TRANSTHYRETIN EXPRESSION IN THE RAT-BRAIN - EFFECT OF THYROID FUNCTIONAL-STATE AND ROLE IN THYROXINE TRANSPORT

Rats were made hypo- or hyperthyroid to study the role of thyroid horm ones on cerebral transthyretin (TTR) mRNA expression. TTR mRNA was det ected by Northern blot in rat liver, choroid plexus and meninges but n ot in cultured astrocytes or cultured cerebral endothelial cells. No c hanges were found in the levels of TTR mRNA in liver, choroid plexus o r meninges in hypo- or hyperthyroid rats compared with the controls. I n order to investigate the main route of thyroxine transport from bloo d to brain, the distribution of [I-125]thyroxine in the brain was stud ied after intravenous (i.v.) and intraventricular (i.v.c.) injection b y both direct counting and autoradiography. While distribution of [I-1 25]thyroxine could be seen throughout the brain parenchyma after i.v. injection, the labelling was confined to the CSF spaces after i.v.c ad ministration. When protein synthesis was inhibited by cycloheximide tr eatment and [I-125]thyroxine was injected intravenously, the uptake of [I-125]thyroxine in the choroid plexus decreased while the uptake in the cerebral cortex increased. This indicates that thyroxine is transp orted into the brain primarily through the blood-brain barrier and not via the choroid plexus and CSF. We discuss the possibility that TTR h as a role in the distribution of thyroxine throughout the brain.