Lithium induces brain-derived neurotrophic factor and activates TrkB in rodent cortical neurons: an essential step for neuroprotection against glutamate excitotoxicity.

Mechanisms underlying the therapeutic effects of lithium for bipolar mood disorder remain poorly understood. Recent studies demonstrate that lithium has neuroprotective actions against a variety of insults in vitro and in vivo. This study was undertaken to investigate the role of the brain-derived neurotrophic factor (BDNF)/TrkB signaling pathway in mediating neuroprotection of lithium against glutamate excitotoxicity in cortical neurons. Pretreatment with either lithium or BDNF protected rat cerebral cortical neurons from glutamate excitotoxicity. The duration of treatment required to elicit maximal neuroprotection by BDNF (1 day) was much shorter than that by lithium (6 days). K252a, an inhibitor of Trk tyrosine kinases, and a BDNF neutralizing antibody suppressed the neuroprotective effect of lithium. Treatment of cortical neurons with lithium increased the cellular BDNF content in 3 days and the phosphorylation of TrkB at Tyr490 in 5 days, suggesting that long-term lithium administration enhances BDNF expression/secretion, leading to the activation of TrkB receptor. Lithium failed to protect against glutamate excitotoxicity in cortical neurons derived from homozygous and heterozygous BDNF knockout mice, although lithium fully protected cortical neurons prepared from wild type mice littermates. Taken together, these data suggest that the BDNF/TrkB pathway plays an essential role in mediating the neuroprotective effect of lithium.