Is rat brain content of large neutral amino acids (LNAAs) a reflection of plasma LNAA concentrations?

The phenotypic spectrum of paediatric neurotransmitter diseases and infantile parkinsonism

Paediatric neurotransmitter diseases are a group of inherited disorders attributable to a disturbance of neurotransmitter metabolism. The monoamines, catecholamines and serotonin, also called biogenic amines, are neurotransmitters with multiple roles including psychomotor function, hormone secretion, cardiovascular, respiratory and gastrointestinal control, sleep mechanisms, body temperature and pain. Given the multiple functions of monoamines, disorders of their metabolism comprise a wide spectrum of manifestations, with motor dysfunction being the most prominent clinical feature. The severity of the clinical manifestations ranges from mild to severe. Patients with severe and intermediate phenotypes may present with infantile parkinsonism that differs in a number of aspects from the parkinsonism in nigrostriatal degeneration. Analysis of monoamine metabolites and pterins in spinal fluid assists in the diagnosis of these disorders. Treatment options include tetrahydrobiopterin supplementation, L: -dopa, 5-hydroxytryptophan, and medications that potentiate monoamine transmission. Response to treatment is variable.

Aromatic L-amino acid decarboxylase deficiency: clinical features, treatment, and prognosis

BACKGROUND

Deficiency of aromatic L-amino acid decarboxylase (AADC) is associated with severe developmental delay, oculogyric crises (OGC), and autonomic dysfunction. Treatment with dopamine agonists and MAO inhibitors is beneficial, yet long-term prognosis is unclear.

OBJECTIVE

To delineate the clinical and molecular spectrum of AADC deficiency, its management, and long-term follow-up.

RESULTS

The authors present six patients with AADC deficiency and review seven cases from the literature. All patients showed reduced catecholamine metabolites and elevation of 3-O-methyldopa in CSF. Residual plasma AADC activity ranged from undetectable to 8% of normal. Mutational spectrum was heterogeneous. All patients presented with hypotonia, hypokinesia, OGC, and signs of autonomic dysfunction since early life. Diurnal fluctuation or improvement of symptoms after sleep were noted in half of the patients. Treatment response was variable. Two groups of patients were detected: Group I (five males) responded to treatment and made developmental progress. Group II (one male, five females) responded poorly to treatment, and often developed drug-induced dyskinesias.

CONCLUSIONS

The molecular and clinical spectrum of AADC deficiency is heterogeneous. Two groups, one with predominant male sex and favorable response to treatment, and the other with predominant female sex and poor response to treatment, can be discerned.

Effects of pure macronutrient ingestion on plasma tryptophan and large neutral amino acids

The role of tryptophan and its competitor large neutral amino acids, proposed earlier for serotonin synthesis following carbohydrate or protein ingestion, was reassessed in relation to a recent study investigating serotonin release, including the so far unknown effects of fats. In the present study, meals of either carbohydrates, casein, or lard, were supplied to rats for 30 min and blood samples collected every 15 min to follow the changes in plasma large neutral amino acids. In response to carbohydrates, amino acid levels fell and the ratio tryptophan over sum of other amino acids increased. Following casein ingestion, all amino acids were enhanced, tryptophan somewhat less, leading to a decreased ratio. The lard meal induced a slight decrease in some amino acids while the ratio remained constant. Only in response to casein, and partly to carbohydrates, did a consistent relation appear between the previously observed serotonin changes and the ratio. These data suggest that a relationship between the ratio and the previously observed serotonin changes is not always encountered because the release is not obligatorily coupled to synthesis and is subject to behavioral influences. It remains that serotonin release is affected by the composition of the meal through peripheral metabolic mechanisms.

Distribution volume of 3-O-methyl-6

The distribution volume (DV) of 6-[F-18]fluoro-L-DOPA (FDOPA) in the cerebellum recently has been linked using positron emission tomography (PET) to plasma large neutral amino acid (LNAA) concentrations in monkeys. In this article the authors provide additional experimental support for this relation by directly measuring the DV as the steady-state tissue to plasma radioactivity ratio in rats using a labeled LNAA analog 3-O-methyl-6-[F-18]FDOPA (OMFD), a compound that has no known specific enzyme or receptor interactions in brain tissue. The measured DV for OMFD (tissue OMFD concentration/plasma OMFD concentration) was found to be inversely related to plasma LNAA concentrations. The relation (DV = 1.5-0.00094*[LNAA], R--2 = 0.79) resulted in an 8% DV decrease per 100 nmol/mL plasma LNAA increase within the observed range of 330 to 510 nmol/mL. This was similar to recent noninvasive observations with FDOPA PET in vervet monkeys and with 6-[F-18]Fluoro-m-tyrosine PET in squirrel monkeys. The OMFD striatum to cerebellum (Str/Cb) ratio was greater than 1.0 for all measurements, averaging 1.09 +/- 0.04, and was approximately equal to the Str/Cb LNAA ratio of 1.12 +/- 0.05. This current study verifies the variation of DV of OMFD or FDOPA as a function of plasma LNAA concentrations and suggests the possibility of using OMFD for measuring cerebral LNAA noninvasively with PET.

Effects of amitriptyline on rat plasma and brain content of monoamine precursors and other large neutral amino acids

Amitriptyline caused decreased concentrations in rat plasma of the amino acids tyrosine and tryptophan, which are precursors to monoaminergic neurotransmitters, and also of those other large neutral amino acids (LNAAs) (valine, phenylalanine, leucine and isoleucine) with which tyrosine and tryptophan compete for the carrier-mediated transport from plasma into the brain. The various LNAAs were, however, not decreased to the same extent. Thus, the relative concentrations, calculated as the ratio of each one of them to the total concentration of LNAAs, were also influenced by amitriptyline. The relative concentrations of tryptophan were decreased while the relative concentrations of phenylalanine and leucine were increased. Those of tyrosine, valine and isoleucine were not influenced by amitriptyline. The relative concentrations of the LNAAs in plasma are considered to be of importance to how much of each of them that will be transported into the brain. Based on this assumption one would have expected that the amitriptyline-induced changes in the relationships between different LNAAs would have caused changes in their brain concentrations in accordance with their relative concentrations in plasma. However, amitriptyline caused decreased concentrations of all LNAAs in rat whole brain. This finding cannot be explained by altered relationships between the LNAAs but must be explained by other mechanisms. Nevertheless, this effect of amitriptyline on brain LNAA concentrations might be of importance to monoamine function.

Potential treatment of amyotrophic lateral sclerosis with gabapentin: a hypothesis

OBJECTIVE

To provide the biochemical rationale for the use of the new anticonvulsant agent gabapentin as a treatment for amyotrophic lateral sclerosis (ALS).

BACKGROUND

ALS is a neuropathologic disorder of the central nervous system characterized by a progressive loss of upper and lower motor neurons. Although the etiopathology of ALS is incompletely known, it is hypothesized that glutamatergic neurotransmission is related to neuropathology. Glutamate is an excitatory amino acid neurotransmitter that is cytotoxic when overexpressed at synaptic terminals, probably through a calcium-related mechanism. The concentration of glutamate in cerebrospinal fluid is increased in patients with ALS. The increased extracellular concentrations of glutamate may be caused by a decreased capacity of glutamate transport in brain tissue and/or abnormal glutamate metabolism. Recent success with the glutamate release inhibitor riluzole in well-controlled clinical trials supports the excitotoxic mechanism of neuropathology in patients with ALS. POTENTIAL TREATMENT FOR ALS: Gabapentin has demonstrated neuroprotective properties in a model of chronic glutamate toxicity in vitro. Although the neuroprotective mechanism of action of gabapentin is currently unknown, it is hypothesized here that gabapentin decreases the rate of formation of glutamate derived from the branched-chain amino acids (BCAAs) leucine, isoleucine, and valine. The proposed decrease in formation of glutamate from BCAAs may decrease the pool of releasable glutamate and therefore compensate for diminished glutamate uptake capacity and/or abnormal glutamate metabolism in patients with ALS.

CONCLUSIONS

Based on this rationale, it is proposed that gabapentin may provide a beneficial effect in the treatment of patients with ALS.

Relationship between plasma and brain large neutral amino acids in rats fed diets with different compositions at different times of the day

Large neutral amino acids (LNAAs) compete with each other for carrier-mediated transport through the blood-brain barrier into the brain. The relative plasma concentration, expressed as the ratio of each LNAA to the sum of LNAAs, is considered the main regulator of brain LNAA concentrations. In order to investigate the consistency of this assumption throughout a 24-h period, we have compared the relationship of plasma LNAAs to brain LNAAs among groups of rats fed diets containing various amounts of protein (in order to obtain a wide range of plasma LNAA levels) at two different phases of the light/dark cycle (0900 and 2100 hours). The relationship between plasma and brain LNAAs was found to be dependent on both diet and the time of day. Similar plasma amino acid concentrations in the morning and in the evening contrasted with different brain concentrations. Furthermore, previous findings that brain LNAA concentrations are influenced by plasma amino acid concentrations were confirmed.

Diurnal rhythms in rat brain large neutral amino acids (LNAAs), monoamines and monoamine metabolites

To obtain reliable information on the diurnal rhythms in rat brain large neutral amino acid (LNAA) concentrations, animals were killed, in groups of six, every one h throughout one 24 h period. By using such short sampling intervals, it was possible to detect a statistically significant diurnal variation in the whole brain concentration of each LNAA and to give a more detailed description of these diurnal variations than previously presented. Furthermore, the diurnal variations in whole brain concentrations of the precursor amino acids, tyrosine and tryptophan, were compared with corresponding variations in the respective monoamines and their main metabolites. Similar diurnal patterns were demonstrated for tryptophan, serotonin and 5-hydroxyindoleacetic acid. No such correspondence was detected for tyrosine, dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid.