Loss of neurotransmitter receptors by hyperphenylalaninemia in the HPH-5 mouse brain.

The binding of the muscarinic acetylcholine antagonist quinuclinidylbensilate to its specific receptors was measured by quantitative autoradiography in the brain of the HPH-5 mouse, a phenylalanine hydroxylase-deficient mouse mutant, as a model for human PKU. Three types of response to a hyperphenylalaninemic condition were observed: no effect as in the putamen; a gradual decrease over time such as in several areas of the cerebral cortex and the hippocampus; a transient increase, followed by a decrease, such as in the frontal area of the cerebral cortex. Of particular significance is the effect on the CA1 and CA3 layer of the hippocampus, since this structure has been implicated in the acquisition and storage of long-term memory. Hyperphenylalaninemia leads to a decrease in neurotransmitter receptor density and, therefore, to a decrease in connectivity, which may form the basis for the mental retardation in this condition.