Couple of very interesting tidbits here that might helpfully improve our futures. For one, it might help target medications for certain features of depression long term, such as destruction of ability to feel motivation. In particular as a side-effect of some antidepressants, a long term reduction of glutamate is observed, which may either require modifying current antidepressants or supplementing them to consider this kind of finding.

Some additional considerations is that this further supports ketamine as helpful for a lot of people experiencing low motivation. Ketamine, as suggested here, is a "rapid acting" sort of antidepressant that acts on glutamate. So the mechanism would directly be in probably improving innate motivations by increasing the amount of free glutamate in the brain. As the review states about 1/3rd of participants generally do not improve with any antidepressant, but Ketamine and glutamate regulation shows considerable and rapid effect for even treatment resistant people. As this neurology study might demonstrate, this may be due to long term inhibition of the function of glutamate by other antidepressants.

Finally, competition was the primary motivating factor in action. Of course this is on healthy participants. Perhaps - and this would be interesting - in some sense parts of depression could be facilitated by a lacking of fair and interesting social competition for people. A primary motivating factor in many for seeking pleasure in gaming, in lieu of other aspects of life, could be a direct response to a perceived or real lack of success or ability to succeed in real life, leading to a lack of motivation to pursue things in real life.

I certainly hope we soon have more research on this and, if it's anything like my guesses here, regulating glutamate might be the kind of long-term cure-all we all really need. Of course there are many worrying side-effects from recreational or high dose users you can review here, and probably in future other alternatives that have fewer risk of allergic reaction and CNS symptoms (low/high BP, etc) can be developed.

As a final note, there are glutamate antagonists used for parkinson's. There are interesting neurological overlaps with parkinson's and depression, with early expression of the illness also exhibiting as depression. Low doses of antagonists for parkinson's may in future prove effective if modified for the purpose of treating motivation effects of depression. As some studies in 2012 and later have suggested.