Abstract
The regulatory influence of thyroid hormone on glucose-6-phosphate dehydrogenase (G6-PDH) and 6-phosphogluconate dehydrogenase (6-PGDH) activity as well as the ontogeny and relative distribution of these enzymes have been investigated in certain regions of the rat central nervous system. In adult female rats, little differences in the activities of G6-PDH and 6-PGDH were noted in various areas of the nervous system when enzyme activity was expressed per g of tissue. Studies on the ontogenesis of these hexose monophosphate shunt enzymes revealed minor changes in activity in the cerebellum between 1 day and 1 year of age. In the cerebral cortex, enzyme activity remained relatively constant at 0–60 days of age but increased significantly between 60 days and 1 year. Experimental cretinism, induced by thyroidectomy of 1-day-old rats with 100 μCi 131I, interfered with the activities of G6-PDH and 6-PGDH in developing cerebral cortex and cerebellum. Whereas 50 μCi 131I had little or no effect on these brain enzymes, 200 μCi of the radioisotope produced 30–50% inhibition of enzyme activity. Progressive increases in brain DNA as well as depression of body and brain growth resulted from increasing the dose of 131I. l-Triiodothyronine (T 3) treatment of neonatally thyroidectomized rats in early life produced significant increases in brain G6-PDH and 6-PGDH activity. However, when the initiation of T 3 treatment was delayed until cretinous rats had reached adulthood, the hormone had no appreciable effect on enzyme activity. The T 3-induced increases in brain hexose monophosphate shunt enzymes in neonatally thyroidectomized rats were prevented by cycloheximide, an inhibitor of protein synthesis, suggesting that new enzyme synthesis may be involved in the observed stimulation of brain enzyme activities. Data indicate that thyroid hormone plays an important role in the regulation of G6-PDH and 6-PGDH in the developing cerebral cortex and cerebellum.
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