Characterization of [¹²⁵I]β-endorphin binding sites in the rat caudal dorsomedial medulla

[¹²⁵I]β-endorphin bound to high affinity (K(d) = 0.25 nM) 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-endorphin₁-₃₁ β-endorphin₁-₂₇ > DAMGO > naloxone > N- acetyl-β-endorphin₁-₃₁ > 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.