Acute or chronic lithium does not affect agonist-stimulated inositol trisphosphate formation in rat brain in vivo

The high affinity inositol transport system--implications for the pathophysiology and treatment of bipolar disorder

The 'inositol-depletion hypothesis' postulates that the therapeutic effects of lithium are due to inhibition of inositol monophosphatase, which leads to depletion of brain cells of myo-inositol and consequently to dampening of phosphoinositide (PI) signaling. This article examines the potential relevance of an alternative mechanism for inositol depletion: inhibition of myo-inositol uptake that proceeds via the sodium/myo-inositol cotransport (SMIT). We discuss recent in vitro experiments that show a pronounced downregulation of SMIT after chronic treatment with lithium, carbamazepine, and valproate at therapeutically relevant concentrations. It is concluded that downregulation of SMIT could represent a common mechanism of action of mood stabilizers.

Lithium reduces tau phosphorylation: effects in living cells and in neurons at therapeutic concentrations

BACKGROUND

The mechanism of action of lithium remains to be determined satisfactorily. Recent studies suggested a possible role in inhibiting glycogen synthase kinase-3 (GSK-3), previously shown to phosphorylate the protein tau. Tau is expressed mainly in neurons, where it functions to stabilize microtubules in a phosphorylation-dependent manner.

METHODS

Neurons and transfected non-neuronal cells were treated with lithium and the phosphorylation of tau at multiple epitopes examined by western blotting and by immunocytochemistry. Using green fluorescent protein as a tag we examined the effects of lithium on phosphorylated tau in living cells.

RESULTS

Lithium reversibly reduced tau phosphorylation at therapeutic concentrations, and even at high concentrations did not alter neuronal morphology. Green fluorescent protein tagged-tau when phosphorylated by GSK-3 was diffusely distributed; treatment with lithium resulted in association with microtubules and then bundle formation. Removing lithium allowed observation of the dissolution of bundles and gradual dissociation of tau from microtubules in living cells.

CONCLUSIONS

Lithium may have multiple effects in brain, but at least one action is demonstrated to be a relative inhibition of GSK-3-induced tau phosphorylation. These results carry implications for future studies of the actions of mood-stabilizing drugs and indeed of the molecular mechanisms of affective disorders.