This is not a comprehensive list, simply some articles I have found along the way.
“Methylation also plays a role in the GABA and glutamate balance in a variety of ways. For one, if there is impairment in the methylation pathway, then folate doesn’t get utilized and it can break down into glutamate. Additionally, if you are not methylating properly you may not be able to suppress microbes like viruses or make enough T cells to fight them off, which means they will linger around to interfere with the GAD enzyme.
Methylation may be impaired due to nutritional deficiencies, toxins, genetic mutations, or Candida overgrowth. Methylation is also heavily influenced by the Krebs cycle and vice versa, so a problem in this cycle can also impede methylation, and consequently GABA production. The Krebs cycle can also be impaired by Candida overgrowth, as well as bacterial overgrowth. If methylation is impaired, then it is even more important to manage glutamate levels.” http://www.holistichelp.net/blog/how-to-increase-gaba-and-balance-glutamate/