Table 2.

A synthesis of how zinc ions modulate the interactions between opioid receptors ligands and their receptors is presented below

		type of ligand	endogenous / exogenous (for agonists)	name	receptor	effects	brain structure	concentration	reference
zinc ion actions		agonists	endogenous	enkephaline	δ-receptor specific agonist	reduced both affinity and number of binding sites		physiological concentration	Ogawa N. et al., 1985 [43]
			exogenous	[3H] DAGO [([Tyr-D-Ala-Gly-Methyl-Phe-Glyol]-enkephalin	μ-receptor	reduced		micromolar concentration	Tejwani G.A., Hanissian S.H. 1990 [42]
					μ-receptor	reduced			Fowler C.B. et al., 2004 [78]
				[3H] DSTLE ([Tyr-D-Ser-Gly-Phe-Leu-Thr]-enkephalin)	δ-receptor	slighly reduced		micromolar concentration	Tejwani G.A., Hanissian S.H. 1990 [42]
				3H-met-enkephalinamide (2-D-ala-5-L-methionine)	δ-receptor	reduced	hippocampus, the cerebral cortex and the basal ganglia of the (rat)	endogenous concentrations	Stengaard-Pedersen K., 1982 [79]
				[3H] EKC (ethylketocyclazocine)	κ-receptor	slighly reduced		micromolar concentration	Tejwani G.A., Hanissian S.H. 1990 [42]
				(+)-[3H]pentazocine	σ2-receptor	slighly reduced	hippocampus (rat)	milimolar concentration	Connor M.A., Chavkin C. 1992 [83]
		antagonist		[3H]naloxone	μ-receptor	reduced affinity, with no effect on the number of binding sites	hippocampus, cortex, midbrain, striatum (rat)	dose-dependent manner	Hanissian S.H., Tejwani G.A. 1990 [85]
reduced zinc concentration compared to physiologic conditions	metallothionein peptide 1 (specific zinc chelator)	agonists	exogenous	(+)-[3H]pentazocine	σ2-receptor	non-changed	hippocampus (rat)		Connor M.A., Chavkin C. 1992 [83]
	zinc deficiency effect compared to normally-fed animals	antagonist		[3H]naloxone	μ-receptor	increased	isolated brain membranes (rat)		Essatara M.B., 1984 [72]