An animal model of early-treated PKU

Phenylketonuria (PKU) is a genetic disorder in which the hydroxylation of phenylalanine (Phe) to tyrosine is severely disrupted. If PKU is left untreated, severe mental retardation results. The accepted treatment is to restrict dietary intake of Phe. It has generally been thought that cognitive impairments are prevented if levels of Phe in plasma are maintained at or below five times the normal level. However, we recently documented that children treated early and continuously for PKU or children mildly hyperphenylalaninemic, who have levels of Phe in plasma approximately three to five times normal, still have cognitive impairments. These impairments are specific to the functions of frontal cortex (A. Diamond, W. Hurwitz, E. Lee, W. Grover, and C. Minarcik, unpublished observations). To investigate the mechanism underlying these cognitive deficits, an animal model of this condition was developed and characterized. Thirty-six rat pups were divided into three groups. The first group was treated pre- and postnatally with Phe and α-methylphenylalanine (a phenylalanine hydroxylase inhibitor). The second group was injected postnatally with Phe and α-methylphenylalanine. The third group received postnatal control injections. The mild plasma Phe elevations in the two experimental groups produced significant behavioral and neurochemical effects. Both experimental groups were impaired on a task dependent on frontal cortex, delayed alternation. Levels of dopamine, homovanillic acid (HVA), norepinephrine, and 5-hydroxyindole acetic acid (5- HIAA) were measured in medial prefrontal cortex, anterior cingulate cortex, striatum, and nucleus accumbens. The largest neurochemical reductions observed were in HVA and were in the two frontal cortical areas (medial prefrontal cortex and anterior cingulate cortex). There were modest reductions in HVA in the nucleus accumbens but no significant changes in HVA, or in any other metabolite or neurotransmitter, in the striatum. The levels of 5-HIAA were also reduced in all brain regions examined. There was no effect on norepinephrine in any of the four regions examined. Reduced levels of HVA in medial prefrontal cortex were the only neurochemical effect that significantly correlated with every measure of performance on the delayed alternation task. This study provides evidence of deleterious effects from mild elevations in the levels of Phe in plasma previously considered small enough to be safe. These effects include impaired performance on a cognitive task dependent on frontal cortex and reduced HVA levels in frontal cortex. It is not possible to rule out a role for other neurotransmitter systems from this study alone, but the results of other investigations suggest that the behavioral impairment is due to the alterations in the dopamine system in frontal cortex.