Modulation of mu, delta and kappa opioid receptors in rat brain by metal ions and histidine

Abstract

The effect of zinc (Zn 2+) and several other trace elements was studied on the binding of the opioid receptor agonists [ 3H] DAGO ([Tyr- d-Ala-Gly-Methyl-Phe-Olyol]-enkephalin) a, [ 3H] DSTLE ([Tyr- d-Ser-Gly-Phe-Leu-Thr]-enkephalin) and [ 3H] EKC (ethylketocyclazocine), which are specific for the mu, delta and kappa opioid receptors, respectively, in the cerebral cortex of the rat. Physiological concentrations of zinc were inhibitory to mu receptor binding, whereas the delta and kappa receptors were relatively insensitive to this inhibition. Scatchard analysis, using these opioid agonists, revealed curvilinear plots; concentrations of zinc equal to or less than the IC 50 (the concentration of cation which caused 50% inhibition of the binding of opioid ligand to its receptor), increased the K D (the dissociation constant) of all three subtypes of receptor, with no effect on the B max (the maximum number of binding sites) and abolished the high affinity sites of the delta and kappa receptors. Copper, cadmium and mercury also inhibited the binding of these ligands to their receptors. Histidine was most effective in preventing the inhibitory effects of zinc and copper, whereas it was less effective on cadmium and without any effect on the inhibition caused by mercury. Magnesium and manganese were stimulatory to opioid receptor binding, whereas cobalt and nickel had dual (stimulatory and inhibitory) effects. Non-inhibitory concentrations of zinc significantly decreased the stimulatory effects of magnesium and manganese on the mu and delta receptors, suggesting that part of the effect of zinc was through prevention of the actions of stimulatory cations. The reducing reagent dithiothreitol partially protected against inhibition by zinc and the oxidizing reagent dithiobisnitrobenzoic acid potentiated the inhibitory effects of zinc on the binding of [ 3H] DSTLE and [ 3H] DAGO. These results suggest that physiological concentrations of free zinc can inhibit mu but not delta and kappa receptors. The lower susceptibility of delta and kappa receptors to inhibition by zinc may be due to the presence of fewer sulfhydryl-groups in these receptors, or due to their inaccessibility to binding by zinc. Relative physiological concentrations of free zinc and histidine may govern the extent of binding of the mu ligands to their receptors.