Large neutral amino acids levels in primate cerebrospinal fluid do not confirm competitive transport under baseline conditions.

In rodents, transport of large neutral amino acids (LNAAs) across the blood brain barrier (BBB) and blood-cerebrospinal fluid (CSF) barrier is mediated by high affinity carriers. Net brain LNAA levels are thought to be determined mainly by this competitive transport from plasma. Since the affinity for LNAA transport at the BBB in primates is considerably higher than in rodents, brain influx and by extension LNAA brain levels, should be even more dependent on competitive transport. Given that LNAA levels in CSF and brain interstitial fluid are usually similar, we analyzed serum and CSF of fasted subjects (n=24) undergoing spinal anesthesia and calculated brain influx and transporter occupancy using a conventional model of transport. Despite predicted near-full transporter saturation (99.7%), correlations between CSF levels and brain influx were modest, limited to tyrosine (r=0.60, p<0.002) and tryptophan (r=0.50, p<0.01) and comparable to correlations between CSF and serum levels. We also analyzed serum and CSF in (n=5) fasted vervet monkeys. Tyrosine and phenylalanine levels in CSF were positively correlated with those in serum, but correlations with calculated brain influx, which takes competition into account, were weaker or absent. We conclude that in primates i) baseline CSF LNAA levels do not confirm competitive transport, ii) brain LNAA levels should not be estimated on the basis of serum indices alone. This has implications for amino acid challenge studies and for neuropsychiatric disorders associated with dysregulated LNAA transport in which quantitative information about brain LNAA levels is needed.

Relationships between large neutral amino acid levels in plasma, cerebrospinal fluid, brain microdialysate and brain tissue in the rat.

Experimental limitations may preclude direct measurement of large neutral amino acid (LNAA) levels in brain tissue. Some data suggest that serum or cerebrospinal fluid (CSF) may provide an index of LNAA brain levels. We examined this in a series of experiments in rats, administering tyrosine, phenylalanine or valine IP 60min prior to harvesting of blood, CSF or brain tissue or during in vivo microdialysis of the brain. Serum indices of the administered LNAA generally showed a significant (r>0.8) correlation with brain tissue levels but the linear relationships varied significantly across brain regions, the LNAA and its dose. Increases in levels of an administered LNAA were consistently greater in CSF than in brain tissue. In contrast, changes in LNAA levels in brain tissue and in vivo microdialysate were generally comparable. We confirm that changes in serum and CSF LNAA levels can support limited, qualitative inferences about changes in brain tissue LNAA levels; quantitative inferences should not be drawn without prior validation under relevant experimental conditions.

Cerebrospinal fluid concentrations of large neutral and basic amino acids in Macaca mulatta: diurnal variations and responses to chronic changes in dietary protein intake.

In rats, dietary protein intake influences brain concentrations of tryptophan, tyrosine, and other large neutral amino acids (LNAAs) and the neurotransmitters to which they are linked. Few experiments have examined these dietary protein-amino acid relationships in nonhuman primates, in relation to time of day or dietary protein content. We therefore examined the effect in monkeys of changes in chronic protein intake on 24-hour plasma and cerebrospinal fluid (CSF) concentrations of LNAAs (tyrosine, phenylalanine, branched-chain amino acids) and basic amino acids. Juvenile male monkeys (Macaca mulatta) consumed for sequential 4-week periods diets differing in protein content (approximately 23% --> approximately 16% --> approximately 10% --> approximately 6% protein [percentage of energy]). The daily ration was presented as a morning meal of fruit and an afternoon meal of fruit and a commercial diet to mimic feeding patterns in the wild. During week 4 on each diet, blood and CSF were sampled repeatedly over a 48-hour period via indwelling catheters. Plasma and CSF LNAA concentrations varied markedly with time of day and dietary protein content, showing up to 4-fold variations. Diurnal variations in plasma and CSF basic amino acids were smaller in magnitude and generally not strongly linked to dietary protein content. A measure of the competitive transport of LNAAs across the blood-brain barrier, calculated using plasma concentrations of the LNAAs and their blood-brain barrier kinetic constants, predicted the observed CSF concentration of each LNAA examined remarkably well, except for phenylalanine. Based on observations in rats, the variations in the CSF concentrations of the LNAAs in monkeys may be large enough to influence metabolic and signaling pathways in brain to which they have been linked.

Reduction of large neutral amino acid concentrations in plasma and CSF of patients with maple syrup urine disease during crises.

Neurological dysfunction is common in patients with maple syrup urine disease (MSUD). However, the mechanisms underlying the neuropathology of this disorder are poorly understood. We determined the concentrations of all amino acids in plasma of patients with MSUD during crises (with severe CNS symptoms) and after recovery in the hope of detecting possible alterations of these levels during metabolic decompensation. Blood samples obtained from 11 children with MSUD aged 1 month to 7 years and from 10 age-matched controls (5 months to 6 years) with no evidence of metabolic disease were examined for their amino acid content by high-performance liquid chromatography. We observed that leucine, isoleucine and valine concentrations were respectively 30, 9 and 3 times higher than normal values, whereas the concentrations of the large neutral amino acids (LNAA) phenylalanine, tyrosine, tryptophan and methionine were significantly lower during metabolic decompensation as compared to the controls. In addition, concentrations of leucine, but not of valine or isoleucine, were inversely related to the LNAA concentrations in plasma. The concentrations of these amino acids in plasma returned to normal values when patients were clinically well. CSF amino acid concentrations also showed decreased amounts of LNAA and increased concentrations of branched-chain amino acids. It is possible that the decrease in plasma concentrations of LNAA may lead to a deficit of these essential amino acids in the brain as well as of their products such as proteins and neurotransmitters, a fact that might be related to the neurological dysfunction of MSUD.