Amino acid neurotransmitters: Glutamate & GABA

What is glutamate?

The main excitatory transmitter in the CNS

What is GABA?

The main inhibitory transmitter in the CNS

What molecule is glutamate synthesised from?

Glutamine

What molecule is GABA synthesised from?

Glutamate

What type of receptors do glutamate and GABA activate?

Ionotropic and metabotropic receptors

Describe the distribution of glutamate

Found in pyramidal neurones

Describe the synthesis of glutamate

Glutamate in the CNS comes from either:

- Krebs cycle

- Glutamine

TCA cycles

Alpha-Oxo-glutarate is converted to glutamate using a reduction reaction

Glutamine

Glutamine made by glial glial cells and taken up by neurons.

Describe the degradation of glutamate

1. Glutamate is stored in synaptic vesicles

2. Glutamate is released from vesicles by Ca2+ dependent exocytosis.

3. When glutamate is released, it can be taken up by glial cells (astrocytes specifically) OR into nerve cells by excitatory amino acid transport (EEAT) proteins.

4. In astrocytes, glutamate is converted to glutamine and and recycled back to neurones via transporters (GlnT).

5. Glutamine is converted back into glutamate in neurons.

6. Glutamate is taken up into synaptic vesicles by vesicular glutamate transporters (VGluT).

Describe ionotropic receptors

They are ligand-gated ion channels

- Multi subunit receptors

- Heterogeneous receptors

- Affects physiological function and pharmacology

- Rapid cellular effects

Describe metabotropic receptors

These are G protein-coupled receptors

- Hetero and homodimers

- Activate second messenger systems

- Slower effects on synaptic transmission

- May be "autoreceptors" located presynaptically on nerve terminals

Describe GABAa receptor

A Cl- ion channel

GABA is the agonist

Muscimol can also act as an agonist

Bicuculline can act as an antagonist

Its effect happens in milliseconds

It has an inhibitory effect by causing fast hyperpolarisation

Describe GABAb receptor

A GPCR (K+/ Ca2+)

GABA is the agonist

Baclofen can also act as an agonist

Bicusaclofen/CGP can act as an antagonist

Its effect happens in milliseconds - minutes

It has an inhibitory effect by causing fast hyperpolarisation

Describe GABAc receptor

A Cl- ion channel

GABA is the agonist

Muscimol can also act as an agonist

TPMpA can act as an antagonist

Its effect happens in milliseconds

It has an inhibitory effect by causing fast hyperpolarisation

Describe the synthesis of GABA

Glutamic acid decarboxylase (GAD) converts glutamate into GABA.

Describe the degradation of GABA

GABA is converted into succinic semialdehyde via GABA-T (GABA transaminase).

Succinic acid semi-aldehyde is then converted into succinate via succinic dehydrogenase.

Describe the distribution of GABA

Mainly found in short local interneurons

Some found in longer projection neurones

What enzyme can inactivate GABA?

GABA-transaminase (GABA-T)

Describe the structure of ionotropic GABA receptors

It is a pentameric receptor. Has 2 alpha subunits, 2 beta subunits and a gamma subunit.

Name the ionotropic receptors for GABA

GABAa

GABAc

Name the metabotropic receptors for GABA

GABAb

Explain what is meant by inhibitory transmission at a synaptic level

This means the cell becomes hyperpolarised. Therefore the membrane potential moves further from resting membrane potential. Membrane potential becomes more negative. This makes it hard to generate an action potential (AP).

This happens as:

The binding of GABA to the ionotropic receptor (GABAa) causes the channel to open and allow Cl- channels into the cell

What is the orthosteric site?

Where the agonist binds to the receptor.

What is the allosteric site?

A site where other substrates can bind to the receptor which is not where the agonist binds

Describe the different sites on GABAa receptors

GABAa have the orthosteric site (where GABA binds) and allosteric/ modulatory sites where other substrates can bind.

What is the effect of allosteric binding on ionotropic receptors ?

Drugs can bind to allosteric sites to "turn up" or "turn down" effects of endogenous neurotransmitters.

The drugs (channel modulators) cannot open the channel by themself - only can modulate binding of endogenous neurotransmitter.

For example allosteric binding of channel modulators to GABAa receptors can affect transport of Cl- ions when GABA is bound to the orthosteric site.

What drugs can bind to GABAa receptors?

1. Anaesthetics

2. Barbiturates (used more before benzodiazepines made)

3. Benzodiazepines - for anxiety relief, hypnotic

4. Neurosteroids - metabolites of progesterone and deoxycorticosterone.

Give an example of anaesthetics which bind to GABAa receptors

Etomidate & propofol

Give an example of a barbiturate which bind to GABAa receptors

Pentobarbital (has sedative/anticonvulsant effects)

What is the effect of GABAb receptors presynaptically?

They close calcium channels presynaptically to reduce transmitter release (autoreceptor).

- Has an inhibitory effect as stops vesicular release of neurotransmitter.

What is the effect of GABAb receptors postsynaptically?

They open potassium channels postsynaptically causing a slow hyperpolarization

- Has an inhibitory effect

Describe GABAb receptor mechanism

Gio coupled receptor

It couples to calcium channels. Closes calcium channels so there is less calcium dependent release of neurotransmitter. (pre-synapse). Post synaptically, they open K+ channels. Open K+ channels then there is slow hyperpolarisation.

What is baclofen used to treat?

Baclofen is used to treat spasticity - rigidity in limbs and muscles.

This acts through the GABAb receptor by acting as an agonist.

However, it crosses the BBB so can cause sedation.

What are the different ionotropic glutamate receptors?

There are 3 types NMDA, AMPA, KAINATE.

They are named after the after the ligands which binds to them.

Describe the structure of ionotropic glutamate receptors

Tetrameric (4 subunits)

NMDA: 7 different sub-units

AMPA: 4 different sub-units

KAINATE: 5 different sub-units

Main ONE glutamate binds to NMDA

Explain what is meant by excitatory transmission at a synaptic level

When the membrane potential becomes transiently more positive. This makes it more likely for an action potential to occur.

It is responsible for fore fast excitatory neurotransmission.

This occurs because...

Glutamate binds to NMDA receptor which allows Na+ into the cell.

Describe metabotropic glutamate receptors

There are 8 different GPCRs split into 3 groups (groups 1-3).

Can have homo and heterodimers

Their neurotransmission is slow.

Can be found post and pre-synaptically.

Grouped by sequence, and the 2nd messenger systems that they are coupled to.

No drugs currently work through metabotropic receptor.

How does glutamate bind to so many different receptors?

The structure of glutamate is flexible. It can rotate its bonds along 2 different axis. Therefore, it can adopt different confirmations and have 9 different rotamers.

Describe the different sites on NMDA receptors

NMDA receptors have receptor site (orthosteric) and several modulatory sites (allosteric)

What are two selective NMDA receptor antagonists?

Ketamine memantine.

How do pre-synaptic receptors on NMDA receptors control neurotransmitter release?

1. Enhance glutamate release

they can increase glutamate release

by increasing Ca2+ influx.

Mg2+ block disappears when cell depolarised.

2. Inhibits glutamate release

they can decrease glutamate release

by decreasing Ca2+ influx.

Mg2+ can block Ca2+.