L2: The space in between

Is there a gap? Chemical vs electrical synapses (what was thought)

1. In late 1800→ most scientists believed that the nervous tissue was a continuous mesh wehere one cell pysicically joined the other

   • mainly coz microscopes were insufficient to resolve fine neuronal processes

2. But the fact there is a gap arises naturaull between the presynaptic and postsynaptic terminals

Camilllo Golgi developed…

The black reaction:

• Staining method randomly labelled individual neurons in their entirety

• realing the polarised strcuture of axons and denrites

But due to Golgi’s interpretation

• thought the neurons were a continuous network

• a functional syncytium

  • → could not see the synaptic gap

But what did Santiago Ramon y Cajal think

• Used Golgi’s method

• oughtin emerging concenptual advances

• → argued that neurons are discrete entities separated by gaps

But was Golgi entirely wrong?

• Some neurons are connected physically  via electrical synapses or gap junctions

• where cytoplams are continuous through aligned channels

  • → made of proteins called connexins

What do these connexins allow

• rapid, bidirectional flow of ions

Role of these

1. abundanat in development

2. Critical for synchronising certain neural circuits in invertebrates and mammals

   1. → e.g in inhibitory networks for oscillations

Final proof of a presence of the gap/ synaptic cleft

Charles Herrington:

• Demonstrated unidirectional spinal reflexes

• delay directionalilty and ability of presynaptic depolarisaion to cause postsynaptic hyperpolarisation 

  • could only be explained by the existence of a chemical cleft or synpase

subsequeny disocveries further confirms the chemical nature of neurotransmission cementing the neuron doctrine

Neutransmitter clearance from the cleft: why must this happen

• must be cleared rapidly to preserve temporal precision

Ways clearance can occur

1. Diffusion away

2. Enzymatic breakdown

   • acetylcholine broken down by acetylcholinesterase)

3. Reuptake via specialised plasma membrane transporters

Reuptake is especially important: how does it work

1. Transporters use the sodium electrochemical gradient to import neurotransmitters

2. energetic cost is paid by the pumps that maintain this gradient for the membrane potential

If neurotransmitters are on astrocytes…

neurotransmitters are:

1. taken up

2. broken down

3. precursors are shuttled back to neurons

IF neurotransmitters are on the presynaptic terminal

Neurotransmitters are not degraded

instead

• immedicately repacked into vesicles

What does this ensure

• continuous transmission

Why is clearnace deliberately slow in some synapses?

• allows spillover to neirghbouring synapses and extrasynaptic receptors

Pre-synaptic protein of interest: alpha-Synulcein→ features

• 140-amino acid

• SNCA-encoded presynaptic protein

Pre-synaptic protein of interest: alpha-Synulcein→ what is its role

• regulates neurotransmitter release 

and

• reuptake 

by

• interacting with SNARE proteins

Pre-synaptic protein of interest: alpha-Synulcein: what happens when it is mutated

• misfolds into Lewy bodies

  • aggregates that spread and disrupt synaptic function

  • → defining Parkinson’s disease and Lewy body dementia

Three man chemical groups of neurotransmitters

1. Amino acids

   • glutamate (main excitatory)

   • GABA (main inhibitory)

   • glycine (another inhibitory)

2. Esters

   • ACh→ for neuromuscular junction and for the brain

3. Monoamines

   • Dopamine

   • noradrenaline

   • adrenaline (catecholamines)

   • serotonin

   • histamine

They have different properties

• Amino acids and acetylcholine→ act quickly

• Monomines→ act more diffusely and more slowly

  • this is why they are sometimes called neuromodulators

some transmitter are produced by only…

• small groups of neurons

However, they can still be effect across the entrire brain because…

• their axons project widley

Example 1: Acetylcholine:

• in nuclei of the basal forebrain and brain stem

but

• its projections influence sleep-wake cycles and memory

• its los→ alzherimers

Example 1: Acetylcholine: organophosphate poising

• illustrates the danger of disrupting acetylcholine lcearnace

What happens:

• prevents acetylcholinesterase from breaking it down

• leads to uncontrolled signalling

• eventually respiratory-failture

Example:2 Serotonin

• produced by neurons of the raphe nuclei in the brainstem

• equally widespread projections

Example:2 Serotonin, what does it regulate

• mood, appetite, pain and sleep

Example:2 Serotonin, what does dysfunction of serotonergic systmes cause

• associated with depression, anxiety and schizophenia

Example:2 Serotonin: Drugs to prolong serotonin’s action

• SSRIs (e.g fluoxetine/Prozac)

• prolong serotonin action in the cleft by blocking reuptake, improving symptoms of mood disorders

Example 3: Dopamine

• small localised production→ wide-spanning impact:

  • neurons in substantia nigra progect to the striatum in the nigrostriatal pathway

  • essential for movement

  • DEGREDATION→ PArkinson’s disease

Example 3: Dopamine: neurons in the neighbouring ventral tegmental area…

• project more broadly to limbic and cortical regions

  • where disruption of dopamine transmission is linked to psychosis and schizophenia

Example 3: Dopamine: Another→ small hypothalamic population projects…

• the pituitary glands

• regulates hormone release for lactation

Example 3: Doapmine: the catecholamine family forms…

• a biochemical chain from tyrosine

• through L-SOPA → dopamine→ noradrenaline→ adrenaline

• drugs can act at multiple steps in this pathway:

  • L-DOPA therapy in Parkinson’s

  • Enzyme inhibitors used for hypertension or addition

Neuropeptides and promiscuous release: Dale’s principle

• The same chemical transmitter is released from all the synaptic terminals of a neuron

However:

• This is far too simple

Why is it far too simple?

• Neuropeptides add complexity

What are neuropeptides

• short amino acid chains

• longer than classical transmitters

• shorter than hormones

Examples of neuropeptides

1. substance P

2. vasopressin

3. somatostatin

4. endorphins

5. glucagon-like peptide-1 (GLP-1)

Where are they synthesised and released

1. Synthesised in soma

2. transported to terminal

3. often co-released with fast transmitters like glutamate

Roles of Peptides

1. act at low concentrations

2. produce long-lasting effects

→play key roles in physiology and disease

Example: GLP-1

• naturally released from gut cells and brainstem neurons

• after food intake

→ SIgnals satiety

Example: GLP-1:How does it signal satiety

• acting on postsynaptic receptors in the hypothalamus

Example: GLP-1: theraputic agonists

e.g Ozempic

• mimic this peptide

• have much longer half-life

• proloning ‘full'ness’

→ reduciing food intake

Neurones also engage in

1. Co-transmission

2. Co-release

1. Co-transmission

• maining separate vescicle pools for different transmitter

2. co-release

• single vesicle contains more than one transmitter

  • e.g glutamate and GABA

This promiscuity of signalling shows that…

• neurotransmission is rarely as simple as one neuron, one transmitter

New generation of genetically encoded tool allows us to…

e.g ‘sniffers’

• allows us to directly visualise neurotransmitter release in real time

Example: GABA sniffer

• derived from fluorecent bacterial proteins

• glows green when it detets GABA in the synaptic cleft

When combined with electrophysiology, this provides …

• a simultaneous readout of pre-and postsynaptic activity

These approaches reveal that…

• neurotransmitter signalling is:

  • messy, promiscuous and diverse

modern tools let us capture that complexity in action