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CHARACTERIZATION OF THE k‐SUBTYPE OF THE OPIATE RECEPTOR IN THE GUINEA‐PIG BRAIN

1 In homogenates of guinea‐pig brain, the characteristics of the binding of [3H]‐ethylketazocine, an agonist for the putative k‐receptor, were determined by estimation of the affinity and capacity of binding, by competitive inhibition for the binding site by unlabelled ligands and by selective protection of the binding site from alkylation by phenoxybenzamine. 2 At 25°C the maximum number of binding sites for [3H]‐ethylketazocine was about 14pmol/g fresh brain, of which about 50% were high affinity sites. 3 In competition experiments, the high affinity binding of [3H]‐ethylketazocine to the k‐binding site was readily displaced by several k‐agonists but not by the selective μ‐ligand, D‐Ala2, MePhe4, Glyol5‐enkephalin or the selective δ‐ligand, D‐Ala2, D‐Leu5‐enkephalin. In contrast, the k‐agonists tested so far exhibit a high degree of cross‐reactivity with the μ‐binding site but somewhat less with the δ‐binding site. Similar specificities were observed in protection experiments. 4 The approximate proportions of the three subtypes of opiate receptor in the guinea‐pig brain are 25% μ‐binding sites, 45% δ‐binding sites and 30% k‐binding sites. 5 The endogenous opioids, Met‐enkephalin, Leu‐enkephalin and porcine β‐endorphin have only a low affinity for the k‐binding site.