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Cerebral serotonin regulation by phenylalanine analogues and during hyperphenylalaninemia
Insights
Severe hyperphenylalaninemia in infant rats reduces brain serotonin. This deficiency is caused by impaired tryptophan transport and tryptophan hydroxylase suppression, but can be reversed by tryptophan injection.
Area of Science:
- Neuroscience
- Biochemistry
- Pharmacology
Background:
- Phenylketonuria (PKU) is a genetic disorder characterized by severe hyperphenylalaninemia.
- Brain serotonin (5-HT) synthesis is dependent on tryptophan availability and tryptophan hydroxylase (TPH) activity.
- Understanding the mechanisms of 5-HT deficiency in hyperphenylalaninemia is crucial for potential therapeutic interventions.
Purpose of the Study:
- To investigate the mechanisms underlying cerebral serotonin deficiency in rat models of hyperphenylalaninemia.
- To differentiate the roles of tryptophan transport and TPH activity in 5-HT reduction.
- To evaluate the efficacy of tryptophan supplementation in restoring 5-HT levels.
Main Methods:
- Infant rats were treated with p-chlorophenylalanine (p-cl phe) and/or phenylalanine (phe), or alpha-methylphenylalanine (alpha-mephe) and phe.
- Brain tryptophan concentrations, TPH activity, and serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels were measured.
- The effects of tryptophan injection on 5-HT levels were assessed in hyperphenylalaninemic rats.
Main Results:
- p-cl phe-induced hyperphenylalaninemia did not lower brain tryptophan but reduced 5-HT due to TPH suppression.
- alpha-mephe alone suppressed TPH activity transiently; however, alpha-mephe plus phe led to 5-HT reduction via hyperphenylalaninemia and inhibited tryptophan transport.
- Tryptophan injection successfully restored cerebral 5-HT levels despite persistent hyperphenylalaninemia.
Conclusions:
- Cerebral serotonin deficiency in hyperphenylalaninemia models can result from TPH suppression or impaired tryptophan transport.
- The specific mechanism of 5-HT reduction depends on the pharmacological agents used to induce hyperphenylalaninemia.
- Tryptophan supplementation represents a viable strategy to counteract serotonin deficits in hyperphenylalaninemia.
Abstract:
Severe hyperphenylalaninemia induced in infant rats by 3 days of treatment with p-chlorophenylalanine (p-cl phe) plus phenylalanine (phe) did not lower the tryptophan concentration of the brain, and the cerebral serotonin (5-HT) deficiency was attributable entirely to the known suppression to tryptophan hydroxylase (TPH) by p-cl phe. The decrease in 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) was thus no more pronounced than in rats which, treated with p-cl phe alone, were devoid of hyperphenylalaninemia. Suppression of TPH was found to also underlie the decrease in cerebral 5-HT caused by treatment with alpha-methylphenylalanine (alpha-mephe) alone: a 22% loss of midbrain TPH activity was detectable 24 hr after an injection only, reverted toward the normal during the next 2 days, and was clearly unrelated to the weak competitive inhibition of the enzyme by alpha-mephe in vitro. However, alpha-mephe (unlike p-cl phe), when administered together with phe, did not suppress TPH, nor did it counterbalance the reduction of cerebral tryptophan uptake by excess phe. Thus the 5-HT diminution in the rat model of phenylketonuria produced by treatment with alpha-mephe plus phe was attributable to hyperphenylalaninemia and the inhibition of tryptophan transport to the brain. Injection of tryptophan was found to restore the cerebral 5-HT level in the face of persistently severe hyperphenylalaninemia.