| Abstract: | The action of the antithyroid drugs methimazole (MMI) and propylthiouracil (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. 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. 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. 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. Three groups of receptors have been, thus, postulated, based upon their different sensitivity towards alterations in membrane disulphide bridges in equilibrium SH] equilibrium: Group I, including imipramine recognition sites and the uptake system for 5-HT; Group II, including 5-HT1-receptors; Group III, including 5-HT2-and D2-receptors. |
