Ionic zinc may function as an endogenous ligand for the haloperidol-sensitive σ2 receptor in rat brain

In the search for an endogenous a transmitter, whose existence was previously suggested by release studies, we tested the effects of releasable substances known to be present in the hippocampus, and we determined that ionic zinc may function as an endogenous ligand for the haloperidol-sensitive σ₂ site. Zn²⁺ displaced 1,3-di(2-[5-³H]tolyl)guanidine ([³H]DTG) from two binding sites in rat brain membranes, with an IC₅₀ for the high affinity site of 110 ± 3 μM and for the low affinity site of 20 ± 4 mw. The σ₁-selective ligand (+)-[³H]pentazocine was only weakly displaced from rat brain membranes by Zn²⁺ (IC₅₀ = 1.4 ± 0.05 mM). These results indicate that the Zn²⁺-sensitive σ binding site corresponds to the σ₂ site. The interaction between Zn²⁺ and the σ₂ site may have physiological significance, because ionic zinc is present in synaptic vesicles in the brain and may function to regulate binding at the σ₂ site. To test this hypothesis, we measured the effects of metallothionein peptide 1, a specific zinc chelator, on the actions of the putative endogenous a ligand(s) released in the hippocampus by focal electrical stimulation. Release of the endogenous σ ligand(s) was measured by competition with specific radioligand binding in live hippocampal slices. High frequency, focal, electrical stimulation of the zinc-containing mossy fibers in the hilar region of the hippocampus caused a decrease in the specific binding of [³H]DTG, (+)-[³H]3-(3-hydroxyphenyl)-N-(1-propyl)piperidine, or (+)-[³H]pentazocine to a sites. The decrease in [³H]DTG binding was largely blocked by metallothionein peptide 1, whereas the decrease in (+)-[³H]pentazocine binding was unaffected. These results suggest that Zn²⁺ may act as an endogenous ligand at σ₂ sites in the rat hippocampus.