## Neurobiological Basis of Alcohol Withdrawal ### Mechanism of Alcohol's CNS Effects **Key Point:** Alcohol is a CNS depressant that enhances GABAergic (inhibitory) neurotransmission and suppresses glutamatergic (excitatory) neurotransmission through allosteric modulation — it does NOT directly alter neurotransmitter synthesis or release. ### Chronic Alcohol Exposure and Receptor Adaptation With chronic alcohol use, the brain undergoes homeostatic adaptation: 1. **GABA-A receptor downregulation** — reduced number and sensitivity of inhibitory receptors 2. **Glutamate receptor upregulation** — increased number and sensitivity of excitatory receptors 3. **Altered intracellular signalling** — changes in cAMP and protein kinase C pathways ### Withdrawal Syndrome Pathophysiology When alcohol is suddenly removed: - Loss of GABAergic inhibition (due to downregulated receptors) - Unopposed glutamatergic excitation (due to upregulated receptors) - Result: **hyperexcitability** manifesting as tremor, autonomic instability, seizures, and delirium tremens ### Why Benzodiazepines Work **Clinical Pearl:** Benzodiazepines bind allosterically to GABA-A receptors and enhance chloride influx, restoring inhibitory tone during the period when endogenous GABA signalling is compromised. This is the gold-standard pharmacological management. ### Why Option 3 is INCORRECT **High-Yield:** Alcohol does NOT directly increase glutamate release or decrease GABA synthesis. Instead: - Alcohol **enhances** GABA-A receptor function (allosteric modulation) - Alcohol **inhibits** NMDA glutamate receptors (antagonism) - Chronic exposure causes **compensatory receptor changes** — not changes in neurotransmitter production This is a common misconception in exam questions. The pathology is receptor-level adaptation, not altered synthesis. | Mechanism | Acute Alcohol Use | Chronic Alcohol Use | Withdrawal | |-----------|-------------------|-------------------|------------| | GABA-A receptors | Enhanced | Downregulated | Insufficient inhibition | | Glutamate receptors | Inhibited | Upregulated | Unopposed excitation | | Clinical effect | CNS depression | Tolerance develops | Hyperexcitability | **Mnemonic:** **GABA-DOWN, Glu-UP** = Chronic alcohol causes GABA receptor downregulation and glutamate receptor upregulation, leading to withdrawal hyperexcitability when alcohol is removed. [cite:Harrison 21e Ch 394]
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