Thyroid hormone increases astrocytic glutamate uptake and protects astrocytes and neurons against glutamate toxicity

Abstract

Thyroid hormone (T₃) regulates the growth and differentiation of rat cerebellar astrocytes. Previously, we have demonstrated that these effects are due, at least in part, to the increased expression of extracellular matrix molecules and growth factors, such as fibroblast growth factor-2. T₃ also modulates neuronal development in an astrocyte-mediated manner. In the mammalian central nervous system, excitatory neurotransmission is mediated mainly by glutamate. However, excessive stimulation of glutamate receptors can lead to excitotoxicity and cell death. Astrocytic glutamate transporters, GLT-1 and GLAST, play an essential role in the clearance of the neuronal-released glutamate from the extracellular space and are essential for maintaining physiological extracellular glutamate levels in the brain. In the present study, we showed that T₃ significantly increased glutamate uptake by cerebellar astrocytes compared with control cultures. Inhibitors of glutamate uptake, such as L-PDC and DL-TBOA, abolished glutamate uptake on control or T₃-treated astrocytes. T₃ treatment of astrocytes increased both mRNA levels and protein expression of GLAST and GLT-1, although no significant changes on the distribution of these transporters were observed. The gliotoxic effect of glutamate on cultured cerebellar astrocytes was abolished by T₃ treatment of astrocytes. In addition, the neuronal viability against glutamate challenge was enhanced on T₃-treated astrocytes, showing a putative neuroprotective effect of T₃. In conclusion, our results showed that T₃ regulates extracellular glutamate levels by modulating the astrocytic glutamate transporters. This represents an important mechanism mediated by T₃ on the improvement of astrocytic microenvironment in order to promote neuronal development and neuroprotection.