Localization of triiodothyronine in nerve ending fractions of rat brain.

Radioactive triiodothyronine reaching the rat brain after intravenous administration is rapidly and selectively taken up in the nerve ending fraction. A concentration gradient of radioactivity from brain cytosol to synaptosomes is observed at 5 min, increases linearly over the first hour, and is maintained for at least 10 hr. Radioactivity in the synaptosomes is due to triiodothyronine (90%) plus a single unidentified metabolite (10%). Approximately 85% of the synaptosomal radioactivity is released by osmotic disruption of the particles. The process of selective uptake, concentration, and retention of triiodothyronine in nerve terminals of the rat brain may be related to the sympathomimetic and behavior-altering effects of the thyroid hormones.