Modulation of cerebral catecholamine concentrations during hyperphenylalaninaemia

Approaching altered inhibitory control in phenylketonuria: A functional MRI study with a Go-NoGo task in young female adults

Subtle executive function deficits, particularly regarding inhibitory control, have been reported in patients with phenylketonuria (PKU) despite early dietary treatment. Purpose of this study was to assess whether young female adults with PKU exhibit altered neural activity underlying such deficits, particularly in a fronto-parietal cognitive control network (CCN). Behavioural data and functional magnetic resonance imaging (fMRI) data were acquired during a Go-NoGo task in 16 young adult patients with PKU and 17 control subjects. Hypothesis-driven analyses of behavioural and fMRI data in the CCN were supplemented by exploratory whole brain activation analyses. PKU patients exhibited a trend towards higher errors of commission. Patients exhibited marginally increased activation associated with inhibitory control in only one CCN core region (right middle frontal gyrus, p = .043). Whole brain analyses revealed widespread relatively increased activation in adults with PKU in the main task contrast (NoGo > Go). This increased activation was mainly observed outside the CCN and largely overlapped with the default mode network (DMN). In conclusion, only subtle inhibitory control deficits and associated brain activity differences were observed in young adults with PKU. Thus, this work adds to the notion that this particular population seems to be only slightly affected by such cognitive deficits. While there were also only minimal increases when compared to healthy subjects in brain activity in a cognitive control network, we observed more widespread activation increases outside this network. These results support the assumption of DMN dysfunction in PKU.

Tackling frontal lobe-related functions in PKU through functional brain imaging: a Stroop task in adult patients

BACKGROUND

Profound mental retardation in phenylketonuria (PKU) can be prevented by a low phenylalanine (Phe) diet. However, even patients treated early have inconsistently shown deficits in several frontal lobe-related neuropsychological tasks such as the widely accepted Stroop task. The goal of this study was to investigate whether adult patients exhibit altered brain activation in Stroop-related locations in comparison to healthy controls and if an acute increase in blood Phe levels in patients has an effect on activation patterns.

METHODS

Seventeen male, early-treated patients with classic PKU (mean ± SD age: 31.0 ± 5.2 years) and 15 male healthy controls (32.1 ± 6.4 years) were compared using a color-word matching Stroop task in a functional magnetic resonance imaging (fMRI) study at 3T. Participants were scanned twice, and an oral Phe load (100 mg/kg body weight) was administered to patients prior to one of the fMRI sessions (placebo-controlled). Activity in brain regions that are known to be involved in Stroop tasks was assessed.

RESULTS

PKU patients exhibited poorer accuracy in incongruent trials. Reaction times were not significantly different. There were no consistent differences in BOLD activations in Stroop-associated brain regions. The oral Phe administration had no significant effect on brain activity.

CONCLUSIONS

Neither a generally slower task performance nor distinctively altered functioning of brain networks involved in a task representing a subset of dopamine-dependent executive functions could be proven. Decreased accuracy and inconsistent findings in posterior areas necessitate further study of frontal-lobe functioning in PKU patients in larger study samples.

Phenylketonuria: no specific frontal lobe-dependent neuropsychological deficits of early-treated patients in comparison with diabetics

Neuropsychologic studies have shown that even phenylketonuric patients treated early suffer from phenylalanine-related deficits in all age periods, from childhood to adulthood. This study was performed to determine whether phenylketonuric children show specific frontal lobe-dependent deficits when compared with diabetic patients. The comparative study included 42 phenylketonuric patients, 10 to 18 y of age [mean 14.7 (years, months), SD 2.9], and 42 diabetic patients matched for sex, age, and socioeconomic status. Patients were assessed for intelligence quotient (Culture Fair Intelligence Test), information processing (Wisconsin Card Sorting Test, Trail-Making Test), and selective (Stroop task) as well as sustained attention (Test d-2). Phenylketonuric patients had significantly poorer results than the diabetic patients. Within all tests, however, this was due to reduced performance speed, not to deficits in specific functions. Patients did not show deficits in insight and learning. The selection abilities and the sustained attention of the phenylketonuric patients were not impaired. Performance speed and blood phenylalanine levels were negatively correlated. Elevated phenylalanine levels may cause an imbalance in neurotransmitter metabolism. However, this seems to refer to a global neurotoxic effect rather than to specific effects on the dopaminergic system, which would affect specifically the activation of the frontal lobes.

Evidence for the importance of dopamine for prefrontal cortex functions early in life

There is considerable evidence that dorsolateral prefrontal cortex subserves critical cognitive abilities even during early infancy and that improvement in these abilities is evident over roughly the next 10 years. We also know that (a) in adult monkeys these cognitive abilities depend critically on the dopaminergic projection to prefrontal cortex and (b) the distribution of dopamine axons within dorsolateral prefrontal cortex changes, and the level of dopamine increases, during the period that infant monkeys are improving on tasks that require the cognitive abilities dependent on prefrontal cortex. To begin to look at whether these cognitive abilities depend critically on the prefrontal dopamine projection in humans even during infancy and early childhood we have been studying children who we hypothesized might have a selective reduction in the dopaminergic innervation of prefrontal cortex and a selective impairment in the cognitive functions subserved by dorsolateral prefrontal cortex. These are children treated early and continuously for the genetic disorder, phenylketonuria (PKU). In PKU the ability to convert the amino acid, phenylalanine (Phe), into another amino acid, tyrosine (Tyr), is impaired. This causes Phe to accumulate in the bloodstream to dangerously high levels and the plasma level of Tyr to fall. Widespread brain damage and severe mental retardation result. When PKU is moderately well controlled by a diet low in Phe (thus keeping the imbalance between Phe and Tyr in plasma within moderate limits) severe mental retardation is averted, but deficits remain in higher cognitive functions. In a four-year longitudinal study we have found these deficits to be in the working memory and inhibitory control functions dependent upon dorsolateral prefrontal cortex in PKU children with plasma Phe levels 3-5 times normal. The fact that even infants showed these impairments suggests that dopaminergic innervation to prefrontal cortex is critical for the proper expression of these abilities even during the first year of life. To test the hypothesis about the underlying biological mechanism we have created the first animal model of early and continuously treated PKU. As predicted, the experimental animals had reduced levels of dopamine and the dopamine metabolite, homovanillic acid (HVA), in prefrontal cortex and showed impaired performance on delayed alternation, a task dependent on prefrontal cortex function. Noradrenaline levels were unaffected; however some reduction in serotonin levels and in dopamine levels outside the prefrontal cortex was found. If prefrontal cortex functions are vulnerable in children with a moderate plasma Phe:Tyr imbalance because of the special properties of the dopamine neurons that project to prefrontal cortex, then other dopamine neurons that share those same properties should also be vulnerable in these children. The dopamine neurons in the retina share these properties (i.e. unusually high firing and dopamine turnover rates), and we have found that PKU children with plasma Phe levels 3-5 times normal are impaired in their contrast sensitivity, a behavioural measure sensitive to retinal dopamine levels.

Deficits in selective and sustained attention processes in early treated children with phenylketonuria--result of impaired frontal lobe functions?

Twenty normally intelligent children with early treated phenylketonuria (PKU) (IQ: mean = 101.4, SD = 10.0; age: mean = 10 years 11 months, SD = 1.3 years) and 20 healthy controls, matched for age, sex and IQ, were assessed for their selective (Stroop Task) and sustained attention (Test-d-2). Using positron emission tomography an activation of the frontal lobe during the Stroop task had previously been demonstrated. In addition to the Stroop Task and the Test-d-2, a short-term memory test as a "non-frontal-lobe-function-task" was administered to all subjects. Group comparisons demonstrated that PKU children had specific deficits in selective and sustained attention, which were significantly correlated with the concurrent serum phenylalanine concentration.

CONCLUSION

The results give evidence that even dietary treated children with PKU were suffering from impaired attentional control mechanisms in spite of a normal IQ. The deficits might be the result of impaired frontal lobe functions.

Deficient cumulative learning: an animal model of retarded cognitive development

Biological insults that produce profound mental retardation (MR) in humans have generally been found to produce little cognitive dysfunction in animal models. Based on the fact that impaired transfer of learning is one of the hallmark characteristics of mentally retarded humans, we proposed that this discrepancy may largely reflect the common use of a single learning task as the critical cognitive index rather than an assessment of cumulative learning. Consistent with this hypothesis, rats exposed to prenatal hyperphenylalaninemia (a model of maternal PKU) evidenced significant impairment when tested on a series of 10 problems designed to allow for positive transfer of learning. This same treatment, however, did not alter learning rate of the individual tasks that comprised this series when presented singly to experimentally naive animals. Deficient transfer of learning contributed significantly to the impairment observed in the maternal PKU group. These results support the hypothesis that the assessment of cumulative learning is an important component of animal models of impaired cognitive development.

Cognitive profile of rats exposed to lactational hyperphenylalaninemia: correspondence with human mental retardation

The present study was designed to provide further information on the enduring cognitive effects of experimental phenylketonuria (PKU) in rats, produced by the administration of alpha-methylphenylalanine and phenylalanine on postnatal days 3-21. These rats evidenced: (1) impaired learning set formation, (2) stimulus perseveration, particularly after an error, and (3) difficulty in utilizing the less salient features of their environment in mastering discrimination problems. In contrast, long-term memory function and the ability to form simple associations did not differ from controls. This pattern of intact and impaired cognitive functions bears remarkable similarity to that of mentally retarded humans and neonatally hyperphenylalaninemic rhesus monkeys, thus affirming the use of rats to study mental retardation. In addition, possible reasons for the mildness of the impairments commonly observed in animal models of severe mental retardation syndromes are discussed. We suggest that transfer of learning paradigms that assess the animal's ability to use information acquired in other problems are more likely to uncover significant cognitive impairments in such models than are procedures that test only the animals' ability to solve a single problem.

Cerebral serotonin regulation by phenylalanine analogues and during hyperphenylalaninemia

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.

Cerebral glycine content and phosphoserine phosphatase activity in hyperaminoacidemias

Chronic hyperphenylalaninemia maintained with the aid of a suppressor of phenylalanine hydroxylase, alpha-methylphenylalanine, increases the glycine concentration and the phosphoserine phosphatase activity of the developing rat brain but not that of liver or kidney. Similar increases occur after daily injections with large doses of phenylalanine alone, while tyrosine, isoleucine, alanine, proline, and threonine, were without effect. Treatment with methionine, which increases the phosphoserine phosphatase activity of the brain and lowered that of liver and kidney, left the cerebral glycine level unchanged. When varying the degrees of gestational or early postnatal hyperphenylalaninemia, a significant linear correlation was found between the developing brains' phosphoserine phosphatase and glycine concentration. Observations on the uptake of injected glycine and its decline further indicate that coordinated rises in the brain's phosphoserine phosphatase and glycine content associated with experimental hyperphenylalaninemia denote a direct impact of phenylalanine on the intracellular pathway of glycine synthesis in immature animals.

Developmental changes of cerebral phenylalanine uptake from severely elevated blood levels

Brain phenylalanine concentrations at plasma levels raised to that in phenylketonuric subjects were studied in rats from fetal through postnatal life. Suppression of the hepatic phenylalanine hydroxylase with alpha methylphenylalanine, and injections of age-adjusted doses of phenylalanine on the next day, assured the persistence of the same elevation of plasma levels for at least four hours prior to assay. The net phenylalanine uptake determined under these conditions underwent several-fold decreases between the fourth day and the end of the suckling period, and by about the age of 30 days it was as low as in adulthood. The development of transport properties studied here could contribute to the change with age in the vulnerability of the brain to the same degree of hyperphenylalaninemia and, since the cerebral phenylalanine uptake may decrease to non-damaging levels during childhood, it is pertinent to defining the age at which the rigorous diet of phenylketonurics might be safely relaxed.