Aha, found it!

Here is what I read:

ON A PRECAUTIONARY NOTE, two amino acid chelates of magnesium (magnesium aspartate, magnesium glutamate) MUST be avoided in treating depression. Each of these amino acid complexes of magnesium has been reported to worsen depression. Aspartate and glutamate act as neurotransmitters in the brain and retina by facilitating the transmission of from neuron to neuron. Too much aspartate or glutamate in the brain kills certain neurons by allowing the influx of too much calcium into the cells. This influx triggers excessive amounts of free radicals, which kill the cells. The neural cell damage that can be caused by excessive aspartate and glutamate is why they are referred to as "excitotoxins." They "excite" or stimulate the neural cells to death. Aspartate is a principal ingredient in the highly maligned Aspartame sweetener. To avoid aggravating Depression with super-sweeteners, instead use Stevia.

It's down near the bottom of this article -

magnesium article

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