Sulfhydryl groups in receptor binding of thyrotropin-releasing hormone to rat amygdala

Binding of [3H]-[3-Me-His2]thyrotropin-releasing hormone [( 3H]MeTRH) to TRH receptors in rat amygdala was decreased by sulfhydryl reagents in a time-, temperature-, and concentration-dependent manner. A pronounced reduction in receptor density, with little or no change in binding affinity, was apparent following disulfide bond reduction by dithiothreitol (DTT), alkylation of thiol groups by N-ethylmaleimide (NEM), and their oxidation by 5,5'-dithiobis (2-nitrobenzoic acid). Heavy metals (Cd2+, Hg2+), which complex with reactive -SH residues, also potently inhibited binding. The pharmacological specificity of residual [3H]MeTRH binding in chemically modified amygdala membranes was the same as that in control preparations. Sequential exposure to thiol reagents, in the presence or absence of cations, revealed possible additive effects. Pretreatment of membranes with TRH (10(-8) - 10(-6) M), and its continued presence during modification, afforded protection against DTT and NEM. These results indicate the possible importance of thiol groups in the maintenance of TRH receptor conformation.