ion
channels, and thus alter the response to vesicular release.

Neuronal excitability –
GPCRs located on the soma / dendrites can regulate
membrane polarisation, synaptic integration and spiking patterns in response to sustained depolarisation

Question 38

How does neuromodulation affect the output of neuronal circuits?

Answer 38

the frequency of motor rhythm and lags between segments in a motion such as walking can be regulated by descending neuromodulation
neuromodulation can turn circuits on and off and modulate the output they produce

Question 39

what are features of neuropeptides?

Answer 39

packaged in large dense core vesicles (LDCVs)

released in response to high-frequency firing

may reach receptors far from the release site

act via metabotropic receptors

do not undergo reuptake, but are broken down by extracellular peptidases

nature of signaling requires chemical diversity (>50 neuropeptides)

Question 40

What is NMDA receptor dependent long term potentiation

Answer 40

glutamate release activates AMPA/Kainate receptors on the dendritic spines of CA1 pyramidal neurons
evokes postsynaptic EPSPs simultaneously across many cells
frequent strong stimulation leads to large increase in evoked response due to residual Ca2+
Ca2+ decays rapidly
but there is persistent increase in the size of the evoked response that lasts hours
this form of synaptic plasticity is activity-dependent and synapse-specific

Question 41

What is NMDA receptor-dependent learning?

Answer 41

LTP depends on NMDA receptors because they are able to detect high-frequency input and provide intracellular signal to induce plasticity because

NMDAR show voltage-dependent Mg2+ block – at resting membrane potential,
Mg2+ can block the passage of ions through the receptor. Even if the receptor
is activated by glutamate, this Mg2+ block prevents ion flow, and NMDAR
contribute little to basal synaptic transmission. If the membrane is
depolarised, the Mg2+ is expelled from the channel.

Ca2+ permeability – the coincidence of glutamate release and postsynaptic
depolarisation leads to an NMDAR-mediated Ca2+ influx, which provides an
intracellular signal for plasticity

Tetanic stimulation initially produces depolarisation of the postsynaptic neuron via
AMPA/kainate receptors, but subsequent depolarisation and continued glutamate
release recruits NMDAR

Question 42

How can the expression of LTP be mediated postsynaptically?

Answer 42

by an increase in the
response to transmitter release - Ca2+ influx through NMDAR activates
Calcium/Calmodulin-dependent Kinase II (CaMKII), which increases AMPA currents
in 2 ways:

It phosphoryaltes AMPA channels, and increases their conductance

It favours the insertion/retention of AMPA receptors in the membrane