Schizophrenia clinical + neurobio

What do we mean when we say that schizophrenia is a heterogenous brain disease?

It’s characterized by deficits in information integration across MULTIPLE cognitive domains (perception, language, emotion, and behavior)

What are the positive + negative symptoms of schizophrenia?

positive (basically, added behaviors, thoughts, or perceptions that are NOT normally there - ex. psychosis)

• hallucinations (sensory experience in the ABSENCE of external stimulus

• delusions (false belief that is not shared by the culture - it is personal)

• thought disorder (vague, rambling, disorganized)

• disordered behavior - inconsistent, purposelessness

  • the purposelessness is LINKED to dysfunction in the reward and motivation symptom (mesolimbic pathway —> VTA to nucleus accumbens) because behavior is no longer goal-directed

negative (deficits - missing or reduced abilities that normally ARE there):

• energy

• emotion (flat affect/no facial expressions, lack of speaking/alogia, avolition/lack of emotion)

• pleasure

  • the negative symptoms look a lot like depression

cognitive

• memory deficits

• attention (for both selective attention and sustained attention), executive control, perceptual issues

  • selective attention: ability to focus on one specific stimulus or task (tuning out everything else) —> linked to acetylcholine (allowing us to focus on what’s relevant and ignore the other stuff)

  • sustained attention: vigilance/maintaining focus on a task or stimulus over long periods of time —> linked to norepinephrine (which can plateau obviously)

You need to have at least 2 of the following (delusions, hallucinations, disorganized/incoherent speech, catatonia, and negative symptoms) to meet the diagnostic critera for schizophrenia

• you HAVE to have either delusions, hallucinations, or incoherent speech though

• symptoms have to be continuous for at least 6 months

how would schizophrenia compare to other disorders, like bipolar, on multiple psychopathological dimensions?

schizophrenia has less characteristics of mania or depression (even though the negative symptoms can resemble depression, it’s not the primary feature of the disorder) in favor of cognitive impairment + psychosis

bipolar disorder involves a similar degree of psychosis, but it involves a LOT more mania and relatively more depressive characteristics

What is the clinical course of schizophrenia?

prodromal (early adolescence) - usually the disease is not identified for what it is during this time, but the signs make much more sense in hindsight

• we see subtle changes (ex. acting odd, social withdrawal, strange affect/beliefs, paranoid statements) but nothing TOO drastic

acute index episode (around the time of the first hospitalization)

• happens between 18-25 (late adolescence/early adulthood)

• this is where we see a clear break of reality and the onset of symptoms

relapsing, remitting course

• symptoms plateau around 5 years after the diagnosis

• medications can improve both the short-term/acute (ex. during psychotic episode) AND long-term outcome

Why is there a value in identifying populations who are at risk of developing schizophrenia early on?

• people can go from normal to prodromal and then go back to normal (called remission of prodromal symptoms), remain in prodromal (maintenance of subthreshold symptoms), or progress to psychosis

• if we learn what keeps people in the prodromal/subthreshold zone, or causes remission/going back to normal, maybe we can develop gene therapies, drugs, or other treatments to “artificially” achieve that same effect

What is derailment in schizophrenia (related to the disordered thoughts aspect)? What else do we see in relation to disordered thoughts in schizophrenia

we see a loose association between various ideas - as in, they aren’t completely random - so it’s basically derailment

• we also see grandiosity (false self-importance/delusions of grandeur) and perseveration (repeating the same thing over and over), and neologisms (newly-coined, made up words) in addition to derailment/loose associations between things

• the derailment of thoughts represents the lack of goal-directed behavior (dysfunction of mesolimbic/VTA-NAcc and mesocortical pathways)

What are some cognitive findings that hint at a much more fundamental deficit?

we see deficits in eye movements (with eye tracking of a swinging pendulum) and decreased discrimination of odors

SENSORY PROCESSING DEFICITS:

pre-pulse inhibition fails:

• this is when we have one small stimulus (beep) + one big stimulus (loud clap), where the small stimulus coming first is meant to warn you/dampen the response to the second stimulus. t

  • the brain has already been primed for the second stimulus and the startle response should be diminished

• if you still overreact, that points to a deficit in sensory info processing

diminishing of P300 wave:

• the P300 wave isn’t specific to any specific stimulus - it rises when we detect a specific stimulus out of background noise, as in a disturbance to the background

• the amplitude of the wave is REDUCED in schizophrenia patients compared to normal patients, which shows a deficit in SELECTIVE ATTENTION

What effect do antipsychotics have on pre-pulse inhibition in mice?

Increase pre-pulse inhibition (so we end up seeing the expected/NORMAL diminished response to the second stimulus when the brain has been primed with the first stimulus)

Is schizophrenia a developmental disorder?

• strong familial predisposition

• shares a susceptibility gene with a developmental syndrome with a high rate of psychosis

  • susceptibility is different from CAUSALITY though. There is no causal mutation like alpha synuclein for huntington’s\

• we also see alterations in developmentally-relevant gene transcripts and proteins

• also, heritability is high (we see a MUCH higher lifetime risk of schizophrenia for monozygotic twins compared to dizygotic/fraternal twins —> indicates a high contribution of genetics towards the overall phenotype

What increases schizophrenia risk?

• increased risk of schizophrenia for when the mother experiences stress, malnutrition, viruses, etc

• minor physical (ectodermal) abnormalities —> this tracks because neural crest cells are the precursor to both neurons and dermal cells, so this indicates that maybe there was some developmental issue while the child was a fetus.

  • this also makes sense because we see migrational abnormalities in multiple parts of cortex (ex. prefrontal, parietal, and temporal)

• delayed developmental milestones (ex. clumsiness)

• subtle cognitive, emotional, and motor deficits in childhood

Talk about the performance on cognitive tests (an intelligence scale) for 7 year olds who later developed schizophrenia, their unaffected siblings, and control children?

the control children showed consistent results for multiple parts of the test (ex. vocabulary, comprehension) while the siblings of the schizophrenic patients were below that (reduced performance) + showed variable levels of performance (same with the 7 year olds who later developed schizophrenia

• this suggests that we see subtle cognitive deficits before the actual disorder emerges

• siblings might share some risk genes, even if they never develop the illness

Talk about neuropathology in schizophrenia - what do we see?

• absence of focal pathology (since it impacts multiple cognitive domains, we can’t just localize to one region)

• migrational abnormalities in cortices

• absence of normal cerebral asymmetries (where both hemispheres might try tot take over each other’s functions, which leads to disorganization and inefficiency

• diffuse loss of synapses

  • increased neuronal density without neuronal loss, which points to lack of dendritic arborization

  • reductions in axon terminal density

  • decreased gene expression of proteins associated with synaptic function, like BDNF

we see much faster/enhanced GLOBAL synaptic pruning in schizophrenic patients compared to controls

What are some abnormalities we can see through structural imaging?

1) enlargement of lateral ventricles

2) global (not restricted to one region) atrophy —>

3) decreased volume specifically in:

• medial temporal lobe (amygdala, hippocampus)

• cerebellum

• basal ganglia

• thalamus

we also mainly see effects in association cortices, not primary and secondary sensory cortices

• these are the areas that integrate sensory input + are responsible for higher order thinking

• for example, if you lose the ability to distinguish between different smells (as is what occurs in schizophrenia), this is an example of losing higher-order sensory processing (as opposed to losing the ability to detect smells in general)

When patients are actively hallucinating (auditory hallucinations) and we conduct imaging on them, what do we see?

• concentrated activation in temporal (hippocampus) and parietal lobes (which is responsible for multisensory integration)

• in auditory hallucinations, the language parts of the brain are also being activated (ex. Broca’s area)

  • what’s interesting about Broca’s area is that it becomes active during speech production - but also for internal speech (internal monologue)

  • in schizophrenia, your brain just loses the ability to tell between self-generated speech and external speech, which is part of what drives auditory hallucinations/”hearing voices”

Can we model auditory hallucinations in a computer?

Yes. We can use a computer to model neural networks, with 3 layers to simulate the working memory loop involved in speech production

• input (sensory, ex. auditory info from ears)

• hidden (working/”manipulatable” memory, where information is stored or maintained)

• response generation (interpretation) —> modeling Wernicke’s

In a “healthy” model, the network receives an input (a sequence of words or phrases) and outputs something very similar (80% correct). The errors we wee are omission or substitution errors

• in a model with a lot of pruning (reduced number of synapses), we see that the model “heard” a word where none was spoken, pointing to auditory hallucination (sensory experience in the absence of an external stimulus

Is having excess synaptic connectivity a positive? We know that too little is negative, because it can lead to hallucinations (ex. auditory hallucinations)

Yes, we see a bell-shaped curve for optimal synaptic connectivity, measured by the percentage of synapses pruned.

• too many synapses —> excessive noise, conflicting signals, inefficient neural processing

  • we can’t filter relevant from irrelevant information

• too few synapses —> insufficient integration of multiple sensory inputs, poor working memory and hallucination-like errors

  • we will we see a reduction in the words detected while processing external speech (when too many synapses are pruned), but an INCREASE in hallucinated words during silence

    • we don’t see an increase in hallucinations (and a corresponding SHARP decline in the actual words that are detected) with neuronal pruning (death), but only in synaptic pruning

    • the percentage of reduction in synaptic density (amount of synapses pruned) is connected to the stages of schizophrenia (and whether someone stays sub-threshold or even goes into remission entirely)

      • more reduction in synapses = more severe schizophrenia

Does schizophrenia result from an imbalance between excitation and inhibition?

Yes, we see a reduction in inhibitory signaling from GABAergic interneurons within the cortex (reduction in their activity) —> potentially too much excitation

• this explains the symptoms of schizophrenia that arent neatly explained by the “excess of dopamine” hypothesis

What did we see in a genome-wide association study where we tried to “pair” or discover the connection between genetic markers and traits of schizophrenia? What does this have to do with different alleles of the relevant gene(s)?

We saw a big association for schizophrenia on chromosome 5, which contains C3 and C4 genes

• these are proteins that are involved in synaptic pruning/microglial disgestion of synapses (and as we know, reduction in synaptic pruning can lead to the development of hallucinations) —> they work together

• levels of C4 expression are higher in schizophrenia patients than in controls

• we have three different forms or variants of the C4 gene:

  • AL-AL is the “bad” version/polymorphism - we see a 30% risk of schizophrenia with 2 AL alleles and a 10-20% risk with one AL allele

    • knocking out C4 leads to reduced synaptic pruning

What can lead to a decrease in inhibitory interneuron activity, like what we observe in schizophrenia? What does this have to do with neuronal migration during development of a fetus?

hypoxia exposure in-utero (as we know, fetuses can be put at greater risk for schizophrenia in the womb if exposed to maternal stress or infection/illness, but this is another way)

• inhibitory/GABAergic interneurons are particularly vulnerable to a loss of oxygen

• this is because most GABAergic interneurons migrate to the cortex during development (they don’t originate in the cortex itself)

  • the cortex needs excitation to be controlled, and for information processing to occur smoothly

  • if the GABAergic interneurons don’t migrate properly or die due to hypoxia, we can see LESS inhibitory interneurons in cortex —> may be a reason behind cognitive deficits/sensory processing issues

  • even when we transplant hypoxia progenitor (ex. multipotent stem cells) into a host without hypoxia, we see that they fail to avoid the striatum

    • think of the striatum like an obstacle on the way, which hinders the GABAergic neurons. If the striatum fails to repel these neurons, they get STUCK!!!

Why do GABAergic interneurons get stuck at the striatum instead of making it to the cortex during development?

Because of lack of semaphorin-3 (a repelling factor active during development, which hypoxia inhibits)

• sempahorin-3 tells the GABA neurons not to come to striatum/get stuck there and go to cortex instead (attracted there by NRG-1 and ErB4)

  • we can also see the same issue with problems with NRG-1 itself (via polymorphisms or issues with its receptors)

What are dysbindin and DISC-1, and how do they contribute to a risk of schizophrenia?

dysbindin deficiency —> reduced glutamate release

• why is this relevant when schizophrenia leads to too much INHIBITION relative to excitation?

  • because we NEED excitatory input (from the PFC) to excite the GABAergic neurons (ex. in the lateral amygdala)

  • this allows them to do their job of inhibiting brain structures locally

  • without this we can have symptoms like paranoia, which is a big part of having schizophrenia

    • PCF —> amygdala is more relevant for stress, i am just providing an example

DISC-1 —> affects neural proliferation, migration, and differentiation which we know is affected in schizophrenia

• mutations or altered DISC-1 function leads to impaired neurodevelopment

How can we explain symptoms in terms of known risks/causes

• computer modeling

• imaging (ex. PET scans)

• post-mortem studies (via autopsies?)

we can also look at neuronal or glial function

• activity at synapses (too much synaptic pruning?)

• balance of excitation vs inhibition —> skewed towards excitation in schizophrenia

What leads to the positive symptoms of schizophrenia (besides synaptic pruning)? Which drugs can mimic these effects?

excess dopamine levels in the basal ganglia (ex. the striatum)

• coke and amphetamines can mimic these effects (lead to a psychotic episode indistinguishable from schizophrenia)

What indirectly affects dopamine levels - causing them to be excessive in schizophrenia patients?

reduced inhibition of the PFC, which is responsible for “top-down” inhibition of the limbic system (ex. amygdala, hippocampus) and the striatum

• this is because NMDA receptors on GABAergic interneurons fail (we see HYPOfunction)

• these interneurons don’t fire as much

  • the striatum is FULL of GABAergic MSNs that are excited by the prefrontal cortex (via glutaminergic input)

  • these send inhibitory input to other structures, like the substantia nigra and (indirectly) the VTA

  • without that inhibition, the substantia nigra will release too much dopamine into the striatum, which leads to overactivity of the receptors

Why is the “excess dopamine hypothesis” an oversimplification?

because we see overactivity of D2 (inhibitory) dopamine receptors in the striatum and underactivity of D1 (excitatory) receptors in prefrontal cortex

• overactivity of D2 in striatum/mesolimbic pathway—> filters out too much normal information

  • the brain finds it hard to distinguish real sensory input, like words, from irrelevant stuff/background noise —> thought to underlie positive symptoms like hallucinations

• we also see a DECREASE in goal-directed behavior because of how the striatum indirectly inhibits the VTA, which projects to the nACC and releases dopamine there

• underactivity of D1 in PFC/mesocortical pathway —> decrease in working memory, leads to cognitive deficits

  • high numbers of D1 receptors are actually associated with WORSE working memory, because this is compensating for a deficiency of dopamine in the brain

In summary, psychosis reflects the hyperactivity of D2 receptors in the striatum; cognitive deficits reflect HYPOactivity of D1 receptors in cortex

Does the affinity of drugs (such as antipsychotics) for D2/inhibitory DA receptors correlate with their potency/efficiency? What is one limitation of these drugs, and what drug can we use to address that limitation?

Yes - the more affinity they have for the receptor the more potent they are (which means we need a smaller dose)

• because they are antagonists, they will end up reducing the effects caused by D2 receptor overactivity (ex. hallucinations)

One limitation of these drugs is that they are good at treating positive (but not negative) symptoms, or cognitive symptoms (more concerned with D1 receptors and their UNDERACTIVITY - so even if these drugs had “universal affinity”/powerful generalized binding to multiple receptor subtypes, as they are antagonists, they wouldnt be likely to fix that problem

• however, D1 receptor agonists in cortex were shown to improve working memory and cognitive function when we added them to the treatment (alongside D2 receptor antagonists)

• we saw maintained activation of the prefrontal cortex with this agonist

What is stepholidine, and what are its opposing effects on D1 and D2 receptors?

Stepholidine behaves as an antagonist for D2 receptors in the basal ganglia/striatum (reducing positive schizophrenia symptoms, like hallucinations) and an agonist for D1 receptors in the prefrontal cortex (increasing their activity + improving cognitive function and working memory)

Summarize the glutamate hypothesis of schizophrenia (which suggests that glutamate might be more central to schizophrenia than dopamine). How does it end up creating a cycle?

• dysfunction of NMDAR on GABAergic neurons

• lower GABA transmission/inhibitory signals

• Overactivity of D2 receptors/too much striatal dopamine —> positive symptoms of schizophrenia (ex. hallucinations)

Lowered glutamic/glutaminergic transmission can lead to widespread effects on multiple neural circuits + can also explain what is going on beyond the positive and cognitive symptoms (the latter of which are caused by D1 receptor hypoactivity)

We end up seeing a cycle

• reduced prefrontal cortex dopamine activation (via D1 receptor underactivity)

• the PFC, which normally inhibits DA neurons in the VTA, will stop inhibiting the DA neurons as much if it is underactive

• D2 receptors in the striatum will be overactive

• D2 receptor activation will inhibit cortical glutamate neurons (which release glutamate locally - PFC is releasing glutamate within itself).

• less NMDA receptor activation

• less activation of D1 receptors in the PFC (which is mediated by NMDA receptors)

What are the ways that you can get hypoactive NMDAR activation (beyond inhibition via overactive D2 receptors in the striatum)?

• decreased spines on cortical pyramidal neurons (downstream of the cortical GABAergic neurons), which receive and send projections to other regions of the brain

• increased EAAT levels (this is a glutamate transporter that leads to reduction in extracellular/synaptic glutamate)

• less NR1 mRNA (which is a subunit of the NMDA receptor and likely assists with its function)

• alterations in scaffolding proteins, like PSD-95 (which holds NMDA receptors in their place)

What happens if we knock out NR-1, a subunit of the NMDA receptor, postnatally in mice?

The NMDAR/NR-1 knockout mice show MORE anxiety (ex. in the elevated plus maze test)

• group housed mice display a blunted effect (though it is still there) because they are more resilient than single-housed mice

We also saw decreased pre-pulse inhibition for the NR-1 knockout mice, which showed an inability to ignore irrelevant stimuli in the environment/that these mice became more overwhelmed easily

!! Remember that the NMDA receptors are located on GABAergic interneurons in cortex!

• they receive inputs from pyramidal glutaminergic neurons (also in the cortex) and release GABA

Do we see reduced GAD-67 and parvalbumin (calcium-binding protein) staining in the NR-1/NMDAR knockout mice?

Yes - which makes sense; without NMDA receptors (which bring in calcium influx), we will see less paravalbumin staining

• if we give animals a NMDA receptor antagonist (which is similar in function to a knockout), we can also see a schizophrenia-like phenotype

• for example, we see less pre-pulse inhibition, which is also seen in schizophrenic patients and NR-1 knockout mice —> but when we stop giving the antagonist (PCP), we see that the deficit leaves. So the phenotype CAN be rescued

We also see reduced GAD-67 (enzyme that converts glutamate to gaba)

• because its expression is activity-dependent. Less GABAergic neuronal activation = less need to produce GABA to meet the needs of the cortex = less GAD-67 expression = less GABA

• So we can use GAD67 as a measure of GABA neuronal functionality, which is DECREASED in schizophrenia patients

How does ketamine, an NMDA antagonist, work?

it can induce hallucinations in people (like angel dust) by:

• increasing striatal dopamine acutely (which will lead to an immediate increase in dopamine signaling + lead to more positive symptoms like hallucinations)

• increasing D1R receptors chronically - there isn’t enough dopamine to activate these receptors (since they are underactive), so it is a compensatory mechanism. More D1Rs = more indication of insufficient dopamine

Ketamine is an NMDA antagonist, meaning that it leads to reduced PV (an indication of calcium influx/the presence and activity of NMDARs) and GAD-67 expression (since less activation of the NMDARs on GABAergic neurons = less need to produce GABA via this enzyme)

• the reduction is dose-dependent (higher doses = more blocking of NMDARs)

• after 24 hours on ketamine we can see cell body shrinkage

In small doses it can work as an anti-depressant.

On the AX cognitive test (where participants had to press a button only in response to a certain stimulus), what did ketamine participants and schizophrenia patients have in common?

ketamine participants had a similar high error rate to the schizophrenia participants —> this means that we saw the same subtle cognitive deficits in both groups (in attention, working memory)

Which is more central to the development of schizophrenia: glutamate or DA?

glutamate, because knocking out or blocking NMDA receptors (via an antagonist) can lead to:

• less activation of D1 receptors in cortex, leading to upregulation of these receptors as a compensatory mechanism

• decreased GAD-67 (GABA-producing) enzyme in the brain, decreased parvalbumin (which we can use to check for the presence of Ca2+, since it’s a calcium-binding protein) in GABAergic neurons

• decreased pre-pulse inhibition (hallucinations) —> prepulse inhibition is a measure of the brain’s ability to filter out irrelevant stimuli, and if the person is still startled by the second noise, it means they don’t filter out sensory stimuli properly

  • this can contribute to hallucinations because internal thoughts might not be properly distinguished from external voices/input

• increase in anxiety/paranoia (abnormalities in amygdala functioning, increased activation of the HPA axis)

Summarize the circuitry involved in schizophrenia (showing that the excess in dopamine leads to opposite effects in the mesostriatal and mesocortical pathways). Which symptoms does this entire circuitry drive?

NMDA receptor hypofunction in prefrontal cortex (on the GABAergic neurons) leads to:

• decreased DA activity in cortex

• upregulation of DA1 receptors

• decreased inhibition of the substantia nigra neurons (via the GABAergic neurons in prefrontal cortex) —> these are downstream EXCITATORY pyramidal neurons, still in cortex.

  • these neurons become hyperexcitable because they aren’t being inhibited well

  • we see an INCREASE in the dopamine released by the substantia nigra/VTA to the striatum (which contain the dopaminergic neurons)

  • the striatum has LOTS of MSNs with D2 receptors —> we see excessive activation of the inhibitory D2 receptors

    • less goal-directed behavior

    • positive symptoms (ex. hallucinations)

This is what drives the positive (via D2Rs) + cognitive (via D1Rs) symptoms

How might manipulating glycine and D-serine (amino acids) serve as a treatment for schizophrenia? hint: think about NMDARs

• necessary cofactors for NMDARs —> facilitate NMDAR function by binding to them

  • this means that they enhance learning and memory in animals, and might help with CBT

When we block uptake of glycine, it leads to an increase in synaptic glycine, which allows for increased NMDAR receptor binding (which is important, since glycine serves as a cofactor)

• the glycine transporter inhibitor protects against the increase in cortical dopamine levels + locomotion increase seen with a model of schizophrenia (amphetamines)

  • reduces sensitization (increased response) to dopamine

What is the nicotinic theory of schizophrenia - does acetylcholine play a role in schizophrenia + have a use in treating schizophrenia symptoms? What theory shows that using an agonist for nicotinic acetylcholine receptors might be helpful?

We do know that a failure to properly “activate” or excite GABAergic neurons in cortex via NMDARs leads to less GABA inhibition of VTA DA neurons and more D2 receptor activation, which leads to increased psychosis and locomotion/movement

However, a lot of people with schizophrenia self-medicate using cigarettes (which contain nicotine, an ACh agonist) AND we see genetic linkage to the alpha-7 nicotinic acetylcholine receptor (nAChR)

—> these nicotinic acetylcholine receptors are found in brain regions + specific neuron types that are connected to schizophrenia

• for example, they are found on the GABAergic prefrontal cortex neurons, and can work to increase GABA release even in the absence/reduction of NMDAR functioning

• because of this, we can use alpha-7 nAChR agonists (many of which are very structurally varied from each other)

What effect do alpha-7 nAChR agonists have on GABA neuron firing?

Positive effect (administering them leads to an increase in the firing of GABA neurons) —> a lower dose is most optimal for GABA neuron function/firing rate though

• we also see enhanced pre-pulse inhibition in a-7 agonist rats (where response to the second stimulus is inhibited, showing increased sensory filtration)

  • a-7 agonists work against NMDAR antagonists

  • allosteric modulators (so not direct agonists) also work to decrease the effects of ketamine-induced schizophrenia (remember that ketamine can cause subtle cognitive deficits)

What is a brief summary of some other meds/their effects?

• meds that have a broad affinity for a lot of receptors in the brain (DA, 5-HT, NA, ACh)

• dopamine (D2) and serotonin receptor antagonist (high affinity for 5-HT2 receptors, which are EXCITATORY) —> antipsychotic effects

• agonist for muscarinic acetylcholine receptors (which are G-protein coupled)