How NAC might help with depression, bipolar disorder, OCD, and anxiety

There is actually some decent evidence that NAC does a good job. So how does it work?

NAC is N-acetyl cysteine. If your brain gets stressed out, cysteine gets depleted which causes multiple problems. NAC can give your brain the cysteine it may need. Cysteine can also help reduce glutamate levels. Glutamate is an excitatory neurotransmitter, whose levels may be too high in bipolar disorder & OCD & anxiety. 

[NAC] is an easy way to deliver an amino acid called cysteine, which otherwise does not easily reach brain cells. … Glutathione (GSH, L-gamma-glutamyl-L-cysteinylglycine) is the predominant anti-oxidant in the cytoplasm of cells. Virtually all cells require glutathione for viability and function. Glutathione is synthesized from three amino acids in a two-step process, beginning with the combination of glutamic acid & cysteine and ending with the addition of glycine. … Glycine & glutamic acid are plentiful in cells, so it is the availability of cysteine that controls the reaction rate. 
http://www.psycheducation.org/depression/meds/n-acetylcysteine.htm

Cysteine assists in the regulation of neuronal intra- and extracellular exchange of glutamate through the cystine–glutamate antiporter. Whereas this antiporter is ubiquitous throughout all cell types, in the brain it is preferentially located on glial cells. The dimer, cystine, is taken up by astrocytes and exchanged for glutamate, which is released into the extracellular space. This free glutamate appears to stimulate inhibitory metabotropic glutamate receptors on glutamatergic nerve terminals and thereby reduce the synaptic release of glutamate. 
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3044191/