Lithium was introduced into psychiatry 50 years ago and remains to be the most commonly used drug for the treatment of manic depressive illness. The precise mechanisms underlying its clinical efficacy remain to be defined. In a recent study, we explored the neuroprotective effects of lithium against excitotoxicity elicited by glutamate, a major excitatory amino acid neurotransmitter involved in the synaptic plasticity and pathogenesis of neurodegenerative and neuropsychiatric disorders. We found that long-term exposure to lithium chloride dramatically protects cultured rat cerebellar granule cells (CGCs), cerebral cortical and hippocampal neurons against glutamate-induced excitotoxicity which involves apoptosis mediated by N-methyl-D-aspartate (NMDA) receptors. In CGCs, the lithium neuroprotection occurs at therapeutically relevant concentrations (0.5-5.0 mM) and requires treatment for 6-7 days for complete protection to occur, while a 24-hr treatment is ineffective. The protection is specific for glutamate-induced excitotoxicity and can be attributed to inhibition of NMDA receptor-mediated calcium influx. Our results suggest that modulation of glutamate receptor hyperactivity represents, at least in part, the molecular mechanisms by which lithium alters brain function and exerts its clinical efficacy in the treatment for manic depressive illness. These novel actions of lithium also suggest that excessive glutamatergic neurotransmission may be the pathogenic mechanism underlying bipolar illness. We have studied the role of expression of genes involved in pro- apoptosis and cytoprotection. In CGCs, treatment with neuroprotective concentrations of lithium induces a time- dependent increase in the levels of mRNA and protein of the cytoprotective gene Bcl-2. Conversely, the levels of mRNA and protein of the pro-apoptotic genes, Bax and p53 are decreased by lithium. In contrast, glutamate treatment induces a rapid increase in the levels of Bax and p53 mRNA and protein. Pretreatment with lithium suppresses the glutamate-induced increase in Bax and p53 and maintains Bcl-2 at an elevated level. Additionally, glutamate induces a rapid, reversible decrease in the activity of a cell survival factor, Akt, through enhanced dephosphorylation due to activation of protein phosphatase PP2A. In contrast, lithium activates the PI 3-K/Akt signalling pathway and enhances the phosphorylation of glycogen synthase kinase-3. Pretreatment with lithium facilitates the recovery of glutamate-induced loss of Akt activity. Moreover, glutamate treatment induces a persistent decrease in the level of phospho-cyclic AMP response element binding protein (p-CREB). This glutamate-induced loss of p-CREB is due to activation of protein phosphatase PP1 and is effectively suppressed by long-term lithium pretreatment. In a more recent study we found that glutamate induces a robust activation of AP-1 binding, Jun N-terminal kinase (JNK) and p38 kinase. Suppression of these glutamate-induced activities using selective inhibitors results in neuroprotection, suggesting their roles in excitotoxicity. Moreover, long-term lithium-induced neuroprotection is concurrent with inhibition of glutamate-induced activation of AP-1 binding, JNK and p38 kinase. Collectively, our results suggest that glutamate excitotoxicity involves induction of pro-apoptotic genes and suppression of cytoprotective gene expression. Moreover, lithium neuroprotection is due, at least in part, to suppression of these glutamate-induced effects. In light of recent clinical observations that there are neuroanatomical and morphological abnormalities in the frontal cortex of the brain of bipolar patients, we have extended our studies to include primary cultures of cortical neurons prepared from embryonic rats. These cortical neuronal cultures are highly vulnerable to glutamate-induced apoptotic insults. Pretreatment with subtherapeutic or therapeutic concentrations of LiCl for 6 days robustly protects against glutamate excitotoxicity in these cultures. Thus, significant protection was achieved at 0.1-0.2 mM with a nearly complete protection at 1.0 mM. Interestingly, valproate, another mood-stabilizer, also induces a dose- and time-dependent protection against glutamate excitotoxicity. The lithium neuroprotection in cortical neurons is associated with a reduction in NMDA receptor-mediated calcium influx, and this lithium-induced action is correlated with a selective decrease in tyrosine phosphorylation at position 1472 of the NR2B subunit of the receptor. The latter is preceded by a reduction in the activation of Src, which is a member of the Src tyrosine kinase family and is involved in NR2B subunit tyrosine phosphorylation. The neuroprotective effect of lithium is blocked by a brain-derived neurotrophic factor (BDNF) neutralizing antibody and K252a, a Trk receptor antagonist, suggesting a role of BDNF-TrkB signaling in neuroprotection. In support of this notion, we found that long-term lithium increases the level of intracellular BDNF, triggers the release of BDNF and activates TrkB, the receptor for BDNF. Moreover, the lithium neuroprotection is completely blocked by either heterogenous or homogenous knockout of the BDNF gene. Using rat cortical cell and cerebellar granule cell cultures, we also demonstrated that lithium enhances the proliferation of their progenitor cells detected by co-labeling with BrdU and nestin. Furthermore, the decrease in neuronal progenitor proliferation induced by glutamate, glucocorticoids or haloperidol is antagonized by lithium pretreatment, suggesting a prominent role of this drug in neurophysiology and neuropathology. We have expanded from our in vitro cell culture studies by using animal models of cerebral ischemia and Huntington's disease. We found that subcutaneous injection of rats with LiCl for 16 days reduces the size of ischemic brain infarct volume by more than 50% in rats subjected to permanent occlusion of the left middle cerebral artery. In a more recent study, we employed a rat focal ischemia paradigm in which the left middle cerebral artery is subjected to transient occlusion followed by reperfusion and found that lithium is neuroprotective even when administrated after the onset of the ischemic attack. This lithium neuroprotection is dose-dependent in the range of 0.5 to 3 mEq given subcutaneously. Moreover in ischemic rats treated with lithium, the level of cytoprotective heat shock protein 70 in neurons of the cerebral cortex is markedly increased compared with the control. In the Huntington's disease animal model, we injected quinolinic acid, a partial agonist of the NMDA receptor, into the left side of rat striatum. This treatment results in about 70% lesion of the striatum which involves the loss of GABAnergic neurons expressing dopamine D-1 receptors. This quinolinic acid-induced lesion requires activation of the transcription factor NF-kB and induction of p53, c-Myc and cyclin D1 and is protected by metabotropic receptor agonists and prostaglandin A1. Our results show that chronic lithium treatment for 16 days or one day pretreatment decreases the size of striatal lesion by 40-50%. In addition, lithium neuroprotection effects are associated with over-expression of Bcl-2 in affected areas and other brain structures. The neuroprotective effect is also correlated with suppression of QA-induced DNA damage and caspase-3 activation. Additionally, the lithium neuroprotection is accompanied with an enhanced cell proliferation in the striatum near the site of quinolinic acid injection. Thus, our in vitro and in vivo studies raise the possibility that lithium, in addition to its use for bipolar depressive illness, may have expanded use for the treatment of neurodegenerative diseases, particularly those linked to excitotoxicity.
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