Selectivity of ligand binding to opioid receptors in brain membranes from the rat, monkey and guinea pig

Abstract

Conditions for the equilibrium binding to opioid receptor of [ 3H[sufentanil (μ selective), [ 3H][D-Pen 2, D-Pen 5]enkephalin (δ selective), and [ 3H]U69,593 (κ selective) were established in membranes from rat brain cerebrum, monkey cortex, or guinea pig cerebellum. The selectivity index of various opioid alkaloids and peptides in binding to the μ, δ, or κ opioid receptors was expressed as the ratio of their EC 50 values in displacing two selective radiolabeled ligands: [ 3H]sufentanil/[ 3H](D-Pen 2, D-Pen 5)enkephalin (selectivity: μ/δ), [ 3H]sufentanil/[ 3H]U69,593 (selectivity: μ/κ), or [ 3H][D-Pen 2, D-Pen 5]enkephalin/[ 3H]U69,593 (selectivity: δ/κ). High resolution in binding selectivity was observed: in rat brain the μ/δ selectivity for Tyr-D-Ala-Gly-(Me)Phe-Gly-ol and sufentanil were 0.02 and 0.03, whereas for [D-Pen 2, D-Pen 5]enkephalin and ICI 174,864 they were 1200 and 998. Compared to μ opiates, the specific binding of δ and κ agonists was less sensitive to sodium. The results describe a routinely applicable methodological approach for the assessment of selective ligand binding to the μ, δ and κ opioid receptors in rodent and monkey brain membranes.