Opioid receptor binding in rat spinal cord.

The interaction of various radioligands with spinal opioid receptors has been characterized under variable experimental conditions. Binding to mu, delta, and kappa sites was measured in all (cervical, thoracic, lumbar) segments. The apparent affinity constant (K) of [3H]Ethylketocyclazocine (EKC) was similar in Tris, 2.09 (+/- 1.06) X 10(8) M-1, and phosphate buffer, 2.16 (+/- 0.02) X 10(8) M-1, when its interaction with delta and mu sites was blocked. Without blocking ligands, EKC binding was resolved in two components: K1 = 1.01 (+/- 0.21) X 10(9) M-1 and K2 = 0.95 (+/- 0.61) X 10(7) M-1. Likewise, the binding of [D-Ala2, MePhe4, Gly(ol)5]enkephalin (DAGO) or [D-Ala2, D-Leu5]-enkephalin (DADLE) alone was represented by a 2-site model. By adjusting the radioligand and receptor concentration or by the addition of blocking ligands, binding was represented by a 1-site model for DAGO, K = 4.35 (+/- 1.41) X 10(8) M-1, and DADLE, K = 2.44 (+/- 0.08) X 10(8) M-1.