Effect of transition or heavy metals on [ 3H]haloperidol binding in rat striatal membranes in vitro

Abstract

Our previous experiments have shown that several metal cations affect dopaminergic uptake and release processes in synaptosomes in vitro. It is thus possible that other membrane-related steps of neurotransmission, such as receptor binding, are affected as well. We studied the effect of Mn 2+, Cu 2+, Cd 2+, Zn 2+, Hg 2+, Pb 2+ and of two organometals, methyl mercury and triethyl lead, on [ 3H]haloperidol binding in the striatal P 2 fraction assuming that such a study would reveal direct effects of the ions on dopaminergic D 2 receptor binding. According to non-linear curve fitting and Scatchard analysis, [ 3H]haloperidol bound to two sites in striatal tissue. The K d of the higher affinity site was 0.14 ± 0.05 nM and the B max 226.3 ± 50.3 fmol/mg protein. The respective values for the lower affinity sites were 2.49 ± 0.56 nM and 678.3 ± 111.4 fmol/mg protein. Among the divalent cations, Hg 2+ (IC 50 0.7 μM) and Cu 2+ (IC 50 2.9 μM) inhibited the high affinity [ 3H]haloperidol binding most potently. The inhibition by Cu 2+ was due to a decrease in the binding affinity (increase in the K d) while the number of binding sites remained unchanged. Zn 2+ inhibited the binding by 41.8% and Cd 2+ by 38.7% at 10 μM concentration while Pb 2+ and Mn 2+ did not affect binding significantly at this or lower concentrations. Methyl mercury (IC 50 0.9 μM) and triethyl lead (IC 50 2.6 μM) inhibited binding as well. Both these organometallic cations decreased the binding affinity but did not change significantly the number of binding sites. The results suggest that particularly Cu 2+, Hg 2+, methyl mercury and triethyl lead are able to decrease striatal dopaminergic D 2 receptor binding if they reach the receptors in the brain.