## Mechanism of Lithium Action **Key Point:** Lithium's primary mechanism involves inhibition of inositol monophosphatase, an enzyme in the phosphatidylinositol (PI) cycle, leading to the **inositol depletion hypothesis**. ### The Inositol Depletion Hypothesis Lithium blocks inositol monophosphatase, which normally recycles inositol-1-phosphate back to free inositol. This causes: 1. Depletion of intracellular free inositol 2. Reduced regeneration of phosphatidylinositol (PI) 3. Decreased availability of PIP₂ (phosphatidylinositol 4,5-bisphosphate) 4. Impaired signal transduction through the PI second-messenger system 5. Reduced neuronal excitability and neurotransmitter release ### Secondary Effects Lithium also: - Inhibits glycogen synthase kinase-3β (GSK-3β), affecting protein kinase C and CREB signaling - Modulates gene expression through effects on transcription factors - Enhances neuroprotection and neuroplasticity **High-Yield:** The inositol depletion hypothesis is the **most widely accepted mechanism** and is tested frequently in NEET PG. This is distinct from its effects on monoamines (which are secondary). **Clinical Pearl:** Because lithium depletes inositol, dietary inositol supplementation does NOT reverse lithium's therapeutic effect — the drug's action is irreversible within the therapeutic window. [cite:KD Tripathi 8e Ch 12]
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