(Adapted from a FB post, and also further input from comments on that post.)
There is certainly a glutamate involvement in Autism.
Reducing free glutamate (most glutamate is "bound" within foods and so is fine) is very important to controlling symptoms in at least some ASD (Autism Spectrum Disorder) cases, if not all. But I've got no data on the prevalence.
Walsh mentions glutamate but to my surprise it's to do with reduced glutamate activity at NMDA receptors (Nutrient Power p. 115).
However I think the effect I'm mentioning here (of free glutamate) doesn't contradict that, but it is an entirely separate mechanism driven by an overly permeable blood-brain barrier, driven in turn by an overly permeable gut barrier.
For anyone dealing with Autism who's not seen it, check out the TEDx talk by a biochemist who finally resolved what I assume was PDD-NOS in her daughter. It's compelling (and for those of us with PDD-NOS kids, very emotional viewing):
To very, very briefly summarise that video: Gluten contains a lot of free glutamate. So does casein (both those also have many separate effects on gut health and other things of course).
Reducing it drastically can have a profound affect on ASD children. There's plenty more information about this on the Internet but the bottom line is that processed foods are the main source (though walnuts are pretty high, as is broccoli. FYI Soy sauce is off the charts high).
Free glutamate can over-stimulate neurons and result in manic and OCD behaviours commonly seen in ASD.
Theanine is a very interesting supplement with regards to free glutamate. It's mainly found in tea ("normal" tea; Camellia chinensis) and is considered very safe since it's just a simple amino acid and shows almost no toxicity: https://examine.com/supplements/theanine/#summary7-1
“In animal research, L-theanine appears to be remarkably safe as very high doses have failed to cause toxicity".
The mechanism of action is essentially to slow glutamine release in the NMDA receptors, but also a very mild competitive inhibition of glutamate uptake in the gut.
The effect is calming but yet not sleep inducing. In fact it's popular to "stack" it with caffeine or other stimulants (I really like it with my Bulletproof coffee in fact!) to get a smoother "lift".
I first came across it on a blog post by an ND who was recommending it for helping with sleep problems in small children (it aids sleep onset but probably not sleep maintenance).
Note there is "Suntheanine" which is simply a brand name of the process used to isolate the L form (many molecules come in two mirror images, the L form of Theanine is the one needed for the effect) thereby giving a pure supplement. It's apparently the form usually used in studies.
It's important to note that for almost any stimulus, some people will react in completely the opposite way to the majority.
A paradoxical response to Theanine is not unknown and this article explains what is probably happening (a "GABA shunt"):
http://www.holistichelp.net/blog/how-to-increase-gaba-and-balance-glutamate/
So be wary to watch for the actual response when supplementing.