Thyroid Hormone Homeostasis in Brain: Possible Involvement of Adrenergic Phenomenon in Adult Rat

Abstract

Background: Imbalance in thyroid hormone concentrations has been linked with profound neurobehavioral alterations in the adult. Peripheral hypothyroidism is associated with a phenomenon of central thyroid hormone homeostasis in adult rat. This central homeostasis mechanism could be maintained by adrenergic interplay due to close physiological association between sympathetic nervous system activity and thyroid hormones. The central homeostasis is characterized by increased cerebrocortical synaptosomal T(3) content, deiodinase type II (DII) activity, and cAMP content. Methods: We injected specific alpha- and beta-adrenergic receptor (AR) agonists and antagonists along with an anti-thyroid drug to find out any AR-mediated action on central homeostasis. Results: The alpha(2)-AR agonist did not alter the onset of central homeostasis, but prolonged its duration. Similar prolongation was observed with alpha(2)-AR antagonist and alpha-AR agonist, but these compounds amplified the normal anti-thyroid drug-induced rise in cerebrocortical T(3) content on the day of onset of central homeostasis. Injections of the alpha- AR antagonist did not cause any perturbations. All these observations have been supported by parallel changes in cerebrocortical DII activity, cAMP and [Ca(2+)](i) content. Conclusion: There emerges a close correlation between cerebral T(3) content, DII activity, cAMP and [Ca(2+)](i) content that are regulated by the AR system. Thus, thyroid hormone homeostasis in the adult mammalian brain is maintained primarily by the beta-adrenergic pathway along with an unexpected pharmacological involvement of the alpha-ARs.