The causes of autism are widely debated, but a substantial body of research exists that shows differences between autistic and non-autistic individuals in the structure of the brain, 'electrical wiring' and the level of certain neurotransmitters in blood and spinal fluid. One such neurotransmitter is serotonin (5) which is instrumental in the control of sleep, mood, some types of sensory perception, body-temperature regulation and appetite. It also affects the rate at which hormones are released and has something to do with tissue inflammation. Individuals with Autism are known to have increased levels of serotonin in their blood. They produce, absorb or metabolize serotonin differently than those who are not autistic.
Researchers have found abnormalities in the synthesis of serotonin in certain structures of the brain and an overall difference in serotonin levels for the whole brain. In a study carried out at the Children's Hospital of Michigan PET Scan Center (1), researchers discovered at least some of the brain structures that have abnormal serotonin synthesis. The brain structures that make up the dentatothalamocortical pathway were found to have abnormal serotonin synthesis. This is a neural pathway involved in sensory integration and speech. Sensory integration in autistic patients is often very poor so that only one type of sense at a time can be processed. For example, if an individual with autism is looking at something, s/he will not necessarily notice that a fire alarm is ringing or someone is calling his/her name. In addition to poor sensory integration, verbal communication is also impaired in autistic individuals.
The values for overall serotonin synthesis in the brains of autistic patients are also significantly different. Dr. Harry Chugani, the Director of the Children's Hospital of Michigan PET Scan Center (1) reports that "[I]n nonautistic children, the values for serotonin synthesis change during development, such that the values for children between 3 and 8 years old are three times higher than those measured in adults. In the autistic children between the ages of 3 and 8, serotonin synthesis values were only 1 * higher than the adult values." This would suggest that especially for autistic children, medication that mimics serotonin would be helpful in targeting specific behaviors that are characteristic of autism.
Not all behaviors associated with autism are affected by changes in the values of serotonin synthesis, but those that arise from reduced 5-HTT (3) (the serotonin transporter) such as aggression and repetitive behavior are alleviated. In the November, 1999 Archives of General Psychiatry (4) , a study was published which found that autistic adults who were given a diet low in the amino acid tryptophan (an amino acid that the body converts to serotonin) experienced an increase in the intensity of autism behaviors displayed such as pacing, rocking, and self-harm. It was observed that their ability to interact socially did not change, but that they were more anxious.
The same research group did a similar study in which fluvoxamine (Luvox) was given to autistic adults. This drug increases serotonin levels in the brain. When taking fluvoxamine, these individuals were less aggressive and more adept at reciprocal communication.
Serotonin has a definite connection with the severity of autistic symptoms. Changes in the levels of serotonin synthesis do not seem to affect all symptoms, but instead target certain behaviors such as self-harm or withdrawal. It's likely that these behaviors are coping mechanisms for weak or overwhelmingly strong sensations that are alleviated with higher values of serotonin synthesis. When sensory integration is improved, the need for the coping mechanisms are reduced.
The brain is very adaptable (5) , so why is it that serious injury, illness, and strokes can be overcome without the pervasive developmental difficulties associated with autism? Serotonin and other neurotransmitters contribute to the ability of neurons to communicate with each other, the rest of the Central Nervous System and the body.
Differences in the levels of these chemicals are detected in a range of psychiatric disorders so there is nothing unusual in finding that a seemingly minor change in the level of a chemical can so drastically impair the ability of neurons to communicate effectively.
2) Evidence of Linkage between the Serotonin Transporter and Autistic Disorder
3) Serotonin Transporter (5-HTT) gene variants associated with autism?
4) Autism: The Serotonin Connection
4)Pervasive Developmental Disabilities