Characterization of [ 125I]β-Endorphin Binding Sites in the Rat Caudal Dorsomedial Medulla

Abstract

[ 125I]β-endorphin bound to high affinity (K d = 0.25 n M) receptors in the caudal dorsomedial medulla of rats with a B max of 97 fmol/mg protein. The relative potency for displacement of [ 125I]β-endorphin binding was: β-endorphin 1–31 > β-endorphin 1–27 > DAMGO > naloxone > N-acetyl-β-endorphin 1–31 > U50488 > DPDPE. The B max for [ 3H]DAMGO binding was 81 fmol/mg protein, indicating that most [ 125I]β-endorphin binding corresponds to μ-opioid receptors. [ 3H]DAMGO binding was not influenced by lesioning noradrenergic nerve terminals in the caudal dorsomedial medulla. Our findings indicate that β-endorphin interacts primarily with μ-opioid receptors in the caudal dorsomedial medulla. These receptors are not affected by noradrenergic denervation.