Schizophrenia (A*)

Question 1

Define schizophrenia.

What is the normal age of onset?

Describe the inheritance pattern of schizophrenia.

Answer 1

• Schizophrenia is a severe psychiatric disorder characterised by distortion of thoughts, perception and mood.
• Onset is usually in adolescence or early adulthood.
• Many genes are involved in schizophrenia, so the inheritance is non-mendelian. The hereditary component is strong.

Question 2

Describe the symptoms of schizophrenia.

Answer 2

Symptoms of schizophrenia are classified as either positive (type 1) symptoms or negative (type 2) symptoms.

• Positive symptoms involve the presence of abnormal thoughts and behaviours:

1 - Delusions.

2 - Hallucinations.

3 - Inserted thoughts.

4 - Disorganised speech.

5 - Motor symptoms.

• Negative symptoms involve the absence of normal thoughts and behaviours:

1 - Reduced expression of emotion.

2 - Social withdrawal.

3 - Poverty of speech (absence of useful information in speech).

Question 3

What is the dopamine hypothesis of schizophrenia?

Answer 3

The dopamine hypothesis of schizophrenia states that dopaminergic hyperactivity is primary dysfunction in schizophrenia.

Question 4

What is treatment-resistant schizophrenia?

Answer 4

Treatment-resistant schizophrenia is a form of schizophrenia that is not effectively treated by at least two trials of antipsychotic drugs.

See A card 24 for details on clozapine treatment for treatment-resistant schizophrenia.

Question 5

What type of drugs are most antischizophrenic drugs?

Answer 5

Most antischizophrenic drugs are D2 receptor antagonists.

Question 6

List 3 pieces of evidence for dopamine hyperactivity in patients with schizophrenia.

Answer 6

Evidence for dopamine hyperactivity in patients with schizophrenia include:

1 - Drug-naive schizophrenia patients show increased dopamine synthesis.

2 - Basal and amphetamine-induced dopamine release is elevated.

• Amphetamine induces type I symptoms.

3 - Dopamine antagonism significantly improves symptoms of schizophrenia.

• There is a correlation with acute episodes and responsiveness to drug treatment.

Question 7

List 2 problems with the dopamine hypothesis of schizophrenia.

Answer 7

Problems with the dopamine hypothesis of schizophrenia include:

1 - Dopaminergic neuroleptics take weeks to work.

2 - Dopaminergic neuroleptics, with the exception of clozapine, only reduce positive symptoms.

Question 8

List 2 hypotheses for the aetiology of schizophrenia other than the dopamine hypothesis.

Answer 8

Other theories for the aetiology of schizophrenia include:

1 - Glutamate hypothesis.

2 - Neurodevelopmental hypothesis.

3 - Dopamine imbalance hypothesis.

*2 and 3 are sometimes considered as one single model.

Question 9

What is the glutamate hypothesis of schizophrenia?

Answer 9

The glutamate hypothesis states that hypoactivity of glutamatergic transmission is the primary dysfunction in schizophrenia.

Question 10

List 2 pieces of evidence for the glutamate hypothesis of schizophrenia.

Answer 10

Evidence of the glutamate hypothesis:

1 - PCP is a noncompetitive NMDA channel blocker which induces both type I and type II symptoms of schizophrenia (better evidence than amphetamines for the dopamine hypothesis because amphetamines only induce only type I symptoms).

• PCP also intensifies psychosis (hallucinations or delusions) in schizophrenic patients.
• Ketamine is another example of an NMDA antagonist that produces a similar effect.

2 - NMDA knockdown mice expressing 5-10% of the wild type NMDA receptors show increased type 2 symptoms.

• Social interaction is decreased.
• Mating frequency is decreased.
• Escape behaviour is decreased.
• These symptoms can be reversed by antipsychotic drugs.

Question 11

Describe the interaction between glutamatergic and dopaminergic neurones according to the glutamate hypothesis.

How does this contribute to positive and negative symptoms of schizophrenia?

Answer 11

Interaction between glutamatergic and dopaminergic neurones according to the glutamate hypothesis:

• Principal glutamatergic neurones in the cortex are underactive in schizophrenia and have secondary effects on dopamine transmission.
• Principal neurones make both horizontal connections in the cortex with other principal neurones (hypoactivity of these connections contributes to the cognitive deficits) and vertical connections downwards into subcortical areas (hypoactivity of these connections underlies psychosis).
• The principal neurones make two connections in subcortical areas:

1 - Direct innervation of the mesocortical pathway.

• Since the glutamatergic innervation is hypoactive, the dopamine activity in the mesocortical pathway decreases (opposes the dopamine hypothesis which points towards dopamine hyperactivity - see card 20 for more details).

2 - Indirect innervation of the mesolimbic pathway via inhibitory GABAergic interneurones.

• Since the glutamatergic innervation is hypoactive, the GABAergic interneurone activity between the principal neurones and the mesolimbic pathway is decreased. This decrease in GABAergic inhibition means the dopamine activity in the mesolimbic pathway increases.
• There is therefore an imbalance in dopaminergic activity; the mesocortical pathway is hypoactive, driving negative symptoms, and the mesolimbic pathway is hyperactive, driving positive symptoms.

Question 12

Describe the sensorimotor gating deficit in schizophrenia.

What is the mechanism that underlies this deficit in sensorimotor gating?

How does this contribute to the positive and negative symptoms of schizophrenia?

Answer 12

• Schizophrenics have difficulties with filtering innocuous sensory input (example: filtering out the ticking of a clock).
• Schizophrenics are therefore said to have fragmented thinking and sensory overload.
• There are also deficits prepulse inhibition, which is the idea that a weak ‘prepulse’ stimulus is able to inhibit a later strong stimulus (e.g. a startle response produced by a loud noise is usually dampened the second time the loud noise occurs).

Mechanism:

• All sensory information enters the brain via the thalamus.
• The thalamus makes excitatory connections with the cortex which allows for the processing of behavioural responses to the sensory information.
• In order to filter out unimportant information, the cortex relays information to the thalamus through the cortico-striato-thalamic loop.
• Both direct and indirect pathways of the basal ganglia receive excitatory cortical glutamatergic input (hypoactive in schizophrenia) and inhibitory dopaminergic input (hyperactive in schizophrenia).
• The thalamus receives input from the direct and indirect pathways of the basal ganglia in the striatum (not just involved in movement!), completing the cortico-striato-thalamic loop. The direct pathway decreases sensory gating (opens the filter) whereas the indirect pathway increases sensory gating (closes the filter).
• Hypoactive excitatory glutamatergic transmission at the direct pathway leads to a decrease in excitatory stimulation of the thalamus, which in turn results in an increase in gating (closing of filter), causing negative symptoms.
• Hypoactive glutamatergic transmission at the indirect pathway leads to a decrease in inhibitory stimulation of the thalamus, which in turn results in a decrease in gating (opening of filter), causing positive symptoms. This is confounded by hyperactive dopaminergic transmission which, in the indirect pathway, exerts an inhibitory effect.

Question 13

List 3 novel drug approaches for schizophrenia.

Answer 13

Novel drug approaches for schizophrenia:

1 - NMDA receptor potentiation.

2 - AMPA PAMs (AKA AMPAkines - prolong AMPA receptor opening time).

3 - mGluR agonists.

See A* cards 24 and 26 for details on these - and more!

Question 14

List 2 types of antischizophrenic drugs that potentiate NMDA receptor activity.

Answer 14

Antischizophrenic drugs that potentiate NMDA receptor activity:

1 - Glycine agonists, such as the prodrug D-serine.

2 - Glycine uptake inhibitors, specifically those targeting GLYT1 such as bitopertin (see A* card 24 and 26 for details).

Question 15

List 3 pieces of evidence that indicate a neurodevelopmental problem in schizophrenia.

Answer 15

Evidence that indicates a neurodevelopmental problem in schizophrenia includes:

1 - There are structural brain abnormalities in schizophrenic patients that can be seen using imaging.

2 - There is a correlation with pre-and post-natal problems.

3 - There is a classical onset in late adolescence / early adulthood.

Question 16

List the neurodevelopmental deficits in schizophrenia.

Answer 16

Neurodevelopmental deficits in schizophrenia:

1 - Frontal and temporal lobe dysfunction.

• Causes impairment of cognitive tasks requiring attention, memory and executive functions.
• Causes a decrease in IQ.
• These symptoms persist despite improvement of type 1 and type 2 symptoms with antischizophrenic drugs.

2 - Structural changes in the brain.

• Enlarged ventricles that are indicative of tissue loss.
• Reduced tissue mass in the temporal lobe and prefrontal cortex (limbic / limbic-association areas).

3 - Abnormal distribution of glial cells in cortical areas.

• Normally, glia are lined up in a particular pattern in the cortex.
• Abnormal distribution is perhaps due to defective migration.
• Might result in defective neurone guidance.

4 - Low glucose utilisation.

5 - Reduced cerebral blood flow.

Question 17

List 5 causes of neurodevelopmental deficits in schizophrenia.

Answer 17

Causes of neurodevelopmental deficits in schizophrenia include:

1 - Maternal immune activation (see A* card 22).

2 - Severe malnutrition in utero.

3 - Alcohol exposure in utero.

4 - Obstetrical complications.

5 - Genetic predisposition.

Question 18

Why is the onset of schizophrenia in late adolescence / early adulthood?

Answer 18

• The onset of schizophrenia is in late adolescence / early adulthood because brain development does not complete until after puberty, especially the frontal cortex.
• Type 1 symptoms only manifest after this development finishes. Type 2 symptoms do occur during the course of development, but are less noticeable and generally don’t lead to diagnosis.

Question 19

In which region of the brain is the primary defect in schizophrenia?

Answer 19

The primary defect in schizophrenia is in the frontal lobe.

Question 20

What is deficit syndrome?

What causes deficit syndrome?

How does this influence our understanding of the pathophysiology of schizophrenia?

Answer 20

• Deficit syndrome is a behavioural disorder that has symptoms reflecting the negative symptoms of schizophrenia.
• Deficit syndrome arises from damage to the dorsolateral prefrontal cortex, a relatively common occurrence in RTAs.
• Experimentally, this has also been induced in animals by damaging the mesocortical dopamine pathway, which has connections to the frontal lobe.
• This implies that dopaminergic hypofunction (not hyperfunction as the dopamine hypothesis suggests) has a role in the negative symptoms in schizophrenia (see card 11 for details).

Question 21

Describe the relationship between dopamine activity in the mesocortical pathway and the mesolimbic pathway.

What is this hypothesis known as?

Give an example of a novel drug approach that makes use of this hypothesis.

What is the advantage of this drug approach?

Answer 21

• The mesocortical pathway is thought to exert negative feedback on the mesolimbic pathway.
• Negative feedback from the mesocortical pathway results in a decrease in dopamine turnover and D2 receptor expression in the mesolimbic pathway.
• Hypofunction of the mesocortical pathway in schizophrenia (due to glutamate hypoactivity) therefore results in a loss in negative feedback to the mesolimbic pathway.
• The mesolimbic pathway (already hyperactive due to glutamate hyperactivity) therefore becomes even more hyperactive in schizophrenia.
• This is the dopamine imbalance hypothesis, which explains both positive and negative symptoms of schizophrenia.
• The message here is that both glutamate hypoactivity and mesocortical hypoactivity (which itself is caused by glutamate hypoactivity) contribute the mesolimbic hyperactivity.
• Dopamine stabilizers such as aripiprazole (Abilify) are D2 partial agonists.
• In the hypoactive mesocortical pathway, the partial agonist activity results in an overall receptor stimulation, acting as a functional agonist.
• In the hyperactive mesolimbic pathway, the partial agonist activity results in an overall receptor inhibition, acting as a functional antagonist.
• Partial agonists like these are better for side effects because they avoid complete blockade of other dopaminergic pathways.

Question 22

A*:

Describe the role of maternal immune activation in the development of schizophrenia.

Answer 22

A*:

• Specifically, maternal exposure to toxoplasmosis, influenza and rubella are associated with greater foetal risk of schizophrenia.
• Maternal immune activation upregulates immunological factors such as interleukins, IFN-gamma, TNF-alpha, GMCF and CRP, which predispose the foetus to schizophrenia by disturbing the development of normal brain connectivity.
• Foetal exposure to IL-6, for example, is associated with changes in both transient and stable gene expression such as crystallins, sc4mol, aldh1a1 and atoh7, that are linked to schizophrenia (Garbett et al., 2012). This response serves a protective function against environmental stressors, but sacrifices normal neuronal development.
• Some of these proteins, notably crystallins, continue to exert a protective effect after the neurodevelopmental disability has occurred. For example, in Alzheimer’s mice, knockout of the crystallin gene worsens symptoms of Alzheimer’s disease, and this effect can be reversed by administration of crystallin.
• The therapeutic benefits of crystallin has also been demonstrated in encephalomyelitis and multiple sclerosis in preclinical studies. Since mutations to the crystallin gene Crybb2 are also associated with schizophrenia (Heermann et al., 2019), investigations into the therapeutic potential of crystallin as a symptomatic treatment for Alzheimer’s disease is warranted.

Question 23

A* (from A* glutamate cards):

What is the NMDA hypofunction hypothesis of schizophrenia?

Answer 23

• NMDA antagonists such as PCP 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 24

A*:

Describe the mechanism of action of clozapine for the treatment of schizophrenia.

What is the prevalence clozapine-resistant schizophrenia?

List 4 drugs developed to enhance the effects of clozapine in non-clozapine-resistant schizophrenia

Give an example of a novel treatment for clozapine-resistant schizophrenia.

Answer 24

• Clozapine is an atypical / second generation neuroleptic that is considered a multi-acting receptor-targeted antipsychotic (MARTA). It is the gold standard drug for treatment-resistant schizophrenia.
• Second generation neuroleptics have the advantage of achieving greater therapeutic effect (especially improved treatment of negative symptoms compared to first generation neuroleptics) whilst causing fewer extrapyramidal motor effects such tardive dyskinesia (which is caused by long-term exposure to various neuroleptics).
• Clozapine is an antagonist at numerous receptors, including 5-HT2A, 5-HT2C and D2 receptors. Its mechanism of action is unclear, and various hypotheses have been proposed since its discovery in 1958.
• Dziedzicka-Wasylewska et al. (2008) proposed that clozapine mediates a neuroleptic effect through dimerisation of striatal D1 and D2 receptors. However, this hypothesis relies on the notion that D1 and D2 receptors are coexpressed in striatal medium spiny neurones (MSNs). This has attracted some controversy in the past decades, as others have found only limited levels of D1/D2R coexpression, with estimations that 95% of striatal medium spiny neurones show no evidence of coexpression (Betrán-González et al., 2008). If dimerisation of D1/D2Rs were to contribute significantly to the therapeutic mechanism of clozapine, one would expect greater levels of D1/D2R coexpression.
• It has been estimated that 1/3 of schizophrenia patients are resistant to clozapine, however the mechanisms underlying clozapine resistance are unclear. If the hypothesis proposed by Dziedzicka-Wasylewska et al. holds water, one could speculate that clozapine resistance is a product of low striatal D1/D2R coexpression, as this would reduce the number of receptors able to undergo dimerisation. Elucidation of the mechanism of action of clozapine will help to explain clozapine resistance, which, in turn, will facilitate the development of more robust therapies for treatment-resistant schizophrenia.
• DAAO is an enzyme that metabolises D-amino acids such as D-serine.
• The DAAO inhibitor, sodium benzoate, is an emerging adjuvant treatment for clozapine-resistant schizophrenia. Although the exact mechanisms are unclear, sodium benzoate is thought to produce a clinical effect by increasing the availability of NMDA coagonists such as D-serine (by reducing DAAO-mediated D-serine metabolism), thereby increasing NMDA activation. Sodium benzoate is also thought to exert a therapeutic effect through an antioxidant action. A combination of sodium benzoate and clozapine has been shown to significantly improve symptoms of clozapine-resistant schizophrenia compared to clozapine alone, and is generally well-tolerated, making it an ideal candidate for drug treatment.
• Besides treating clozapine-resistant schizophrenia, numerous treatments have been / are being developed to enhance clozapine treatment in (typical) schizophrenia. Examples include:

1 - D-serine (an agonist of the glycine binding site on NMDA receptors).

2 - D-cycloserine (a partial agonist for the glycine site on NMDA receptors).

3 - Glycine (an agonist of the glycine binding site on NMDA receptors).

4 - Sarcosine (a GLYT1 inhibitor and NMDA coagnoist - see card 26 for more details).

• These drugs are coagonists for NMDA receptors that potentiate NMDA activation when coreleased with glutamate. As well as acting as a coagonist at NMDA receptors, sarcosine also increases synaptic glycine, further potentiating NMDA transmission. Although the exact mechanisms are unknown, they are thought to improve symptoms of schizophrenia by increasing glutamate transmission, which is reduced in schizophrenia according to the glutamate hypothesis. Although showing no / few side effects and good safety profiles (although see A* card 26 for more details safety), these drugs have seen mixed results for the treatment of schizophrenia as adjuvant therapies to clozapine, and have failed to meet primary efficacy endpoints in the majority of clinical trials, sometimes worsening symptoms. These drugs show greater promise as adjuvant therapies for non-clozapine neuroleptics or alone.

Question 25

A*:

How long is the delay between administration of a neuroleptic and clinical effect?

What might cause this effect?

Answer 25

• Despite causing relatively rapid changes to central dopamine transmission, neuroleptic drugs show a therapeutic latency of approximately 2-3 weeks. This challenges the classical idea of the dopamine theory, which posits that dopamine hyperactivity is the primary dysfunction in schizophrenia.
• This suggests that the therapeutic action of neuroleptic drugs involves some form of neuroplastic change, as these processes are usually relatively slow and occur over a similar timescale to that of the therapeutic latency of neuroleptic drugs.
• It has been shown that haloperidol, a commonly used D2 antagonist neuroleptic drug, induces changes to neuronal physiology and connectivity in the striatal indirect pathway (Sebel et al., 2016). This led to differential degeneration and reinforcement of corticostriatal networks following 2 weeks of drug exposure in mice, a process which has been suggested to be governed by L-type Ca2+ channels. This is not unlikely considering polymorphisms of L-type Ca2+ channels are associated with schizophrenia.
• Elucidation of the mechanisms underpinning this remodelling process will help to explain the mechanism of action of D2 antagonists - the primary mode of treatment for schizophrenia. A clear understanding of these mechanisms will, in turn, facilitate the development of novel therapeutic targets.

Question 26

A*:

Describe 6 emerging glutamatergic drug treatments for schizophrenia.

Answer 26

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 27

From NO and purinergic signalling A* cards:

Describe a potential purinergic target for schizophrenia.

How can this target be used to treat schizophrenia?

Answer 27

• Purinergic hypothesis of schizophrenia: dysfunction in purinergic signalling results in a decrease in adenosine transmission which underlies the imbalance between glutamatergic and dopaminergic transmission (Lara and Souza, 2000).
• The hypothesis posits that dysregulation of glutamatergic signalling arises from dysfunctional A2A receptor activity, which disrupts glutamate homeostasis through disturbed astrocyte-neurone signalling.
• In mice models of schizophrenia, antagonism of neuronal A2A receptors is beneficial for cognitive function but antagonism of astrocytic A2A receptors is detrimental to cognitive function (Matos et al., 2015).
• Purinergic miscommunication has been shown to lead to decreased presynaptic glutamate release, and downregulation of NMDA receptors.
• The purinergic hypothesis also suggests that other neurotransmitters, such as 5-HT, noradrenaline, dopamine and acetylcholine become dysregulated due to defective purinergic signalling. Collectively, these physiological changes are thought to underlie both positive, negative and motor symptoms of schizophrenia.
• Over the past decade, evidence has accumulated to suggest that stimulation of A2A receptors has potential to improve symptoms of schizophrenia. It is well-documented that A2A receptors form functional dimers with D2 receptors in the indirect striatal pathway of the basal ganglia. Here, A2A receptor activation antagonises D2 receptor function. Since schizophrenia is associated with dopaminergic hyperactivity, the therapeutic action of A2A stimulation in schizophrenia may be through attenuation of striatal dopaminergic transmission.
• Stimulation of A2A receptors would, in theory, be particularly useful for treating positive symptoms of schizophrenia. That is, the presence of abnormal thoughts and behaviours such as delusions and hallucinations. In schizophrenia, hyperactivity of dopaminergic transmission at the striatal indirect pathway leads to a disinhibition of thalamus, which receives inhibitory innervation from the basal ganglia in what is known as the cortico-basal ganglia-thalamo-cortical loop. This, in turn, results in a decrease in thalamic sensory gating (opening of filter), causing positive symptoms. Indeed, the efficacy of A2A receptor stimulation has been evidenced in clinical trials:
• Dipyridamole is an adenosine uptake inhibitor. Its primary function is as a vasodilator and anti-platelet drug, however is has also shown efficacy for the treatment of positive symptoms of schizophrenia as an adjuvant therapy (Wonodi et al., 2011). Although showing a good safety profile, dipyridamole is contraindicated in patients suffering from respiratory diseases since it is associated with increased risk of bronchospasm. Furthermore, the vasodilatory effect of dipyridamole effect makes it unsuitable for use in patients suffering from hypotension, and in geriatric patients, dipyridamole use is associated with increased risk of orthostatic hypotension. The ubiquity of A2A receptors makes diverse side effects an inevitable consequence of orthosteric agonism, hence the development of positive allosteric modulators (PAMs) of A2A receptors with high CNS penetrability will likely result in a preferable side effect profile. Various A2A PAMs have recently been discovered, however much of the research focus for these drugs has been directed towards inflammatory conditions.

Question 28

From neuropeptides A* cards (the first point is relevant to schizophrenia):

A*:

List 3 potential therapeutic roles of NAAG-targeting drugs.

Answer 28

1 - NAAG peptidase inhibitor ZJ43 reduces PCP-induced motor symptoms of schizophrenia.

• NAAG is a partial agonist at NMDA receptors, meaning in low concentrations, NAAG has an antagonistic effect, and in high concentrations has an agonistic effect. In schizophrenia, it has been hypothesised that decreased expression of NAAG underlies the hypoglutamatergic state. This has been evidenced by functional imaging studies comparing NAA and NAAG expression in schizophrenia patients and healthy individuals. Furthermore, a negative relationship exists between NAAG expression and positive and negative symptoms scale (PANSS) (Jessen et al., 2013). Hence, ZJ43 is thought to improve symptoms of schizophrenia by reducing NAAG metabolism, which in turn alleviates NAAG-mediated inhibition of glutamatergic transmission at NMDA receptors. Also see below for general neuroprotective functions of NAAG.

2 - NAAG peptidase inhibitor 2-PMPA protects against neuronal apoptosis and neurite degeneration in diabetic neuropathy.

• The mechanisms underlying the therapeutic effect of NAAG in diabetic neuropathy are unclear, however cell culture models of diabetic neuropathy have shown that mGluR activation leads to an increase in synthesis of NAAG, which in turn inhibits proapoptotic caspase enzymes. This could underlie the neuroprotective effect (and also see point 3 below).

3 - NAAG peptidase inhibitor 2-PMPA and GCPII inhibitors protect against motoneurone death in amyotrophic lateral sclerosis, a neurodegenerative disease characterised by motor deficits arising from degeneration of upper and lower motoneurones.

• 2-PMPA: Owing to NAAG’s neuroprotective effect, NAAG peptidase inhibitors have been proposed to have therapeutic potential for the treatment of neurodegenerative diseases such as ALS. NAAG is a potent agonist at mGluR3s. Activation of mGluR3s is known to reduce glutamate release through action at inhibitory mGluR3s, preventing excitotoxicity and stimulating release of growth factors such as TGF-beta that prevent neuronal apoptosis (and also see point 2 above).
• GCPII inhibitors: This has a dual mechanism. Firstly, inhibition of GCPII leads to increased concentrations of NAAG, which as described above, exerts a neuroprotective effect. Secondly, NAAG is metabolised into NAA and glutamate. By inhibiting NAAG metabolism, glutamate concentrations decrease, protecting against excitotoxicity.