Serotonin (5-Hydroxytryptanamine) originates in the midbrain region where the cerebral hemispheres and thalamus-hypothalamus are bridged to the spinal cord, and distributed throughout the brain. The thalamus regulates sensory integration and motor integration. The hypothalamus regulates body temperature, emotions, hunger, thirst, and Circadian rhythms. Many of the functions associated with these structures are affected in autism. For example, poor sensory integration and dysfunctional emotional behavior are characteristic of autism. It's difficult to know whether differences in the structure of the thalamus and the hypothalamus cause these symptoms or if they affect production of serotonin levels which in turn triggers autistic symptoms.
Autism is a complicated disorder that affects many different aspects of a person and cannot be traced to one unique cause. Autism is increasingly thought to have causes rooted in a range of problems. Research has uncovered evidence of genetic abnormalities, viruses are thought by some to cause autism, as are toxins that pollute the environment. Autism is also attributed to physical abnormalities of various brain structures. These can be divided into two classifications: neural structure differences and abnormal brain chemistry. (2) The connection between the latter two is most significant to an understanding of serotonin production and synthesis. The former possible causes are likely to cause the abnormalities in neural structure and brain chemistry. These abnormalities in turn bring about the behaviors that are characteristic of autism. Serotonin levels are only one of autism's many contributing factors. This neurotransmitter is not necessarily a cause, however, but abnormal levels of serotonin can be understood as a primary symptom that is the base for observable symptoms. Therefore, drug therapy can limit certain symptoms that are related to serotonin levels, but it does not eliminate the causes of autism.
The role of serotonin is to regulate mood, regulate food intake by stimulating GIT smooth muscles, regulate sleep, and to stimulate or inhibit the release of various hormones. (2) As can be seen, serotonin functions in a variety of ways and works in conjunction with other chemicals. One implication of this multi-functioning capacity is that we cannot accurately determine how much to alter serotonin production, absorption or synthesis in order to produce the desired effect. Because many different brain structures with varying functions are affected in autism and symptoms vary from individual to individual, drugs affecting the serotonergic system can only be part of how autism is dealt with.
Some controversy over the relevance of serotonin levels to autism exists. A study published in the November Archives of General Psychiatry, researchers found that low levels of serotonin . (3) in adults contributed to a lessening in the intensity of autistic symptoms. Other researchers have the idea that low levels of serotonin would have the opposite effect and induce abnormal mood regulation. Celia M. Bibby states that autistic children have abnormally high levels of serotonin and that high levels of serotonin in general cause more intense autistic symptoms (4,5) . It's important to remember, however, that normal serotonin change according to age and are generally higher in children than they are in adults.
In the studies described in the earlier report, serotonin levels had varying affects on the patients that were part of the studies; however, both children and adults with autism have been known to benefit from drug therapy that affects serotonin levels in the blood. "There will always be a part of the brain that uses serotonin in a different way, a receptor that activates neurons differently." (5) The needs of autistic patients vary so widely that it is difficult to determine what the exact effects of serotonin levels will be in autism patients.
2) Overview of Autism: Causes ,
3) Autism: the Serotonin Connection ,
4) Autism: A Conditioned Response to Biochemical Toxicity?" ,
5) Mechanisms of LSD: a Glimpse into the Serotonergic System ,