[On the relevance of protein kinase C to lithium therapy in bipolar disorder]

The discovery of lithium's efficacy as a mood-stabilizing agent revolutionized the treatment of patients with bipolar disorder and after 5 decades this drug continues to be the mainstay of treatment of this disorder. Valproate, which is dissimilar structurally to lithium, shares most of the effects of lithium at the level of protein kinase C (PKC). Both drugs reduce the activity of PKC, though via different mechanisms. In comparison to patients with major depressive disorder, schizophrenia, or healthy controls, PKC activity is significantly elevated in manic patients, suggesting that changes of PKC activity may be a central pathological trait of the illness. The precise physiological role of PKC activity in the regulation of mood is unclear. The enzyme modulates cellular responses via phosphorylation of numerous substrate proteins. Such substrates of PKC include cytoskeletal proteins, neurotransmitter and hormone receptors, G proteins, GAP-43, MARCKS etc. Further studies are required to clarify any causal role of CPK changes in bipolar-disorder.