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Lithium, the simplest xenobiotic in the modern pharmacopoeia, has a complex mechanism of action that has not been completely elucidated even after more than 50 years of clinical use. Psychotropics modifying monoamine neurotransmission form the basis of modern psychopharmacology. The current paradigm of neuropsychiatric therapeutics implies that because xenobiotics that affect the neurotransmission of dopamine are most efficacious, this receptor-drug exemplar defines the disease. Multiple lines of investigation have shown that beyond simple monoamine balance, the interaction between multiple signaling cascades, neurotrophic and neuroprotective systems is not only involved in the pathogenesis of mood disorders, but also the cellular and molecular means of the action of lithium.155 Additionally, the understanding that a particular receptor only undergoes a single agonist-receptor activation has been replaced by the growing comprehension that agonist binding causes activation of multiple pathways of downstream signaling, with a myriad of results.25 Lithium illustrates this new paradigm through its multiple direct and indirect effects.
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Clinically, the therapeutic effects of lithium and similar mood-stabilizing pharmaceuticals become evident only after chronic administration, so their mechanism of action is unlikely solely the result of acute biochemical interactions. One of the central processes involved in the therapeutic effects of lithium, and potentially the pathogenesis of mood disorders, is its interaction with the β form of glycogen synthase kinase 3 (GSK-3). GSK-3β is a serine/threonine kinase originally described with the regulation of glycogen synthesis in response to insulin.25 Subsequently it has been shown to influence multiple systems, including gene transcription, neuronal function and neurogenesis, synaptic plasticity, and the circadian cycle, as well as cellular structure, apoptosis, and cell death. In fact, GSK-3β phosphorylates more than 100 substrates, with a suggestion of many more.101 Each of these systems are implicated in the pathophysiology of mood disorders.25,155
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Postmortem tissue from the ventral prefrontal cortex of patients with major depressive disorder showed elevated GSK-3β activity as well as decreased activity in the GSK-3β modifying enzyme, Akt (also known as protein kinase B).102,103 GSK-3β is inadequately inhibited in association with mood disorders and is inhibited in humans treated with lithium.101 Overactivity of this enzyme is associated with neuronal degeneration and sensitivity to apoptotic stimulation. Dysregulation of GSK-3β is implicated in tumor growth and the neurofibrillary tangles of Alzheimer disease.56,191 It also is a key regulator of neuronal cell fate, with a proapoptotic effect in many settings.5,22,24,26,101,118,131,161 It is involved in regulating the activity of β-catenin, Jun, and cyclic adenosine monophosphate (cAMP) response element-binding protein (CREB), transcription factors important in embryonic patterning, cell proliferation, neuronal modeling and plasticity, neuronal signal transduction, memory consolidation, and cytoskeletal remodeling. Its other targets include transcription factors such as c-Jun, nuclear factor activated T-cells, proteins bound to microtubules (Tau, microtubule-associated protein 1B, kinesin light chain), cell cycle mediators (cyclin D), and metabolic regulators (glycogen synthase, pyruvate dehydrogenase).153,155 Hypoxia contributes to increased GSK-3β activity, which may be counteracted or inhibited with mood-stabilizing drugs. Vascular depression, or depression after stroke, is an organic model of major depression.94,133 The finding that this depressive state responds similarly to intervention with mood stabilizers lends further support to the GSK-3β hypothesis.70,79,90,150,190
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Many older generation antipsychotics are now known to affect GSK-3β signaling in mice.119 GSK-3β activity is regulated not only by first- and second-generation antipsychotics but also through 5-HT neurotransmission and activation of 5-HT2A receptors as well as through monoamine-affecting antidepressants which modify these neurotransmitters.22, 23, 24, 25, 26, 27, and 28,117 Atypical antipsychotics and antagonists of D2 dopamine receptors, as well as 5-HT2A serotonin receptors, may exert some of their utility through inhibition of GSK-3β activity.22,25
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The link between neuropsychiatric illness and these centrally placed mediators of signaling comes from exploration of dopamine neurotransmission (Fig. 72–1). Scaffolding proteins, the β-arrestins, are traditionally associated with the termination of G protein–coupled receptor (GPCR) signaling and desensitization. After a receptor is stimulated, GPCRs are phosphorylated, leading to the β-arrestin recruitment that uncouples the receptor from the G protein and leads to GPCR internalization. β-Arrestins also act as scaffolds for the formation of a protein complex that allows for GPCRs to signal independently from G proteins.22,24,26,71 Dopaminergic neurotransmission through GPCRs is mediated through a complex that involves Akt, β-arrestin 2 (βArr2), and protein phosphatase 2A (PP2A). Akt is a serine/threonine kinase that is additionally regulated through phosphatidylinositol-mediated signaling.25 Akt activity results from an equilibrium between phosphorylation (activation) and dephosphorylation (inactivation).27 Regulation of GSK-3β activity is achieved through phosphorylation of its N-terminal serine residue, leading to inhibition.118 The Akt-βArr2-PP2A complex, when activated, dephosphorylates (deactivates) Akt, which leads to the activation of GSK-3β through loss of inhibition (loss of phosphorylation.)25,27,70
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Lithium is a direct inhibitor of GSK-3β that can also inhibit its activity indirectly through a mechanism involving Akt activation.26,27,48,101 Lithium activates Akt by disrupting assembly of the Akt-βArr2-PP2A complex through displacement of the magnesium cofactor required for assembly.25, 26, and 27 Through disruption of this complex, which normally promotes Akt inactivation, lithium promotes Akt activation, leading to increased phosphorylation (inactivation) of GSK-3β. This is demonstrated in β-arrestin knockout mice, which, unable to create this complex, are resistant to the behavioral effects of lithium.25,27,70,101
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Identification of the Akt-βArr2-PP2A signaling complex as a target of lithium lends credence to the use of lithium as an adjunct to enhance the action of atypical antipsychotics and antidepressants in poorly responsive subjects by acting through Akt–GSK-3β signaling.25,27,101
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Inhibition of GSK-3β by lithium is thought to be neuroprotective by modifying the downstream targets and effectors of GSK-3β activity.5,23,27,70,91,110,150,162 Lithium is implicated in the neuroprotective modulation of the bcl-2 gene, which is known for its role in preventing apoptosis and in downregulation of the proapoptotic protein p53. Lithium increases bcl-2 concentrations in cultured nervous tissue of both rats and humans.49,129 Additional support comes from patients undergoing long-term therapy with either lithium or valproic acid (VPA) who show prefrontal cortex volumes significantly greater than in patients not treated with either agent, suggesting a protective effect in humans.47,162 Further evidence points toward a neuroprotective and neurotrophic effect of lithium with evidence to support benefit in such diverse neurodegenerative conditions as Parkinson disease (in a mice model of Parkinson disease using N-methyl-4-pheynyl-1,2,3,6-tetrahydropyridine {MPTP}),198 Huntington disease,168 amyotrophic lateral sclerosis (ALS),68 stroke and multiple sclerosis,23,47 traumatic brain injury,199 and Alzheimer disease.5,19,22,25,42,131,136
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It would be naive to assume that lithium has a single mechanism of action, and it is plausible that other mechanisms, such as the inhibition of inositol monophosphatases, also contribute to its pleiotropic effects on behavior. Compelling evidence has bridged a link between the current molecular targets and downstream regulators of the effects of lithium with the classic proposed mechanism of action, the inositol-depletion hypothesis.
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Among the first proposed mechanisms of action of lithium was the inositol-depletion hypothesis.90 Inositol is a six-carbon sugar that forms the backbone of a number of cellular signaling mechanisms. Lithium treatment results in a decreased myoinositol (the most biologically active stereoisomer of inositol) concentration in the cerebral cortex.34,90,131 Abnormalities in regional brain myoinositol concentrations are thought to occur in bipolar patients. This theory is partially supported by experimental magnetic resonance spectroscopy data.173,197 Myoinositol is phosphorylated to form phosphatidyl inositol (PIP), which is further phosphorylated and combined with diacylglycerol (DAG) to form phosphatidyl 4,5-bisphosphate (PIP2). Upon stimulation of a cell, G protein–coupled receptors activate phospholipase C (PLC), which hydrolyzes PIP2 to release the secondary messengers DAG and inositol 1,4,5-trisphosphate (IP3).34,92 Each of these secondary messengers in turn initiates a cascade of events, including activation of protein kinase C, which is important for calcium homeostasis and neurotransmitter release,131,173 as well as independent mobilization and regulation of intracellular calcium.149,162,173 Many extracellular signals, including some serotonin receptor subtypes, neurotrophin signaling pathways, receptor tyrosine kinase pathways, and G protein–mediated signaling, activate PLC to exert their actions.80,154,155
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Serial dephosphorylation of IP3 leads to regeneration of myoinositol and recycling of the inositol pool. Two enzymes involved in this pathway are inhibited by lithium. The first enzyme, inositol 1,4-bisphosphate 1-phosphatase (IPPase), dephosphorylates the bisphosphate to inositol monophosphate (IMP). The second enzyme, inositol 1-monophosphatase (IMPase), dephosphorylates IMP to myoinositol4 (Fig. 72–2).
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The inhibition of IMPase is interesting and important. Lithium inhibits IMPase by “uncompetitively” binding to the enzyme–substrate complex and preventing the release of phosphate. It performs this function by displacing a magnesium ion from the active site after hydrolysis. This is the same mechanism by which lithium directly inhibits GSK-3β.48 Uncompetitive refers to the inhibitor binding to the enzyme-substrate complex; the higher the concentration of the substrate, the more the enzyme is inhibited.70,86,127 Uncompetitive inhibitors only bind to the enzyme-substrate complex, inhibiting the reaction at that point. Uncompetitive inhibitor kinetics are related proportionally to the concentration of the enzyme–substrate complex and cannot be overcome by increasing the concentration of the substrate, unlike a competitive inhibitor.150 This supports a theory about the pathophysiology of bipolar disorder involving an excess of myoinositol and is one reason why the mood-stabilizing effects of lithium are thought to occur only in bipolar patients.90 The nature of the action of lithium serves as a regulator to preferentially block pathologic signaling caused by excessive myoinositol while leaving the normal signaling intact. As described, IMPase is an important step in the cellular recycling of the inositol pool and is inhibited by lithium.
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Myoinositol is also generated de novo from glucose-6-phosphate by inositol synthase, which forms IMP. The inhibition of IMPase by lithium subsequently leads to myoinositol depletion by preventing the conversion of the newly synthesized IMP to myoinositol. Interestingly, VPA also inhibits inositol synthase, illustrating a potential mechanism for the synergy of these complementary mood stabilizers.60,115 A third mechanism of intracellular diminution of inositol by lithium (as well as VPA and carbamazepine) is the effect of lithium on reducing activity and transcription of the sodium myoinositol transporter, preventing the uptake of exogenous myoinositol by the cell. This mechanism of inhibition may be overcome by increased extracellular concentration of myoinositol.90
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The result of these effects is depletion of the inositol pool available to the cell, causing a series of events at different points in the signal transduction cascade that leads to differential gene transcription and expression. This sequence ultimately is responsible for some of the observed clinical effects of lithium on the central nervous system (CNS).170 Experimental data using dextroamphetamine as a model for clinical mania demonstrate increased regional inositol signaling in the human brain that is attenuated by pretreatment with lithium, lending support to the hypothesis.31
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The inositol depletion hypothesis, while an original attempt to explain in molecular terms the therapeutic effects of lithium, does not fully elucidate nor replicate the clinical disease or response to therapy. In vivo studies with more drastic inositol depletion than that which occurs from lithium therapy fail to replicate predicted behavioral patterns. Studies in knockout mice lacking various isoforms of inositol monophosphatase fail to replicate the antidepressant or antimanic effects of lithium. The model remains an attractive one, and the flaws may represent species variation with validity more in humans with bipolar disorder than the mouse model illustrates.23,131,169 However, there may be a synergy with other proposed mechanisms of lithium’s effect. Using a yeast model, it was shown that GSK-3β is required for de novo inositol synthesis, and that loss of GSK-3β activity leads to inositol depletion. This finding links the two targets of not only lithium, but other mood stabilizing pharmaceuticals.12
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In summary, although the precise mechanism of action is unknown, some common features of investigation have emerged. The potential targets, widely found and disparate in function, all seem to be inhibited by lithium in an uncompetitive fashion, most commonly through displacement of a divalent cation, usually Mg2+. The systems affected by this inhibition vary widely. Downstream targets seem to modulate secondary cell messengers and intracellular signal transduction, transcription factors and gene expression, and neuronal plasticity and cellular differentiation. Further study is needed to elucidate the complex interaction of these pathways with the action of lithium to form an integrated hypothesis.