synapses and neurotransmitters

Synapse

Junction between 2 neurons allowing signals to pass from one to the other

Synaptic transmission

Process of signalling via synapses

How many synapses are in the brain?

100 trillion

How many neurons in the brain?

86 billion

Neuron doctrine vs reticular theory

Whether the brain is made of separate cells (neuron doctrine) or whether neurons are connected into a continuous network (reticular theory)

Evidence for neurons in late 1800s

Golgi stain by Ramon y Cajal

Evidence for neurons around 1900

Sherrington had physiological evidence from study of reflexes

Evidence for neurons around 1950s

Electron microscopy

Electrical synapses

Formed of gap junctions that allow current to pass directly between neurons
Pores connect the 2 cytoplasms together so that current and charge can pass

Diameter of a gap junction

1-2nm

What is transmitted through an electrical synapse?

Current, charge and after-hyperpolarisation

What does a gap junction transmit?

Upward and downward changes in membrane potential

How can you tell if neurons are connected by gap junctions?

Small molecules like dyes diffuse from one neuron to the other
Stimulate one neuron and record on the other neuron
Both hyperpolarising and depolarising stimuli are passed from one neuron to the other

What blocks hyperpolarising and depolarising stimuli being passed from one neuron to the other?

Deleting a connexin gene

shakB2 mutant

Deletes a connexin gene

What are electrical synapses good for?

Fast communication
Synchronising neurons

How were chemical synapses discovered?

2 isolated frog hearts
Stimulate vagus nerve
Heart rate slows
Remove fluid sample
Add fluid to recipient heart
Heart rate slows

Post-synaptic cell of a motor neuron

Skeletal muscle

Post-synaptic cell of an autonomic neuron

Hormonal gland, smooth muscle or heart

Autonomic nervous system

A subdivision of the peripheral nervous system. Controls involuntary activity of visceral muscles and internal organs and glands.

Synaptic vesicle size

40-50nm

Dense core secretory granule size

100nm

What type of NT is in a synaptic vesicle?

Small molecule

What type of NT is in a dense core secretory granule?

Peptide

How are synaptic vesicles filled?

Transporter proteins at the presynaptic terminal

How are dense core secretory granules filled?

By the ER/golgi sectretory apparatus

Synaptic vesicles or dense core can be recycled?

Synaptic

First stage of chemical synaptic transmission

Package neurotransmitter in vesicles and put them at the pre-synaptic terminal

Second stage of chemical synaptic transmission

Action potential arrives and voltage gated calcium channels open

Third stage of chemical synaptic transmission

Calcium influx causes vesicles to fuse to membrane and release NTM

Fourth stage of chemical synaptic transmission

NTM diffuse across the synaptic cleft and activate receptors on the postsynaptic cell

What happens to synaptic vesicles once they have released the NTM?

Vesicle is taken up again by endocytosis

How do vesicles fuse to the membrane?

Via SNARE proteins
Calcium binds to synaptotagmin and causes a conformational change that makes the SNAREs attach together forcing the vesicles to fuse to the plasma membrane

What is a target for toxins?

SNARE proteins

Activation of an ionotropic receptor?

Directly depolarise or hypoerpolarise the posysynaptic cell

Activation of a G-protein coupled receptor?

Activates G-proteins and may activate a signalling cascade which can lead to ion channels opening

Does the NTM enter the postsynaptic cell?

No

Fifth stage of synaptic chemical transmission?

NTMs are removed from the cleft
They are actively taken up by transporters for recycling either into presynaptic neuron or glia
They diffuse away
They are destroyed in the synaptic cleft by enzymes

Similarities between electrical and chemical synapses

Both are plastic but chemical are mroe so
Both allow summing up inputs by the post-synaptic neuron

What type of synapses are most?

Chemical

Differences between electrical and chemical synapses?

Signals pass in both directions in electrical but one in chemical
Signals are passed directly in electrical but can be radically transformed in chemical
Electrical is faster

What NTM does NMJ use?

Acetylcholine

How does the MNJ achieve efficient transmission?

Largest synapses in the body - large s.a
Large number of active zones on presynaptic cell
Postsynaptic cell contains junctional folds densely filled with NTM receptors
Active zones and junctional zones are precisely aligned to maximise transmission

How did we figure out NTMs are released from vesicles?

Stimulate motor nerve and record fro muscle
The evoked responses are all integer multiples of a spontaneous potential (spontaneous MEPP)

MEPP

Miniature end plate potential

Axodendritic

The axon of 1 neuron synapsing onto the dendrite of another neuron

Axoaxonic

Axon synapsing onto another axon

Axosomatic

Axon onto cell body

Dendrodendritic

Dendrite to dendrite

Dendrosomatic

Dendrite onto cell body

Criteria for NTM

Present in presynaptic terminals
Be released in response to stimulation
Act on the posysynaptic neuron
Blocking the neurotransmitter should prevent synaptic transmission

How do we tell if a neurotransmitter is present?

Immunostaining

How to tell if the neurotransmitter is released?

Collect fluid around neurons after stimulating them

Amino acid NTM

Glutamate, glycine and GABA

Amine NTM

ACh and monoamines

Where are amino acid and amine NTM stored?

Synaptic vesicles

Where are peptide NTMs stored?

Secretory granules

What size are amino acid and amine NTM?

100-200 Da

What size are peptide NTMs?

1000-3000 Da

What can amino acid and amine NTM bind to?

Ligand gated ion channels or G-proteins coupled receptors

What can peptide NTM bind to?

G-protein coupled receptors

Co-transmitter

Small molecule transmitter that is often released with peptide NTMs

Ionotropic receptors

Directly depolarise or hyperpolarise the postsynaptic cell

G-protein coupled receptors

Multiple possible second messengers which allow for amplification \= signalling cascade

Convergence with NTM

Each receptor can activate different downstream effectors

Divergence with NTM

Different receptors can activate the same downstream effector

What is the most common excitatory NTM in the CNS?

Glutamate

Where is glutamate found?

In all neurons of the CNS

AMPA receptors

An ionotropic glutamate receptor that is a non-specific cation channel that sodium and potassium can pass through - not calcium
Mediates fast excitatory transmission
Glutamate binding triggers Na/K currents leading to EPSPs

NMDA receptor

Ionotropic glutamate receptor
Often co-exists with AMPA receptors
Voltage depedent magnesium block so only opens when neuron is already depolarised
Lets calcium in as well as Na/K
Leads to downstream signalling
Functions as a coincidence detector

Coincidence detector

When a neuron is activated right after it was already activated - importance for learning

mGluRs

Metabotropic receptors of glutamate
Allow flutamate to sometimes be inhibitory like in the retina

How many subunits in AMPA and NDMA receptors?

4

GABA

An inhibitory neurotransmitter in the brain.

What is GABA synthesised from?

Glutamate

What enzyme converts glutamate to GABA?

Glutamic acid decarboxylase

How is the action of GABA terminated?

Uptake into presynaptic terminals and glia

How does GABA inhibit?

Produces IPSPs via GABA gated chloride channels (GABAa receptor) if the membrane potential is above chloride Nernst potential

IPSPs

Inhibitory postsynaptic potentials

Too much inhibition via GABA will cause what?

Coma or loss of conciousness

Too little inhibition via GABA will cause what?

Seizures

Allosteric drug

Binds to a site other than where ligand or agonist binds

Allosteric drugs of GABAa receptor

Ethanol
Benzodiazepines
Barbiturates
Neurosteroids

Benzodiazepines

Treat anxiety
E.g. Diazepam

Barbiturates

Sedatives and anti-convulsants

Neurosteroids

Metabolites of steroid hormones
E.g. progesterone

GABAb receptor

Metabotropic receptor for GABA that activation might lead to:
Open K+ channels
Close calcium channels
Trigger other second messengers

Autoinhibitory GABAb

Inhibits itself to stop releasing too much GABA

Glycine

Inhibits neurons via glycine-gated chloride channel
Glycine binds and opens the chloride channel
Also binds NMDA glutamate receptors

What does an inhibitory synapse do?

Blocks the propagation of an EPSP towards the soma

Inhibition presynaptically

1. Action potental arrives
2. GABA release
3. Inactivation of calcium channels
4. Less calcium enters
5. Less neurotransmitter released
6. Reduced effect on postsynaptic membrane

What makes up ACH?

Acetyl CoA (acetic acid) and choline

What enzyme creates ACH?

Choline acetyltransferase in the cytoplasm of cells

How to tell if a neuron is cholinergic?

Test whether the enzyme choline acetyltransferase is present

Where is acetyl coA produced?

Mitochondria by cellular respiration

What is Ach destroyed by?

Acetylcholinesterase

What happens to choline once ach is broken down?

Taken back up into presynaptic cleft by a choline transporter

nAChRs

Nicotinic acetylcholine receptors
Ach gated Na+/Ca2+ channel
Found in NMJ and CNS

mAChRs

5 types of GPCRs found in CNS and ANS

M1, M3 and M5

Excitatory via Gq