Glutamate as a Neurotransmitter (A*)

Question 1

What proportion of neurotransmitters in the body is made up of glutamate?

Answer 1

Glutamate represents ~90% of all neurotransmitters in the body.

Question 2

List 4 brain functions that rely on glutamate transmission.

Answer 2

Glutamate transmission is responsible for:

1 - Learning / memory.

2 - Pleasure.

3 - Pain.

4 - Anxiety.

Question 3

Describe the glutamate synthesis cycle.

Answer 3

1 - Glutamine is converted into glutamate by glutaminase.

2 - Glutamine synthase reconverts glutamate back into glutamine.

Question 4

Which neurotransmitters are synthesised from glutamate?

Which enzymes are involved?

Answer 4

GABA and glycine are synthesised directly from glutamate:

1 - GAD converts glutamate into GABA.

• Glutamate also contributes to the Krebs cycle when it is converted into alpha-oxoglutarate:

2 - Transaminase converts glyoxylate, a byproduct of the Krebs cycle, into glycine.

3 - Transaminase converts oxaloacetate, an intermediate product of the krebs cycle, into aspartate (an NMDA receptor agonist).

Question 5

Describe the process of glutamate recycling.

Answer 5

1 - Once synthesised, glutamate is packaged into vesicles using specific, high-affinity vesicular transporters known as VGLUTs.

2 - Following glutamate release into the synapse, glutamate reuptake occurs via Na+-dependent excitatory amino acid transporters (EAAT) into both the presynaptic terminal (where it is reused) and surrounding astrocytes.

3 - In astrocytes, glutamate is reconverted into glutamine, which is then rereleased via glutamine transporters.

4 - The glutamine reenters the presynaptic terminal via extrasynaptic glutamine transporters, where glutamine reenters the glutamate synthesis cycle.

Question 6

List the subtypes of excitatory amino acid transporters.

Describe their distribution in the brain.

Answer 6

1 - EAAT-1 is primarily glial, and is found in the cerebellum.

2 - EAAT-2 is mixed glial and neuronal, and is found in the forebrain.

3 - EAAT-3 is primarily neuronal, and is found in the cortex.

4 - EAAT-4 is found in cerebellar Purkinje fibres.

5 - EAAT-5 is primarily neuronal and is found in the retina.

Question 7

Give an overview of the glutamate receptor subtypes. Include:

Are these ionotropic or metabotropic?

Are these excitatory or inhibitory?

Do these receptors mediate slow or fast transmission?

Answer 7

Ionotropic glutamate channels:

1 - NMDA (fast excitatory).

2 - AMPA (fast excitatory).

3 - Kainate (fast excitatory).

Metabotropic glutamate channels:

1 - Group 1 (slow excitatory).

2 - Group 2 (slow inhibitory).

3 - Group 3 (slow inhibitory).

Question 8

Which ions pass through ionotropic glutamate receptors?

In which direction do these ions move through the channels?

Answer 8

Ions passing through ionotropic glutamate receptors include:

1 - Na+ (influx).

2 - Ca2+ (influx).

3 - K+ (efflux).

Question 9

How does the speed of transmission differ between NMDA and AMPA receptors?

Answer 9

NMDA receptors mediate slower excitatory synaptic responses than AMPA receptors (although both are still fast because they are ionotropic).

Question 10

How many subunits does an NMDA receptor have?

Describe their distribution.

How many genes encode these receptors?

Answer 10

An NMDA receptor is tetrameric (has 5 subunits).

NMDA receptor subunits include a combination of:

1 - NR1.

• Expressed in almost all neurones.
• Derived from a single gene.
• Has 8 splice variants.

2 - NR2.

• Has a more restricted expression than NR1.
• Derived from 4 genes.

3 - NR3.

• Distribution unknown.
• Derived from 2 genes.

Question 11

Which NMDA receptor subunit is the modulatory site?

Answer 11

The NR2 subunit is the modulatory site for the NMDA receptor.

Question 12

Which coagonist is required, along with glutamate, to activate NMDA receptors?

What is the role of Mg2+ in NMDA receptor activation?

Answer 12

• Both glycine and glutamate are required to activate NMDA receptors.
• Mg2+ blocks NMDA receptor ion channels, so excitation must be sufficient so as to be able to remove the Mg2+ ion before the channel can open.
• Membrane depolarisation through other channels such as AMPA receptors can help achieve this.

Question 13

List the binding sites on the NMDA receptor.

Answer 13

Binding sites on the NMDA receptor include:

1 - Glutamate binding site.

2 - Glycine binding site.

3 - Polyamine binding site.

4 - Open channel blocking site.

5 - Zn2+ binding site (not the ion channel!).

6 - Mg2+ binding site (not the ion channel!).

Question 14

List 5 agonists and 3 antagonists for the NMDA receptor glutamate binding site.

Answer 14

NMDA receptor agonists for the glutamate binding site include:

1 - Glutamate.

2 - Aspartate.

3 - Quinolinate (at some receptor subtypes).

4 - L-homocysteine.

5 - Cysteinesulphinate

NMDA receptor antagonists for the glutamate binding site include:

1 - Quinolinate (at some receptor subtypes).

2 - 2R-CPPene.

3 - D-AP5.

Question 15

What is strychnine?

How does it interact with the NMDA receptor?

Answer 15

• Strychnine is a competitive antagonist of the glycine receptor.
• It is unable to bind to the glycine binding site of the NMDA receptor because the glycine binding site on the NMDA receptor is pharmacologically distinct from the glycine receptor.

Question 16

Give an example of an agonist (other than glycine) and antagonist for the glycine binding site of the NMDA receptor.

Answer 16

• An agonist for the glycine binding site of the NMDA receptor is D-serine.
• An antagonist for the glycine binding site of the NMDA receptor is kynurenic acid derivatives.

Question 17

On which subunit of the NMDA receptor is the polyamine binding site found?

What is the effect of polyamines binding to the polyamine binding site on the NMDA receptor?

What is a potential pathological process that might be caused by this binding site?

Answer 17

• The polyamine binding site of the NMDA receptor is found on the NR2B subunit.
• Binding of polyamines to this site increases the ability of glutamate and glycine to open ion channels (modulatory effect).
• In some cases, this might mediate excitotoxicity (for details see card 22).

Question 18

What is the effect of Zn2+ and Mg2+ on NMDA receptor function?

Answer 18

• Zn2+ can cause a voltage-dependent or voltage-independent block of NMDA receptors.
• Mg2+ can cause a voltage-dependent block of NMDA receptors.

Question 19

List 4 non-competitive voltage-dependent blockers of NMDA receptors other than Zn2+ and Mg2+.

To which binding site do these ligands bind?

Answer 19

Non-competitive voltage-dependent blockers of NMDA receptors include:

1 - Ketamine.

2 - Memantine

3 - Dizocilpine.

4 - Phencyclidine.

• These ligands bind to the OPEN ion channels of NMDA receptors (hence they are voltage-dependent).

Question 20

Describe the mechanism of excitotoxicity, using ischaemia as an example.

Answer 20

In the case of ischaemia (there are other triggers other than ischaemia):

1 - Ischaemia decreases a neurone’s energy supply.

2 - A decrease in energy supply to a neurone results in excessive cell depolarisation.

• This happens because ischaemic insult results in a decrease in ion gradient homeostasis across the neuronal membrane, which in turn is due to a reduction in energy supply to fuel the ion pumps.

3 - Excessive cell depolarisation results in excessive glutamate release and excessive influx of ions such as Ca2+.

4 - Water follows, resulting in excessive swelling and subsequent neuronal death.

• Other cytotoxic molecules released as a result of ischaemia (e.g. free radicals and lipases) also contribute to neuronal death.
• Glutamate is stored in very high concentrations in glutamatergic neurones. If these concentrations were to be applied to the extracellular space, neurones would quickly die of excitotoxicity.

Question 21

List 2 pathologies that might involve excitotoxicity, resulting in further damage.

Answer 21

Pathologies that involve excitotoxicity include:

1 - Stroke (due to hypoxia, cardiac arrest etc.).

2 - Seizures.

Question 22

List 3 clinically used NMDA-targeting drugs.

What are they used for?

Answer 22

Clinically used NMDA-targeting drugs include:

1 - Ketamine (an NMDA antagonist used for anaesthesia).

2 - Memantine (an NMDA antagonist used to treat Alzheimer’s. NB. this is strange because NMDA blockade usually has deleterious effects on Alzheimer’s).

3 - NR2B antagonists (in clinical trials for Parkinson’s and neuropathic pain).

Question 23

What is the glutamate hypothesis of schizophrenia?

What pharmacological targets are useful for the treatment of schizophrenia?

Answer 23

• The glutamate hypothesis of schizophrenia suggests that hypofunction of cortical NMDA receptors underlie schizophrenia.
• See schizophrenia lecture for more info and for dopamine hypothesis.
• Glycine binding sites are useful for treating schizophrenia, particularly the negative symptoms of schizophrenia.
• Inhibitors of GLYT1 (glycine transporter 1 - reuptakes glycine from the synapse to terminate glycine signalling) are also in clinical trials for schizophrenia.

Question 24

How many subunits does an AMPA receptor have?

Answer 24

An AMPA receptor has 4 subunits: GluR1-4.

Question 25

How many subunits does a kainate receptor have?

Answer 25

A kainate receptor has 5 subunits:

• GluR 5-7.
• KA 1-2.

Question 26

Give an example of a family of drugs that act as selective antagonists for both AMPA and kainate receptors.

List 3 examples of these drugs.

Answer 26

• Quinoxalinediones are selective antagonists for both AMPA and kainate receptors.

1 - NBQX.

2 - CNQX.

3 - DNQX.

Question 27

List 4 AMPA and kainate-based treatments.

Which conditions are these therapies used to treat?

Answer 27

AMPA and kainate-based treatments:

1 - Quinoxalinediones.

• Used to treat stroke and pain.

2 - GluR5 antagonists.

• Used to treat neuropathic pain.

3 - AMPA negative allosteric modulators (NAMs).

• Used to treat epilepsy, Parkinson’s and migraines.

4 - AMPA positive allosteric modulators (PAMs / AMPAkines).

• Used to treat Alzheimer’s and schizophrenia. Idea is to upregulate neuronal growth factors to induce repair.

Question 28

Describe the structure of metabotropic glutamate receptors.

How are metabotropic glutamate receptors grouped?

Answer 28

• Metabotropic glutamate receptors contain 7 transmembrane domains (like all GPCRs).
• All metabotropic glutamate receptors are homodimers.
• The grouping is based on their pharmacology and mode of intracellular signalling.

Question 29

List the metabotropic glutamate receptors, including the group they are classified in.

To which signalling pathways are these receptors coupled?

Answer 29

• Group 1 (coupled to Gq):

1 - mGlu1.

2 - mGlu5.

• Group 2 (couples to Gi/o):

1 - mGlu2.

2 - mGlu3.

• Group 3 (coupled to Gi/o):

1 - mGlu4.

2 - mGlu6.

3 - mGlu7.

4 - mGlu8.

Question 30

What are the functions of the groups of metabotropic glutamate receptors?

Answer 30

• Group 1 metabotropic glutamate receptors:

1 - Involved in synaptic plasticity.

2 - Regulate proliferation, differentiation and survival of neural stem cells.

• Group 2 metabotropic glutamate receptors:

1 - Involved in nociceptive signalling and pain modulation.

2 - Involved in in emotional processing

• Group 3 metabotropic glutamate receptors:

1 - Involved in presynaptic inhibition.

Question 31

List 5 metabotropic glutamate receptor-based treatments.

Which conditions are these therapies used to treat?

Answer 31

Metabotropic glutamate receptor-based treatments:

1 - mGlu2 and mGlu3 (group 2) agonists.

• Used to treat anxiety and depression.

2 - mGlu5 antagonists:

• Used to treat anxiety and depression.

3 - mGlu5 NAMs:

• Used to treat anxiety, pain and addiction.

4 - mGlu5 PAMs:

• Used to treat schizophrenia.

5 - mGlu4 PAMs:

• Used to treat Parkinson’s.

Question 32

List 3 examples of glutamate-based treatments that target glutamate transporters.

Answer 32

1 - Enhancing EAAT2 function, e.g. with beta-lactam antibiotics (which upregulate EAAT2 expression), might be helpful for treating motoneurone disease.

2 - EAAT3 inhibitors might be useful for treating schizophrenia.

3 - VGLUT inhibitors might be useful for treating epilepsy and neurodegenerative diseases.

Question 33

A*:

List 4 roles of astrocytes in the control of glutamatergic activity.

Answer 33

The role of astrocytes in the control of glutamatergic activity (from Hertz et al., 2004):

1 - Astrocytes are involved in the synthesis of glutamate precursors.

2 - Astrocytes are involved in glutamate reuptake from the synapse.

3 - Astrocytes can release glutamate into the synapse.

4 - Astrocytes protect against excitotoxicity by disposing of excess glutamate (by conversion into other products such as glutamine and glutathione).

• Glutathione is a neuroprotective antioxidant that protects against oxidative stress.

Question 34

A*:

Describe the mechanisms of glutamate release from astrocytes.

Answer 34

Mechanisms of glutamate release from astrocyte (from Malarkey et al., 2007):

1 - Astrocytes can release glutamate by Ca2+-induced exocytosis.

• Ca2+ release from intracellular stores and from influx through ion channels is believed to trigger exocytosis.
• Glutamate loading into vesicles occurs in astrocytes in the same way as in neurones. For example, VGLUTs load glutamate into vesicles and vacuolar type proton pumps establish a proton gradient across the vesicle to drive glutamate loading through VGLUTs.
• Exocytosis in astrocytes is mediated by the same SNARE proteins involved in glutamate exocytosis in neurones, and astrocytes also have the same Ca2+ sensor, synaptotagmin.

2 - Astrocytes can release glutamate by exchange with cysteine in the cystine-glutamate antiporter.

• Cysteine uptake into astrocytes is necessary for the production of glutathione.
• However, there is controversy over whether there is a significant contribution to total glutamate release through these antiporters.
• Some research indicates that glutamate released through these antiporters plays a role in decreasing the excitatory postsynaptic effect of glutamate by binding to group 2 mGluRs.
• Studies have shown that this could be used as a potential target for treating cocaine withdrawal (Moran et al., 2005).

3 - Astrocytes can release glutamate through P2X7 receptors.

• This occurs in a Ca2+-independent, voltage-independent manner.
• Activation of P2X7 receptors is therefore a potential mechanism by which ATP can contribute to glutamate release, and therefore contribute to cell signalling in response to stress.

4 - Astrocytes can release glutamate through volume-regulated anion channels in response to cell swelling.

• This is thought to play a role in glutamate signalling during pathology, e.g. inflammation.
• An example of a specific damage signal that can lead to glutamate through this process is ATP, which can cause cell swelling by increasing Ca2+ influx through P2X receptors.

5 - Astrocytes can release glutamate through connexin gap junctions known as ‘hemichannels’.

• This mode of glutamate transport is thought to be particularly important in glutamate signalling between glia (rather than release into the synaptic cleft).

Question 35

A*:

What is astrocyte reactivity?

How does it affect glutamate signalling?

Give an example of a neuropsychiatric condition that involves astrocyte reactivity.

Answer 35

• Astrocyte reactivity is a response to various pathologies characterised by a series of changes to astrocyte morphology and signalling.
• Specifically astrocytes undergo proliferation and hypertrophy, and upregulate inflammatory mediators.
• Astrocyte reactivity can reduce glutamate uptake from the synapse due to downregulation of EAATs.
• Astrocyte reactivity also tends to increase glutamate release due to disturbed ion homeostasis resulting from changes in metabolism that affect the functioning of ion pumps.
• This results in overstimulation of glutamatergic pathways and contributes to excitotoxicity, which in turn impairs various cognitive functions.
• Astrocyte reactivity is thought to contribute to cognitive dysfunction in Alzheimer’s disease.

Question 36

A*:

Give an example of a glutamate-based drug treatment that targets astrocytes.

What condition can this used to treat?

Answer 36

• Beta lactam antibiotics such as ceftriaxone increase expression of EAAT-2 transporters.
• Increasing expression of glutamate transporters can protect against excitotoxicity and prevent against overstimulation of glutamatergic pathways.
• Ceftriaxone was shown to improve learning and memory in rats with cognitive dysfunction (Koomhin et al., 2012), and improved tau pathology in AD rats (Zumkehr et al., 2015).

Question 37

A*:

Describe the pathophysiology of amyotrophic lateral sclerosis / Lou Gehrig’s disease.

Give an example of a glutamate-targeting drug that is used to treat ALS.

Answer 37

• Amyotrophic lateral sclerosis involves degeneration of both upper and lower motoneurones.
• On the cellular level, it is characterised by inflammation, free radical production, dysfunction of mitochondria and continuous, excessive glutamate transmission and other excitatory amino acids.
• Excessive glutamate transmission is thought to be due to defective EAAT2 transporters.
• Excessive glutamate activity causes excitotoxicity, leading to the apoptosis.
• Riluzole is a drug used to treat ALS.
• It blocks glutamate transmission by:

1 - Blocking NMDA receptors.

2 - Influencing posttranslational modification of EAAT2 transporters, which is defective in ALS.

Question 38

A*:

What is the NMDA hypofunction hypothesis of schizophrenia?

Answer 38

• NMDA antagonists such as phencyclidine and ketamine were shown to cause degeneration of grey matter volume in the cortex in a manner similar to what is seen in schizophrenia. (Olney and Farber, 1995).
• This is thought to be because the NMDA antagonists in the cortex reduced activity of GABAergic thalamic interneurones that are innervated by the cortical glutamatergic neurones.
• The thalamic GABAergic interneurones innervate thalamic glutamatergic neurones, which project their axons back into the cortex.
• Disinhibition of these thalamic glutamatergic neurones by reduced thalamic GABAergic activity results in excessive glutamatergic transmission in the cortex.
• It is thought that the degeneration of cortical grey matter in schizophrenia is caused by excitotoxicity mediated by excessive glutamate release. This, in turn is due to hypofunction of NMDA receptors expressed on thalamic GABAergic neurones.
• This is known as the NMDA hypofunction hypothesis of schizophrenia.

Question 39

A*:

What is memantine?

Where is its binding site on the NMDA receptor?

Describe the mechanism of action of memantine for the treatment of Alzheimer’s disease.

Answer 39

• Memantine is a non-competitive voltage-dependent blocker of the NMDA receptor that binds to the open ion channel (hence it is voltage-dependent).
• It is thought to bind to the same site on the NMDA receptor as Mg2+, since administration of Mg2+ decreases the potency of memantine.
• Other drugs that block NMDA receptors usually have deleterious effects on Alzheimer’s disease, since Alzheimer’s disease is characterised by hypoactivity of glutamatergic pathways.
• The mechanism of action for memantine is thought to involve blockade of pathological NMDA hyperactivity but not normal physiological NMDA activity.
• The exact mechanism is unclear, but this likely relates to its competition with Mg2+, since memantine and Mg2+ share a common binding site. This is not true for other similar drugs like ketamine and phencyclidine.
• Also, memantine has neuroprotective functions:

1 - Memantine inhibits amyloid beta accumulation (Rogowski and Wenk, 2003).

2 - Memantine protects against loss of inhibitory cholinergic interneurones that innervate excitatory glutamatergic neurones. Preventing disinhibition means preventing excitotoxicity (Johnson and Kotermanski, 2006).

Question 40

A*:

Describe 6 emerging glutamatergic drug treatments for schizophrenia.

Answer 40

Emerging glutamatergic drug treatments for schizophrenia (mention why potentiating glutamate transmission is useful - glutamate hypothesis):

1 - D-amino acids.

• As described above, D-serine, although not yet approved for the treatment of schizophrenia, is thought to provide some efficacy for the cognitive symptoms of schizophrenia as a supplement to non-clozapine neuroleptics. It is generally well tolerated, however some studies indicate that byproducts of D-serine metabolism may cause nephrotoxicity (Maekawa et al., 2005). Also, D-cycloserine can be neurotoxic in high doses, limiting potential for treatment requiring high dosages. Sarcosine is associated with respiratory complications at high doses, and may also cause motor symptoms, again limiting options for treatment. The safety of glycine requires further investigation, but is currently thought to cause minimal / no side effects.

2 - GLYT1 inhibitors (e.g. sarcosine, as described above).

GLYT1 is a reuptake transporter for glycine that transports glycine from the synapse into pre-and postsynaptic neurones, as well as glia. It facilitates the termination of glycine transmission at both NMDA receptors (where glycine is a coagonist with glutamate) and glycine receptors. Therefore, inhibition of GLYT1 postpones the termination of glycine at its receptors, thereby potentiating glutamate transmission. As described in A* card 24, sarcosine shows no / few side effects and good safety profiles. This is true for most GLYT1 inhibitors, although some have been shown to cause dizziness, requiring reduction of the administered dosage.

3 - EAAT3 inhibitors.

• EAAT3 is a glutamate transporter that transports glutamate from the synapse primarily into neurones. Analogous to GLYT transporters for glycine, EAAT transporters terminate glutamate transmission at glutamate receptors. Hence, inhibition of EAAT increases synaptic glutamate, potentiation transmission at all types of glutamate receptor. Of particular interest in schizophrenia is the potentiation of NMDA receptors.

4 - mGlu2 and 3 agonists.

• Mixed mGlu2/3 (group II metabotropic) agonists are thought to potentiate glutamate transmission at NMDA receptors containing the NR2B subunit. This effect has been shown to be mediated by suppressing GSK3 signalling, a signalling pathway which is known to be negatively associated with neurogenesis, neuroplasticity and neuroprotection (see depression lecture for details on signalling pathway). These drugs are also associated with restoration of physiological neurone morphology, such as forming dendritic spines, which are lost in some glutamatergic neurones in schizophrenia due to a defective pruning process. One prominent mGlu2/3 agonist, LY2140023, significantly improved both positive and negative symptoms of schizophrenia compared to a placebo in a phase II trial. However, the drug did not meet its primary efficacy endpoint, was associated with weight gain and convulsions and showed no improvement in extrapyramidal symptoms, and hence did not meet phase III trials. Today, the focus on targeting mGluRs has changed from orthosteric agonists to PAMs, as these only exert an effect when present with an endogenous agonist (and therefore show better safety profiles), generally show greater specificity for the receptor compared to orthosteric ligands (and therefore show fewer side effects) and have reduced potential for desensitisation. *

Question 41

A*:

List 4 emerging glutamatergic treatments for Parkinson’s disease.

Answer 41

Emerging glutamatergic treatments for Parkinson’s disease:

1 - D-serine has been shown to significantly improve both motor and nonmotor symptoms of Parkinson’s as an adjuvant therapy, and is currently in stage 4 clinical trials.

• This may be explained by the finding that animal models of Parkinson’s disease show reduced D-serine in the substantia nigra. and similar findings have been made in the cerebrospinal fluid of Parkinson’s disease patients.
• These findings may open avenues for other NMDA-based treatments, such as glycine, the GLYT-1 inhibitor, sarcosine and the partial agonist, D-cycloserine.

2 - NR2B antagonists.

• Dysfunction of the nigrostriatal neurones in Parkinson’s is thought to increase glutamatergic transmission through NMDA receptors with NR2B subunits. Restoration of physiological levels of glutamatergic transmission at these receptors by way of NR2B antagonists initially showed efficacy for treating symptoms of Parkinson’s disease in animal models, but showed limited efficacy for the treatment of Parkinson’s disease in clinical trials. NR2B antagonists did significantly improve treatment-induced dyskinesia, however since these drugs have a tendency to cause amnesia and dissociation, these drugs are unlikely to be pursued.

3 - AMPA PAMs.

Potentiation of glutamatergic signalling at AMPA receptors has been shown to drive synthesis of BDNF. Hence, it has been hypothesised that administration of AMPA positive allosteric modulators (PAMs) can enhance proliferation and cause neuroprotection in the substantia nigra. This has been evidenced by studies which have shown that the neuroprotective effect of AMPA PAMs can be reversed by administration of a BDNF inhibitor. In spite of the excitotoxic effect produced by other potentiators of ionotropic glutamate receptors, AMPA PAMs have been shown to protect against excitotoxicity - a mechanism thought to play a major role in the loss of nigral cells in Parkinson’s. One group demonstrated that a class of AMPA PAMs, biarylpropylsulfonamides, exerted a neuroprotective effect in nigral cells derived from rodents which were infused with 6-OHDA - a substance used experimentally to cause neurodegeneration. Due to their modulatory action at allosteric sites, AMPA PAMs are also less likely than orthosteric agonists to cause unwanted side effects. Despite the potential clinical benefit of AMPA PAMs for Parkinson’s, most research effort into these drugs has been focussed on Alzheimer’s disease and schizophrenia.

4 - mGlu4 positive allosteric modulators (PAMs) have shown efficacy in recent years for treating the motor symptoms of Parkinson’s disease. The drugs have potential to reduce the required dosage of L-DOPA, thereby protecting against L-DOPA-induced dyskinesia. Despite success in preclinical models, only one mGlu4 PAM, foliglurax, has entered clinical trials due to its favourable pharmacokinetic properties. Research into foligurax was discontinued as it did not meet efficacy endpoints. The therapeutic potential behind targeting mGlu4 receptors lies in their ability to reduce neurotransmission in the indirect pathway of the basal ganglia. This pathway is inhibitory to movement, therefore inhibition of the pathway by potentiation of mGlu4 transmission has potential for treating the hypokinetic (poverty of involuntary motor activity) and bradykinetic (slow voluntary motor activity) motor symptoms of Parkinson’s disease. Generally, mGlu4 PAMs show high specificity for the mGlu4 receptor, therefore the drugs have been well-tolerated in preclinical models and in the foligurax trial. Furthermore, mGlu4 PAMs also show a good safety profile. All other previously developed mGlu4 PAMs have been shown to produce either anxiolytic or antidepressant effects in preclinical models using the elevated plus maze and forced swim tests. Hence, the development of mGlu4 PAMs with more desirable pharmacokinetic properties is warranted for the treatment of Parkinson’s disease, anxiety and depression.