Abstract
Rat littermates made hyperphenylalaninaemic by daily injections of α-methylphenylalanine
plus phenylalanine exhibit only slight reductions in body weight but a considerably
affected cerebellar development. Weight and size of the cerebellum are reduced. The
DNA content (µg/mg wet weight and total cerebellar DNA) is decreased. Morphometric
measurements reveal a smaller area of all cerebellar layers; the external granular
layer being most, the white matter least affected. The cell packing density of the
cells in the granular layer is decreased whereas Purkinje cells are lined up closer
when compared with the controls. A small reduction in Purkinje cell number is accompanied
by considerably reduced estimated total cell number of the internal granular layer.
The incorporation of 3H-thymidine into cerebellar tissue is decreased at 5 days p. p., but increased at
15 days. No significant alteration is found in the kidney. Results are discussed in
relation to a limited protein synthesis of developing neurons resulting from "cellular
undernutrition" of other amino acids that compete with the high phenylalanine levels.
A comparison is made with the effects of "systemic undernutrition" on cerebellar development.
Phenylketonuria (PKU) is one of the most striking clinical examples of impairing brain
development by an amino acid imbalance. Experimental hyperphenylalaninaemia (hyperphe)
can be elicited in suckling rats by the combined injection of phenylalanine and α-methylphenylalanine,
an inhibitor of phenylalanine hydroxylase (Greengard et al 1976). This treatment brings
about changes in the composition of serum and brain, which are similar to those found
in human PKU (Delvalle et al 1978, Lane et al 1980), unspecific side effects of the
inhibitor have not been reported (Kelly and Johnson 1978, Lane el al 1980).
Hyperphe is accompanied by reduced serum concentrations of other essential amino acids
(Linneweh and Ehrlich 1960, 1962). Furthermore, high levels of phenylalanine were
shown to restrict the carrier mediated transport of large neutral amino acids into
the brain (Agrawal et al 1970, Aoki and Siegel 1970, Hughes and Johnson 1976, Neame
1961, Oja 1972). The resulting amino acid imbalance disturbs the normal function of
the protein synthesis system of nerve cells which is reflected in increased RNAse
activity (Roberts and Morelos 1976, 1980), polyribosome disaggregation (Copenhaver
et al 1973, Hughes and Johnson 1977, Roberts and Morelos 1980, Taub and Johnson 1975),
altered t-RNA-coupling to amino acids (Hughes and Johnson 1976, 1977, 1978, Taub and
Johnson 1975), impaired incorporation of labelled amino acids into proteins (Agrawal
et al 1970, Aoki and Siegel 1970, Berger et al 1980, Hughes and Johnson 1976, Oja
1972) and changed structure as well as pattern of neuronal proteins (Lane and Neuhoff
1980, Rodrigues and Borisy 1979). The most fascinating feature of hyperphe is its
exclusive and specific effect on the differentiating brain; neither any other developing
organ nor the adult brain have been shown to be influenced to a comparable extent.
Neuropathological studies of PKU-brains are few. Microencephaly and reduced content
and stainability of the white matter are the most striking features (Alvord et al
1950, Crome and Pare 1960, Malamud 1966). In a previous paper (Huether et al 1982)
a desynchronisation of certain parameters of myelination and reduced myelin yields
have been shown in brain and spinal cord of hyperphe rats. But in accordance with
other reports (Figlewicz and Druse 1980) the gross chemical composition of isolated
myelin was not found to be largely affected. This suggests that myelination is not
likely to be the primary defect in PKU.
In this study the effect of experimental hyperphe on the developing cerebellum is
investigated. Special attention has been given to biochemical and morphological parameters
of proliferation and cell numbers which are known to be affected considerably by other
metabolic disorders, such as undernutrition (Balázs et al 1979, Barnes and Altman
1973) and hormonal imbalances (Bohn and Lauder 1980, Cotterrell et al 1972, Gombos
1980, Gourdon et al 1973, Nicholson and Altman 1972).
Key words
Phenylketonuria - Hyperphenylalaninaemia - Cerebellum - Brain development - Undernutrition