P11 is a protein found in the brain of humans and other mammals which has been implicated in the regulation of mood. A mouse study from 2006 found that low levels of p11 are likely to prevent the raising of serotonin levels from immediately curing clinical depression. The protein is thus a strong candidate target molecule for new antidepressant drugs.

Reference

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Svenningsson P, Chergui K, Rachleff I, Flajolet M, Zhang X, Yacoubi ME, Vaugeois JM, Nomikos GG, Greengard P. Alterations in 5-HT1B receptor function by p11 in depression-like states. Science 2006;311(5757):77–80. PMID 16400147

See also

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Associated story on National Public Radio

Biochemicals | Mood disorders

As quoted in the article by Svenningsson etal, p11 alters the Serotonin (specifically 5-HT 1B) receptors in depression-like states (downregulated in untreated cases and upregulated in treatment with either drugs or ECT) in animal models, measured by tail suspension tests (for depression) and thigmotaxis (for anxiety); it was also seen in brain tissues (and Hela cells) in depressed patients. This indeed is a breakthrough as a target for newer drugs in depression and depression-like states; that said, this is probably just scratching the surface, but sure is a beginning.