Intraperitoneal injections of p-chlorophenylalanine (pCPA) were given to 7 1 rats in the first 7 weeks of life, decreasing brain 5-hydroxytryptamine concentrations to 20% of control values. The brain weight was significantly decreased, evident after 2 weeks of treatment, and also 52 days after the last injection. The retarded brain growth was not due to a general growth inhibition only: body weight was decreased initially, but from Day 24 the experimental animals gained weight more rapidly than the controls, resulting in a significantly higher body weight from Day 67.
The decrease in brain growth was accompanied by behavioral changes, evident later in life. The first test was given 4 weeks after the last $PA-injection. The changes observed were moderate, but consistent in all the tests. There were no learning deficits or clear motivational changes. In all the tests there was evidence of a reduced arousal level in the pCPA-treated rats: latencies were long, exploration was low, habituation to new environment was fast, and the reactivity was low.
Rats fed or injected phenylalanine have frequently been used as an animal model for the human disease phenylketonuria (PKU) (Karrer & Cahilly, 1965). A previous experiment indicated that rats fed L-phenylalanine show a reduced ability to sustain arousal, and it was suggested that this might be a "basic" deficit in these rats (Hole, 1972). However, the biochemical alterations produced by feeding excess L-phenylalanine are not identical with the alterations in the disease; for instance, there is a great increase in the serum tyrosine levels in t h s preparation, in contrast to the normal or subnormal level in the disease (Rendina,