Protective effect of tryptophan and 5-hydroxytryptophan on experimental phenylketonuria induced with phenylalanine+ p-cholorophenylalanine in rats.

An experimental phenylketonuria-like syndrome was produced in rats by oral administration of 1-phenylalanine (Phe, 500 mg/kg) and dl-p-chloro-phenylalanine (pCPA, 100-125 mg/kg) daily from the 2nd-3rd day of life to the age of 42 days. The effects of added dl-5-hydroxytryptohan (5HTP, 5 mg/kg) or 1-tryptophan (Trp, 100-125 mg/kg) were also studied. The groups receiving Phe+pCPA gained less weight than normal and less than the Phe controls. When tested during the last week of treatment the Phe+pCPA rats made more errors in the swim maze without changes in swimming times, and they showed significantly poorer learning of conditioned shock avoidance. These effects were partly antagonized by 5HTP or Trp. One to three weeks after the end of the treatments similar differences still remained in conditioned shock avoidance (and Trp and 5HTP prevented the effect of pCPA), but no more in maze learning, motor activity, or exploration on open field. Brain 5HT and 5-hydroxyindoleacetic acid (5HIAA) were lowered in all the pCPA treated rats during the treatment but returned to normal after it. Trp and 5HTP in the doses used only partly corrected the decrease of 5HT and 5HIAA. Brain noradrenaline did not change much.