| Abstract: | Abstract— Using the method of least squares, a logistic curve was fitted to the data points for DNA content in neonatal rat cerebellum versus postnatal age (day 0 is the day of birth). The resultant equation was differentiated to give an expression for the rate of cerebellar DNA accumulation in units of ng/h per mg wet cerebellum. The DNA accumulation rate in control rats increased from 77.0 at 2 days of age to a maximum of 108 at 7 days of age and declined thereafter to a minimum of 16.3 on day 15. Thyroxine treatment significantly (P < 0.05) increased the rate to 89.8 (117% of control) at 2 days of age, and a significant elevation was maintained to 6 days of age at which time a maximum rate of 115 (114% of control) was attained. The rate was significantly decreased below control at 9 and 12 days of age, and reached a minimum of 9.22 on day 15. The developmental pattern for the activity of cerebellar thymidylate synthetase (EC 2.1.1.6), in units of pmol/h per mg wet cerebellum, closely paralleled the pattern for DNA accumulation rate in both control and thyroxine-treated animals. In controls, thymidylate synthetase activity increased from 98.6 at 2 days of age to a maximum of 125 at 7 days of age and declined thereafter to a minimum of 30.0 at 15 days of age. In thyroxine-treated animals, the activity was significantly increased to 118 (122% of control) at 4 days of age and remained significantly elevated through 6 days of age at which time a maximum activity of 154 (115% of control) was attained; thereafter, the activity was significantly decreased below control and reached a minimum of 16.9 (56.3% of control) on day 15. The developmental pattern for the activity of cerebellar thymidine kinase (EC 2.7.1.21) did not parallel the DNA accumulation rate quite so closely, in neither treated nor control animals, as did the pattern for thymidylate synthetase activity. These data suggest that thymidylate synthetase activity in the developing rat cerebellum may be more important for maintenance of replicative DNA synthesis than is thymidine kinase activity. In addition, the thyroxine-induced acceleration of the increase and subsequent decline in rate of DNA accumulation and in the activities of thymidylate synthetase and thymidine kinase in developing rat cerebella is probably the result of alterations in the number of external granular cells undergoing replicative DNA synthesis. |
