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. We have previously shown that long-term treatment of cerebellar granule cells (CGCs) with relatively low doses of lithium increases the mRNA level of the immediate early gene product, c-Fos, and up-regulates the expression and function of m3-muscarinic receptors in these neurons. Since AP-1 and cAMP-responsive element (CRE) are key elements of transcriptional regulation for a variety of proteins with neurophysiological importance, we have investigated the effects of lithium on gene transcription in cultured CGCs and in rats by measuring transcription factor binding to AP-1 and CRE sites. We found that treatment of CGCs with lithium chloride induces a time- and concentration-dependent increase in AP-1 and CRE binding activities. Chronic dietary treatment of rats with lithium carbonate for 4 weeks also significantly increases AP-1 and CRE binding activity in the frontal cortex, hippocampus, amygdala and cerebellum. p-CREB, Jun D and Fos family proteins are present in the AP-1 binding sites in the rat brain and in CGCs. In a parallel 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 CGCs, cerebral cortical and hippocampal neurons against glutamate-induced excitotoxicity which involves apoptosis mediated by N-methyl-D-aspartate (NMDA) receptors. This 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. The long-term effects of lithium are not due to down-regulation of NMDA receptor sites, nor are they related to its known ability to block inositol monophosphatase activity. 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. In a related study, we found that lithium also robustly protects against carbamazepine and phenytoin-induced apoptosis and modestly inhibits age-induced death of CGCs. However, unlike the protection against glutamate excitotoxicity, this effect of lithium on anticonvulsant-induced neurotoxicity does not require long-term treatment and occurs at relatively high concentrations (less than or equale to 5 mM). This neuroprotective effect of lithium is inhibited by a phosphatidylinositol 3-kinase (PI 3-K) inhibitor, LY294002. The inhibition of carbamazepine neurotoxicity by lithium may have clinical implications, as combined treatment with lithium and carbamazepine is known to potentiate the effect of either drug alone in the treatment of manic depressive 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. The ratio of Bcl-2/Bax is increased by about 6-fold after treatment with 3 mM LiCl for 7 days. 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. Lithium also blocks glutamate-elicited cytochrome c release from mitochondria and the cleavage of the caspase-3 substrate, lamin B. In a parallel study, we found that glutamate induces a rapid, reversible decrease in the activity of a cell survival factor, Akt, through enhanced dephosphorylation due to activation of protein phosphatase(s). In contrast, lithium activates the PI 3-K/Akt signalling pathway and enhances the phosphorylation of glycogen synthase kinase 3-beta. Pretreatment with lithium facilitates the recovery of glutamate-induced loss of Akt activity. Taken together, our results suggest that lithium-induced up-regulation of cytoprotective gene products and down-regulation of pro- apoptotic gene products play a prominent role in its neuroprotective actions. We have expanded from our in vitro cell culture studies by using animal models of cerebral ischemia and Huntingtons 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 occlusion of the left middle cerebral artery. The focal ischemia-induced neurological deficits such as abnormal posture and hemiplegia, measured at 24 hr after middle artery occlusion, are significantly reduced by chronic lithium pretreatment. In the Huntingtons 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 and c-Myc. Our results show that chronic lithium treatment or one day pretreatment decreases the size of striatal lesion by 40-50%. Moreover, lithium neuroprotection effects are associated with over-expression of Bcl-2 in affected areas and other brain structures. 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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