The catecholaminergic and serotonergic neurotransmitter systems are implicated in the pathophysiology of attention-deficit/hyperactivity disorder (ADHD). The amino acid tyrosine is the precursor for ...synthesis of the catecholamines dopamine and norepinephrine, while tryptophan is the precursor of serotonin. A disturbed transport of tyrosine, as well as other amino acids, has been found in a number of other psychiatric disorders, such as schizophrenia, bipolar disorder and autism, when using the fibroblast cell model. Hence, the aim of this study was to explore whether children with ADHD may have disturbed amino acid transport.
Fibroblast cells were cultured from skin biopsies obtained from 14 boys diagnosed with ADHD and from 13 matching boys without a diagnosis of a developmental disorder. Transport of the amino acids tyrosine, tryptophan and alanine across the cell membrane was measured by the cluster tray method. The kinetic parameters, maximal transport capacity (V(max)) and affinity constant (K(m)) were determined. Any difference between the two groups was analyzed by Student's unpaired t-test or the Mann Whitney U test.
The ADHD group had significantly decreased V(max) (p = 0.039) and K(m) (increased affinity) (p = 0.010) of tryptophan transport in comparison to controls. They also had a significantly higher V(max)of alanine transport (p = 0.031), but the Km of alanine transport did not differ significantly. There were no significant differences in any of the kinetic parameters regarding tyrosine transport in fibroblasts for the ADHD group.
Tryptophan uses the same transport systems in both fibroblasts and at the blood brain barrier (BBB). Hence, a decreased transport capacity of tryptophan implies that less tryptophan is being transported across the BBB in the ADHD group. This could lead to deficient serotonin access in the brain that might cause disturbances in both the serotonergic and the catecholaminergic neurotransmitter systems, since these systems are highly interconnected. The physiological importance of an elevated transport capacity of alanine to the brain is not known to date.
Human fibroblast cells are an advantageous model to study the transport of amino acids across cell membranes, since one can control the environmental factors. A major problem in all earlier studies ...is the lack of precise and detailed knowledge regarding the expression and functionality of tyrosine transporters in human fibroblasts. This motivated us to perform a systematic functional characterization of the tyrosine transport in fibroblast cells with respect to the isoforms of system-L (LAT1, LAT2, LAT3, LAT4), which is the major transporter of tyrosine. Ten (
n
=
10) fibroblast cell lines from healthy volunteers were included in the study. Uptake of L-U-
14C tyrosine in fibroblasts was measured using the cluster tray method in the presence and absence of excess concentrations of various combinations of inhibitors. This study demonstrated that LAT1 is involved in 90% of total uptake of tyrosine and also around 51% of alanine. Not more than 10% can be accounted for by LAT2, LAT3 and LAT4 isoforms. LAT2 seems to be functionally weak in uptake of tyrosine while LAT3 and LAT4 contributed around 7%. 10% could be contributed by system-A (ATA2 isoform). Alanine consequently inhibited the tyrosine transport by up to 60%. Tyrosine transport through the LAT1 isoform has a higher affinity compared to system-L. In conclusion, the LAT1 isoform is the major transporter of tyrosine in human fibroblast cells. Competition between tyrosine and alanine for transport is shown to exist, probably between LAT1 and LAT2 isoforms. This study established fibroblast cells as a suitable experimental model for studying amino acid transport defects in humans.
To further evaluate the controversial putative association between a Ser9Gly variant in the first exon of the dopamine D3 receptor gene (DRD3) and schizophrenia.
Swedish patients with schizophrenia ( ...n=156) and control subjects ( n=463) were assessed for the DRD3 Ser9Gly variant. Meta-analyses including previous and the present Swedish case-control results were performed.
No significant difference between the Swedish patients and controls were found, but there was an association between DRD3 Ser9Gly Ser/Ser and homozygous genotypes and response to anti-psychotic drugs. This finding was supported by an incomplete meta-analysis. In a meta-analysis of all case-control studies comprising 8761 subjects the association between DRD3 Ser9Gly homozygosity and schizophrenia ( =4.96, degree of freedom=1, p <0.05, odds ratio=1.10, 95% confidence interval=1.01-1.20) persisted. However, the previously proposed association between the Ser/Ser genotype and schizophrenia was not significant (chi2 =2.71, degree of freedom=1, p>0.05, odds ratio=1.08, 95% confidence interval=0.99-1.17).
Whereas the present Swedish case-control analysis did not yield any evidence for association with the diagnosis, the present meta-analysis suggests that the DRD3 gene confer susceptibility to schizophrenia. Reasons for the discrepancies between prior studies are discussed.
Schizophrenia involves neural catecholaminergic dysregulation. Tyrosine is the precursor of catecholamines, and its major transporter, according to studies on fibroblasts, in the brain is the L-type ...amino acid transporter 1 (LAT1). The present study assessed haplotype tag single-nucleotide polymorphisms (SNPs) of the SLC7A5/LAT1 gene in 315 patients with psychosis within the schizophrenia spectrum and 233 healthy controls to investigate genetic vulnerability to the disorder as well as genetic relationships to homovanillic acid (HVA) and 3-methoxy-4-hydroxyphenylglycol (MHPG), the major catecholamine metabolites in the cerebrospinal fluid (CSF). Moreover, the involvement of the different isoforms of the system L in tyrosine uptake and LAT1 tyrosine kinetics were studied in fibroblast cell lines of 10 patients with schizophrenia and 10 healthy controls. The results provide suggestive evidence of individual vulnerability to schizophrenia related to the LAT1 SNP rs9936204 genotype. A number of SNPs were nominally associated with CSF HVA and MHPG concentrations but did not survive correction for multiple testing. The LAT1 isoform was confirmed as the major tyrosine transporter in patients with schizophrenia. However, the kinetic parameters (maximal transport capacity, affinity of the binding sites, and diffusion constant of tyrosine transport through the LAT1 isoform) did not differ between patients with schizophrenia and controls. The present genetic findings call for independent replication in larger samples, while the functional study seems to exclude a role of LAT1 in the aberrant transport of tyrosine in fibroblasts of patients with schizophrenia.
Autism is a developmental, cognitive disorder clinically characterized by impaired social interaction, communication and restricted behaviours. The present study was designed to explore whether an ...abnormality in transport of tyrosine and/or alanine is present in children with autism. Skin biopsies were obtained from 11 children with autism (9 boys and 2 girls) fulfilling the DSM-IV diagnostic criteria for autistic disorder and 11 healthy male control children. Transport of amino acids tyrosine and alanine across the cell membrane of cultured fibroblasts was studied by the cluster tray method. The maximal transport capacity,
V
max and the affinity constant of the amino acid binding sites,
K
m, were determined. Significantly increased
V
max for alanine (
p
=
0.014) and increased
K
m for tyrosine (
p
=
0.007) were found in children with autism. The increased transport capacity of alanine across the cell membrane and decreased affinity for transport sites of tyrosine indicates the involvement of two major amino acid transport systems (L- and A-system) in children with autism. This may influence the transport of several other amino acids across the blood–brain-barrier. The significance of the findings has to be further explored.
Aberrant tyrosine transport is a repeated finding in fibroblasts from schizophrenic patients. The transport aberration could lead to disturbances in the dopaminergic and noradrenergic ...neurotransmitter systems. Tyrosine and tryptophan are the precursors of the neurotransmitters dopamine and serotonin. Disturbed dopaminergic, noradrenergic and serotoninergic systems are implicated as causes of bipolar disorder. Hence, the aim of this study was to explore whether patients with bipolar disorder have an aberrant transport of tyrosine and/or tryptophan. Fibroblast cell lines from patients with bipolar type-1 disorder (
n
=
10) and healthy controls (
n
=
10) were included in this study. All patients fulfilled the DSM-IV diagnostic criteria. The transport of amino acids across the cell membranes was measured by the cluster tray method. The kinetic parameters, maximal transport velocity (
V
max
) and affinity constant (
K
m
) were determined. A significantly lower
V
max
for tyrosine (
p
=
0.027) was found in patients with bipolar type-1 disorder in comparison to healthy controls. No significant differences in
K
m
for tyrosine and in the kinetic parameters of tryptophan between patients with bipolar type-1 disorder and healthy controls were observed. The decreased tyrosine transport (low
V
max
) found in this study may indicate less access of dopamine in the brain, resulting in disturbed dopaminergic and/or noradrenergic neurotransmission, that secondarily could lead to disturbances in other central neurotransmitter systems, such as the serotoninergic system. However, as sample size was small in this study and an age difference between patients and controls existed, the present findings should be considered as pilot data. Further studies with larger sample number are needed to elucidate the transport aberration and the significance of these findings.
Efficacy and tolerability of Hypericum LI 160 was compared to fluoxetine and placebo in mild to moderate Major Depression (DSM-IV) in a 4-week randomized, double-blind trial. One hundred and ...sixty-three outpatients from 15 general practitioner centers received either 900 mg Hypericum LI 160, 20 mg fluoxetine, or placebo daily. Amelioration was measured by the Hamilton and the Montgomery-Asberg Depression scales. Response and remission rates and global ratings by investigators and patients were measured. Adverse event reports, laboratory screening, vital signs, physical exams and ECG were collected. No significant differences could be observed regarding efficacy measures except for remission rate (Hypericum 24%; fluoxetine 28%; placebo 7 %). Hypericum was significantly better tolerated than fluoxetine. Hypericum LI 160 or fluoxetine were not more effective in short-term treatment in mild to moderate depression than placebo.
Aberrant tyrosine transport across the fibroblast membrane, as measured by lower
V
max and/or lower
K
m is a repeated finding in patients with schizophrenia. The aim of this study was to investigate ...the importance of two major transporters, the L- and A-systems and tyrosine transport in fibroblast cell lines from patients with schizophrenia and healthy volunteers. Fibroblast cell lines,
n
=
6 from healthy volunteers and
n
=
6 from patients with schizophrenia, were included in the study. Uptake of 14-C
l-tyrosine in fibroblasts was measured using the cluster tray method in absence and presence of inhibitors. The uptake of tyrosine by the L-system was evaluated with the inhibitor 2-aminobicyclo heptane-2-carboxylic acid (BCH) and the A-system with the inhibitor nonmetabolized methyl-aminoisobutyric acid (MeAIB). Using 14-C MeAIB the functionality of system A isoform 2, ATA2, was tested. BCH inhibited the uptake of tyrosine with 90%, showing that tyrosine transport in fibroblasts is mainly transported by the L-system. Not more than 10% could be contributed by the A-system. Excess of MeAIB did not influence tyrosine kinetics. Moreover, MeAIB kinetics did not differ between the patients and the controls. In conclusion, aberrant tyrosine transport observed in patients with schizophrenia is probably linked to the one of the L-systems and does not seem to involve the ATA2 transporter.
Compared to healthy controls, unmedicated schizophrenic patients had significantly higher plasma concentrations of taurine, methionine, valine, isoleucine, leucine, phenylalanine, and lysine. Except ...for taurine, these amino acids share the L-transport system for neutral amino acids. In the patients, homovanillic (HVA) acid levels in CSF were decreased and the plasma levels of the amino acids competing with tyrosine and tryptophan for transport into the brain, were all negatively correlated to the CSF concentrations of HVA and 5-HIAA. These findings could be explained by a change in the affinity of the L-system or by a decrease in its overall capacity in schizophrenia. Raised plasma levels of the competing amino acids may limit the brain uptake of tyrosine, leading to a diminished dopamine turnover, and resulting in a compensatory development of supersensitive dopamine receptors.
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