Preparation of 3H-[3-M3-His2]TRH as an improved ligand for TRH receptors

[3H]-3-methyl-His2]thyrotropin releasing hormone ([3H]MeTRH) binds to sites in the sheep anterior pituitary gland which appear to be the same as those occupied by [3H]TRH and which can therefore be identified as TRH receptors. In competition experiments performed in parallel, both ligands gave the same number of binding sites, 15 pmol/g wet weight, and showed the same pharmacology for 19 TRH analogs ranging over more than 5 orders of magnitude in potency. The apparent dissociation constant of binding of [3H]MeTRH was about 3.5 nM compared to 29 nM for [3H]TRH. Kinetic experiments with [3H]MeTRH yielded a rate constant for association of 1.4 x 10(7) M-1 min-1 and rate constants for the biphasic dissociation of 5 x 10(-2) min-1 (fast phase) and 7 x 10(-3) min-1 (slow phase). Detailed methods are described for preparation of [3H]MeTRH by reduction of the dehydroproline precursor and its purification by ion exchange and antibody affinity chromatography. The major advantage of the new ligand is that its higher affinity of binding gives relatively less non-specific binding than is obtained with [3H]TRH, particularly in central nervous tissue.