Normalization of dendritic spine numbers in rat hippocampus after termination of phenylacetate injections (PKU model).

Increased numbers of apical dendritic spines are present on hippocampal CA1 pyramidal cells in rats injected with phenylacetate from 2 to 21 days of life if animals are sacrificed at 20-30 days. However, if sacrificed at 60-90 days, spine counts are not significantly different from saline injected controls. These results suggest that this increased spine density at 3-4 weeks represents retardation of normal maturational spine loss rather than an actual hyperplasia, and is reversible upon termination of the phenylacetate injections. Implications for human pathologic series of phenylketonuria are discussed.