NEURO: Neurotransmitter Systems I: Glutamate

Question 1

What are the criteria of a neurotransmitter?

Answer 1

• The molecule must be synthesised and stored in the presynaptic neurone.
• The molecule must be released by the presynaptic axon terminal upon stimulation.
• The molecules must produce a response in the postsynaptic cell.

Question 2

Describe the synthesis of glutamate.

Answer 2

This type of neurotransmitter is known as an amino acid transmitter because their precursors are all amino acids.

Glutamine is converted to glutamate via the action of phosphate-activated glutaminase. It is synthesised in the nerve terminals.

Question 3

Describe the transport of glutamate.

Answer 3

Glutamate is transported into vesicles by vesicular glutamate transporters (VGLUT).

There is an H+-Glu transporter on the vesicle membrane. We harness the passive process of H+ moving down its concentration gradient to get more glutamate into the vesicle.
Thus, the intracellular environment of the vesicles is really acidic.

Question 4

List the different ionotropic receptors.

Answer 4

• AMPA receptors
• NMDA receptors
• Kainate receptors

Question 5

What is the response when glutamate activates the different ionotropic receptors?

Answer 5

With AMPA, we get the influx of Na+ and the efflux of K+.

With NMDA, we get the influx of Na+ and Ca2+, and the efflux of K+.

With Kainate, we get the influx of Na+ and the efflux of K+.

Question 6

Describe AMPA receptors.

Answer 6

There are 4 subunit types (plus alternative splice variants):

• GluA1
• GluA2
• GluA3
• GluA4

The molecule is referred to as hetero-tetrameric, or the ‘dimer of dimers’. This is because there are normally 2 pairs of two types of subunits. The most common orientation is that one pair is GluA2, and the other pair is GluA1/3/4.

There are four orthosteric binding sites; however, only two sites need to be occupied for the channel to be opened.
The current increases as more binding sites are opened.

The presence of GluA2 subunits prevents Ca2+ flow. Thus, they protect the brain against excitotoxicity.

Question 7

Describe NMDA receptors.

Answer 7

There are three subunit types (plus alternate splice variants):

• GluN1 (or NR1)
• GluN2 (or NR2)
• GluN3 (or NR3)

It is also hetero-tetrameric. The most common orientation is a pair of GluN1 subunits plus GluN2 (or 3). GluN3 subunits are inhibitory to NMDA receptor function.

NMDA receptors have a unique function: not only are they ligand-gated, but they are also voltage-gated.

We have two ligands: glutamate (major) and Glycine/D-serine. All the sites must be occupied for the channel to open.

With the voltage-gating, there is a molecule of Mg2+ that is blocking the ion channel at rest. It’s only in a depolarised neurone that the Mg2+ would exit the NMDA receptor and allow ions to flow. This depolarisation occurs by the initial stimulation of AMPA receptors, which will allow Na+ into the post-synaptic cell, depolarising it in the process.

Question 8

How are NMDA receptors dependant on AMDA receptors?

Answer 8

On the post-synaptic cell membrane, the AMPA receptors are activated first. They allow Na+ into the cell, depolarising it, and thus activating the NMDA receptors. The NMDARs allow both Na+ and Ca2+ into the cell, further depolarising the cell.

Furthermore, the Ca2+ traffics more AMPARs to the cell membrane surface, enlarging the signal. It also activates an enzyme called CamKinase II (CamKII) which phosphorylates AMPARs, allowing them to pass more current through. Effectively, it increases their permeability.

Question 9

Describe how the NMDA and AMPA receptors play a role in long-term potentiation, and what implications that mechanism has.

Answer 9

Because of the actions of the NMDARs and AMPARs, we now have more receptors on our membrane, and we have an increased ion flow. This causes an increased response induced in the post-synaptic cell.

This potentiation is deemed long-term because we’ve had more proteins inserted into the membrane and phosphorylated. These changes aren’t readily reversible.
This is thought to be a very important mechanism in what is known as synaptic strengthening and also in learning and memory (mostly mediated by calcium influx).

Question 10

Describe Kainate receptors.

Answer 10

There are 5 subunit types:

• GluK1 (GluR5)
• GluK2 (GluR6)
• GluK3 (GluR7)
• GluK4 (KA1)
• GluK5 (KA2)

They used to be differently named because they were thought to be AMPA receptors.

The receptor is tetrameric and can be made up of homomers or heteromers.
GluK1-3 can form homomers or heteromers. GluK4 and 5 can only form heteromers with GluK1-3 subunits.

It is a ligand-gated ion channel, although we don’t know exactly how many molecules of glutamate are required for the channel to open (since it’s hard to properly crystalise the receptor to study that).

Question 11

Describe metabotropic receptors.

Answer 11

These receptors are G protein-coupled receptors (GPCRs). They form dimers on the membrane, and there are three types of dimers they can make:

• homomers
• heteromers within groups (e.g. mGlu1 and 5)
• heteromers outside of groups (e.g. mGlu2 and 5-HT2A)

There are 8 subtypes of the receptor (mGlu1-8), and they are divided into 3 subgroups (based on their sequence homology).

GROUP 1: mGlu1, mGlu5
GROUP 2: mGlu2, mGlu3
GROUP 3: mGlu4, mGlu6, mGlu7, mGlu8

Group 1 is predominantly found post-synaptically, while Groups 2 and 3 are predominantly pre-synaptically.

They also bind to different G proteins:

• Group 1 binds to αGq/11, ultimately increasing intracellular Ca2+ release. They contribute to long-term potentiation and therefore plasticity.
• Groups 2 and 3 bind to αGi, ultimately decreasing cAMP formation. They inhibit futher neurotransmitter release.

Question 12

What are the ways in which the glutamate signal can be terminated?

Answer 12

We can get glutamate diffusion away from the synapse, or reuptake by the EAATs (excitatory amino acids transporters) pre-synaptically.

Question 13

What is excitotoxicity, and what effect does it have on the neurons?

Answer 13

Excitotoxicity occurs when we have too much excitation. This happens under pathological conditions.

We could have damage to the vesicular glutamate transporter so that the glutamate stored isn’t released. This build up of glutamate in the vesicle increases its concentration, causing the Glut-H+ transporter to switch and release the glutamate out into the cytosol. This increases the concentration of the glutamate intracellulary, which causes the EAATs to work backwards as well.
These leak out the glutamate into the synapse without the presence of a stimulus. This leads to the uncontrolled induction of an action potential in the post-synpatic cell (what’s significant about this is the uncontrolled levels of Ca2+).

Excessive Ca2+ can have multiple implications:

• it can cause mitochondrial damage (which can stop cellular respiration taking place)
• it can cause oxidative stress (by formation of free radicals)
• it can cause apoptosis in cells (due to previous reasons)

A stroke can cause excitotoxicity, and autism and Alzheimer’s have been linked to it as well.