THYROID-HORMONES AND CARBOHYDRATE-METABOLISM OF BRAIN IN THE TELEOST, CHANNA-PUNCTATUS .1. EFFECT OF T4 AND THIOUREA ON SUCCINIC-DEHYDROGENASE (SDH) ACTIVITY AND PROTEIN-CONTENT

The brain of the fishes (C. punctatus) subjected to cold acclimation (15 .+-. 1.degree.) in darkness exhibited low succinic dehydrogenase (SDH) activity after 3 and 7 days and high protein content after 7 days when compared with their warm-acclimated (32 .+-. 1.degree.) counterparts. In warm-acclimated fishes maintained in complete darkness, T4 [thyroxine] (0.5 .mu.g/g body wt/day) depressed the enzyme activity after 5 and 7 days of treatment and reduced the protein content after 3 days. But neither in cold acclimated fishes maintained in darkness for 3, 5 and 7 days nor in warm-acclimated fishes maintained in 12 h dark and 12 h daylight for the same periods did T4 induce a significant change in the same biochemical parameters. It appears that T4 action in this fish is dependent on acclimation temperature and light:dark regimes. When warm-acclimated control fishes maintained in complete darkness were compared with those maintained in 12 h dark and 12 h daylight, the enzyme activity was higher and the protein content lower in the former than in the latter. The results suggest that natural photoperiod regulates the thyroid activity in vivo. In vitro studies revealed that the presence of T4 (3.12 .mu.M) in the incubating medium stimulated the enzyme activity of brain homogenates possibly due to direct action on mitochondria. Immersion of fishes in thiourea solution (1 mg/ml) for 3, 5 and 7 days resulted in enhancement of enzyme activity after 7 days of treatment and of protein content after 5 days of treatment. Thyroid hormones in vivo appear to have an inhibitory effect on the carbohydrate metabolism of the nervous tissue.