Mechanisms of L-Triiodothyronine-Induced Inhibition of Synaptosomal Na+-K+-ATPase Activity in Young Adult Rat Brain Cerebral Cortex

The role of thyroid hormones (TH) in the normal functioning of adult mammalian brain is unclear. Our studies have identified synaptosomal Na(+)-K(+)-ATPase as a TH-responsive physiological parameter in adult rat cerebral cortex. L-triiodothyronine (T3) and L-thyroxine (T4) both inhibited Na(+)-K(+)-ATPase activity (but not Mg(2+)-ATPase activity) in similar dose-dependent fashions, while other metabolites of TH were less effective. Although both T3 and the β -adrenergic agonist isoproterenol inhibited Na(+)-K(+)-ATPase activity in cerebrocortical synaptosomes in similar ways, the β -adrenergic receptor blocker propranolol did not counteract the effect of T3. Instead, propranolol further inhibited Na(+)-K(+)-ATPase activity in a dose-dependent manner, suggesting that the effect of T3 on synaptosomal Na(+)-K(+)-ATPase activity was independent of β -adrenergic receptor activation. The effect of T3 on synaptosomal Na(+)-K(+)-ATPase activity was inhibited by the α2-adrenergic agonist clonidine and by glutamate. Notably, both clonidine and glutamate activate Gi-proteins of the membrane second messenger system, suggesting a potential mechanism for the inhibition of the effects of TH. In this paper, we provide support for a nongenomic mechanism of action of TH in a neuronal membrane-related energy-linked process for signal transduction in the adult condition.