Thymidine Phosphorylase Activity in the Optic Tectum of Developing Chick Embryos and the Effects of 3′,5′‐Cyclic Nucleotides and Monosodium Glutamate on Enzyme Activity

Changes in activities of thymidine phosphorylase (EC 2.4.2.4.) in the optic tectum of developing chick embryos were determined and the effects of the addition of adenosine 3′,5′‐monophosphate or guanosine 3′,5′‐ monophosphate to the test medium were also determined. In addition, injection of 30 mg of monosodium glutamate onto the chorioallantoic membrane was assessed for effect on thymidine phosphorylase activity in the optic tectum. Thymidine phosphorylase activity was highest at the earliest times studied, peaking at the 10th day of incubation, but decreasing sharply until the 12th day, at which time a new level of activity was maintained until after the 18th day. Between the 18th day and 1 day post‐hatching there was a steady decrease in activity. The addition of adenosine 3′,5′‐monophosphate (cAMP) to the test medium resulted in decreased enzyme activity at 10 and 16 days of incubation, but no change at 1 day post‐hatching; guanosine 3′,5′‐monophosphate (cGMP) in the test medium resulted in decreased activity at 16 days, but in no change at 10 days or 1 day post‐hatching. The injection of monosodium glutamate into the egg before 10 days of incubation resulted in a decrease in enzyme activity only at 10, 18, and 20 days of incubation. Thymidine phosphorylase activity during optic tectum development has two phases (days 8‐10 and days 12‐18), each followed by a decrease in enzyme activity. The addition of cAMP to the test medium lowers activity during both phases, and cGMP only affects the second phase. This suggests two mechanisms for control of the enzyme, one present during both phases and one only during the second phase. That a single injection of MSG decreases enzyme activity at 10, 18, and 20 days of incubation, but not days 12‐16 or 1 day post‐hatching, suggests that MSG has not only immediate but also long‐term biochemical effects on prenatal brain development.