Opioid ligand binding sites in the spinal cord of the guinea-pig

Document Type

Journal Article

Publication Date

1-1-1986

Journal

Neuropharmacology

Volume

25

Issue

5

DOI

10.1016/0028-3908(86)90170-X

Keywords

binding sites; opioid binding; opioid delta receptors; spinal cord; spinal cord kappa receptors; spinal cord mu receptors

Abstract

The properties of opioid binding sites in membranes from the spinal cord of the guinea-pig were analyzed in experiments employing radiolabeled opioid ligands, selective or partially selective for μ, δ and k-type binding sites. Incubation was conducted at 37°C in a quasi-physiological modified Krebs medium, containing sodium and magnesium. The types of binding sites were discriminated on the basis of their affinities for [3H]-d-Ala2-MePhe4-Gly5-ol]enkephalin ([3H]DAGO), [3H-d-Ala2-d-Leu5-enkephalin, and [3H]ethyketocyclazocine and the relative potencies of the displacing ligands, DAGO, [d-Ser2-Leu-5]enkephalyl-Thr and tran-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzene-acetamide methanesulfonate hydrate (U50488H), which are selective for μ, δ and k type binding sites respectively. In membranes from whole spinal cord, k type sites comprised about 60%, μ about 30% and δ about 10% of the total of μ, δ and k binding sites. Binding sites of the μ type were also found in the lumbo-sacral region of guinea-pig spinal cord, in contrast to earlier reports of their absence from this tissue. Morphine showed a better than 500-fold selectivity for μ over k sites in spinal cord, while nalbuphine and (-)1-cyclopentyl-5-(1,2,3,4,5,6-hexahydro-8-hydroxy-3,6,11-trimethyl-2,6-methano-3-benzazocin-11-yl)3-pentanone methanesulfonate (WIN 44441-3) showed about a 10-fold selectivity for μ sites. The drug U50488H had about a 150-fold greater affinity for k than μ-type binding sites. © 1986.

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