**5. Subcellular levels of L-triiodothyronine (L-T3) and L-thyroxine (L-T4) in adult rat brain cerebral cortex**

As the brain approaches adulthood, nuclear iodothyronine concentrations gradually decreases reaching a plateau and maintains it, and the TH levels increase within nerve terminals of adult vertebrates [1,18-21]. It also demonstrated decrease in L-T3-binding in adult brain compared to developing brain.

Although, evidence of transportation 125I-L-T3 and 125I-L-T4 within the nerve terminal was demonstrated following intravenous injection in adult rat brain [10,18,19,22], its euthyroid concentrations and subcellular distribution was never been evaluated until recently [13,23]. Intravenous administration of [125I]-L-T4 in rats followed by thaw mount autoradiography showed distribution of L-T4 in selective areas of adult brain in a saturable manner. Gradually L-T4 was concentrated more within nerve terminals fractions, where L-T4 was monodeiodinated to produce L-T3, the active form of TH [10]. L-T4 and L-T3 transportation within neurons are shown to occur by two different mechanisms. L-T3 is actively taken up in a saturable manner, while L-T4 transportation occurs by diffusion and in a non-saturable way. L-T4-transporation within the neuron is dependent upon L-T4-concentration gradient between extracellular and intracellular compartments and is maintained by high deiodination rate of L-T4 to L-T3 [24]. Role of transthyretin has also been described as a major binding protein in cerebrospinal fluid. Transthyretin has been implicated to facilitate L-T4 transportation across the blood-brain-barrier and finally into the brain. Recently MCT-8 has been ascribed to be the most effective TH transporter [25]. These MCT-8 are 12 transmembrane spanning proteins, and in particular plays a major role for very specific transportation of L-T3 within the neurons followed by the active conversion of the prohormone L-T4 to L-T3 by the D-II within the CNS [26]. D-II is essentially important for the conversion of the prohormone L-T4 into the active L-T3 within the CNS. However, understandings of the levels of THs within the neurons are imperative. This information is crucial to explore the role of L-T3 in neural signal transmission in mature brain. To help meet this requirement the following study was performed to quantify and compare the levels of THs in adult rat brain cerebral cortex.
