I was just reading a free issue of "Delicious Living" (deliciouslivingmag.com), distributed by my local supermarket. I have no idea how reliable a source this is, but it had some statements that relate to your first question. In an article about dealing with depression, it quoted a nutritional consultant saying that "...folic acid helps maintain healthy levels of neurotransmitters in the brain, including the neurotransmitter serotonin, a hormone that affects mood."
It also quoted the same nutritional consultant saying "The artificial sweetener aspartame can block the formation of serotonin and can cause headaches, insomnia, and depression in individuals who may be serotonin-deprived."