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

Abstract

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 microM) reduced the inhibitory potency of Cd2+ on the binding but was ineffective against CH3Hg+ and Cu2+. Zn2+ at 0.3 to 10 microM significantly enhanced the specific binding of [3H]GBR 12783 and [3H]cocaine by 42 to 146%. Zn2+ (3 microM) 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.