L-tyrosine and L-tryptophan transport in red blood cells in normal subjects. Effects of other amino acids, temperature and medium of incubation

In vivo metabolism and partitioning of 6-[18F]fluoro-L-meta-tyrosine in whole blood: a unified compartment model

Physiological quantification of dynamic PET data requires the determination of an input function, preferably from plasma. A compartmental model relating a parent radiotracer, its radiolabelled metabolites and their exchange between plasma and erythrocytes is presented. This model allows for the time course of radioactivity measured in whole blood to be transformed into the time course of the radiotracer in plasma. The utility of this approach is illustrated with blood data collected on 30 human subjects injected with 6-[18F]fluoro-L-meta-tyrosine (FmT), a pre-synaptic dopaminergic radiotracer. A three-compartment four-parameter model is shown to yield significantly better fits to the blood data than related lower and higher order models. This model is found to be robust to measurement noise, and yet sensitive to metabolic changes induced by pretreatment with carbidopa. For FmT, the between-subject variations are shown to be small enough to warrant the use of a population-based correction; tissue time-activity curves were simulated to verify that this correction does not significantly affect the precision and accuracy of the derived rate constants. The unified blood model can be adapted for radiotracers other than FmT as long as the blood partition ratio of the parent radiotracer differs from that of its metabolites and/or the rate at which they equilibrate between plasma and erythrocytes is different.

Effects of serotonin precursors on the negative feedback effects of glucocorticoids on hypothalamic-pituitary-adrenal axis function in depression

In order to investigate the relationships between brain serotonergic turnover and hypothalamic-pituitary-adrenal (HPA) axis function in unipolar depression, the authors measured intact adrenocorticotropic hormone (ACTH) and cortisol levels in baseline conditions and after combined dexamethasone (1 mg PO) and L-5-hydroxytryptophan (L-5-HTP, 200 mg PO) administration in 13 minor, 17 simple major, and 17 melancholic subjects. L-5-HTP significantly enhanced post-DST ACTH and cortisol secretion in major--but not in minor--depressed subjects. Major depressed subjects with or without melancholia exhibited significantly higher post-DST ACTH and cortisol responses to L-5-HTP than minor depressed subjects. L-5-HTP administration converted some major depressed ACTH or cortisol suppressors into nonsuppressors. L-5-HTP stimulated ACTH or cortisol secretion to the same extent in major depressed HPA-axis suppressors and nonsuppressors. It is concluded that L-5-HTP loading may augment ACTH and, consequently, cortisol escape from suppression by dexamethasone in major but not in minor depressed subjects. The findings show that serotonergic mechanisms modulate the negative feedback of glucocorticoids on central HPA-axis regulation. It is hypothesized that the higher L-5-HTP-induced post-DST HPA-axis hormone responses in major depression reflect upregulated 5-HT2 receptor-driven breakthrough secretion of pituitary ACTH from suppression by dexamethasone.

L-tyrosine and L-tryptophan membrane transport in erythrocytes and antidepressant drug choice

In the treatment of depression, when antidepressant drug choice is made according to alterations of erythrocyte membrane transport of L-tyrosine and L-tryptophan in the individual patient, the clinical results are superior to those obtained when drugs are prescribed according to the physician's judgment. This is demonstrated by comparing three experimental groups: I, 100 patients treated in relation to their L-tyrosine and L-tryptophan transport; II, 30 patients treated according to the clinician's experience; III, 38 subjects treated against the L-tyrosine and L-tryptophan transport indications. In these groups, the frequency of patients improved by more than 70% is 77%, 47%, and 16%, respectively.

Influences on cortisol and noradrenergic turnover of healthy controls and depressed patients during L-tryptophan loading

To investigate the interrelationships between the noradrenergic and serotonergic systems and the hypothalamic-pituitary-adrenal (HPA) axis, we measured the excretion of 3-methoxy-4-hydroxyphenylglycol (MHPG) and free cortisol (UFC) in 24-h urine both before and after loading with 2 g L-tryptophan (L-TRP) orally in 24 healthy controls and 33 depressed patients categorized according to DSM-III. Loading with L-TRP reduced MHPG excretion significantly by 56% in major depressives, whereas in healthy controls and in minor depressives no significant effects were observed. Although basal MHPG did not differ among these study groups, loading with L-TRP resulted in MHPG excretion in major depressives being significantly lower than in healthy controls and minor depressives. Loading with L-TRP increased UFC significantly by 49% only in major depressives with associated features. After L-TRP those patients exhibited significantly increased UFC as compared with all other depressives and controls, whereas basal UFC values did not differ among the study groups. In baseline conditions there were no significant correlations between MHPG and UFC excretion.