Subcellular distribution of thyrotropin-releasing hormone (TRH) in rat brain and hypothalamus.

TRH localization in rat brain and hypothalamic tissue was studied by determination of immunoassayable TRH in subcellular fractions prepared from homogenates of these tissues. In both hypothalamus and brain, most of the TRH was found in a crude mitochondrial fraction and its concentration was higher in the synaptosomal fraction isolated by density gradient centrifugation. Electron microscopy was used to verify the composition of the fractions. Exposure of the brain crude mitochondrial fraction to osmotic shock resulted in solubilization of TRH which then was sedimented by centrifugation at 96,000 X g. TRH added to brain homogenates in vitro was rapidly destroyed by incubation at 37 degrees C, but no endogenouse TRH was lost by such incubation. The localization of TRH of hypothalamic and brain tissue to subcellular fractions consisting largely of synaptosomes and synaptic vesicles is compatible with a neurotransmitter function for this substance.