Tinnitus has a weird direct connection to NMDA. There were efforts to develop an antagonist (neramexane) as a treatment, unfortunately the trial failed. Strangely many, including me, get often an actual worsening of tinnitus while using an antagonist (or, to be exact, probably when passing a certain threshold. While under stress, a tiny dose of memantine would help, but any stronger antagonist makes it worse again- indicating a non-linear somewhat).
Somehow I think now that the NMDA system is far too complex to just speak about under- or overactivity. Probably it has more to do with balancing of receptor activity (there are even 5 or so subunits) throughout the brain, in relation to overall glutamate (you'll never have all NMDARs blocked or you'd be in anaesthesia, and antagonism leads to release of more glutamate through probably autoreceptors, which then at least causes more AMPAR activation, but maybe also a shift in the activity of the non-blocked NMDARs etc). What does this mean? I can't say.
Edited by dopamimetiq, 09 March 2016 - 12:27 PM.