## Neurobiological Basis of Autism Spectrum Disorder ### The Excitation-Inhibition (E-I) Imbalance Hypothesis **Key Point:** The glutamatergic-GABAergic imbalance hypothesis is the most well-supported neurobiological model of ASD. This theory proposes that autism results from an imbalance between excitatory (glutamate) and inhibitory (GABA) neurotransmission in the brain, leading to excessive excitation relative to inhibition. ### Evidence Supporting E-I Imbalance 1. **Genetic findings:** Mutations in genes encoding GABA~A~ receptors, GABA synthesis enzymes, and glutamate receptors are overrepresented in ASD. 2. **Neuroimaging:** Reduced GABA levels in motor and prefrontal cortex correlate with symptom severity. 3. **Cellular studies:** Altered balance of excitatory and inhibitory synapses in cortical circuits of individuals with ASD. 4. **Behavioral correlates:** Excessive excitation may explain sensory hypersensitivity, repetitive behaviors, and social difficulties; insufficient inhibition may impair impulse control and attention. ### Other Implicated Systems (Secondary) | System | Role in ASD | |--------|-------------| | Serotonergic | Involved in mood, social behavior; dysfunction may contribute but NOT primary mechanism | | Dopaminergic | Implicated in reward, motivation, repetitive behavior; secondary to E-I imbalance | | Cholinergic | No primary role; not a major focus of ASD neurobiology | **High-Yield:** The E-I imbalance hypothesis explains why some ASD individuals respond to GABAergic medications (e.g., valproate) and why seizures are common comorbidities (both reflect cortical hyperexcitability). **Clinical Pearl:** This model integrates genetic, neurochemical, and behavioral findings and is increasingly used to guide therapeutic development (e.g., GABA~A~ agonists, glutamate antagonists). [cite:Rubenstein & Merzenich, 2003; DSM-5 pp. 50–51]
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