Gpp(NH)p promotes the formation of a low affinity state for the δ opioid agonist-receptor complex

Abstract

We previously demonstrated an association time-dependent dissociation of the δ opioid agonist, DADL, from its receptor that suggested multi-step formation of a high affinity complex. We have now examined the effect of the GTP analog, Gpp(NH)p, on the dissociation of [ 3H]DADL from these same sites on bovine hippocampal synaptic membranes. Gpp(NH)p accelerates dissociation of this complex in a dose-dependent manner resulting in the transformation of a high affinity form to a lower affinity state. The Gpp(NH)p effect was more pronounced when it was added at the time of initiation of dissociation. Under these conditions Gpp(NH)p engendered biphasic off-rates that were no longer association time-dependent. The addition of Gpp(NH)p at the onset of association did not prevent (but retarded) the formation of the high affinity state. Steady state homologous displacement binding curves demonstrated single site [ 3H]DADL binding in the absence of Gpp(NH)p but in its presence an improved fit for a two site model was obtained. In summary, on the basis of both kinetic and equilibrium binding data Gpp(NH)p appears to transform the δ opioid agonist-receptor complex into a low affinity state.