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Whole blood and urinary levels of serotonin (5-hydroxytryptamine; 5-HT) and the derivative urinary 5-hydroxyindoleacetic acid (5-HIAA) were measured in normal and autistic subjects. An association was tested between autism and a marker coding for the 5-HT2A serotonergic receptor gene. Significant group (high urinary 5-HT and low whole blood 5-HT in autism) and age effects (urinary 5-HT decrease with age) were found. Moreover, whole blood 5-HT levels were correlated with clinical state. No differences in allele and genotype frequencies for the 5-HT2A receptor marker were found in this autistic population compared with age-matched healthy students.

We tested for an association between autism and genes coding for enzymes involved in monoaminergic metabolism and for a linked marker, c-Harvey-Ras-1 (HRAS 1), using restriction fragment length polymorphisms. We did not find evidence of an association between autism and genes coding for tyrosine hydroxylase, dopamine-beta-hydroxylase (DBH), and tryptophan hydroxylase. However, we report a positive association between autism and the locus containing the gene for HRAS-1.