Thermodynamic analysis of rat brain opioid mu-receptor-ligand interaction.

Opioid mu-receptors are membrane bound receptors. The mechanism by which they transduce their biological effect into the inner compartment of the postsynaptic cell is still not fully understood. The present study was attempted to the measurement of changes of the thermodynamic parameters of the recep­ tor — agonist/antagonist interaction. We have set up the binding assays of a mu-receptor agonist (H-dihydromorphine) as well as an antagonist (^-na­ loxone). The saturation isotherms of both ligands have been assayed at various temperatures and from the resulting KD values the standard changes of Gibbs energy, enthalpy and entropy have been calculated. While the binding of the mu-receptor agonist H-dihydromorphine appears to be entropy driven {A? = 230 J m o r ' K ' ) and endothermic (AH = 19kJmor'), the binding of the mu-receptor antagonist H-naloxone is apparently driven by a decrease of standard enthalpy (4H = — 27kJmol ' ; i.e. the reaction is exothermic) and is also characterized by an increase of standard entropy (zl5° = 76 J mol ' K ' ) . The maximal number of 'H-naloxone binding sites has to be determined by incubation at 0—4°C. The present data do not support the view that opioid mu-receptors transduce their biological signal through the adenylatecyclase system by a mechanism similar to beta-adrenergically stimulated adenylatecy­ clase.