Effect of CH₃HgCl and several transition metals on the dopamine neuronal carrier; peculiar behaviour of Zn²⁺

CH₃Hg⁺ and metal ions inhibited the specific binding of (1-[2-(diphenylmethoxy)ethyl]-4-(3-phenyl-2-[1-³H]propenyl) piperazine) ([³H]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 μM) reduced the inhibitory potency of Cd²⁺ on the binding but was ineffective against CH₃Hg⁺ and Cu²⁺. Zn²⁺ at 0.3 to 10 μM significantly enhanced the specific binding of [³H]GBR 12783 and [³H]cocaine by 42 to 146%. Zn²⁺ (3 μM) increased the affinity of all pure uptake inhibitors tested and of the majority of the substrates for the [³H]GBR 12783 binding site. Dissociation experiments revealed that Zn²⁺ both inhibited and enhanced the [³H]GBR 12783 binding by recognizing amino acids located close to or in the radioligand binding site. Micromolar concentrations of Zn²⁺ noncompetitively blocked the [³H]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 [³H]GBR 12783 binding site; low concentrations of Zn²⁺ could allow a protection of these -SH groups.