Glutamate-mediated calcium signaling: A potential target for lithium action

Bipolar disorder and schizophrenia are two debilitating mental health disorders associated with both severe impairment and increased suicide risk. Several lines of evidence indicate that these disorders are associated with disturbances in the glutamate system. For almost half a century, lithium has been the most effective drug for treatment of mood disorders. Lithium is still used mainly on empiric grounds and its molecular mechanisms of action are still largely unknown. This study was designed to explore the effects of continuous lithium exposure, in therapeutically relevant concentration, on the glutamate-mediated Ca2+ response in rat primary hippocampal neurons. We show that lithium treatment is associated with multiple perturbations in calcium signaling. Lithium attenuated calcium release after activation of both metabotropic glutamate receptors (mGluR)1/5 as well as muscarinic cholinergic receptors, two different Gq-coupled receptors. The attenuation of the calcium response was, for mGluR5 receptors, found to be associated with a downregulation of the plasma membrane expression of this receptor. Lithium also attenuated calcium influx after activation of the N-methyl-D-aspartate receptor, without affecting its cell surface expression. Furthermore lithium treatment was associated with a decrease in intracellular calcium concentration and a reduction of calcium content in intracellular stores. Thus we have shown that lithium attenuates the effects of glutamate-mediated calcium signaling and regulates intracellular calcium levels as well as calcium turnover in hippocampal neurons. These effects can be expected to influence the communication within and between neurons in a variety of ways since calcium may be considered as the most common and the most versatile signaling molecule in neurons.