Characterization and distribution of 3H-(3MeHis2)thyrotropin releasing hormone receptors in rat brain.

The characteristics and distribution of putative thyrotropin releasing hormone (TRH) receptors were studied in rat central nervous system using the TRH analogue 3H-(3MeHis2)TRH as a radiolabeled ligand. The analogue had a dissociation constant of 2.3 +/- 0.2 nM and a receptor density of 34 +/- 2 fm/mg protein in whole brain, homogenates. An association rate constant ot 1.6 x 10(-3) min-1nM-1 and a biphasic dissociation with rate constants of 2.6 x 10(-3) min-1 and 1.3 x 10(-4) min-1 were observed. The brain was dissected into ten regions, and detectable levels of binding were found in all regions. The highest levels were found in amygdala/piriform cortex area and the septal region, and the lowest levels were found in the cerebellar and cerebral cortex. Competition curves showed the methylated analogue to have approximately 7-fold higher affinity for the receptor than TRH. The higher affinity, along with lower nonspecific binding, accounts for the much improved sensitivity of the binding assay of the methylated analogue (70-80% specific binding) as compared to 3H-TRH (15-20% specific binding) and enables one to work with much lower tissue amounts. Use of the tritiated analogue will greatly aid in further studies of TRH receptors.