The effect of hyperphenylalaninaemia on the muscarinic acetylcholine receptor in the HPH‐5 mouse brain

Summary Previous studies on the effect of hyperphenylalaninaemia on the development of the muscarinic acetylcholine receptor in the cerebrum of the rat, using α‐methylphenylalanine‐induced hyperphenylalaninaemia, have shown a gradual and steady decrease in the number of binding sites for this neurotransmitter. The HPH‐5 mouse, a phenylalanine hydroxylase mutant, can be hyperphenylalaninaemic without the use of a hydroxylase inhibitor. By employing quantitative autoradiography using [ 3 H]quinuclinidylbenzilate to label muscarinic acetylcholine receptors, a refined analysis of this decrease in neurotransmitter binding sites can be made. The decrease was confirmed and is therefore due to the hyperphenylalaninaemia per se and not to the use of the inhibitor. Various areas of the brain reacted differently to hyperphenylalaninaemia, from no change (putamen) to a gradual decrease (external layer of the olfactory bulb, parietal, occipital and cingulate areas of the cerebral cortex, CA 1 and CA 3 layer of the hippocampus) to a decrease preceded by a transient increase (frontal area of the cerebral cortex, caudate nucleus). The extent of these changes depends on the duration of exposure to hyperphenylalaninaemia as well as on the degree of brain maturation, but can even be observed in the brain of the adult mouse on a hyperphenylalaninaemic regimen for 11 days. Since the hippocampus has been shown to be involved in the long‐term storage of information, damage to this structure by hyperphenylalaninaemia may provide a clue to the global mental retardation observed in untreated PKU.