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Lithiums Mechanism in Bipolar Disorder: Fact-Checking the Inositol Depletion Hypothesis

“Research has suggested individuals with BD show heightened intracellular calcium levels. A systematic review and meta-analysis by Harrison et al. (2021) confirmed there is strong evidence pointing to elevated intracellular calcium levels in individuals with BD and that calcium dysregulation is a likely contributor to neuronal hyperexcitability. It is therefore particularly relevant to understand how lithium acts on second messenger systems, more specifically its inhibition of the enzyme Inositol Monophosphatase (IMPase). Originating from the work of Berridge et al. (1982), the inositol depletion hypothesis suggests lithium’s inhibition of IMPase reduces the recycling loop of myo-inositol, which hinders the regeneration of phosphatidylinositol-4,5-bisphosphate (PIP2). In practice, this inhibition cripples the phosphatidylinositol recycling loop, leading to reduced stimulus-evoked production of inositol-1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG) (Martinowich et al., 2009). Therefore, weakening of the IP₃ pathway causes diminished calcium release from the endoplasmic reticulum (Sade et al., 2016). It also decreases protein kinase C (PKC) translocation, by interfering with DAG signals (Harwood, 2004). Without the reduction of calcium release and PKC, it is thought they would contribute to dysregulated neuronal activity present in BD, highlighting lithium’s mood-stabilising effects (Hahn & Friedman, 1999). It is therefore argued BD arises from irregularities in calcium-regulated functions, indicating lithium acts by balancing out the effects of calcium related dysfunctions (Meltzer, 1986), and this appears to support views of lithium’s therapeutic effects partly arising via depletion of inositol (Ochoa, 2021). The inositol depletion hypothesis is presented to be one of the most widely accepted in relation to lithium’s therapeutic efficacy in BD (Ochoa, 2021). Nevertheless, evidence supporting and discussing these intracellular mechanisms are derived from in vitro or animal models (e.g. Sade et al., 2016), limiting direct causal inference in human BD. Therefore, longitudinal neuroimaging studies, which track brain changes in individuals who respond and those who do not respond to lithium, may strengthen causal claims by establishing whether calcium changes lead to symptom improvements. However, lithium’s mechanisms of action are not limited to those mentioned above. Lithium is presumed to target Glycogen Synthase Kinase-3 beta (GSK-3β), an enzyme involved in many signalling pathways (Vanessa et al., 2025). Research has suggested lithium inhibits GSK-3β, which is argued to regulate pathways and modulate how signals are processed (Thangavel et al., 2025). However, the extent to which this inhibition contributes to mood stabilising effects in BD remains inconclusive, given such interpretations rely on indirect evidence over causal clinical data. A lesser explored possible mechanism (when comparing to research for calcium dysregulation or neurotransmitters), is the apparent modulation of nitric oxide (NO) signalling. An interesting human clinical study by De Sousa et al. (2014) demonstrated increased levels of NO following lithium treatment but also reported changes in NO levels did not correlate with clinical improvements. Even so, their findings allow room to question what broader role lithium’s effects on NO could play in oxidative stress and neuroprotective pathways. Malhi et al. (2013) report that lithium reduces oxidative stress and inhibits apoptotic pathways, which serves as protection for neurons against damage that may occur from repeated mood episodes. Holistically, the evidence suggest lithium produces large-scale neurobiological effects, that support mood stability in BD, but it remains highly debated whether these intracellular changes are causal or epiphenomenal. Though exact mechanisms of action are not yet fully understood, thus far, the gathered evidence points to lithium possibly modulating neurotransmission, intracellular signalling and playing a part in neuroprotection. Such mechanisms frame BD as a disorder of dysregulated networks, over simple chemical imbalance. Having said this, calcium dysregulation is arguably the most compelling explanation for lithium’s mood stabilising properties and BD neurobiology.”
Partially accurate
Confidence: High Checked on April 23, 2026

Summary

Elevated intracellular calcium levels in bipolar disorder are confirmed by a systematic review and meta‑analysis, and lithium’s inhibition of inositol monophosphatase and the resulting reduction in IP₃‑mediated calcium release and PKC activity are well documented. However, the claim that lithium raises nitric oxide levels and that this change lacks clinical correlation is not substantiated by the provided sources, and the extent of lithium’s GSK‑3β inhibition remains inconclusive. Consequently, while the calcium‑related mechanisms are supported, the broader mechanistic assertions are only partially verified.

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pmc.ncbi.nlm.nih.gov
pubmed.ncbi.nlm.nih.gov
nature.com

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