The equilibrium receptor binding properties and the pharmacological specificity of the spinal receptors for thyrotropin-releasing hormone (TRH) were determined. [3H]MeTRH bound to a single class of high-affinity (dissociation constant, KdS = 5.0; 6.0 and 5.5 nM), saturable (Bmax = 21.5, 32.6 and 130.7 fmol/mg protein) binding sites for TRH in homogenates of the rat, guinea pig and rabbit spinal cord respectively. [3H]MeTRH receptor binding was competitively but differentially inhibited by TRH analogs. The inhibition constants (Kis) in the three spinal cord preparations were: MeTRH (4.2-5.9 nM); TRH (14.2-33.9 nM); RX77368 (113-122 nM); CG3703 (117-142 nM); MK-771 (122-140 nM); CG3509 (10-32 microM); NVal2-TRH (32-56 microM) and TRH free acid (37-73 microM). These data have shown that the parent tripeptide. TRH, and its methylated analog. MeTRH, are the most potent displacers of [3H]MeTRH receptor binding, and that N-terminus modifications (as in CG3703, CG3509). C-terminus modifications (as in RX77368, TRH free acid) and both N- and C-terminus modifications (as in MK-771) of TRH result in markedly reduced affinity for the TRH receptor. Although, CG3703 and CG3509 have been previously found to be almost equally effective in the treatment of spinal cord injury, we have found that CG3703 has a significantly higher (70- to 283-fold) affinity than CG3509 for spinal TRH receptors. Interestingly, although RX77368 and MK-771 appear to have similar TRH receptor affinities to CG3703, the former analogs have shown less beneficial effects in animal models of spinal injury.(ABSTRACT TRUNCATED AT 250 WORDS)