Ketamine and presumably MXE aren't something I would really look at for depression despite all the recent studies that have come out. There is a lot of evidence pointing to negatives for sustained usage, especially in abuse settings. There's also the fact that some people that browse this forum might take this info as an excuse to run out and k-hole themselves, so right now I'll say that this is a very bad idea until some sort of peer reviewed study comes out regarding an appropriate dosing regime (yes I know researchers some have posted short term human trials). So, there might be some novel antidepressants that come out in the next decade that use this mechanism of action if it all pans out.
http://www.nature.com/clpt/journal/v91/n2/full/clpt2011244a.html Review of the current relationship between ketamine and depression.
Summed up: some types NMDA antagonism are associated with improvements in depression, but
ketamine is a dirty as hell drug so its surprising efficacy could just as easily be a result of its AMPA potentiating actions or a combination of various mechanisms. In fact in most animal studies NBQX (a AMPA antagonist) blocked its antidepressant effects. Zinc and magnesium do block NMDAR in addition to more than a handful of other actions in the body, but I'll say short of cases of deficiency, megadoses won't be of much help for tolerance or depression though a multivitamin a day never hurts.
But, rather than deal with the legal issues associated with ketamine. Why not drink some cough syurp!?!? But, seriously DXM if not OTC and unpatentable would probably be pushed harder than anything else as a new antidepressant. Its safe, well characterized, has multiple relevant mechanisms of action, and on paper looks like the next best thing aside from getting off the couch and eating right you could do for depression.
http://www.ncbi.nlm.nih.gov/pubmed/22401777
http://www.ncbi.nlm.nih.gov/pubmed/22339643
http://www.sciencedirect.com/science/article/pii/S0306987711002246