Effect of CH 3HgCl and several transition metals on the dopamine neuronal carrier; peculiar behaviour of Zn 2+

Abstract

CH 3Hg + 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 CH 3Hg + > Cu 2+ > Cd 2+ > Zn 2+ > Ni 2+ = Mn 2+ = Co 2+, suggesting that -SH groups are chiefly involved in this inhibition. Five millimolar dithiothreitol reversed the rather stable block of the specific binding produced by Cd 2+ or Zn 2+. An increase in the concentration of Na +, or addition of either K + or Ca 2+ reduced the inhibitory effects of metal cations, except Cu 2+. Zn 2+ (3 μM) reduced the inhibitory potency of Cd 2+ on the binding but was ineffective against CH 3Hg + and Cu 2+. Zn 2+ at 0.3 to 10 μM significantly enhanced the specific binding of [ 3H]GBR 12783 and [ 3H]cocaine by 42 to 146%. Zn 2+ (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 Zn 2+ 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 Zn 2+ 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 +, Ca 2+ and metal ions could recognize some -SH groups located in the [ 3H]GBR 12783 binding site; low concentrations of Zn 2+ could allow a protection of these -SH groups.