Background: The catecholaminergic and serotonergic neurotransmitter systems are implicated in the\r\npathophysiology of attention-deficit/hyperactivity disorder (ADHD). The amino acid tyrosine is the precursor for\r\nsynthesis of the catecholamines dopamine and norepinephrine, while tryptophan is the precursor of serotonin.\r\nA disturbed transport of tyrosine, as well as other amino acids, has been found in a number of other psychiatric\r\ndisorders, such as schizophrenia, bipolar disorder and autism, when using the fibroblast cell model. Hence, the aim\r\nof this study was to explore whether children with ADHD may have disturbed amino acid transport.\r\nMethods: Fibroblast cells were cultured from skin biopsies obtained from 14 boys diagnosed with ADHD and from\r\n13 matching boys without a diagnosis of a developmental disorder. Transport of the amino acids tyrosine,\r\ntryptophan and alanine across the cell membrane was measured by the cluster tray method. The kinetic\r\nparameters, maximal transport capacity (Vmax) and affinity constant (Km) were determined. Any difference between\r\nthe two groups was analyzed by Student�s unpaired t-test or the Mann Whitney U test.\r\nResults: The ADHD group had significantly decreased Vmax (p = 0.039) and Km (increased affinity) (p = 0.010) of\r\ntryptophan transport in comparison to controls. They also had a significantly higher Vmaxof alanine transport (p =\r\n0.031), but the Km of alanine transport did not differ significantly. There were no significant differences in any of\r\nthe kinetic parameters regarding tyrosine transport in fibroblasts for the ADHD group.\r\nConclusions: Tryptophan uses the same transport systems in both fibroblasts and at the blood brain barrier (BBB).\r\nHence, a decreased transport capacity of tryptophan implies that less tryptophan is being transported across the BBB\r\nin the ADHD group. This could lead to deficient serotonin access in the brain that might cause disturbances in both\r\nthe serotonergic and the catecholaminergic neurotransmitter systems, since these systems are highly interconnected.\r\nThe physiological importance of an elevated transport capacity of alanine to the brain is not known to date.
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