http://archpsyc.ama-assn.org/cgi/content/abstract/63/8/856
Basically that gives some credit to the theory that high levels of free glutamic acid (which is the metabolite of MSG) affects depression.
Im not in the hospital ATM and ive not got my account set up yet to let me onto the medical trial databases etc so I cant do a proper search, but simply theoretically knowing that glutamic acid levels affect dopamine and prolactin levels directly you should be able to see there will be a depressant affect, amongst others.
Hawker
Let me guess, you got that link from here
http://www.msgtruth.org/depress.htm