Demonstration of a slowly dissociating form of bovine hippocampal synaptic membrane opiate receptors.

We studied the binding characteristics of opiate receptors in synaptic plasma membranes isolated from bovine hippocampus. On the basis of kinetic binding studies the interaction of [3H][D-Ala2,D-Leu5]enkephalin (DADL) with its receptor was an extremely slow process. Rates of association and dissociation were determined and a pseudo-first order rate constant for association calculated to be 5.68 x 10(5) l/mol . s at 25 degrees C. The rate of dissociation (t 1/2 = 70 min) was accelerated by GTP and changed from linear to biphasic. The kinetically derived equilibrium dissociation constant (0.29 nM) was considerably lower than the KD obtained from Scatchard and Hill plots (1.24 nM). Measurement of DADL association as a function of temperature yielded a linear Arrhenius plot. Finally, competition binding assays revealed that delta-specific agonists exhibited relatively high potency in displacing [3H]DADL from synaptic plasma membranes receptors whereas mu-specific agonists and antagonists were less effective. These results with bovine hippocampus may be explained by the formation of a slow-dissociating, high affinity agonist conformation of the delta-opiate receptor which has been predicted for the system by the cyclic-allosteric and ternary complex models.