Opiate Receptor-Mediated Inhibition of Catecholamine Release in Primary Cultures of Bovine Adrenal Chromaffin Cells

Abstract: Primary cultures of chromaffin cells from bovine adrenal medulla were used to evaluate the ability of several opiates to reduce the release of catecholamines induced by stimulation of nicotinic receptors. Etorphine, β-endorphin, Met-enkephalinArg⁶,Phe⁷], and the synthetic peptide d -Ala²,Me-Phe⁴,Met(O)^s-ol]enkephalin inhibited the acetylcholine-induced release of catecholamines with an IC₃₀ varying from 10^?7 to 1 × 10^?6M. The effect was stereospecific because levorphanol (IC₃₀= 7.5 × 10^?7M) was approximately two orders of magnitude more potent than dextrorphan. Morphine (μ-receptor agonist), d -Ala², d -Leu⁵]enkephalin (δ-receptor agonist), ethylketazocine (k -receptor agonist), and N-allylnormetazocine (σ-receptor agonist) were at least 100–1000 times less potent than etorphine. Diprenorphine (IC₅₀= 5 × 10^?7M) and naloxone (IC₅₀= 10^?6M) antagonized the effect of etorphine. High-affinity, saturable, and stereospecific binding sites for ³H]etorphine, ³H]dihydromorphine, ³H-d -Ala²,d -Leu⁵]enkephalin, ³H]ethylketazocine, and for ³H]N-allylnormetazocine, ³H]diprenorphine, and ³H]naloxone were detected in chromaffin cell membranes and in membranes obtained from adrenal medulla homogenates. However, the number of binding sites for ³H]etorphine and ³H]diprenorphine was 10–70 times higher than the number of sites measured with the other ³H ligands. The rank order of potency of these compounds for the displacement of ³H]etorphine binding correlates (r = 0.90) with the rank order of potency of the same compounds for the inhibition of acetylcholine-induced catecholamine release. These data suggest that a stereoselective opiate receptor (different from the classic μ-, δ-, k -, or σ-receptor) with high affinity for etorphine, diprenorphine, β-endorphin, and Met-enkephalinArg⁶,Phe⁷] modulates the function of the nicotinic receptor in adrenal chromaffin cells.