[Regulation of opioid receptors by thyroliberin analogues]

The ability of thyroliberin (TRH) to interact with opioid receptors (OR) was studied using radioligand analysis. TRH did not influence specific binding of [3H]-naloxone, but increased affinity of high affinity binding sites for the ligand in a dose-dependent manner. TRH also decreased affinity of low- affinity binding sites. Kinetic analysis of low-affinity binding sites suggests the existence of at least two subpopulations of OR, which differed in their affinity to naloxone and mode of interaction with TRH. TRH acted as non-competitive and competitive inhibitor of receptor binding sites with the lowest and moderate affinity, respectively. The allosteric pattern of TRH influence on OR with high and the lowest affinity to naloxone was suggested. TRH analogues were estimated for their ability to change in OR binding characteristics. The level of [3H]-DADL specific binding was not influenced by the peptides tested but the affinity was changed. Blind control experiment showed the ability of the TRH relative substances to increase in affinity of d-receptors could be ranked in the row: dihydroorotyl-hystidyl-prolinamide > TRH > methionyl-asparagyl-phenylalaninamide. This is consistent with ability of these compounds to influence the dopaminergic events.