Biological Bases of Behavior: Schizophrenia, Neurodevelopment, and Circuit Dysfunction, and Psychosis Notes

Overview of Schizophrenia as a Neurodevelopmental Disorder

• General Characteristics and Prevalence:

  • Schizophrenia is categorized as a neurodevelopmental disorder.

  • Lifetime prevalence is approximately $1\%$.

  • Typical Onset:

    • Men: Late teens to early $20s$.

    • Women: $20s$ to early $30s$.

• Major Symptom Domains:

  • Positive Symptoms: Additions or distortions of normal experience.

  • Negative Symptoms: Loss or reduction of normal functioning.

  • Cognitive Symptoms: Core deficits that often precede the onset of psychosis and predict functional outcomes.

• Heritability and Pathophysiology:

  • Heritability estimates approach $80\%$.

  • The disorder involves abnormalities in:

    • Brain development.

    • Synaptic connectivity.

    • Neurotransmission.

    • Large-scale brain networks.

Clinical Symptom Domains and Associated Circuitry

• Positive Symptoms (Psychosis):

  • Manifestations: Hallucinations, delusions, disorganized thought, disorganized behavior, and paranoia.

  • Key Circuitry: The Mesolimbic dopamine pathway.

    • Originates in the Ventral Tegmental Area (VTA) and projects to the nucleus accumbens.

    • Associated with aberrant salience attribution and reward processing.

• Negative Symptoms:

  • Manifestations: Flat affect, anhedonia (loss of pleasure), avolition (loss of motivation), social withdrawal, and alogia (poverty of speech).

  • Key Circuitry: Prefrontal Cortex (PFC) and related networks, including the medial and dorsolateral PFC.

    • Involved in reduced motivation, emotional expression, and goal-directed behavior.

• Cognitive Symptoms:

  • Manifestations: Deficits in working memory, attention impairment, executive dysfunction, processing speed deficits, and impaired cognitive flexibility.

  • Key Circuitry: Frontoparietal and executive control networks.

  • Clinical Significance: Cognitive deficits serve as the strongest predictor of long-term functional outcomes.

Neurodevelopmental Trajectory and Environmental Contributors

• Early Vulnerability:

  • Genetic risk.

  • Prenatal and perinatal complications.

  • Early cognitive and social differences observed in childhood.

• Adolescence (Critical Window):

  • Process of synaptic pruning.

  • Myelination and network maturation.

  • Increase in stress sensitivity.

  • Emergence of prodromal symptoms.

• Transition to Psychosis:

  • Dopamine dysregulation.

  • Network dysfunction.

  • Cognitive decline and functional deterioration.

• Environmental Contributors:

  • Cannabis exposure.

  • Urbanicity (living in urban environments).

  • Childhood trauma and social adversity.

  • Inflammation and stress.

Genetics and Polygenic Risk

• The Polygenic Nature of Schizophrenia:

  • There is no single "schizophrenia gene."

  • Risk is contributed by thousands of small-effect variants.

  • Polygenic Risk Scores (PRS): An aggregate measure of cumulative genetic vulnerability. PRS is associated with:

    • Earlier symptom onset.

    • Cognitive vulnerability.

    • Neurodevelopmental abnormalities.

• Major Genetic Systems and Implicated Genes:

  • Synaptic Plasticity / Glutamate Signaling: Genes like $GRIN2A$ and $DLG2$. Relevant to cognitive dysfunction and cortical dysconnectivity.

  • GABAergic Inhibition: Genes like $GAD1$ and $GABRB2$. Relevant to Excitation/Inhibition (E/I) imbalance and cortical synchrony.

  • Dopamine Regulation: Genes like $DRD2$ and $COMT$. Relevant to salience attribution and psychosis.

  • Calcium Channel Signaling: Gene $CACNA1C$. Relevant to cognitive and affective regulation.

  • Immune / Synaptic Pruning: Genes like $C4A$ and MHC genes. Relates to excessive pruning during adolescence.

  • Neurodevelopment: Genes like $DISC1$ and $NRG1$. Relates to abnormal brain maturation.

• Gene x Environment Interaction:

  • Genetic vulnerability interacts with stress, trauma, social adversity, cannabis exposure, and inflammation.

Synaptic Pruning and E/I Imbalance

• Normal Adolescent Development:

  • A critical window where synaptic pruning improves network efficiency.

  • Relies on balanced glutamate and GABA signaling for coordinated cortical communication.

• Proposed Mechanism in Schizophrenia:

  • Excessive Synaptic Pruning: Occurs in frontal and temporal cortical circuits during adolescence.

  • $C4A$-Mediated Synaptic Tagging: Leads to microglial over-pruning.

  • Reduced Inhibitory Regulation: Disruption in GABA leads to a cortical E/I imbalance.

• Functional Consequences:

  • Noisy and inefficient cortical signaling.

  • Cognitive dysfunction and impaired salience processing.

  • Downstream dopamine dysregulation leading to the emergence of psychosis.

Dopamine Dysregulation Models

• Normal Function: Dopamine is responsible for salience attribution, reward prediction, prediction error signaling, motivation, and reinforcement learning.

• Mesolimbic Hyperdopaminergia (VTA → Nucleus Accumbens):

  • Excessive assignment of salience to stimuli.

  • Internally generated experiences are perceived as meaningful or external.

  • Primary driver of positive symptoms (hallucinations and delusions).

  • Hippocampal dysregulation may contribute to this striatal dopamine hyperactivity.

• Mesocortical Hypodopaminergia (VTA → Prefrontal Cortex):

  • Reduced dopamine in the PFC.

  • Leads to reduced executive control, working memory deficits, reduced motivation, and cognitive dysfunction.

• Modern Perspective:

  • Dopamine dysfunction is circuit-specific.

  • It may emerge downstream from E/I imbalance, cortical dysconnectivity, or hippocampal dysregulation.

Large-Scale Brain Network Dysfunction

• Salience Network (SN):

  • Components: Insula and Anterior Cingulate Cortex (ACC).

  • Function: Detects important stimuli and switches attention between internal and external data.

  • Schizophrenia Pathology: Aberrant salience attribution and misidentification of internal experiences.

• Default Mode Network (DMN):

  • Components: Medial PFC and Posterior Cingulate Cortex (PCC).

  • Function: Self-referential thought, internal mentation, and autobiographical processing.

  • Schizophrenia Pathology: Excess internal activity, altered resting states, and failure to suppress the DMN during cognitive tasks, leading to intrusive self-generated experiences (hallucinations).

• Central Executive Network (CEN):

  • Components: Dorsolateral Prefrontal Cortex (DLPFC) and Parietal Cortex.

  • Function: Working memory, cognitive control, and executive functioning.

  • Schizophrenia Pathology: Impaired cognitive control and reduced top-down regulation.

• Key Concept: Dysconnectivity among these large-scale networks underlies both psychosis and cognitive dysfunction.

Neuroimaging Findings

• Structural Neuroimaging Findings:

  • Enlarged lateral ventricles.

  • Reduced gray matter volume and cortical thinning.

  • Reduced hippocampal volume.

  • Abnormalities in the DLPFC, superior temporal gyrus, and ACC.

  • Subtle abnormalities may precede psychosis, with accelerated gray matter loss occurring around the first episode.

• White Matter Abnormalities (DTI Findings):

  • Reduced white matter integrity and altered myelination.

  • Frequently Affected Tracts:

    • Uncinate fasciculus (Frontal ↔ Temporal).

    • Cingulum bundle (Limbic ↔ Executive networks).

    • Arcuate fasciculus (Language processing).

    • Corpus callosum (Interhemispheric communication).

    • Fornix (Hippocampus ↔ PFC).

  • Cognitive Dysmetria: Schizophrenia involves "dysconnectivity" or abnormal integration of perception and thought.

• Functional Neuroimaging Findings:

  • Hypofrontality: Reduced activation in the PFC (especially DLPFC) during working memory and executive tasks.

  • Inefficient PFC Recruitment: Mild tasks may cause increased activation, but complex tasks show reduced activation compared to healthy controls.

  • PET Imaging: Increased striatal dopamine release during acute psychosis.

  • Hallucinations: Auditory hallucinations are linked to activation in the superior temporal gyrus, auditory cortex, and language-related regions.

Cognitive Dysfunction and Impact

• Common Deficits:

  • Working memory, attention, processing speed, executive functioning, cognitive flexibility, and verbal learning.

• Neurobiological Correlates:

  • DLPFC dysfunction and hypofrontality.

  • Frontoparietal dysconnectivity.

  • Impaired gamma synchrony and E/I imbalance.

• Functional Impact:

  • Cognitive impairment is the primary predictor of: academic and occupational functioning, independent living, and social functioning.

  • These deficits often persist even after positive symptoms (psychosis) respond to medication.

Prodrome and Early Intervention

• Clinical High Risk (CHR) / Prodrome Features:

  • Social withdrawal and functional decline.

  • Cognitive changes and attenuated (mild) psychotic symptoms.

  • Increased stress sensitivity.

• Benefits of Early Intervention:

  • Associated with better functional outcomes and reduced relapse.

  • Shorter duration of untreated psychosis (DUP) leads to better long-term prognosis.

  • Approaches: Coordinated specialty care, family intervention, CBT for psychosis (CBT-p), and cognitive remediation.

Treatment and Pharmacological Implications

• Dopamine $D_2$ Blockade:

  • Standard antipsychotics reduce mesolimbic dopamine signaling.

  • Effective for positive symptoms but less effective for cognitive dysfunction and negative symptoms.

• Side Effect Tradeoffs:

  • First-Generation Antipsychotics: High risk of Extrapyramidal Symptoms (EPS) due to $D_2$ blockade in the basal ganglia.

  • Second-Generation Antipsychotics: Lower EPS risk but higher metabolic risk (weight gain, diabetes).

  • CATIE Study: Found no clear overall superiority of second-generation over first-generation antipsychotics; treatment must be individualized.

• Therapeutic Interventions:

  • Cognitive Remediation Therapy (CRT): Targets working memory and attention through repeated cognitive practice and strategy coaching.

  • Psychotherapy: Includes CBT-p and Acceptance & Commitment Therapy (ACT).

Ethics and Decision-Making Capacity

• Capacity Assessment:

  • Diagnosis of schizophrenia alone does NOT determine decisional capacity.

  • Most individuals retain the capacity to make their own decisions.

• Cognitive Barriers to Capacity:

  • Attention, working memory, and executive functioning deficits may affect comprehension.

  • Capacity is most likely to be impaired during acute psychosis or severe cognitive disorganization.

• Clinical Supports for Ethics:

  • Use of visual aids, repetition, and simplified language.

  • Structured consent procedures and multimedia tools.

  • Principle: Support autonomy and only use proxy decision-making when absolutely necessary.