EFFECT OF CH3HGCL AND SEVERAL TRANSITION-METALS ON THE DOPAMINE NEURONAL CARRIER - PECULIAR BEHAVIOR OF ZN2+

CH3Hg+ and metal ions inhibited the specific binding of (1-[2-(diphenylmethoxy)ethyl]-4-(3-phenyl-2-[1-H-3]propenyl) piperazine) ([H-3]GBR 12783) to the dopamine neuronal carrier present in membranes from rat striatum with a general rank order of potency CH3Hg+>CU2+>Cd2+>Zn2+>Ni2+=Mn2+=Co2+, suggesting that -SH groups are chiefly involved in this inhibition. Five millimolar dithiothreitol reversed the rather stable block of the specific binding produced by Cd2+ or Zn2+. An increase in the concentration of Na+, or addition of either K+ or Ca2+ reduced the inhibitory effects of metal cations, except CU2+.Zn2+ (3 mu M) reduced the inhibitory potency of Cd2+ on the binding but was ineffective against CH3Hg+ and CU2+.Zn2+ at 0.3 to 10 mu M significantly enhanced the specific binding of [H-3]GBR 12783 and [H-3]cocaine by 42 to 146%. Zn2+ (3 mu M) increased the affinity of all pure uptake inhibitors tested and of the majority of the substrates for the [H-3]GBR 12783 binding site. Dissociation experiments revealed that Zn2+ both inhibited and enhanced the [H-3]GBR 12783 binding by recognizing amino acids located close to or in the radioligand binding site. Micromolar concentrations of Zn2+ noncompetitively blocked the [H-3]dopamine uptake but they did not modify the block of the transport provoked by pure uptake inhibitors. These findings suggest that Na+, K+, Ca2+ and metal ions could recognize some -SH groups located in the [H-3]GBR 12783 binding site; low concentrations of Zn2+ could allow a protection of these -SH groups.