Selective effects of thiol reagents on the binding sites for imipramine and neurotransmitter amines in the rat brain

Abstract

1 The action of the antithyroid drugs methimazole (MMI) and propylthiouracyl (PTU) on the binding of [³H]-imipramine, [³H]-5-hydroxytryptamine ([³H]-5-HT) (to 5-HT₁-receptors) and [³H]-spiperone (to 5-HT₂-, D₂-receptors) of rat brain membranes has been examined. The synaptosomal uptake of [³H]-5-HT was also studied. 2 Micromolar concentrations of the disulphide bond reducing agents MMI, PTU, dithiothreitol (DTT) and mercaptoethanol increased both the binding of [³H]-imipramine and the uptake of [³H]-5-HT. In contrast, they decreased the number of 5-HT₁-receptors, and did not affect 5-HT₂- and D₂-sites. 3 Reaction with membrane-bound sulphydryl (SH) groups by micromolar concentrations of N-ethylmaleimide (NEM), hydroxymercuribenzoic acid (PCMB), or Ellman's reagent (DTNB) decreased the binding of [³H]-imipramine, the number of 5-HT₁-receptors, and the uptake of [³H]-5-HT. Millimolar concentrations of NEM were necessary in order to decrease partially 5-HT₂- and D₂-receptors. 4 The effects of NEM on imipramine recognition sites and on the uptake of 5-HT could be prevented by DTT; protection was not obtained in other receptor systems. 5 Three groups of receptors have been, thus, postulated, based upon their different sensitivity towards alterations in membrane [disulphide bridges SH] equilibrium: Group I, including imipramine recognition sites and the uptake system for 5-HT; Group II, including 5-HT]-receptors; Group III, including 5-HT₂- and D₂-receptors.