Effect of thyroid hormones on benzodiazepine receptors in neuron-enriched primary cultures

The effect of administration of thyroid hormones on central benzodiazepine receptors was investigated using neuron-enriched primary cultures obtained from the neopallium of 16-day-old embryonic rats. Addition of L-triiodothyronine for 3 days decreased the maximal number of benzodiazepine receptor binding sites without any change in affinity at 10(-5) and 10(-6) M. L-Thyroxine administered for 3 days had the same effect at 10(-5) M. No significant change was observed over periods of less than 3 days, a finding indicating that this inhibition was not a direct in vitro effect. This down-regulation seems to be a direct modulatory effect of thyroid hormones on cerebral cortical neurons. Addition for 3 days of D-thyroxine and D-triiodothyronine, which are physiologically inactive isomers of the thyroid hormones, did not induce any significant alterations in benzodiazepine receptors. The decrease in number of cerebral cortical neuronal benzodiazepine receptors due to L-isomers of thyroid hormones may be related to the convulsions and anxiety observed in thyrotoxicosis in humans.