From http://www.gaps.me/preview/?page_id=20 :
Gluteomorphins & Casomorphins or opiates from gluten and casein.
Gluten is a protein present in grains, mainly wheat, rye, oats, barley. Casein is a milk protein, present in cow, goat, sheep, human and all other milk and milk products. In the bodies of children and adults with autism and schizophrenia these proteins do not get digested properly due to the fact that their digestive systems are full of abnormal microbial flora and hence unhealthy. As a result of misdigestion gluten and casein turn into substances with similar chemical structure of opiates, like morphine and heroin. There has been quite a substantial amount of research done in this area by Dohan, Reichelt, Shattock, Cade and others, where gluten and casein peptides, called gluteomorphins and casomorphins, were detected in the urine of schizophrenic patients and autistic children. Incidentally, these substances were also found in patients with depression and rheumatoid arthritis. These opiates from wheat and milk get through the blood-brain barrier and block certain areas of the brain, just like morphine or heroin would do, causing various neurological and psychiatric symptoms. Based on this research the gluten and casein free diet (GFCF diet) has been developed.